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NWS-COMETHydrometeorology Course
15 – 30 June 1999
Meteorology Primer
Presented by: Pete StamusTues, Wed, 15-16 June 1999Hydromet 99-2
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Peter A. StamusResearch Associate - Senior Meteorologist
NOAA/Forecast Systems Laboratory
and
CSU/Cooperative Institute for Research in the Atmosphere (CIRA)
303-497-6100
303-497-7262 (fax)
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Purpose of the primer
• Basic understanding of meteorological processes.
• Starting point for the rest of Hydromet
• To give you a semester-long Introduction to Meteorology course in 8 hours.
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Atmosphere Structure
Fun facts
• Standard atmosphere– Very long term average for mid-latitudes
– Average surface pressure 1013 mb
– Average surface temperature 59 oF
• 1/2 of the mass of the atmosphere (500 mb)
below 6 km (3.7 miles)
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Atmosphere Structure
Fun facts
• Lapse rate (decrease in temperature in the vertical)
Troposphere:
+15 oC (at sfc) to ~ -50 oC (at 10 km) -6.5 oC / km
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Water vapor in the atmosphere
The most important parameter we attempt to measure and forecast.
• Clouds• Precipitation• Energy Transfer
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Evaporation and Condensation
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Evaporation and Condensation
• Evaporation– Fast molecules escape, slower remain
cooling process
• Condensation– Slower molecules collide, form droplets,
droplets fall, faster molecules remain
warming process
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Evaporation and Condensation (cont.)
• The Evaporation/Condensation process transfers heat energy to the atmosphere– Latent Heat of Condensation
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Evaporation and Condensation (cont.)
Fun facts
• Wind enhances evaporation
• Warm water evaporates faster than cool water
• Air temperature effects evaporation rate– Cool air, slower molecules, condensation more likely,
slows evaporation
• Warm air can hold more water vapor before saturation than cold air
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Saturation Vapor Pressure
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Relative Humidity and Dew Point
Pressure at 1000 mb
Parcel A Parcel B
T = 10 oC (50 oF)
es = 12.3 mb
e = 12.3 mb
T = 20 oC (68 oF)
es = 23.7 mb
e = 12.3 mb
RH = (e / es) x 100 = 100% RH = (e / es) x 100 = 52%
Therefore: Td = 10 oC for Parcel B
Dew point = Temperature to which air must be cooled at constant pressure to reach saturation. It is a measure of the air’s actual water vapor content.
Relative Humidity is a measure of the degree of saturation of the air.
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Energy Budget
• Incoming solar
• Emitted long-wave
• Transfer with latitude
• Long-term balance
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Energy Transfer with latitude
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Daily and Seasonal Energy Balance
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Lab 1
Basic Surface Features/Moisture
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Atmospheric Pressure
• Pressure = total weight of air above
• Air is compressible, so gravity concentrates most air molecules near the surface
• Atm pressure decreases with height
rising air cools, sinking air warms
• Greatest pressure variation in vertical, but smaller horizontal variations produce winds and weather systems
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Pressure and terrain
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Pressure and volume
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Pressure and volume (cont.)
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Typical 500 mb map
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Lab 2
3-D Atmospheric Structure
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Wind
• Differential heating of land/ocean leads to
pressure differences in the atmosphere
• Pressure differences are forces that lead to
atmospheric motions
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Wind (cont.)
• Newton’s Laws of Motion
– First Law:
Objects at rest remain at rest and objects in motion remain in motion,
provided no force acts on the object
– Second Law:
Force equals mass times the acceleration produced
F = ma
• To determine wind direction and speed, need to know the
forces that affect horizontal movement of the air
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Wind (cont.)
• Forces that lead to the wind– pressure gradient force (PGF)
– Coriolis force (C)
– centripetal force (c)
– gravity (g) -- doesn’t effect horizontal motions
– friction (F)
Net Force = PGF + C + c + g + F
• If these forces add to zero, then
(1) The air remains at rest; or,
(2) The air remains in motion along a straight path at a constant speed
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Wind (cont.)
• pressure gradient force (PGF)– Moves air from higher pressure to lower pressure
• Coriolis force (C)– Apparent force due to the Earth’s rotation
– Acts to turn wind to the right in the Northern Hemisphere
• centripetal force (c)
– Inward directed, keeps parcels rotating around pressure centers
• gravity (g)
– Always acts downward; vertical motions only
• friction (F)
– Acts opposite to the direction of motion; retards motion
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Typical Flow
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Idealized surface flow
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Lab 3
Wind