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Guido Cervone EOS 121 Lecture 4


of pressure is the mercury barometer. invented by Evangelista Torricelli in 1643. ... of the column of mercury in a barometer, which at sea level averages 76 cm ... – PowerPoint PPT presentation

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Title: Guido Cervone EOS 121 Lecture 4

Chapter IV
Guido CervoneEOS 121 Lecture 4
The atmosphere contains a tremendous number of
gas molecules being pulled toward Earth by the
force of gravity. These molecules exert a force
on all surfaces with which they are in contact,
and the amount of that force exerted per unit of
surface area is pressure.
The standard unit of pressure is the pascal
(Pa). Meteorologists in the U.S. use the
millibar (mb), which equals 100 Pa. Canadian
meteorologists use the kilopascal (kPa), equal
to 1000 Pa, or 10 mb. Air pressure at sea level
is roughly 1000 mb (100 kPa) or more precisely,
1013.2 mb.
The enclosed air molecules move about continually
and exert a pressure on the interior walls of the
container (a). Pressure can increase by
increasing the density of the molecules (b) or
increasing the temperature (c). If the air in
the container is a mixture of gases, each gas
exerts its own specific amount of pressure,
referred to as its partial pressure. The total
pressure exerted is equal to the sum of the
partial pressures. This relationship is known as
Daltons law.
Surface pressure is the pressure actually
observed at a particular location, whereas sea
level pressure is the pressure that would exist
if the observation point were at sea level. Sea
level pressure allows us to compare pressure at
different locations taking into account
differences in elevation. To correct for
elevation, add 1 mb per 10 meters. For
high-elevation sites, this method is unreliable
because we must account for compressibility of
the atmosphere.
Pressure does not decrease at a constant rate.
It decreases most rapidly at low elevations and
at greater heights. Pressure decreases with
altitude by about half for each 5.5 km.
The Equation of State (Ideal Gas Law) p
?RT where p is pressure expressed in pascals, ?
(rho) is density in kilograms, R is a constant
equal to 287 joules per kilogram per kelvin, T is
temperature in kelvins. The equation tells us if
the air density increases while temperature is
held constant, the pressure will increase, and at
constant density, an increase in temperature
leads to an increase in pressure.
The standard instrument for the measurement of
pressure is the mercury barometer invented by
Evangelista Torricelli in 1643. Barometric
pressure is often expressed as the height of the
column of mercury in a barometer, which at sea
level averages 76 cm (29.92 in). To convert
barometric heights to millibars 1 cm 13.32
mb 1 inch 33.865 mb
An alternative instrument for the observation of
pressure is the aneroid (without liquid)
barometer which contains a collapsible chamber
from which some of the air has been removed. The
weight of the atmosphere presses on the chamber
and compresses it by an amount proportional to
the air pressure. Aneroid devices that plot
continuous values of pressure over extended
periods are called barographs.
An isobar is a line that connects points having
exactly the same sea level pressure drawn at
intervals of 4 mb on surface weather maps. The
spacing of the isobars indicates the strength of
the pressure gradient, or rate of change in
pressure. A dense clustering of isobars
indicates a steep pressure gradient (a rapid
change in pressure with distance), while widely
spaced isobars indicate a weak gradient.
A weather map showing the distribution of sea
level air pressure. The pressure is relatively
low over the northeastern U.S. and eastern
Canada, and the highest and lowest pressure on
the map are only within about 4 percent of each
If the air over one region exerts a greater
pressure than the air over an adjacent area, the
higher-pressure air will spread out toward the
zone of lower pressure as wind. The pressure
gradient gives rise to the pressure gradient
force, which sets the air in motion. For pressure
gradients measured at constant altitude, we use
the term horizontal pressure gradient
force. Everything else being equal, the greater
the pressure gradient force, the greater the
wind speed.
The vertical pressure gradient force and the
force of gravity are normally of nearly equal
value and operate in opposite directions, a
situation called hydrostatic equilibrium.
The Hydrostatic Equation ?p
-? g
where ?p refers to a change in pressure, ?z
refers to a change in altitude, and -? g refers
to density and the acceleration of gravity.
Two columns of air with equal temperatures,
pressures, and densities (a). Heating the column
on the right (b) causes it to expand upward. It
still contains the same amount of mass, but it
has a lower density to compensate for its greater
height. Because the pressure difference between
the base and top is still 500 mb, the vertical
pressure gradient is smaller.
The gradual poleward decrease in mean temperature
