Transcript Atmospheric Pressure

Section 02
Lesson 5/6/7
Atmospheric Pressure, Temperature and Density
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Definition of Atmospheric Pressure
Measurement of Pressure
QFF, QFE, QNH
Pressure Variation at Surface Level
Pressure Variation with Height
Density
International Standard Atmosphere
Gas Laws
Atmospheric Pressure
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Pressure is defined as force per
unit area.
Atmospheric pressure is the force
exerted at the earth’s surface by a
column of air above it.
Pressure at a point acts uniformly
in all directions.
Units of pressure are:
 millibar
(mb).
 hectopascal (hPa).
 1mb = 1 hPa
Atmospheric Pressure
Atmospheric pressure is defined as the
force per unit area exerted against a
surface by the weight of the air column
above that surface.
The pressure at point
"X" increases as the
weight of the air above
it increases. The
pressure at point "X"
decreases if the weight
of the air above it
decreases.
Measurement of
Atmospheric Pressure
Vacuum
Mercury
barometer
Aneroid Barometer
Barograph Recorder
QFE
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Is the prevailing atmospheric/aerodrome
pressure at the airfield elevation
QFF
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Is the local station barometric pressure
adjusted to mean sea level assuming an
isothermal column of air at station
temperature
It is only used on surface synoptic charts
It is expressed to one decimal point.
QNH
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Is a pressure setting which when set on an
altimeter sub-scale will cause it to read the
airfield elevation above mean sea level.
ISA conditions are assumed for the air
column
It is always expressed in integer values.
Pressure Variation
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Pressure varies widely from day to
day.
Usual range of variation,
 970
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Lowest recorded,
 870
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mb to 1030 mb.
mb, Typhoon Tip, Oct. 1979.
Highest recorded,
 1084
mb, Agata, Siberia, Dec. 1968.
Extremes
of Surface
Pressure
Variation
60
Horizontal Pressure Variation
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Isobars
Highs/Anticyclones
Ridges
Lows/Cyclones
Troughs
Pressure Gradient
Pressure Gradient Force
Isallobars
Isobars
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A line drawn on a weather map
connecting points of equal
pressure is called an isobar.
The isobars are generated from
mean sea level pressure reports.
The pressure values are given in
hectopascals (or millibars).
Surface Isobar Charts
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Reporting stations are not all at
Mean Sea Level.
Local Barometric Pressure is
adjusted to Mean Sea Level using
an Isothermal column of air.
Mean Sea Level Pressure plotted
on Synoptic Charts is known as the
QFF.
Typical Surface Pressure Patterns
COL Weather over land
TROUGH - formed by the
extension of a low
COL - a zone of slack
pressure gradients with
calm or light variable
winds
RIDGE - formed by the
extension of an
anticyclone
Winter: Poor vis; radiation fog
Summer: CB/TS in late afternoon
Patterns of Pressure Distribution
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Anticyclones or highs: are regions where the pressure at its centre is highest relative to its
surroundings. The circulation is clockwise in the northern hemisphere and anticlockwise in the
southern hemisphere.
 Ridge: a region of isobars extending away from a high centre with no sharp curvature. Pressure
along the line of the ridge is higher than its surroundings.
Patterns of Pressure Distribution
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Depressions or lows (or cyclones): are regions where the pressure at its centre is lowest relative to
its surroundings. The circulation is anticlockwise in the northern hemisphere and clockwise in the
southern hemisphere.
 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
 Cols: is a region of nearly uniform pressure situated between a pair of highs and a pair of lows.
Pressure
Variation
with
Height
Pressure changes aloft
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Pressure decreases with height
Pressure also varies horizontally aloft.
This is due to horizontal variations of mean
temperature in layers in the atmosphere.
Pressure changes aloft
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Rate of pressure decrease with height decreases
significantly nearer the tropopause
Pressure is determined by the density of air
above
Density
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Density = Mass per unit volume
(kg/m3)
When air is heated, density
becomes less
With an increase in height, both
pressure and density decrease
Density variations
Density of dry and moist air
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Dry air = 78% N2 , 21% O2
Molecular weight: N2 = 28
O2 = 32
= 60
H20 = 18
Therefore moist air is less dense
than dry air
The Relationship between Pressure,
Temperature and Density
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Air behaves according to the Gas Law
 Pressure
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= Temperature x Density x Gas Constant
•p=TxxC
 or leaving out the constant
•pTx
Pressure is directly proportional to Temperature
and Density
Gas Law Continued
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If Temperature held constant:
p
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Thus air at a higher pressure is more dense than air
at a lower pressure.
If the Pressure is held constant:
T
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
x  = Constant
Thus at a given atmospheric pressure, air that is cold
is more dense than air that is warm.
Summary of conclusions from Gas Laws
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Density of air
 Increases
 i.e.
with increasing air pressure
decreasing altitude
 Increases
if the temperature is decreased
 Decreases if the air pressure decreases
 i.e.
increasing altitude
 Decreases
if the temperature increases.
Summary of conclusions from Gas Laws
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Note that the controlling factor in the atmosphere is
air pressure.
 The
rapid decrease of pressure with height
overcomes the tendency of the decreasing
temperature with height to increase density.
Pressure and Height
Calculations
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At sea level pressure 1013hPa
and 15°C
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At 500 hPa
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1hPa = 27 ft height change
1hPa = 50 ft height change
Radiosonde data is used to
calculate the “thickness” of the
atmosphere at various locations
International Standard Atmosphere
MSL Temperature
15°C; 288K
MSL Pressure
1013.25 mb, 29.92 ins Mercury.
MSL Density
1225 gm/m3
Temperature
Decreasing 1.98°C/1000 ft (6.5°C/km) to 11000 m (36090 ft).
Isothermal at –56.5°C to 20,000 m (65617 ft).
Increasing at 0.3°C/1000ft (1°C/km) to 32,000 m (104987 ft).
Standard Pressure
Levels
Pressure (mb)
1000
700
500
400
300
250
200
150
Approx. Height
Mean sea level
10,000 ft.
18,000 ft.
24,000 ft.
30,000 ft.
34,000 ft.
39,000 ft.
45,000 ft.