Clouds

Clouds are simply high fogs, mist, or haze. They form when air above the surface cools below its dew point. The shape of a cloud depends on the air movement that forms it. If air movement is mainly horizontal, clouds form in layers. They are called stratiform clouds. If air movement is mainly vertical, clouds grow upward in great piles. These are called cumuliform clouds.

At temperatures above freezing, clouds are made entirely of water droplets. Below freezing, clouds are usually mixtures of snow crystals and supercooled water. Supercooled water is water that has cooled below 0 degrees Celsius without freezing. When supercooled droplets encounter something to freeze on, such as an airplane or ice nuclei, they form snow and ice crystals. Below a temperature of about –18 degrees Celsius, clouds are almost entirely snow and ice crystals.

There are four families of clouds. The heights in the table below are measured above the surface, not above sea level.

The table includes three simple cloud names – cirrus, stratus, and cumulus. These three names represent the three main cloud types. All the other clouds are combinations or variations of these types.

Cirrus clouds are thin, feathery, or tufted. They are so high that they are always made of ice crystals. All of the high family of clouds are of the cirrus type. Stratus clouds are low sheets or layers of cloud. Cumulus clouds are formed by vertically rising air currents. More pictures of cumulus clouds are shown here and here. They are piled in thick, puffy masses.

Cirrostratus is high, thin, smooth or fibrous sheets of ice-crystal clouds. They sometimes cause halos, or rings, around the sun or moon. Cirrostratus clouds often mean the approach of rain or snow. Stratocumulus clouds are layers made up of round puffs. They often cover the whole sky, especially in the winter. Cirrocumulus clouds are small globular patches of cloud made of ice crystals.

The prefix alto (high) and the word nimbus (rain cloud) are used in cloud names. Altocumulus clouds are like stratocumulus clouds. Their puffs look smaller because they are farther away (higher). Altostratus clouds are stratus clouds that occur at a higher level. They are gray or bluish and produce no halo around the sun or moon. Nimbostratus clouds are dark, gray layers of cloud that produce steady rain. Fog can be considered as a low stratus cloud in contact with the ground. When the fog lifts, it usually becomes true stratus.

Orographic clouds, as the name implies, are produced by the flow of air interacting with mountainous terrain. Cap clouds form when air containing water vapor is uplifted on the windward slide of the slope and reaches saturation producing liquid water cloud droplets and a cloud which can "cap" the summit.

Lenticular clouds are lens-shaped clouds that can result from strong wind flow over rugged terrain. At the time of this photo, the winds were blowing around 30-40 mph from right to left, forming several lenticular clouds. Sometimes they stack up like pancakes in multiple layers as are several depicted in this first photo. The strong flow produces a distinct up and down wavelike pattern on the lee side of the mountain or large hill and the lenticular clouds tend to form at the peaks of these waves. They sometimes are very round and the edges are so well defined that they resemble flying saucers. This close up sequence shows a large lenticular cloud at various stages of illumination as the sun moved lower on the horizon and lit the cloud from below. Another lenticular cloud can be seen in the background of the last frame of the sequence. These photos were taken on January 25, 1999 in Plymouth, NH, by James D. Rufo, a PSC meteorology student. Lenticular clouds are often placed into the middle cloud category since they are most common at those altitudes.

Another "specialty" cloud is one that can develop due to Kelvin-Helmholtz (K-H) instability waves and subharmonic resonance with other waves in the atmosphere. This can result in an intertwined or spiral cloud pattern as shown in this picture, which was also taken by James D. Rufo. H-H instability is the result of strong wind shear. K-H clouds that form in early stages can resemble well-organized waves that appear to be breaking like ocean waves. Another type of cloud can be formed from the vapor contained in the exhaust of a jet engine of an airplane when they are flying at high enough altitudes where cold temperatures cause the vapor to turn into ice crystals like cirrus clouds. These clouds are called "contrails" (short for "condensation trails") and look like lines in the sky. The photo shows two contrails. The one on the lower right was formed by a jet that flew a few minutes ahead of the jet which formed the contrail in the center. The newer contrail is narrower and hasn't had the chance to diffuse like the older one.

The shapes of clouds show how the air is moving through them. For example, the air in stratiform clouds flows, for the most part, horizontally. Air in a growing cumulus clouds moves upward because it is buoyant. The air is buoyant because its temperature is warmer than the surrounding air. To what height will the cumulus cloud be warmer and more buoyant? To answer this question, it is necessary to know how temperature changes as air rises in a cloud.

Rising dry air cools at a rate of 1 degree Celsius for every 100 meters. This is the dry-adiabatic lapse rate. The cooling is caused only by the air expanding. The air expands as it rises because it is surrounded by lower pressure. Similarly, sinking air is compressed at it encounters higher pressure. This raises the temperature at the same rate, 1 degree Celsius for every 100 meters. Thus, if an air parcel rises 100 meters and then sinks 100 meters its final temperature will be the same as its starting temperature.

