What is air pressure?
1. The weight of the atmosphere as it pushes down upon Earth’s surface exerts a force
per unit of area = air pressure.
2. Air pressure anywhere on the Earths surface depends on the weight of the air above therefore, air pressure decreases as elevation increases.
3. Air pressure decreases about 50% every 5 km up in elevation.
4. Most noticeable when you experience a change in air pressure.
A. Ex: airplane or elevator – rapid ascent – the air pressure outside your body
rapidly decreases and your eardrums may hurt or pup because of greater
internal pressure.
5. We use barometers to measure air pressure.
6. Humidity affects the air pressure.
A.
B.
C.
D.
The more water vapor the air contains the lighter the air is.
Water molecules have less mass to them vs. nitrogen and oxygen.
Dry Air: about 99% oxygen and nitrogen.
Humid Air: 97% oxygen and nitrogen.
1. Water vapor pushes out an equal volume amount of nitrogen and
oxygen.
7. Normally a decrease in air pressure = warmer, humid air = rain and snow.
8. An increase = drier, cooler weather = fair weather.
9. High air pressure = steadily increases toward the center.
A. The pressure in the high is greater than the surrounding air pressure.
10. Low air pressure = air pressure steadily decreases toward the center.
A. The pressure in the low in lower than the surrounding air pressure.
11. Use isobars to plot differing air pressures.
A. A line that joins points having the same air pressure.
B. A closed isobar is one that forms a closed loop on a weather map.
C. Isobars that are close together show that the air pressure is changing rapidly in
the area.
12. In a high pressure system and a low-pressure system the winds spin in opposite
directions.
A. To see which one does which way we are going to do a lab.