From Evaporation to Flood: How Moisture and Air Pressure Make Weather

Start with the supply. Solar energy drives evaporation from oceans, lakes and soil, and the water vapor rises, cools, condenses into clouds and later falls again as precipitation.

This is where the vapor comes back down. Small droplets or ice crystals in a cloud collide and merge until air currents can no longer hold them, and about 505,000 cubic kilometres of water falls each year, averaging 990 millimetres worldwide.

Terrain decides where rain lands. Air forced up a mountain cools to its dew point, so windward uplands of Hawaii receive over 100 inches of rain a year while leeward Waikiki gets under 20.

Water bodies can seed weather too. Cold air crossing at least about 100 kilometres of warmer lake water picks up moisture and drops it as snow or rain on the downwind shore, most strongly around the Great Lakes.

Pressure systems steer the pattern. Sinking, dry air in a high suppresses cloud formation, bringing clear skies and, in temperate latitudes, warm dry weather in summer and cold dry weather in winter.

A permanent high-pressure belt sits near 30 degrees north and south, where air that rose at the equator sinks back down. The steady dry, sunny conditions it produces coincide with major deserts including the Sahara and the Atacama.

When a strong high stalls, it can cap the atmosphere like a lid. The 2021 Pacific Northwest heat dome showed this pattern, with compression further heating the trapped air while the system blocked cooling breezes and clouds.

The opposite case is a low, where pressure is lower than its surroundings and cloud, wind and possible rain or storms are common. Winds circulate counterclockwise in the Northern Hemisphere and clockwise in the Southern because of the Coriolis effect.

Lows often travel along boundaries between unlike air masses. Cold fronts can bring thunderstorms and heavy precipitation, warm fronts are usually preceded by gradual rain and fog, and even a quiet front shows a noticeable wind shift as it passes.

A rotating low becomes a cyclone. Tropical cyclones are warm-core systems fueled by condensation over warm ocean water, while extratropical cyclones form along mid-latitude temperature gradients and typically last two to six days as the jet stream steers them.

Cyclones can also feed long corridors of vapor. An atmospheric river is thousands of kilometres long, can carry more water than the largest river on Earth, and is the main cause of extreme precipitation and flooding along mid-latitude westerly coastlines.

The path ends where water overwhelms the land. Floods take river, coastal, groundwater and flash forms, and because floodplains long attracted settlement, a large share of major cities and farmland now sits in areas of significant flood risk.