List of Local Winds | Atmospheric Circulation | Climatology | Geography

Here is a list of local winds:- 1. Land and Sea Breezes 2. Mountain and Valley Breezes 3. Chinook and Foehn 4. Harmattan 5. Sirocco 6. Mistral 7. Bora 8. Blizzard.

1. Land and Sea Breezes:

Land and sea breezes, representing a complete cycle of diurnal winds, are in fact, monsoon winds at local scale because they change their direction twice in every 24-hour period. These local diurnal monsoon winds very commonly known as land and see breezes are found in the coastal areas wherein sea breeze blows from sea to land, during day time and land breeze moves from land to sea during night due to differential heating and cooling of land and water.

Sea Breeze:

Land is heated more quickly than the adjacent sea during day time, with the result the warm air over the adjacent land expands and thus low pres­sure is ‘developed while high pressure is developed over adjacent sea. The pressure gradient causes circu­lation of relatively cool air from sea to adjacent land (fig. 35.15).

Sea breezes begin to flow usually between 10-11 a.m. and become most active in early afternoon usually between 1-2 p.m. with maximum velocity ranging between 10 to 20 kilometres per hour and are terminated by 8 p.m. at night. The average depth of sea breeze system ranges between 1000-2000 metres in the coastal regions of the tropical areas while its depth is between 200 and 500 m near the lakes.

The cooling effect of sea breezes reaches 50 to 65 km inland in the tropical regions while 15 o 50 km in the middle latitudes. The velocity of these winds varies spatially e.g. the velocity varies from 25 to 50 km per hour in the temperate areas while sometimes sea breezes become stormy in the tropical areas. Sea breezes have cooling effects on the coastal land as the temperature drops by 5°C to 10°C, with the result weather becomes pleasant. Sea breezes are most active during summer season.

Land Breeze:

After sunset the sea breezes are weakened because the day time low pressure over land is weakened due to rapid loss of heat through outgoing long-wave radiation from the land. Consequently, the position of day time high and low pressure is reversed. Now high pressure is developed on land against low pressure on the adjacent sea with the result air starts moving from land to sea during night (fig. 35.15B). Land breezes are comparatively weaker than sea breezes. These are dry winds.

2. Mountain and Valley Breezes:

Mountain and valley breezes also known as upvalley (during daytime) and down-valley (during night) breezes are, in fact, local as well as diurnal (periodic winds the directions of which are reversed during 24 hours). The slopes and valley doors in the mountainous regions are more heated through insola­tion during daytime than the free atmosphere at the same elevation. Consequently, the warm air moves upslope (upward). This upslope moving breeze during daytime is called valley breeze (fig. 35.16).

Valley breezes reach mountain peaks and yield precipitation through cumulus clouds. The valley slopes and upper parts are cooled due to loss of heat through outgoing long-wave radiation and thus cool air descends through the valley slopes.

Such wind is called down-valley or mountain breeze. The mountain breezes cause inver­sion of temperature in the valleys. This is why the valley floors are characterized by frost during night while the upper parts are free from frost in cold areas.

3. Chinook and Foehn:

Warm and dry local winds blowing on the leeward sides of the mountains are called chinook in the USA and foehn in Switzerland. These local vertical winds are of cyclonic origin and largely influence the weather conditions of the affected areas locally. The winds associated with the cyclones after descending through the eastern slopes of the Rockies become warm and dry and thus give birth to chinook.

The winds ascend through the western slopes of the Rockies mountains and thus are cooled at the dry adiabatic rate of 5.5°F per 1000 feet (10°C per 1000 metres). These winds after reaching higher height become saturated (due to lowering of temperature and hence increase of relative humidity) and yield precipitation.

The latent heat of condensation released after precipitation is added to the ascending winds, with the result the temperature of the ascending winds decreases at the moist adiabatic rate of 3°F per 1000 feet (fig. 35.17). The westerly winds after crossing over the Rockies descend through the eastern slopes and thus are heated at the dry adiabatic rate of 5.5°F per thousand feet. These warm and dry winds after reaching the foothill zones of the eastern slopes of the Rockies are called Chinooks.

Chinook winds are more common during winter and early spring along the eastern slopes (leeward side) of the Rocky Mountains from Colorado (USA) in the south to British Columbia (Canada) in the north. Normally, the actual temperature of chinooks is 40°F (4.4°C) but the actual temperature of the affected areas is below freezing point during winter months. Thus, chinook becomes very warm in comparison to prevail­ing subzero temperature.

