The weather is a combination of sunshine and cloud, dampness (humidity), rain or snow (precipitation), wind and calm, warmth and cold (temperature) on a particular day.

Depending on where you are, one day may be very much like the next, or it may change greatly. But over the years, the day-to-day weather forms a recognisable pattern called the climate. In this book you will find the expected patterns of weather across the world during April.

Why the weather varies across the world

Weather patterns over the earth are produced because the sun heats the earth more in some places than others. Whenever the land or the ocean becomes warm, it shares its heat with the air above. This warmed air then expands, becomes less heavy and rises, leaving room for more air to flow in to take its place. Whenever the land or ocean is cold, the air touching it loses heat and cools down, becomes more dense, sinks and spreads out.

In these ways the air begins to circulate. The circulation is felt as winds. Because it is generally warmer in the tropics than at the poles, the circulation of air involves the whole lower atmosphere.

Air also flows between places where the air pressure is lower than average (low pressure areas called Lows) and places where the air pressure is higher than average (high pressure areas called Highs).

The pattern in the tropics

The world's biggest Low is caused by hot air rising near the equator (it is called the Equatorial Low). Its position moves north and south during the year as it follows the movement of the overhead sun (see page 4) and the latitude of greatest heating. The Equatorial Low is the heat 'engine' that powers the entire global circulation. Cloud and rain are produced by rising air, making the Equatorial Low also the world's main rain-producing area and the place of the great tropical rainforests.

With air rising near the equator, air has to sink elsewhere to keep the atmosphere in balance. This happens at about latitude 30°­35° in each hemisphere. The sinking air creates high pressure regions (known as Subtropical Highs). In this sinking air, cloud cannot form and it is nearly always dry. Here lie many of the world's great deserts.

Air continually flows from the Subtropical Highs to the Equatorial Low. This creates the trade winds.

The position of the Equatorial Low and Subtropical Highs changes in a very predictable way during the year and gives places in the tropics very predictable seasons: wet when the Equatorial Low is nearby and dry when the band of Subtropical Highs is close. The tropics are hot throughout the year.

The pattern in the mid-latitudes

The mid-latitudes are the regions between the tropics and the Arctic or Antarctic. Here the winds flow from west to east very quickly (forming the prevailing westerly winds) and, just as in a fast-flowing river, disturbances, or eddies are an important feature.

A downward-spiralling eddy produces a temporary High (known as an anticyclone) with settled weather; an upward-spiralling eddy produces a temporary Low (called a depression) with unsettled weather. These anticyclones and depressions form and re-form, shifting position as they are carried along by the Westerly Winds (Westerlies). Lows, in particular, draw in air first from the tropics and then from the poles. This is why the mid-latitudes have the most changeable weather in the world.

The effect of the great continents

Lows and Highs can also be formed by heat and cold. It gets very hot over the centres of large continents each summer because they are far from the cooling effects of the oceans. These continents are therefore places where the hot air rises and thus where seasonal Lows form. The summer Lows draw in moist air from the nearby oceans and produce thundery rainstorms.

The same places get very cold in the winter and form seasonal Highs. Winter Highs are associated with mostly dry weather. These changing Highs and Lows add to the complexity of weather and may give rise to monsoons.

APRIL

In April the centre of the Equatorial Low lies close to the equator. Hot, thundery weather is to be expected in this area. Near the tropics, the Subtropical Highs produce drought.

Over the northern continents the cold winter Highs are fading away, and continental areas are heating up enough to begin to experience their first thunderstorms; some tornadoes are even possible. Within the Arctic Circle temperatures are now rising above freezing.

Over the southern continents there is a general cooling, and in the mid-latitudes, a hint of autumn.

During April, lands near the equator show a daily rhythm of rainfall, rather than the prolonged deluge that comes with the monsoon.

In many parts of the northern hemisphere people think that the weather gets drier after April. This is not because it rains less, but because the warmth of the air makes the rain dry up faster, and there is an end to the clinging damp of winter.

But, from the Sahara Desert to northern India the heat is building up under a clear sky. In South East Asia it is not quite as hot, but the humidity is high and so the air feels very sticky. But there is worse, much worse, to come in May.