Transcript No Slide Title - De Anza College

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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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