ATMOSPHERIC PRESSURE

ATMOSPHERIC PRESSURE
Air has weight and thus exerts pressure on eartl1 surface. The pressure exerted by atmosphere as a rest of its weight above a unit area of the earth's surface called the atmospheric pressure.

It is expressed in mil bars (mb) and measured with a mercury barometer. 11 average atmospheric pressure at sea level is 1013.25 m
Pressure varies with both temperature and altituc decreasing logarithmically with height. Two princiF types of barometer are Mercury barometer and Anerc barometer. Mercury barometer is a very accurate instr ment, though large and cumbersome. Atmospheric presssure is read in inches and centimetres; thus 29.92 incl1
(76 em) of mercury are equivalent to 1013 mb at sea lev one inch of mercury is equivalent to 33.2 mb. Anerl barometer is an instrument consisting of a metal box fr( which the air is virtually exhausted. The sides are flexil and expand and contract with changes in air pressure. The movements are amplified and registered by a needle a calibrated circular dial. The instrument is light a portable.
DISTRIBUTION OF PRESSURE Air pressure decrea: with altitude. For about every 275 metr:es of rise in elevation, pte mercury column falls 1/30 of its height. 1 rate of drop of mercury becomes less and less w increasing height and beyond 50 kIn, decrease is extrem slight.