Air pressure is the pressure exerted by the weight of air above Exerted in all directions (up, down, and sideways) The air pressure pushing down on an object exactly balances the air pressure pushing up on the object Average Air Pressure at sea level is: 1 kg/cm 2 = mb = 1 atm = in. of mercury

differences in air pressure causes the Earth’s winds and weather changes air pressure is affected by: temperature water vapor elevation

 Barometer: device used for measuring air pressure  Unit:  millibars (mb)  inches of mercury (in. Hg)  Atmospheres (atm.)  Torricelli: invented the mercury barometer in 1643

Types of barometers Mercury When air pressure increases, the mercury in the tube rises When air pressure decreases, the mercury in the tube goes down Aneroid recording mechanism providing a continuous record of pressure change over time

 Pressure—measured in mb (millibars)—is given by the number to the upper right of the circle  For example, suppose it’s mb  Only the 10s/1s/0.1s are shown—so it is represented as Pressures below 1000 mb would start with high numbers, such as 964 for mb

Barometric Pressure Add a decimal between the last two digits 13.8 Add a 9 or 10 in front to fit on the scale (ranges from to )

1) 196………………………… mb 2) 423………………………… mb 3) 895………………………… mb

Formula : inches x 33.9 = millibars Coversion Table n Hgmb

 The gravitational attraction between Earth and atmospheric gases causes particles of gas to be pulled toward the center of Earth. State of the Atmosphere Air pressure increases as you near the bottom of the atmosphere because of the greater mass of the atmosphere above you. Atmospheric pressure decreases with height because there are fewer and fewer gas particles exerting pressure. The density of air is proportional to the number of particles of air occupying a particular space.

State of the Atmosphere

 Temperature, pressure, and density are related. State of the Atmosphere In the atmosphere, temperature is directly proportional to pressure. If an air mass maintains a certain density, as temperature increases or decreases, pressure does, too.

 The relationship between temperature and density is inversely proportional. State of the Atmosphere If an air mass maintains a certain pressure, as temperature increases, density decreases, and as temperature decreases, density increases.

 In most atmospheric interactions, however, neither density nor pressure remains unchanged. State of the Atmosphere Temperature varies with changes in both pressure and density. Temperature is proportional to the ratio of pressure to density, which decreases with increasing altitude.

Temperature Inversion State of the Atmosphere –A temperature inversion is an increase in temperature with height in an atmospheric layer. –This can happen when the lower layers of the atmosphere lose heat to Earth’s surface and become cooler than the air above them. –A temperature inversion can act like a lid to trap pollution under the inversion layer. –In all cases, the presence or absence of inversions can have a profound effect on weather conditions.