WWW LWC web site


Scheme of work

The structure of the atmosphere

The atmospheric heat budget

Factors affecting insolation

Planetary surface winds

The general atmospheric circulation

Equatorial climates

Tropical wet and dry climates

Tropical monsoon climates

Tropical storms

Air masses affecting the UK



Urban climates

Climate change in the last 20.000 years

Evidence of climate change

Global warming?

Effects of global warming


The general atmospheric circulation system

Introduction video

Hadley cells

This area of greater heat acts as zone of thermal lows known as the intertropical convergence zone (ITCZ) . The Intertropical Convergence Zone draws in surface air from the subtropics. When this subtropical air reaches the equator, it rises into the upper atmosphere because of convergence and convection . It attains a maximum vertical altitude of about 14 kilometers (top of the troposphere ), and then begins flowing horizontally to the North and South Poles. Coriolis force causes the deflection of this moving air in the upper atmosphere, and by about 30 degrees of latitude the air begins to flow zonally from west to east. This zonal flow is known as the subtropical jet stream . The zonal flow also causes the accumulation of air in the upper atmosphere as it is no longer flowing meridionally . To compensate for this accumulation, some of the air in the upper atmosphere sinks back to the surface creating the subtropical high pressure zone (STHP) . From this zone, the surface air travels in two directions. A portion of the air moves back toward the equator completing the circulation system known as the Hadley cell . This moving air is also deflected by the Coriolis effect to create the Northeast Trades (right deflection) and Southeast Trades (left deflection) . The surface air moving towards the poles from the subtropical high zone is also deflected by Coriolis acceleration producing the Westerlies . Between the latitudes of 30 to 60 degrees North and South, upper air winds blow generally towards the poles. Once again, Coriolis force deflects this wind to cause it to flow west to east forming the polar jet stream at roughly 60 degrees North and South. On the Earth's surface at 60 degrees North and South latitude, the subtropical Westerlies collide with cold air travelling from the poles. This collision results in frontal uplift and the creation of the subpolar lows or mid-latitude cyclones . A small portion of this lifted air is sent back into the Ferrel cell after it reaches the top of the troposphere. Most of this lifted air is directed to the polar vortex where it moves downward to create the polar high .

The Ferrel and Polar cells

The Ferrel cell occurs at higher latitudes (between 30 and 60 N and S) and is responsible for the climate types occurring in the mid latitudes. Air is pulled towards the poles forming the warm southwesterlies in the northern hemisphere and northwesterlies in the southern hemisphere. This deflection is due to the Coriolis Force. As the winds travel over the oceans they pick up moisture. At around 60 N and S they meet cold air from the poles. The warm air is less dense and so it rises over the denser cold air. This uplift causes low pressure at the surface resulting in unstable mid latitude depressions which are characteristic of our Cool Temperate Western Maritime climate.

On reaching the tropopause some of the air returns to the tropics as part of the Ferrel Cell whilst some is diverted polewards as part of the Polar Cell.

On the surface at the north and south poles, descending air from the Polar Cell results in high pressure. This causes air to move towards the mid latitude low pressure belt.

We can look at the progress of the moving wind and pressure belts.

In addition to the three cell model it is necessary to add upper air westerly air streams (Rossby waves) and jet streams

Rossby waves follow a wavy undulating pattern as they travel around the Earth's upper atmosphere. They occur between 4 and 6 times in both hemispheres stretching from polar latitudes to the tropics. They are possibly caused by the north south mountain ranges of the Rockies and the Andes. The waves have considerable variation in amplitude.

Within the upper westerly winds are bands of fast moving air (up to 250 km/hr) called jet streams. It can be several hundreds of km in width but only 1 - 2 km in height. They are the product of a large temperature gradient between two air masses.

There are two main types of jet streams.

Polar Front jet stream (PFJS) associated with the meeting of cold polar and warm tropical high above the Atlantic between 40 and 60 N and S. The exact location varies, but it marks the division between the Polar and Ferrel cells and helps explain the formation of mid latitude depressions .
Sub tropical jet stream (STJS) associated with the poleward ends of the Hadley Cell at approx 25 N and 35 S. However, in summer above west Africa and southern India this jet may become easterly. This is due to higher temperatures over land over the more southerly sea areas.


The movement of the ITCZ will determine the length of the wet and dry season within the tropics.