Wind Pressure Sys. Structure of a Depression Anticyclone Weather Services TRS




Atmosphere and Atmospheric Pressure


The Atmosphere

Most of the weather changes take place in the lower layer of the atmosphere, which is known as the TROPOSPHERE.

This layer extends about 11 miles-high over the equatorial regions and about 5 miles high over the Polar Regions.

The layer above the troposphere is known as the STRATOSPHERE where there is little water vapour and the lower part appears to be isothermal.

There is a rise in temperature towards its upper limit.

The boundary between the troposphere and the stratosphere is known as the TROPOPAUSE.

About 20 miles above the earth the stratosphere gives way to the OZONOSPHERE, where there is a high concentration of ozone which absorbs ultra violet radiation.

The IONOSPHERE starts about 50 miles above the earth.

The density of the atmosphere decreases with height.

Approximate percentage volumes of the various gases forming dry air in the troposphere are nitrogen 78%, oxygen 21%, argon 0.9%, carbon dioxide 0.03%, with the balance being made up of traces of hydrogen, helium, neon, krypton, radon, xenon and ozone.

As most of the weather changes take place in the troposphere, it is in this region that the changes in temperature arc most important.

Main features of the troposphere

As altitude increases, air pressure decreases due to the decreased weight of air above. With less pressure, the density decreases. More than three-fourths of the air is concentrated within a layer averaging about 7 statute miles thick, called the troposphere. This is the region of most weather, as the term is commonly understood.

The top of the troposphere is marked by a thin transition zone called the tropopause, immediately above which is the stratosphere. Beyond this lie several other layers having distinctive characteristics. The average height of the tropopause ranges from about 5 miles or less at high latitudes to about 10 miles at low latitudes.

Sun as energy source

The sun, at a distance of about. 93,000,000 miles and at a temperature of about 6,000C, is the principal source of light and heat for the earth. The heat from the sun travels to the earth in the form of short wave radiation, which passes through the atmosphere without appreciably warming it.

On striking the earth some of the heat will be absorbed to warm the earth. The heat received at the earth from the sun is known as INSOLATION.

The amount of insolation per unit area varies with latitude. It can be seen in the sketch below that a band of rays has to heat a very much larger area in a high latitude than it does in a low latitude.

The increase in the temperature of the earth will depend, amongst other things, on the amount of insolation and the specific heat of the earth.

Assuming that the insolation is constant, a surface with a high specific heat warms and cools les s quickly than a surface with a low specific heat.

For instance, the sea temperature in non-tidal waters hardly changes in the 24 hours.

A contributing factor to this small change is its depth. It will be found that, in general, sea temperatures are less than the temperatures of adjacent land by day and greater by night. There is a similar difference in summer and winter.

The amount of insolation will depend on the suns altitude and the degree of cloud cover. It may be noted that the SOLAR CONSTANT, which is the intensity of solar radiation at the outer boundary of the atmosphere, is 1.39kw/m2.

The height of the station under consideration above sea level will be a factor influencing the heating, as will the prevailing wind. Air which his flowed over warm surfaces will have a smaller cooling effect than air which has flowed over cold surfaces.

As the earth is a warm body it radiates heat. This radiation will cool the surface.

If the earth is radiating heat at a greater rate than it is receiving it, the net result will be a cooling of the surface. The converse is also true.

In the diagram below the curves of insolation and radiation are shown to increase and decrease at a regular rate for case of illustration.

Quite clearly the temperature falls until point A is reached when it begins to rise. The rise continues to point B and then, once again, the temperature falls.

It will be noted that the lowest temperature occurs shortly after sunrise whereas the maximum temperature occurs about 1400 hours local time, and not when insolation is minimum and maximum. A similar lag takes place on an annual basis. In northern latitudes the lowest temperatures do not usually occur until February and the maximum temperatures occur in July and early August.

The sea temperature lags even further behind the sun. The minimum occurring in March and the maximum in September.

It must be understood that all the above general statements can be considerably modified by local weather conditions, particularly cloud amounts.

Nature of solar radiation

Heat is transferred to the air surrounding the Earth by one or more of the following means: -

1. Radiation

2. Conduction

3. Convection

4. Turbulence.



The suns rays being in the shorter wavelength do not appreciably heat up the air, rather the heating is done of the water vapour particles in the atmosphere,

The sun does however heat the Earth, and the Earth after getting heated up re-radiates heat. This earth heat is in the form of long waves. If there is no cloud cover then the heat is transferred to space, but with a cloud cover the heat is radiated back on the earth.

Thus on a cloudless night the heat radiated out by the Earth is considerable and a lot of surface cooling takes place.


The heat is transferred from particle to particle. Thus, air, which is in contact with a warm surface, is warmed; this occurs by day when the sun is shining. Air, which is in contact with a cold surface, is cooled; this occurs on clear nights.

Air masses acquire their characteristics by the same process.


 Air, which is warmed, expands and consequently its density decreases. This makes it lighter than the un-warmed air surrounding it and the warmer air rises.

Convectional processes take large amounts of warm air and water vapour from the surface to the upper levels. When the water vapour condenses into water drops and precipitation occurs, the latent heat remains. Most of the atmospheric heating takes place in this way.

Conversely, air, which is colder than the surrounding air, has a greater density, and as it is heavier, it sinks. An example of this is the Katabatic wind.


 Air, which flows over a rough surface tends to be deflected upwards. The Rising air will be replaced by some air from levels up to 600m giving an interchange of air between the surface and 600m.

The rising air carries its warmth (acquired by conduction) with it, and the falling brings its coolness.


