Blankets of volatile gases and liquids near and above the surface of the earth are of prime importance, along with the solar energy, for the sustenance of life on earth. These are distributed and recycled throughout the atmosphere and hydrosphere of the planet.

The Earth is surrounded by a relatively thin atmosphere consisting of a mixture of gases, primarily molecular nitrogen (77 percent) and molecular oxygen (21 percent). This gaseous envelope, commonly called the air, also contains much smaller amounts of gases such as argon, carbon dioxide, methane, and water vapour, along with minute solid and liquid particles in suspension.

It is not surprising that the Earth, as a small planet (with a rather weak gravitational field) at fairly warm temperatures (due to its proximity to the Sun), should lack the most common gases in the universe, hydrogen and helium. Whereas both the Sun and Jupiter are dominantly composed of these two elements, they could not be retained long on the Earth and would rapidly evaporate into interplanetary space. It is surprising, however, that more than 20 percent of the Earth’s atmosphere is composed of oxygen, a highly reactive gas that, under most planetary conditions, would have combined with other chemicals. The two parts per million of methane in the atmosphere, which is far out of chemical equilibrium, is actually of biogenic origin (produced in the digestive tracts of cows, for example).

The atmosphere extends from the surface of the Earth to heights of thousands of kilometres, where it gradually merges with the solar wind—a stream of charged atomic particles that flows outward from the outermost regions of the Sun. The composition of the atmosphere is more or less constant with height to an altitude of about 100 kilometres.

The atmosphere is commonly described in terms of distinct layers, or regions. Most of the atmosphere is concentrated in the TROPOSPHERE, which extends from the surface to an altitude of about 15 kilometres. The behaviour of the gases in this layer is controlled by convection. This process involves the turbulent, overturning motions resulting from buoyancy of near-surface air that is warmed by the Sun. Convection maintains a vertical temperature gradient (i.e., temperatures decline with altitude) of roughly 6º C per kilometre (1o C per 165m) through-out the troposphere. At the top of the troposphere, which is called the tropopause (zone of transition between troposphere and stratosphere), temperatures fall to about -60º C (-76º F). The troposphere is the region where virtually all water vapour exists and where all weather occurs.

The dry, tenuous stratosphere lies above the troposphere and extends to an altitude of about 50 kilometres. Convective motions are weak or absent in the stratosphere; motions instead tend to be horizontally oriented. The temperature in this layer increases with altitude.

In the upper stratospheric regions, absorption of ultraviolet light from the Sun breaks down oxygen molecules; recombination of oxygen atoms with O2 molecules into ozone (O3) creates the ozone layer, which shields the lower ecosphere from harmful radiations of short-wavelength ( Ultra violet radiations).

Above the relatively warm stratopause (zone of transition between stratosphere and mesosphere) is the even more tenuous mesosphere, in which temperatures again decline with altitude, reaching roughly -85º C at the mesopause. Temperatures then rise with increasing height through the overlying layer known as the thermosphere.

Above about 100 kilometres, in the ionosphere, there is an increasing fraction of charged, or ionized, particles. Spectacular visible auroras are generated in this region, particularly along circular zones around the poles, by episodic precipitation of energetic particles.

The general circulation of the Earth’s atmosphere is driven by solar energy, which falls preferentially in equatorial latitudes. Atmospheric redistribution of heat pole ward is strongly affected by the Earth’s rapid rotation and the associated Coriolis force at non-equatorial latitudes (which adds an east-west component to the direction of the winds), resulting in about three latitudinal cells of circulation in each hemisphere. Instabilities produce the characteristic high-pressure areas and low-pressure storms of the mid-latitudes as well as the fast, eastward-moving jet streams of the upper troposphere that guide the paths of storms.

The oceans are massive reservoirs of heat, and their slowly changing currents and temperatures also influence weather and climate, as in the so-called El Nino episodes

The Earth’s atmosphere is not a static feature of the environment. Rather its composition has evolved over time in concert with life and continues to change as human activities alter the ecosphere. Roughly halfway through the history of the Earth, the atmosphere’s unusual complement of free oxygen began to develop owing to photosynthesis by blue-green algae and subsequently evolving plant life. Accumulation of oxygen eventually made it possible for respirating animals to move out onto the land.

The Earth’s climate at any location varies with the seasons, but also there are longer-term variations in global climate. Volcanic explosions, such as the 1991 eruption of Mount Pinatubo in the Philippines, can inject great quantities of particulates into the atmosphere, which remain suspended for years, decreasing atmospheric transparency and resulting in measurable cooling worldwide. Rare, giant impacts of asteroids and comets can have even more profound effects. The dominant climate variations observed in the recent geologic record are the ice ages, which are linked to small variations in the Earth’s geometry with respect to the Sun.

The Sun is believed to have been less luminous during the early history of the Earth, so if other planetary conditions were identical with those of today, the oceans would have been frozen. But it is expected that there was much more carbon dioxide in the Earth’s atmosphere during earlier periods, which would have enhanced greenhouse warming. In this phenomenon, heat radiated by the surface is trapped by gases such as carbon dioxide in the atmosphere and reradiated back to the surface, thereby warming it. There is presently 10 times more carbon dioxide buried in carbonate rocks in the Earth’s crust than in the atmosphere, in sharp contrast with Venus, whose atmospheric evolution followed a different course.

The amount of carbon dioxide in the atmosphere is rising steadily, and has evidently increased by more than 10 percent in the last 30 years owing to the burning of fossil fuels (e.g., coal, oil, and natural gas) and the destruction of tropical rain forests, such as that of the Amazon River basin. A further doubling by the middle of the 21st century could lead to a global warming of a few degrees, which would have profound effects on the sea level and on agriculture.

Of more immediate concern is the impact of human activities on the stratospheric ozone layer. Complex chemical reactions involving traces of man-made chlorofluorocarbons have recently created temporary holes in the ozone layer, particularly over Antarctica, during polar spring. More disturbing, however, is the discovery of a growing depletion of ozone over temperate latitudes, where a large percentage of the world’s population resides, since the ozone layer serves as a shield against ultraviolet radiation, which has been found to cause skin cancer.

Composition & Structure of Atmosphere

Earth’s atmosphere is mainly consisted of nitrogen, oxygen, and argon, which together constitute the major gases of the atmosphere. The remaining gases are often referred to as trace gases. The below table shows the composition of Dry atmosphere.

Composition of Earth’s Atmosphere
Gas	Volume
Nitrogen (N2)	78.08%
Oxygen (O2) 20.95%	20.95%
Argon (Ar)	0.93%
Carbon dioxide (CO2)	0.04%

Apart from the above gases dry atmosphere also contains traces of Neon, Helium, Krypton, Nitrous oxide etc.

