Tag Archive: Contrail

Cirrus Clouds

Jet stream cirrus is by no means the only kind of cirrus cloud. Cirrus clouds come in many different forms. The thing they have in common is that they are all made of ice crystals, and thus they are all in cold parts of the atmosphere – generally below -20°C, or -4°F. At these temperatures, the ice nuclei that turn supercooled water droplets into ice are generally common enough that at least a few droplets freeze. Since ice can grow by direct sublimation of water vapor at humidities where supercooled water would evaporate, the ice particles grow rapidly at the expense of the water droplets.

cirrus streakToday I’m going to talk about (and illustrate) those types of cirrus that are closest to the original meaning of the word: latin cirrus, or curl of hair. This original meaning is maintained in the fact that cirrus clouds often appear fibrous or silky.

Cirrus are sometimes considered warning clouds, because they are often the first sign of an approaching warm front. In such a front relatively warm, most air is pushed upward along a sloping surface, with colder, drier air below. The warm air cools as it is forced upward, and at its highest, which is the first part we normally see, it is very cold indeed – cold enough that when the moistest patches cool enough to produce condensation in the form of tiny liquid water droplets at temperatures well below freezing, the droplets freeze almost at once.

Mares' tail cirrusThe air has cooled enough by then that it is more than saturated relative to ice. The ice crystals formed by freezing of drops grow very rapidly and fall, the fall streaks being drawn out by the winds at various heights of the air. The result may be what is called mares’ tails, or it may be less well defined, but the fibrous or silky appearance is pronounced. This appearance is not always due to an approaching front, but if the cirrus is increasing with time and gradually covers the whole sky, it’s best to prepare for a storm.

contrailsA hundred years ago that was the prevailing type of scattered cirrus, but today contrail cirrus is probably more common. Contrail cirrus differs from warm front cirrus in that the original water cloud is formed because burning of fossil fuel produces even more water vapor than it does carbon dioxide. As a result, planes actually add water vapor to the air. In addition, the eddies formed by planes moving through the air may cause local lifting and cooling enough to cause condensation.

If humidity is such that ice can grow while water drops evaporate, and the temperature is low enough that a portion of the water drops freeze, the result is a persistent contrail that grows with time. In areas with heavy air traffic, the contrails of multiple airplanes may actually produce a sheet of cirrus that covers most of the sky. More often streaks of cirrus clouds follow the planes.

Although we sometimes refer to jet contrails, the only real connection with jets is that most of the planes flying that high today are jets. In fact, contrails first became a problem during World War II when propeller bombers, flying high to avoid detection, found themselves leaving cloud trails that shouted “we’re here!” to antiaircraft batteries on the ground.

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?