Increase of height above sea level generally means a decrease of temperature. The rate at which the temperature changes with height is known as the lapse rate, an average value being about 0.7C/100m.


The lapse rate can vary and the environment lapse rate will be referred to when a particular air mass is under consideration. This rate will be dependent on many factors and will vary with the altitude, although it is usually between the S.A.L.R. and D.A.L.R. Plotting temperatures of the air at various heights will give environment curves.


If a volume of air rises to a region of lower pressure, the volume will increase and following the gas laws the temperature of the volume will fall. The change of temperature is solely due to expansion or contraction and no heat has been given to or received from the adjacent air. This change of temperature is known as an Adiabatic Temperature Change.


When dry air is forced to rise it has been found that it decreases its temperature by 1C/100m and this is known as the Dry Adiabatic Lapse Rate. With dry air, which is forced down, an increase at a similar rate is found.

Dry air is any air, which is not saturated.


Latent heat is the heat necessary to change 1 kilogram of water to 1 kilogram of Vapour at saturation temperature. If a quantity of water changes to vapour an amount of latent heat will have been required.

This sort of thing will happen with air at a temperature above dew point (note however that the higher the temperature of the air the more water vapour it will normally contain- unless some artificial means have been employed to reduce the water vapour content).

The latent heat that has been originally required to change the water to its vapour state has not been lost but is retained in the mass of the water vapour. Now when the water vapour is cooled and the water condenses back the latent heat is released and this is known as the latent heat of condensation.

The latent heat of vapourisation is generally accepted as 540 calories/gramme of water.

When the cooling of the air to its dew point temperature is caused by the air rising the latent heat released will modify the overall cooling rate.

For the air that is rising:

D.A.L.R. (-) 1.0C/100m

Latent Heat (+) 0.5C/100m (average for sea level in middle latitudes)

S.A.L.R. (-) 0.5C/100m

The S.A.L.R. is a variable quantity depending on the latent heat of the condensing water vapour. The S.A.L.R. is low near the equator and high in Polar Regions.

It will also increase with height.


If air, which his been forced to rise (or fall) from its initial level tends to return to that level it is said to be stable. This condition occurs when the environment lapse rate is less than the S.A.L.R.

The figure shows in black the environment curve of some tropical air, whose lapse rate is about 0.3C/100m.

Suppose some dry air at point A is forced upwards, it will cool at the D.A.L.R. and follow the red line.

When the rising air reaches the level BB1 it will be much colder than the environment or surrounding air.

As it is colder it is denser, and will want to return to A, a tendency shown by the dotted line.

If instead of the air rising from A, it fell, then at the level CC, the failing air would have warmed adiabatically to a considerably higher temperature than the environment or surrounding air.

It would then be less dense and the tendency would be for it to return to A as indicated.

If the air forced to rise or fall from A were saturated instead of dry, the path indicated by the green line would be followed. Again, for the same reasons as are given above, the tendency is for the air to return to A.


If air, which has been forced to rise (or fall) from its initial level tends to continue its upward (or downward) movement, it is said to be unstable. This condition occurs when the environment lapse rate is greater than the D.A.L.R.


The figure shows in black the environment curve of some polar air whose lapse rate is about 1.2C/100m.

Suppose some saturated air at point A is forced upward it will cool at the S.A.L.R. and follow the green line. When this rising air reaches the level BB1, it will be warmer than the environment or surrounding air. As it is warmer it will be less dense and the tendency is for it to continue upwards.

If, instead of the air rising from A, it fell, then at the level CC1, the falling air would be at a lower temperature than the environment or surrounding air. It would then be denser and it would continue to fall.

If the air forced to rise or fill from A were dry instead of saturated, the path indicated by the red line would be followed. Again, for the reasons given above, the rising or falling air continues to rise or fall.


 This condition occurs when the environment lapse rate is the same as the D.A.L.R., if dry air is under consideration, or the. S.A.L.R. in the case of saturated air. The air which has been forced to rise or fall will have no tendency to continue its upward or downward movement nor will it have any tendency to return to its original position.


 This occurs when stable conditions exist for dry air and unstable conditions exist when the air is, saturated. The figure illustrates this.


The environment curve has a lapse rate between the S.A.L.R. and the D.A.L.R. The red line representing dry air forced up or down follows a similar pattern to earlier figure for stable air, whilst the green line representing saturated air forced up or down follows a similar pattern to figure for unstable air.

This conditionally unstable condition is quite common, the air below the condensation level being stable and that above being unstable


 The usual tendency for the temperature to fall with height is sometimes reversed. When the temperature increases with height an inversion is said to exist. An inversion also exists if a layer of air is isothermal.

 Inversions may occur at any level at all. They frequently, occur at ground level on clear nights. They also occur just above cloud layers.

As an inversion gives rise to very stable conditions, convection and turbulence are damped out and there will be no upward movement of air through the inversion.

 Inversions may be caused in anticyclone conditions by subsiding air warming adiabatically to a temperature above that of the lower layers of air.

The figure shows a curve for the environment where there is an inversion above the surface.

Dry and saturated air forced up from A will clearly return showing stable conditions, whereas if forced up from B (or down from A) unstable conditions exist.

Atmospheric Pressure

The atmosphere by reason of its weight exerts a pressure on the surface of the earth. This pressure is normally measured in millibars, the mean value at sea level being around 1013 mb.

This pressure is in certain places semi-permanently above the mean, while in other places it is semi permanently below the mean. These places are referred to as regions of high and low pressure respectively. There are also temporary areas of high or low pressure.