Cloud

A cloud is a beautiful sight in the sky. It is made up of tiny drops of water or ice crystals. These tiny particles form when air cools down or gets enough moisture. Clouds can appear in many shapes and sizes.

We see clouds in Earth’s atmosphere. Scientists who study clouds call this field nephology. This is a part of meteorology, the science that helps us understand weather. Clouds have special names based on where they are in the sky and what they look like. These names use both Latin and common words.

Clouds play a role in Earth’s climate. They can reflect sunlight, which cools the planet. They can also trap heat from the ground, which can warm it. The height, shape, and thickness of clouds decide if they cool or warm the planet. Clouds higher up in the atmosphere have less impact. But clouds closer to the ground have a bigger impact on how warm or cool our planet stays.

Etymology

The word "cloud" comes from old English words clud or clod, which meant a hill or a big piece of stone. In the 1200s, people began using this word for rain clouds because they looked like piles of rock. This new use replaced the old English word weolcan, which was the word for clouds.

Homospheric nomenclatures and cross-classification

This table shows different ways to name clouds. It uses special names for clouds close to the ground and those higher up. Both styles are approved by the World Meteorological Organization. They help us learn about the shape and height of clouds and sort them into groups. This includes common low clouds and some special types found above the main cloud layer. For example, cumulus clouds can be different sizes and reach different heights.

History of cloud science

Main article: Timeline of meteorology

Long ago, people looked at clouds and other weather. Around 340 BC, a thinker named Aristotle wrote a book called Meteorologica. He was the first to call rain and clouds "meteors," meaning things high in the sky. This helped create the word meteorology, the study of clouds and weather.

In the 1800s, two scientists studied clouds. Luke Howard in England and Jean-Baptiste Lamarck in France tried to sort out cloud types. Howard used easy Latin names, and his work became popular. Lamarck used French names, which were harder, so his system didn’t catch on. Later, Howard’s ideas became the official way to name clouds.

Formation

Clouds are found high in the sky where the air is cold. When air cools to a special temperature called the dew point, it can't hold any more water vapor. This makes the water vapor turn into tiny droplets or ice crystals, forming clouds.

One way this happens is when warm air rises and cools. This can happen when the sun heats the ground, making the air above it rise. When this air cools, it can form clouds. Another way is when air is pushed up by weather patterns or when it moves over mountains. These actions help create the conditions needed for clouds to form.

Tropospheric classification

Classification in the troposphere groups clouds by their shape and height. There are ten main types.

Clouds in the troposphere have five main shapes. These shapes help scientists study clouds from satellites. They are listed from least to most unstable:

• Nonconvective stratiform clouds appear in stable air and have flat, sheet-like shapes. They can form at any height. Examples include high-level cirrostratus, mid-level altostratus, low-level stratus, and multi-level nimbostratus.
• Cirriform clouds look like thin, wispy strands. They form high in the sky where the air is stable. Examples include cirrus clouds.
• Stratocumuliform clouds have both layered and puffy shapes. They form in stable air with some convection. Examples include cirrocumulus, altocumulus, and stratocumulus.
• Cumuliform clouds appear in isolated piles or tufts. They form in unstable air. Examples include cumulus clouds.
• Cumulonimbus clouds are large, towering clouds. They form in very unstable air and can reach high into the sky. These clouds often have flat tops.

Levels and genera

Clouds form at different heights in the sky. They are grouped into three levels based on their height:

High-level

High clouds form between 3,000 to 7,600 meters in polar regions, 5,000 to 12,200 meters in temperate regions, and 6,100 to 18,300 meters in tropical regions. All cirriform clouds are high clouds. Examples include:

• Cirrus (Ci) – These are thin, wispy clouds made of ice crystals. They often appear as delicate white strands in the sky.
• Cirrocumulus (Cc) – These are small, white puffy clouds that appear in rows or groups.
• Cirrostratus (Cs) – These are thin, icy clouds that can create halos around the sun or moon.

Mid-level

Mid-level clouds form from near the surface up to about 4,000 meters near the poles, 7,000 meters at midlatitudes, and 7,600 meters in the tropics. Examples include:

• Altocumulus (Ac) – These are mid-level clouds that appear in patches or sheets. They are made of water droplets and ice crystals.
• Altostratus (As) – These are gray or blue-gray clouds that often cover the sky. They can bring light rain or snow.

Low-level

Low clouds form from near the surface up to 2,000 meters. Examples include:

• Stratocumulus (Sc) – These are low, layered clouds that appear in patches or sheets. They can bring light rain.
• Cumulus humilis – These are small, fair-weather clouds with flat bases and rounded tops. They do not bring rain.
• Stratus (St) – These are flat, gray clouds that can resemble fog. They can bring light drizzle or snow grains.

