Climatology/Wind

Introduction
Wind is the flow of gases on a large scale. On the surface of the Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the Sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space. Winds are commonly classified by their spatial scale, their speed, the types of forces that cause them, the regions in which they occur, and their effect. The strongest observed winds on a planet in the Solar System occur on Neptune and Saturn. Winds have various aspects: velocity (wind speed); the density of the gas involved; energy content or wind energy. Wind is also an important means of transportation for seeds and small birds; with time things can travel thousands of miles in the wind. Winds can shape landforms, via a variety of aeolian processes such as the formation of fertile soils, such as loess, and by erosion. Dust from large deserts can be moved great distances from its source region by the prevailing winds; winds that are accelerated by rough topography and associated with dust outbreaks have been assigned regional names in various parts of the world because of their significant effects on those regions. Wind also affects the spread of wildfires.

Factor Affecting Wind Motion

 * 1) Horizontal Pressure (Gradient)
 * 2) Rotation of the Earth (The coriolis Force)
 * 3) Frictional Forces
 * 4) Centrifugal action of wind

Horizontal Pressure (Gradient)
Pressure gradient  is the  change  in  the  air pressure  per  unit  of  distance  travelled  along  a  certain line. In another words, it is an average change in barometric pressure per unit of distance along a certain linear direction in a given region. The pressure gradient  determines  the  direction  as  well  as  the  intensity  of  the  wind  blow. The winds always  blow  from  the  high  pressure  areas  to  the  low  pressure areas.Pressure  gradient  can  be  calculated  by  the  change  in  the  isobaric  values  (barometric  pressure generally in millibars per km (distance)).

Rotation of the earth - The Coriolis Force
Coriolis, practically speaking is not a force but it an effect which is observed on a mass of body in a rotating system. It results from the rotational movement of the earth and the movement of air in relation to the earth. It acts perpendicular to the axis of the earth.It is determined by the mass of the  body  and  its  rate  of  rotation. The earth  rotates  from  west  to  east  on  its axis. Hence, the Coriolis  force operates  in  north-south  direction. The Coriolis  force  is  zero  at  the  equator  and maximum  at  the  poles. This concept  was  first  explained  by  French  engineer  G.  G.  Coriolis in 1835, thus it is known by his name. One rotation  takes  about  24 hours. The velocity  of  the  rotation  of  the  earth  is 1670  km/hour  (the  circumference  is  about 40000 km) along the equator which comesdown to half (835 km/hour) along 600 north and south latitudes.We  know  that  the  earth  is not  a  non-rotating body but, it rotateson its axis. When the air moves equatorward on the spinning earth in the northern hemisphere, the air is deflected to the right as the surface itself moves forward. The same thing also happens when the air is moving from equator to pole, i.e., rightward turning in the northern hemisphere.

Frictional Force
Air friction is the resistance to motion of air in relation to the surface roughness and irregularity  through  which  the wind  is  blowing. Frictional force  reduces the  velocity  of  the wind  near  the ground. Therefore, wind  is  most  affected  by  the  friction  andit  is  maximum  near the  ground. Above the  ground,  the  friction  is  reduced  very drastically.The surface irregularity and undulation is impacting the blowing winds. Within a height of about one km from the ground, the wind is creating eddies and straight blow is  heavily  affected. After that  the  velocity  is  increasing. Right moving  arrows  shows  the direction as well as the velocity of the winds.

Centripetal Force
Centripetal force operates at right angles to the blowing wind. It is the inward pulling force i.e. towards the  centers  of  rotation. Vertically above about a height of around 5.5 km, the  air  pressure  is  approximately  500  mb. By this  height,  the  friction  force  is almost terminated except over the highlands and mountains. Coriolis force  is  equal  to  centripetal  force  plus  pressure  gradient  force. But in case  of  high  pressure  where  anticyclone  is  developed  in  upper  troposphere,  pressure  gradient force is equal to Coriolis force plus centripetal force. In these conditions, the winds are blowing parallel to the isobars. It is called geostrophic winds by which the jet stream is developed.

Classification of Wind
They are classified in two broad categories:-
 * Permanent Winds or Invariable Winds or Planetary Winds
 * Variable Winds

Permanent Wind
Direction of winds remains more or less same throughout the year though their area change seasonally, such winds are called permanent winds these are related to thermally and dynamically induced pressure belts and rotation of the earth ,hence they are called planetary winds. these winds include trade winds,Westerlies and polar winds.

Winds in the Tropical Region
Areas extending between 30°N and 30°S latitude are include in tropical zone.trade winds blow from the subtropical high pressure belt to the Equatorial low pressure belts.there is a belt of calm or doldrum characterized by feeble air circulation.

Equatorial Westerlies
The westerlies, anti-trades,prevailing westerlies, are prevailing winds from the west toward the east in the middle latitudes between 30° and 60° latitude. They originate from the high-pressure areas in the horse latitudes and trend towards the poles and steer extratropical cyclones in this general manner. The westerlies are strong, especially in the Southern Hemisphere, in areas where land is absent, because land amplifies the flow pattern, making the current more north-south oriented, slowing the westerlies. The strongest westerly winds in the middle latitudes can come in the roaring forties, between 40° and 50° latitude. The westerlies play an important role in carrying the warm, equatorial waters and winds to the western coasts of continents, especially in the southern hemisphere because of its vast oceanic expanse.

Doldrums
The doldrums, usually located between 5° north to 5° south of the equator, are also known as the Intertropical Convergence Zone or (ITCZ ). The trade winds converge in the region of the ITCZ, producing convectional storms that produce some of the world's heaviest precipitation regions.The ITCZ moves north and south of the equator depending on the season and solar energy received. The location of the ITCZ can vary as much as 40° to 45° of latitude north or south of the equator based on the pattern of land and ocean. The Intertropical Convergence Zone is also known as the Equatorial Convergence Zone or Intertropical Front.

