V. Wind

Wind, in climatology, the movement of air relative to the surface of the Earth. Winds play a significant role in determining and controlling climate and weather. Generally, air in horizontal motion is known as wind. Horizontal flow of air takes place along the pressure gradient. Vertical movement is noticed but negligibly small compared to horizontal movement as the height of the atmosphere is only for few kilometres. However vertical movement or uplift of air only causes significant weather changes in cloud formation and rain. The wind velocity depends upon the rate of change of pressure per unit distance between two places at same elevation, called as pressure gradient. The moving air exerts a force or pressure on the objects, which obstruct it.

Origin of wind

Due to differential heating of earth surface, a low pressure is developed over warmer place and high pressure is developed over colder place. The air moves horizontally from high pressure area to low pressure area along the pressure gradient. This horizontal flow of air is called wind. The direction of wind first is decided by the direction of pressure gradient (perpendicular direction of isobars).

Forces acting on wind

1. Pressure gradient force (P):

This force causes the air movement in the direction of low pressure. It acts in the perpendicular direction of isobars.
Wind direction is deflected from its normal course (pressure gradient) due to combined effects of Coriolis force (2), Fractional force (3), Centrifugal force (4), and Gravity (5).

2. Coriolis force (C):

An apparent force resulting due to earth rotation, which deflects the wind. It is also called as Coriolis Effect, means the effect on wind due to the earth rotation. The Coriolis force always acts at right angle to the direction of the air motion to right in the northern hemisphere and to the left in the southern hemisphere (Figure 5.1). The Coriolis force is zero at equator and maximum at poles. This force tends to deflect the wind at right angle to its line of motion.

_Figure 5.1. Effect of Coriolis force on wind_

Figure 5.1. Effect of Coriolis force on wind

3. Frictional force:

The force which results due to friction between moving air and surface and it increases with surface roughness. It acts in opposite direction of wind motion and reduces the wind velocity.

_Figure 5.2. Effect of Frictional force on wind_

Figure 5.2. Effect of Frictional force on wind

4. Centrifugal force (Cfg):

The pressure gradient and Coriolis forces are present, but when the isobars are curved, there is a third force – the centrifugal force. This apparent force, pushes objects (particles) away from the center of a circle. The centrifugal force alters the original two-force balance and creates the non-geostrophic gradient wind.

Geostrophic wind:

It is the wind, which blows parallel to isobars in a straight line, with a balance between Coriolis force and pressure gradient force acting on it. These winds are observed at higher levels, where there is no frictional force due the surface roughness.

_Figure 5.3. Effect of centrifugal force on wind_

Figure 5.3. Effect of centrifugal force on wind

5. Gravity (g):

It always acts downwards on the parcel of air. It plays an important role in vertical motion of air. It has no horizontal movement of air.

Gradient wind

It may be defined as the wind that results with a balance between pressure gradient force on one hand and the coriolis force and centrifugal force on the other hand.

Air current

The vertical movement of air is called as air current. It includes eddies, convectional currents, convergence rise and subsidence.

Beaufort’s scale

The Beaufort scale, officially known as the Beaufort wind force scale, is a descriptive table. It depicts the force of wind by a series of numbers from 0 to 12. Actually, the Beaufort scale goes all the way to 17, but the last five numbers only apply to tropical typhoons.

_Table 5.1. Beaufort scale values and descriptions_

Table 5.1. Beaufort scale values and descriptions

Definitions:

Isotach: It is the line on weather map, which joins the places of equal wind speed. Isotach are drawn on weather map to represent places with equal wind speed.
Wind ward: It is the direction, from which wind is coming.
Lee ward: It is the direction towards which wind is blowing.
Vorticity: Whirling motion of fluid particles is known as vorticity.
Haze: Decrease in the visibility of atmosphere due to dust and smoke particles is termed as haze.
Dust evil: It is a whirling dust column of height 1 km or more over the earth surface.
Gale: It is a wind having velocity between the ranges of 28-55 km.
Squall: It is a sudden increase in wind speed (greater than 50 km/hr) for at least one minute and then decreases.
Gust: It may be defined as an eddy moving down with mean speed greater than 25 to 30 km/hr.
Lull: It may be defined as an eddy moving up with mean wind speed less than 5 km/hr.

