Chapter 4: Atmospheric pressure Flashcards
Atmospheric pressure is
the weight of a column of air exerted on a surface of a unit area by the activity of the molecules composing the fluid (air).
The pressure of the atmosphere at a point on the earth’s surface is therefore equivalent to
the weight of the whole column of air standing on unit area at that point.
if one considers a point at some height
above the surface, it seen that the pressure at that height is equal to
the weight of the air above
A pressure of 1000 mb (100 kPa) results from the weight exerted by
10,000 kg of air overlying one square meter of surface, accelerated by
gravity of 10 m sec-2
.
The huge pressure does not crush us because
it is exactly balanced by outward pressure from the inside of our bodies.
Ears popping due to change in altitude are the result of
the pressure difference between the inside of our heads and the surrounding air.
Pressure decreases with altitude, due to
the reduction of the mass of overlying air with height.
Pressure is 700 mb at
3,000 m
pressure is 500 mb at
5500m
pressure is 300 mb at
10,000 m
In SI units the pressure is measured by
Pascals.
One Pascal =
one Newton per square meter [1 Pa = 1 N/m2]
In meteorology the unit of pressure, which is a
force per unit area, is Pascal
In meteorology the unit of
pressure, which is a force per unit area, is Pascal. This unit is very small so
that the pressure is usually expressed by
millibars (mb) or hectoPascal s
(hPa).
The relation between mb, hPa and Pascal is the following:
1 mb= 1 hPa = 100 Pa
The relation between mb, hPa and Pascal is the following:
1 mb= 1 hPa = 100 Pa.
Also the pressure is measured in
millimeters and inches of mercury (1 in = 254 mm)
……………………. is used to measure atmospheric pressure
simple barometer
A simple
barometer, which is called
Torricelli barometer or simply a mercury barometer
Torricelli correctly described the atmosphere as
a vast ocean of air that exerts a pressure on us and all things about us.
the weight of the mercury in the column equaled
the weight of that extended from the ground to the top of the atmosphere
The length of mercury column varies with
temperature.
Therefore,
Meteorologists determine the length of mercury column under
standard conditions.
Under standard conditions the temperature is that of melting ice
0 c
Under standard conditions the temperature is that of melting ice (0cand the gravitational acceleration is
9.80665 m/s2
Under standard conditions the temperature is that of melting ice (0oC) and the gravitational acceleration is 9.80665 m/s2. On the sea level the pressure
under standard conditions is equal
760 mm (1013.25 mb)
air is also measured by
aneroid barometer
aneroid means
without liquid
Instead of having a mercury column held up by air pressure, the
aneroid uses a
partially evacuated metal chamber. A chamber, being very
sensitive to variations in air pressure, changes shape, compressing as the pressure
increasing and expands as pressure decreases.
An advantage of the aneroi d
barometer
it can be easily connected to a recording mechanism
An advantage of the aneroi d
barometer is that it can be easily connected to a recording mechanism. The
resulting is a new instrument, which is called the
barograph.
The barograph
records the pressure continuously with passage of time
In the atmosphere the amount of pressure is determined by the effect of two
factors
temperature and density
A temperature variations of the air affect the
density of the air (the mass of the air per unit volume)
As temperature decreases
the air molecules are
closely packed (greater density)
As temperature decreases
the air molecules are closely packed (greater density), so the air pressure is
high.
The pressure at any given altitude in the atmosphere depends mainly at the
weight of the air directly above that point
With ascending through the
atmosphere, the air becomes
less dense because the lesser amount (weight) of the
air above, the result is a decrease in pressure
The rate at which the pressure
decreases with height is
not a constant
The rate at which the pressure
decreases with height is not a constant. The rate is greatest near
the earth’s
surface and with increasing altitude the rate of pressure decreasing becomes less
and less
The rate is greatest near the earth’s
surface and with increasing altitude the rate of pressure decreasing becomes less
and less (this is because
the amount of gases decreases rapidly in the lower
troposphere where in the first 5 km concentrated 50% of the whole amount of
gases, the pressure at this height is only half its value near the earth’s surface).
The atmospheric pressure is reduced approximately one-half for each
5-kilometer
increase in altitude.
Mean sea level (MSL)
Since pressure varies with altitude, we cannot readily compare station
pressures between stations at different altitudes. To make them comparable, we
must adjust them to some common level.
To compute sea level pressure, it is necessary to determine the
station pressure. We then add to this value the weight of air, which would occupy a
column of unit area of cross-section between the station and MSL
to express the pressure at a station of height 3 km into
the sea level pressure, (see figure 3.3), we can use the basic principle of
sea level pressure reduction
for 3 km the pressure will decrease
300 mb
If the rate of pressure decreasing with height is 10 mb for each 100
pressure is measured
700 mb at this height
for 3 km the pressure will decrease
300 mb. If the rate of pressure decreasing with height is 10 mb for each 100
pressure is measured 700 mb at this height, then the sea level pressure for
this station is
700 + 300 or 1000 mb.
To meet the need of weather forecaster, a network of observing
stations has been built up, which covers the greatest part of the earths
surface. The density of the network depends on different factors such as th
difficulties in climate and terrain, including about 70% of the earths surface
covered by oceans, the distribution of population, etc.
In studying the weather, it is necessary to obtain observations from
very large meteorological stations simultaneously and at regular intervals.
Isobars.
lines of equal pressure
Low
a center of pressure surrounded on all sides by higher pressure; also called a cyclone. Cyclonic curvature is the curvature of isobars to the left when you stand with lower pressure to your left
High
a center of pressure surrounded on all sides by lower pressure; also called an anticyclone. Anticyclone curvature is the curvature of isobars to the right when you stand with lower pressure to the left.
Trough
an elongated area of low pressure with the lowest pressure along a line marking maximum cyclonic curvature
Ridge
an elongated area of high pressure with the highest pressure along a line marking maximum anticyclonic curvature.
Col
the neutral area between two highs and two lows. It is also is the intersection of a trough and a ridge.
