Download pressure gradient force

Part 1. Energy and Mass
Chapter 4.
Atmospheric Pressure and Wind
Introduction
Pressure = Force per unit area
Gases exert equal pressure in all directions
Average atmospheric pressure is controlled by the
“weight” of overlying air--it decreases with height
• Average sea level air pressure is 1013.25 mb
Air pressure changes depending on the air
density and temperature
Dalton’s Law: Where several different gases are
mixed, the total gas pressure is equal to the sum of the
partial pressures of the individual gases
Air pressure is less at a higher elevation
(p2) than at a lower elevation (p1)
Gravity is always trying to pull the air downward toward the
Earth’s surface
Air pressure
decreases with
elevation according
to this curve
Meteorologists use
air pressure as a
measure of
elevation in the
atmosphere (i.e.,
the 500 mb level or
the 200 mb level)
The Equation of State (Ideal Gas Law)
For a gas, the following measurable parameters
are inter-related:
• Pressure
• Temperature
• Density
Changes in air pressure occur with changes in
air temperature or density (or both)
Molecular movement in a
sealed container
Pressure increases by increasing density (b)
or temperature (c)
Aneroid barometer (left)
and its workings (right)
A barograph
continually
records air pressure
through time
The distribution of air pressure is important
for determining weather patterns
Air always tries to move from higher pressure
to lower pressure
The greater the pressure difference between
high and low pressure, the greater the force
trying to move the air
Isobars = lines of equal air pressure
• Pressure gradient = change in pressure with
distance
• Steep pressure gradients are represented by closely
spaced isobars
Sea level air pressure depicted on a weather map
Low
pressure
gradient
area
(calm)
High
pressure
gradient
area
(windy)
Air pressure measurements made at high elevations must be corrected to
give the air pressure at sea level
Pressure Gradient Force
Initiates air motion
– High to lower pressure
– Wind speed reflects gradient
Horizontal Pressure Gradients
Usually small across large spatial scales
Vertical Pressure Gradients
Usually greater than horizontal gradients
• Pressure always decreases with altitude
Hydrostatic Equilibrium = Force of gravity
balances vertical air pressure gradient
• Local imbalances in hydrostatic equilibrium
cause updrafts and downdrafts
Heating causes a
density decrease in
a column of air
Both air
columns are
at the same
temperature
All columns have
the same total mass
Warmer air has
lower density and
therefore greater
column height
The air in
the right
column is
warmer than
the air in the
left column
500 mb height contours for May 3, 1995
Upper air
pressure
maps depict
the height to
the specific
air pressure
level (such
as the height
to the 500
mb air
pressure
level)
Lines of
equal
elevation
500 mb
elevation
5280 m
500 mb
elevation
5880 m
Upper air heights decrease with latitude
Colder air in south, 500 mb
level at lower elevation
Warmer air in south, 500 mb
level at higher elevation
Forces that Affect the Speed and Direction
of Winds
1) The Pressure Gradient Force (pgf): Air tries
to move from areas of high pressure to areas of
low pressure; a larger pressure gradient gives a
larger pgf and faster winds
2) The Coriolis Force
Free-moving objects are affected by the Earth’s
rotation; the coriolis force causes an apparent
deflection to the right in the northern
hemisphere and to the left in the southern
hemisphere
• The coriolis force is greater at high latitudes
than at low latitudes
• The faster the air is moving, the greater the
coriolis force on the air
Coriolis Deflection
3) The Friction Force
The friction force acts in the opposite direction
from the direction of movement of the air; it
acts to slow the air movement
• Air friction if greatest near the Earth’s
surface
• Above an elevation of 1.5 km (1500 m or
about 4500 ft), air friction is negligible
Winds in the Upper Atmosphere are affected
by only the pressure gradient force and the
Coriolis force
When the pressure gradient force balances with
the Coriolis force, the result is the geostrophic
wind (parallel to the isobars)
Free Atmosphere (no friction) Pressure Gradient
This plot shows the direction of the pressure gradient force at the
500 mb level. The pgf is always perpendicular to the isobars.
Geostrophic Flow Development
(a) Air
particle
starts
moving
(c) Faster
air
movement
results in a
larger
Coriolis
force
(b) As air
starts
moving, it
starts being
affected by
the Coriolis
force
(d) When
the pgf and
the Coriolis
force
become
equal and
opposite, the
geostrophic
wind results
If the pgf and Coriolis forces never balance,
Supergeostrophic and Subgeostrophic Flow
results
Supergeostrophic and subgeostrophic flows
follow curved air pressure contours
• Supergeostrophic flow occurs in ridges
• Subgeostrophic flow occurs in troughs
These flows are called Gradient flows
High pressure “ridge”
Supergeostrophic flow
Low pressure “trough”
Subgeostrophic flow
Gradient Wind
Cyclones, Anticyclones, Troughs, and Ridges
High pressure areas (anticyclones)
• Clockwise motion in northern hemisphere
• Descending air
• Clear skies
Low pressure areas (cyclones)
• Counterclockwise motion in northern
hemisphere
• Ascending air
• Clouds
Upper atmosphere
• Ridges = surface anticyclones
• Troughs = surface cyclones
Due to friction, near
surface air crosses
isobars at an angle
Northern and Southern
Hemisphere
anticyclonic air
patterns
Northern and Southern
Hemisphere cyclonic
air patterns
Ridges and troughs in the northern hemisphere
Maps depicting troughs, ridges, cyclones, and anticyclones
Measuring Wind
Wind direction indicates direction from which
wind blows
Azimuth = degree of angle from 0 to 360o
Wind vanes indicate wind direction
Anemometers record wind speed
Aerovanes indicate wind speed and direction
An azimuth
An aerovane