Transcript Winds - De Anza College

Winds
Professor Jeff Gawrych
De Anza College
Principle forces
Q: What drives the weather in the atmosphere?
The uneven heating of the earth
Q: What causes the weather to change?
Pressure differences and winds
We must analyse the predominant forces in the
atmosphere, namely:
– pressure and pressure gradients,
– gravity
– rotation of the earth
– friction
Pressure differences
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Determines the direction and speed of winds
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Can help explain general circulation of
atmosphere (next lecture)
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These differences are brought on by
Temperature and density changes
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Gas law
p=RT
p-Pressure (Pa),  - density (g m-3)
R - Gas Constant (287 J K-1 kg-1)
T - Temperature (K)
Cold air is more dense and warm air is less dense, so cold air sinks
and warm air rises!!!
It’s the difference
in pressure that
causes a fluid
like air to move.
Pressure Gradient Force
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Pressure gradient:
– Difference in pressure between different
locations.
– Force blows from higher pressure to lower
pressure
– Force is directed perpicular to isoba
– Stronger presure gradients produce stronger
winds
Pressure Gradient Force
The Rotation of the Earth
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Rockets, migrating birds, and large scale
weather systems are all deflected due to
the rotation of the Earth.

The Earth’s rotation causes
– Rotational movement that deflects the winds.
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The Coriolis Force is the name of this
rotational force that deflects motion.
Coriolis Force
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Affects direction, not speed of object
Maximum at the poles
Zero at the equator
Not the reason the toilets flush a certain way
in each hemisphere.
Always acts to the right of the motion in
 Northern Hemisphere.
Always acts to the left of the motion in the
 Southern Hemisphere.
3 main types of wind

1) Geostrophic:
– winds aloft, where frcition is minimal
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2) Surface winds
– Frition cannot be ignored
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3)Local/regional winds
– Slope induced winds, thermal circulations,
etc,monsoon, etc.
Geostrophic Flow

Assumes there is no friction at all!
– Not 100% realistic, but neraly true at high
altitudes
PGF and Coriolis Force in balance
 Flow is parallel to isobars
 The jet stream is nearly geostophic

Geostrphic Flow
Surface Flow
Aloft, winds are nearly geostrophic
At surface, friction alters direction of winds
Why do low-pressure systems
cause rain?
Aloft: flow is counter clockwise in NH and
flow is geostrophic
 Surface: flow is ~Clockwise, but NOT
geostrophic,
 Instead flow is inward towards low causing
convergence
 Leads to clouds/rain
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Why do high-pressure systems
cause clear conditions?
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Aloft: flow is clockwise in NH and geostrophic
Surface: flow is ~clockwise but NOT geostrophic
Instead flow is outward from high causing
divergence
Leads to sinking motion (subsidence) and clear
skies
Sinking air WARMS and DRIES OUT
Surface Winds

Friction is now a major factor, and slows
down the wind.
– Trees, buildings, monuntqains,air
resistance, etc. all contribute to friction

Winds DO NOT blow parallel to isobars,
but
– Inward towards low pressure
– Outward from high pressure
Slope Winds
Day: warmer air rises and ascends upslope. Often causes cloud formation
Night: Cooler air descends downslope.
Slope flow II:
Slope flow significantly
alters precipataion patterns
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Thermal Circulations
– Localized events
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sea breeze
land breeze
country breeze
– Surrounding areas
heat/cool at different
rates.
– Surface winds blow
from the colder place
towards the warmer
place
Monsoon circulation