Connecting Concept:  Difference in Density of Air Air is less dense at Equator Over equatorial regions, where the surface is being heated strongly throughout the year and air warmed by contact with it is expanding and rising, the air all the way up to the tropopause is less dense than air to the north and south. Thus, density of the air is maximum at the equator. But here,you must note that almost same amount of atmospheric mass exists at both equator and poles but only the density of the air is less at equator and greater at poles Poles Exert more gravitational pull on atmospheric gases Gravity increases from equator to poles as the earth is not a perfect sphere. That means the gravitational force is more over poles. Hence the atmosphere is pulled with more force near the poles and leads to contraction of the atmosphere. The centrifugal force due to Earth’s rotation is maximum at Equator Because the speed of the rotating earth is greatest at the equator the atmosphere tends to bulge out due to friction and Coriolis force.

Structure of Atmosphere

The atmosphere can be studied as a layered entity – each layer having its own peculiar characteristics:

 

Troposphere

Troposphere is the lowest portion of Earth’s atmosphere and contains approximately 80% of the atmosphere's mass and 99% of its water vapour and aerosols. The average depth of the troposphere is approximately 17 km in the middle latitudes. The characteristic features of the Troposphere are its greatest density. In addition to nitrogen and oxygen, carbon dioxide, and water vapour (nearly all of the water vapour contained in the atmosphere is concentrated in the troposphere) and of numerous particles of various origin.

Thickness of Troposphere

It thickness of the Troposphere is maximum at equator, deeper in the tropics, up to 20 km , andshallower near the polar regions, at 7 km in summer, and indistinct in winter. In India, it is taken to be around 16 Kilometers.

Chemical Composition of Troposphere

The chemical composition of the troposphere is essentially uniform, with the notable exception of water vapour. The amount of water vapour decreases strongly with altitude. Thus the proportion of water vapour is normally greatest near the surface and decreases with height.

Temperature of Troposphere

Temperature of the troposphere decreases with height. The rate at which the temperature decreases is called the Environmental Lapse Rate (ELR). The environmental lapse-rate (ELR) isabout 0.6°C per every 100 meters. Temperature decreases at an early uniform rate with increased altitude.

Tropopause

The boundary between troposphere and stratosphere, called the tropopause, is a temperature inversion. Tropopause refers to the altitude at which the fall in the temperature is stalled. This layer separates the troposphere from the stratosphere (the second layer of the atmosphere). This layer is usually quiet and no major movement of air takes place in it. Its height at Tropic of Cancer and

Tropic of Capricorn is roughly 10 to 15 km, highest at the equator 18 km and at the poles it is about 8 km above the earth. In India, the tropopause is generally at a height of around 16 km. The altitude of the tropopause varies with the variations of sea — surface temperature, season, latitude, and weather systems, such as the passage of cyclones and anticyclones. So, Tropopause is not a hardlined boundary. The higher is the temperature of the lower layers, the higher is the height of this layer, the layer is lower where there is a cyclone below it. Also note that the tops of cumulonimbus clouds often float in his region.

Stratosphere

The stratosphere is the second major layer of Earth’s atmosphere, just above the troposphere, and below the mesosphere. It is called stratosphere because it is stratified in temperature, with warmer layers higher up and cooler layers farther down. Top of the stratosphere has a temperature of about −3°C, just slightly below the freezing point of water. This is in contrast to the troposphere near the Earth’s surface, which is cooler higher up and warmer farther down. This Inversion begins in tropopause.

Why there are no Vertical Winds in Stratosphere?

The increase in the temperature with height in the stratosphere makes this region very stable place where the air tends not to overturn vertically. Thus vertical winds are almost absent in Stratosphere.In contrast with the atmosphere, where the vertical wind speeds are often several meters per second,in the stratosphere, they are seldom more than a few centimetres per second. The result is that it

takes air a very long time to be transferred from the bottom of the stratosphere, unless there is a thrust of gases such as that during the highly explosive volcanic eruptions. The inability of the air tomix in vertical direction is also the principal reason why the Ozone depleting Chloro-Fluoro Carbons take so long to reach the altitudes where the Sun’s energy is sufficient enough to break them apart. This also implies that some of the ozone depleting substances will still be there a centuries later from now.

Connecting Concept : Aviation & Jet Streams in Stratosphere Stratosphere is free from the violent weather changes which occur below in the Troposphere. So, it is preferred by commercial airliners. The commercial airliners typically cruise at altitudes of 9–12 km in the lower reaches of the stratosphere. They do this to optimize fuel burn. Jet liners, however, face another menace in stratosphere, namely jet streams. Jet streams are high velocity horizontal air currents. The main jet streams are located near the tropopause, the transition between the troposphere (where temperature decreases with altitude) and the stratosphere (where temperature increases with altitude). The location of the jet stream is extremely important for aviation. Jet streams are NOT always harmful for aviation. They are beneficial and used commercially as it reduced the trip time and fuel consumption. Commercial use of the jet stream began in 1950s when an aeroplane flew from Tokyo to Honolulu at an altitude of 7,600 meters cutting the trip time by over one-third. It also nets fuel savings for the airline industry.

Ozone Layer in stratosphere

As discussed above, the Ozone layer is contained within the stratosphere. In this layer ozone concentrations are about 2 to 8 parts per million, which is much higher than in the lower atmosphere but still very small compared to the main components of the atmosphere. It is mainly located in the lower portion of the stratosphere from about 15–35 km, through the thickness varies seasonally and geographically. About 90% of the ozone in our atmosphere is contained in the stratosphere.

The Ozone layer absorbs ultraviolet radiation from the sun and converts it into heat and chemical energy. It is this activity that is responsible for the rise in temperature. The layer is NOT of uniform thickness. Height at the equator is maximum and lowest at the poles.

Ionosphere

Ionosphere is called so because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere. Ionosphere stretches from 50 to 1,000 km and typically overlaps both the exosphere and the thermosphere. It has practical importance because it influences, for example, radio propagation on the Earth. It is also responsible for auroras.

Temperature in Ionosphere

Ionosphere is also known as THERMOSPHERE because of the high temperatures because of the high temperatures prevailing there as much as 870°C over the equator and 1427°C over the north pole, the temperature near the upper boundary of the thermosphere may become higher than 1000-1500°C. Along with temperature rise sharp changes caused by the corpuscular and ultraviolet solar radiation are observed in it.

Various Layers in Ionosphere

We note that the ionization depends primarily on the Sun and its activity. This means that the amount of ionization in the ionosphere varies greatly with the amount of radiation received from the Sun. This is the reason that there are changes in the Ionosphere and there are diurnal effect and seasonal effects. The activity of the Sun is associated with the position of earth in the revolutionary orbit, sunspot cycle, with more radiation occurring with more sunspots. Radiation received also varies with geographical location (polar, auroral zones, mid-latitudes, and equatorial regions).

