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".

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.

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.

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}

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)

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

There are several distinct sub-classes of altocumulus

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.

•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.