What are Winds?
Air flows from areas of high pressure to areas of low pressure. Horizontal
movement of the air is called wind. It is nature's attempt to balance
inequalities in air pressure. The vertical or nearly vertical movement of air
is referred to as air current. Winds and air currents manage a system of
circulation in the atmosphere.
What are the FACTORS AFFECTING DIRECTION AND VELOCITY OF WIND?
Direction and speed of wind are controlled by a combination of
factors. These are the pressure gradient force, the coriolis force, altitude,
latitude, rotation of earth, the centripetal acceleration and friction.
1. Pressure Gradient: We have already studied about the
pressure gradient. The greater the difference in pressure between two points,
the steeper is the pressure gradient and the higher is the wind speed. Since
the direction of the force is from higher to lower pressure area and
perpendicular to the isobars, the wind blows parallel to the gradient and at
right angles to the isobars.
2. Coriolis Force: The earth rotates on its
inclined axis. If it did not, winds would follow the direction of the pressure
gradient. But the rotation produces another force other than the pressure
force. It is called the 'Coriolis force', which deflects the air.
The deflection is the least at the Equator and greatest at the
Poles. This tends to turn the flow of air by changing its direction from its
original straight path. The wind starts deflecting to its right in the Northern
Hemisphere. In the Southern Hemisphere it starts deflecting to its left from
its original path. Thus a wind blowing from north becomes north-easterly in the
Northern Hemisphere. A wind blowing from south becomes south-easterly in the
Southern Hemisphere, e.g.. North East Trade winds. This is due to maximum speed
of rotation at the Equator, hence the deflection is less.
3. Rotation of the Earth: The
rotation of the earth causes the air at the Poles to be thrown away towards the
Equator. In theory, this should result in air piling up along the Equator to
produce a belt of high pressure, whilst at the Poles low pressure should
develop. But what actually happens is
explained under the heading Pressure Belts in this Chapter. It is to be noted
that temperature
and rotation together affect the pressure pattern.
4. Altitude: Vertically, pressure decreases
with increase in altitude. The rate of decrease with height is however, not
constant. Many factors like the slope of the land, presence of winds and the
temperature affect the pressure of air vertically. As stated above, at ground
level the pressure varies between 940 mb to 1040 mb. At a height of 3000
metres, the pressure may be just 700 mb. This means the rate of decrease is 100
millibars per 1000 metres of height gained. At high altitudes, there is a cover
of thin air which has both low pressure and a low capacity to hold heat. This
is one of the reasons that mountaineers take oxygen supplies with them while
going up high altitudes.
5. Latitude: When the temperature of a place
rises, air expands; this makes the air less dense, and so its pressure
decreases. When the temperature falls, air becomes dense and its pressure
increases. Temperature and pressure are inversely related. It is because of
high temperature that we find a belt of low-pressure around the earth at the
Equator. At the Poles, because of extremely low temperature, high pressure
exists.
There are in-blowing winds and uplifting air currents in the hot
regions of low pressure. There are out-blowing winds and sinking air currents
in the cold regions of high pressure. The uplifting air currents are called
convection currents.
The more the pressure, the more is the speed of the wind. Because
of the 'migration' of the sun, the heat zones are not constant. So the winds
direction and speed are affected.
6. Friction: Near the earth's surface wind
does not move freely in a horizontal plane due to irregularities in the earth's
surface. Friction determines the angle at which the air will flow across the
isobars. It may also alter wind direction. Over relatively smooth ocean
surface, friction will be low and the air will move at comparatively lower
angle to the isobars and hence at a greater speed. Over rugged terrain,
friction will be high and therefore, the angle of the air flow will also be
high and the speed much reduced.
What are the TYPES OF WINDS?
There are some winds which blow throughout the year from one
latitude to the other in response to the latitudinal differences in air
pressure. These are known as Permanent or Prevailing or Planetary Winds.
Certain winds reverse their direction periodically with season and are called
Periodic Winds. There are certain winds in different parts of the world which
flow in comparatively small area and have special characteristics. These are
called Local Winds.
Discuss about PERMANENT WINDS?