There are also mechanisms that disturb the ionosphere and decrease the ionization. There are disturbances such as solar flares and the associated release of charged particles into the solar wind which reaches the Earth and interacts with its geomagnetic field.

Accordingly, Ionosphere has been divided into different sets of layers during day and night which are shown in this graphic:

D Layer

The D layer explains why the AM Radio gets disturbed during day time, but quite smooth in night time. We see in the above graphics that the D layer is the innermost layer, 60 km to 90 km above the surface of the Earth. At this layer, the net ionization effect is low, but loss of wave energy is great due to frequent collisions of the electrons. This is the reason that the high-frequency (HF)radio waves are not reflected by the D layer but suffer loss of energy therein. The absorption is small at night and greatest about midday. This causes the disappearance of distant AM broadcast band stations in the daytime.

E-Layer

The E layer is the middle layer, 90 km to 120 km above the surface of the Earth, with primary source of ionization being soft X-ray (1-10 nm) and far ultraviolet (UV) solar radiation ionization of molecular oxygen (O2). This layer disappears in the night because primary source of ionization is no longer present. The practical value of this layer is that it reflects long radio-waves back to earth, which enables them to be received at a distance, rather than disappear into space. It is also known as HEAVISIDE-KENNELLY LAYER.

Importance of E-Layer

The E layer is a region of the ionosphere which influences long-distance communications by strongly reflecting radio waves in the 1-3 megahertz. It is also called E region, Heaviside layer, or Kennelly-Heaviside layer. This region reflects radio waves of medium wavelength and allows their reception around the surface of the Earth. The layer approaches the Earth by day and recedes from it at night.

In technical terms, it is a cylinder of relativistic electrons gyrating in the magnetic field, which produces a self field strong enough to dominate the externally applied field and produces half reversal in the system. Since the mid ’20s, another connection regarding the ionosphere has been hypothesized that lightning can interact with the lower ionosphere. According to this theory, thunderstorms could modulate the transient, localized patches of relatively high-electron density in the mid-ionosphere E layer, which significantly affects radio wave propagation.

F-Layer

The F LAYER extends from about 200 km to more than 500 km above the surface of Earth. The E layer allows the penetration of short-radio waves, which continue until they reach the APPLETON LAYER.

Appleton layer reflects short-radio waves (which have penetrated the HEAVISIDE KENNELLY LAYER) back to earth. This is also supposed to be the region where polar AURORAS occur and where most of the meteors burn themselves out.

Exosphere

The exosphere lies above the altitude of 800 kilometer and it needs further studies. Characteristic of exosphere is an extreme rarefaction of the air; gas particles, moving with tremendous velocities, nearly fail to meet one another and there takes place an outflow of gas particles into the interpreter space.

ATMOSPHERE (On the basis of composition)
	HETEROSPHERE	HOMOSPHERE
1.	Gases not evenly mixed	Gases evenly mixed.
2.	Height Begins at 80 km & extends up to 10,000 km	Extends from the earth’s surface up to an altitude of 80 km.
3.	Solar constant is measured at 480 km	N2 & O2 are the Main Gases
4.	Above 480 Exposure (outer space). It contains individual atoms of light gases, weakly bound by gravity	CO2 helps in maintaining global temperatures.

Temperature Atmosphere and Insolation

Sun is the major source of energy for the entire earth system. The earth does receive very small proportions of energy from other stars and from the interior of the earth itself (volcanoes and geysers provide certain amount of heat energy). However, when compared with the amount received from the sun, these other sources seem insignificant.

The energy emitted by the sun which reaches the surface of the earth is called Insolation. The sun, amass of intensely hot gases, with a temperature at the surface be 6000°C emits radiant energy in the form of waves, which consists of very short wavelength x-rays, gamma rays, and ultraviolet rays;the visible light rays and the longer infrared rays. The earth receives only about one two-thousand millionth of the total insolation poured out by the sun, but this is vital to it; the amount received at the outer limit of the atmosphere is called Solar Constant. Thus Solar Constant is the rate per unit area at which solar radiation is received at the outer limit of the atmosphere.

Radiation as method of Heat Energy Transfer

Radiation is the process by which most energy is transferred through space from the sun to the earth. Radiation is given off by all bodies including earth and human being.

The hotter is the body, shorter are the waves. We can simply say that the radiation from Sun comes to earth in the form of smaller waves and earth being cooler body, gives off energy in the form of long-wave. These are then radiated back to the atmosphere. This Long-Wave Radiation from the earth’s surfaces heats the lower layers of the atmosphere. It is evident that the atmosphere is primarily heated from below by radiation from the heated Earth surface.

The atmosphere of the earth does not heat up directly as solar radiation is in the form of short waves and air cannot absorb the short waves. The earth absorbs the short wave energy and then radiates in the form of long wave terrestrial radiation that can be absorbed by the air. So, air heats up when comes in contact with the surface of the earth.

Convection as a method of heat transfer

Heat energy transfer that involves the movement of a fluid (gas or liquid) is termed as convection. Fluid in contact with the source of heat expands and tends to rise within the bulk of the fluid. Cooler fluid sinks to take its place, setting up convection currents. This is the principle of natural convection in many domestic hot-water systems and space heaters.

Conduction as Method of Heat Transfer

Conduction is the means by which heat is transferred from one part of a body to another or between two touching objects. Heat flows from the warmer to the cooler (part of a) body in order to equalize temperature.

Advection as Source of Heat Transfer

Advection is the horizontal heat transfer within the atmosphere. Obviously the wind is the transfer agent of advection. Wind brings about the horizontal movement of large portions of lower atmosphere.

Latent Heat of Condensation

A proportion of the solar energy is used to change liquid water from rivers, lakes, and oceans to water-vapour. The solar energy used to do this is then stored in the water-vapour as latent or potential energy. Later the water-vapour in the atmosphere may change to form liquid water again through a process called CONDENSATION. The energy released through this process is known as the Latent Heat of Condensation. Like other means of heat transfer in the earth system, latent heat of condensation plays a major role in warming of the atmosphere and in addition, is as source of energy for STORMS.

Earth’s Albedo

The ratio between the total solar radiation falling (incident) upon a surface and the amount reflected without heating the earth, is called ALBEDO (expressed as a decimal or as a percentage).The earth's average albedo is about 0.4 (40 percent) ; that is , 4/10 of the solar radiation is reflected back into space. It varies from 0.03 for dark soil to 0.85 for a snow-failed. Water has a low albedo (0.02) with near-vertical rays, but a high albedo for low-angle slanting rays. The figure for grass is about 0.25.Over-pastured land and bare soil are more reflective of solar radiation than are crops and vegetation.

A desert is much more reflective than a savanna or forest. If economic pressure on soil and vegetation increases, and drought then occurs, the effect overall is to increase the albedo of the surface.

