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.

Snowflakes

In the air, vapor’s swirling,
On the pond, folks are curling,
The vapor makes drops, the drops freeze and pop,
And six-sided snowflakes fall down.

On the lake, skates are gliding,
Overhead, clouds are hiding,
Ice in the sky is growing, oh, my,
And six-sided snowflakes fall down

Snowflakes could be square or five pointed,
Or octagons, or spherical, you know,
But water with water is jointed
So that only six arms can grow.

On the slopes, skiers swish on,
Snowflakes hide stars to wish on,
They fall through the air, and catch in your hair,
The six-sided snowflakes fall down.

The rhyme above can be sung, to the tune of “Winter Wonderland.” But it’s also a fairly good outline of why snowflakes look the way they do.

A water molecule is made up of one oxygen atom and two hydrogen atoms. The hydrogen atoms are not in a straight line with the oxygen atom, but are angled, like a bent line with the oxygen at the bend.

Ice, being crystallized water, is made up of water molecules in three-dimensional order. The water molecules in an ice crystal are held together by what are called hydrogen bonds — each hydrogen atom links not only with the oxygen in the water molecule, but with the hydrogen atom of a neighboring molecule. Given the shape of the water molecule, the easiest way the molecules can form an ordered structure is a hexagonal lattice. I’m not going to try to draw it, but there is a good drawing in this reference.

Most snowflakes actually start out as water droplets in clouds. A few droplets encounter ice nuclei as the temperature drops below freezing, and freeze into ice droplets. Sometimes the droplets explode to make many ice particles as they freeze, and each bit of ice can nucleate another droplet.

If ice and water are side by side at subfreezing temperatures, the ice will suck up water vapor from the water. The growth on the ice will be strongest at the sites where the crystal lattice juts out farthest, so the frozen droplet rapidly grows into something like a very short bit of a hexagonal pencil. The edges and corners of this hexagonal prism grow fastest, and sometimes even sprout arms.

Why are snowflakes often symmetrical, but different from each other? The type of growth is determined by the temperature and moisture of the air at the moment of growth. As each snowflake follows a slightly different path through the cloud, it will encounter a different sequence of growth than any other snowflake. At the same time, all of its six arms see the same sequence. The result is a snowflake that is fairly symmetrical but different from any other snowflake.

Very simple snowflakes – usually simple hexagonal plates or needles – may look very similar to each other. But the more complex dendritic snowflakes are generally one of a kind, because each has had a unique path through the cloud that spawned them.

We have snow of the ground now, here in Fairbanks, and many other areas farther south will soon. If you live in snow country, invest in a small hand lens and enjoy the myriad shapes of the snowflakes.

(Photos are from Bentley’s collection of snowflake images.)