Atmospheric Pressure AND Winds PDF

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Atmospheric Pressure and Winds - factors and concept by manocha sir...

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ATMOSPHERIC

PRESSURE

AND

WINDS:

PLANETARY WINDS, FACTORS AFFECTING WINDS,

GENERAL

CIRCULATION,

JET

STREAMS Atmospheric pressure is the force exerted by the weight of the air (unit: pascal). One standard atmospheric pressure- 101.325 KPA. It is defined as the force per unit exerted against a surface by the weight of the air above that surface. It exists because air molecules have mass and are constantly being pulled toward the Earth by gravity. Measured with an instrument called a “barometer”, hence also called barometric pressure. Air pressure changes with height and also when air warms up or cools down. Changes in air pressure cause changes in the weather. Decreases with increasing altitudes at the rate of 0.1’ per 600 feet but this rate of decreasing also varies seasonally, spatially & also on a daily basis. Standard sea-level atmospheric pressure is 1013.2 mb. The highest sea level pressure (1075.2 mb). Recorded at Irkutsk in Siberia – 14 Jan, 1893. Lowest sea level pressure (877 mb)—Marina Islands.

Inverse relationship between temperature & pressure i.e, when thermometer is high, barometer is low & vice versa. Lines joining points having some atmospheric pressure at a given time – Isobars. Rate of change of pressure per unit horizontal distance is called pressure gradient.

SIGNIFICANCE: 1. Exerting an equal outward pressure that balances the inward pressure of the atmosphere. 2. Variation in pressure in atmosphere creates our atmospheric circulation & thus plays a major role in determining our weather & climate. 3. Differences in atmospheric pressure creates winds 4. Movement of winds drives ocean current. 5. Atmospheric pressure works its way into several of earth’s system. 6. High values of atmospheric pressure produces clear & stable weather, while low pressure brings bad weather. 7. Weather at a particular station is largely controlled more by pressure in adjoining areas, rather than the actual pressure recorded over there.

PRESSURE VARIATION—DIURNAL AND SEASONAL DIURNAL VARIATIONS: 1. Principle reasons—insolational heating & radiational cooling.

2. There are 2 maximum & 2 minima of pressure during 24 hrs— semidiurnal oscillation. 3. Hardly noticeable in middle-latitudes. 4. Larger amount of variation at places near sea-level in the tropical regions. 5. Relatively smaller at high latitude regions. 6. Magnitude larger during day over continental landmass. 7. Irregularities in middle- latitude regions unsettled weather conditions predominate (cyclones), while the opposite in tropical regions. SEASONAL/ANNUAL VARIATION: 1. Larger in tropical regions 2. Smallest in equatorial regions (no variation in amount of insolation received). 3. Over continents—high pressure—during cold season—over oceans— during warm season. FACTORS AFFECTING AIR PRESSURE: On the basis of mode of genesis a. THERMAL: Effects of temperature—heated air—it expands—density decreases—low pressure vice versa. Ex- equatorial low pressure belt, polar high pressure belts.

b. DYNAMIC: Pressure gradient forces. Rotation of earth. Ex- subtropical high pressure belt, subpolar low pressure belt. 1. ALTITUDE: a. Maximum air density at earth’s surface b. Air density declines with height (pull of earth’s gravity is less). c. Fewer number of gas molecules at higher altitudes—means fewer molecular collisions and a declining air pressure. 2. AIR TEMPERATURE: a. Earth not heated uniformly because of unequal distance of insolation, differential heating and cooling of land and water surfaces. b. Air pressure is low in equatorial regions and it is higher in polar regions (gases when heated; become less dense, expand in volume and rise). c. Inverse relationship between air temperature and air pressure. 3. WATER VAPOUR CONDENSATION: a. Atmospheric air heated—air molecules becomes more active—increase space between neighbouring molecules—air density reduces—water vapour concentration affects atmospheric pressure—because molecular weight of water less than average molecular weight of air—water evaporates, enter atmosphere as gas—volume of wet air weighs less than an—volume of dry air—humid air is less dense and exerts less pressure than dry air areas.

