Transcript The Causes of Weather

The Causes of Weather
May 2013
Air Masses
 The air over a
warm surface can
be heated causing
it to rise above
more dense air.
 Air Mass: A very
large mass of air
that has nearly
uniform properties
such as
temperature,
humidity and
pressure.
High Pressure Systems
 When an air mass
cools over an ocean or
a cold region of land, a
high pressure
system forms.
 As the air mass cools,
the air mass becomes
more dense.
 When the air mass
contracts, it draws in
surrounding air from
the upper atmosphere.
Low Pressure Systems
 Air masses that travel
over warm land or
oceans may develop
into low pressure
systems.
 When an air mass
warms, it expands and
rises. As it rises, it
cools.
 Water vapour in the
air may condense,
producing clouds or
precipitation.
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Land and Sea Breezes
 Land and Sea Breezes
 Land breeze : a type of wind that
blows from the land to the ocean.
 Land breezes usually occur at night.
 Air moves because of differences in
air pressure over the land.
 Winds at the surface flow from high
to low pressure creating a land
breeze.
Land and Sea Breezes
 Sea Breeze: A wind from the sea that
develops over land near coasts.
 Formed by increasing temperature
differences between the land and
water.
 Air temperature gets cooler relative
to nearby locations as one moves
closer to a large body of water.
Land and Sea Breezes
 The sea has a greater heat capacity
than land and can absorb more heat
than the land.
 The strength of the sea breeze is
directly proportional to the
temperature difference between the
land and the sea.
 (Land and Sea Breezes activity)
The Coriolis Effect and Wind
 The Coriolis effect is a
change in the direction
of moving air, water,
or any objects on
Earth’s surface due to
Earth’s rotation.
 As Earth rotates, any
location at the equator
travels much faster
than a location near
either of the poles.
Global Wind Systems
 Wind systems are
wide zones of
prevailing winds.
There are three
major wind
systems, which
occur in both
hemispheres.
 Trade Winds
 Prevailing
Westerlies
 Polar Easterlies
Jet Streams
 A large temperature gradient in upper-level air,
combined with the Coriolis effect, results in strong
westerly winds called jet streams.
 A jet stream is a narrow band of fast-moving wind.
 A jet stream can have a speed up to 300 km/h or
greater at altitudes of 10 km to 12 km.
 Storms form along jet streams and
generate large-scale weather systems
Shipwrecks Reveal Ocean Currents
Fronts
 A front is a zone that develops as a result
of the meeting of two air masses with
different characteristics.
 Each air mass has its own temperature and
pressure.
 An approaching front means a change in
the weather, and the extent of the change
depends on the difference between
conditions in the air masses.
 Fronts usually bring precipitation.
Fronts
Weather Fronts
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