How does Rain Form?

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.

Why?

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.

Fog, Fog and Fog

Freezing fog. That term has been used by the local radio station lately to refer to ice fog. (At least, that’s what I think they mean.) There are at least three different kinds of fog made of oxygen dihydride (water.) None of them are well described by the term freezing fog.

The first and commonest, which I will refer to as warm fog, is certainly not freezing fog. It is composed of very small drops of liquid water, with the temperature above freezing. This kind of fog is what is  stable: the droplets do not collide, grow and fall out, and seeding is useless. Many low-level clouds are exactly like this kind of fog, and they very rarely initiate rain. The only situation in which this type of fog could produce anything that might possibly be called freezing fog is if it is carried over a surface – road, wire, or tree branch – which is well below freezing. This might happen in Alaska if we have had a week at 40 below and we suddenly get a warm fog, but it is certainly not common.

The second kind of fog, which produces ice storms and can be dissipated by seeding, is supercooled fog. This is a fog made up of liquid droplets which are below freezing temperature. It is very common in clouds well above the ground, where it is responsible for aircraft icing.

Liquid water? Below freezing?

Ice melts at the freezing point, but water does not automatically freeze. Ice has an ordered crystal structure, and you can think of liquid water at temperatures below freezing as needing a little shove to get the molecules into the right order. Something that helps produce that order is known as a freezing nucleus. The best nucleus as actually a splinter of ice, but there are many other possibilities. A reasonably large volume of water usually has some impurities that will act as ice nuclei at temperatures only a little below freezing. Also, if a tiny droplet hits almost anything it will freeze. But that same droplet, floating in the air, may remain unfrozen at temperatures quite a bit below freezing.

The colder it is the more things are available to act as nuclei, and in clouds, the most dangerous temperatures for icing are generally above 0°F. So fogs of temperatures below freezing but above 0°F are very likely to be supercooled fogs. They can be dissipated by seeding, but they can also be responsible for ice buildup on streets, wires and branches. (Ice storms can also be caused by rain falling through sub-freezing air, but supercooled fog alone is enough.)

Fogs at temperatures between 0°F and -20°F are actually quite rare in nature. Below -20°F, and especially at temperature approaching and below -40°F, a third type of fog may appear: ice fog.

Ice fog is made up of tiny spherical droplets, and looks just like any other fog. The difference is that the droplets are ice. You could call ice fog frozen fog, though not freezing fog. In nature, ice fog is pretty well confined to temperatures below -40°F, as water droplets freeze without needing a nucleus at around that temperature. A source of water is needed, so natural ice fog tends to occur around herds of caribou or warm springs. (Yellowstone was actually used for some early ice fog research.)

In built-up areas, combustion produces not only water vapor, but some ice nuclei. Consequently some water droplets freeze and some do not, and the ones that freeze are able to grow a bit by vapor growth from the evaporation of those that don’t freeze. Some ice fogs at relatively warm temperatures may even grow into well-formed ice crystals, and produce some of the optical effects often associated with ice crystals.

You are very unlikely to see ice fog unless you live in an area where 40 below temperatures are common, but fog at temperatures below freezing is likely supercooled fog. Supercooling, by the way, is very important in the formation of most raindrops — but I’ll talk about that some other time.