Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Further Reading: Chapter 07 of the text book
Outline
- Geostrophic Winds
- Cyclones and Anti-Cyclones
- Upper Atmospheric Circulation
- Jet Streams
Myneni
Lecture 18: Winds Aloft
Mar-02-07
(1 of 9)
Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
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Myneni
Lecture 18: Winds Aloft
Mar-02-07
(2 of 9)
Geostrophic Winds-1
Previously, we talked about pressure gradient forces and the Coriolis Force
We want to continue to look at how these forces affect winds
Before we looked at how the Coriolis force affects winds at the surface. What happens to winds aloft
where there is no frictional force? Produces Geostrophic winds
L
1004mb
1008mb
1012mb
CF
1016mb
PGF
H
Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 18: Winds Aloft
Mar-02-07
(3 of 9)
Geostrophic Winds-2
– These winds occur when the only important forces are the Pressure gradient force
and the coriolis force
– Originally the winds blow down the PGF
– As they do so, they now have a velocity so they are affected by the Coriolis force
– This makes the winds veer to the right (in the northern hemisphere)
– The winds continue to veer to the right until the coriolis force is in the opposite
direction as the pressure gradient force
– Now there are no net forces, i.e. the winds continue blowing in the direction they’re
going
– The net effect is that for geostrophic winds, they blow parallel to isobars, not
perpendicular to them
– The other important result is that for geostrophic winds, the Coriolis force is always
equal to and in the opposite direction from the pressure gradient force
Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 18: Winds Aloft
Mar-02-07
(4 of 9)
Cyclones-1
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Almost all winds except for those at the surface are geostrophic
Knowing this, how can we determine the direction the winds are blowing just by looking
at a map of isobars?
First, we when we look at maps of isobars, we typically see “cyclones” and “anticyclones”
L
PGF
CF
Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 18: Winds Aloft
Mar-02-07
(5 of 9)
Cyclones-2
– For a cyclone, we know the pressure gradient points into the center perpendicular to
the isobars
– Because the winds are geostrophic, we know by definition that the coriolis force has
to be equal and in the opposite direction of the PGF
– If we are in the NH, we know the CF is to the right of the winds, i.e. the winds are to
the left of the CF, i.e. the winds blow to the north
– On the other side of the cyclone we can do the same thing and we find the winds blow
to the south
– In general, we find that in the northern hemisphere winds blow counterclockwise
around cyclones
Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 18: Winds Aloft
Mar-02-07
(6 of 9)
Anti-Cyclones
H
CF
PGF
For an anti-cyclone, we know the
pressure gradient points out of the
center perpendicular to the isobars
Because the winds are geostrophic, we
know by definition that the coriolis
force has to be equal and in the
opposite direction of the PGF
If we are in the NH, we know the CF is
to the right of the winds, i.e. the
winds are to the left of the CF, i.e.
the winds blow to the south
On the other side of the cyclone we can
do the same thing and we find the
winds blow to the north
In general, we find that in the northern
hemisphere winds blow clockwise
around cyclones
Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Upper Atmospheric Circulation
Myneni
Lecture 18: Winds Aloft
Mar-02-07
(7 of 9)
- Pressure decreases much less rapidly in a warm
atmosphere relative to a cold atmosphere.
- Therefore, at a fixed height, there is higher
pressure in the tropics compared to the poles
- Thus, a pressure gradient exists in the upper
atmosphere from the tropics to the poles
Upper Air Westerlies: Blow in a complete circuit
around the Earth, from 25N to the poles. At high
latitudes, the westerlies form a circum-polar spiral
circling the polar low.
Tropical High Pressure Belt: High altitude part of
the surface sub-tropical high belt, slightly shifted to
the equator.
Equatorial Easterlies: A zone of low pressure, with
light easterly winds called equatorial easterlies.
Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Rossby Waves
Myneni
Lecture 18: Winds Aloft
Mar-02-07
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The smooth westward flow of the upper air
westerlies can form undulations (disturbances),
forming Rossby waves, especially when the cold
polar air comes into contact with warm tropical air.
In these undulations, warm air pushes poleward
and tongues of cold air can be brought
equatorward.
Eventually, the tongue of cold air can be pinched
off, leaving it a far southerly location.
This Rossby wave circulation is an important
mechanism by which heat is transported to the
polar regions!
This is also the reason why the weather at mid-latitudes is so variable, as pools of warm moist
tropical air clash with cold and dry polar air masses.
Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Jet Streams
Myneni
Lecture 18: Winds Aloft
Mar-02-07
(9 of 9)
Narrow belt of high velocity winds (~100-250 mph)
Located at high altitudes.
Polar Front Jet Stream: The polar jet is found in
both hemispheres, somewhere between 30-60N,
with intense westerly winds.
Subtropical Jet Stream: Located just above the
subtropical high pressure cells, in both hemispheres,
this westerly jet is a result of conservation of angular
momentum (the picture).
Tropical Easterly Jet Stream: It is a summer-time jet, seen in Northern hemisphere only, over
southeast Asia, India and Africa.