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Chapter 19 The Atmosphere in
Motion
 Page 412
 What special gear is the hang glider
wearing?
 What force is keeping the hang glider aloft?
 What other kind of activities depend on this
force?
 What causes winds?
Chapter 19.1
Air Pressure and the Wind
What is air pressure?
 A column of air that reaches from sea level
to the tom of the atmosphere pushes down
at 147lbs per square inch
 Air pressure – is the weight of the overlying
atmosphere
 It is exerted in all directions
 Force pushing on your body is balanced
by your body pushing out
 The higher you go into the atmosphere the lover
the pressure
 Barometers – used to measure air pressure
– Aneroid – evacuated can with read outs
attached and barograph
– Mercury p 414
 Units of air pressure
– inches or millimeters – used to read mercury
– millibars used on weather maps – these are
adjusted to sea level pressure due to altitude
differences
Why does air pressure Change
 Elevation – higher up less air pressure
 Temperature – the warmer (more spread
out) the less Air P.
 Water Vapor – the more water vapor the
lower the A.P. – H2O molecules weigh
less than the N & O molecules in the air
 Isobars – lines of equal pressure –
closed isobar forms a closed lop on a
map
 If pressure increases toward center it is a
High Pressure Center
 Low pressure center – pressure is lower
 Pressure Gradient – how close the isobars
are. Close together – large gradient
What makes the wind blow
 Difference in A.P. – the greater the
difference the faster the winds. The closer
the isobars the faster the wind
 A wind blows form High P. to low P.
– e.g. Hot air on an island rises (form L.P.) Cool
air from the water (H.P) rushes in, this is known
as a sea breeze
Measuring surface wind direction
and speed.
 Wind vane – measure the direction from
which the wind is blowing – Winds are
named by the direction they come from
– e.g. westerly or sea breeze
 Anemometer – measure wind speed
Chapter 19.2
Factors affecting Wind
Coriolis Effect
 If the earth did not spin and was perfectly smooth air would
flow strait from high pressure areas (poles) directly to low
pressure areas (equator)
 Coriolis effect causes objects traveling freely over the
earth to curve
– In n. hemisphere curve right
– In s. hemisphere curve left
– The effect is greater near the poles
– The effect is also greater in long diastase travel vs.
short travel
– The effect increases with increased speed
 Friction – with the earth’s surface slows winds
down. The smooth surface(water) the faster
wind can blow the higher coriolis effect
 The coriolis effect is a strong factor in determining
wind direction as is a Pressure gradient speed
 Coriolis causes winds to blow counter
clockwise in a low, clockwise in high
 Jet stream – swift winds due to low friction – direct
path of storms
Global Wind Patterns
 Non rotating earth would allow warm air to rise at the
equator, move to the poles and sink to travel back to the
equator, resulting in large circulation cell
 The coriolis effect (Rotation of earth) prevents this from
happening
 3 cell circulation model
 Weakness of 3Celled model
– gives simplified view of circulation between 30 and
60
– No effect given to continents or season
– Simplifies upper level winds
B. Description of Wind and
Pressure Belts
 ITCZ (intertropical convergence zone) occurs at the
equator
– hot and humid with little or no wind
– rain is common
– historically been called the doldrums
 Between 20 and 35 air sinks forming subtropical highs,
location of earth’s deserts. Known as horse latitudes
 Between the doldrums and the horse latitudes are the
easterly trade winds
– Warm and steady (speed and direction) winds
Chapter 19.3
Continental and Local Winds
Effects of Seasons and
continents
 Seasons, land masses and topography causes
winds to vary from global patterns depicted in the
3 cell model
 Hot air above land in summer creates lowpressure. Cold air above oceans in summer
creates high pressure
– e.g. Atlantic Highs bring winds off the ocean into
the eastern sea board. Winter is reversed
 Monsoons – winds caused by seasonal pressure
changes
Local winds – extent 100 km or less include
sea breeze, land breeze, mountain breeze
and valley breeze
 Sea – land breeze - During day land is
warmer creating low pressure above`
causes the cool air (H) to blowing from
the water (sea) and night it reverse
 Mountain Breeze – at night the air
against the mountain cools more than
the surround air and sinks down hill
 Valley breeze – mountain heats the air
causing it to rise and the air from the
valley flows up hill