The rise of temperature in the affected areas by 40°F (4.4°C) after the arrival of chinooks within 24 hours is not unusual. Sometimes, temperature rises by 30° to 40°F within few minutes. Thus, snow present on the ground surface melts away due to sudden rise in temperature as if by magic. This is why chinook is also called as ‘snow eater’.

This is the impact of chinooks that green pastures are open in a narrow strip along the eastern slopes of the Rockies even during winter season. The rise in temperature due to chinooks also helps in early sowing of spring wheat in the USA.

A warm and dry wind similar to chinook is called ‘foehn’ along the northern slopes of the Alps Mountains. These are more common during spring and autumn in Switzerland. The weather becomes quite pleasant in the valleys due to melting of snow because of increase in temperature by 40°F after the arrival of foehn winds. This is why valleys of Switzerland are called ‘climatic oasis’ during winter season. These winds help in early sowing of spring wheat, ripening of grapes and check autumn frost.

4. Harmattan:

The warm and dry winds blowing from north­east and east to west in the eastern parts of Sahara desert are called harmattan. These winds become ex­tremely dry because of their journey over Sahara desert. While blowing over Sahara desert these winds pick up more sands mainly red sands.

The western coast of Africa is warm and moist and hence the weather becomes unpleasant because the weather con­ditions characterized by high temperature and high relative humidity become injurious for human health. The weather becomes suddenly dry and pleasant at the arrival of harmattan as the relative humidity of the air is remarkably reduced due to high temperature and hyperaridity of harmattan.

This is why harmattan is known as ‘doctor’ in the Guinea coastal area of western Africa. In fact, harmattan is very dusty and stormy wind blowing with so gusty speed that trees are up­rooted. These winds are usually associated with dust storms resulting into marked reduction in the visibil­ity.

Harmattan becomes more vigorous during summer months. In fact, harmattan is a special type of north­east trade wind. It becomes extremely warm wind because of hot and dry desert of Sahara. Similar warm, dry, very strong and dust-laden winds arc called ‘brick-fielder’ in Victoria province of Australia, ‘black-roller’ in the Great Plains of the USA, ‘shamal’ in Mesopotamia and Persian Gulf, and ‘norwester’ in New Zealand.

5. Sirocco:

Sirocco is a warm, dry and dusty (full of sands) local wind which blows in northerly direction from Sahara Desert and after crossing over the Mediterra­nean Sea reaches Italy, Spain etc. Sirocco becomes very strong and active at the time of the origin of cyclonic storms over the Mediterranean Sea. It be­comes extremely warm and dry while descending through the northern slopes of the Atlas Mountain.

There are different local names for sirocco in Africa e.g., khamsin in Egypt (UAR), gibli in Lybia, chilli in Tunisia etc. The warm and dry dusty winds in the Arabian Desert are called ‘simoom’. Sirocco, while passing over the Mediterranean Sea picks up moisture and yields rainfall in the southern part of Italy where the rain associated with sirocco is called ‘blood rain’ because of fallout of red sands with falling rains.

It may be remembered that sirocco while blowing through Sahara Desert picks up red sands which settle down with rains in south Italy. It is apparent that sirocco is very much injurious to agricultural and fruit crops.

6. Mistral:

Mistral is a cold local wind which blows in Spain and France from north-west to south-east direc­tion. These winds are more common and effective during winter season because of development of high pressure over Europe and low pressure over Mediter­ranean Sea. They become extremely cold when they blow through central plateau and descend into Rhone valley on the southern coast of France.

While blowing through the narrow valley of Rhone they become stormy northerly cold winds. The average velocity of mistral is 56-64 km per hour but sometimes it becomes 128 km per hour. These stormy winds adversely affect air-flights, the arrival of mistral causes sadden drop in air temperature to below freezing point.

7. Bora:

Bora is an extremely cold and dry north-easterly wind which blows along the shore of the Adriatic Sea. Bora becomes more effective in north Italy where it descends through the southern slopes of the Alps and blow in southerly direction. The velocity of bora ranges between 128 km and 196 km an hour. Unlike mistral, bora is relatively moist wind because it picks up moisture while coming from over the Adriatic Sea.

8. Blizzard:

Blizzard is a violent stormy cold and powdery polar wind laden with dry snow and is prevalent in north and south Polar Regions, Siberia, Canada and the USA. The visibility becomes remarkably low because of snow and ice crystals. The velocity ranges between 80-96 km an hour.

The arrival of these winds causes sudden drop in air temperature to subfreezing level, thick cover of snow on the ground surface and onset of cold waves. These winds reach the southern states of the USA because of the absence of any east-west mountain barrier. They are called ‘norther’ in the southern USA and ‘burran’ in Siberia.