Earth’s Atmosphere
Atmospheric Pressure
* The weight of the air above
you. The more air that is above
you the more pressure you will
* Sinking air pushes down
more- cold air is usually high
* Rising air pushes down lesshot air is usually low pressure.
Where is pressure greater, at top of a
mountain or at sea level?
Sea level
So ….
Atmospheric pressure is greatest near Earth’s surface
Decreases as you move out to space
Can vary from locations at the same level due to the density of the air
Cold Air vs. Warm Air
Cold air is denser than warm air so has a higher pressure
Warm is less dense so has a low pressure
These masses of warm/cold air used to predict weather
Measures air pressure
Measured in millibars
Movement of Air
 Wind
 The
movement of air molecules from an area of high
pressure to an area of low pressure
 Temperature
differences on Earth’s surface create
areas of different pressure and the winds that
circulate our planet
Causes of temperature difference
Earth is curved rather than flat so
more solar radiation is received at
the equator
Air above equator is heated more
than any place on Earth … heated air
rises (low pressure)
Air above poles receives less
radiation so air is cooler … colder air
sinks (high pressure)
The Coriolis Effect
Due to Earth’s rotation
Causes air masses moving south in
Northern Hemisphere to be turned
westward from their original paths
These wind patterns determine the
weather and also when ships/planes
can travel
Review …..
What 2 things determine our wind patterns on Earth ?
Differences in heating
Coriolis effect
Global Wind Systems
Trade winds
Jet Streams
Located near top of troposphere
Flow west to east
2 jet streams in each hemisphere
Resemble fast moving rivers
Average speed between 97-185 km/h
Their positions in altitude and latitude from day to day / season to season
Jet Stream
Local Wind Systems
Land Breezes
Movement of air towards water
Sea Breezes
Movement of air towards land
Mountain vs. Valley Breezes
During DAY, heated air along mountain slopes rises = Valley Breeze
At NIGHT, cooled air along mountain slopes moves downward = Mountain

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