Weather and climate, Year 7: more practice

A rain gauge

提示It collects the water in a container.

为什么Rainfall is measured in millimetres (mm) of depth collected over a period such as a day.

Millimetres (mm)

提示It is a depth, not a volume.

为什么One millimetre of rain is a layer of water one millimetre deep over the ground.

So direct sunlight does not heat it and give a false reading

提示Air temperature is not the same as how hot a thermometer in the sun gets.

为什么Weather stations use a screen that lets air through but blocks the sun, so the reading reflects the air itself.

Convectional rainfall

提示Common on hot summer afternoons.

为什么The rising warm air cools, water vapour condenses into cloud, and heavy showers or thunderstorms often follow.

Frontal rainfall

提示Named after the boundary between the two air masses.

为什么The lighter warm air rides up over the heavier cold air, cools and condenses. Many UK wet spells are frontal.

The leeward side

提示The air has already lost its moisture by then.

为什么Air rises on the windward side and drops its rain. Descending on the other side, it warms and holds its moisture. This is the rain shadow.

Rise and cool

提示Water vapour only condenses when it is cooler.

为什么Rising air cools, vapour condenses into droplets, and clouds form. When the droplets grow heavy enough, they fall as rain.

Cold

提示Think where it began its journey.

为什么Polar maritime air is cold and showery. It crosses the sea on its way south, picking up moisture.

Hot and dry

提示It begins over warm land, far from the sea.

为什么Continental means it has formed over land, so it carries little moisture. In summer it can bring heatwaves.

Different air masses with different temperatures and moisture keep arriving

提示Each air mass brings its own character.

为什么Britain sits where polar and tropical, sea and land air meet, so conditions can change within days or hours.