3. +
Wind and Air Pressure
Wind is the result of horizontal differences
in atmospheric pressure.
Atmospheric (air) pressure is the force
exerted against a surface by continuous
collision of gas molecules.
Atmosphere is without walls but the force of
gravity prevents its escape.
6. +
Measuring Air Pressure
Air pressure is measured in millibars.
1 millibar (mb) = 100 newtons/m2
1013.25 mb = sea level pressure
The
mercury barometer is the standard pressure-
measuring instrument.
Air pressure pushes on a pool of mercury, which forces
the mercury up a tube.
High pressure: The mercury goes further up the
tube.
Low pressure: The mercury stays lower.
11. +
Pressure Changes with Altitude
Air pressure is defined as the force exerted
against a surface by continuous collision of
gas molecules.
Air pressure decreases with altitude.
Because of the decrease of pressure above you
Air pressure increases with depth.
Because of the increase of pressure above you
14. +
Why Does Air Pressure Vary?
Recall that variations in air pressure causes the
wind to blow, which in turn causes changes in
temperature and humidity
Water vapor affects air pressure.
The amount of water vapor reduces the density of
air.
Cold dry air has higher pressure.
Warm, dry air has higher pressure than equally
warm, moist air.
15. +
Why Does Air Pressure Vary?
The movement of air can also cause
variations in air pressure.
Convergence occurs when a net airflow into a
region causing pressure to rise.
Divergence results when there is a net outflow
of air from a region and surface pressure drops.
16. +
Why Does Air
Pressure
Vary?
Temperature
influences air
pressure.
19. +
Factors Affecting Wind- Pressure
Gradient Force
The force that generates winds results from
horizontal pressure differences. Greater
differences the greater the wind blows
Pressure gradient force
Spacing of the isobars indicate the amount of
pressure change occurring over a given
distance.
24. +
Factors Affecting Wind- Pressure
Gradient Force
Isobars on a surface chart:
Low-pressure systems (L) that occur in the
middle latitudes are called midlatitude cyclones.
Produce stormy weather
High-pressure systems (H) are called
anticyclones, which tend to be associated with
clearing conditions.
25. +
Factors Affecting Wind- Coriolis
Force
TheCoriolis force cannot generate wind; it
modifies airflow.
The Coriolis force causes all free flowing
objects including wind:
In the Northern Hemisphere, a deviation to the
right is due to the Earth’s rotation.
In the Southern Hemisphere, the deviation is to
the left.
26. +
Factors Affecting Wind- Coriolis
Force
The Coriolis force is a deflecting force.
It is always directed at right angles to the
direction of airflow.
It controls only wind direction, not speed.
It is affected by wind speed (the stronger the
wind, the greater the force).
The Coriolis force is strongest at the poles and
is nonexistent at the equator.
28. +
Factors Affecting Wind- Friction
Remember pressure gradient
force is the primary driving force
for winds. Air moves from
high to low concentrations
but wind speeds don’t
increase indefinitely.
Friction significantly influences airflow
near Earth’s surface, but its effect is
negligible at higher altitudes.
30. +
Winds Aloft
Geostrophic flow occurs when the pressure
gradient force and the Coriolis force are balanced.
Thewind flows parallel to the isobars.
They flow in relatively straight paths.
Wind Direction is directly related to prevailing
pressure patterns. So if we know direction we can
establish a rough approximation of the pressure
distribution.
33. +
Winds Aloft
Curved airflow
and the gradient
wind
Curved airflow:
Winds around cells
of high pressure or
low pressure follow
curved paths.
34. +
Winds Aloft
Gradient winds blow at a constant speed.
They are parallel to the curved isobars.
Centers of low pressure are called cyclonic; winds
flow counter-clockwise in the Northern Hemisphere.
A trough is the result of isobars curving to form
elongated regions of low pressure.
Centers of high pressure are anticyclonic; winds flow
clockwise in the Southern Hemisphere.
A ridge is the result of isobars curving to form
elongated regions of high pressure.
Opposite is true in the Southern Hemisphere
41. +
How Winds Generate Vertical Air Motion
Factors that promote vertical airflow:
Friction
Increased friction causes a drop in wind speed
resulting in a pileup of air upstream from the
ocean to land.
Convergence can result in cloudy weather.
Decreased friction causes and increase in wind
speed from the land to the ocean.
Subsidence and divergence results in clearing
weather.
42. +
How Winds Generate Vertical Air Motion
Vertical airflow is associated with cyclones
and anticyclones.
46. +
Wind Measurement
Wind direction is always measured from
the direction it is coming.
A North wind blows north to south.
A East wind blows east to west.
A prevailing wind consistently blows
more often than not from one direction.
48. +
Wind Measurement
A wind vane shows wind direction.
Wind speed is often measured with a cup
anemometer, which has a dial that shows
wind speed.
An aerovane is a combination of wind
vane and anemometer.