Atmospheric Pressure and Winds PDF

Atmospheric Pressure refers to the force per unit area exerted against a surface by the weight of the air above that surface. The average atmospheric pressure at sea level is 1013.25 mb or 760 mm. These variations cause horizontal movements of air called winds. MEASUREMENT OF PRESSURE Pressure is n...

Atmospheric Pressure refers to the force per unit area exerted against a surface by the weight of the air above that surface. The average atmospheric pressure at sea level is 1013.25 mb or 760 mm. These variations cause horizontal movements of air called winds. MEASUREMENT OF PRESSURE Pressure is normally measured in millibars. The variations in pressure are shown on maps by means of Isobars. These are lines joining the places having the same barometric pressure. Pressure Gradient The rate of change of atmospheric pressure between two points on the earth's surface is called pressure gradient. FACTORS AFFECTING ATMOSPHERIC PRESSURE 1. Altitude The atmospheric pressure decreases with height. or altitude. The atmospheric pressure is highest at sea level. This is because at higher altitudes the air is thinner or less dense than the air at the sea level. At high altitudes, atmospheric pressure and available oxygen get so low that people feel breathless., Mountain climbers use bottled oxygen when they ascend very high peaks. They also take time to get used to the altitude because quickly moving from higher pressure to lower pressure can cause decompression sickness. 2. Temperature Atmospheric pressure decreases with increase in temperature. This is because when the temperature rises, air expands. The molecules of air move far apart (become less dense) and hence exert less pressure. The Equatorial region has a low-pressure belt, whereas the Polar regions have high pressure belts. 3. Water Vapour Water vapour concentration affects atmospheric pressure because the molecular weight of water (18 g/mol) is less than the average molecular weight of air (about 29 g/mol). humid air is less. dense and exerts less pressure than dry air. 4. Rotation of the Earth Due to the rotation of the earth, bulk of the air at the Poles is thrown away towards the Equator. Since the Equatorial region receives great amount of heat throughout the year, the air becomes warm and light and therefore, it rises and creates low pressure. WORLD PRESSURE BELTS The distribution of atmospheric pressure across the latitudes is termed global horizontal distribution of pressure. Its main feature is its zonal character known as pressure belts. On the earth's surface, there are in all seven pressure belts. (i) Equatorial Low-Pressure Belts This low-pressure belt extends from 0 to 5° North and South of Equator. Due to the vertical rays of the sun here, there is intense heating. This low-pressure belt is also called as doldrums because it is a zone of total calm, i.e., without any breeze. (ii) Sub-tropical High-Pressure Belts At about 30°North and South of Equator lies the area where the ascending Equatorial air currents descend. This area is thus an area of high pressure. It is also called the Horse latitude. (iii) Circum-polar Low-Pressure Belts These belts located between 60° and 70° in each hemisphere are known as Circum-polar Low-Pressure Belts. Due to earth's rotation, the winds surrounding the Polar region blow towards the Equator. (iv) Polar High-Pressure Areas At the North and South Poles, between 70° to 90° North and South, the temperatures are always extremely low. These areas of Polar high pressure are known as the Polar Highs. These regions are characterised by permanent Ice Caps. SHIFTING OF PRESSURE BELTS If the earth had not been inclined towards the sun, the pressure belts, as described above, would have been as they are. The shifting of the pressure belts causes seasonal changes in the climate, especially between latitudes 30° and 40° in both hemispheres. WINDS Air flows from areas of high pressure to areas of low pressure. Horizontal movement of the air is called wind. Factors Affecting Direction and Velocity of wind Direction and speed of wind are controlled by a combination of factors. 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 higher is the wind speed. 2. Coriolis Effect 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 Effect or 'Coriolis Force, which deflects the air. 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. 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. 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. The prevailing wind 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. 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 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 have fixed velocity and are regular. (iii) 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 Low-Pressure Belts. The Polar Easterlies The winds that originate in the North and South Polar regions and blow towards Circum-polar Low Pressure Zone are known as Polar Winds. Characteristics of Polar Winds (1) 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. PERIODIC WINDS Periodic winds blow at regular intervals or in regular cycles. They are winds that result from localised differences in pressure and temperature. 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. Winter Monsoons During winter season, the conditions are just reverse of those of summers. A high pressure develops over a big landmass stretching from Central Asia up to north- west Indian plain. At the same time a low pressure zone develops in the Indian Ocean. LOCAL WINDS Local winds are restricted to a certain place only. They may be warm or cold depending upon the area from which they blow. 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. Foehn and Chinook A strong warm wind develops on the leeward side of the Alps. Due to regional pressure gradient, air is forced to cross the barrier. As the air ascends the southern slopes of the Alps, it expands and cools. 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. Cyclones A Cyclone is a portion of the atmosphere in which the pressure is lowest in the centre. The winds blow inward in the opposite direction. Tropical Cyclones Cyclones are associated with turbulent weather conditions, thick cloud cover, strong winds and rainfall. Tropical cyclones, thus, cause heavy damage to property and loss of human lives. The central part of a cyclone is a calm region and is known as the eye of the cyclone surrounded by a turbulent vortex. 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. Anticyclones An anticyclone is a fine atmospheric condition. There is High Pressure in the core or centre and Low Pressure around it. Winds blow gently outwards.