MET 10 Lecture 7
Air Pressure and Winds
Chapter 6
Dr. Craig Clements
San Jose State University
Hydrologic Cycle
Atmospheric Pressure
Air pressure is simply the mass of air above a given level.
As we climb in elevation there are fewer air molecules above
us; atmospheric pressure decreases with height.
What causes air pressure to change in the horizontal?
Why does air pressure change at the surface?
A simplified model
Dots represent air molecules
We assume that:
1. Air molecules are not crowded
close to the surface
2. Air density remains constant from
surface to top of column.
3. Width of column does not change
4. Air is unable to freely move into or
out of the column.
Two air columns, each identical mass, have same surface pressure.
Important concept:
Warm air aloft is normally associated with high atmospheric
pressure, and cold air aloft is associated with low pressure.
Becoming more stable
Horizontal difference in temperature creates
a horizontal difference in pressure. The
pressure difference establishes a force
(called Pressure Gradient Force) that
causes the air to move from higher pressure
toward lower pressure.
Air above a region of surface high pressure is more dense than air above a
region of surface low pressure (at the same temperature). (The dots in each
column represent air molecules.)
A barometer is an instrument
that measures atmospheric
pressure.
The mercury barometer.
The height of the mercury
column is a measure of
atmospheric pressure.
Sea-level pressure vs. station pressure
The barometer reading at a particular location is called
station pressure. This is the pressure that has been
corrected for temperature, gravity, and instrument error.
Since pressure varies with altitude, the pressure at stations
of different altitudes must be corrected in order to compare
them. This adjusted pressure is called sea-level pressure.
The top diagram (a) shows
four cities (A, B, C, and D)
at varying elevations above
sea level, all with different
station pressures.
The middle diagram (b)
represents sea-level
pressures of the four cities
plotted on a sea-level
chart.
The bottom diagram (c)
shows isobars drawn on
the chart (dark lines) at
intervals of 4 millibars.
Surface and Upper-Air Charts
Isobaric maps: maps of constant pressure are constructed to
show height variations along a constant pressure surface
(isobaric surface).
Lines of constant pressure = isobars
Contour lines– lines that connect points of equal elevation
above sea level.
Lines of low height = region of low pressure
Lines of high height = region of high pressure
Lines of constant temperature = isotherms
Surface and Upper-Air Charts
What does upper-air mean?
At what level do we consider upper-level?
Typically, we discuss surface pressures and upper-air.
when we refer to upper-air levels we use pressure as our
height coordinate.
For example, common levels we use:
850 mb = ~5,000 ft above sea level.
700 mb = ~10,000 ft above sea level
500 mb = ~16,000 ft above sea level (5000 m)
300 mb = ~29,000 ft above sea level (9000 m)
The 300 mb level is the jet stream level.
Areas of low pressure (L) and high pressure (H) are shown.
Arrows indicate wind direction– the direction from which the
wind is blowing.
Areas of high pressure are also called anti-cyclones.
Ridge
• When the height contours bend strongly to the north,
this is known as a Ridge.
• Strong ridges are accompanied by warm and dry
weather conditions at the surface.
Trough
• When the height contours bend strongly to the south,
(as in the diagram below), it is called a trough.
• Strong troughs are typically preceded by stormy
weather and colder air at the surface.
Why the wind blows
Newton’s Laws of Motion.
Newton’s first law of motion states that an object at rest
will remain at rest and object in motion will remain in
in motion as long as no force is exerted on the object.
Newton’s second law of motion states:
that the force exerted on an object = its mass times the
acceleration produced.
F= ma
To determine which direction the wind will blow we must
identify and examine all the forces that affect the
horizontal movement of air.
Why the wind blows
To determine which direction the wind will blow
we must identify and examine all the forces
that affect the horizontal movement of air.
These forces include:
1.
2.
3.
4.
