Cool air turns vapour to steam

Derek Syms, of Auckland, asked:

Water boils at 100degC and turns into steam. But clothes on a clothesline steam when the ambient temperature is only 18degC. Why is this?

John Campbell, a retired physicist at the University of Canterbury, responded:

There is an intermediate step which you need to observe first. When water evaporates it turns to water vapour, which is just individual water molecules suspended in air. Water vapour is invisible. As the water vapour cools due to the surrounding air, the air cannot support so much water vapour so it condenses out to water again (i.e. small water droplets), forming steam.

Water vapour is invisible to our eyes. If you look closely at a boiling kettle, the vapour near the spout is invisible (or nearly so, apart from turbulent changes of density which cause light scattered from objects behind the vapour to be distorted).

Air at 100degC can hold more water vapour than at lower temperatures. As the hot mixture of air and vapour cools in the surrounding cooler air, water droplets are formed and the plume becomes visible as ‘‘steam’’. This forms some centimetres from the spout.

A similar example is a jet engine. It burns fuel and so gives out water vapour at very high temperatures. As this plume cools again, water droplets (or even ice) are formed.

It is a common observation, when air conditions at high altitude are right, to see a contrail behind a jet. Note that the visible ‘‘steam’’ is some distance behind the aircraft.

It is basically impossible for a water droplet (or a single crystal) to form starting from nothing. The pressure inside a drop depends inversely on its radius, so it is energetically not possible for drops to condense in very pure air. (In that state it is called supercooled.) So water droplets form on impurities. This can be ions from the jug water, unburnt fuel in the jet engine, or, in the case of rain, on dust in the air. That is why the air appears much clearer just after rain. All the dust has formed water droplets and fallen out of the sky.

Due to energy from sunshine, or some other source, water is evaporating from anything that is wet (liquid), thus turning it into invisible water vapour.

Once the air it is dissolved in cools, the excess water vapour turns into visible steam (water droplets). I have just observed this coming from my fence when the sun came out after rain.

Contrails (short for ‘‘condensation trails’’) are lineshaped clouds produced by aircraft engine exhausts or changes in air pressure, typically at aircraft cruise altitudes several miles above Earth’s surface. They are composed primarily of water, in the form of ice crystals.