WEATHER - CLOUDS

Question: How Do Clouds Form?

Clouds require key atmospheric ingredients to form.

• water
• dust particles
• temperature or pressure changes

Answer:
Water molecules must have a surface on which to collect. The air is constantly full of water. When you look into the sky and see a cloud, it is actually moisture you are seeing. Most of the time, water vapor in the air cannot be seen unless it collects and condenses to form a cloud.

Clean air (without any dust or particles) will not produce clouds without supersaturation (or relative humidity above 100%). Luckily for us, Mother Nature and humans put lots of Cloud Condensation Nuclei or dust into the atmosphere. This dust comes from sources such as volcanoes, cars, sea spray from the ocean, and fires. Other particles in the atmosphere, including bacteria, can also play a role in serving as condensation nuclei.

When temperatures or pressures decrease, the air cannot hold as much water. 

The lower the temperature, the greater the condensation of gaseous water to liquid water. Note: I have simplified the language on air "holding water", but there are inherent misconceptions that can be developed as a result. 

Clouds are classified according to their height above &

appearance (texture) from the ground.

Cloud roots & translations summarize the components 

of this Classification System:

1)  Cirro - curl of hair, high.

2)  Alto - mid

3)  Strato - layer

4)  Nimbo - rain, precipitation

5)  Cumulo - heap

High-level clouds:  

High-level clouds occur above about 20,000 feet and are given the prefix "cirro-". Due to cold tropospheric temperatures at these levels, the clouds primarily are  composed of ice crystals,  and often appear thin, streaky, and white (although a low sun angle, e.g., near sunset, can create an array  of color on the clouds).

   

The three main types of high clouds are:

 cirrus, cirrostratus, and cirrocumulus. 

Cirrus clouds are wispy, feathery, and composed entirely of ice crystals. They often are the first sign of an approaching warm front or upper-level jet streak.

Cirrostratus clouds form more of a widespread, veil-like layer (similar to what stratus clouds do in low levels).  When sunlight or moonlight passes through the hexagonal-shaped ice crystals of cirrostratus clouds, the light is dispersed or refracted (similar to light passing through a prism) in such a way that a familiar ring or halo may form. As a warm front approaches, cirrus clouds tend to thicken into cirrostratus, which may, in turn, thicken and lower into altostratus, stratus, and even nimbostratus. 

Cirrocumulus clouds are layered clouds permeated with small cumuliform lumpiness. They also may line up in streets or rows of clouds across the sky denoting localized areas of ascent (cloud axes) and descent (cloud-free channels). 

Mid-level clouds:

The bases of clouds in the middle level of the troposphere, given the prefix "alto-", appear between 6,500 and 20,000 feet. Depending on the altitude, time of year, and vertical temperature structure of the troposphere, these clouds may be composed of liquid water droplets, ice crystals, or a combination of the two, including supercooled droplets (i.e., liquid droplets whose temperatures are below freezing). 

The two main type of mid-level clouds are altostratus and altocumulus. 

Altostratus clouds are "strato" type clouds (see below) that possess a flat and uniform type texture in the mid levels. They frequently indicate the approach of a warm front and may thicken and lower into stratus, then nimbostratus resulting in rain or snow. However, altostratus clouds themselves do not produce significant precipitation at the surface, although sprinkles or occasionally light showers may occur from a thick alto-stratus deck. 

Altocumulus clouds exhibit "cumulo" type characteristics (see below) in mid levels, i.e., heap-like clouds with convective elements.  Like cirrocumulus, altocumulus may align in rows or streets of clouds, with cloud axes indicating localized areas of ascending, moist air, and clear zones between rows suggesting locally descending, drier air.  Altocumulus clouds with some vertical extent may denote the presence of elevated instability, especially in the morning, which could become boundary-layer based and be released into deep convection during the afternoon or evening. 

 Low-level clouds:

Low-level clouds are not given a prefix, although their names are derived from "strato-" or "cumulo-", depending on their characteristics. Low clouds occur below 6500 feet, and normally consist of liquid water droplets or even supercooled droplets, except during cold winter storms when ice crystals (and snow) comprise much of the clouds.

The two main types of low clouds include:

• stratus, which develop horizontally
• cumulus, which develop vertically

Stratus clouds are uniform and flat, producing a gray layer of cloud cover which may be precipitation-free or may cause periods of light precipitation or drizzle.  Low stratus decks are common in winter in the Ohio Valley, especially behind a storm system when cold, dismal, gray weather can linger for several hours or even a day or two.

Stratocumulus clouds are hybrids of layered stratus and cellular cumulus, i.e., individual cloud elements, characteristic of cumulo type clouds, clumped together in a continuous distribution, characteristic of strato type clouds. Stratocumulus also can be thought of as a layer of cloud clumps with thick and thin areas. These clouds appear frequently in the atmosphere, either ahead of or behind a frontal system.

Nimbostratus clouds are generally thick, dense stratus or stratocumulus clouds producing steady rain or snow . 

