The Atmosphere in
Motion
What is air pressure?
It is the weight of the atmosphere
above as it pushes down on earth’s
surface.
Air pressure surrounds us from all
sides.
Air pressure decreases as you go
up in the atmosphere. The balloon
above will expand as a result.
We use a barometer to measure air
pressure.
What are the 3 things that cause
changes in air pressure?
Elevation. Higher elevations exert
less pressure.
Humidity. More humid air exerts
less pressure.
Temperature. Warmer air exerts
less pressure.
How do we plot pressure
differences?
By using isobars on a weather map
How does wind blow?
Wind blows from High to Low
Pressure
But the Coriolis Force causes wind
to deflect to the right in the
Northern Hemisphere and to
the left in the Southern
Hemisphere
Will the Coriolis Force have an
effect on your bathtub, toilet, or
kitchen sink?
Absolutely NOT! Coriolis only takes
effect when there are large
distances, high velocities, and
large time scales!
The Coriolis Effect is greatest near the
poles and the least near the Equator.
The effect is greater on fast moving
objects.
The effect does not depend on an
object’s direction of movement.
It is only noticeable for objects
traveling great speeds over great
distances.
This causes wind directions around
high and low pressure systems
to change.
Northern Hemisphere
Clockwise
Counter-Clockwise
This causes wind directions around
high and low pressure systems
to change.
Southern Hemisphere
Clockwise
Counter-Clockwise
Friction of wind with the ground will
cause Coriolis Effect to be
weakened. Winds will flow more
perpendicular to isobars.
Northern Hemisphere
Clockwise
Counter-Clockwise
The Jet Stream is a high altitude stream of
air at the boundary between warm air
and cold air.
Global Wind Patterns
Global Wind Patterns are the result of
earth’s rotation and temperature
(pressure) differences around the
globe
If the Earth did not rotate, this is what the
global wind patterns would look like.
Winds would only respond to
temperature (pressure) differences.
Because the Earth rotates, this is what the
global wind patterns look like. Winds
respond to temperature (pressure)
differences and Earth’s rotation.
The rising motion of air at low pressure
areas and descending motion of air at
high pressure areas creates Hadley
cells.
If the Earth did not rotate, one large
circulation cell would occur in each
hemisphere.
Because Earth rotates, the circulation
pattern looks like this. We call this the
Three-celled Circulation Model
Weaknesses of the Three-celled Model
It assumes that surface winds are
uniform. Surface winds can be
determined by local High and
Low pressure weather systems
It assumes that all of the
Earth’s surface is the same. In
actuality, Earth has oceans
and landmasses.
It assumes that upper-level
winds travel north or south.
Actually, upper level winds are
primarily westerly, except near
the equator where the Coriolis
effect is weak.
Prevailing winds are the result of the global
circulation model.
Horse Latitudes
Doldrums
Horse Latitudes
The Horse Latitudes, which have little or no
wind, got their name because horses
were dumped overboard to lighten the
ship’s load and conserve fresh water.
Horse Latitudes
Doldrums
Horse Latitudes
The Doldrums, which have little or no wind,
got their name because sailors would
get trapped there in between the trade
winds for days or weeks. Hence, the
saying “stuck in the doldrums.”
Horse Latitudes
Doldrums
Horse Latitudes
El Niño occurs when the tropical easterlies
(trade winds) die off about every 3-5
years. Go here for the latest
information.
Northeast Trade
Winds
Southeast Trade
Winds
Tracking El Niño was the NOVA video that
we saw in class. Click here for the
latest Topex-Poseidon El Niño Sea
Surface Temperature Image.
The warm flux of ocean
water from the 19971998 El Niño
Local Wind Patterns
The Indian Monsoon
Sea Breezes during the day
Rising warm air over the land is replaced by cooler air
from the ocean. The wind goes from the ocean to the
land, High to Low.
Valley Breeze during the day
Rising warm air over the valley flows uphill as a valley
breeze.
Land Breezes during the night
Rising warm air over the ocean is replaced by cooler air
from the land. The wind goes from the land to the ocean,
High to Low.
Mountain Breezes during the night
Cool air at the top of the mountain sinks and causes a
mountain breeze.
The Indian Monsoon
Increased rainfall occurs over India in the Summer
months. This is because the warmer continent
creates rising air and a sea breeze pattern.
Cooler, moist air from the Indian Ocean brings
monsoon rains to the Indian landmass.
The Indian Monsoon
Decreased rainfall occurs over India in the Winter
months. This is because the colder continent
creates sinking air and a land breeze pattern.
Rain stays offshore over the warmer Indian
Ocean.