Around the equator radiation from the Earth's surface heats the lower layers of the atmosphere, causing them to expand and rise. This effect creates a permanent low-pressure zone (called the doldrums), with light to non-existent winds.
The light warm air rises and eventually cools, spreading north and south to form convection currents. At around latitudes 30° North and 30° South the air in these currents sinks, creating two belts of high pressure, called the horse latitudes.
Like the doldrums, the horse latitudes are regions of light winds and calms. The dry, subsiding air and therefore stable atmospheric conditions of the horse latitudes tend to give rise to huge deserts on the Earth's surface - the Sahara, for example.
From the horse latitudes, air currents (winds) flow outwards across the Earth's surface. Those that flow towards the equator are the Trade Winds, and those moving towards the poles are the Westerlies.
The Westerlies eventually meet cold air currents (the Polar Easterlies) flowing from the poles - areas of high atmospheric pressure caused by the sinking of cold, dense air.
The regions between 30° and 65° North and South are transition zones with changeable weather, contrasting with the stable conditions in the tropics.
The weather in these transition zones is influenced by the formation of large depressions, or cyclones, which result from the intermingling of polar and subtropical air.
Although there is a continual heat exchange between the tropics and the poles, winds do not blow directly north-south. The Coriolis effect, caused by the rotation of the Earth on its axis, deflects winds to the right of their natural direction in the Northern Hemisphere, and to the left in the Southern Hemisphere. (The Coriolis effect also deflects ocean currents in a similar way.)
The paths of winds and the positions of the dominant low- and high-pressure systems also undergo seasonal changes. These result from the 23° tilt of the Earth's axis, which causes the Sun to move northwards and southwards (as seen from the Earth) during the year. At the equinoxes (on about 21 March and 23 September) the Sun is overhead at the equator, and solar radiation is equally balanced between the two hemispheres. But on about 21 June, the summer solstice in the Northern Hemisphere, the Sun is overhead at the Tropic of Cancer (23° North), and on 21 December, the winter solstice in the Northern Hemisphere, the Sun is overhead at the Tropic of Capricorn (23° South).
The overall effect of these changes in heating is that the wind and pressure belts move north and south throughout the year. For example, Mediterranean regions come under the influence of the stable atmospheric conditions of the horse latitudes in summer, giving them hot, dry weather, but in winter the southward shift of wind belts brings cooler weather and cyclonic rain to Mediterranean lands.
The astronomical dates pertaining to seasons do not coincide exactly with the actual seasons, however, because the Earth's surface is slow to warm up and cool down. As a result the summer months in the middle latitudes are June, July and August. Similarly winter in the Northern Hemisphere occurs in December, January and February.
Winds are also affected by the fact that land heats up and cools faster than does water. Rapid heating of coastal regions during the day creates an area of relatively low air pressure on land, into which cooler air from the sea is drawn. At night, the land cools rapidly and cold air flows from the land towards the relatively warmer sea.
Differential heating of the land and sea also leads to the development of huge air masses over the continents and oceans.
There are four main types of air masses. Polar maritime air is relatively warm and moist, because it is heated from below by the water. Polar continental air, by contrast, is cold and mainly dry in winter, but warm in summer when the land heats quickly. Tropical maritime air is warm and moist, whereas tropical continental air, such as that over the Sahara Desert, is warm and dry.
The movements of these air masses and their interactions with adjacent masses along the boundaries known as fronts have an important effect on the weather in transitional areas.
Depressions form along the polar front, the boundary between the polar and tropical air masses in the middle latitudes. They begin when undulations or waves develop in the front; warm air then flows into pronounced undulations, thereby forming depressions.
The forward arc of the undulation is called the warm front, and the following arc is the cold front.
Depressions are low-pressure air systems and winds are therefore drawn towards their centers. But the deflection caused by the Coriolis Effect makes winds circulate around rather than blow directly into the centre of a depression.
The wind circulation in depressions (cyclones) is in an anticlockwise direction in the Northern Hemisphere and clockwise in the Southern Hemisphere.
On weather maps depressions appear as a series of concentric isobars (lines joining places with equal atmospheric pressure - analogous to contour lines of height on land maps), with the lowest pressure at the centre. When the isobars are close together the pressure gradient is steep, and the steeper the pressure gradient, the stronger are the winds, which tend to blow parallel to the isobars.
The formation of depressions is closely related to the paths of the jet streams in the upper atmosphere. On charts of the higher atmospheric layers, a poleward ripple in the westward-flowing jet stream usually indicates a depression below.
The flow of the jet streams affects the development of depressions.
When a jet stream broadens, it tends to suck air upwards, intensifying the low pressure below and causing wet, windy weather.
When a jet stream narrows, it tends to push air down, thereby raising the pressure below. The jet streams are strongest in winter, when the temperature difference between polar and tropical regions is greatest; therefore the pressure gradient between these two regions is also steepest in winter.
When a jet stream becomes strongly twisted, waves may break away. The jet stream soon connects up again, however, cutting off blocks of cold or warm air from the main flow. Such stationary blocks can bring spells of unseasonal weather, such as the so-called "Indian summer".
Within a depression warm air flows upwards over cold air along the warm front. Because the gradient is gradual, the clouds ahead of the warm front are usually stratiform in type.
Along the cold front cold air undercuts the warm air, causing it to rise steeply; as a result, towering cumulonimbus clouds often form behind the cold front.Because the cold front moves faster than the warm front, the warm air is gradually pushed upwards, or occluded. Bands of cloud linger for some time above occluded fronts, but the depression soon weakens or is replaced by another.