In contrast to layered, horizontal stratus, cumulus clouds are more cellular (individual) in nature, have flat bottoms and rounded tops, and grow vertically. In fact, their name depends on the degree of vertical development. For instance, scattered cumulus clouds showing little vertical growth on an otherwise sunny day used to be termed "cumulus humilis" or "fair weather cumulus," although normally they simply are referred to just as cumulus or flat cumulus.

Latin Root		Translation		Example
cumulus stratus cirrus nimbus		heap layer curl of hair rain		fair weather cumulus altostratus cirrus cumulonimbus

eap") refers to piled-up clouds. Strato (meaning "layer") refers to flat, wide, layered clouds. 

Type of Cloud (Genus)	Abbreviation	Appearance	Composition	Altitude (height)
Cumulo-nimbus =Thunderheads	Sb	Can cause lightning, thunder, hail, strong rains, strong winds, and tornadoes		Near ground up to 75,000 feet (Vertical clouds)
Cirro-stratus	Cs	Thin, wispy, appears in sheets. Located above thunderheads		Above 18,000 feet (High-altitude clouds)
Cirrus	Ci	Thin, wispy, filamentous, or curly	Mostly composed of ice crystals	Above 18,000 feet (High-altitude clouds)
Cirro-cumulus	Cc	Small, puffy, patchy and/or with a wavelike appearance		Above 18,000 feet (High-altitude clouds)
Alto-cumulus	Ac	Medium-sized puffy, patchy, scattered clouds - often in linear bands		6,500 - 20,000 feet (Middle-altitude clouds)
Alto-stratus	As	Thin, uniform		6,500 - 20,000 feet (Middle-Alttude clouds)
Strato-cumulus	Sc	Broad and flat on the bottom, puffy on top,		Below 6,500 feet (Low-altitude clouds)
Cumulus	Cu	Puffy and piled up.		Below 6,500 feet (Vertical clouds)
Stratus	St	Uniform, flat, thick to thin layered clouds will ill-defined edges	Mostly composed of liquid droplets	Below 6,500 feet (Low-altitude clouds)
Nimbo-stratus	Ns	Uniform, dark, flat, low, featureless clouds that produce precipitation	Mostly composed of liquid droplets	Below 6,500 feet (Low-altitude clouds)
Fog	Very low stratus clouds		Mostly composed of liquid droplets	In contact with the ground (Ground-hugging clouds)

Other types of clouds: Mammatus clouds are dark clouds shaped like sagging pouches. These clouds often appear after a tornado.
Orographic clouds are clouds that are formed as moist air rises over mountains or other major geographic features. The air floats up the side of the mountain and cools quickly, condensing and turning into a cloud.
A pileus cloud is a smooth cloud that is found over or on the top of a major geographic feature, like a mountain.
A contrail (short for CONdensation TRAIL) is a cloud-like vapor trail that forms behind some aircraft when flying in cold, clear, humid air. The contrail forms from the water vapor contained in the jet's engine exhaust.

 STANDARD CLOUD TYPE CHART

Cirrus (above)	Cirrostratus (above)	Cirrocumulus (above)	Altostratus (above)
Altocumulus (above)	Stratus (above)	Stratocumulus (above)	Nimbostratus (above)
Cumulus (above)	Cumulus congestus (above)	Cumulonimbus (above)	Wall cloud (above)
Shelf cloud (above)	Fractus (scud) (above)	Mammatus (above)	Contrails (above)
	Cloud chart showing the different types of high, mid, and low-level clouds, as well as a number of other interesting

What are clouds?
A cloud is a large collection of very tiny droplets of water or ice crystals. The droplets are so small and light that they can float in the air. 

How are clouds formed?
All air contains water, but near the ground it is usually in the form of an invisible gas called water vapor. When warm air rises, it expands and cools. Cool air can't hold as much water vapor as warm air, so some of the vapor condenses onto tiny pieces of dust that are floating in the air and forms a tiny droplet around each dust particle. When billions of these droplets come together they become a visible cloud. 

Why are clouds white?
Since light travels as waves of different lengths, each color has its very own unique wavelength. Clouds are white because their water droplets or ice crystals are large enough to scatter the light of the seven wavelengths (red, orange, yellow, green, blue, indigo, and violet), which combine to produce white light. 

Why do clouds turn gray?
Clouds are made up of tiny water droplets or ice crystals, usually a mixture of both. The water and ice scatter all light, making clouds appear white. If the clouds get thick enough or high enough all the light above does not make it through, hence the gray or dark look. Also, if there are lots of other clouds around, their shadow can add to the gray or multicolored gray appearance.
 
Why do clouds float?
A cloud is made up of liquid water droplets. A cloud forms when air is heated by the sun. As it rises, it slowly cools it reaches the saturation point and water condenses, forming a cloud. As long as the cloud and the air that its made of is warmer than the outside air around it, it floats!

