Most of the rain that falls over the continents started out as snow or hail. It may be quite warm by the time it reaches the ground, but it started out as ice.


aftermath of an ice storm, from Morguefile

Aftermath of an ice storm. The raindrops were supercooled, and froze as they struck the twigs.

Cloud droplets, it turns out, do not coalesce easily. Most of them, especially in continental clouds, are so small that the air keeps them from actually colliding. The result is that most continental clouds do not rain unless they are tall enough to reach well above the freezing level.

But what if ice and liquid water are both present in a cloud?

No, that’s not impossible. Water can exist as a liquid at temperatures below freezing. It’s not stable, and any small particle around which ice can form will result in rapid freezing, but such particles are rare in the free atmosphere. They do, however, occur. Furthermore, they are activated at different temperatures, and the colder the air, the more particles that can act as freezing nuclei.

At temperatures below freezing but above about -15°C (5°F) such particles are rare enough that most clouds are made up of supercooled water droplets. These are the temperatures at which pilots dread having to go through clouds, because the plane acts as a giant freezing nucleus and any droplets that strike it are instantly turned to ice. Below about -20°C (-4°F) there are generally enough ice nuclei that the cloud is made up of ice crystals, or glaciated.

What happens if both ice and supercooled water are present?

Water evaporates more easily than ice at temperatures below freezing. The result is, water evaporates from the supercooled droplets and condenses on the ice crystals, which grow rapidly at the expense of the droplets. Since they grow more rapidly, they begin to fall faster, and accumulate still more water droplets by collision. If they get large enough to continue falling into the warmer air below the freezing level, they will (usually) melt and reach the ground as rain.

If there are really strong updrafts, as is often the case in a thunderstorm, they may actually be carried upward through the supercooled part of the cloud. They may collect several layers of ice, too much to melt once they fall out of the updraft. The result is hail.

If there is a layer below the cloud which is warm enough to melt the snowflakes but the ground itself is below freezing, the result may be freezing rain. But in most cases freezing rain started out as snowflakes that thawed, but then fell back into colder conditions near the ground.