Cirrus Clouds thin and wispy The most common form of high-level clouds are thin and often wispy cirrus clouds. Typically found at heights greater than 20,000 feet, cirrus clouds are composed of

ice crystals. Cirrus generally occur in fair weather and point in the direction of air

movement at their elevation.

Cirrostratus Clouds sheet-like and nearly transparent

Cirrostratus are sheet-like, high-level clouds composed

of ice crystals. Though cirrostratus can cover the entire sky and be up to

several thousand feet thick, they are relatively

transparent, as the sun or the moon can easily be seen

through them. Sometimes the only indication of their presence is given by an

observed halo around the sun or moon.

                                    

• Cirrocumulus clouds appear as small, rounded white puffs. The small ripples in the cirrocumulus sometimes resemble the scales of a fish. A sky with cirrocumulus clouds is sometimes referred to as a "mackerel sky."

Altocumulus Clouds

These clouds may appear as parallel bands or rounded masses. Typically a portion of an altocumulus cloud is shaded, a characteristic which makes them distinguishable from the high-level cirrocumulus. Altocumulus clouds result from the gradual lifting of air in advance of a cold front. The presence of altocumulus clouds on a warm and humid summer morning is commonly followed by thunderstorms later in the day.

Altostratus clouds are gray or blue-gray middle level clouds composed of ice crystals and water droplets. These clouds usually cover the entire sky. In the thinner areas of the cloud, the sun may be dimly visible as a round disk. Altostratus clouds often form ahead of storms that will produce continuous precipitation.

Stratus clouds are uniform grayish clouds that often cover the entire sky. They resemble fog that does not reach the ground. Usually no precipitation falls from

stratus clouds, but sometimes they may drizzle. When a thick fog "lifts," the resulting clouds are low stratus.

Nimbostratus Clouds are dark, low-level clouds accompanied by light to moderately falling precipitation.

Low clouds are primarily composed of water droplets since their bases generally lie below 6,500 feet. However, when temperatures are cold enough, these clouds may also

contain ice particles and snow.

Stratocumulus Clouds generally appear as a low, lumpy layer of clouds that is sometimes accompanied by weak intensity precipitation. Stratocumulus vary in color from dark gray to

light gray and may appear as rounded masses, rolls, etc., with breaks of clear sky in between.

Fair Weather Cumulus Clouds have the appearance of floating cotton and have a lifetime of 5-40 minutes. Known

for their flat bases and distinct outlines, fair weather cumulus exhibit only slight vertical growth, with the cloud tops

designating the limit of the rising air.

When the top of the cumulus resembles the head of a cauliflower, it is called cumulus congestus or towering cumulus. These clouds grow upward, and they can develop into a giant cumulonimbus, which is a thunderstorm cloud.

Cumulonimbus Clouds are much larger and more vertically developed than fair weather cumulus. They can exist as

individual towers or form a line of towers called a squall line. Fueled by vigorous convective updrafts, the tops of

cumulonimbus clouds can easily reach 39,000 feet or higher.