Download Atmospheric Pressure: Definition, Measurement, and Variation and more Lecture notes Meteorology in PDF only on Docsity!
Atmospheric Pressure 2-
y
2.1 Definition and Pressure
Measurement
2.1.1 Definition
Pressure acts in all directions, up and sideways as well as down, but it is convenient in meteorology to regard atmospheric pressure as the weight of an air column acting on unit area, see fig. ME 2.1.
The units of pressure are Force divided by Area (or Force per Unit Area N/m 2 or Pa (Pascal)). The unit of pressure in meteorology is the hecto Pascal (hPa) which replaces the millibar (mb) which was in use in former times. However the units are of identical magnitude, therefore:
1 mb = 1 hPa
2.1.2 The Mercury Barometer The Italian physicist Torricelli invented the Mercury barometer in the 17th century, see fig. ME 2.2.
Atmospheric pressure forces mercury to rise in an evacuated glass tube.
The unit of pressure was then mm Hg (Hg is the abbreviation for mercury). This unit has been replaced by the SI-unit hecto Pascal, hPa.
The reading of a mercury barometer has to be manually corrected for the temperature of the mercury column and gravity at each site.
2 Atmospheric Pressure
Fig. ME 2.1 Atmospheric pressure
Surface area (A)
Column of air with weight (W)
Pressure at this level = WA
Fig. ME 2.2 Mercury barometer
Vacuum
Mercury column
Atmospheric pressure
Atmospheric pressure
2-2 Atmospheric Pressure
y
760 mm Hg = 1013 hPa
In the USA barometric pressure is still measured in inches Hg:
29.92 inches Hg = 1013 hPa.
2.1.3 Aneroid Barometer The aneroid barometer consists of a flexible metallic capsule which is partially evacuated. It compresses under small increases in air pressure and the capsule compression/ expansion is converted by a lever system to drive a pointer on an amplified scale. It is a much more convenient and portable system than a mercury barometer, see fig. ME 2.3.
2.1.4 The Barograph The barograph is an aneroid barometer used in conjunction with a recording drum. Instead of a needle and graduated scale the lever mechanism moves a pointer which leaves an ink trace on a scaled recording paper wrapped around the drum. This leaves a permanent record of the pressure
variations. This is used to provide a meteorologist with the pressure tendency, or rise and fall of pressure over time and is an important forecasting tool, see fig. ME 2.4.
2.2 Pressure Reduction to Mean
Sea Level and Pressure Systems
2.2.1 Pressure at the Surface QFE The atmospheric pressure measured on a barometer at an airfield is known as the QFE or Aerodrome Pressure.
The QFE varies widely, both from day to day and place to place, due to variations in the weight of the air column above the surface
2.2.2 Pressure at Mean Sea Level QFF Since QFE is measured at station level, it is not immediately usable for meteorological purposes because of the differences in altitude of the observing stations. In order to
Fig. ME 2.3 Aneroid barometer
Graduated scale
Aneroid capsule
Lever system
Fig. ME 2.4 Barograph
Amplifying Protective case levers
Aneroid cell Ink trace
Record paper on cylinder
Noon 10 8 6 1020 1015 1010 1005 1000 995
2-4 Atmospheric Pressure
y
northern hemisphere and clockwise in the southern hemisphere.
Trough A trough is a region of isobars extending away from a low centre and may have sharp curvature. Pressure along the line of the trough is lower than its surroundings.
Col A col is a region of nearly uniform pressure situated between a pair of highs and a pair of lows.
Wind circulation around pressure systems is usually along the isobars, see fig. ME 2.6.
2.3 Vertical Pressure Distribution
2.3.1 Variation of Pressure with Height The pressure at any level in the atmosphere is equal to the weight of air column above a surface of 1m 2. It follows therefore that pressure must reduce with height in the
atmosphere as the weight of the air column above reduces. How much it reduces depends upon the gravitational acceleration and density of the air column, see fig. ME 2.7. From fig. ME 2.7 it can be seen that the most
rapid decrease in pressure with height is in the troposphere up to about 500 hPa at which point about 50% of the atmosphere is below this level. The rate of pressure decrease with height begins to decrease significantly as one approaches the tropopause. The pressure will of course be determined by the density of the air above.
Table ME 2.1 shows the height change at various altitudes for a pressure change of 1 hPa.
Fig. ME 2.6 Wind circulation around pressure systems
Col
L
H
Ridge Trough
1002 1017
1017
1014
1014
1011
1011
1005
1008
1008
1002 1008 1005
Fig. ME 2.7 Variation of pressure with height
Above 99.9 %
Above 99 %
Above 90 %
Mt. Above 50 % Everest
50
40
30
20
10
0 100 300 500 700 900 0
1 hPa
5 hPa 10 hPa 25 hPa 50 hPa Altitude (km)
Pressure (hPa)
Atmospheric Pressure 2-
y
2.4 Atmospheric Density
2.4.1 Density
Density is defined as mass per unit volume, expressed in kg/m 3. When air is heated it expands, and the same mass (or weight) occupies a larger volume and so density becomes less. As pressure decreases with height the density of the air is decreasing as well. This will also be dependent on the temperature of the air as warm air is less dense than cold air and warm columns of air will be taller than cold air columns. Assume two columns of air both have the
same height and surface pressure. If one column is cooled while the other is heated, the colder column will be more dense and contain less air above the fixed datum of 10 000 ft. The warm column will expand, decreasing in density and pushing more air above the fixed datum, see fig. ME 2.8.
The pressure at 10 000 ft in the cold column will therefore be less than the pressure at 10 000 ft in the warm air column. This will create pressure differences aloft and result in winds aloft.
The rate of decrease of pressure with height is therefore greater in the cold air than in the warm air, so at any height above the surface the pressure aloft will be lower in the cold air than in the warm air.
Table ME 2.1 Height variation for 1 hPa pressure change at various heights
MSL
Height Height difference for a change of 1 hPa
18 000 ft (500 hPa) 39 000 ft (200 hPa)
27 ft 50 ft 100 ft
8 800 ft 8 000 ft 7 200 ft
10 000 ft 9 000 ft
10 000 ft
Warm/low density Cold/high density
500 hPa
700 hPa
11 000 ft
1000 hPa
Fig. ME 2.8 Density variations