Earth’s Energy Budget

Earth’s Energy Budget can be discussed in terms of incoming heat energy and outgoing heat energy.

These are as follows:

Incoming Heat Energy

This is made of :

• Solar radiation (99.97%)
• Geothermal energy (0.025%)
• Tidal energy (0.002%)
• Fossil fuel consumption (about 0.007%)
• Minor Sources: remains part

Outgoing Heat Energy

The average albedo (reflectivity) of the Earth is about 0.3, which means that 30% of the

incident solar energy is reflected into space, while 70% is absorbed by the Earth and

reradiated as infrared. This 30% of the incident energy is reflected, consisting of 6% reflected from the atmosphere, 20% reflected from clouds and 4% reflected from the ground (including

land, water and ice). The remaining 70% of the incident energy is absorbed, out of 51% is absorbed by land and water, and then emerges in the following ways:

Temperature inversion

Under special circumstances the normal trend of decrease in temperature with increasing height is reversed under special circumstances. Some of the situations leading to temperature inversion:

1)      Frontal situation of middle latitudes represents case of temperature inversion as in a front, warm air rises over cold air.

2)      On a clear night when heat is radiated from the earth surface, the air near to the surface is cooled by conduction of heat to cold ground. The lower layer of the atmosphere is cooler than air aloft.

3)      At night the colder denser air on the upper slopes of a valley side descends into the valley bottom displacing warmer air aloft thereby making the valleys coler than the hill tops.

4)      When air is subsiding in an anticyclone the air is warmed adiabatically & this air is warmer than the air at the ground level creating a case of temperature inversion.

PRESSURE & WINDS

WINDS

• Horizontal flow of air is known as winds and this flow is caused by combination of pressure gradient, Coriolis force and frictional forces caused by uneven heating, rotation of earth and distribution of land and oceans.
• Pressure-gradient force is the rate of change of pressure with respect to distance is the pressure gradient. The larger the pressure gradient force the stronger will be the resulting wind. (i.e. closer Isobars)
• Coriolis Force-  As winds move north or south they are deflected due to the rotation of the Earth. As the Earth spins on its axis, a person standing on the equator moves from west to east at around 1600km/hour. At the poles, on the other hand, that person would not move at all, just spin around in place. So, the equator and anything on it moves west to east faster than any other place on Earth. The west to east motion decreases from the equator to the pole.

As winds move away from the equator, their west to east momentum carries them to the east of a true poleward trajectory. In the northern hemisphere they are deflected to the right. In the southern hemisphere they are deflected to the left.

The strength of the coriolis force is zero at the equator, half its maximum strength at 30° latitude, and maximum at the poles. Fast winds and winds covering the greatest distances are deflected the most.  In the absence of friction (approximated in the upper atmosphere), the coriolis force would cause the winds to blow parallel to isobars, in circles, clockwise around high pressure and counterclockwise around lows in the northern hemisphere.  These isobar parallel circular winds, geostrophic winds, only occur in the upper atmosphere, away from friction with the Earth's surface.

• Friction Force:  Friction with the Earth's surface only affects wind speed up to altitudes around 500 m (1640 ft).  Friction prevents geostrophic winds at low altitude.  Rather, low level winds move at an angle across isobars.

The net effect of these forces is that near surface winds spiral outward away from high pressure centers and inward toward low pressure centers. 

• - In the northern hemisphere, where winds are deflected to the right, winds spiral clockwise around high pressure and counterclockwise around low pressure 
• - In the southern hemisphere, where winds are deflected to the left, winds spiral counterclockwise around high pressure and clockwise around low pressure

• Low pressure centers are zones of convergence, with winds spiralling inward.  These are called cyclones. Tornadoes and hurricanes are strong cyclonic storms. High pressure centers are zones of diveregence, with winds spiralling outward.  These are called anticyclones.
• Friction with Earth's surface slows the movement of air and alters wind direction. It is greatest at the surface and its influence generally extends to an elevation of 1 - 3 km. Over the sea surface the friction is minimal.
•  Upper-air winds, called geostrophic winds, blow parallel to the isobars and reflect a balance between the pressure-gradient force and the Coriolis effect. Upper-air winds are faster than surface winds because friction is greatly reduced. Friction slows surface winds, which in turn reduces the Coriolis effect. The result is air movement at an angle across the isobars toward the area of lower pressure.

Pressure Belts

The pressure exerted by the atmosphere as a result of its weight on the surface of the earth, expressed in millibars (1000 mb = 1 bar = 1 million dynes per square centimeter).

The average pressure over the earth’s surface at sea-level is 1013.25 mb, equivalent to the weight of a column of 29.92 inches (76.0 centimeters) of mercury at 0°C or to a weight of air 14.66 pounds per square inch or 1033.3 gram per square centimeter.

Two factors leading to the formation of high and low pressure are thermal and dynamic.

A study of the distribution of air pressure reveals that the air pressure is not uniformly distributed over the Earth’s surface. In order to destroy this uneven distribution of pressure, winds are caused. In other words, winds are caused due to uneven distribution of pressure. Winds move from high to low pressure areas.

• Thermal Control: The sunrays make different angles at different latitudes. Where the rays are more vertical, the amount of insolation received is more, i.e., the place with more vertical rays is heated more than that where the sunrays are less vertical. Such a place, which is hot, heats the air with the result that the air pressure decreases. In this way many belts of high and low pressures are produced.
• Dynamic Control: The Earth is rotating about its axis. Due to this rotation the pressure belts shift from their ideal positions. For example, the winds at poles shift towards equator.

Direction of Winds

Had the Earth not been rotating about its axis the winds would have blown in the direction of pressure gradient. The earth on account of its rotation produces a force known as Coriolis force after the name of the Mathematician Coriole. The force displaces the winds from the direction of wind (or pressure gradient). Ferrel formulated a law to deduce the direction of winds in the northern and southern hemispheres. It is known as Ferrel’s law.

 According to Ferrel’s law, if we stand with our faces in the direction in which the wind is blowing the wind will turn towards our right hand in the northern hemisphere and towards our left hand in the southern hemisphere.

Buys Ballot’s Law:

It relates the wind direction to the position of air pressure areas. According to this law, if we stand with our backs in the direction in which the wind is blowing we will find the area of low pressure towards our left hand in the northern hemisphere and towards our right hand in the southern hemisphere.

MOVEMENT OF PERMANENT WINDS

Hadley Cell

• The air at the Inter Tropical Convergence Zone (ITCZ) rises because of the convection currents caused by low pressure. Low pressure in turn occurs due to high insolation. Hence, winds converge at this low pressure zone.
• This converged air rises along with the convective cell. It reaches the top of the troposphere up to an altitude of 14 km, and moves towards the poles which results into accumulation of air at about 30° N and S.
• Formation of Subtropical High- Part of the accumulated air sinks to the ground and forms a subtropical high. Another reason for sinking is the cooling of air when it reaches 30° N and S latitudes.
• Down below near the land surface the air flows towards the equator as the easterlies. The easterlies from either side of the equator converge in the Inter Tropical Convergence Zone (ITCZ).
• These circulations from the surface upwards and vice-versa are called cells.
• Such a cell in the tropics is called Hadley Cell.

