What is the difference between noctilucent clouds and cirrus clouds? When is the best time to see noctilucent clouds, and can they be forecast?
Cottage Grove, Minnesota
Cirrus clouds and noctilucent clouds are both ice-crystal clouds. They can both be very thin and fibrous, displaying lacey or filigreed detail and sometimes wavelike structure. The similarity ends there.
Cirrus clouds can be thick enough to block the sun or thin enough that blue sky shows through. Figure 1 shows a typical example of cirrus clouds. They are visible any time during the day and often provide rosy sunrises and sunsets. They are difficult to see at night unless the moon is out. Noctilucent clouds are so tenuous that they are usually seen only during deep twilight, not far above the horizon, and against a dark sky. Their color is often a distinctive silvery blue, unlike cirrus, which are white during the day or pink near sunrise and sunset. Figure 2 is a striking example of noctilucent clouds.
Caption: Figure 1. Ordinary cirrus clouds.
Caption: Figure 2. Noctilucent clouds, photographed just after midnight (Mountain Daylight Time) on July 2, 2011, with the skyline of Edmonton, Alberta (53.5°N latitude), in the foreground.
Cirrus clouds usually occur high in the troposphere (the lowest layer of the atmosphere), day and night, and in all seasons, usually at altitudes between 26,000 and 43,000 feet. Tropical cirrus clouds extend higher. Noctilucent clouds appear only in the summer hemisphere, from three to five weeks before summer solstice to seven to nine weeks afterward, usually at latitudes higher than 50°, and only at the top of the mesosphere. (The mesosphere lies above the stratosphere, which, in turn, lies above the troposphere.) Typical altitudes range from 51.6 to 52.8 miles. Figure 3 illustrates optimum conditions for viewing noctilucent clouds. They are best seen in deep twilight, when the sun is 6° to 16° below the horizon. Under these conditions, the troposphere and stratosphere are in the earth's shadow, but the mesosphere is still in sunlight, which illuminates the clouds. The atmosphere is so thin in the mesosphere (the pressure near the top is less than 0.001% of that at the earth's surface) that there are few molecules to scatter blue light, and so even the sunlit part of the sky appears black, thus bringing these gossamer clouds into prominence.
Caption: Figure 3. The geometry of sun and earth that makes noctilucent clouds visible. The sun must be more than 6° below the horizon so that these very high clouds, still in sunlight, may be viewed against a dark sky. Diagram not to scale.
Before an ice crystal cloud can form, the temperature must be low enough and the water vapor supply great enough for the deposition of water molecules on ice nuclei, or microscopic particles upon which ice crystals can form directly from vapor in the air.
In the troposphere, cirrus clouds form readily below about –22°F, provided the vapor supply is sufficient. At –22°F, a concentration of 1.5 grams of water vapor per kilogram of dry air is sufficient to form a cirrus cloud. The required vapor supply quickly decreases as the temperature falls. The supply of vapor in the troposphere comes almost exclusively from evaporation at the ground and from bodies of water. Convective updrafts are very efficient at lifting moisture from near the ground to high in the troposphere, particularly in thunderstorms.
In the mesosphere, noctilucent clouds form only at very low temperatures, below about –207°F. Such temperatures are the most extreme registered anywhere in the atmosphere. For comparison, the temperature of dry ice is –110°F. The only place such extreme cold occurs is at the top of the mesosphere and at high latitudes. The only time is a few weeks around the time of summer solstice.
There are two main sources of water vapor in the mesosphere. Some water vapor reaches the mesosphere from the troposphere far below. This vapor enters the stratosphere through what is called the “cold trap” at the tropopause (the top of the troposphere). Much of the vapor that would otherwise reach the stratosphere through the tops of very tall thunderstorms condenses in the very cold air at the tropopause. The vapor that survives the cold trap has a concentration of only a few parts per million (ppm) by volume; it diffuses slowly upward through the stratosphere to the mesosphere. In the upper stratosphere, chemical reactions occur that destroy methane and create water vapor molecules. These, too, diffuse upward into the mesosphere. These two sources together can raise the concentration of water vapor molecules in the lower mesosphere to a maximum of six to seven ppm. One obstacle to water vapor molecules reaching the top of the mesosphere is the destruction of some of them by intense ultraviolet radiation from the sun above 75 km. The water vapor mixing ratio at the height of noctilucent clouds is probably a few ppm. Such air is a thousand times drier than air in the middle of the Sahara desert.
The crystals in cirrus clouds range in size from 10 to 2000 μm (1 μm = one-millionth of a meter). The most common shapes are bullet rosettes, aggregates (multiple crystals stuck together), hollow columns, and hexagonal plates. Both the size and shape depend upon the temperature and the amount of water vapor present. In noctilucent clouds, the crystals do not have hexagonal shapes as they do in cirrus clouds. The mostly likely crystal habit is a special form of ice called cubic ice. The crystals are exceptionally tiny: the size range is from less than 0.025 μm (subvisible) to 0.100 μm.
To summarize, though cirrus clouds and noctilucent clouds both contain ice crystals, their visual appearance, the frequency of observation, the atmospheric conditions accompanying their formation, and the shapes and sizes of the ice crystals within them are strikingly different.
Weatherwise Contributing Editor THOMAS SCHLATTER is a retired meteorologist and volunteer at NOAA's Earth System Research Laboratory in Boulder, Colorado. Submit queries to the author at firstname.lastname@example.org, or by mail in care of Weatherwise, Taylor & Francis, 325 Chestnut St., Suite 800, Philadelphia, PA 19106.