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Pressure Systems
What is air pressure?
 It is caused by the weight of all the air in
the atmosphere pushing down on Earth.
 It is also known as atmospheric pressure.
 Air pressure changes with the height and
also when air warms up or cools down.
 Changes in air pressure cause changes
in weather.
Differences in air pressure
 Low Pressure
 When air rises, it leaves behind an area of
lower pressure, because the upward-moving
air is not pressing down so hard on the
Differences in air pressure
 High Pressure
 Areas of high pressure are formed where air
is sinking back down, and so pushing down
Differences in air pressure
Differences in air pressure
 Since there are many areas of high and
low pressure above the Earth’s surface
due to uneven surface heating:
 Air moves from high pressure to low
pressure forming winds.
 As a result, the greater the difference between
high pressure and low pressure areas, the
greater the wind speed is.
Highs and Lows
 Pressure is different
all over the world.
 Lows are areas of low
pressure with the
lowest pressure at the
 Lows usually bring
wet, cloudy weather.
 Highs are areas of
high pressure with
the highest pressure
at the center.
 Highs bring dry,
sunnier weather.
 The ways that these
pressures change
from day to day
affects the weather
Measuring Air Pressure
 Air pressure is measured in millibars
(mb) on a barometer.
 The simplest kind of barometer is a
mercury barometer and pressure is
measured in (inches) of mercury
 Air exerts a pressure although we are not
aware of it.
 Pressure is a force, or weight, exerted on a
surface per unit area and is measured in
Pascals (Pa)
 Atmospheric pressure is measured using
a barometer
 These usually measure in millibars (mb)
 1 mb= 100 Pa
 A mercury barometer measures the
pressure by noting the length of mercury
which is supported by the weight of the
 1 cm of mercury is equal to 13.33 mb
 An aneroid barometer is a more compact
way to measure atmospheric pressure.
 Air blows from areas of high pressure to
regions of low atmospheric pressure.
 A pressure system is a region of the Earth’s
atmosphere where air pressure is unusually
high or low.
 High and low pressure systems form (and die)
constantly due to thermodynamic interactions
of the atmosphere and water
High Pressure Systems
 In a high-pressure system, air pressure is
greater than the surrounding areas.
 This difference in air pressure results in
wind or moving air.
 In a high pressure area, air is denser
than in areas of lower pressure.
 The result is that air will move from the highpressure area to an area of lower density, or
lower pressure
High Pressure Systems
 High pressure systems are associated
with clear, cool weather
 Around high pressure systems, winds
flow clockwise in the northern
hemisphere and counterclockwise in the
southern hemisphere.
 Air moving from high pressure to low
pressure follows a spiraling route due to the
rotation of the Earth beneath the moving air.
Low Pressure Systems
 Receives winds from high pressure
 As winds blow in, the air can be uplifted
 The components of storms are attracted
to regions of low pressure
 For this reason, heavy precipitation and
overcast conditions are often associated
with low-pressure systems.
Pressure Systems on
 On weather maps, low-pressure systems
are shown with a capital L, unless they
are tropical storms or tropical cyclones
 In these cases, the symbols for the weather
conditions would be used.
 High-pressure systems are shown with
an H
Pressure Systems on
 Isobar Maps
 Show where pressures are relatively high
and low and show gradual or dramatic
changes in pressure over a distance.
 Isobar
 A line that connects locations at equal