Fundamental Of Physical Geography Chapter 10 Atmospheric Circulation And Weather Systems

The characteristics are: 
i. It deflects the wind to the right direction in the northern hemisphere and to the left in the southern hemisphere. The deflection is more when the wind velocity is high.

ii. The Coriolis force is directly proportional to the angle of latitude.

iii. It is maximum at the poles and is absent at the equator.

The three types of planetary winds are:

i. Trade wind 
ii. Westerlies
iii. Polar winds.

The air with distinctive characteristics in terms of temperature and humidity is called an airmass. It is defined as a large body of air having little horizontal variation in temperature and moisture.

The air masses are classified according to the source regions. There are five major source regions.

These are:

(i) Warm tropical and subtropical oceans;

(ii) The subtropical hot deserts;

(iii) The relatively cold high latitude oceans;

(iv) The very cold snow covered continents in high latitudes;

(v) Permanently ice-covered continents in the Arctic and Antarctica.

The weight of a column of air contained in a unit area from the mean sea level to the top of the atmosphere is called the atmospheric pressure. 

The seven pressure belts of the earth are :

i. Equatorial low-pressure belt.

ii. Sub-tropical high-pressure belt (Northern Hemisphere)

iii. Sub-tropical low-pressure belt (Southern Hemisphere)

iv. Sub-polar low-pressure belt (Northern Hemisphere)

v. Sub-polar low-pressure belt (Southern Hemisphere)

vi. Polar high (Northern Hemisphere)

vii. Polar high (Southern Hemisphere).

The heated air rises up and its water vapour condenses and falls as rain. Thus, the regions of high temperature and low pressure remain cloudy and get comparatively more rain.

The relationship between temperature and pressure is summarised as:

i. High temperature - Low pressure

ii. Low temperature - High pressure

(b) Horse Latitudes : The sub-tropical belts or the regions lying between 30° and 35° are known as horse latitudes. This zone is characterized by weak and variable winds and is calm. Hence, the sailors were compelled to throw away some of the horses over board in order to lighten the ships, so that the ships could move. This part, therefore, came to be called Horse latitudes. But now both the sub-tropical belts are known by this name.

Mountain and valley breezes are periodic winds formed due to variation in gravity. They are formed in the mountain slopes.the 
(a) Mountain Breezes: They occur during nights when the air at upper slopes becomes dense, compressed and heavy due to more cooling and ground radiation. At the same time lower slopes are relatively warmer and have low pressure. Now heavy air descends, it is termed as mountain breeze or gravity winds.
(b) Valley Breezes: They occur during the day time. Due to insolation, valley bottom gets heated, air becomes light and it ascends upslopes. It is termed as valley breeze.
Mountain Valley Breezes.

Fig. Mountain Valley Breezes.

(b) Loo : Loo winds are hot and dry surface winds. They move in the northern plains of India during summer season especially in the months of May and June. They move from west to east. They cause loss of lives in the plains of Bihar and U.P.

The pattern of planetary winds largely depends on:

(i) latitudinal variation of atmospheric heating;

(ii) the emergence of pressure belts;

(iii) the migration of belts following an apparent path of the sun;

(iv) the distribution of continents and oceans;

(v) the rotation of the earth.

The Mistral is the name given to the strong, northerly or north-westerly wind experienced on the shores of the north-west Mediterranean. It is most prevalent during the winter. The wind is strong and may sometimes have a speed of over 100 km per hour. It is also very cold and harmful to plant life.

Fig. Mistral
The Foehn is the name given to the hot dry wind which blows down the leeward slopes of mountains.
Foehn winds often blow with great velocity and cause much discomfort. They cause snow to disappear very quickly and thus make pasture available for animals sooner than would otherwise be the case. Similar winds blowing eastwards across the Prairies of North America from the Rockies are known as Chinook Winds.

ii. Polar Air Masses : Polar air masses are located over warmer high latitude oceans of North Pacific, North Atlantic and the entire sweep of Southern Hemisphere surrounding Antarctica. The air overlying these areas providing warmer currents is a highly efficient evaporator of readily available moisture. So polar maritime is less cold and more moist. North America and North Europe experience this type of airmass.

