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There are two ingredients needed for clouds to form: water and nuclei.

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There are two ingredients needed for clouds to form: water and nuclei.

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We see clouds almost daily, yet they are complicated and varied. In fact, the simple presence of a cloud tells us little about the weather without learning more details.

Clouds are the visible aggregate of minute particles of water and/or ice that form when water vapor condenses in the atmosphere.

Clouds can grow very tall or appear flat as a pancake. They are typically white in color but can also be different shades of gray or brilliant yellow, orange, or red. They can weigh tens of millions of tons yet float in the atmosphere.

Clouds can be harbingers of good weather or bad. Their absence can be a good thing after a flooding rain or a bad thing during a drought. They provide relief from the heat of direct sunlight but also trap warmth, leading to higher temperatures.

Precipitation from clouds helps crops to grow but can reduce visibility and make travel dangerous.

They come in infinite shapes and sizes yet we often recognize more familiar objects or animals.

Clouds can be carried along by winds of up to 150 mph (240 km/h) or can remain stationary while the wind passes through them.

They can form behind high flying aircraft or can dissipate as a plane flies through them.

They are not confined to Earth but are found on other planets as well.

Learn about clouds and how they form to become "Cloudwise".

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Water is always present in Earth’s atmosphere in some form. However, water molecules on their own are too small to bond together in the formation of cloud droplets. They need a "flatter" surface, an object with a radius of at least one micrometer (one millionth of a meter) on which they can condense. Those objects are called cloud condensation nuclei.

These nuclei are minute solid and liquid particles found in abundance in the atmosphere, such as smoke from fires or volcanoes, ocean spray, or tiny specks of wind-blown soil. They are hygroscopic, meaning they attract water molecules, and are about 1/100th the size of a cloud droplet upon which water condenses.

This means that every cloud droplet has a speck of dirt, dust, or salt crystal at its core. However, even with its condensation nuclei, a cloud droplet is still essentially made up of pure water.

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In an ideal atmosphere, the saturation level of a parcel with a surface temperature of 85°F and a dew point of 65°F will cool to the saturation point at about 4,000 feet in elevation.

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The presence of water-attracting nuclei alone is not enough for a cloud to form; the air temperature must also be below the dew point, the point of saturation where evaporation equals condensation.

Air can reach the point of saturation in a number of ways. The most common way is through air rising from the surface up into the atmosphere and cooling.

As a block of air (a “parcel”) rises, it expands due to the lower pressure higher in the atmosphere. This expansion results in cooling. When the temperature changes due to expansion or contraction of the parcel rather than from heat transfer between the parcel and surrounding atmosphere, this is called the adiabatic process.

For each 1000-foot increase in elevation that the parcel rises, its air temperature will decrease by 5.5°F. (9.8°C per kilometer) until it reaches saturation. This is called the “dry lapse rate”.

Once the parcel reaches saturation temperature (100% relative humidity), water vapor will condense onto the cloud condensation nuclei, resulting in the formation of a cloud droplet.

However, the atmosphere is in constant motion. As air rises, drier air is entrained, or mixed into the rising parcel, and both condensation and evaporation are continually occurring. As a result, cloud droplets are constantly forming and dissipating. When more water condenses on nuclei than evaporates from them, clouds form and grow. Conversely, if there is more evaporation than condensation, clouds dissipate. This is why clouds appear and disappear as well as constantly change shape.

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Four latin terms form the basis for the naming of clouds:

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Four latin terms form the basis for the naming of clouds:

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Cirrus : fibrous or hair-like

Cumulus : a heap or pile

Stratus : a horizontal sheet or layer

Nimbus : rain-bearing

The prefix Alto is used to indicate medium altitude clouds.

Terms, and basic classifications first proposed by Luke Howard in 1803.

http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cld/cldtyp/home.rxml - good online guide

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High Level:

Cloud base above 6000m

Are all forms of cirrus (ice clouds)

Medium Level:

Cloud base 2000-6000m

Low level:

Cloud base below 2000m (within boundary layer)

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Cirrus (Ci): White, delicate, fibrous in appearance. Forms in patches or narrow bands. May for comma-shaped streaks or “mare’s tails” (cirrus uncinus)

Cirrus clouds are formed entirely of ice crystals. These grow and evaporate slowly, leading to soft edges to clouds.

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Cirrocumulus (Cc) : Thin white patch or sheet of cloud; appears dappled or rippled. Dappling results from convective overturning within the cloud, ripples from gravity waves.

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Aircraft contrails: condensation from aircraft exhaust. May dissipate quickly, or be very long-lived depending on conditions.

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Altostratus (As): A greyish sheet of cloud, may be fibrous or uniform in appearance. Thin enough in parts to make out the sun, but no halo.

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Altocumulus (Ac): White or grey patches arranged in sheets. Shape and texture are variable.

There are several distinct sub-classes of altocumulus

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Altocumulus lenticularis : White or grey lenticular (lens shaped) clouds formed by the lifting of air over a topographic barrier

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Altocumulus castellanus(Ac cas): white or grey, broken cumulus-like clouds; upper part appearing castle-like. Sometimes arranged in lines.

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Altocumulus undulatus (Ac und): white or grey patches or sheets of cloud with an undulating or rippled appearance.

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Cumulus (Cu): Brilliant white to grey, dense detached clouds. Forms clumped or heaped (cauliflower-like) shapes, usually with sharp outlines and flat base. Field of Cu often have bases all at same (lifting condensation) level.

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Cumulus humilis (Cu hum): small cumulus, of limited vertical extent, may have a flattened appearance. Also called fair-weather cumulus

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Cumulus mediocris : cumulus, of moderate vertical extent.

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Cumulus congestus: crowded (congested) field of cumulus or greater vertical extent. May produce rain.

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Pileus : cap clouds that form above large cumulus as the upward motion of the convective cloud distorts the layer of air above (pileus is latin for skull-cap)

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Cumulonimbus (Cb) : huge towering cloud, dark base and white sides. Associated with heavy rain, thunderstorms, and hail. Frequently has an anvil shaped top.

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Mammatus : smooth, rounded shapes sometimes formed on the underside of cumulonimbus; they result from downdrafts within the cloud.

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Stratocumulus (Sc) : white or grey sheet of cloud, usually formed in mounds or rolls.

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Stratus (St) : grey featureless layer of cloud with a uniform base. Often associated with drizzle or snow.

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Nimbostratus (Ns) : Dark grey, featureless, thick layer of cloud. Associated with prolonged precipitation. Commonly forms in frontal systems

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•Clouds play an important controlling role in the global radiation budget.

Reflection of incoming solar (short-wave) radiation

Absorption of both solar and thermal infra red (long-wave) radiation (incoming & outgoing)

Emission of infra red radiation (up and down)

•The altitude, type, and thickness of cloud, along with that of clouds above & below determines whether the local net effect is to warm or cool the air & surface below.

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Deep convective clouds play a major role in mixing boundary layer air – along with moisture, aerosol particles, and gases (both natural and man-made pollutants) – up into the free-troposphere.

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•Clouds provide an environment within which aqueous phase chemical reactions can take place within the atmosphere.

•Aerosol particles can be substantially modified within clouds

-Aqueous phase reactions with gases dissolved in droplets

-Coalescence of droplets brings multiple aerosol particles together

• Chemically different aerosol may react

• On evaporation of droplet, a single aerosol particle is formed, containing material from all contributing droplets.