Chapter 19 The Atmosphere in Motion
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
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
Continental and Local Winds
Effects of Seasons and continents
Seasons, land masses and topography causes winds to
vary from global patterns depicted in the 3cell model
Hot air above land in summer creates low-pressure.
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