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.

Ice Fog: Ground Level Contrails

Know how a contrail forms?

No, it has very little to do with particles produced by a jet engine (or a propeller engine, for that matter.) The culprit is water vapor.

Burning any hydrocarbon fuel, no matter how cleanly, produces two gasses: carbon dioxide and water vapor. If the fuel is dirty or combustion is incomplete or very hot, other things may be produced—sulfur compounds, nitrogen oxides, carbon monoxide, particulates—but the energy we get out of burning hydrocarbons comes from combining oxygen from the air with the hydrogen and carbon that make up the bulk of the fuel.

An oxygen atom plus two hydrogen atoms is a molecule of water. A carbon atom plus two oxygen atoms is a molecule of carbon dioxide. The definition of clean combustion is combustion in which only these two compounds are produced.

Fuels vary in their ratio of carbon to hydrogen. Coal has about equal quantities of each, and when cleanly burned produces more than three times its weight in carbon dioxide and somewhat less than its weight in water. Gasoline has about 2 atoms of hydrogen to one of carbon, and produces a little less carbon dioxide but more than its weight in water. Straight hydrogen does not produce carbon dioxide, but produces a whopping nine times its weight in water.

In most climates, we can ignore the water, at least near the ground. But the air can only hold so much water, and the amount it can hold decreases rapidly with temperature. What’s more, the limit on how much it can hold differs depending on whether ice is present. It is perfectly possible for air to have more moisture than it could hold if ice were present, but not enough that moisture condenses out in cloud droplets. In fact, this is very common at high elevations.

Further, cloud droplets can have a temperature well below freezing, but still be liquid droplets. They can be triggered into freezing by ice nuclei, the most effective nucleus being a sliver of ice. This is how wing icing on airplanes occurs, by flying through what are called supercooled clouds—clouds of liquid water drops at subfreezing temperatures..

At very low temperatures, below about –40, an ice nucleus is not necessary, as a droplet will freeze spontaneously.

Now imagine an airplane flying in air with a temperature below –40 (true of most commercial flights today) with a moisture content not high enough for cloud formation, but high enough that ice crystals can grow. The engine exhaust contains large concentrations of water vapor—enough to cause condensation of droplets just behind the plane. Since the temperature is below –40 these droplets will freeze very rapidly. Once they are frozen they gather in water from the air around them. The result is a contrail that is not only visible, but grows.

Most areas don’t have ground temperatures below –40 very often—but here in Fairbanks, Alaska, we do. Automobiles leave contrails in these conditions. More, many of the pollutant particles we spew into the air act as ice nuclei at temperatures a little warmer than –40, so the combined persistent contrails—ice fog—can occur well above –40. It’s fog made of ice particles, rather than water droplets. It’s densest just behind each vehicle, making it hard to see the tail lights of the car ahead.

Growth from vapor makes well-formed crystals that produce optical effects like sun dogs, halos, and ice pillars. We have those, but not in ice fog. Ice fog particles are basically frozen droplets, and while they are crystalline, they do not generally have the clearly defined facets necessary for them to act as prisms. So ice fog looks just like fog.

Sad to say, my photos of ice fog all seem to be slides that have not been digitized. Does anyone have a good photo of ice fog or contrails I could put on this page?

Geophyscal Christmas Carols 3

Here Comes Ice Fog (to the tune of “Here Comes Santa Claus”)

Here comes ice fog, here comes ice fog,
Right down Airport Way,
Hazing street lights, hiding tail lights,
Darkening the day.
Pickup contrails flow together
Wreathing through the town
Temp’rature’s at minus forty
And it’s going down.

Plug your car in, heap the blankets
On its shivering hood.
Tires are square and steering’s stiff and
Traction’s not so good.
Peering through the frosted windshield
Hours before the dawn –
Let’s all head off to Hawaii
‘Til this weather’s gone!

All of these “Geophysical Christmas Carols” were originally written for the Christmas party at the Geophysical Institute, where I worked for many years. They echo both genuine geophysical research and life in Alaska. Enjoy!