The Trade Winds
The winds blowing from the Sub-Tropical High Pressure area (30°N
and S) towards the Equatorial Low Pressure belt are the extremely steady winds
known as the trade winds.
The name trade comes from the German word trade meaning 'track'.
To blow trade means 'to blow steadily in the same direction and in a constant
course'.
North and South of the Equatorial Belt are the Trade Winds in the
zone lying between 5° and 30° North and South. In other words, they
cover almost the entire area between 30°N and 30°S latitudes on
both sides of the Equator. The Trade Winds are a result of a pressure gradient
from the Sub-Tropical Belt of High Pressure to the Equatorial Belt of Low
Pressure.
In the Northern Hemisphere, the wind moving towards the Equator,
is deflected by the earth's rotation to flow south-westward. Thus, the
prevailing wand there is from the North-East, and it has been named as the
'North-East Trade'. In the Southern Hemisphere, deflection of the wind is towards
the left, this causes the 'South-East Trades'.
Characteristics of Trade Winds: They blow from Sub-tropical High Pressure to Equatorial Low Pressure.
(i) Since they are
warm winds, they pick up moisture and are responsible for heavy rainfall on
eastern sides of tropical lands.
(ii) They are called
North-East Trades in Northern Hemisphere and South East Trades in Southern
Hemisphere. The winds and pressure belts move a few degrees north and south
along with the movement of the overhead sun.
(iii) They have fixed velocity
and are regular.
(iv) They are permanent or
prevailing winds.
THE WESTERLIES
The Westerlies or the Prevailing Westerly Winds blow between 35°
and 60° North and South latitudes from the Sub-Tropical High Pressure Belts
towards the Sub-Polar Ix)w-Pressure Belts.
In the Northern Hemisphere, the Westerlies generally blow from the
south-west to the northeast, and in the Southern Hemisphere from the north-west
to the south-east. These are on-shore winds on the west coasts and off-shore
winds on their east coasts. The on-shore winds bring rainfall while the
off-shore wands do not bring rainfall.
Westerlies bring heavy rainfall to the eastern coasts of
continents lying within the Tropics. On the western coasts of continents, these
winds do not bring any rainfall. It is because here they are 'off-shore' winds
or winds blowing just parallel to the shores. Therefore, the western areas
within the tropics suffer from aridity. The great deserts of the Sahara,
Kalahari, Atacama and the Great Australian Deserts all lie on the western
margins of the continents, within the tropical latitudes. Characteristics of
Westerlies
(i) They blow from
Sub-tropical High Pressure to Sub-polar low.
(ii) They are very
strong wands, and most often blow from the western side of land mass.
(iii) They are interspersed by
cyclones and cause light drizzle.
(iv) They arc stronger in the
southern hemisphere as there is absence of land mass.
THE POLAR EASTERLIES
The wands that originate in the North and South Polar regions and
blow towards Circum-polar Low Pressure Zone are known as Polar Winds. They
start from Polar High Pressure Zone, and originate from ice capped land-mass in
Arctic and Antarctic latitudes. In the Northern Hemisphere, they blow from the
north-east, and are called the North- East Polar Winds; and in the Southern
Hemisphere, they blow from the south-east and arc callcd the South-East Polar
Winds.
They are also deflected to the west in both hemispheres and hence
are known as Polar Easterlies.
Characteristics of Polar Winds
(i) They are very
cold winds.
(ii) They are also
referred to as Polar easterlies from the direction in which they blow.
(iii) When they blow over
oceans they become warm.
WHAT ARE PERIODIC WINDS?
Periodic winds blow at regular intervals or in regular cycles.
They are winds that result from localised differences in pressure and
temperature. For example, land and sea breezes and the seasonal wands.
LAND AND SEA BREEZES
They are caused by differential rate of heating of the land and
the sea.
In day time land gets heated faster than the adjoining sea. This
creates a low pressure zone
on the land and high pressure zone over the sea. Thus the winds
blow from sea to land and are called Sea Breezes. At night reverse of this
happens and winds blow from land to sea and are called Land Breezes.
