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How Heat Transfer Creates
Weather Systems
Professor Jeff Gawrych
De Anza College
Since the Tropics
are warm and the
Polar Regions are
cold, air will start
to circulate.
We know...
 Earth is warmed via...
– Radiation: amount of sunlight absorbed
– Conduction: significant, but minor contribtuion
– Convection: Air movement (wind)/Weather systems
–Greenhouse Effect: Traps outgoing radiation
 Air is a poor conductor of heat
 Earth cools primarily by emitting longwave, IR
radiation
 Therefore, daily temperatures are determined by
Daily temperatures
 Day-to-day variation in temepratures is mainly to
do with convection and the G.H. effect
 Exercise: Make a list of some things that you
think are necessary to accurately forecast
tomorrow’s high and low temperatures in San
Jose.
 Maybe it will help to think along these lines:
–Why will the high temperature probably NOT be 110
Daily temperatures
 A few answers:
– The most imporant factor in determing on how hot or
cold it can get in a given day is the profile of the air mass
above us.
 We get this info from weather balloons
 So knowing the current conditions is essential!
–Cloud cover: During the day, clouds block incoming
sunlight. At night, they enhance the greenhouse effect
–Winds: winds limit how much a place can warm up or
cool off
–Land use: Urban vs. Rural locations: Urban areas get
warmer during the day/stay warmer at night
Air Masses
 A general description of the atmosphere over a
certain area.
– Tells temperature and moisture profile.
 Type of air mass over a region tells what the
weather may be like.
 Air masses originate in specific areas and can
dominate the climate by not moving for long
periods of time
 .
– Remember: climate is what you expect, weather is what
Air masses
 Name of air masses tells you where it originated

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

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c = continental = dry
m = marine = moist
T = tropical = warm
P = polar = cold
A = Arctic = extremely cold
 Examples
cT: continental tropical (warm and moist)
cP: continental polar (cold and dry)
mT: marine tropical ( warm and moist)
mP: marine polar (cold and moist)
Pressure Differences
 Determines the direction and speed of winds
 Can help explain circulation patterns around
the world
 General guidelines:
– High pressure: clear skies/fine weather
– Low pressure: cloudy skies/changeable
weather
 We will see why this is later...
It’s the difference
in pressure that
causes a fluid
like air to move.
What causes pressure to
change?
 Temperature changes!!
 Therefore Density change
Formally, the equation of state describes this
relationship:
p=RT
p-Pressure (Pa)
 - density (g m-3)
R - Gas Constant (287 J K-1 kg-1)
T - Temperature (K)
Cold air is ____ dense and warm air is ____ dense, so
cold air _____ and warm air _____!!!
Fronts
 Separate different air masses of different
densities. Cold air is more dense than warm
aircold air sinks, warm air rises.
 So when a cold front passes, it forces lifting.
– Cold fronts bring cold air into region
– Warm fronts bring warm air into region
Cold Front
Warm front