Clouds

[Definition | Formation | Experiment #1 | Experiment #2 | Classification]

Dictionary Definition

A visual body of very fine water droplets or ice particles suspended in the atmosphere at altitudes ranging up to several miles above sea level. (i.e. the white balls of fluff you see in the sky).

Formation

Explanation

A cloud is formed when water is transported vertically through evaporation and lifting. Moisture trapped in the ground or in lakes, seas, and oceans ascends to the sky, forming a cloud. The wind patterns then move the water to another location where precipitation occurs. The three main processes that create this are convection, orographic lifting, and dynamic lifting.

Convection: A water source, such as the ground or a body of water, is heated by the sun and evaporation takes place. The warm surface heats the air above it, causing the warm air to rise and take the water vapor along with it. The higher the temperature is, the more water vapor that can be taken with it. The rising air cools as it goes higher and expands, causing water droplets to condense out as the air expands. The same thing happens to a cold glass in a warm room, when the glass produces beads of water on the outside.
Orographic Lifting: Air with water vapor travels to high altitudes where the water vapor condenses out to form water droplets. However, this time the water vapor rises because it gets a running start up a convenient slope, not because the water around it warms (as in convection). Because of low-level winds, the air containing the water vapor is moving horizontally along the surface until it comes to a slope of a mountain or some other feature. When the moving moist air encounters the slope, it rises and cools as it increases in altitude. Eventually it reaches the condensation level (the point where it expands), and a cloud is formed.
Dynamic Lifting: To get to the condensation level by this method, the warm air mass moves up the slope of a colder air mass. Since the colder air mass is compressed by gravity, it is more dense than the warmer air mass. When the mass of cold air moves toward a stationary warm air mass, the cold air mass pushes underneath the other because of the difference in density. The cold air mass pushes upward to transport the warm air to high altitudes. Also, when a warm air mass moves toward a stationary cold air mass, it can still get up to higher altitudes. Since the warm air cannot force its way under the cold air, because it is less dense, it climbs on top the cold air and slowly gets higher and higher until it reaches the condensation level.

Experiment #1: Homemade Cloud

What You'll Need:

bottle
warm water
sheet of foil

Preparation:

Put the foil into the freezer until extremely cold.

Directions:

Rinse the empty bottle with warm water.
Pour out the water, leaving about an inch in the bottle.
Remove the foil from the freezer, fold it, and place it on top of the bottle.
Watch as a cloud of water droplets forms in the bottle.

What's Happening?

The warm water turns into gas and then the gas cools and forms a cloud of tiny droplets. No rain falls, though, because the cloud is not cold enough.

Experiment #2: Rain Cloud

What You'll Need:

tea kettle
empty tray
ice cube tray

Preparation:

Put the ice cube tray with water in the freezer until frozen.

Directions:

With help from an adult, boil some water in a teakettle.
Set the empty tray below the cloud to catch the rain.
Hold a tray of ice cubes above the cloud.
Watch as rain falls into the empty tray.

What's Happening?

Before you put the ice cube tray over the cloud, the cloud is warm and no rain falls. But when you add the cold of the ice cubes, the cloud gets cold and the rain falls.

Classification

There are many different types of clouds, and not all of them produce rain. It depends on the temperature and position of the cloud (colder makes rain). Below is a listing of the ten different types of clouds, in groups based on their average altitudes.

Cloud Type	Description	Possible Precipitation	Other Comments
High-Cloud Family (Average Height 6 to 12 kilometers)
Cirrus	Detached clouds of delicate, fibrous structure. Generally white in the daytime.	None	Often colored bright red or yellow before sunrise or after sunset.
Cirrocumulus	Layers or patches of thin ripples, small tufts, or globular masses. White without any darker parts.	None	Relatively rare and always associated with Cirrus or Cirrostratus clouds.
Cirrostratus	A thin whitish veil that may give the sky a milky look, or have a fibrous structure. When the sun shines through, it casts a shadow.	None	When a Cirrostratus cloud drifts in front of the sun or the moon, a ring with red inside and blue outside is formed. (called a halo)
Middle-Cloud Family (Average Height 2 to 6 kilometers)
Altocumulus	One or more nonfibrous layers which cannot be fused or shaded.	Wisps of rain or snow (virga)	When a Altocumulus cloud drifts in front of the sun or the moon, a ring with blue inside and red outside is formed. (called a corona)
Altostratus	A sheet or layers of fibrous, striated, or uniform clouds. Gray or bluish color.	Light rain or snow	Altostratus Clouds do not create shadows when the sun shines through them.
Low-Cloud Family (Average Height .8 to 2 kilometers)
Stratocumulus	One or more nonfibrous layers. Grayish color with darker parts, and a distinctly visible outline.	Light to heavy rain or snow	Stratocumulus clouds have a wavy appearance when they cover the entire sky.
Stratus	Uniform clouds of indefinite shape. Give the sky a hazy appearance.	Drizzle	Stratus clouds become fog if they rest on the ground.
Nimbostratus	A low, dark-gray, shapeless, and wet-looking cloud layer. Appears to be dimly lit from within.	Steady rain or snow	Low, ragged clouds or bad weather are often below a layer of Nimbostratus clouds, and may merge.
Vertically Developed Family (Average Height .5 kilometers)
Cumulus	Dense clouds shaped like domes or towers. Brilliantly white when lit by the sun at top, with dark bases.	Infrequent light rain or snow	Cumulus clouds generally appear in the morning and dissolve toward evening, when over land.
Cumulo-nimbus	Massive clouds in the form of billowing masses. Upper fibrous parts often in the shape of an anvil.	Heavy rain, snow, or hail	Cumulo-nimbus clouds are associated with thunderstorms and violent winds. They form in the late afternoon.