Why low pressure brings clouds, and warmer nights
A low on the weather map usually means wind, cloud and rain. It forms when air far overhead spreads apart and pulls surface air upward, partly lifting the weight pressing down. As that air rises it cools into cloud, which cuts daytime heat but traps warmth after dark.
A low-pressure area is simply a region where air pressure is lower than its surroundings, marked with an L on English-language weather maps. Winds circle it anticlockwise in the northern hemisphere and clockwise in the southern, because the Coriolis effect acts in opposite directions. Large rotating lows with inward-spiralling winds are called cyclones, and the process that creates or strengthens them is cyclogenesis.
Most lows begin high in the troposphere. On the eastern side of big upper-level troughs in the westerly winds, or ahead of smaller shortwave troughs, the winds aloft diverge. Air below rises to fill the gap, which reduces pressure at the ground. Rising air cools as it expands, and once it drops below its dew point, clouds form. Those clouds reflect sunlight by day and absorb outgoing heat by night, so temperatures swing less than under clear skies. Stronger lows bring stronger winds.
Heat can do the job too. Deserts and continents warm quickly under strong sun, and the lighter air rises to create thermal lows. Across large landmasses these draw moist ocean air inland, driving monsoons. Over warm tropical seas, organised thunderstorms can build a warm-core system, and with sea temperatures of at least 26.5 degrees Celsius down to about 50 metres, moist air and little wind shear, it can become a tropical cyclone.
Lows come in many sizes. Extratropical and polar cyclones are the giants, while polar lows are small, fierce storms over high-latitude oceans with winds of at least 17 metres per second, hard to detect and dangerous to ships and offshore platforms. One study counts about 234 significant extratropical cyclones forming each northern winter. Globally, lows occur most often over the Tibetan Plateau and downwind of the Rocky Mountains, and in Britain and the Netherlands they are known as depressions.
Source: Low-pressure area
