Definition of Mistral in Geography

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Local Winds

Local Winds and Its Influence On Climate

Local Winds are produced due to local variability in temperature and pressure conditions. Thus, they are more localized in their extent and cover limited horizontal and vertical dimensions and confined to the lower levels of the troposphere. However, it is important to remember that some of the local winds can have very large dimensions like, Northers of North America, which originate in Arctic Canada and reach as south as the Gulf of Mexico.

Types of Local Winds

1. Periodical-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.

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

Land and Sea Breeze

The only difference between Land and Sea Breeze and Monsoon is that the former is generated by the diurnal variation of pressure while later (Monsoon) is generated by seasonal variation of pressure and due to this reason, the Land and Sea Breeze are sometimes known as diurnal Monsoon.

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 flow over the land.
The 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 helps in moderation of coastal temperature and due to the sea breeze, coastal regions record a drop of 5-10 ⁰C in their temperatures. It also frequently causes late afternoon rainfall in these coastal areas, particularly during summer.
The Sea breezes become operative from 9-10 AM in the morning and can remain operative till 8 PM during summers and they reach their maximum intensity in the mid-afternoon when their velocity ranges 10-20 km/hr.
During evening and morning, when the temperature contrast between land and sea is at a minimum, the sea breeze dies down as the intensity of cool sea breeze is directly proportional to thermal contrast over the land and sea.
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.

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 breezes is also lesser compared to the Sea Breeze.
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.

The significance of Land and Sea Breeze

They help in coastal navigation in case of small crafts.
They bring greater relief to inhabitants of those areas by reducing temperature and humidity.

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.
Valley Breeze is generally more intense and more persistent and on occasions, these breezes reinforce the prevailing winds on the windward slope of the mountain and cause heavy orographic rainfall.

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 characterized by frost during the night while the upper part/ hill-side is free from frost in cold areas.

Frequency

Mountain Breeze is more common in cold season while valley breeze has a greater preference for Summer Season, in summer the valley breeze continues even after the sunset and sometimes they persist throughout the night.

Hot Local Winds

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

Chinook

These are warm and dry winds blowing on the eastern slopes (leeward side) of the Rocky Mountain. They are the result of adiabatic heating which occurs due to downslope compression on the leeward side, as the mountain barrier creates frictional drag which tends to pull the air from the higher level down on the leeward and air forced down is heated adiabatically and at the same time its relative humidity is also lowered.
The temperature in Chinook is so warm that it can remove the underlying snow cover/ice and sometimes these winds are so dry that in spite of their below freezing temperatures the entire snow cover on the ground disappears, by process of sublimation. Thus, these winds are also known as Chinook, which literally means Snow Eater.
Chinook leads to the melt-down of the water and due to the sudden rise in temperature the water is evaporated, thus there occurs a reduction in the soil moisture, which is a negative aspect of Chinook.

Weather associated with Chinook

Ordinarily, a Chinook wind is accompanied by the cyclonic activity which produces Cloud and precipitation on the windward side of the Rocky Mountain Range.
The latent heat released into the air through the condensation process warms the air and which passes across the mountain range and since the air has lost its moisture it becomes drier.
During winter Great Plain of North America are very cold and frozen, Chinook with its arrival increases the temperature and brings relief to the people and at the same time, the rise in temperature due to Chinook also helps in the early sowing of spring wheat in the USA.

Foehn

Foehn is dry and warm wind resulting due to adiabatic heating on the leeward side of the Mountain range.
These winds are more common on the northern side of Alps in Switzerland and with the arrival of these winds, there is a rapid rise in temperature.
The low relative humidity and high temperature are due to the adiabatic heating of the down-slope winds.
The Foehn winds are present throughout the winter and due to the presence of such winds the temperature increases, and valleys of Switzerland are called Climatic Oasis during the winter season.

