Air The mixture of gases, mostly nitrogen and oxygen, that surrounds the Earth.

Air mass A large body of air of a similar temperature and moisture content. Air masses are separated by fronts.

Air pressure The weight of air pressing down on the Earth. Air pressure is low where air masses are rising. The rising air will cool and condense, forming rain clouds. Pressure is higher where air masses are sinking. High air pressure is usually accompanied by dry weather.

Anticyclone A large air mass that rotates around a centre of high pressure, an area of sinking air. Anticyclones rotate clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere (opposite to an cyclone). Anticyclones typically result in stable, fine weather with clear skies.

Avalanche A rapid flow of snow down a slope.

Barometer An instrument that measures air pressure.

Beaufort Scale A scale used to measure the strength of the wind, where 0 is complete calm and 12 is a hurricane.

Cloud A visible cluster of water droplets, ice crystals or both. The droplets are so small that they can remain in the air. Different types of cloud form at different heights and under different conditions.

Condensation The process of a gas changing into a liquid when it is cooled. Water vapour condenses more easily if the air contains small particles, such as dust, salt or smoke, for it to collect around.

Cyclone A large air mass that rotates around a centre of low pressure, an area of rising air. Cyclones rotate anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere (opposite to an anticyclone). They are often described as depressions or lows. As the rising air cools and condenses, clouds form, so the weather is often cloudy and wet. Tropical cyclones (another name for hurricanes) are low pressure systems that form over warm tropical waters. They are characterized by strong winds and heavy rain.

Dew Droplets of water that cover the ground overnight. Dew forms when the ground cools down faster than the air, causing moisture in the air to condense.

Drought A period where there is significantly less rainfall than normal. When an area with an arid or semi-arid climate goes without rain for a lengthy period of time, this is not necessarily as a drought, because such conditions are expected.

Evaporation The process in which a liquid changes into a gas when it is heated up or when the air above it is dry.

Flood An overflow of water on to areas of surrounding land. Flooding is often caused by particularly heavy rain swelling the level of water in rivers, which then burst their banks, or a storm at sea causing seawater to surge on to coastal areas.

Fog A thick layer of cloud-like moisture formed by water vapour cooling at ground level. Fog is thicker than mist.

Freezing rain Liquid rain that freezes on contact with surfaces, such as the ground, trees, electrical wires, aircraft and cars. 

Front A boundary between two air masses. A warm front occurs when warm air flows up and over colder, heavier air. This makes the moisture in the rising air condense and fall as rain. A cold front occurs when cold air pushes against warm air, forcing it upwards where it forms clouds.

Frost A thin layer of sparkling ice crystals that forms when the air is below freezing and dew freezes on the ground.

Hail Precipitation that falls as ice. Hail forms in large cumulonimbus clouds where water droplets are tossed about by strong upward-moving air currents. Each time a raindrop is pushed into the freezing top of the cloud a thin layer of ice forms around it. When the drop is too heavy to be lifted by the currents, it will fall to the ground.

Humidity The level of moisture in the air. Air with lots of water molecules in it is very humid. Warm air can hold more moisture than cool air.

Hurricane A massive, spinning storm of wind and rain. Hurricanes form when warm, moist air from the oceans is stirred up into a huge, spinning mass. Strong winds and rain swirl around a calm centre, the “eye” of the storm. The spiralling winds are caused by the force of the Earth rotating on its axis.

Lightning A visible spark of electricity produced during a thunderstorm. Electricity can build up inside clouds when ice and water molecules bump against each other and become electrically charged. The light we see is a strip of air being suddenly heated by the electricity. Fork lightning travels from clouds to the ground. Sheet lightning occurs within clouds.

Meteorology The study of the weather.

Mist A thin layer of cloud lying at ground level. Mist is thinner than fog.

Mistral The cool northerly wind that blows down the Rhône Valley in southern France. It occurs when low pressure in the northern part of the Mediterranean Sea draws in air from the north, which is funnelled between the Alps and the Massif Central.

Precipitation Water that falls in the form of rain, hail, sleet or snow. The temperature inside and outside a cloud determines which form precipitation will take.

Rain Liquid water that falls from clouds.

Rainbow An arch of colours in the sky that forms when the Sun shines from behind you at rain falling in front of you. The raindrops split the light into all the different colours of the spectrum.

Sleet Snow that falls through warm air and is partially melted when it reaches the ground.

Snow Precipitation that falls to the ground as clumps of ice crystals, called snowflakes. All snowflakes have six sides, but no two are exactly alike.

Thunder The loud sound that accompanies a flash of lightning. The heat of the flash makes the air around it expand so fast it makes a loud boom.

Tornado A twisting column of air that extends from the base of a cloud to the ground, like an upside-down funnel. A tornado's winds are so powerful, it can pick up people, cars and even houses.

Water cycle The process by which water circulates from the land or oceans to the atmosphere and back again. When it is warmed by the Sun, water evaporates and rises. As it does so it cools and condenses, forming clouds. Rain and snow fall to the ground, where it flows into rivers, lakes and seas and the water cycle continues.

Weather Weather describes the conditions and changes that take place in the lower atmosphere, up to about 20 km (12.5 miles) above the Earth’s atmosphere. It includes temperature, wind speed and direction, cloud type and cover, rain, hail, snow, frost, ice, drought and storms.

Wind Wind is air moving from one place to another. The Sun makes some regions of air warmer than others. When warm air rises, cooler air flows in to take its place.

Consultant: Ian Fairchild