Evaporation and transpiration

For every 1 °C of warming, air can hold about 7% more water vapour.

Why wet skin feels cold

Step out of a swimming pool on a breezy day and you shiver, even in summer. NOAA uses this example to explain evaporation. Turning liquid water into vapour takes energy, and the water on your skin takes that energy from you. Sweating works the same way: the body spends its own heat to evaporate the sweat, and cools down.

In the water cycle the main energy source is the Sun. Sunlight warms the sea surface, and some molecules gain enough energy to break away from their neighbours into the air. Because oceans cover more than 70% of Earth's surface, they supply most of the vapour. The USGS estimates that about 90% of the moisture in the atmosphere comes from evaporation from oceans, seas, lakes and rivers, and about 10% from plants.

Warm air holds more

There is a ceiling on how much vapour air can hold, and it depends on temperature. Meteorologists describe it with the saturation vapour pressure. The NWS formula is es = 6.11 × 10^(7.5T ÷ (237.3 + T)) hectopascals, with T in °C. At 15 °C it gives about 17.1 hPa; at 25 °C about 31.7 hPa, nearly double. Each extra degree raises the ceiling by about 7%, the figure NASA quotes for the atmosphere as a whole.

This is why the model sends up more dots as you raise the temperature. It also explains a pattern NOAA describes in a warming climate: warmer air takes up more water before it rains, so dry spells can last longer, and when rain does come it can be heavier. The same 7% rule makes humid tropical air a huge store of energy, which is part of what powers hurricanes.

Plants as pumps

Transpiration is evaporation from leaves. Water enters a plant through its roots, travels up the stem and leaves through tiny pores called stomata on the underside of the leaves. NOAA notes that only about 1% of the water passing through a plant is used for growth; the remaining 99% goes into the air. A forest is therefore a large, quiet pump that lifts water from the soil into the sky. Hydrologists often combine the two processes and call them evapotranspiration.

A little vapour comes from ice. On cold, dry, windy days snow can sublimate, turning straight into vapour, which is why a snowbank can shrink without ever melting. The USGS counts this as a very small share of the moisture in the air.

Power stations use evaporation on purpose. The USGS describes cooling towers in which hot water from the plant is sprayed inside so that evaporation cools it before it goes back into a river, because hot water would harm the river's wildlife. The white plume above a cooling tower is not smoke; it is vapour condensing again into droplets in the cooler air. Across the whole planet, the USGS adds, the amount of water evaporating each year is about the same as the amount falling back as precipitation.