Earth Science 19.1 Understanding Air Pressure
Understanding Air
Pressure
Earth Science 19.1 Understanding Air Pressure



Of the various elements of
weather and climate, changes in
air pressure are the least
noticeable.
When you listen to a weather
report, you listen to the
temperature, the precipitation,
and humidity.
Although you might not perceive
day-to-day or hour-to-hour
changes in air pressure, these
changes are very important in
producing changes in our weather.
Earth Science 19.1 Understanding Air Pressure




Variations in air pressure from
place to place can generate winds
that have hurricane force like the
ones at right.
Winds, in turn, bring changes in
temperature and humidity.
Air pressure is one of the most
basic of weather elements and is
major factor in predicting
weather.
Air pressure is closely tied to the
other elements of weather in a
cause-and-effect relationship.
Air Pressure Defined



Air pressure is simply the
pressure exerted by the weight
of the air above.
Average air pressure at sea level
is about 1 kilogram per square
centimeter.
This pressure is roughly the
same pressure that is produced
by a column of water 10 meters
in height.
Air Pressure Defined



You can calculate that the air
pressure exerted on a 50 cm by
100 cm school desk exceeds
5000 kilograms, which is about
the mass of a 50 passenger
school buses.
Why then doesn’t the desk
collapse under the weight of the
air above it.
Air pressure is exerted in all
directions; down, up and
sideways. The air pressure
pushing down on an object
balances against the air pressure
pushing up on the same object.
Air Pressure Defined



Imagine a tall aquarium has the
same dimensions as the desktop
we just mentioned in the
previous example.
When the aquarium is filled to a
height of 10 meters, the water
pressure at the bottom equals 1
atmosphere, or 1 kilogram per
square meter.
Now imagine what will happen if
you place this aquarium on top of
the desk. The desk collapses
downward because the pressure
downward is greater than the
pressure in all directions.
Air Pressure Defined


If you could place the desk
inside the aquarium however, the
desk would not collapse but sink
to the bottom because the water
pressure is exerted in all
directions on it, not just
downward.
The desk, like your body, is built
to withstand the pressure of 1
atmosphere.
Measuring Air Pressure




When meteorologists measure
atmospheric pressure, they use a
unit of measure called millibars.
Standard air pressure is 1013.2
millibars.
You might have heard the phrase
“inches of mercury” which is used
by weathermen to describe
atmospheric pressure.
This expression dates to 1643
when an Italian scientist named
Torricelli invented the mercury
barometer.
Measuring Air Pressure




A barometer is a device used for
measuring air pressure.
Torricelli correctly described
the atmosphere as a vast ocean
of air that exerts pressure on us
and all objects around us.
To measure this force, he filled
a glass tube, closed at one end,
with mercury. He then put the
tube, upside down, into a dish of
mercury as shown at right.
The mercury flowed out of the
tube until the weight of the air
pressure exerted on the surface
of the mercury was equal with
the weight of the mercury in the
column.
Measuring Air Pressure



As a result, when air pressure
increases, the mercury in the
tube rises.
When air pressure decreases,
the mercury in the tube goes
down.
With some modern
improvements, the mercury
barometer is still the standard
instrument used today for
measuring air pressure.
Measuring Air Pressure




The need for smaller and more
portable instruments for
measuring air pressure led to the
invention of the aneroid
barometer.
The aneroid barometer uses a
metal chamber with some air
removed.
This partially emptied chamber is
very sensitive to variations in air
pressure.
This chamber changes shape and
compresses as the air pressure is
increases, and it expands as the
air pressure decreases.
2 examples of aneroid barometers
Measuring Air Pressure


One advantage of an aneroid
barometer is that it can be easily
attached to a recording device.
This type of device provides a
continuous record of pressure
changes over time.
Factors Affecting Wind





As important as vertical motion is,
far more air moves horizontally, a
phenomena called wind. What
causes wind however?
Wind is the result of horizontal
differences in air pressure.
Air flows from areas of high
pressure to areas of lower
pressure.
You may have experienced this
flow of air when you open a
vacuum-packed can of coffee or
tennis balls.
The noise you hear is the air
rushing from the higher pressure
outside the can to the lower
pressure inside the can.
Factors Affecting Wind





