Air Masses

• When the air remains over a homogenous area for a sufficiently longer time, it acquires the characteristics of the area. The homogenous regions can be the vast ocean surface or vast plains and plateaus.
• The air with distinctive characteristics in terms of temperature and humidity is called an air mass. It is a large body of air having little horizontal variation in temperature and moisture.
• Air masses form an integral part of the global planetary wind system.
• They extend from surface to lower stratosphere and are across thousands of kilometers.

Source regions

• The homogenous surfaces, over which air masses form, are called the source regions.
• The main source regions are the high pressure belts in the sub tropics (giving rise to tropical air masses) and around the poles (the source for polar air masses).
• Source Region establishes heat and moisture equilibrium with the overlying air mass.
• When an air mass moves away from a source region, the upper level maintains the physical characteristics for a longer period. This is possible because air masses are stable with stagnant air which do not facilitate convection. Conduction and radiation in such stagnant air is not effective.

Conditions for the formation of Air masses

• Source region should be extensive with gentle, divergent air circulation (slightly at high pressure).

• Areas with high pressure but little pressure difference or pressure gradient are ideal source regions.
• There are no major source regions in the mid-latitudes as these regions are dominated by cyclonic and other disturbances.

Air masses based on Source Regions

• There are five major source regions. These are:

1. Warm tropical and subtropical oceans;
2. The subtropical hot deserts;
3. The relatively cold high latitude oceans;
4. The very cold snow covered continents in high latitudes;
5. Permanently ice covered continents in the Arctic and Antarctica.

• Accordingly, following types of airmasses are recognised:

1. Maritime tropical (mT);
2. Continental tropical (cT);
3. Maritime polar (mP);
4. Continental polar (cP);
5. Continental arctic (cA).

• Tropical air masses are warm and polar air masses are cold.
• The heat transfer processes that warms or cools the air takes place slowly.

Cold Air Mass

• A cold air mass is one which is colder than the underlying surface and is associated with instability and atmospheric turbulence.

Cold source regions (polar air masses)

• Arctic Ocean – cold and moist
• Siberia – cold and dry
• Northern Canada – cold and dry
• Southern Ocean – cold and moist

Warm Air Mass

• A warm air mass is one which is warmer than the underlying surface and is associated with stable weather conditions.

Warm source regions (tropical air masses)

• Sahara Desert – warm and dry
• Tropical Oceans – warm and moist

Influence of Air Masses on World Weather

• The properties of an air mass which influence the accompanying weather are vertical distribution temperature (indicating its stability and coldness or warmness) and the moisture content.
• The air masses carry atmospheric moisture from oceans to continents and cause precipitation over landmasses.
• They transport latent heat, thus removing the latitudinal heat balance.
• Most of the migratory atmospheric disturbances such as cyclones and storms originate at the contact zone between different air masses and the weather associated with these disturbances is determined by characteristics of the air masses involved.

Classification of Air Masses

• Broadly, the air masses are classified into polar and tropical air masses.
• Both the polar and the continental air masses can be either of maritime or continental types.

Continental Polar Air Masses (CP)

• Source regions of these air masses are the Arctic basin, northern North America, Eurasia and Antarctica.
• These air masses are characterized by dry, cold and stable conditions.
• The weather during winter is frigid, clear and stable.
• During summer, the weather is less stable with lesser prevalence of anticyclonic winds, warmer landmasses and lesser snow.

Maritime Polar Air Masses (MP)

• The source region of these air masses are the oceans between 40° and 60° latitudes.
• These are actually those continental polar air masses which have moved over the warmer oceans, got heated up and have collected moisture.
• The conditions over the source regions are cool, moist and unstable. These are the regions which cannot lie stagnant for long.
• The weather during winters is characterized by high humidity, overcast skies and occasional fog and precipitation.
• During summer, the weather is clear, fair and stable.

Continental Tropical Air Masses (CT)

• The source-regions of the air masses include tropical and sub-tropical deserts of Sahara in Africa, and of West Asia and Australia.
• These air masses are dry, hot and stable and do not extend beyond the source.
• They are dry throughout the year.

Maritime Tropical Air Masses (MT)

• The source regions of these air masses include the oceans in tropics and sub-tropics such as Mexican Gulf, the Pacific and the Atlantic oceans.
• These air masses are warm, humid and unstable.
• The weather during winter has mild temperatures, overcast skies with fog.
• During summer, the weather is characterized by high temperatures, high humidity, cumulous clouds and convectional rainfall.