The combined phenomenon of southern oscillation and El Nino is known as ENSO. 

Atmospheric pressure varies from place to place and from time to time. This variation depends on the following factors:

Fig. Simplified general circulation of the atmosphere

The possible reasons for the formation of subtropical high pressure over 30°N and S latitudes are as under.
i. The air at the Inter-Tropical Convergence Zone (ITCZ) rises because of convection caused by high insolation and a low pressure is created. To this low pressure, the winds from tropics-the easterlies, converge. The converged air rises upto the height of 14 km and moves towards the North and South Poles. This causes accumulation of air at about 30°N and S. Part of the accumulated air sinks to the ground and forms a subtropical high pressure.
ii. The cooling of air when it reaches 30°N and S latitudes is also another reason for the sinking down below near the land surface. The air flows towards the equator as the easterlies. The easterlies from the side of the equator converge in the ITCZ. In the middle latitude, the circulation is that of sinking cold air that comes from the poles and the rising warm air that blows from the sub-tropical high. Thus it forms the subtropical high pressure over 30°N and S latitudes.

The features are: 

i. It affects the speed of the wind.

ii. It is greatest at the surface and its influence generally extends upto an elevation of 1 - 3 km.

iii. Over the sea surface the friction is minimal.

When two different air masses meet, the boundary zone between them is called a front.

The process of formation of the fronts is known as frontogenesis.

There are four types of fronts:
(a) Cold;
(b) Warm;
(c) Stationary;
(d) Occluded

Air masses of different densities do not mix readily and tend to retain their identities as far as temperature and moisture are concerned. The boundary zone of the convergence separating the two air masses are called fronts. When a warmer and lighter airmass moves against a cold and more dense airmass, the former rides up over the later. It is called a warm front. If the cold air mass forces its way under a mass of warm air and pushes the latter upward, the front will be called a cold front.
In the mid-latitudes along the front of the warm and cold air masses temperate cyclones are formed.

Fig. Warm and Cold fronts.

The air masses are classified according to the source regions. There are five major source regions.

These are:

(i) Warm tropical and subtropical oceans;

(ii) The subtropical hot deserts;

(iii) The relatively cold high latitude oceans;

(iv) The very cold snow covered continents in high latitudes;

(v) Permanently ice covered continents in the Arctic and Antarctica.

The air at the Inter-Tropical Convergence Zone (ITCZ) rises because of convection caused by high insolation and a low pressure is created. The winds from the tropics converge at this low-pressure zone. 

The conditions favourable for the formation and intensification of tropical storms are:

(i) Large sea surface with temperature higher than 27° C;

(ii) Presence of the Coriolis force;

(iii) Small variations in the vertical wind speed;

(iv) A pre-existing weak low-pressure area or low-level cyclonic circulation;

(v) Upper divergence above the sea level system.

Followings are the characteristics of trade winds: 

The tropical cyclones originate in tropical seas. The conditions for the formation of the tropical cyclones are :

1. Large sea surface with the higher temperature, more than 27°C.

2. Small variation in the vertical wind speed.

3. A pre-existing weak low pressure area.

4. Upper divergence above the sea level system.

Over the Indian Ocean-especially in Bay of Bengal 50% cyclones are formed from a preexisting shallow low pressure area.

The eye of a cyclone is a region of tropical cyclone where a torrential rain occurs and the high velocity is there. The winds reach maximum velocity in this region as high as 250 km/hour. Around the eye is the eye wall where there is a strong spiralling ascent of air to greater heights in the troposphere.
Intensive Rainfall occurs to the left of the Cyclone. Maximum rainfall occurs close to the centre of the storm. Secondary, maximum of rainfall occurs 2º away from Primary maximum to the right of the storm centre. Slow moving/big size cyclones give more rainfall, whereas, fast moving/small size ones give less rainfall. More than 90% of rainfall is limited within 200 Km radius of the storm.

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The two types are:
i. High-Pressure Belt
ii. Low-Pressure Belt