results in denser air occurring at high
latitudes. As indicated by the hydrostatic
equation, pressure drops more rapidly with height
at high latitudes and lowers the height of
the 500 mb level. The dashed lines depict the
height of the 500 mb level as they would be drawn
on a 500 mb weather map.
A 500 mb map with height contours labeled in
decameters ranging from 5880 m in the south to
5220 m in the extreme northwest. Contours for 500
mb maps are drawn at 60 m intervals. These maps
depict the varying heights of pressure
levels. Where height contours are close, the
pressure gradient force is large.
The rotation of Earth gives rise to the Coriolis
force which causes an apparent deflection
(turning) of the wind to the right in the
Northern Hemisphere and to the left in the
Southern Hemisphere. The Coriolis force is zero
at the equator and increases to a maximum at the
poles. The Coriolis force acting on any moving
object increases with the objects
speed. However, the force changes only the
direction of a moving object, never its speed.
The other factor that influences the movement of
air is friction. Air in contact with the surface
experiences frictional drag, which decreases wind
speed. Friction is important within the lowest
1.5 km of the atmosphere (planetary boundary
layer). Air in the free atmosphere, above 1.5
km, experiences negligible friction.
The Equation of Motion ?v / ?t Fp Fc
Ff where Fp stands for pressure gradient, Fc
stands for the Coriolis effect, and Ff stands
for friction. The equation of motion says the
acceleration of a mass of air is the sum of the
accelerations of these three forces.
A stationary parcel of air in the upper
atmosphere subjected to a south-to- north
pressure gradient force (a). If the parcel is
tethered to an imaginary pole, no movement of the
parcel can take place. Once the imaginary cord is
cut, the horizontal pressure gradient accelerates
the parcel northward (b). Initially, when the
wind speed is low, the Coriolis force is small.
As the parcel speeds up, the strength of
the Coriolis force increases and causes greater
displacement to the right (c). The wind speed
increases the Coriolis force sufficiently to
cause the air to flow perpendicular to the
pressure gradient force. The air flow
becomes unaccelerated, with unchanging speed and
direction known as geostrophic flow (or
geostrophic wind).
In common pressure distributions the height
contours curve and assume varying distances from
one another. In the absence of friction, the air
flows parallel to the contours constantly
changing direction and therefore undergoing an
acceleration. In order for the air to follow the
contours, there must be a continual mismatch
between the pressure gradient and Coriolis
forces. This movement is known as gradient flow
(or gradient wind).
Supergeostropic flow (a) occurs in the upper
atmosphere around high-pressure systems. As the
air flows, it is constantly turning to its right.
This turning motion occurs because the Coriolis
force has a greater magnitude than the pressure
gradient force (as represented by the length of
the dashed arrows). Observe the changing
direction of the four solid arrows 1 through 4.
Subgeostrophic flow (b) occurs in the upper
atmosphere around low-pressure systems. The
pressure gradient force is greater than
the Coriolis force and the air turns to its left
in the Northern Hemisphere.
Geostrophic flow cannot exist near the surface.
Friction slows the wind, so that the Coriolis
force is less than the pressure gradient force.
The air flows at an angle to the right of the
pressure gradient force in the Northern
Hemisphere (a) and to the left in the Southern
Hemisphere (b).
Enclosed areas of high pressure marked by roughly
circular isobars or height contours are called
anticyclones. The wind rotates clockwise around
anticyclones in the Northern Hemisphere, as the
Coriolis force deflects the air to the right and
the pressure gradient force directs it outward.
In the boundary layer, the air spirals out of
anticyclones (a), while in the upper atmosphere
it flows parallel to the height contours (b). In
the Southern Hemisphere, the flow is
counterclockwise (c) and (d).
Closed low-pressure systems are called cyclones.
Air spirals counterclockwise into
surface cyclones in the Northern Hemisphere (a)
and rotates counterclockwise around an
upper-level low (b). The flow is reversed in the
Southern Hemisphere (c) and (d).
Elongated zones of high and low pressure are
called ridges (a) and troughs (b), respectively.
Direction is always given as that from which the
wind blows, so that a westerly wind is one from
the west. It is often expressed by its azimuth,
the degree of angle from due north, moving
clockwise. A simple device for observing wind
direction is the wind vane. Wind speeds are
measured with anemometers that have rotating cups
mounted on a moving shaft. Looking like an
airplane without wings (right), an aerovane
indicates both wind direction and speed.
The next chapter examines atmospheric moisture.
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