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MET 1010
Introduction to Weather
Dr. Arturo Rodriguez
Miami Dade College
North Campus
Air Pressure & Winds
Chapter 6
Air Pressure & Winds
Air Pressure & Winds
Atmospheric Pressure
Air moves in response to horizontal
differences in pressure.
Air pressure is simply the mass of air above
a given level.
Atmospheric pressure always decreases
with increasing height.
Air pressure, air density (the mass of air in a
given volume), and air temperature are all
Air Pressure & Winds
Measuring Air Pressure
Air pressure can be defined as the
force exerted by the air molecules over
a given area.
Instruments that detect and measure
pressure changes are called
barometers. In meteorology, the bar is
a unit of pressure that describes a
force over a given area.
Air Pressure & Winds
By definition, a bar is a force of
100,000 newtons acting on a surface
area of 1 square meter. A newton is
the amount of force required to move
an object with a mass of kilogram so
that it increases its speed at a rate of 1
meter per second each second.
Air Pressure & Winds
Because surface pressure changes are
normally small, the unit of pressure most
commonly found on surface weather maps
is the millibar (mb), where one millibar is
one-thousandth of a bar.
The unit of pressure designed by the
International System (SI) of measurement is
the pascal (Pa), where 1 pascal is the force
of 1 newton acting on a surface of 1 square
meter. A more common unit is the
hectopascal (hPa), which equals 1 millibar.
Air Pressure & Winds
A common pressure unit used on TV and
radio weather broadcasts is inches of
mercury. At sea level, standard
atmospheric pressure is
1013.25 mb=1013.25hPa=29.92in.Hg
Standard atmospheric pressure at sea
level is the pressure exerted by a column of
mercury 29.92 in. (760 mm) high, having a
density of 1.36 X 104 kg/m³, and subject to
an acceleration of gravity of 9.80 m/sec².
Air Pressure & Winds
Fig. 6.3 compares pressure readings in
millibars and inches of mercury.
Fig. 6-3, p.145
Air Pressure & Winds
Because we measure atmospheric
pressure with an instrument called
barometer, atmospheric pressure is
also referred to as barometric
Evangelista Torricelli invented the
mercury barometer in 1643.
Fig. 6-4, p.146
Air Pressure & Winds
The most common type of home
barometer is the aneroid barometer.
The altimeter and barograph are two
types of aneroid barometers.
Altimeters measure pressure, but are
calibrated to indicate altitude.
Barographs are recording aneroid
Fig. 6-5, p.146
Aneroid Barometer
 Uses a small, flexible metal box called an aneroid cell.
 The aneroid cell is made from an alloy of beryllium and
 The capsule is prevented from collapsing by a strong spring.
 Small changes in external air pressure cause the cell to expand
or contract.
 This expansion and contraction drives mechanical levers such
that the movements of the capsule are amplified and displayed
on the face of the aneroid barometer.
Aneroid Barometer
More Aneroid Barometers
Fig. 6-6, p.147
Air Pressure & Winds
Pressure Readings
Fig. 6.7 gives the station pressure
measured at four locations only a few
hundred kilometers apart. To properly
measure horizontal changes in
pressure, barometer readings must be
corrected for altitude. Altitude
corrections are made so that a barometer
reading taken at one elevation can be
compared with a barometer reading taken
at another.
Air Pressure & Winds
Station pressure observations are
normally adjusted to a level of mean
sea level – the level representing the
average surface of the ocean. The
adjusted reading is called sea-levelpressure. The size of the correction
depends primarily on how high the
station is above sea level.
Air Pressure & Winds
Near the earth’s surface, atmospheric
pressure decreases on the average by
about 10mb for every 100 m increase
in elevation (about 1 in. of Hg for each
1000 ft rise). This decrease in
atmospheric pressure with height
(10mb/100m) occurs when the air
temperature decreases at the standard
lapse rate of 6.5 C/1000 m.
Fig. 6-7, p.147
Air Pressure & Winds
Notice in Fig. 6.7 the isobars, lines
connecting points of equal pressure. Drawn
as solid dark lines at intervals of 4 mb, with
1000 mb being the base value.
With its isobars, the bottom chart (Fig. 6.7c)
is now called a sea-level-pressure chart,
or simply a surface map. When weather
data are plotted on the map, it becomes a
surface weather map.
Air Pressure & Winds
Surface and Upper-Air Charts
Fig. 6.8a is a simplified surface map
that shows areas of high and low
pressure and arrows that indicate wind
direction – the direction from which
the wind is blowing.
The large blue H’s on the map indicate
the centers of high pressure, which
are also called anticyclones.
Air Pressure & Winds
The large red L’s represent centers of
low pressure, also known as
depressions, mid-latitude cyclones ,
or extra-tropical cyclones because
they form in the middle latitudes,
outside the tropics.
The solid dark lines are isobars with
units in millibars.
Fig. 6-8, p.148
Air Pressure & Winds
Notice that the surface winds tend to
blow across the isobars toward
regions of low pressure. In the
Northern Hemisphere the winds blow
counterclockwise and inward
toward the center of the lows and
clockwise and outward form the
center of the highs.
Air Pressure & Winds
Fig. 6.8b shows an upper-air chart for the
same day as the surface map in Fig. 6.8a.
The upper-air-map is a constant pressure
chart because it is constructed to show
height variations along a constant pressure
(isobaric) surface, which is why these maps
are also known as isobaric maps. This
particular isobaric map shows height
variations at a constant level of 500 mb
(which is about 5600 m or 18,000 ft above
sea level). Hence, this map is called a 500millibar map.
Air Pressure & Winds
The solid dark lines on the map are contour
lines – lines that connect points of equal
elevation above sea level. Although contour
lines are height lines, they illustrate pressure
much like isobars do. Consequently,
contour lines of low height represent a
region of lower pressure, and contour
lines of high height represent a region of
higher pressure. See Focus Section on
page 149.
Air Pressure & Winds
Notice on the 500-mb map (Fig. 6.8b) that
the contour lines typically decrease in value
from south to north. The reason for this fact
is illustrated by the dashed red lines, which
are isotherms – lines of equal
temperature. Observe that colder air is
generally to the north and warmer air to the
south, and recall that cold air aloft is
associated with low pressure, warm air
aloft with high pressure.
Air Pressure & Winds
The contour lines are not straight, however,
they bend and turn, indicating ridges
(elongated highs) where the air is warmer
and indicating depressions or troughs
(elongated lows) where the air is colder.
The arrows on the 500-mb map show the
wind direction. Unlike the surface winds
that cross the isobars, the winds in the 500mb chart tend to flow parallel to the contour
lines in a wavy west-to-east direction.
Surface and upper-air charts are valuable
tools for the meteorologist.
Air Pressure & Winds
Surface maps describe where the centers
of high and low pressure are found, as well
as the winds and weather associated with
these systems.
Upper-air charts are extremely important in
forecasting the weather. The upper-level
winds not only determine the movement of
surface pressure systems, but they
determine whether these surface systems
will intensify or weaken.