Introduction:

Local winds occur on a small spatial scale, their horizontal dimensions typically several tens to a few hundreds of kilometres. They also tend to be short-lived lasting typically several hours to a day. There are many such winds around the world, some of them cold, some warm, some wet, some dry. There are many hazards associated with the winds.

Planetary winds or Primary Winds are those which blow extensively over continents and oceans. The winds that blow constantly throughout the year and blow constantly in a particular direction. There are types of permanent winds namely Trade winds, Easterlies and Westerlies.

Body:

Types and Impact of local winds on the weather:

Periodical winds: The winds originating from diurnal temperature and pressure variation are known as Periodical and they generally complete their cycle in a day/ 24 hour like Land & Sea Breeze and Mountain & Valley Breeze.

Land and Sea Breeze: Land and Sea Breeze is generated by the diurnal variation of pressure. Due to this reason, the Land and Sea Breeze are sometimes known as diurnal Monsoon.

Land Breeze:

• At night reversal of sea breeze may occur but with somewhat weaker characteristics as the temperature and pressure gradient are less steeper during the night.
• During night land breeze is established since land cools to a temperature lower than the adjacent water setting up a pressure gradient from land to sea
• The horizontal and vertical extent of the Land Breeze helps in moderation of temperature of a coastal area during night time as it maintains regular circulation
• Land Breeze usually attains its maximum intensity in the early morning hours and dies out soon after sunup.

Sea Breeze:

• The sea breeze develops along seacoasts or large inland water bodies when the land heats much faster than the water on a clear day and a pressure gradient is directed high over the water to low over the land.
• Impact of Sea breeze rapidly declines landward and impact is limited to 50km.
• Land- Sea Breeze system is very shallow as the average depth of the land and sea breeze, varies from 1000-2000M in tropical regions and over the lakes, the depth is even lesser.
• Sea Breeze brings cool marine air and thus help in moderation of coastal temperature and due to the sea breeze, coastal regions record a drop of 5-10 0C in their temperature
• It also frequently causes late afternoon rainfall in these coastal areas, particularly during summer.
• Due to the location nearer to the lakes, places experience the Lake Effect like Chicago, due to its location near a lake presents a typical example of lake effect- where lakeside areas are cooler than the much warmer outlying areas in the summer.

Mountain and Valley Breeze: These winds develop over areas with large differences in relief and majorly caused by the temperature gradient that exists between Mountain Slopes and valleys.

Valley Breeze:

• Due to the intense insolation during the daytime, the slopes of the mountain heat up rapidly but the free atmosphere above the lowlands is not heated to some extent.
• As the valleys receive comparatively lesser insolation so relatively high pressure sets up in the valleys while along the mountain slopes due to more heating the warm air is uplifted, and low pressure sets up.
• Thus, the air moves from the Valleys towards the slopes (High pressure to the low pressure) and this upslope movement of air is known as valley breeze.
• Valley breezes are also known as Anabatic Wind.
• Weather associated with the Valley Breeze
• This type of upslope winds in the Mountainous region may cause occasional and afternoon thundershowers on warm and humid days.
• Sometimes, the valley breezes are also accompanied by the formation of cumulus cloud near mountain peaks or over slopes and escarpments.

Mountain Breeze:

• On mountain-sides under the clear night sky, the higher land (upslope land) radiates heat and is cooled and in turn cools the air in contact with it. The cool denser air flows down the mountain slope due to the pressure difference since the valley is warmer and at relatively lower pressure.
• This flow of the air is termed as Mountain Breeze and they are also known as Katabatic wind.
• Weather associated with the Mountain Breeze
• By the morning the mountain breeze produces temperature inversions and valley bottom becomes colder than the Mountain Slopes.
• Thus, the valley floors are characterised by frost during the night while upper part/ hill-side are free from frost in cold areas.

Non-Periodical winds: Only present during a season and are classified as Hot and Cold Winds.

Hot Local Winds: Hot Local winds are produced generally by the mechanism of downslope compressional heating also known as adiabatic heating. The examples of the Hot Local Winds include Chinook, Harmattan, Foehn, Sirocco, Norwester, Brickfielder, Khamsin, Santa Ana, Loo etc.

• Chinook:
  • Chinook is the name of hot and dry local wind, which moves down the eastern slopes of the Rockies in U.S.A. and Canada.
  • The literal meaning of chinook is snow eater as they help in melting the snow earlier.
  • During winter Great Plain of North America are very cold and frozen, Chinook with its arrival increase the temperature and bring relief to the people and at the same time, the rise in temperature due to Chinook also helps in early sowing of spring wheat in the USA.
  • They keep the grasslands clear of snow. Hence they are very helpful to ranchers.
• Foehn:
  • Foehn is strong, dusty, dry and warm local wind, which develops on the leeward side of the Alps mountain ranges.
  • Regional pressure gradient forces the air to ascend and cross the barrier.
  • Ascending air sometimes causes precipitation on the windward side of the mountains.
  • After crossing the mountain crest, the Foehn winds start descending on the leeward side or northern slopes of the mountain as warm and dry wind.
  • The temperature of the winds varies from 15°C to 20°C, which help in melting snow.
  • Thus making pasture land ready for animal grazing and help the grapes to ripe early.
• Harmattan:
  • Harmattan, hot, dry wind that blows from the northeast or east in the western Sahara.
  • It usually carries large amounts of dust, which it transports hundreds of kilometres out over the Atlantic Ocean.
  • The dust often interferes with aircraft operations and settles on the decks of ships.
  • The interaction of the Harmattan with monsoon winds can cause tornadoes.
  • Humidity drops to as low as 15 per cent, which can result in spontaneous nosebleeds for some people.
  • The wind can also increase fire risk and cause severe crop damage
• Loo:
  • Loo is hot and dry winds, which blow very strongly over the northern plains of India and Pakistan in the months of May and June.
  • Their direction is from west to east and they are usually experienced in the afternoons.
  • Their temperature varies between 45°C to 50°C.
  • They have desiccating effects and are considered as environmental hazards.
  • It causes heat waves and can cause heat stroke.

Cold Local Winds: Cold local winds are dust-laden winds and as they have a temperature below freezing point, they create Cold Wave condition. The examples of Cold Local winds include-Mistral, Bora, Northers, Blizzard, Purga, Laventer, Pampero, Bise etc.

• Mistral:
  • Mistrals are most common local cold winds.
  • They originate on the Alps and move over France towards the Mediterranean Sea through the Rhone valley.
  • They are very cold, dry and high-velocity winds.
  • They bring down temperature below freezing point in areas of their influence.
  • People in these areas protect their orchards and gardens by growing thick hedges and build their houses facing the Mediterranean sea.
• Bora:
  • These are cold and dry north-easterly winds which blow from the mountains towards the eastern shore of Adriatic Sea.
  • Bora is more effective in North Italy since here it descends the southern slopes of the Alps, although due to descend it gets adiabatically heated still its temperature is very low in comparison to the coastal area and these are the typical example of fall winds.
  • Bora has often associated with the passage of a temperate Cyclone and at times the Bora winds themselves attain the hurricane force at the foot of the mountain and may cause disastrous impacts on properties.
• Blizzard:
  • Blizzard is cold, violent, powdery polar winds (pick dry snow from the ground)
  • They are prevalent in the north and south polar regions, Canada, USA, Siberia etc. Due to the absence of any east-west Mountain barrier, these winds reach to the southern states of USA.

Conclusion:

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.