Section 02 Lesson 567 Atmospheric Pressure Temperature and

Section 02 Lesson 5/6/7 Atmospheric Pressure, Temperature and Density n n n n 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 n n 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: u millibar (mb). u hectopascal (h. Pa). u 1 mb = 1 h. Pa

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 baromet er

Aneroid Barometer

Barograph Recorder

QFE n Is the prevailing atmospheric/aerodrome pressure at the airfield elevation

QFF n n n 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 n n n 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 n n Pressure varies widely from day to day. Usual range of variation, u 970 n Lowest recorded, u 870 n mb to 1030 mb. mb, Typhoon Tip, Oct. 1979. Highest recorded, u 1084 mb, Agata, Siberia, Dec. 1968.

Extremes of Surface Pressure Variation

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Horizontal Pressure Variation n n n n Isobars Highs/Anticyclones Ridges Lows/Cyclones Troughs Pressure Gradient Force Isallobars

Isobars n n n 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 n n n 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 u 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. u 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 u 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. u 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 u Cols: is a region of nearly uniform pressure situated between a pair of highs and a pair of lows.

Pressu re Variatio n with Height

Pressure changes aloft n n n 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 n n Rate of pressure decrease with height decreases significantly nearer the tropopause Pressure is determined by the density of air above

Density n n n Density = Mass per unit volume (kg/m 3) 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 n n n Dry air = 78% N 2 , 21% O 2 Molecular weight: N 2 = 28 O 2 = 32 = 60 H 20 = 18 Therefore moist air is less dense than dry air

The Relationship between Pressure, Temperature and Density n Air behaves according to the Gas Law u Pressure n = Temperature x Density x Gas Constant • p=Tx x. C F or leaving out the constant • p Tx Pressure is directly proportional to Temperature and Density

Gas Law Continued n If Temperature held constant: up n n Thus air at a higher pressure is more dense than air at a lower pressure. If the Pressure is held constant: u. T n 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 n Density of air u Increases F i. e. with increasing air pressure decreasing altitude u Increases if the temperature is decreased u Decreases if the air pressure decreases F i. e. increasing altitude u Decreases if the temperature increases.

Summary of conclusions from Gas Laws n Note that the controlling factor in the atmosphere is air pressure. u The rapid decrease of pressure with height overcomes the tendency of the decreasing temperature with height to increase density.

Pressure and Height Calculations n At sea level pressure 1013 h. Pa and 15°C u n At 500 h. Pa u n 1 h. Pa = 27 ft height change 1 h. Pa = 50 ft height change Radiosonde data is used to calculate the “thickness” of the atmosphere at various locations

International Standard Atmosphere

Standard Pressure Levels