The unequal heating of Earth’s
surface generates pressure
differences.
Solar energy is therefore the
ultimate source of energy for the
creation of wind.
If Earth did not rotate and their
were no friction what-so-ever
between moving air and earth’s
surface, air would flow in a
straight line from areas of high
pressure to areas of low pressure.
But both factors, Earth’s rotation
and friction, do exist and the flow
of air is therefore not so
straightforward.
Three factors combine to control
wind: pressure differences,
Coriolis effect, and friction.
Pressure Differences




Wind is created by
differences in pressure: the
greater these differences
are, the greater the wind
speed is.
Over Earth’s surface,
variations in air pressure are
determined from barometric
readings taken at hundreds of
weather stations.
These pressure data readings
are shown on a weather map
using isobars.
Isobars are lines on a map
that connect places of equal
air pressure.
Pressure Differences





The spacing of isobars indicates
the amount of pressure change
over a given distance.
These pressure changes are
expressed as the pressure
gradient.
A steep pressure gradient, like
a steep hill, causes great
acceleration of a parcel of air. A
less steep gradient causes a
slower acceleration.
Closely spaced bars indicate a
steep pressure gradient and
high winds.
Widely spaced bars indicate a
weak pressure gradient and
light winds.
Pressure Differences





The pressure gradient is the
driving force of wind.
The pressure gradient has both
magnitude (strength) and
direction.
It’s magnitude is reflected in
the spacing of the isobars. The
closer the spacing, the stronger
the winds.
The direction of force is always
from areas of high pressure to
areas of low pressure and at
right angles to the isobars.
Friction affects wind speed and
direction while Coriolis effect
affects wind direction only.
Coriolis Effect




The weather map at right
shows typical air movements
associated with high and low
pressure systems.
Air moves out of regions of
higher pressure and into the
regions of lower pressure.
However, wind does not cross
the isobars at right angles as
one would expect.
This change in direction
results from the rotation of
the earth and is called the
Coriolis effect.
Coriolis Effect



The Coriolis effect describes
how Earth’s rotation affects
moving objects.
All free-moving objects or
fluids, including the wind, are
deflected to the right of their
path of motion in the
Northern Hemisphere.
In the Southern hemisphere,
they are deflected to the left.
Coriolis Effect



Imagine the path of a rocket
launched from the North Pole
toward a target located at the
equator.
The true path of the rocket is
straight, however, in the time
it would take for the rocket to
fly from the North pole to the
equator, the Earth would have
rotated underneath the
rocket by 15 degrees. The
rocket arrives at a spot to the
right of where it was intended
because the Earth moved
under it while it flew.
The counterclockwise rotation
of the Northern hemisphere
causes this path to deflect.
Coriolis Effect


This apparent shift in
direction is attributed to the
Coriolis effect.
This deflection
 Is always directed at right
angles to the direction of
airflow
 Affects only wind direction
and not wind speed
 Is affected by wind speed; the
greater the wind speed the
greater the deflection
 Is strongest at the poles and
weakens toward the equator,
becoming nonexistant at the
equator
Friction




The effect of friction on wind is
important only within a few
kilometers of Earth’s surface.
Friction acts to slow air
movement, which changes wind
direction.
To illustrate friction’s effect on
wind direction, first think about a
situation in which friction does
not play a role in wind’s direction.
When air is above the friction
layer, the pressure gradient
moves across the isobars. As soon
as air starts to move, the Coriolis
effect acts at right angles to
motion. The faster the wind
speed, the greater the deflection.
Friction





The pressure gradient (PGF) and
Coriolis effect (CF) balance in highaltitude air, and wind generally
flows parallel to isobars.
The most prominent features of
airflow high above the friction layer
are the jet streams.
Jet streams are fast-moving rivers
of air near the tropopause that
travel between 120 and 240
kilometers per hour in a west-toeast direction.
One such jet stream is situated
over the polar front, which is the
zone separating cool polar air from
warm subtropical air.
Jet streams were first encountered
by high-flying bomber pilots during
World war II.
Friction





For air close to Earth’s surface ,
the roughness of the terrain
determines the angle of airflow
across the isobars.
Over the smooth ocean surface,
friction is low, and the angle of
airflow is small.
Over rugged terrain, where
friction is higher, winds move
more slowly and cross the isobars
at greater angles.
Friction causes winds to flow
across the isobars at angles as
great as 45 degrees.
Slower wind speeds caused by
friction decrease the Coriolis
effect.