☁️ Cloud cover
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How much of the sky is covered by cloud — measured in percent and split across low, mid and high layers.
Cloud cover is the fraction of the sky obscured by cloud, usually given as a percentage from 0% (clear) to 100% (overcast). This page also breaks cloud into low, mid and high layers, because clouds at different heights are made of different things, form in different ways, and tell you different things about the weather to come.
Clouds form when air rises, expands and cools until the water vapour it carries condenses onto tiny particles into droplets or ice crystals. Air is lifted in several ways: by heating from the ground, by being forced up over mountains, by converging at weather fronts, and by turbulence. Where the air sinks instead, clouds evaporate and skies clear.
Low clouds (below about 2 km), such as stratus and cumulus, are made of water droplets and bring drizzle, showers and grey overcast skies. Mid-level clouds (2-6 km), like altostratus and altocumulus, often signal an approaching weather system. High clouds (above 6 km), such as wispy cirrus, are made of ice crystals and can be the first sign of a front a day or more away.
Cloud is the great regulator of temperature. By day it reflects sunlight and keeps the surface cooler; by night it acts like a blanket, trapping heat and keeping temperatures up. This is why clear nights are colder and more prone to frost and fog, while cloudy nights stay mild. Cloud cover also governs how much solar energy reaches solar panels and how strong the UV is.
Traditionally, observers estimated cloud cover in eighths of the sky, called oktas. Today, automated ceilometers fire a laser upward to detect cloud base and amount, and satellites map cloud across whole continents. Forecast models predict cloud at each level from humidity and vertical motion, which is what the layered figures here represent.
They represent the fraction of the sky covered by cloud. 0-10% is clear, around 50% is partly cloudy, and 90-100% is overcast. The layered values show how much of each is low, mid or high cloud.
Clouds trap heat radiating from the ground, acting like a blanket. Clear nights let that heat escape to space, so temperatures fall further and frost is more likely.
Yes. Many cloud types, especially thin high cloud or fair-weather cumulus, produce no rain. Rain needs clouds deep and moist enough for droplets to grow large enough to fall.
Only partly. Thick storm cloud blocks most UV, but thin or broken cloud transmits the majority, so sun protection is still needed on overcast days.