Trade winds
Trade winds blow in a belt lying between5°N-30°N in the northern hemisphere and 5°S-30°S in the southern hemisphere.As we all know that air travels through high pressure to low pressure. There is low pressure on the equator, while high pressure at the sub tropics. So, air moves from tropics toward equator. Due to corolis force and rotation of earth winds move toward left hand side in southern hemisphere and right hand side in northern hemisphere.

Polar Winds
Polar winds begin near the North and South Poles. Frigid air in the winter sinks toward the ground creating a high pressure area at the poles. These winds occur in both hemispheres.The polar easterlies Winds are also called Polar Hadley cells. they are dry, cold prevailing winds ,blow from the high-pressure areas of the polar highs at the North and South Poles towards low-pressure areas within the Westerlies at high latitudes.

Variable Wind
It is type of local winds.Local Winds are produced due to local variability in temperature and pressure condition. Thus, they are more localised in their extent and cover limited horizontal and vertical dimensions and confined to the lower levels of the troposphere.

Surface winds
Surface friction plays an important role in the speed and direction of surface winds.it moves over the ground, wind blows across the isobars into a center of low pressure and out of a center of high pressure.the effects of surface friction, the winds, locally, do not always show the speed and direction that would be expected from the isobars on the surface weather map. These variations are usually due to geographical features such as hills, mountains and large bodies of water.Except in mountainous regions, the effect of terrain features that cause local variations in wind extends usually no higher than about 2000 feet above the ground.

Land and sea breezes
It is caused by the differences in temperature over land and water. The sea breeze occurs during the day when the land area heats more rapidly than the water surface. This results in the pressure over the land being lower than over the water. The pressure gradient is often strong enough for a wind to blow from the water to the land.The land breeze blows at night when the land becomes cooler.Then the wind blows towards the warm, low-pressure area over the water.Land and sea breezes are very local and affect only a narrow area along the coast.

Mountain Winds
Hills and valleys substantially distort the airflow associated with the prevailing pressure system and the pressure gradient. Strong up and down drafts and eddies develop as the air flows up over hills and down into valleys.lines of hills and mountain ranges will act as a barrier. If there is a pass in the mountain range, the wind will rush through this pass as through a tunnel with considerable speed. Daytime heating and nighttime cooling of the hilly slopes lead to day to night variations in the airflow. At night, the sides of the hills cool by radiation. The air in contact with them becomes cooler and therefore denser and it blows down the slope into the valley. This is a mountain winds, also called katabatic wind or mountain breeze. If the slopes are covered with ice and snow, the katabatic wind will blow, not only at night, but also during the day, carrying the cold dense air into the warmer valleys. The slopes of hills not covered by snow will be warmed during the day. The air in contact with them becomes warmer and less dense. This is an anabatic wind or valley breeze. In mountainous areas, local distortion of the airflow is even more severe. Rocky surfaces, high ridges, sheer cliffs, steep valleys, all combine to produce unpredictable flow patterns and turbulence.

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,, Khamsi, 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.

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 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.

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.these winds arrive in 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 which blows from the Sahara Desert over the central Mediterranean, southern Italy and Spain etc.the Sirocco descend through the northern slope of the Atlas Mountain they become extremely warm and dry. These winds carry red sand particle from Sahara due to which they cause red colour rainfall in southern Europe and this rainfall is called as Blood Rain.Sirocco winds are known as Levech in Spain and Leste in Morocco and Madeira, Gibli in Lybia, Chilli in Tunisia.

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 condition 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 they have a temperature below freezing point, they create Cold Wave condition.examples of Cold Local winds are - Mistral, Bora, Blizzard, Purga, Laventer, Pampero, Bise etc.

Mistral
It is a cold and dry wind which blows in the 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. they pass through the narrow Rhome Valley, they turn into stormy northerly cold winds and their average velocity which is 55-65 km/hr to 128km/hr.

Boro
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 ,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.

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.

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

Jet Stream
Jet Stream are defined as swift geostrophic air streams in the upper troposphere that meander in relatively narrow belts.It exist in the higher levels of the atmosphere at altitudes ranging from 20,000 to 40,000 feet or more. A jet stream in the mid latitudes is generally the strongest.Jet Streams extend from 20 degrees latitude to the poles in both hemispheres.jet Streams develop where air masses of differing temperatures meet.So, usually surface temperatures determine where the Jet Stream will form.Greater the difference in temperature, faster is the wind velocity inside the jet stream.

Types of Jet Streams
There are three types of jet stream:-

Sub Tropical Jet Streams
It is developed in winter and early spring. Their maximum speed approaches 300 knots which are associated with the merger with polar-front jets. A subsidence motion accompanies subtropical jets and gives rise to predominantly fair weather in areas they pass over. Sometimes they drift northward and merge with a polar-front jet.

Tropical Easterly Jet Stream
It occurs near the tropopause over Southeast Asia, India, and Africa during summer. This jet implies a deep layer of warm air to the north of the jet and colder air to the south over the Indian Ocean.

Polar-Night Jet Stream
It meanders through the upper stratosphere over the poles. They are present in the convergence zone above the sub polar low pressure belt.

Characteristics of Jet Streams

 * Its genesis is associated with the thermal contrast of air cells, for example Hadley cell, Ferrel cell.
 * The meandering or the whirl movement of the Jet Stream is called Rossby Wave.
 * Equatorial extension of the Jet Stream is more in winter because of the southern shift of the pressure belts.
 * During winters, the thermal contrast increases and the intensity of the high pressure centre at the pole increases. It intensifies the formation of Jet Streams, its extension as well as its velocity.