4. EARTH’S ROTATION: a. Also cause convergence & divergence b. Generates centrifugal forces c. Results in deflection of air from original place—decrease in pressure d. Sub- polar regions (low pressure) and sub- tropical (high pressure regions) are created as result of earth’s rotation. DISTRIBUTION OF AIR PRESSURE: HORIZONTAL: a. High pressure—Atmospheric pressure of planet is greater than its surrounding environment. Winds flow outward due to higher density air near their center & friction with land. Winds flow clockwise around a high pressure area in Northern hemisphere and counter clockwise in Southern hemisphere (Coriolis effect) Weather’s system—anticyclone (high pressure) b. LOW PRESSURE—atmospheric pressure at sea level is below that of surrounding location. Counter clock wise in northern hemisphere (wind moves inward & is deflected right from the center of high pressure) and clockwise is southern hemisphere (wind moves inward & is deflected left from center of high pressure) also called a cyclonic formation or a depression (low pressure).

HORIZONTAL DISTRIBUTION: EQUATORIAL LOW-PRESSURE BELTS: a. Located between latitudes 5N and 5S b. But this zone is not stationary because of seasonal shift of this belt with northward (summer solstice) and southward (winter solstice) migration of sun. c. During northern summer- extends- 20 N in Africa & to North of tropic od=f cancer in Asia. Southern summer extends—10-20S. d. Thermally induced—maximum insolation—ground surface intensely heated--- lowermost air due to contact gets heated—expands—warm air rises upward—cause low pressure. e. Thus, convectional currents are set up in atmosphere throughout year. f. Warm & moist air moves upward—great potential for tremendous amount of energy in the form of latent heat of condensation0—creates clouds. g. Zone of convergence of northeast & southeast trade winds. h. Winds here are light & variable with frequent calms thus it is called doldrums. This zone is hence called the ITCZ belt which is key component of global circulation system. i. Diurnal variation of pressure is a distinctive feature & is more pronounced.

SUB TROPICAL HIGH-PRESSURE BELT:

a. 25 degrees and 35 degrees north and south latitudes. b. Broken into a number of high pressure cells or centres of actions. High temperature almost throughout the year. c. Dynamically produced due to rotation of earth. d. Rotation of earth—sinking of air & settling of winds. e. Convergence of winds at higher altitude above this zone results in subsidence of air from higher altitudes—descent of winds—contraction in their volume—high pressure (due to rarefied air molecules at top). f. That is why this zone is characterised by anticyclonic conditions which cause atmospheric stability & aridity—reasons for existence of hot deserts in western parts of continents (25-35 degrees north & south). g. These latitudes between 25-35 degrees north and south are called “horse latitudes”. SUB POLAR LOW-PRESSURE BELT: a. 60-65 degrees north and south latitudes. b. Dynamically induced because of low temperature throughout the air. c. Resulted from spreading of surface air outwards from this zone due to earth’s rotation & hence thinning causes air to spread. d. More broken in northern hemisphere and more regular in southern hemisphere due to land sea differentials. For example—the centres of these

low pressures lie in the Aleutian Islands in Pacific Ocean and between Greenland & Iceland’s in the Atlantic.

POLAR HIGH-PRESSURE BELTS: a. Exists in polar areas. b. Prevalence of very low temperature throughout the year causes operation of both dynamic and thermal factors to operate. VERTICAL DISTRIBUTION: a. Air pressure decreases with increasing elevation—more diffused—less density—low pressure. b. In fact, at top of mount Everest (elevation-8848m), the air pressure is only about 1/3rd the pressure at sea level. SEASONAL DISTRIBUTION: JANUARY— 1. Because of apparent movement of sun towards the tropic of capricorn, the equatorial trough of low pressure is displaced well to south of the equator. (Jan- summer month) 2. Areas of lowest pressure occur in South Africa, Australia. 3. Sub-Tropical high-pressure belt in northern hemisphere is located well to north, especially over cold continents. Isolated cells of high pressure united

to form a continuous belt. The north-eastern part of Siberia in Asia becomes the coldest region on earth’s surface with highest pressures. 4. During summer of southern hemisphere sub-tropical high-pressure belt lie at at 30-40 degrees south (pacific, Atlantic, Indian ocean).

JULY: a. Aleutian & Icelandic lows disappears from ocean which developed high pressure during winters have extensive low-pressure cells. b. Low pressure belts shift northwards. c. Landmasses extensively have low pressure cells now. d. Low pressure in Asia develops. e. STHPB continuous in southern hemisphere. f. STHPB more developed over oceans (Pacific & Atlantic) in northern hemisphere....