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.
Today 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.
The 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.
A 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.