Rain, snow and the freezing level

Millions of cloud droplets have to join up to make a single raindrop.

Why most clouds never rain

Look up on a day with fair-weather clouds and you are looking at tonnes of water that will never reach you. Cloud droplets are tiny, typically about 0.02 mm across, and they fall so slowly that the gentle updrafts holding the cloud up are enough to keep them there. The USGS explains that a drop has to grow until its fall speed beats the updraft before it can leave the cloud as precipitation.

Growing is hard work. A raindrop holds as much water as millions of cloud droplets. In warm clouds the droplets grow by collision and coalescence: bigger droplets fall a little faster, bump into smaller ones and merge. In clouds that reach below freezing there is a faster route, the Bergeron process. Ice crystals and supercooled droplets sit side by side, and the crystals grow at the droplets' expense. The crystals fall as snow, and often melt on the way down into rain. Much of the rain in middle latitudes starts as snow.

Raindrops are not teardrops

Drawings show raindrops as teardrops, but the USGS describes something different. Drops smaller than about 1 mm in radius are round. As they grow, air pushing up from below flattens the bottom, and by 2 to 3 mm across they look like a hamburger bun. At about 4 to 5 mm the dent in the bottom opens into a parachute, and the drop breaks into smaller drops. That is why raindrops never grow to the size of marbles.

Rain or snow? Follow the 0 °C level

Air usually gets colder with height. The standard atmosphere used by NASA cools about 6.49 °C per kilometre. So if it is 15 °C at sea level, the 0 °C level is about 15 ÷ 6.49 ≈ 2.3 km up; at 5 °C it drops to about 0.8 km. Snow falling from above that height melts as it passes through it. When the 0 °C level reaches the ground, snow arrives as snow. Watch the mountain in the model: the white cap grows and shrinks as you move the temperature, because only the ground above the 0 °C level stays snowy.

Real air does not always follow the standard. Sometimes a warm layer sits above cold air near the ground. Snow that melts in a warm layer can refreeze into ice pellets (sleet) before reaching the ground. If the drops remain liquid below freezing, they freeze on contact with cold surfaces, producing freezing rain. Forecasters watch these layers closely because a difference of one or two degrees decides between a wet road and a skating rink.

On average the world receives about 980 mm of precipitation a year over oceans and land together, according to NOAA. It is very unevenly shared. The USGS notes that in 2017 Colombia received about 3,240 mm while Egypt received about 18 mm. A useful conversion: 1 mm of rain on 1 square metre is exactly 1 litre of water.