NWSCOMET Hydrometeorology Course 23 February 9 March 2000

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NWS-COMETHydrometeorology Course23 February 9
March 2000

• Meteorology Primer

Presented by Pete Stamus Wed-Thurs, 23-24
February 2000 Hydromet 00-2
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• Peter A. Stamus
• Research Associate - Senior Meteorologist
• CSU/Cooperative Institute for Research in the
  Atmosphere (CIRA)
• and
• Colorado Research Associates (CORA)
• 303-415-9701 x224
• 303-415-9702 (fax)
• stamus_at_co-ra.com

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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 StructureFun 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 StructureFun 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 atmosphereThe 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 affects 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
Parcel B
Parcel A
Pressure at 1000 mb
T 10 oC (50 oF) e 12.3 mb es 12.3 mb
T 20 oC (68 oF) e 12.3 mb es 23.7 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 airs 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.)

• Newtons 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) -- doesnt 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 Earths 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