Fronts

• Fronts are the typical features of midlatitudes weather (temperate region – 30° – 65° N and S). They are uncommon (unusual) in tropical and polar regions.
• Front is a three dimensional boundary zone formed between two converging air masses with different physical properties (temperature, humidity, density etc.).
• The two air masses don’t merge readily due to the effect of the converging atmospheric circulation, relatively low diffusion coefficient and a low thermal conductivity.

Front Formation

• The process of formation of a front is known as Frontogenesis (war between two air masses), and dissipation of a front is known as Frontolysis (one of the air masses win against the other).
• Frontogenesis involves convergence of two distinct air masses. Frontolysis involves overriding of one of the air mass by another.

• In northern hemisphere Frontogenesis (convergence of air masses) happens in anti-clockwise direction and in southern hemisphere, clockwise direction. This is due to Coriolis effect.
• Mid-latitude cyclones or temperate cyclones or extra-tropical cyclones occur due to frontogenesis.

General Characteristics of Fronts

• The temperature contrast influences the thickness of frontal zone in an inversely proportional manner, i.e., two air masses with higher temperature difference do not merge readily. So the front formed is less thick.
• With a sudden change in temperature through a front, there is a change in pressure also.
• Front experiences wind shift, since the wind motion is a function of pressure gradient and Coriolis force.
• The frontal activity is invariably associated with cloudiness and precipitation because of ascent of warm air which cools down adiabatically, condenses and causes rainfall.
• The intensity of precipitation depends on the slope of ascent and amount of water vapour present in ascending air.

Classification of Fronts

• Based on the mechanism of frontogenesis and the associated weather, the fronts can be studied under the following types.

Stationary Front

• When the surface position of a front does not change (when two air masses are unable to push against each other; a draw), a stationary front is formed.
• The wind motion on both sides of the front is parallel to the front.
• Warm or cold front stops moving, so the name stationary front.
• Once this boundary resumes its forward motion, becomes a warm front or cold front.

Weather along a stationary front

• Cumulonimbus clouds are formed. Overrunning of warm air along such a front causes frontal precipitation.
• Cyclones migrating along a stationary front can dump heavy amounts of precipitation, resulting in significant flooding along the front.

Cold Front

• Such a front is formed when a cold air mass replaces a warm air mass by advancing into it or that the warm air mass retreats and cold air mass advances (cold air mass is the clear winner).
• In such a situation, the transition zone between the two is a cold front.
• Cold front moves up to twice as quickly as warm fronts.
• Frontolysis begin when the warm air mass is completely uplifted by the cold air mass.

Weather along a cold front

• The weather along such a front depends on a narrow band of cloudiness and precipitation.
• Severe storms can occur. During the summer months thunderstorms are common in warm sector.
• In some regions like USA tornadoes occur in warm sector.
• Produce sharper changes in weather. Temperatures can drop more than 15 degrees within the first hour.

Cloud formation along a cold front

• The approach of a cold front is marked by increased wind activity in warm sector and the appearance of cirrus clouds, followed by lower, denser altocumulous and
• At actual front, dark nimbus and cumulonimbus clouds cause heavy showers. A cold front passes off rapidly, but the weather along it is violent.

Warm Front

• It is a sloping frontal surface along which active movement of warm air over cold air takes place (warm air mass is too weak to beat the cold air mass).
• Frontolysis (front dissipation) begin when the warm air mass makes way for cold air mass on the ground, i.e. when the warm air mass completely sits over the cold air mass.

Weather along a warm front

• As the warm air moves up the slope, it condenses and causes precipitation but, unlike a cold front, the temperature and wind direction changes are gradual.
• Such fronts cause moderate to gentle precipitation over a large area, over several hours.
• The passage of warm front is marked by rise in temperature, pressure and change in weather.

Clouds along a warm front

• With the approach, the hierarchy of clouds is—-cirrus, stratus and nimbus. [No cumulonimbus clouds as the gradient is gentle]
• Cirrostratus clouds ahead of the warm front create a halo around sun and moon.

Occluded Front

• Occlusion: Meteorology a process by which the cold front of a rotating low-pressure system catches up the warm front, so that the warm air between them is forced upwards.
• Such a front is formed when a cold air mass overtakes a warm air mass and goes underneath it.
• Frontolysis begin when warm sector diminishes and the cold air mass completely undertakes the warm sector on ground.
• Thus, a long and backward swinging occluded front is formed which could be a warm front type or cold front type occlusion.

Weather along an occluded front

• Weather along an occluded front is complex—a mixture of cold front type and warm front type weather. Such fronts are common in west Europe.
• The formation Mid-latitude cyclones [temperate cyclones or extra-tropical cyclones] involve the formation of occluded front.

Clouds along an occluded front

• A combination of clouds formed at cold front and warm front.
• Warm front clouds and cold front clouds are on opposite side of the occlusion.