Buys-Ballots’ Law
If we stand with back toward wind direction, the low pressure will be on the left hand and high pressure will be on right hand in northern hemisphere and vice-versa in southern hemisphere.

Types of air circulation

Wind movement in the atmosphere is classified into three circulations.
1. Permanent circulation: It includes the planetary/permanent wind system, where, permanent or continuous exchange of air takes place over macro scale. Trade winds, westerlies and easterlies are primary circulations. It is a regular circulation.
2. Periodic circulation: It is the periodic/seasonal exchange of air, which takes place over synoptic scale. Therefore, it is also called synoptic scale circulation e.g. Monsoons, cyclones, anticyclones and air masses. It is named as periodic/seasonal circulation.
3. Local circulation: It is the air exchange which takes place over mesoscale and microscale. It includes local winds, land breeze, sea breeze, mountain winds and valley winds. It is also called as local circulation. The wind is named with the direction from which it is blowing. So, wind coming from east is called easterly.

Planetary winds/Permanent winds

Wind systems of the world

The wind belts found on the earth’s surface in each hemisphere are:
a. Doldrums
b. Trade wind belt
c. Prevailing westerlies
d. Polar easterlies

_Figure 5.4. Global wind circulation_

Figure 5.4. Global wind circulation

1. Doldrums
Owing to continuous heating of the earth by insolation, pressures are low and winds converge and rise near the equator. This intertropical convergent zone is known as ‘Doldrums’.
a) These are the equatorial belts of calms and variable winds.
b) The location is 5°S and 5°N latitudes.
c) Wind is light due to negligible pressure gradient.
d) Mostly, there are vertical movements in the atmosphere.
e) The atmosphere is hot and sticky.

2. Trade winds (Tropical Easterlies)
a) The regular high temperature at the equator results in a high pressure forming in the upper levels of the equator.
b) Then, the air is transferred to the northward and southward directions until 35° North and South in both the hemisphere.
c) Due to this reduction in surface pressure on the equator (doldrums) there is an increase in pressure at 35°N and 35°S which are known as horse latitude (sub-tropical high).
d) As a result, the winds flow from the horse latitude to the equatorial region.
e) While moving, these winds are deflected by Coriolis force to the right in northern hemisphere and to the left in southern hemisphere.
f) These winds flow from 35°N to the equator in NE direction in the northern hemisphere and from 35°S to the equator in SE direction in the southern hemisphere. These are known as ‘Trade winds’. These are known as ‘Tropical easterlies’.
g) These are most constant winds in force and direction and flow over nearly half the globe.

3. Anti-trade winds
a) This is a supplementary wind system of the earth which is effective at higher levels.
b) This system works in opposite direction to the surface winds.
c) The anti-trade winds mostly flow from land to ocean and brings no rain.

4. Prevailing Westerlies
a) The winds that flow from sub-tropical high to the low-pressure area about 60-70° latitudes in both the hemispheres are known as ‘Prevailing westerlies’.
b) In the northern hemisphere the direction of Prevailing westerlies is SW and in southern hemisphere NW.
c) These winds are forceful and are irregular as compare to the trade winds in the tropical regions.
d) High precipitation zone

5. Polar Easterlies / Polar winds
a) A permanent high pressure exists on the poles.
b) From these high pressure Polar Regions, cold winds flow to areas at about 60-65° latitudes in both the hemispheres.
c) The winds flow in NE direction in the northern hemisphere and in SE direction in the southern hemisphere.

Periodic winds/monsoons

Monsoon is a periodic circulation. The term monsoon is derived from Arabic word ‘mausim’ or from the Malayan ‘monsin’, means ‘season’. The word monsoon is used for such a circulation, which reverses its direction after every six months i.e. from summer to winter and from winter to summer. In summer, the moist wind blows from ocean to land and in winter, cold and dry wind blows from land to ocean. Monsoon circulation involves 1800 change in the direction of winds. The principal monsoons of the world are the summer and winter monsoons of Asia and the monsoon circulations over west and east Africa. Asian monsoon system (100S to 800N) consist three distinct regional systems:
1. Indian monsoon
2. Japanese monsoon
3. Malayam monsoon
The detail of different monsoons is given in the chapter of monsoon.