Air rising in a cloud does not cool as fast as rising dry air does. On the average, it cools at 0.6 degrees Celsius for every 100 meters. Why does it cool at a slower rate? The condensing water releases heat to the air, which makes the air cool more slowly. In the same way, sinking air in clouds warms 0.6 degrees Celsius for every 100 meters because the evaporation of cloud droplets slows its warming. Meteorologists call the rate of temperature change of a rising or sinking saturated parcel the moist-adiabatic lapse rate.

Cumulus clouds and other clouds with vertical development form when rising air currents are buoyant, or lighter than the surrounding air. How can this happen when air cools as it rises? It can, if the temperature of the surrounding air decreases even faster with height. Suppose a cloud is rising through a layer of air with a lapse rate of 1 degree Celsius for each hundred meters. Since the air in the cloud is cooling at only 0.6 degrees Celsius for each 100 meters, it will be 0.4 degrees Celsius warmer than the surrounding air after rising 100 meters, 0.8 degrees Celsius warmer after rising 200 meters, and so on. The rising air in the cloud is warmer than the surrounding air even though it gets cooler as it rises. Since the cloud is warmer, it is also lighter or less dense. That is, the air in the cloud is buoyant. The cloud can continue to grow. Meteorologists say that the air surrounding the cloud is unstable.

If a shallow layer or air is unstable, cumulus clouds can form. If a deep layer of air is unstable, thunderclouds, or cumulonimbus clouds, might form. From these come lightning, thunder, and heavy showers. Violent thunderstorms can have hail, strong winds, and even tornadoes. They are the tallest of all clouds that can span all cloud layers and extend above 60,000 feet. They usually have large anvil-shaped tops (as shown) which form because of the stronger winds at those higher levels of the atmosphere. The "cb" depicted in the photo had a base at around 3,000 feet and it extended upward to around 30,000 feet - small compared to most thunderstorms which are associated with really severe weather. Sometimes, these strong cumulonimbus clouds can have appendages protruding from the base of the cloud, which are called "mammatus" clouds because they resemble the mammary glands of mammals. They indicate that the atmosphere is quite unstable and can also be an indicator of impending severe weather. The picture of mammatus clouds, shown here, was taken by Mark Gibbas, a PSC meteorology alumnus, at Acadia National Park.

Rising buoyant air currents form cumulus clouds. These clouds often appear in the late morning or early afternoon on bright sunny days. They have flat bases and billowy tops. Their shape reveals how these clouds form and grow.

Cumulus clouds form over heated ground. The ground is warm enough so that rising air remains buoyant even though it is cooling at the dry-adiabatic lapse rate. The flat cloud base shows where the water vapor began to condense. This height is called the condensation level. Here the temperature is about equal to the dew point.

Suppose the temperature and dew point of the air at the ground are known. Then the condensation level can be found. For dry air the rate of cooling by expansion is 1 degree Celsius for every 100 meters. As the air rises, its dew point falls at a rate of 0.2 degrees Celsius for every 100 meters.

Here is an example: At the surface the air temperature is 20 degrees Celsius, the dew point is 12 degrees Celsius, and the difference between these temperatures is 8 degrees Celsius. However, 100 meters higher the air temperature is 19 degrees Celsius and the dew point is 11.8 degrees Celsius. The difference is only 7.2 degrees Celsius. The two temperatures continue to approach each other at a rate of 0.8 degrees Celsius for each 100-meter rise in altitude.

To find where the dew point will reach the air temperature, divide 8 degrees Celsius (the difference between the air temperature at ground level and the dew point) by 0.8 degrees Celsius (the amount per 100 meters at which the dew point approaches the air temperature).

Meteorologists can also estimate the highest possible cloud top. They know that the temperature of the rising air in the cloud starts at the cloud-base temperature and falls with height at the moist-adiabatic lapse rate. They have measurements of the temperature of the surrounding air. The height of the cloud top will be close to the height where the cloud temperature and air temperature are equal. Here, the cloud is no longer buoyant. The rising air spreads out, forming the flat anvil-shaped top characteristic of cumulonimbus clouds.

Layer clouds form in stable air, where motions are mainly horizontal. The atmosphere is stable when the lapse rate of the air surrounding the clouds is smaller than the moist-adiabatic lapse rate. For example, suppose the temperature of the surrounding air is uniform throughout a thick layer. A cloudy rising current would be cooler than the surrounding air as soon as it started rising and cooling. Being colder and therefore heavier than the surrounding air, the air current would sink back down to where it started. In this case, the air cannot easily move up or down and tends to spread out in layers.

Clouds can form in stable air in two ways. First, the air can be forced slowly upward to its condensation level. Air is forced upward when it moves up rising terrain, such as a mountainside, or over a layer of colder, denser air. And second, layer clouds form if radiation or mixing cools a layer of air to its dew point.