Ferrel Cell

• In the middle latitudes the circulation is that of sinking cold air that comes from the poles and the rising warm air that blows from the subtropical high.
• At the surface these winds are called westerlies and the cell is known as the Ferrel cell

Polar Cell

• At polar latitudes the cold dense air subsides near the poles and blows towards middle latitudes as the polar easterlies. This cell is called the polar cell.
• These three cells set the pattern for the general circulation of the atmosphere. The transfer of heat energy from lower latitudes to higher latitudes maintains the general circulation.
• The general circulation of the atmosphere also affects the oceans. The large-scale winds of the atmosphere initiate large and slow moving currents of the ocean.
• Oceans in turn provide input of energy and water vapour into the air. These interactions take place rather slowly over a large part of the ocean.

Doldrums

• The pressure belt between the 0° to 5°N and S is called Equatorial Low Pressure Belt. This belt is characterized by intense heating, with expanding air and ascending convection currents.
• Because the air is largely moving upward, surface winds are light and variable. This region is known as the doldrums.
• The term doldrums has been used by the sailors as it has been marked by erratic weather patterns with stagnant calms and violent thunderstorms.
• Doldrums are belt of calms and variable winds occurring at times along the equatorial trough.

Doldrums are characterized by:

• Low atmospheric Pressure
• High Humidity
• Thunderstorms
• The same area is also called the Intertropical Convergence Zone (ITCZ) or Doldrums. This is the area encircling the earth near the equator where winds originating in the northern and southern hemispheres come together. Please note that the location is not precisely defined as location of the Intertropical convergence zone varies over time.
• Overland, it moves back and forth across the equator following the sun’s zenith point. Over the oceans, where the convergence zone is better defined, the seasonal cycle is more subtle, as the convection is constrained by the distribution of ocean temperatures. Sometimes, a double ITCZ forms, with one located north and another south of the equator. When this occurs, a narrow ridge of high pressure forms between the two convergence zones, one of which is usually stronger than the other. Between 10° and 15° North and South, there are high pressure belts, where air is comparatively dry, light and calm. This region is beneficial to the maritime trade.

Walker Cell

• Warming and cooling of the Pacific Ocean is most important in terms of general atmospheric circulation.
• The warm water of the central Pacific Ocean slowly drifts towards South American coast and replaces the cool Peruvian current. Such appearance of warm water off the coast of Peru is known as the El Nino.
• The El Nino event is closely associated with the pressure changes in the Central Pacific and Australia. This change in pressure condition over Pacific is known as the southern oscillation.
• The combined phenomenon of southern oscillation and El Nino is known as ENSO.
• In the years when the ENSO is strong, large-scale variations in weather occur over the world.
• The arid west coast of South America receives heavy rainfall; drought occurs in Australia and sometimes in India and floods in China.

Classification of Winds

1) Primary Winds

• The winds blowing almost in the same direction throughout the year are called prevailing or permanent winds. These are also called as invariable or planetary winds because they involve larger areas of the globe.
• These are which blow extensively over continents and oceans.

Trade Winds	Westerlies	Polar easterlies
Definition- The trade winds are those blowing from the sub-tropical high pressure areas towards the equatorial low pressure belt. Region- 30°N and 30°S Flow as the north-eastern trades in the northern hemisphere and the south-eastern trades in the southern hemisphere. This deflection in their ideally expected north-south direction is explained on the basis of Coriolis force and Farrell’s law. Trade winds are descending and stable in areas of their origin (sub-tropical high pressure belt), and as they reach the equator, they become humid and warmer after picking up moisture on their way. The trade winds from two hemispheres meet at the equator, and due to convergence they rise and cause heavy rainfall. The eastern parts of the trade winds associated with the cool ocean currents are drier and more stable than the western parts of the ocean.	Definition- The westerlies are the winds blowing from the sub-tropical high pressure belts towards the sub polar low pressure belts. Blow from south­west to north-east in the northern hemisphere and north-west to south-east in the southern hemisphere. The westerlies of the southern hemisphere are stronger and persistent due to the vast expanse of water, while those of the northern hemisphere are irregular because of uneven relief of vast land-masses. Region- 40° and 65°S. These latitudes are often called Roaring Forties, Furious Fifties, and Shrieking Sixties – dreaded terms for sailors. The pole-ward boundary of the westerlies is highly fluctuating. There are many seasonal and short-term fluctuations. These winds produce wet spells and variability in weather.	The Polar easterlies are dry, cold prevailing winds blowing from north-east to south-west direction in Northern Hemisphere and south-east to north-west in Southern Hemisphere. They blow from the polar high-pressure areas of the sub-polar lows.

Monsoon Winds

Monsoon Mechanism

The word monsoon derived from the Arabic word mausim means seasonal winds. In this system, the direction of the winds reverses seasonally. The first thing we note is that Monsoon is typically considered a phenomenon of tropical south Asia, but it is also experienced over parts of North America and Africa.

Mechanism of Monsoon: Traditional View

• Traditionally, monsoon has been considered a result of the differential heating of land and sea.
• In summer, southern Asia develops a low pressure while the pressure over the sea is relatively higher. As a result the air starts flowing towards land from the Indian oceans. The winds coming from ocean carry moisture and thus cause rainfall in summer reason. This is known as the southwest monsoon or summer monsoon

• In winters, the pressure over land is higher than over the sea and consequently the air starts flowing from land to sea. The air coming from land being dry, these winds do not cause rainfall.
• The above explanation is known as the thermal theory of monsoon. This theory explains monsoon as a regional phenomenon but fails to explain the total amount of energy / processes involved in the global monsoon circulation.

Mechanism of Monsoon: Modern View

• The modern meteorologists seek explanation for the phenomenon of monsoon on the basis of seasonal shift in the position of the global belts of pressure and winds. This is also known as Dynamic Theory.
• According to the dynamic theory, monsoons are a result of the shift of the inter-tropical convergencezone (ITCZ) under the influence of the vertical sun. Though the average position of the ITCZ is taken asthe equator, it keeps shifting vertical sun towards with the migration of the vertical sun towards thetropics during the summer of the respective hemisphere.

