Cloud (2024)

Clouds are visible accumulations of tiny water droplets or ice crystals in the Earth’s atmosphere.

Clouds

differ greatly in size, shape, and color. They can appear thin and wispy, or bulky and lumpy.

Clouds

usually appear white because the tiny water droplets inside them are tightly packed, reflecting most of the sunlight that hits them. White is how our eyes perceive all wavelengths of

sunlight

mixed together. When it’s about to rain,

clouds

darken because the water vapor is clumping together into raindrops, leaving larger spaces between drops of water. Less light is

reflected

. The rain

cloud

appears black or gray.

Clouds

form when air becomes saturated, or filled, with water

vapor

. Warm air can hold more water

vapor

than cold air, so lowering the temperature of an air mass is like squeezing a sponge.

Clouds

are the visible result of that squeeze of cooler, moist air. Moist air becomes

cloudy

with only slight cooling. With further cooling, the water or ice particles that make up the

cloud

can grow into bigger

particles

that fall to Earth as precipitation.

Types of Clouds

Because certain types of

clouds

are associated with certain types of weather, it is possible to forecast the

weather

by observing and understanding these different types of

clouds

.

Clouds

are classified into three main groups: cirrus, stratus, and cumulus.

Cirrus

clouds

are wispy, curly, or stringy. They are found high in the

atmosphere

—typically higher than 6,000 meters (20,000 feet)—and are usually made of

ice crystals

.

Cirrus

clouds

usually signal clear, fair

weather

. Their shape often indicates the direction the wind is blowing high in the

atmosphere

.

Stratus

clouds

are horizontal and stratified, or layered.

Stratus

clouds

can blanket the entire sky in a single pattern. They usually occur close to the Earth.

Stratus

clouds

often form at the boundary of a warm front, where warm, moist air is forced up over cold air. This movement produces

clouds

as the moist air is cooled across the entire front. The presence of

stratus

clouds

usually means a chilly, overcast day. If

precipitation

falls from

stratus

clouds

, it is usually in the form of drizzle or light snow.

Cumulus

clouds

are large and lumpy. Their name comes from the Latin word meaning "heap" or "pile." They can stretch vertically into the

atmosphere

up to 12,000 meters (39,000 feet) high.

Cumulus

clouds

are created by strong updrafts of warm, moist air. Most forms of heavy

precipitation

fall from

cumulus

clouds

. The

weather

they bring depends on their height and size. The higher the base of a

cloud

is, the drier the

atmosphere

and the fairer the

weather

will be.

Clouds

located close to the ground mean heavy

snow

or rain.

Variations

Clouds

are also classified according to how high they are in the

atmosphere

and what kind of

weather

they produce.

The prefix "cirro-" refers to

clouds

that lie more than 6,000 meters (20,000 feet) above the Earth. Cirro

cumulus

and cirro

stratus

clouds

are two examples of these “high-level”

clouds

.

The prefix "alto-" indicates

clouds

whose bases are between 2,000 and 6,000 meters (6,500-20,000 feet) above the Earth, such as alto

cumulus

and alto

stratus

clouds

. They are considered "mid-level"

clouds

and are mostly made of liquid water droplets, but can have some

ice crystals

in cold enough temperatures.

The prefix "nimbo-" or the suffix "-nimbus" are low-level

clouds

that have their bases below 2,000 meters (6,500 feet) above the Earth.

Clouds

that produce rain and

snow

fall into this category. ("Nimbus" comes from the

Latin

word for "rain.") Two examples are the nimbostratus or cumulonimbus

clouds

.

Nimbo

stratus

clouds

bring continuous

precipitation

that can last for many hours. These low-level

clouds

are full of moisture.

Cumulonimbus

clouds

are also called thunderheads.

Thunderheads

produce rain, thunder, and lightning. Many

cumulonimbus

clouds

occur along cold fronts, where cool air is forced under warm air. They usually shrink as evening approaches, and moisture in the air evaporates.

Cumulonimbus

clouds

gradually become strato

cumulus

clouds

, which rarely produce rain.

Clouds and Weather

Certain types of

clouds

produce

precipitation

.

Clouds

also produce the bolt of electricity called

lightning

and the sound of thunder that accompanies it.

Lightning

is formed in a

cloud

when positively charged

particles

and negatively charged

particles

are separated, forming an electrical field. When the

electrical field

is strong enough, it discharges a superheated bolt of

lightning

to the Earth. Most of what we consider to be single

lightning

strikes are in fact three or four separate strokes of

lightning

.

The sound of thunder is actually the sonic shock wave that comes when the air, heated by the

lightning

bolt, expands very rapidly. Thunder sometimes sounds like it comes in waves because of the time it takes the sound to travel. Because the speed of light is faster than the speed of sound,

lightning

will always appear before its thunder is heard.

Meteorologists measure cloud cover, or the amount of the visible sky covered by

clouds

, in units called oktas. An

okta

estimates how many eighths of the sky (octo-) is covered in

clouds

. A clear sky is 0

oktas

, while a totally

overcast

or gray sky is 8

oktas

.

Scientists have experimented with a process called cloud seeding for many years.

Cloud

seeding

aims to influence

weather

patterns. Seeds, or microscopic

particles

, are placed in

clouds

. These seeds are artificial cloud condensation nuclei (CCN), which are tiny

particles

of dust, salt, or pollution that collect in all

clouds

. Every

raindrop

and snowflake contains a CCN. Water or ice droplets accumulate around CCN. Scientists hope that

cloud

seeding

will allow people to control

precipitation

.

Extraterrestrial Clouds

Clouds

exist in outer space.

Clouds

on Jupiter, for instance, are divided into three bands in the planet’s

atmosphere

. The highest band, at 50 kilometers above the surface of the

planet

, is mostly clear.

Jupiter

’s middle layer of

clouds

is constantly moving. These storm

clouds

appear as bands and swirls of yellow, brown, and red. Most of these

clouds

are made of droplets of ammonia and

ammonia

crystals, mixed with phosphorus and sulfur. (These

ammonia

storms would be toxic on Earth.)

Beneath

Jupiter

’s thick layer of

ammonia

clouds

lies what some astrophysicists believe is a thin layer of water

clouds

. Scientists think there may be water

clouds

because bursts of

lightning

have been spotted in

Jupiter

’s

atmosphere

.

Interstellar clouds, which exist in the space between

planets

and stars, are not really

clouds

at all.

Interstellar

clouds

are areas where gases and plasma are dense and, sometimes, visible. Astronomers determine what elements are present in

interstellar

clouds

by analyzing the light, or radiation, that comes from them. Most

interstellar

clouds

are made of hydrogen, helium, and oxygen. The

dusty

“milk” of the Milky Way is an

interstellar

cloud

between the

stars

of our galaxy.