Atmospheric Pressure

1. Atmospheric Pressure Chapter 6

2. Learning Targets #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.

3. 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

4. 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.

6. Influence of Temperature and Water Vapor on Air Pressure • 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

7. Water Vapor and Density • 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 O2molecules • Therefore, humid air is less dense than dry air

8. Airflow and Pressure

9. High and Low Pressure Systems