Jet Streams

• Jetstreams are narrow zones at a high altitude in which wind streams reach great speed. They occur where atmospheric pressure gradient is strong. Along a jetstream , pulse like movement of air follow broadly curving tracks .The greatest wind speed occur in the center of the Jet stream with velocity decreasing away from the center .Jet Streams are swift geostrophic air streams in the upper troposphere that meander in relatively narrow belts .These are the strong cores of upper level westerlies which follow a meandering path . The westerly Jet streams occur at an elevation from 9-12Km in the lower middle latitudes but at higher latitude they are found at lower level. Deviations in the path of Jetstream are caused by the disturbing Influence of the continents, particularly in the NH and partially by the travelling centres of high and low pressure .

             

On the Basis of Location aspect, Jet streams are divided into three types and each JetStream influence the Climate and weather of their respective zone :

a)      Polar Front Jet Stream (40-60 degree latitude)

• They are formed above the convergence zone of the surface Polar cold air mass and Tropical warm air mass near the tropopause . They bring the Temperate cyclones from North Atlantic towards the North-Western Europe and hence influence the weather of NW Europe and are responsible for British type of climate . These JS decides the path and direction of Frontal Cyclones .

b)      Sub-Tropical Westerly Jet Stream

• These Jetstreams move in the upper Troposphere at an altitude of 10-12 Km to the North of Subtropical surface HP belt that is above 30-35 degree latitude . These Jet Stream in winter Season bring Western Disturbances or Temperate Cyclones from Mediterranean sea to the Indian Subcontinent and cause rainfall in Northern Gangetic plains of India . These showers are helpful for irrigating Winter wheat in India . Moreover , they replenish the glaciers of the Himalayas by snowfall. During Northern Summers these Jet steams are responsible for dry and hot weather in North India during the month of May and June and only after their withdrawal Tropical Easterly Jet Streams Establish themselves bringing Indian Monsoon. Most of the Tropical Deserts are found Beneath these sub tropical Westerly Jet Stream which cause extreme aridity in the Sub-tropical Latitudes Like Sahara desert , Atacama desert , great Australian deserts etc .

c)       Tropical Easterly Jet Stream

• These Jetstreams are temporary in character and Establish over Indian Subcontinent during Summers when the Sub-Tropical Westerly JS withdrawn from Tibetian Plateau . These JS are responsible for pushing Monsoon airmass into India and thus by and large influence the weather of whole of Indian Subcontinent and giving it a monsoon type climate .

Land Breeze and Sea Breeze

• The land and sea absorb and transfer heat differently. During the day the land heats up faster and becomes warmer than the sea. Therefore, over the land the air rises giving rise to a low pressure area, whereas the sea is relatively cool and the pressure over sea is relatively high.
• Thus, pressure gradient from sea to land is created and the wind blows from the sea to the land as the sea breeze.
• In the night the reversal of condition takes place. The land loses heat faster and is cooler than the sea.
• The pressure gradient is from the land to the sea and hence land breeze results.

 

Valley Breeze and Mountain Breeze

Valley Breeze

In mountainous regions, during the day the slopes get heated up and air moves upslope and to fill the resulting gap the air from the valley blows up the valley. This wind is known as the valley breeze.

During the night the slopes get cooled and the dense air descends into the valley as the mountain wind. The cool air, of the high plateaus and ice fields draining into the valley is called katabatic wind.

Another type katabatic wind occurs on the leeward side of the mountain ranges. The moisture in these winds, while crossing the mountain ranges condenses and precipitates. When it descends down the leeward side of the slope the dry air gets warmed up by adiabatic process. This dry air may melt the snow in a short time.

3) Tertiary Winds or Local Winds

Reason-

Local differences of temperature and pressure produce local winds.

Such winds are local in extent and are confined to the lowest levels of the troposphere.

Some important winds are given below-

Loo

• Harmful Wind
• Region- In the plains of northern India and Pakistan, sometimes a very hot and dry wind blows from the west in the months of May and June, usually in the afternoons.
• Temperature- between 45°C and 50°C.

Foehn or Fohn

• Beneficial Wind
• Region- Foehn is a hot wind of local importance in the Alps.
• It is a strong, gusty, dry and warm wind which develops on the leeward side of a mountain range.
• As the windward side takes away whatever moisture there is in the incoming wind in the form of orographic precipitation, the air that descends on the leeward side is dry and warm (Katabatic Wind).
• Temperature- The temperature of the wind varies between 15°C and 20°C.
• The wind helps animal grazing by melting snow and aids the ripening of grapes.

Chinook

• Beneficial Wind
• Region- USA and Canada, Rocky mountains 
• It is beneficial to ranchers east of the Rockies as it keeps the grasslands clear of snow during much of the winter.

Mistral

• Harmful Wind
• Region- Alps over France towards the Mediterranean Sea.
• It is channeled through the Rhine valley. It is very cold and dry with a high speed.
• It brings blizzards into southern France.

Sirocco

• Harmful Wind
• Region- Sirocco is a Mediterranean wind that comes from the Sahara and reaches hurricane speeds in North Africa and Southern Europe.
• It arises from a warm, dry, tropical air mass that is pulled northward by low-pressure cells moving eastward across the Mediterranean Sea, with the wind originating in the Arabian or Sahara deserts. The hotter, drier continental air mixes with the cooler, wetter air of the maritime cyclone, and the counter-clockwise circulation of the low propels the mixed air across the southern coasts of Europe.
• The Sirocco causes dusty dry conditions along the northern coast of Africa, storms in the Mediterranean Sea, and cool wet weather in Europe.

Beaufort Scale

The Beaufort Scale is defined by the Glossary of Meteorology (AMS) as a system of estimating and reporting wind speeds numerically from 0 (calm) to 12 (hurricane).

BEAUFORT WIND SCALE A scale of wind force, devised by Beaufort in 1805, modified in 1926; it is related to the descriptions of wind effects and estimated velocity at 10 m. above the ground.
Scale No.	Wind	Force (m.p.h)	Observed effects
0	calm	0	Smoke rises vertically
1.	light air	1–3	Wind direction shown by smoke drift, but not by vane
2.	light breeze	4–7	Wind felt on face; leaves rustle; wind vane moves
3.	gentle breeze	8–12	Leaves and small twigs in motion; a flag is extended
4.	moderate breeze	13–18	Raises dust; small branches move
5.	fresh breeze	19–24	Small trees sway; small crests on waves on lakes
6.	strong breeze	25–31	Large branches in motion; wind whistles in telephone wires
7.	moderate gale	32–38	Whole trees in motion
8.	fresh gale	39–46	Breaks twigs off trees
9.	strong gale	47–54	Slight structural damages to houses
10.	whole gale	55–63	Trees uprooted.  Considerable structural damage
11.	storm	64–75	Widespread damage
12.	hurricane	above 75	Devastation

Air Masses

• An air mass is a large body of air with fairly uniform temperature and moisture characteristics. It can be several thousand kilometer in area and it can extend upward to the top and it can extend upward to the top of the troposphere. A given air mass is characterized by a distinctive combination of surface temperature , environmental lapse rate and surface specific humidity .
• Air masses acquire their characteristics in the source region like the temperature and moisture specifically. They move under the influence of pressure gradient and upper level wind patterns.
• They are classified on the basis of the latitudinal position and the nature of the underlying surface of their source regions.

