Mysterious Night-Shining Clouds Getting Brighter
Clouds bright enough to see at night are not as hard to find as they once were.
These so-called night-shining clouds are still rare — rare enough that Matthew DeLand, who has been studying them for 11 years, has seen them only once. But his odds are increasing. [Related: In Images: Reading the Clouds.]
These mysterious clouds form between 50 and 53 miles (80 and 85 kilometers) up in the atmosphere, altitudes so high that they reflect light long after the sun has dropped below the horizon.
DeLand, an atmospheric scientist with NASA’s Goddard Space Flight Center in Greenbelt, Md., has found that night-shining clouds — technically known as polar mesospheric or noctilucent clouds — are forming more frequently and becoming brighter. He has been observing the clouds in data from instruments that have been flown on satellites since 1978.
For reasons not fully understood, the clouds’ brightness wiggles up and down in step with solar activity, with fewer clouds forming when the sun is most active. The biggest variability is in the far north.
Underlying the changes caused by the sun, however, is a trend toward brighter clouds. The upward trend in brightness, DeLand said, reveals subtle changes in the atmosphere that may be linked to greenhouse gases.
Night-shining clouds are extremely sensitive to changes in atmospheric water vapor and temperature. The clouds form only when temperatures drop below minus 200 degrees Fahrenheit (minus 130 degrees Celsius), when the scant amount of water high in the atmosphere freezes into ice clouds. This happens most often in far northern and southern latitudes (above 50 degrees) in the summer when, counter-intuitively, the mesosphere is coldest.
Changes in temperature or humidity in the mesosphere make the clouds brighter and more frequent. Colder temperatures allow more water to freeze, while an increase in water vapor allows more ice clouds to form. Increased water vapor also leads to the formation of larger ice particles that reflect more light.
The fact that night-shining clouds are getting brighter suggests that the mesosphere is getting colder and more humid, DeLand said. Increasing greenhouse gases in the atmosphere could account for both phenomena. In the mesosphere, carbon dioxide radiates heat into space, causing cooling. More methane, on the other hand, puts more water vapor into the atmosphere because sunlight breaks methane into water molecules at high altitudes.
So far, it’s not clear which factor — water vapor or cooling — is causing polar mesospheric clouds to change. It’s likely that both are contributing, DeLand said, but the question is the focus of current research.
Gallery: Reading the Clouds
Let’s take a look at the different types of clouds in the sky, broken up into four groups – high clouds, middle clouds, low clouds and vertical clouds – and what they can tell us about the weather, both now and what’s about to hit.
High clouds form at heights of 15,000 to 40,000 feet (4,500 to 12,000 meters). These are clouds that you only encounter on the top of high mountains or at the cruising altitude of a jet aircraft. Because at the altitudes at which they form the air temperature is below freezing, they tend to be comprised primarily of ice crystals. Like Peter Pan, most forms of high clouds lack the ability to cast shadows.
Cirrus clouds are the most abundant of all high level clouds. Cirrus means a “curl of hair.” These wispy clouds are composed of ice and consist of long, thin streamers that are also called mare’s tails. A few scattered cirrus clouds is a good sign of fair weather. However, a gradually increasing cover of web-like cirrus clouds is a sign that a warm front — the leading edge of a warmer and more humid air mass — is approaching.
Cirrostratus clouds look like thin sheets that spread themselves across the sky. When the sky is covered by these icy shreds they give the sky a pale, white appearance. These clouds can indicate the approach of precipitation. So thin are they that they are translucent, or maybe even a little transparent, so that the sun and moon can be readily seen through them. Also look for a ring or halo surrounding the moon or sun when these clouds are in the sky, sometimes accompanied during the day by colored swatches of cloud called “sundogs,” “mock suns,” or parhelions.
Cirrostratus clouds usually come 12 to 24 hours before a period of rain or snow. Remember: “Circle around the moon, rain or snow soon.”
Another form of high cloud is cirrocumulus. These tend to be large groupings of white streaks that are sometimes seemingly neatly aligned. For most climates these clouds mean a spell of fair weather.
However, during the summertime in the tropics, these clouds may indicate an approaching hurricane. The outer fringe of a hurricane, called the outflow boundary, serves as a very important element in hurricane development because the outflow represents all the energy being released by the hurricane. A powerful hurricane always has good outflow and is accompanied by spiral bands of cirrocumulus flowing out from the center.
Middle clouds form at 6,500 to 20,000 feet (2,000 to 6,000 m). They are comprised of water, and, if cold enough, ice. Middle clouds frequently are opaque and block sunlight, but not always.
Altocumulus clouds are grayish-white with one part of the cloud darker than the other; there is a lot of contrast between light and dark. They are composed of water droplets and can blanket much of the sky in small, puffy, round layers. They resemble the striped patterns of fish scales on a mackerel hence the name “mackerel sky.”
Mackerel skies and mare’s tails formations sometimes appear in the same sky. When that happens, precipitation is sure to follow within 36 hours.
The old sailor’s mnemonic for these kinds of clouds is “Mares tails and mackerel scales, tall ships carry short sails.” Another is “Mackerel sky, storm is nigh,” because if you see altocumulus clouds on a warm, humid morning, be prepared to observe thunderstorms late in the afternoon.
Altostratus clouds are grey and/or blue and cover the entire sky. The sun or moon may shine through an altostratus cloud, but will appear like a hazy and rather diffuse ball. Such clouds usually form ahead of storms that produce steady rain or snow.
Low clouds form below 6,500 feet (2,000 m). These clouds tend to contain chiefly water, but can also be comprised of ice and snow if the weather gets cold enough. Low clouds block sunlight and usually bring precipitation and wind.
Stratocumulus clouds are low, puffy and gray. They appear as masses of puffy clouds with little or no space in between. A sky full of stratocumulus clouds indicates generally dry weather, though on occasion they can produce a brief light shower or sprinkle.
Nimbostratus clouds form a dark gray layer that is so thick that it completely blocks out the moon and sun. Nimbo is derived from nimbus means “rain-bearing” and these clouds will often produce precipitation in the form of a protracted period of rain and/or snow.
Stratus clouds are dull grayish clouds that often stretch across and block the entire sky; stratus means “a layer” and these clouds form flat, unbroken sheets, like a fog that is not on the ground. Stratus clouds produce only mist, drizzle or very light snow.
Vertical Clouds – Cumulus
Vertically developing clouds are the cumulus variety. Cumulus means “a heap” — clouds that are separate, piled-up, fluffy and of different sizes. These puffy clouds are low “fair weather” clouds.
When the top of the cumulus clouds look like the head of a cauliflower, it is called cumulus congestus, or towering cumulus.
As they develop vertically upward they may go from small, fair weather clouds to large, boiling monsters called cumulonimbus, also called thunderheads.
Such clouds are most often associated with cold fronts: When a mass of cool, dry air pushes into a warm, moist air mass, the heavier cool air acts like an atmospheric plow and pushes the warm air up into violent thunderstorms. High winds aloft can make the cloud’s top into a flat anvil-like shape and their bottoms are usually very dark.
These clouds can forecast some of the most severe weather including torrential downpours, vivid lightning, hail and even tornadoes.
Clouds that look like hanging bulges from the skies are called mammatus clouds. More often than not, references statr that such clouds are a forbearer of severe weather, but actually, just the opposite is true: These clouds are formed by sinking air and are sometimes seen after a potent thunderstorm; they signal that a storm is retreating, not approaching.