Local winds

These winds originate due to difference in local features. Local wind is a tertiary circulation over a meso and microscale. It includes the following local wind systems:

1. Land breeze: It prevails during night. The land surface cools to lower temperatures than the temperatures over adjacent water surface. A low pressure is developed over water surface and high pressure is developed over land surface. Due to this pressure gradient between land and sea, air moves from land to sea. This is called land breeze. It is developed along the sea cost. It is daily in occurrence.

_Figure 5.5. Land breeze and sea breeze_

Figure 5.5. Land breeze and sea breeze

2. Sea breeze: It prevails during day time. During day time land surface warms to temperatures higher than near by water surface temperatures. Therefore, a low-pressure area is developed over land surface and high pressure is developed over water surface. Air moves horizontally from sea (water surface) to land along the pressure gradient between sea and land. This movement of air from sea to land is called as sea breeze. It is a reverse of land breeze.

3. Mountain winds: On clear night, the mountain tops radiates energy and cool to lower temperatures as compared to valley. The cool denser air moves down along the mountain slope into the valley. This air movement from mountain to valley is called mountain wind/mountain breeze. It is daily in occurrence. It is also known as katabatic wind.

_Figure 5.6. Diurnal mountain winds variation_

Figure 5.6. Diurnal mountain winds variation

4. Valley wind: During day time in the morning, the mountain top gets energy earlier and warms to temperatures higher than valley temperatures. The warm air over mountain top rises above and generates upslope flow of air from valley. This movement of air from valley to mountain top is known as valley wind. It is also named as anabatic wind.

5. Warm local winds: These are hot and dry winds resulted due to advection of hot and dry air from warm places. Adiabatic warming of air also produces them, as they decent from elevated area to low land area. These are named differently in different parts of the world.

Tornadoes
Tornado is a most violent and destructive whirlwind. It is funnel shaped and its diameter is very small i.e. near about 10 meter. Wind velocity is very high of about 460 mile/hr. Where it touches the ground, there is complete destruction. Tornadoes are experienced in United States of America. It originates due to mixing of dry polar air mass with warm and moist tropical air mass.

Zet stream
It is a stream of very rapid moving air circulating the earth at higher elevation of about 10-12 km i.e. in upper troposphere. Wind speed ranges from 150-300 km.

Effect of wind on crop plants

  1. Transports heat in either sensible or latent heat form from lower to higher altitudes
  2. Wind affects the plant directly by increasing transpiration and the intake of CO2¬ and also causes several types of mechanical damage.
  3. Wind helps in pollination and dispersal of seeds.
  4. Light and gentle winds are helpful for cleaning the agricultural produce.
  5. Hot dry winds frequently do much damage to vegetation in the growing crops by promoting excessive water loss.
  6. Wind has powerful effect on humidity.
  7. Long, continued warm, dry winds injured blossoms by evaporating the secretion of the stigma.
  8. Provides moisture which is necessary for precipitation
  9. Wind prevents frost by disrupting atmospheric inversion
  10. Causes soil erosion

Instruments

1. Anemometer: The science of measuring and recording the direction and speed of wind is called anemometery. The instrument which is used to measure wind velocity is called as anemometer e.g. wind cup anemometer, automatic anemometer etc.
_Figure 5.7. Cup Counter Anemometer_

Figure 5.7. Cup Counter Anemometer

2. Anemograph: The instrument which gives a continuous record of wind velocity with time on a chart.

3. Wind vane: The instrument which is used to measure wind direction is known as wind vane.
_Figure 5.8. Wind vane_

Figure 5.8. Wind vane

4. Aerovane: The instrument, which measures both wind velocity and wind direction.
_Figure 5.9. Aerovane_

Figure 5.9. Aerovane