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