Pressure and Winds

Pressure and Winds
C. David Whiteman
Atmos 3200/Geog 3280
Mountain Weather and Climate
Geisler Group, Dolomites © CD Whiteman
Units
•
•
Pressure:
•
•
•
•
•
mb (hPa)
mph
m/s
km/h
knots
inches of mercury
1
0.4
1.6
0.9
mm of mercury
2
0.9
3.2
1.7
3
1.3
4.8
2.6
4
1.8
6.4
3.5
5
2.2
8.0
4.3
10
4.5
16.1
8.7
pounds per square inch
atmospheres
Winds:
•
•
•
•
Wind speed conversion table
mph (statute miles per hour)
m/s
Beaufort Wind Scale
See Table 5.2
knots (nautical miles per hour)
km/h
See Appendix D for unit conversions
F = 32 + (9/5)C
C = (5/9) (F-32)
Standard atmosphere
Pressure
(hPa = mb)
Typical
height (ft)
Typical
height (m)
1013.25
0
0
1000
370
110
850
4780
1460
700
9880
3010
500
18280
5570
300
30050
9160
Whiteman 2000
Pressure decreases exponentially
with altitude
Pressure changes more rapidly in the vertical than in the horizontal, but
the horizontal variations are important for visualizing traveling pressure
systems.
Highs, lows, ridges, troughs
Sea level pressure analysis
Height of pressure surface analysis
Pressure variation on constant height surface
Height variation on constant pressure surface
How to reduce surface pressure to sea level?
Isobar
Isohypse, isoheight,
contour line
500 mb hemispheric pattern
25 Jan 1999 500 mb height pattern
Mean 500 mb height pattern
Balance of forces, highs and lows, NH
Balance of Forces:
PGF is directed from high to low
and is proportional to pressure
gradient.
Coriolis Force is a function of
speed and latitude and is
directed to right of wind.
Friction Force opposes the wind
and is proportional to surface
drag.
Geostrophic balance - PGF balances CF
Above sfc friction, winds become
parallel to isobars with low on left.
If friction is felt, winds turn across
isobars toward low pressure.
Thus, winds spiral counterclockwise into sfc low and
clockwise out of sfc high.
Leeward, windward, definitions
Convention for naming winds:
Named for the direction from which they blow
Example: A north wind blows from N to S
Example: An east wind blows from E to W
A
A
A
A
sea breeze blows from sea to land
land breeze blows from land to sea
mountain wind blows from mountain to valley
valley wind blows from valley to mountain
If we need more specificity, use the bearing from which the wind blows
Example: a wind from 178° (a south wind)
Example: a wind from 87° (an east wind)
Or use a vector
Will need a legend
or scale
Buys-Ballot rule (Northern Hemisphere)
“If the wind blows into your
back, the Low will be to
your Left (and the high will
be to your right).”
This rule works well if the
wind is above the earth’s
boundary layer, not
channeled by topography,
etc.
500 mb analysis
00Z, 21 Sep 2011
http://www.atmos.albany.edu/weather/difax/2021.gif
Surface analysis
21Z, 20 Sept 2011
http://www.atmos.albany.edu/weather/difax/3096.gif
Transfer of heat by cyclones and anticyclones
Excess of solar radiation relative to long wave loss at equator. Deficit at
poles. Poles get cold, equator gets warm. A zonal temperature gradient
develops. Lows and highs and ocean currents are necessary to transfer heat
from the equator toward the poles.
Cornices, snow fences
Sastrugi, wind flagging
Diurnal variation of wind speed
Winds at the ground
increase in the afternoon as
stronger winds are mixed
down from aloft.
Winds aloft decrease in the
afternoon as the mixing
exposes them to friction at
the earth’s surface.