Monsoons are periodic seasonal winds blowing in the regions of
South East Asia and Northern Australia. The word 'monsoon' is derived from the
Arabic word Mausim meaning 'season'. They develop because of differences in
heating conditions of the continent and the oceans. They are divided into two
wind systems—the Summer Monsoon and the Winter Monsoon.
SUMMER MONSOONS
In summer the land gets more heated than the sea. Hence there
develops a centre of low pressure on the land. Over the adjoining sea. the air
is comparatively cool, and a high pressure develops there. This causes the
winds to blow from the sea to the land. It is the 'Summer Monsoon'.
In May, June and July, the plains of India and China are heated by
the vertical rays of the sun. The intense heat develops a continental low
pressure. During these months, over the Indian Ocean, a high pressure area
develops. So, the winds blow from the Indian Ocean northward and north-westward
into Asia . As they blow from the sea to the land, they bring heavy rainfall to
South-East Asia. The summer monsoon winds blow south-west; so they are known as
the 'South-West Summer Monsoon.'
WINTER MONSOONS
During winter season, the conditions are just reverse of those of
summers. A high pressure develops over a big land mass stretching from Central
Asia upto north-west Indian plain. At the same time a low pressure zone
develops in the Indian Ocean. As the winds blow from the land to the sea, they
bring cold dry weather. They are incapable of producing rain.
When these winds blow over seas and pass over the adjoining land,
they bring some rainfall. The Southern Coromandel Coast (Tamil Nadu) in India
and the Vietnamese Coast and the west coast of Japan get rain from winter
monsoons. The winter monsoon winds blow north-cast; so the monsoon is known as
the 'North-East Winter Monsoons'.
Local
winds are restricted to a certain place only. They may be warm or cold
depending upon the area from which they blow. For example, Harmattan is a hot
local wind of Sahara desert.
Loo: In the
plains of northern India and Pakistan, sometimes a very hot and dry wind blows
from the west in summer in the afternoons. It is known as loo. Its temperature
invariably ranges between 45°C and 50°C. It may cause sunstroke to people.
Foehn and Chinook: A strong warm wind develops on
the leeward side of the Alps. Due to regional pressure gradient, stable air is
forced to cross the barrier. As the air ascends the southern slopes of the
Alps, it expands and cools. Condensation takes place when the air is saturated,
causing rain and snowfall on the higher slopes. However, on descending the
northern slopes, the wind experiences an increase in pressure and temperature.
Due to this air is compressed and warmed. Most of its moisture is lost and it
reaches the valley bottom as a dry. hot wind, called the Foehn. The temperature
of the wind is from 15°C to 20°C. The wind is of use for melting snow and it
hastens the ripening of grapes. Similar kind of wind in the USA and Canada move
down the west slopes of the Rockies and are known as chinooks. The word chinook
literally means 'snow eater'. It is beneficial to ranches
east of the Rockies as it keeps the grasslands clear from snow.
Mistral: During
winter, areas adjacent to highlands may experience a local cold wind which
originates over the snowcapped mountains or highlands and blows down the
valley. These winds have been given local names. The most famous is the mistral
that blows from the Alps over France towards the Mediterranean Sea. Even though
the skies are clear, the mistral brings down the temperature below freezing
point.
These local winds often have a considerable effect on climatic
conditions, notably on the temperature of a place.
WHAT ARE VARIABLE WINDS?
These winds are related to pressure systems and blow in small
areas. They are called variable because they do not blow in any definite
direction and their direction varies with the movement of the pressure system.
Their speed also depends on the intensity of the depression. They last only for
a few days. Two chief types of variable winds are—Cyclones and Anticyclones.
CYCLONES
In low latitudes, an intense depression with a low pressure centre
is known as a Tropical cyclone in the Indian Ocean area, as hurricanes in the
Caribbean, typhoons in China and willy-willies in Australia.
Cyclones on account of the Coriolis Force blow in an
anti-clockwise direction in the Northern Hemisphere and clockwise in the
Southern Hemisphere.