Pressure gradient force
Coriolis force
Centripetal force
Friction
Pressure Gradient Force
When differences in horizontal air pressure exist there is a
net force acting on the air.
This force is the Pressure Gradient Force (PGF) and is
directed from higher toward lower pressure at right angles
to the isobars.
The magnitude of the force is directly related to the pressure
gradient. Steep pressure gradients correspond to strong
forces.
Pressure Gradient is the change of pressure over a given
distance:
Pressure Gradient = pressure difference / distance.
Closer the isobars = steeper the gradient = stronger force
What is the PGF between P1 and P2?
4 mb per 100 km
Coriolis Force
Is an apparent force that is due to the rotation of the earth.
The Coriolis force causes the wind to deflect to the right of
its intended path in the Northern Hemisphere and to the
left of its path in the Southern Hemisphere.
The amount of deflection due to the Coriolis force depends
upon:
1. The rotation of the earth
2. The latitude
3. The object’s speed
The Coriolis force acts at right angles to the wind, and only
influences wind direction not speed.
Except at the equator, a free-moving 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 Coriolis force “behaves” as a real force, constantly tending
to “pull” the wind.
When isobars are widely spaced, the flow is weak; when they
are narrowly spaced, the flow is stronger. The increase in
winds on the chart results in a stronger Coriolis force (CF ),
which balances a larger pressure gradient force (PGF ).
Winds around Lows and Highs
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.
Curved Winds around Lows and Highs
A wind blowing at a constant speed, but parallel to curved
isobars above the level of surface friction is termed:
gradient wind.
A gradient wind blowing around a a low-pressure center is
constantly accelerating because it is constantly changing
direction. This acceleration is called centripetal acceleration.
Centripetal acceleration is directed at right angles to the wind, inward toward
the low center.
The net force acting on the wind must be directed toward the center of the low
in order for the air to keep moving in a counterclockwise, circular path.
This inward-directed force is called centripetal force: imbalance between
Coriolis and PGF.
Winds around Lows and Highs
centripetal force
centripetal force: inward-directed force caused by an
imbalance between the Coriolis force and PGF.
An upper-level 500-mb map showing wind
direction, indicated by lines that parallel the wind.
Surface Winds
Winds on a surface weather map do not blow exactly parallel to the
isobars; instead they cross the isobars moving higher to lower pressure.
The angle at which the wind crosses the isobars varies, but averages 30°.
The frictional drag of the ground slows the wind down.
Wind speeds increase with height above the ground due to lack of friction.
Force of Surface Friction
The effect of surface friction is to slow down the wind so that,
near the ground, the wind crosses the isobars and blows
toward lower pressure.
This phenomenon at the surface produces an outflow of air
around a high. Aloft, the winds blow parallel to the lines,
usually in a wavy west-to-east pattern.
Estimating Wind Direction and Pressure Aloft by Watching Clouds
Upper-level clouds moving from the southwest indicate isobars and winds
aloft.
When extended horizontally, the upper-level chart appears as in (c), where
lower pressure is to the northwest and higher pressure is to the southeast.
Winds in the Southern Hemisphere blow around Highs and Lows
opposite in direction than in the Northern Hemisphere.
Winds and Vertical Air Motion
Hydrostatic Balance
Air does not rush off into space because the upwarddirected pressure gradient force is nearly always
exactly balanced by the downward force of gravity.
When these two forces are in exact balance, the air
is said to be in hydrostatic equilibrium or balance.
When air is in hydrostatic balance, there is no net
vertical force acting on it– no net vertical
acceleration.
Determining Winds
Wind is characterized by its direction, speed, and gustiness.
An onshore wind blows from water to land.
An offshore wind blows from land to water.
A prevailing wind is the name given to wind direction most
often observed during a given time period at a given location.
Wind direction can be expressed in degrees
about a circle or as compass points.
These trees standing unprotected from the wind are often sculpted into “flag” trees.
A wind farm near Tehachapi Pass, California, generates electricity
that is sold to Southern California