Download Air Pressure and Winds-I

Air Pressure and Winds-I
GEOL 1350: Introduction To Meteorology
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Pressure gradient force is in balance
with gravity  Hydrostatic relations
 Means no vertical motion initially 3
How does atmospheric pressure change with altitude ?
Pressure always
decreases with
increasing height.
In the low
atmosphere,
pressure decreases
by about 10 mb
(hPa) for every 100
m increase in
elevation
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Pressure can change as we go up or down in
the atmosphere.
Can pressure change as we move from one
location to another assuming the two
locations are at the same elevation ?
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Two columns of air– same temperature
same distribution of mass
500 mb level
5500 m MSL
1000 mb
1000 mb
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Cool the left column;
warm the right column
The heated column
expands
The cooled
column contracts
500 mb
original 500 mb level
500 mb
1000 mb
1000 mb
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The level of the 500 mb surface changes; the
surface pressure remains unchanged
The level corresponding to
500 mb is displaced downward
in the cooler column
The 500 mb surface is
displaced upward in the
warmer column
new 500 mb
level in warm
air
original 500 mb level
new 500 mb
level in cold
air
1000 mb
1000 mb
The surface pressure remains
the same since both columns
still contain the same mass of air.
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A pressure difference in the horizontal
direction develops above the surface
The 500 mb surface is
displaced downward in
the cooler column
original 500 mb level
Low
The 500 mb surface is
displaced upward in the
warmer column
High
new 500 mb
level in warm
air
new 500 mb
level in cold
air
1000 mb
1000 mb
The surface pressure remains
the same since both columns
still contain the same mass of air.
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Air moves from high to low pressure in middle of
column, causing surface pressure to change.
Difference in pressure in
horizontal direction induces
horizontal winds
500 mb
450 mb
Low
High
550mb
original 500 mb level
500 mb
Surface pressure rises
as air moving into the
column from above
1003 mb
997 mb
Air above moving away
from the column –
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surface pressure drops
Air moves from high to low pressure at the surface
Difference in pressure in
horizontal direction induces
horizontal winds
500 mb
original 500 mb level
Low
High
High
Low
1003 mb
997 mb
500 mb
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Two columns of air– same temperature
same distribution of mass
1000 mb
1000 mb
1003 mb
Heat more
997 mb
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How does the atmosphere converts
heating into motions ?
•  Starting with a uniform atmosphere at rest, we
introduced differential heating
•  The differential heating caused different rates
of expansion in the air columns
•  The differing rates of expansion resulted in
pressure differences along a horizontal
surface.
•  The pressure differences then induce flow.
Heating or cooling of a column of air can establish
horizontal variations in pressure that cause the air
to move horizontally
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Station Pressure and Sea Level Pressure
•  Station pressure
– the pressure
reading at a
particular
location and
elevation
•  Sea-level
pressure
- the pressure
adjusted to
altitude 0 m,
which is the
mean sea level.
Pressure drops
approximately
10 mb for 100
m increase in
height
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Surface Maps
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Surface High and Low Pressure and Winds
Near surface
in the
Northern
Hemisphere,
winds blow
counter
clockwise
around and
towards a low
pressure
center
Clockwise
around and
outwards from
a high
pressure
center
Why?
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Upper-level map and winds aloft
At upper levels, winds blow parallel to the height contours in a wavy west
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to east direction
Pressure gradient force – horizontal difference in
atmospheric pressure causes air to move
500 mb
original 500 mb level
Low P
High P
High
Low
1003 mb
997 mb
500 mb
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Higher water level in A creates high fluid pressure at the
bottom and a net force towards the low fluid pressure at
the bottom of Tank B, causing water to move from A to22B
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The pressure gradient force
  the force that causes wind to blow
  directed from higher towards lower pressure at
right angle to the isobars
  directly related to the pressure gradient – the
change of pressure over a given distance (ΔP/d)
  steep pressure gradients correspond to
stronger pressure gradient force and vice versa
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Computing PGF for each direction
[Stull, Meteorology for Scientists and Engineers]
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SUMMARY
1.  Atmospheric pressure is the pressure exerted by the mass of air
above a region.
2.  A change in surface air pressure can be brought about by
changing the mass above the surface.
3.  Difference in horizontal air pressure produces a horizontal
pressure gradient force (PGF).
4.  The pressure gradient force (PGF) is always directed from higher
pressure toward lower pressure and it is the PGF that causes the
air to move and the wind to blow.
5.  Steep pressure gradients indicate strong pressure gradient
forces and high wind; gentle pressure gradients indicate weak
pressure gradient forces and light winds.
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