Atmosphere The blanket of gases surrounding the Earth.

Climate A long-term pattern of weather. An area’s climate mainly depends on its location on the Earth’s surface, but is also affected by altitude, and by winds and ocean currents, which carry air masses around the globe.

Desert climate A pattern of very dry weather in an area (desert) where less than 25 cm (10 inches) of rain falls in one year.

Dry season A long period of very dry weather. It is one of two seasons that occur in tropical climates.

El Niño A climate pattern that occurs off the west coast of South America every four to seven years. Ocean currents change and warm water moves east. The air above the ocean is warmed, causing rain and storms. More distant areas may suffer droughts as a result.

Equator An imaginary line that runs horizontally around the centre of the Earth.

Hemisphere One half of the Earth’s surface. The Equator divides the Earth into northern and southern hemispheres.

Microclimate The climate of a particular local area. For example, towns are often warmer than the countryside around them. This is because bricks and concrete absorb heat in the day and release it through the night.

Monsoon Winds that change direction between summer and winter. In summer the winds bring torrential rain with them. Monsoons occur where there are great differences in temperature between oceans and land.

Mountain climate A pattern of cold, windy weather that occurs in mountainous regions.

Ozone layer A thin layer of ozone gas that is found in the stratosphere. It absorbs 97–99% of the Sun’s ultraviolet rays.

Polar climate A pattern of cold, dry, windy weather that occurs near the poles.

Seasons Time periods in the year, marked by changes in the weather. The seasons occur because the Earth is tilted on its axis. The half of Earth tilted towards the Sun has summer, while the other half has winter.

Stratosphere The layer of the atmosphere that lies 11–50 km (7–30 miles) above the ground. Jet planes sometimes fly in this layer to avoid storms in the troposphere.

Temperate climate A pattern of weather with warm summers and cool winters.

Trade winds Winds that always blow in the same direction and with the same strength. Near the Equator, the air is warmed by the Sun so that it rises and spreads out. Cool air from the north and south poles blows in to replace it. The trade winds are shifted sideways by the Earth’s rotation so that they always travel slightly east or west.

Tropic of Cancer An imaginary line that runs around the Earth between the Equator and the North Pole.

Tropic of Capricorn An imaginary line that runs around the Earth between the Equator and the South Pole.

Tropical climate A pattern of hot, wet weather throughout the year. Tropical regions lie close to the Equator where the Sun shines directly overhead. There are only two seasons: wet and dry.

Troposphere The layer of the atmosphere that is closest to the Earth. It is sometimes called the “weather layer” because it contains the Earth’s clouds and winds.

Wet season A long period of very wet weather. It is one of two seasons that occur in tropical climates, where it is dry for half the year and wet for the other half.

Consultant: Ian Fairchild