Download 8 October

Measuring Precipitation
Doppler Radar
Transmitter generates energy toward
target, returned energy measured and
displayed
• Brightness of echo = amount/intensity of rain
Doppler: measures speed of horizontal rain
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Doppler Radar
Stepped Art
Fig. 7-33, p. 188
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Doppler Radar
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Doppler Radar
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Doppler Radar
Measuring Precipitation
Measuring from space
Specific satellites designed to assess
clouds, atmospheric moisture, and rain
• TRMM
• CloudSat
Atmospheric Pressure
What causes air pressure to vary
horizontally?
Why does the air pressure change at
the surface?
Atmospheric Pressure
Horizontal Pressure Variations
It takes a shorter column of dense, cold air
to exert the same pressure as a taller
column of less dense, warm air
Warm air aloft is normally associated with
high atmospheric pressure and cold air
aloft with low atmospheric pressure
At surface, horizontal difference in
temperature = horizontal pressure in
pressure = wind
Atmospheric Pressure
Special Topic: Gas Law
P is proportional to T x ρ
P = pressure
T = temperature
ρ = density
Hydrostatic Equilibrium
Force of gravity balances vertical pressure
gradient
• Atmosphere “held” to surface
• Local imbalances = up- and downdrafts
The Role of Density in Hydrostatic Equilibrium
Dense atmosphere experiences greater
gravitational force
Higher temperature air columns are less dense
than cooler ones
Heating = density
decrease in a
column
Columns have
same mass
Lower density =
greater
column height
Atmospheric pressure
Two air
columns
Identical
P, T
Fig. 8-2, p. 195
Atmospheric pressure
One column
warms
compared to
the other
Cooler =
more dense
= smaller
Warmer =
less dense =
taller
Fig. 8-2, p. 195
Atmospheric pressure
Aloft, air flows from
higher pressure to
lower
Fig. 8-2, p. 195
Atmospheric pressure
At the surface, the extra
air increases the pressure
Fig. 8-3, p. 195
Atmospheric Pressure
Daily Pressure Variations
Thermal tides in the tropics
Mid-latitude pressure variation driven by
transitory pressure cells
Diurnal pressure changes
Scale
Pressure is less at p2 than at p1
Atmospheric Pressure
Pressure Readings
Altitude corrections: high altitude add
pressure, 10mb/100m above sea level
Surface and Upper Level Charts
Sea-level pressure chart: constant
height
Upper level or isobaric chart: constant
pressure surface (i.e. 500mb)
High heights correspond to higher than
normal pressures at a given latitude and
vice versa
Sea level charts
Isobars aloft
Chart shows 3
altitudes with
isobars drawn
Developing isobaric maps
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Upper air heights decrease with latitude
500 mb height contours for May 3, 1995
Surface and 500-mbar maps
Isobars & winds at the surface
Isoheights, winds & isotherms at 500
mbar
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Ch. 8 Study materials
Monday (Winds)
Chapter 8 (205-219)
Review
Optional, NOT to be turned in
Questions for review: 1-2, 5-20, 22-23
Questions for thought: 2-3