Accordingly, following types of airmasses are recognized:

1.       Maritime tropical (mT);

2.       Continental tropical (cT);

3.       Maritime polar (mP);

4.       Continental polar (cP);

5.       Continental arctic (cA).

Influence of Air Masses on World Weather

• The properties of an air mass that influence accompanying weather are vertical distribution temperature and the moisture content.
• The air masses carry atmospheric moisture from oceans to continents and cause precipitation over landmasses , Indian Monsoon is an example of Tropical Maritime air mass .
• They transport latent heat, thus removing the latitudinal heat balance.
• The convergence of the  cold polar airmass and the warm tropical maritime airmass lead to the formation of extra-tropical cyclones in the middle latitude which influences the weather conditions of temperate region and gives Mediterranean and British type of climate .

FRONTS

It is the zone along which two contrasting air masses meet, which originated in different source areas, therefore have differing temperatures and humidity characteristics. Best known fronts are Polar and the Intertropical Fronts.

Depression is a mass of air whose isobars form an oval or circular shape, with low pressure at the centre. The air converges at the centre and rises to be disposed off. In a depression, the winds rotate anticlockwise in the northern hemisphere. While in the Southern hemisphere, the circular movement of winds is in a clockwise direction. Depressions are rarely stationary and tend to follow definite tracks. They are most influential over the ocean spreads and they weaken as they move over land areas. They are of two types:

i.          Temperate Cyclones

ii.         Tropical Cyclones

Temperate Cyclones

1.       They are found both on land and sea.

2.       Their isobars are usually V. shaped.

3.       They have a low pressure-gradient.

4.       The wind speed is low and never very strong.

5.       They occupy areas measuring thousands of square km.

6.       They travel from west to east (mainly under Rossby waves).

7.       Rainfall is slow. Sometimes heavy showers take place.

8.       Rainfall continues for many days.

9.       All the sectors (cold front, warm front, warm sector, cold sector) of the cyclone have different temperatures, pressure, humidity etc. characteristics.

10.     There are two fronts (cold and warm) in temperate   cyclone.

11.     More cyclones are produced in winter than in summer.

12.     The direction of winds are rapidly changed at the front. Veering and backing of winds take place.

13.     There is not a single place where winds are inactive and rains are absent.

14.     The energy of the cyclone depends upon the difference of the densities of air masses.

Tropical Cyclones

Tropical cyclone are violent storms that originate over oceans in tropical areas and move over to the coastal areas bringing about large scale destruction caused by violent winds, very heavy rainfall and storm surges. This is one of the most devastating natural calamities. This system incorporates 6 different geographical locations of its regular development:

• Gulf of California (Hurricane)
• Gulf of Mexico (Hurricane)
• Bay of Bengal & Arabian Sea (cyclone)
• Mozambique Channel (Cyclone)
• East China Sea (Typhoon)
• Coral sea (willy willy)

 

Tropical cyclones are formed mainly over the warm tropical seas where water temperatures are above 27⁰ C and there is presence of coriolis effect. Over the tropical seas these areas of low pressure attract winds which under the influence of coriolis spiral and rise upwards. These rising winds are laden with moisture captured from the ocean surface and due to the upward movement the winds get cooled and there is condensation leading to the formation of rain bearing cumulonimbus clouds. It is during this process of condensation that the latent heat is released and this is the source of energy for the cyclone. Following the low pressure depressions the cyclone moves landwards and on the way intensifies due to constant supply of moisture from the ocean. It strikes land and starts weakening as the moisture supply is cut off and the latent heat is no more available to power the cyclone. The centre of the cyclone is known as ‘eye’. The winds at the eye are calm and there is no rainfall.

Some facts about tropical cyclones

• Their isobars are usually complete circles.
• The pressure gradient is steep.
• The wind speed is about 100 km. per hour or even more.
• They have a small area.
• They travel from east to west.
• Rainfall is heavy.
• Rainfall does not last beyond a few fours. If cyclone stays at a place, the rainfall may continue for many days.
• The temperature at the centre is almost equally distributed.
• The fronts are usually absent in tropical cyclones.
• These cyclones are produced in summer than in winter.
• The difference of the densities of air masses does not contribute to the energy of the cyclone. Its energy is derived from the latent heat of condensation.

These cyclones are not formed between 5⁰ N and 5⁰ S of the equator due to absence of coriolis force at the equator.

Anticyclones

Anti-cyclones are the wind systems which have highest air pressure at the centre and lowest at the outer margin and winds blow from centre to outward in clockwise direction in the northern hemisphere and anti-clockwise in the southern hemisphere. Anti-cyclonic circulation is characterised by subsidence and surface divergence and does not favour condensation and cloud formation. Therefore, they are associated with rainless fair weather and atmospheric stability and are called “weather less phenomenon”. Wind system is not fully developed in anti-cyclones because of weak pressure gradient which ranges between 10-20 mb. They are 75% larger than temperate cyclones in size and area and do not have front formation. Sometimes they become so large in size that their diameter becomes 9000 km and can cover nearly half of the USA. Their track is highly variable and unpredictable.

Anti-cyclones are divided into 2 types:

1.       Cold Anti-cyclones: they are thermally induced because they are not developed due to descent of air from above. They originate due to development of high pressure because of very low insolation during winter season in polar areas of the Arctic and Antarctic. After originating in arctic region, cold anti-cyclone advances in easterly and south-easterly direction. Although they are smaller than tropical anti-cyclones in size but move more rapidly than the later. They are of very low thickness and very few of them are higher than 3000m. In winters the cold anti-cyclones originating in the snow covered sub polar regions bring low temperatures and blizzards with them.

2.         Warm anti-cyclones: originate due to descent of air from above which causes its consequent divergence at the surface. Therefore they are dynamically induced. They originate in the belt of sub-tropical high pressure. They are large in size and very sluggish In movement. They are associated with light slow wind, cloudless sky and clear weather. They often influence the weather of south east USA and western Europe. These anti-cyclones are well developed over the oceans. In summer they stagnate over a place and produce high temperature called as “heat waves”. Since they favour clear weather, the diurnal range of temperature is bound to be large.

Enhanced F Scale for Tornado Damage

Dr. T. Theodore Fujita first introduced The Fujita Scale in February 1971. He wanted something that categorized each tornado by intensity and area. The scale was divided into six categories.

The Enhanced F-scale still is a set of wind estimates (not measurements) based on damage. Its uses 28 damage indicators. Each damage indicators is further classified into 8 levels of damage. These estimates vary with height and exposure.

