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 WindsWhich way would the wind blow?
What time of day is shown in this picture?
More Local WindsWhich 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.
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?