Phy Geo (Unit 2-Topic 3) Pressure belts, Wind System, Clouds

Physical Geography (Unit 2-Topic 3) 

Pressure belts

Pressure belts are seasonally identical horizontal pressure variations created in the earth’s atmosphere just above the earth’s surface due to seasonal and spatial variation of energy received by the earth at different places. [Energy received determines the pressure exerted by the atmosphere at a place and variations in pressure exerted determines the direction and velocity of the wind.]

Spatial variation is due to the Geoid (Onion shape) shape of the earth, i.e. the tropics receive maximum energy as they are more or less perpendicular to the sun’s rays whereas the polar regions receive minimum as they make an angle far lesser than 90 degree with the sun’s rays. The temperate regions receive energy somewhere in between the tropics and the polar regions.

Credits: UO Department of Physics

Seasonal variations are due to revolution of the earth on a tilted axis around the sun. The dynamic behaviour of pressure belts (position of the pressure belts oscillating between certain latitudes) is due to seasonal variation in insolation (sun’s energy received).

Credits: NCERT

Low Pressure belt at the equator and High Pressure belt at the poles

• They are both thermally induced pressure belts (they are created due to loss or gain of heat).

Low Pressure Belt (heat gain)

• Equator receives maximum energy. This means the air at the equator becomes less denser and raises. This creates a low pressure at the equator. Winds from the surroundings gush in to fill the gap.

High Pressure Belt (heat loss)

• Here, the air in the upper levels of troposphere cools and becomes more denser. It sinks to the surface and creates a high pressure at the surface.

Sub-Tropical High Pressure belt and Sub-Polar Low Pressure belt

• They are both dynamically induced pressure belts (they are created to earth’s rotation — Coriolis Effect)

Sub-Tropical High Pressure belt

• The ait that raises to the upper levels of troposphere at the equator cools before reaching the 25–35 degree N and S latitudes. This cool dry air sinks at 25–35 degree N and S latitudes creating a high pressure at the surface.

Sub-Polar Low Pressure

• Half of air sinking at both subtropical high and polar high coverage at around 60 degree N and S latitudes and raise. This creates a low pressure belt. The air after raising to the upper levels move towards polar and subtropical regions, adding to the sinking air there.

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Wind System

Wind is the horizontal movement of air caused by :

1. Heating by the Sun
2. Rotation of earth on its axis

The formation of the wind system begins with the sun’s radiation, which is absorbed differently on the earth’s surface. The earth’s surface is heated differently because of scenarios like cloud cover, mountains, valleys, water bodies, vegetation and desert lands.

As a result of this uneven heating, there are bound to be earth surfaces that vary a lot in temperature. Air on surfaces with higher temperatures

will then begin to rise because it is lighter (less dense). As the air rises, it creates low atmospheric pressure. Air on surfaces with cooler temperatures sink (do not rise). The sinking creates higher atmospheric pressure. This behaviour or warm gases or liquids moving upward and being replaced by cooler particles is called Convection. The energy moving during convection is called convectional current.

Anywhere and each time there are differences in atmospheric (air) pressure, there will be winds, because air will move from the high pressure area to the low pressure area. It also means that winds may be even stronger where the difference in the air pressure is greater.

A good example is how a tropical depression forms, where warm air over hot tropical waters rise, and high pressure cold air quickly rushing to fill the space.

Types Of Winds

Local Winds

Local winds are those that are created as a result of scenery such as mountains, vegetation, water bodies and so on. They can move from mild to extreme winds in just hours. Good examples of local winds are sea breezes and land breezes, and mountain and valley breezes. Local winds cover very short distances.

Permanent Winds

They are really large air masses that are created mainly as a result of the earth’s rotation, the shape of the earth and the sun’s heating power.

Trade winds:

This is the belt extending as far as 30° north and south latitude of the Inter-Tropical Convergence Zone (ITCZ).

Westerlies:

This is the belt extending from 30° to 60° latitude from the ITCZ.

Easterlies:

This belt covers from 60° latitudes to the north and south poles.

Coriolis Force

As the winds blow from the north and south towards the equator, their flow path is deflected by the earth’s rotation. When moving objects are viewed in a reference frame, their path looks curved. This is the coriolis effect, and it is simply caused by the earth’s rotation. This effect makes wind systems in the southern side of the equator (southern hemisphere) spin clockwise and wind systems in the northern side (north hemisphere) spin counter-clockwise.