 

FUJITA SCALE			EF SCALE
F Number	Fastest 1/4-mile (mph)	3 Second Gust (mph)	EF Number	3 Second Gust (mph)
0	40-72	45-78	0	65-85
1	73-112	79-117	1	86-109
2	113-157	118-161	2	110-137
3	158-207	162-209	3	138-167
4	208-260	210-261	4	168-199
5	261-318	262-317	5	200-234

HUMIDITY AND PRECIPITATION

It is the degree of water vapours present in the air. For any specified temperature, there is a definite limit to the maximum quantity of moisture that can be held by the air. This limit is known as the saturation point. Humidity can be measured by a hygrometer or sling psychrometer.

Relative Humidity: is the ratio between the amount of water vapour actually present in an air mass and the maximum amount that the air mass can hold at a particular temperature. It is expressed as a percentage. At the saturation point, relative humidity is 100%. It varies inversely with temperature, given a fixed amount of water vapour. Secondly, if an exposed water surface is present, the relative humidity can be increased by evaporation.

Absolute Humidity: is the actual amount of moisture present in air. Amount of water vapor per unit volume of air is usually expressed in grams per cubic meter. It is a measure of the quantity of water that can be extracted from the atmosphere as precipitation occurs. As the absolute humidity cannot remain a constant figure for the same body of air, modern meteorology makes us of another measure of moisture content-specific humidity.

Specific humidity: It is the ratio of the weight of water vapour to the weight of moist air. Expressed in units of grams of water vapour per kilogram of moist air, specific humidity is often used to describe the moisture characteristics of a large mass of air.

Precipitation

Formation of water-particles or ice within the cloud, that fall towards earth’s surface is precipitation. It occurs when condensation takes place rapidly within the cloud. Main types are rain, drizzle, sleet, snow and hail.

i.        Convectional precipitation is caused by heating of moist air in the lower layers of atmosphere, which rises, expands, and is cooled adiabatically to its dew point. Towering cumulonimbus clouds may form. Convection rain is often accompanied by lightning and thunder. In tropical latitudes this type of rain is usually torrential.

ii.       Orographic means, “Related to mountains”. This Precipitation is caused by moisture-laden air being forced to rise over a relief barrier (mountain ranges). As the air rises on the windward side it is cooled at the adiabatic rate. If sufficiently cooled, precipitation results, when the air descends on the leeward side, it gets warmed and dry, having no source from which to draw up moisture. A belt of dry climate, often called a rain shadow may exist on the leeward side. Several of the important dry deserts of earth are of this type like the Patagonia.

iii.      Cyclonic Precipitation (Depression or frontal) occurs when large masses of air of different temperatures meet. The warm moist air of one air mass moves over the cold heavier air of another. Or, it is caused by air rousing through horizontal convergence in an area of low pressure. Cyclonic rain is common throughout the doldrums where the trade winds meet. It is the precipitation along the frontal surfaces of a depression in mid and high latitudes.

Thunderstorm

It is an intense local storm accompanied by lightning and thunder that develops in large cumulonimbus clouds, which result from rapid ascent of air under very unstable conditions. Such conditions occur either at the cold front of a depression or when the ground is intensely heated, and there must always be sufficient moisture in the air for cloud formation.

Measuring Precipitation

Rainfall is usually measured by an instrument called the rain gauge, which can be operated on a simple level merely by setting out a straight side, flat bottomed fan and measuring the depth to which water accumulates in a particular period.

Dew Point

Is the temperature at which the air is fully saturated and below which, condensation normally occurs and water vapour starts to condense to form water droplets. It may be equal to, less than or greater than 0^o C. Dew is the deposition of water droplets on the ground and objects, such as plants near the ground. It occurs when the temperature of the ground surface falls and the air in contact with it is cooled below its dew point. Water vapours from the air or diffused from the soil then gets condensed and are deposited as droplets. The favourable conditions are moist air, light winds and clear skies at night to ensure maximum cooling by radiation.

Frost

Is a weather condition that occurs when the air temperature is at or below 0^oC. Moisture on the ground surface and objects freezes to form an icy deposit. Conditions favourable for its formation are similar to those in the case of dew formation.

Fog

Cloud that collects at the surface of the Earth composed of water vapours that have condensed on particles of dust in the atmosphere. Cloud and fog are both caused by the air temperature falling below dew point. The thickness of fog depends on the number of water particles it contains.

Officially, fog refers to a condition when visibility is reduced to 1 km/0.6 mi or less, and mist or haze to that giving a visibility of 1-2 km or about 1 mi.

Types of Fog

An advection fog is formed by the meeting of two currents of air, one cooler than the other, or by warm air flowing over a cold surface. Sea fogs commonly occur where warm and cold currents meet and the air above them mixes. A radiation fog forms on clear, calm nights when the land surface loses heat rapidly (by radiation); the air above is cooled to below its dew point and condensation takes place. A mist is produced by condensed water particles, and a haze by smoke or dust.

Smog

Also called smoke fog, is a form of fog that occurs in areas where the air contains a large amount of smoke. Smoke particles provide a high concentration of nuclei around which condensation occurs. Condensation can occur around these nuclei even when the air is not saturated and therefore it forms earlier, becomes denser and lasts longer than fog that develops in unpolluted air. The smoke, because of its chemical contents gives an acid taste to fog.

Mist

Is the term for a reduction of visibility between 1-2 km caused by condensation producing water droplets within the lower layers of atmosphere. It is intermediate between fog and haze.

Haze

Is normally formed by water particles that have condensed around nuclei in the atmosphere, but may also be a result of particles of smoke, dust or salt in the air. In meteorology, it is an obscurity of the lower atmosphere that limits visibility to under 2 km but over 1 km.

Clouds

Latin Root	Translation	Example
cumulus stratus cirrus nimbus	heap layer curl of hair rain	fair weather cumulus altostratus cirrus cumulonimbus

Clouds are masses of minute water droplets and/or ice crystals formed by the condensation of water vapour and held in suspension in atmosphere. Condensation which results from cooling, usually takes place around nuclei such as dust, smoke particles and salt. The cooling may be caused by convection, uplift over mountains, or ascent in depressions. Clouds may be present at heights ranging from ground level up to over 13,000 meters.

Types of Clouds

Clouds are classified on the basis of appearance, form and altitude. On the basis of form, there are two major groups: (i) stratiform, or layered types, and (ii) cumuliform or massive, globular types.

Other Classifications

High-Level Clouds: Cloud types include: cirrus and cirrostratus.

Mid-Level Clouds: Cloud types include: altocumulus, altostratus.

Low-Level Clouds: Cloud types include: nimbostratus and stratocumulus.

Clouds with Vertical Development: Cloud types include: fair weather cumulus and cumulonimbus.

Other Cloud Types :Cloud types include: contrails, billow clouds, mammatus, orographic and pileus clouds.

Climatic regions of the World