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NWS-COMETHydrometeorology Course

15 – 30 June 1999

Meteorology Primer

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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)

stamus@fsl.noaa.gov

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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