Transcript Air Pressure and Wind

Air Pressure and
Winds
Atmospheric Pressure
What causes air pressure to change in
the horizontal?
 Why does the air pressure change at the
surface?

Atmospheric Pressure

Horizontal Pressure Variations
 It takes a shorter column of dense, cold
air to exert the same pressure as a taller
column of less dense, warm air
 Warm air aloft is normally associated with
high atmospheric pressure
 Cold air aloft with low atmospheric
pressure
 At surface, horizontal difference in
temperature = horizontal pressure in
pressure = wind
 Basically the same thing happens above
the surface … in the winds aloft
Two air columns,
each with
identical mass,
have the same
surface air
pressure.
Since colder air is more
dense, and takes up less
space… it takes a
shorter column of cold air
to exert the same
pressure as a taller
column of warm air…
So… as column 1 cools,
it shrinks, and as column
2 warms, it must expand.
What happens above
these columns of air?
The pressure differences
aloft causes the air to move
from higher pressure
toward lower pressure.
At the same level in the atmosphere there
is more air above the H in the warm
column than above the L in the cold
column. Warm air aloft is associated with
high pressure and cold air aloft with low
pressure.
As air moves from the top
of column 2 toward the top
of column 1, the pressure
at the surface drops,
And as air moves into
column 1, the surface
pressure rises.
H
L
Surface level charts
are modified to
reflect atmospheric
pressures AS IF
they were at sea
level (approximate
adjustment: 10mb
per 100 meters.
a) Pressure at 4
cities.
b) Pressure
modified to one
level (sea level)
c) Shows isobars
based on one
level
Sea-level isobars drawn so that each
observation is taken into account.
Not all observations are plotted.
Sea-level isobars after smoothing.
The area shaded
gray in the above
diagram
represents a
surface of
constant
pressure, or
isobaric surface.
Because of the
changes in air
density, the
isobaric surface
rises in warm,
less dense air and
lowers in cold,
more-dense air.
Where the
horizontal temperature changes
most quickly, the
isobaric surface
changes elevation
most rapidly.
Changes in elevation of an isobaric surface (500 mb) show up as contour lines on an
isobaric (500 mb) map. Where the surface dips most rapidly, the lines are closer
together (steep gradient – think of topographic maps)
The wavelike patterns of an isobaric surface reflect the changes in air temperature. An
elongated region of warm air aloft shows up on an isobaric map as higher heights and
a ridge; the colder air shows as lower heights and a trough.
Surface map showing areas of high
and low pressure. Notice that the wind
blows across the isobars.
The upper-level (500-mb) map for the
same day as the surface map. Notice that,
on this upper-air map, the wind blows
parallel to the contour lines.
Surface and Upper Level
Charts

Observation: Constant Pressure Surface
 Pressure altimeter in an airplane causes
path along constant pressure not elevation
 May cause sudden drop in elevation
 Radio altimeter offers constant elevation
Newton’s Law of Motion
AN object at rest will remain at rest and
an object in motion will remain in motion
as long as no force is executed on the
object.
 The force exerted on an object equals
its mass times the acceleration
produced.

 Acceleration: speeding up, slowing down,
change of direction of an object.
Forces that Influence Winds

Pressure Gradient Force: difference in
pressure over distance
 Directed almost perpendicular to isobars
from high to low.
 Large change in pressure over short
distance is a strong pressure gradient and
vice versa.
 The force that causes the wind to blow.
The pressure gradient between point 1 and point 2 is 4 mb per 100 km. The net force
directed from higher toward lower pressure is the pressure gradient force.
The closer the spacing of the isobars, the greater the pressure
gradient.
The greater the pressure gradient, the stronger the pressure gradient
force (PGF).
The stronger the PGF, the greater the wind speed.
Forces that Influence Winds

Coriolis Force (Coriolis Effect)
 Apparent deflection due to rotation of the Earth
 Right in northern hemisphere and left in southern




hemisphere
Stronger wind = greater deflection
No Coriolis effect at the equator greatest at poles.
Only influence direction, not speed
Only has significant impact over long distances
Except at the equator, an object heading either east or west (or any other direction)
will appear from the earth to deviate from its path (as the earth rotates beneath it.)
The deviation (Coriolis force) is greatest at the poles and decreases to zero at the
equator.
The effect of surface
friction slows down the
wind so that, near the
ground, the wind crosses
the isobars and blows
toward lower pressure.
Air at the surface
moves from high to
low pressure == wind
(pressure gradient
force - PGF)
Above the level of
friction, air (wind) will
change it’s direction,
due to the Coriolis
Effect, until it
balances with the
PGF and flows
parallel to the isobars
at a steady speed.
Wind blowing under
these conditions is
called geostrophic.
Forces that Influence Winds

Geostrophic Winds
 Earth turning winds (turned by the earth’s
rotation)
 Travel parallel to isobars
 Spacing of isobars indicates speed; close =
fast, spread out = slow
The isobars and contours on an upper-level chart are like the banks along a flowing
stream. When they are widely spaced, the flow is weak; when they are narrowly spaced,
the flow is stronger.
The increase in winds results in a stronger Coriolis force (CF), which balances a larger
pressure gradient force (PGF).
By observing the orientation and spacing of the isobars (or contours) in diagram (a), the
geostrophic wind direction and speed can be determined
Forces that Influence Winds

Gradient Winds Aloft
 Cyclonic: counterclockwise
 Anticyclonic: clockwise
 Gradient wind parallel to curved isobars

Observation: Estimates Aloft
 Clouds indicate direction of winds,
 Allow locating pressure system -- consistent
with cloud location.
Winds and related forces around areas of low and high pressure above the friction level
in the Northern Hemisphere. Notice that the pressure gradient force (PGF) is in red,
while the Coriolis force (CF) is in blue.
Clouds and related
wind-flow patterns
(black arrows)
around low-pressure
areas. In the
Northern
Hemisphere, winds
blow counterclockwise around an
area of low pressure.
In the Southern
Hemisphere, winds
blow clockwise
around an area of
low pressure.
An upper-level 500-mb map showing wind direction, as indicated by lines that
parallel the wind. Wind speeds are indicated by barbs and flags.
- Solid gray lines are contours in meters above sea level.
- Dashed red lines are isotherms in °C.
Stepped Art
Forces that Influence Winds

Winds on Upper-level Charts
 Winds parallel to contour lines and flow west
to east
 Heights decrease from north to south

Surface Winds
 Friction reduces the wind speed which in
turn decrease the Coriolis effect.
 Winds cross the isobars at about 30° into
low pressure and out of high pressure
Winds and air motions associated with surface highs and lows in the Northern
Hemisphere.
(Replacement of lateral spreading of air results in the rise of air over a low pressure and
sinking over high pressure)