Multi-level or moderate vertical

These clouds have bases from near the surface to about 2,400 meters and tops that can reach mid-level heights. Examples include:

• Nimbostratus (Ns) – These are thick, dark gray clouds that bring steady rain or snow over a wide area.
• Cumulus mediocris – These are medium-sized cumulus clouds with flat bases and rounded tops. They usually do not bring rain.

Towering vertical

These large clouds have bases in the low to mid-level range but tops that reach high into the sky. Examples include:

• Cumulus congestus – These are tall cumulus clouds that can bring moderate to heavy showers.
• Cumulonimbus (Cb) – These are massive, towering clouds that can bring thunderstorms, heavy rain, hail, strong winds, and tornadoes.

Distribution

Main articles: Intertropical Convergence Zone, Extratropical cyclone, Cold front, and Warm front

Clouds spread around the world based on where the air moves and how much pressure there is. They are most common near the equator and around the 50th parallels in both the northern and southern hemispheres. This is because warm air near the equator rises and cools, helping clouds form. Farther from the equator, clouds can still appear, but they depend on the air's temperature and moisture.

In areas where air sinks or moves apart, like near the poles and around 30 degrees north and south, there are usually fewer clouds. These places are sometimes called the horse latitudes. Big areas of high pressure, known as subtropical ridges, also help keep these areas clearer by pushing air away from the ground.

Luminance, reflectivity, and coloration

The brightness of a cloud depends on how it reflects and scatters light. Thick, low clouds look very white because the water droplets are close together and reflect sunlight. As you look deeper into the cloud, the light gets weaker, so the bottom can look gray.

Clouds can change color depending on the time of day and the light. During the day, clouds often look white on top and gray underneath. At sunrise or sunset, they can turn red, orange, or pink. At night, clouds look dark unless the moon or lights from Earth shine on them. Sometimes clouds can appear green or blue in strong storms.

Effects

See also: Cloud cover, Cloud feedback, Global dimming, and Climate change

Clouds have important effects on our planet's weather and climate. They help bring rain and snow, which spread water across the Earth. Clouds can also move air up and down, changing weather patterns.

Clouds can both cool and warm the Earth. Their bright tops reflect sunlight away from the ground, which cools the surface. But clouds can also trap heat, like a blanket. This makes it hard to know exactly how clouds will change with our warming climate. Scientists are still studying these effects to better predict future weather and climate changes.

Stratospheric classification and distribution

Polar stratospheric clouds (PSC's) are found in the lowest part of the stratosphere. There is very little moisture above the troposphere, so these clouds are only seen in polar regions during winter when the air is very cold.

PSC's can look different depending on what they are made of and the conditions in the atmosphere, but they are all found at very high altitudes, between about 15,000–25,000 meters (49,200–82,000 feet). They are all classified as one type and do not have different levels or names like clouds closer to the ground. Instead, they have simple descriptive names based on how they look. Some PSC's look like thin, hazy layers, while others have a shiny, colorful appearance.

Mesospheric classification and distribution

Noctilucent clouds are the highest clouds in the sky. They appear about 80 to 85 kilometers above Earth, which is much higher than most other clouds. These clouds glow in the dark after sunset and before sunrise, looking bluish or silvery white, sometimes even red or orange. They are not common and do not change the climate much, but scientists have noticed them appearing more often since the 1800s.

Research shows that during the polar summer, strong upward air movement cools tiny amounts of water vapor until it forms clouds. Tiny particles from burning meteors help these clouds form. There are four main types of noctilucent clouds, each looking different from the others. These clouds are mostly seen near the poles, far from the equator.

Extraterrestrial

See also: Extraterrestrial atmospheres

Clouds are found on many planets beyond Earth. On Venus, thick clouds made of sulfur dioxide cover the planet high above the surface. On Mars, thin clouds made of water-ice can be seen, especially near the poles.

Both Jupiter and Saturn have clouds made from different materials, like ammonia and ammonium hydrosulfide. Uranus and Neptune have clouds made of methane. Saturn's moon Titan also has clouds that scientists think are mostly made of methane. Scientists have found clouds on planets far outside our Solar System, like Kepler-7b, GJ 436 b, and GJ 1214 b.

In culture and religion

Clouds have been important in many stories and beliefs. Ancient Akkadians thought clouds were the sky goddess Antu’s breasts, and rain was like milk from her. In the Book of Exodus, Yahweh guided the Israelites through the desert as a "pillar of cloud" by day and a "pillar of fire" at night. In Mandaeism, special beings called uthras are sometimes said to be in anana ("clouds").

The ancient Greek play The Clouds by Aristophanes shows the philosopher Socrates saying the Clouds are the only true gods. In this play, the Clouds change shapes to show what the viewer sees. In Shakespeare’s Hamlet, a character sees many shapes in the clouds.

In China, clouds are symbols of luck and happiness. Overlapping clouds mean eternal happiness, and colored clouds bring blessings. Cloud gazing, watching clouds and imagining shapes, is a fun activity many people enjoy.