Transcript Document

Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 16: Pressure & Winds
Feb-26-07
(1 of 11)
Further Reading: Chapter 07 of the text book
Outline
- pressure
- pressure gradient force
- land and sea breezes
- coriolis force
Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 16: Pressure & Winds
Feb-26-07
(2 of 11)
Intro
•
•
So far we have talked about thermodynamics (radiation budget and
temperature) and hydrodynamics (moisture and precipitation)
Now we will talk about Dynamics
– Mechanism behind winds, general circulation of the atmosphere, weather systems
•
We will incorporate what we have learned about:
– Temperature and density
– Earth’s rotation
– Moisture, clouds, and precipitation
•
Remember that from the global radiation balance there is a gradient in energy
balance between the equators and poles
– There is a surplus at the equator
– There is a deficit at the poles
•
One way to think of the general atmospheric circulation is that is “trying” to
redistribute energy from the equator to the poles
Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 16: Pressure & Winds
Feb-26-07
(3 of 11)
– Atmosphere has mass
Pressure
– The atmospheric molecules are held near surface by gravity
– Pressure itself reflects the weight of the mass of overlying atmosphere
– Defined as force per unit area at the surface
– Units of Pascal (1 N/m2)
– In the atmosphere, we measure pressure in millibars (mb)
– It is important to remember that the atmosphere exerts pressure on all surfaces
– We don’t “feel” it because it is the same on all sides
1 bar = 1000 mb = 10,000 Pa
Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 16: Pressure & Winds
Feb-26-07
(4 of 11)
Pressure and Altitude
As we move up in the atmosphere,
the number of molecules above us
decreases -> pressure decreases
with altitude
At the surface, the pressure is
typically approximately 1013 mb
Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 16: Pressure & Winds
Feb-26-07
(5 of 11)
Pressure Gradient Force
The pressure at the surface (or any fixed
level) is not the same everywhere.
Because pressure is different at different
locations, we observe pressure gradients.
We can connect regions of equal pressure with lines, called “isobars”
It is the force produced because there is high pressures in one are and low
pressure in another
Pressure gradient and pressure gradient force are always perpendicular to isobars
Magnitude of pressure gradient Force is related to how close the isobars are
As we saw, there is also a pressure gradient force in the vertical; however this is
balanced by the force of gravity so typically we don’t see much vertical
motion because of Pressure Gradient forces
Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Pressure gradient forces can be produced by
many processes. One example is the pressure
gradients due to surface temperatures
Recall thermal differences between ocean & land
During the day, the land is warmer than ocean
At night, the land is cooler than the ocean
Also recall the relation between temperature
and pressure
As air gets warmer, it gets less dense so it rises
As air rises, it produces a vacuum effect, i.e. low pressures
As air gets cooler, it gets more dense so it sinks
As air sinks, it “pushes” down on the surface,
i.e. it produces high pressures
This produces a horizontal pressure gradient
force going from the ocean to the land
The winds then blow down this pressure
gradient, hence the “sea breeze”
What about aloft?
Myneni
Lecture 16: Pressure & Winds
Feb-26-07
(6 of 11)
Land and Sea Breeze-01
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L
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Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 16: Pressure & Winds
Feb-26-07
(7 of 11)
Land and Sea Breeze-02
As the cool air sinks, it produces a
vacuum aloft, hence low pressures
As warm air rises, it creates high
pressures aloft
Hence, aloft the pressure gradient is in
the opposite direction from the
surface
Therefore the winds blow in the opposite
direction
During the night, the temperature
gradient reverses and therefore the
direction of the pressure gradient and
winds reverse -> becomes a “land
breeze”
Relative differences in temperature
produce differences in pressure, this
leads to pressure gradient forces and
affects localized flow of air
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Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 16: Pressure & Winds
Feb-26-07
(8 of 11)
Coriolis Force-01
– Is not a real force; it only appears to affect objects because our reference
frame (the earth) is rotating
– Only becomes important at scales larger than 100km
– Assume we are looking down at the north pole
– If the earth weren’t rotating and we through a ball from the equator to the pole it would
go in a straight line
– Now lets look at what happens when the earth rotates
– We initially throw the ball from A to B
– But in the time it takes for the ball to go from A to B, the earth rotates underneath it.
– Lets see where the ball actually ends up
– During the time of travel, A goes to A’ and B goes to B’
– But the ball, which started at A, has a velocity component the same as A, so it travels
further to the right than B, hence it looks like it’s veered to the right
– It turns out that all objects in motion in the Northern Hemisphere have a Coriolis
force directed to the right of motion
– In the Southern Hemisphere, objects have a Coriolis Force directed to the left of
motion
Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 16: Pressure & Winds
Feb-26-07
(9 of 11)
Coriolis Force-02
Without Rotation
B
With Rotation
B B’
A’
A
A
Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Coriolis Force Movies
– First off, the Coriolis force is always
directed perpendicular to the direction
an object is moving; therefore it can
never increase or decrease the speed of
an object, only change the direction it is
moving
– In addition, remember that the Coriolis
force is always to the right of an object
in motion in the Northern hemisphere
and to the left of an object in motion in
the southern hemisphere
– The Coriolis force is proportional to the
sine of the object’s latitude. Therefore,
at low latitudes, the Coriolis force is
weak; as the object moves to higher
latitudes, the Coriolis force increases
– In addition, the Coriolis force is
proportional to the velocity of an object;
Therefore, as an object’s velocity
increases, the Coriolis force on it
increases
Myneni
Lecture 16: Pressure & Winds
Feb-26-07
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Coriolis Force-03
Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Starting in the northern hemisphere, lets assume that
we have a low pressure center
Winds will start to blow into the low pressure center
because of the pressure gradient force
As the move inward, they become deflected to the
right by the coriolis force; hence we find that at
the surface, winds blow into and
counterclockwise around low pressures
For a high pressure center in the northern
hemisphere, we can see that winds blow out of
the high pressures because of the pressure
gradient force
As they do so, the winds again are deflected to the
right so that in the northern hemisphere, winds
blow out of and clockwise around high
pressures
Now look at the southern hemisphere
For a low pressure center, winds will still blow into
the low pressures but now they are deflected to
the left; therefore winds blow into and
clockwise around low pressures in the southern
hemisphere
Similarly, winds blow out of and counterclockwise
around high pressures in the southern
hemisphere
Myneni
Lecture 16: Pressure & Winds
Feb-26-07
(11 of 11)
Pressure & Winds