Wind Measurement

Winds come in all directions or speeds. They are measured by their directions and speeds.

Depending on the cause of the winds, they can blow in many directions. Some good examples of wind direction are the Easterly winds, which blow from east to west, and westerly winds which blow from west to east. There are also winds that blow from the north, southwards and also those from the south blowing northwards.

Anemometers

Anemometers measure the direction of winds.

The Wind Vane

Another instrument in wind measurement is the wind vane.

Front

This is the boundary between two large air masses. It is the meeting point between cold dense air and warm light air. At a front, warm is pushed upwards because of its less density. A front can be very intense or mild, depending on the temperatures of the two air masses meeting. Air masses with extreme temperatures will produce a fierce front, whereas air masses with little temperature difference will produce very mild front.

Prevailing Winds

This is simply a term used for winds that are most frequent in its’ direction and speed over a specific geographic region. Different geographic areas have different prevailing winds because of the factors that shape them.

Jet Stream

This was discovered in 1940’s (during the World War II). Pilots noticed that their flying speeds were significantly reduced when flying against some areas in the upper troposphere. Jet streams can be found in the upper troposphere. They are very strong winds.

Beaufort Wind Force Scale

In 1805, a British Naval Officer invented this scale to measure wind force. The scale has reading of 0 up to 12 to classify wind force.
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Clouds : Types , Features 

Definition:

A visible aggregation of minute water droplets or ice particles in the air above the ground levels i.e. at the certain elevation is called as the cloud.

There are two major cloud group based on their forms

1.      Strati form Cloud group

These are the clouds present in the form of layers

2.      Cumuli form Cloud group

It consist of globular clouds in the form of cotton balls

The world meteorological organization gave the international cloud atlas consisting of 10 main cloud families. These families are listed according to their heights into 3 groups.

1.      High Clouds

All high clouds are called as ‘cirro’ clouds and they are present at elevation of 5 to 13 kms. 

These clouds are composed of ice  crystals and producephenomenon called ‘halo’ and does not give rainfall/precipitations they are of three types

a.      Cirrus

• These are detached clouds in the form of white delicate filaments or narrow binds. 
• Sometimes they are in the form of white patches.

b.      Cirro Cumulus

• These are the clouds looks like small flakes or globules consisting of white patchy layers of clouds without shading. 
• When these patches are uniformly arranged, it forms a ‘mackerel sky’

c.       Cirro status

These are the transparent whitish clouds in the form of a fibrous veil with smooth appearance.

2.      Middle Clouds

All middle clouds are called as the alto clouds. They are present in the height ranging from 2 to 7 Kms. They of three types

d.      Alto Cumulus

• It’s a white or grey patchy layer of cloud having dark shading on their under surfaces. 
• Sometimes they are found in wavy or parallel bonds where they are referred as sheep or wool packed clouds. 
• Often they are composed of super cooled liquid droplets. They may or may not give rainfall

e.       Alto stratus

• They are grayish or bluish cloud layers having fibrous appearance covering most part of the sky. 
• The sun is vaguely revealed in the form of spot. 
• But the ground shadows are not cast. 
• They give out fine drizzle of snow

f.        Nimbo stratus

• It is a grey cloud layer often very dark, thick enough to blot the sun. 
• The rain or snow falling from this cloud  does not reach the ground and called as virga.

3.      Low Clouds

These are the clouds present below 2 kms of elevation and there are 4 types of cloud families are present in this

g.      Strato cumulus

• It is a low cloud layer which is dull grey in colour consisting of large lumpy masses with brighter intersects

h.      Stratus

• This is a grey cloud layer. 
• The sun is visible and its outline is clearly distinguishable. 
• They are also present in the form of raged patches.

i.        Cumulus

• These are dense detached clouds with sharp outline developing vertically in the forms of mount or tower like cauliflower. 
• The sunlit part is brighter and the base is dark. 
• They represent transition to the cumulo nimbus clouds and are present only during day time

j.        Cumulo nimbus

• These are heavy & dense cloud forms having an anvil head. 
• These are towering clouds from the ground level to intermediate heights. 
• They are associated with heavy rainfall, thunders, lighting.