Cyclones are associated with turbulent weather conditions, cloudy
weather, strong winds and rainfall. Tropical cyclones, thus, cause heavy damage
to property and loss of human lives. The cyclones generally originate in the
tropical region between 8° and 20° N and S. They are more frequent in summer
because of the movement of the Doldrum belt away from the Equator. Most often
they originate in the South China Sea and cause a lot of damage to life and
property In the countries bordering that region. In the Bay of Bengal and the
Arabian Sea, they cause great damage as they are strong winds even though their
range is small.
The central part of a cyclone is a calm region and is known as Lhe
eye of the cyclone surrounded by a turbulent vortex . The force of the winds
depends on the intensity of the low pressure centre and the surrounding high pressure.
The greater the difference, the stronger is the wind.
The weather conditions associated with a tropical cyclone:
1. When there is a
tropical cyclone the air is still, but the temperature and the humidity are
high. There is sudden drop in air pressure.
2. When the front
of the vortex arrives, there are strong winds and thick clouds, then the winds
become violent with great speed. Dense clouds and heavy rain reduce visibility.
3. The eye of the
cyclone heralds a calm condition.
4. When the rear
of the vortex arrives, there are violent winds, thick clouds and heavy rain.
The wind blows in the opposite direction.
A TORNADO: A tornado
is a tropical cyclone, which occurs over land. Its diameter is hardly 300 or
400 metres. It looks like a dark, funnel- shaped cloud extending downward from
the base of the thunderstorm. It has low pressure in the centre, with very
strong winds blowing inwards. The speed of the winds may be as high as 500 to
800 km per hour. Tornadoes arc generally experienced over tropical waters in
North America.
Temperate Cyclones: Temperate cyclones are active over mid-latitudinal region between
35° latitude and 65° latitude in both hemispheres. They are also known as
extra-tropical or wave cyclones. Unlike tropical cyclones which are produced
and developed mainly over the sea, temperate cyclones are produced both on land
and on sea. Whereas the tropical cyclones are limited to a small area and
usually travel from east to west, the temperate cyclones occupy areas measuring
a couple of thousands of kilometres and move from west to east. In a temperate
cyclone, wind speed is low and all the sectors of the cyclone have different
temperatures. The rainfall is slow and continues for many days.
The centre of a tropical cyclones is known as the eye and the wind
is calm at the centre with no rainfall. In a temperate cyclone, there is not a
single place where winds and rains are inactive. Direction of winds rapidly
change at the front. The temperate cyclones are associated with anticyclones
which precede and succeed a cyclone. The approach of a temperate cyclone is
marked by fall in temperature, fall in the mercury level, wind shifts and a
halo around the sun and the moon. A light drizzle follows which turns into a
heavy downpour.
ANTICYCLONES: It is a fine atmospheric
condition. There is High Pressure in the core or centre and Low pressure around
it. Winds blow gently outwards. These winds are clockwise in Northern
hemisphere and anticlockwise in the southern hemisphere. Since they are centres
of high pressure they are found in region of descending air. currents.
JET STREAMS
Jet streams refer to the concentrated bands of rapid air movement
found at the tropopause and the stratosphere, located at 10-15 kilometres above
the surface of the Earth. They are formed near boundaries of adjacent air
masses with significant differences in temperature, such as the polar region
and the warmer air to the south. They are associated with latitudes where the
poleward temperature gradient is particularly strong. Two such zones occur in
each hemisphere. The sub-tropical jet stream occurs at about 30° latitude and
the other, the polar front jet stream is associated with the polar front zone
in each hemisphere. The major jet streams are westerly winds (flowing west to
east) in the Northern Hemisphere.
During the summer, easterly jets are formed in tropical regions,
typically in a region where dry air encounters more humid air at high
altitudes. Low level jets can form wherever low level winds are squeezed
together, typically between an oncoming front and a high pressure cell.
Jet streams transport moisture in the stratosphere, causes
horizontal convergence and divergence which helps in the formation of cyclones
and anti-cyclones, control monsoon rains in tropics and their vertical air
motion.
Meteorologists use the location of the jet stream as an aid in
weather forecasting. The main commercial use of the jet stream is in air
travel, as flight time can be dramatically affected by either flying with or
against the stream. One type of clear-air turbulence is found in a jet stream's
vicinity, which can be a hazard to aircraft.
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