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
Examine the behavior of gases. Gas molecules do not have strong bonds. Rather, they move about freely, filling up all space available to them. If a gas is confined in a container, the molecules will collide, bounce off each other, and will bounce off the sides of the container. This exerts an outward push that we call air pressure.
The atmosphere doesn’t have walls, but is confined below by Earth’s surface and above by the force of gravity preventing it from escaping into space.
We can more specifically define air pressure as the force exerted against a surface by the continuous collision of gas molecules.
Pressure changes with altitude. Let’s consider a cylinder, like the one here, with a movable piston. As more weight is added to the piston, the increased number of molecules per unit volume (density) causes an increase in the pressure exerted on the walls of the cylinder and the gauge. Therefore, we can say that as pressure increases, density increases. We already know that as altitude increases, the density of air decreases (and therefore cools). SO…. We what is the relationship between air pressure and altitude? (as altitude increases, air pressure decreases)
We can also say that the pressure of the gas balances the weight of the piston. This is the same in the atmosphere. The pressure at any given altitude is equal to the weight of the air directly above that point.

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, 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 O2 molecules
Therefore, humid air is less dense than dry air

8. Airflow and Pressure Movement of air causes variations in pressure
When there is a net flow of air INTO a region, it causes a pile up. This forces the air up to allow for the decreased area it now occupies. The air column becomes heavier and taller and creates more pressure. This is called convergence.
When there is a net flow of air OUT of a region, air is pushed down and out. This produces less pressure at the surface.

9. High and Low Pressure Systems