Pressure and Winds The pressure in the atmosphere and the way this causes the global winds.

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Atmopshere :

The Earth's atmosphere is almost completely contained within 80 kilometers, or 50 miles. As air is highly compressible, air at lower altitudes is compressed by air above and so is more dense. From the diagram below, it can be seen that 50% of the Earth's atmosphere is found below 5km. atmospheric pressure

Air movement in the atmosphere may be either vertical or horizontal. Horizontal motion of the air is more commonly know as wind. Horizontal movement of the air is caused by differences in pressure between two areas. The pressures are represented on a map by isobars. These are imaginary lines which join points of equal pressure. The pressure readings are reduced to sea level (average pressure at sea level= 1013 mbar). The pressures are measured in millibars and conventionally drawn at 4- millibar intervals.

The actual pattern of the isobars makes more difference to the weather than the actual values. If the isobars are close together then the pressure gradient is large and we get strong winds.

Here we can see the two most common patterns found over the UK;

This diagram shows an area of low pressure. Low pressure areas are caused by increasing the temperature. This causes the air to heat up and expand, becoming lighter and rising. The wind is blowing towards the centre (rising as it goes). The wind is at a slight angle to the isobars and blows in an anticlockwise direction. The winds found with low pressures are usually strong as there is a steep pressure gradient.
This diagram shows an area of high pressure. High pressure areas are caused by a drop in temperature, causing the air to become more dense and so to fall. The winds found in this case are gentle out- blowing winds. The descending air flows in a clockwise direction.

We can see in these diagrams that the wind doesn'’t blow at right angles to the isobars, i.e. in the direction of the pressure gradient. This is due to friction effects and the coriolis force combined with the effects of gravity and the pressure gradient force.

The Coriolis Force

If the Earth did not rotate at all then the air would move at right angles to the isobars, in the direction of the pressure gradient force.

This picture shows the air rising at the equators where it is heated (convection), causing an area of low pressure and then travelling up to the poles losing heat as it goes (horizontal movement is advection). At the poles it sinks again (subsidence), being cold, and forms an area of high pressure. It then blows outwards from the poles towards the equator along the surface.

But in reality the Earth does rotate and the uneven distribution of land masses and seas also play a part in the air circulation. This means that more than one cell is created. Another consequence of the Earth’s rotation is that air moving towards the pole appears to be deflected to the right. This is the result of the coriolis force.

The coriolis force can be explained by using an example of a roundabout. If we consider person a standing in the middle of the roundabout and trying to throw a ball to person B standing at the edge of the roundabout. If A throws the ball straight at B then by the time the ball has reached the edge of the roundabout B has further round. To the people on the roundabout it looks like the ball has curved round to the right.

In the same way, the Earth’s rotation means that, in the Northern hemisphere, a north-blowing wind appears to move to the right (by 15 degrees of longitude per hour). This explains why the prevailing wind in Britain is a south-westerly, not a southerly. In theory if the coriolis force were the only force acting on the air then it would cause the air to move in a circle.

Air in the upper troposphere

In the upper troposphere the air is unaffected by friction and we can see that there is a balance between the pressure gradient force and the coriolis force. The resultant wind is called the geostrpohic wind. The geostrophic wind blows parallel to the isobars. Buys Ballot was a Dutchman who recognised the existence of the geostrophic wind in 1857. His law states that if you stand with your back to the wind the low pressure is always on the left and the high pressure on the right. (Placing of low and high pressures tell you which direction the pressure gradient wind is blowing. The coriolis is in the opposite direction to the pressure gradient force. So you can work out the direction of the resulting wind.)

This diagram shows the resultant of the pressure gradient wind and the coriolis force, the geostrophic wind, which blows parallel to the isobars.

The resultant wind is also slightly modified by friction from the surface, reducing the coriolis froce and causing the wind to blow at a slight angle to the isobars.

Tricellular Model:

Halley (1686) first suggested and Hadley (1735) explained, the convective cell, seen above, that transfers heat from the equator to the poles by means of convection. This model was adapted by Ferrel in 1856 when he dicovered that there were infact three cells and Rossby (1941) made further refinements to the model. The tricelular model found by Ferrel is still the best model we have.
In this model the air, after crossing the warm oceans in the trade winds and becoming warm and moist, arrives at the Equator (at the ITCZ, inter-tropical convergence zone) and is heated, causing it to rise. The unstable air rises to form very high cumulo-nimbus clouds and afternoon thunderstorms and low-pressures are found. The equator is an area with very gentle winds called the doldrums.
As it rises it cools and moves away from the Equator. Further cooling occurs and increasing density and diversion by the coriolis force cause the air to slow down and subside, bringing an area of high pressure. The latitude at which this occurs is about 300. This area has clear skies and stable weather conditions. When the air reaches the ground the Hadley cell is completed and some of the air returns back to the Equator as the NE trade winds (SE if looking at the southern hemisphere) and some continues towards the poles.
The air which continues towards the poles forms the bottom of the Ferrel cell. This picks up moisture as it crosses the seas and meets cold polar air at a latitude of about 600, this is known as the polar front. This air is then forced upwards and this causes an area of low pressure and brings unstable conditions which produce cyclonic rainfall. The rising air at this stage goes one of two ways. It either travels back towards the equator along the top of the Ferrel cell, or travels up towards the poles where, having cooled down, it descends forming an area of high pressure. The air then returns to the polar front as cold Easterlies.

This pattern shifts during the year as the angle of the Earth to the sun changes, shifting the fronts northwards slightly in the summer and southwards in the winter, so that in July the northern horse latitudes are at around 400. This causes seasonal shifts in weather.

Rossby Waves and Jet Streams :

It has been found that there are very strong winds in the uper troposphere (World War I and II pilots found themselves being blown off course and that Eastward journeys took much less time than westward journeys.) These westely winds are called Rossby waves and blow right the way round the world. They are not straight and have a different number of waves depending on the season.(from three to six normally).
The velocities found in these uppertroposphere winds is not uniform and there are some very fast streams of air found, over 230 km/hr sometimes. These are called jet streams. These help in the rapid transfer of energy around the globe and can dispurse debris from eruptions etc. around the world within a week. There are three main jet streams: