- Slides: 15
WHAT IS AIR PRESSURE? • 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 = 1013. 2 mb = 1 atm = 29. 92 in. of mercury
• differences in air pressure causes the Earth’s winds and weather changes • air pressure is affected by: • temperature • water vapor • elevation
MEASURING AIR PRESSURE � 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 ON MAP � Pressure—measured in mb (millibars)—is given by the number to the upper right of the circle � For example, suppose it’s 1019. 4 mb � Only the 10 s/1 s/0. 1 s are shown—so it is represented as 194 • Pressures below 1000 mb would start with high numbers, such as 964 for 996. 4 mb
REMEMBER HOW TO CONVERT PRESSURE 1) 196……………. . 1019. 6 mb 2) 423……………. . 1042. 3 mb 3) 895……………. . 989. 5 mb
CONVERTING FROM INCHES TO MILLIBAR Formula : inches x 33. 9 = millibars Coversion Table n Hg mb 30. 0 1016 29. 5 999 29. 0 982 28. 5 965 28. 0 948 27. 5 931 27. 0 914
State of the Atmosphere AIR PRESSURE AND DENSITY � The gravitational attraction between Earth and atmospheric gases causes particles of gas to be pulled toward the center of Earth. • 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 AIR PRESSURE AND DENSITY
State of the Atmosphere PRESSURE-TEMPERATURE-DENSITY RELATIONSHIP � Temperature, pressure, and density are related. • 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.
State of the Atmosphere PRESSURE-TEMPERATURE-DENSITY RELATIONSHIP � The relationship between temperature and density is inversely proportional. • If an air mass maintains a certain pressure, as temperature increases, density decreases, and as temperature decreases, density increases.
State of the Atmosphere PRESSURE-TEMPERATURE-DENSITY RELATIONSHIP � In most atmospheric interactions, however, neither density nor pressure remains unchanged. • Temperature varies with changes in both pressure and density. • Temperature is proportional to the ratio of pressure to density, which decreases with increasing altitude.
State of the Atmosphere PRESSURE-TEMPERATURE-DENSITY RELATIONSHIP Temperature Inversion – 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.