Atmospheric PressureChapter 6

#1- Define air pressure and explain why air pressure decreases with an increase in altitude.

#2- Explain what type of surface pressure would result from a cold, dry air mass and a warm, humid air mass.

#3- Explain convergence and divergence.

Learning Targets

Atmospheric Pressure

The pressure exerted by the weight of air above. Average air pressure at sea level is about 1kg/cm2 or

14.7lb/in2

Measuring Atmospheric Pressure

The standard unit of pressure is the pascal (Pa). 1 Pa = 1 N/m2

Meteorologists in the U.S. use the millibar (mb). 1 mb = 100 Pa.

Pressure at sea level is roughly 1000 mb (100 kPa) or more precisely, 1013.2 mb.

Temperature and water vapor influence pressure Temperature-the average molecular motion of a

substance Cold, dry air (Canada)

Slow-moving gas molecules packed closely together As density increases, so does pressure exerted on the

surface Creates high pressure

Warm, humid air (Gulf of Mexico) Gas molecules more widely spaced Density decreases; pressure decreases Creates low pressure

Influence of Temperature and Water Vapor on Air Pressure

Water vapor reduces the density of air Molecular weights of N2 and O2 are lighter than

that of water vapor Mass of air molecules intermixed (take up

same amount of space) As water content increases, lighter water vapor

molecules displace heavier N2 and O2 molecules

Therefore, humid air is less dense than dry air

Water Vapor and Density

Airflow and Pressure

High and Low Pressure Systems