1. Atmospheric (Air) Pressure
I. What is air pressure?
A. Gravity is always pushing the layers of air surrounding the earth down. This push is called air pressure. (weight of a column of air)
B. The upper layers of the air push down on the lower layers of the air. So air pressure is greater on the surface of the earth due to this force (pressure).
II. Method of measurement
A. Air pressure is measured with a barometer – mercury or aneroid
B. Air pressure is measured in INCHES in the English – inches is normal
C. Air pressure is measured in MILLIBARS (mb) in the metric system – mb is normal
D. Converting pressure in inches to pressure in millibars…
Pressure in inches (keep as a decimal) X 34 = Pressure in millibars (tenth)

2. Atmospheric (Air) Pressure
III. Isobars
A. Areas of the same air pressure are connected on a weather maps with lines called isobars.
1. work in intervals of 4 millibars
2. will never cross
3. The closer together they are the stronger the winds and the more severe the weather changes.
B. All pressures on a weather map are given in millibars.

3. Atmospheric (Air) Pressure
IV. Types of pressure
A. Low Pressure (low millibar readings on map)
1. Often called a cyclone
2. Counterclockwise movement around the center in the Northern Hemisphere and clockwise in the Southern Hemisphere.
3. Type of weather is warmer and moister air (bad, stormy weather.
B. High Pressure (high millibar readings on map)
1. Sometimes called an anticyclone
2. Direction of movement is clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.
3. Type of weather is drier and cooler air (nice weather)

4. Atmospheric (Air) Pressure
Predicting the Weather using Air Pressure
1. When air pressure is rising (getting higher), the weather is improving.
2. When the air pressure is dropping (getting lower), the weather is getting worse.
3. When the air pressure is staying the same, the weather isn’t changing.