winds and atmosphere

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Slide 1: 

Pressure and Winds

Air Pressure : 

Air Pressure Air pressure is the pressure exerted by the weight of air above. Average air pressure at sea level is about 1 kilogram per square centimeter, or 14.7 pounds per square inch. Another way to define air pressure is that it is the force exerted against a surface by the continuous collision of gas molecules.

Units and Instruments of Pressure : 

Units and Instruments of Pressure A millibar (mb) equals 100 newtons per square meter. Standard sea-level pressure is 1013.25 millibars. Two instruments used to measure atmospheric pressure are: the mercury barometer, where the height of a mercury column provides a measure of air pressure (standard atmospheric pressure at sea level equals 29.92 inches or 760 millimeters), and the aneroid barometer, which uses a partially evacuated metal chamber that changes shape as air pressure changes.

Slide 4: 

mercury barometer

Wind : 

Wind Wind is the result of horizontal differences in air pressure. If Earth did not rotate and there were no friction, air would flow directly from areas of higher pressure to areas of lower pressure. However, because both factors exist, wind is controlled by a combination of: (1) the pressure-gradient force, (2) the Coriolis force, and (3) friction.

Geostrophic Winds : 

Geostrophic Winds Winds in which the Coriolis force is exactly equal and opposite to the pressure gradient force are called geostrophic winds. Geostrophic winds flow in a straight path, parallel to the isobars, with velocities proportional to the pressure-gradient force. Winds that blow at a constant speed parallel to curved isobars are termed gradient winds. In centers of low pressure, called cyclones, the circulation of air, referred to as cyclonic flow, is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Centers of high pressure, frequently called anticyclones, exhibit anticyclonic flow which is clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. Whenever isobars curve to form elongated regions of low and high pressure, these areas are called troughs and ridges, respectively.

Wind Measurements : 

Wind Measurements Two basic wind measurements, direction and speed, are important to the weather observer. Wind direction is commonly determined using a wind vane. When the wind consistently blows more often from one direction than from any another, it is called a prevailing wind. Wind speed is often measured with a cup anemometer.

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FROM : Kartik Arora Pawas Kalra Nitin Dabas Ritabh Gupta

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