Show description

1. Heavy rain A torrential rainstorm is triggered by air rising and cooling over high ground. 2. Waterlogging Rain falls too fast to soak up because the ground is waterlogged, baked hard or impermeable rock. 4. River rises The waters rise dramatically, often within two hours of the start of the storm. 3. Steep valley The excess water races down steep slopes into the river, driven by gravity. 5. Banks burst Unable to accommodate the deluge, the river abruptly bursts its banks and gushes across the valley floor.

2 Wasilla, Alaska 3 Bonneville flood, North America When the glacial Lake Atna in Alaska breached a natural ice dam around 17,000 years ago, the megaflood released as much as 1,400km3 (336mi3) of water – that’s enough to cover the area of Washington DC in water 8km (5mi) deep! During a wet period 14,500 years ago, Lake Bonneville overflowed its basin. The floodwaters stripped soil, filled a 100m (330ft)-deep canyon to the brim and created immense and powerful waterfalls. 4 This thermal image of the English Channel is evidence of an ancient megaflood that carved out the channel and made Britain an island Yarlung-Tsangpo Gorge, Tibet Gigantic lakes trapped in the Himalayas by ice dams helped erode this majestic gorge – which is possibly Earth’s deepest.

In this weather system, intense exposure to sunlight at the equator causes warm, moist air to rise. As the air rises, it cools again, forming a low-pressure system that results in regular thunderstorms across the region. Above these storms, the jet stream – a current that flows through Earth’s upper atmosphere – carries the air towards higher latitudes until it eventually descends over the tropics to the north and south of the equator. As it falls, it creates a high-pressure system that is responsible for the arid conditions of the Sahara and other deserts that populate this particular region.