How do clouds move?
Clouds move with the wind. High cirrus clouds are pushed along by the jet stream, sometimes traveling at more than 100 miles-per-hour. When clouds are part of a thunderstorm they usually travel at 30 to 40 mph.


Why do clouds form at different heights in the atmosphere?
The characteristics of clouds are dictated by the elements available, including the amount of water vapor, the temperatures at that height, the wind, and the interplay of other air masses. 

How is fog formed?
There are many different types of fog, but fog is mostly formed when southerly winds bring warm, moist air into a region, possibly ending a cold outbreak. As the warm, moist air flows over much colder soil or snow, dense fog often forms. Warm, moist air is cooled from below as it flows over a colder surface. If the air is near saturation, moisture will condense out of the cooled air and form fog. With light winds, the fog near the ground can become thick and reduce visibilities to zero. 

(Graphic Credit: USA TODAY.)

You need warm air! Southerly winds bring warm, moist air over cold ground or snow.	Fog Forms! Moisture condenses into fog as air is cooled from below.

Cloud Chart

Cloud Group	Cloud Height	Cloud Types
High Clouds = Cirrus	Above 18,000 feet	Cirrus Cirrostratus Cirrocumulus
Middle Clouds = Alto	6,500 feet to 18,000 feet	Altostratus Altocumulus
Low Clouds = Stratus	Up to 6,500 feet	Stratus Stratocumulus Nimbostratus

Clouds with Vertical Growth	Cumulus Cumulonimbus
Special Clouds	Mammatus Lenticular Fog Contrails

Cirrus Clouds
Cirrus clouds are the most common of the high clouds. They are composed of ice and are thin, wispy clouds blown in high winds into long streamers. Cirrus clouds are usually white and predict fair to pleasant weather. By watching the movement of cirrus clouds you can tell from which direction weather is approaching. When you see cirrus clouds, it usually indicates that a change in the weather will occur within 24 hours.


Cirrostratus clouds are thin, sheetlike high clouds that often cover the entire sky. They are so thin that the sun and moon can be seen through them. Cirrostratus clouds usually come 12-24 hours before a rain or snow storm.


Cirrocumulus clouds appear as small, rounded white puffs that appear in long rows. The small ripples in the cirrocumulus clouds sometime resemble the scales of a fish. Cirrocumulus clouds are usually seen in the winter and indicate fair, but cold weather. In tropical regions, they may indicate an approaching hurricane.


"Alto" Clouds
Altostratus clouds are gray or blue-gray mid level clouds composed of ice crystals and water droplets. The clouds usually cover the entire sky. In the thinner areas of the clouds, the sun may be dimly visible as a round disk. Altostratus clouds often form ahead of storms with continuous rain or snow.


Altocumulus clouds are mid level clouds that are made of water droplets and appear as gray puffy masses. They usually form in groups. If you see altocumulus clouds on a warm, sticky morning, be prepared to see thunderstorms late in the afternoon.


Stratus Clouds
Stratus clouds are uniform grayish clouds that often cover the entire sky. They resemble fog that doesn't reach the ground. Light mist or drizzle sometimes falls out of these clouds.


Stratocumulus clouds are low, puffy and gray. Most form in rows with blue sky visible in between them. Rain rarely occurs with stratocumulus clouds, however, they can turn into nimbostratus clouds.


Nimbostratus clouds form a dark gray, wet looking cloudy layer associated with continuously falling rain or snow. They often produce precipitation that is usually light to moderate.


Cumulus Clouds
Cumulus clouds are white, puffy clouds that look like pieces of floating cotton. Cumulus clouds are often called "fair-weather clouds". The base of each cloud is flat and the top of each cloud has rounded towers. When the top of the cumulus clouds resemble the head of a cauliflower, it is called cumulus congestus or towering cumulus. These clouds grow upward and they can develop into giant cumulonimbus clouds, which are thunderstorm clouds.


Cumulonimbus clouds are thunderstorm clouds. High winds can flatten the top of the cloud into an anvil-like shape. Cumulonimbus clouds are associated with heavy rain, snow, hail, lightning and even tornadoes. The anvil usually points in the direction the storm is moving.
 

Special Clouds
Mammatus clouds are low hanging bulges that droop from cumulonimbus clouds. Mammatus clouds are usually associated with severe weather.


Lenticular clouds are caused by a wave wind pattern created by the mountains. They look like discs or flying saucers that form near mountains.


Fog is a cloud on the ground. It is composed of billions of tiny water droplets floating in the air. Fog exists if the atmospheric visibility near the Earth's surface is reduced to 1 kilometer or less.


Contrails are condensation trails left behind jet aircrafts. Contrails form when hot humid air from jet exhaust mixes with environmental air of low vapor pressure and low temperature. The mixing is a result of turbulence generated by the engine exhaust.
 

Green Clouds are often associated with severe weather. The green color is not completely understood, but it is thought to have something to do with having a high amount of liquid water drops and hail inside the clouds. In the Great Plains region of the U.S. green clouds are associated with storms likely to produce hail and tornadoes.


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