Download Some Meteorological Background

No.5
Some Meteorological Background
Atmospheric Pressure:
Air
flux
Cold
air
Warm
air
Cold
air
Warm
air
Atmospheric pressure represents the air above a given level. In warm air molecules move
faster and spread further apart, which means that air becomes less dense and an
imaginary column rises. In cold air molecules become slower and crowd closer together.
The air becomes more dense and an imaginary column would shrink. For the same
pressure on the earth surface a shorter column height of cold air is required as of warm
air. Therefore atmospheric pressure decreases faster with height in a cold air column as in
a warm air column. Warm air is usually associated with high atmospheric pressure and
cold air with low atmospheric pressure.
The horizontal temperature difference between warm and cold air creates a horizontal
pressure difference
⇒ pressure gradient force (PGF)
and air is moving from high to low pressure, thus winds blow from high to low pressure.
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Forces that winds experience:
m
m
m
m
pressure gradient force (PGF)
Coriolis force (CF)
centripedal force
friction
Forces influencing winds aloft (above the boundary layer)
Pressure gradient force:
Definition:
pressure gradient = difference in pressure/distance = ∆p/d.
A net force acting on air exists when differences in horizontal air pressure exist. The
pressure gradient force is directed from higher towards lower pressure at right angles to
the isobars (lines indicating the same atmospheric pressure). Closely spaced isobars
indicate steep pressure gradients and strong winds whereas widely spaced isobars mean
gently pressure differences and light winds.
Coriolis force:
The Coriolis force is connected to the earth rotation and acts on all free moving objects
like ocean currents, air crafts and also air molecules. This objects seem to be deflected
from their original straight path since the earth rotates under them. The deviation of the
objects is at the poles greatest and at the equator zero. The deflection is increasing with
the speed of the object, but influences only the direction of the object not its speed.
observed path
actual path
non rotating system
rotating system
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Geostrophic Wind:
L
900mbar
4
PGF
3
2
5
CF
1
912mbar
H
At point 1 the PGF(pressure gradient force) accelerates an air parcel to the lower
pressure. As soon as the parcel starts to move the CF(Coriolis force) deflects the parcel to
the right. At points 2,3,4 the CF becomes larger due to the increasing speed of the air
parcel and the parcel is more and more deflected to the right. At point 5 CF and PGF are
in equilibrium with each other and the parcel=wind is moving in parallel to the isobars at
a constant speed. This is called the geostrophic wind.
In a low pressure system the PGF accelerates the air parcel inward toward the low
pressure center and the CF deflects its path to the right until the air is moving in parallel
to the isobars. We have gradient wind lowing at a constant speed parallel to the curved
isobars. This wind is constantly accelerating as it is constantly changing
direction(antipedal force): a net force is acting on the wind directed toward the center of
the L and the air will keep moving in a circular path. The so-called centripedal force is
the result from the imbalance of the PGF and CF.
Surface winds
Friction near the earth surface reduces the wind speed thus the CF and let winds blow not
parallel to the isobars as the exact balance between PGF and CF will never appear. PGF
is always stronger than CF.
1500m
wind direction
wind direction
0m
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No.5
Lapse Rate:
The lapse rate is the change of air temperature with elevation. The dry adiabatic lapse rate
refers to an air parcel rising adiabatically in the hydrostatic stable atmosphere. The parcel
expands without any exchange of heat when rising, this means the parcel and surrounding
air are always in neutral equilibrium. The dry adiabatic lapse rate is 10°C /1000m. In the
natural atmosphere there is always some water present and the lapse rate is smaller (~6 to
7°C/1000m) for humid air as for dry air due to the condensation heat, when water
condenses at cooler temperature.
Inversion:
An inversion layer is characterized by a negative lapse rate, means a layer of warm air is
on top of a cold air layer. Inversion layers form often during the night, when the earth
surface becomes colder than the surrounding air and the atmospheric right above it cools
down as well.
Potential temperature:
The potential temperature is the actual temperature plus the adiabatic lapse rate at a given
height.
Convection:
Convection provides vertical mixing in the atmosphere.
Process: an air parcel rises due to certain topography. After some time water condenses
and produces condensation heat. The parcel becomes warmer than its environment and is
accelerated upward until in equilibrium with the surrounding air.
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