Extreme Precipitation

What is Precipitation?

When warm air ascends, it transports water vapour. As the air rises, it cools, causing the water vapour to condense (transform from a gas to a liquid) and form clouds. These clouds eventually release precipitation, which falls to Earth as rain, sleet, snow, or hail.

Rainfall Patterns Worldwide

The amount of rainfall varies across the globe. The global circulation system significantly influences these patterns. Regions of low pressure, with rising air, tend to experience high levels of precipitation, while high-pressure areas, with descending air, have lower precipitation.

Globally, heavy rainfall occurs predominantly in a band around the Equator, whereas the tropics are generally drier. Regionally, coastal areas can be especially wet; locally, altitude can affect precipitation levels.

Types of Rainfall

Convectional Rainfall
Intense sunlight heats the ground and the air above it, causing pockets of warm air to rise in convection currents. As this air ascends to higher altitudes, it cools, and the water vapour condenses to form clouds. The resulting rainfall can be heavy and often includes thunderstorms. This type of rainfall is widespread at the Equator, where temperatures are consistently high.

Relief Rainfall
When winds encounter mountains, the warm air is forced to rise, leading to cooling, condensation, and precipitation on the windward side. As the air descends on the leeward side, it becomes dry, creating a rain shadow. In the UK, mountains like the Pennines are typically wetter on their western sides and drier on their eastern sides due to this phenomenon.

Frontal Rainfall
A front is formed when warm air meets cooler air, causing the warm air to rise above the cool air. This leads to the formation of clouds and eventually steady rain. Frontal rainfall is common in the UK, situated at the junction where warm air from the tropics meets cool air from the North Pole.

Extremes of Wet and Dry

Cherrapunji: The Wettest Place on Earth
Cherrapunji, located in the Khasi Hills of northern India, is known for its extreme rainfall, holding the record for the highest monthly rainfall with 9300 mm in July 1861. Moist air from the Bay of Bengal moves across the flat terrain of Bangladesh and ascends rapidly upon encountering the Khasi Hills, causing intense relief rainfall.

The Atacama: The Driest Hot Desert
While all deserts are characterized by low precipitation, the Atacama Desert in Chile is the driest hot desert, receiving an average annual rainfall of only about 15 mm. Some weather stations in the Atacama have never recorded any rain. Positioned on the Tropic of Capricorn, it lies in the rain shadow of the Andes mountains and is influenced by a cold ocean current, which prevents the formation of moist, warm air and clouds. Despite the harsh conditions, some mining towns exist in this arid region.

Antarctica: The Cold Desert
Antarctica, classified as a cold desert, is even drier than the Atacama. The frigid air holds very little water vapour, resulting in minimal precipitation.