Santa Ana

Santa Ana is a dry and warm local wind that occurs in the Santa Ana Valley of Southern California and it is almost similar to Chinook or Foehn with regard to thermal and moisture characteristics.
Unlike Chinook or Foehn, Santa Ana blows as air channel through east-west stretching Santa Valley and due to descend over the western slope of Rockies they become even more hot and dry.
Santa Ana winds are so dry and warm that they pose a great danger to the Californian Orchards and are considered as Climatic Hazard since they desiccate (dry) the soil and vegetation moisture and may even result in widespread forest fires.

Harmattan

These hot and dry wind originate from the Sahara Desert and blow towards the Guinea coast of Africa.
Due to their journey over the Sahara Desert, these winds become extremely dry and as they pass over the Sahara Desert they pick up more sand especially red sand and turn dusty.
As these winds arrive on the western coast of Africa, the weather which is warm and moist before its arrival, turns into pleasant dry weather with low relative humidity, thus bringing great relief to the people. Due to this reason, they are also known as doctor winds in the Guinea coast area of Western Africa.

Sirocco

Sirocco is the warm, dry, dusty local wind that blows from the Sahara Desert over the central Mediterranean, southern Italy and may even reach Spain, etc.
As the Sirocco descend through the northern slope of the Atlas Mountain they become extremely warm and dry
These winds carry red sand particles from Sahara due to which they cause red color rainfall in southern Europe and this rainfall is known as Blood Rain.
Sirocco is so much laden with sands and dust that the atmospheric visibility reduces almost to zero and they are much injurious to the Agriculture and fruit crops.
Sirocco winds are known as Levech in Spain and Leste in Morocco and Madeira, Gibli in Lybia, Chilli in Tunisia.

Khamsin

These are warm, dry, dusty (containing sand particles) local winds that blow in Northern Africa (Egypt) and Arabia.
The temperature of these winds varies from 38⁰C-49⁰C, and this is even more vigorous during late winter and early spring.
These dust storms occur on the forward sides of the temperate cyclone whenever any such cyclone passes through the Mediterranean.

Simoom

Simoom is the intensely hot, dry and dusty wind in the Arabian desert and they come into origin due to the warm Tropical Continental Airmasses.
These winds produce very unpleasant weather.

Dust Devils

Dust devil or the whirlwind form over the hot, dry land due to the intense daytime convection and surface friction.
The dust devils are funnel-shaped and thus resemble a tornado in appearance and sometimes it is so powerful that it can uplift the loosely fixed roofs of the houses and small lighter vehicles.

Loo

It originates from the Thar desert and has north-westerly to a westerly direction.
They dominate during early summer in the months of March to May and create heat waves like conditions in Northern India and adjoining parts.
They have desiccating effects and are considered as environmental hazards.

Cold Local Winds

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

Mistral

It is a cold and dry wind that blows in Spain and France from North-west to South-East direction, mostly occur during winter months.
Due to the presence of the Rhome River, these winds are channelized into the Rhome valley due to which they become extremely cold.
As they pass through the narrow Rhome Valley, they turn into stormy northerly cold winds and their average velocity which is 55-65 km/hr may even reach 128km/hr.
Such stormy cold northerly winds cause a sudden drop in temperature to below freezing point.

Bora

These are cold and dry north-easterly winds that blow from the mountains towards the eastern shore of the 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.
These winds blow with strong gusts with velocities in the range of 128-200 km/hr.
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 the southern states of USA.
Due to the snow and ice particles, the atmospheric visibility reduces almost to zero and the temperatures reduce to sub-freezing levels.
Such Blizzards are of common occurrence in the Antarctic and here the wind velocity may even reach up to 160km/hr.

Levanter

These are strong easterly, moist and damp winds in the southern Spain and Strait of Gibraltar.
These winds are more frequent in early winter to late winter and cause foggy weather.

Pampero

It is a north-westerly cold wind in the Pampas of South America and it is similar to the Northers of North America and Siberia.
Pampero is more active during winters and off the coast of Argentina, it blows from July to September.

Buran

It is an extremely cold north-easterly or easterly wind that blows in central Siberia and eastern Russia.

Definition of Mistral in Geography

Source: https://neostencil.com/local-winds