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Ch 17
Section 2
Weather
Atmospheric pressure
 Atmosphere presses down on you with the equivalent
to one kilogram per square centimeter.
 Pressure is caused by gas molecules colliding with each
other and your skin.
 Pressure decrease as altitude increases
 Air is thinner in upper atmosphere
 Oxygen decreases as altitude increases
 Air planes are pressurized due to this decrease
Global winds and pressure
systems
 Weather patterns
result from complex
global patterns of
wind and pressure.
 Westerlies- winds that
blow from the west in
the middle latitudes
 Tradewinds blow from
the east in the tropics
Jetstreams
 Jetstreams control many weather processes, such as
storm development.
 12 km above surface
 Can travel 500km/h
 Moves north and south with seasons
High and Low pressure
systems
 Subtropical Highs are relatively stable belts of high
pressure near latitudes of 30°.
Coriolis Effect
 When air rises with low pressure and sinks with high
pressure systems.
Air Masses and Weather
Fronts
 Air masses can be polar or tropical and continental or
maritime.
 Air masses interact in zones called weather fronts.
 Warm and cold fronts create different types of
precipitation.
Thunderstorms
 A cumulonimbus cloud can have ice crystals at the top
of the cloud can sometimes form hail.
Downdrafts and Squalls
 The falling rain can sometimes bring cool air with it; the
sinking air current is called a downdraft.
 When a downdraft hits the earth’s surface and spreads
out as windy gusts; it is called a squall.
Downbursts
 When downdrafts hit the surface and spread out very
violently.
 Wind can reach speeds of 260 km/h
Tornadoes
 Intense, short-lived, violent, localized storms that occur
in the mid latitudes.
Tornadoes
 Can move at speeds of 50 km/h across surface
 Wind speeds in funnel can reach 400 km/h
Hurricanes
 Tropical storms that cover vast areas and last for days
 At 118 km/h winds; tropical depression is upgraded to
hurricane.
 Western pacific hurricanes are called Typhoons.