Air Pressure = the weight of the air above you from where you are to the top of the atmosphere.

Air is compressible - Half of all air molecules are below 18,000 feet of altitude; 3/4 are below 36,000 feet, etc. At sea level air exerts a force of about 14 pounds per square inch (enough to crush the soda can.)

Air pressure is measured with a barometer

Air Pressure in the News -

Cold air has higher pressure Cold air is more dense, so it sinks down and pushes harder. –Barometer goes? Hot air expands, becomes less dense, rises, and exerts less pressure. –Barometer goes?

Wind = movement of air Wind always goes from an area of high pressure toward an area of lower pressure. Wylie’s Law - You can’t push with rope; you can’t pull with air! Wind direction is described by where the air comes from - a west wind blows from west toward the east.

Local Winds- Which way would the wind blow? What time of day is shown in this picture?

More Local Winds- Which way would the wind blow? What time of day is shown in this picture?

How do meteorologists predict wind direction?

It’s time you learned about Coriolis Because the Earth rotates on its axis, things moving from North to South or from South to North are deflected. In the Northern Hemisphere, the deflection is to the right. In the Southern Hemisphere, the deflection is to the left.

Figure 3.16 Throwing a ball to someone on a merry go-round does not get to them, they have moved.(a) The equator travels faster than at the poles, it takes a day for both of them to make one full revolution.

Winds Around Pressure Systems

Let’s take one more look at that map.

A Low Pressure System Over Iceland

Why don’t high pressure systems show up on satellite photographs?