Cloud Spotting

A few years ago, a cloud that appeared over Cedar Rapids, Iowa, made international news. It looked nothing like the fluffy or wispy clouds people were used to seeing. Instead, it looked and moved like rough, roiling waves on a stormy sea. Members of the Cloud Appreciation Society, a group of ordinary citizens who are also cloud enthusiasts, had seen the strange cloud before in photos that people had sent to them over the years. The group believed there was enough evidence to warrant the recognition of a brand-new cloud classification.

Thanks to camera phones and the internet, everyday people have been increasingly sharing the most unusual and breathtaking clouds they spot—and helping meteorologists categorize brand-new ones in the process. Steve Cohn, a recently retired meteorologist from the National Center for Atmospheric Research in Colorado, says this type of public enthusiasm helped the World Meteorological Organization decide that it was time to update its International Cloud Atlas. The 121-year-old atlas serves as a global reference for identifying clouds.

A few years ago, a strange cloud appeared over Cedar Rapids, Iowa. This cloud made news around the world. It didn’t look like the fluffy or wispy clouds people were used to seeing. It looked and moved like rough, churning waves on a stormy sea. Members of the Cloud Appreciation Society, a group of everyday people who love clouds, had seen the strange cloud before. People had sent them photos of it over the years. The group believed the evidence was clear. It was time to add a brand-new cloud classification.

More and more, camera phones and the internet allow people to share photos. Everyday people have been sharing the strangest and most beautiful clouds they spot. This helps meteorologists name brand-new clouds. Steve Cohn is a meteorologist who recently retired from the National Center for Atmospheric Research in Colorado. He says such public excitement helped the World Meteorological Organization make a decision. It was time to update its International Cloud Atlas. The 121-year-old atlas is a global reference for identifying clouds.

“The atlas is really important to meteorologists,” says Cohn. “Keeping an eye on the clouds can give us clues about cycles in weather patterns. This helps with forecasts for the coming weeks and seasons. It also helps us understand Earth systems.”

This year, Cohn led the international committee that gave the atlas its first update in 30 years. It identified 12 new cloud classifications, including the wavelike one the Cloud Appreciation Society fought for, which was given the name asperitas.

“The atlas is really important to meteorologists,” says Cohn. “Keeping an eye on the clouds can give clues about cycles in weather patterns. This helps with forecasts for the coming weeks and seasons. It also helps us understand Earth systems.”

This year, Cohn led the international committee that updated the atlas. It was the first update in 60 years. The committee listed 12 new cloud classifications, including the wave-like one spotted by amateur cloud watchers. They named it asperitas.

Clouds form when heat from the sun causes water on Earth to evaporate, or turn from a liquid into a gas. As the water vapor rises into the atmosphere or is carried by wind, the gas cools and condenses back into a liquid. Tiny water droplets form on airborne particles like dust. Billions of these droplets, and sometimes ice crystals, gradually come together to form a visible mass in the sky—a cloud.

No two clouds are exactly alike. The heights and temperatures at which clouds form affect how they look. In England in the early 1800s, an amateur meteorologist named Luke Howard developed a system for classifying the many types of clouds. Howard’s nomenclature—or classification system—works in a way similar to how Latin scientific names are given to plants and animals. Each cloud is named according to a genus—a broad group of related things—that is based on the cloud’s altitude and general appearance. There are 10 of these groups. You may already know some of their names, such as wispy cirrus, fluffy cumulus, blanketing stratus, and thunderous cumulonimbus clouds (see Common Clouds).

What makes clouds form? The sun’s heat causes water on Earth to evaporate, or turn from a liquid into a gas. The water vapor rises into the air, or wind carries it. As this happens, the gas cools and condenses back into a liquid. Tiny water droplets form on particles in the air, like dust. Little by little, billions of these droplets come together. Sometimes ice crystals join them. They form a cloud in the sky.

No two clouds are exactly alike. Clouds form at different heights and temperatures. This affects how they look. In the early 1800s, an amateur meteorologist named Luke Howard studied clouds. He came up with a nomenclature, or classification system, for the many types of clouds. The system is similar to the way Latin scientific names are given to plants and animals. Each cloud is given the name of a genus—a broad group of related things. A genus is based on the cloud’s height and overall look. There are 10 of these groups. You may already know the names of some of them. They include wispy cirrus, fluffy cumulus, blanketing stratus, and thunderous cumulonimbus clouds (see Common Clouds).

Each genus is further divided into several species—smaller groups with similar traits. Clouds’ shapes or internal structures define their species. Species are broken down by variety based on their transparency or arrangement in the sky.

“The species and varieties really describe the character of the cloud,” says Cohn. Take, for example, the species and variety altocumulus castellanus: “It has turrets like a castle,” says Cohn. There’s also altocumulus lenticularis, which forms downwind of mountains. “When the wind flows over mountain ranges, air gets lifted,” says Cohn. “Then, as it continues downwind of the mountain, the air bobs up and down as a wave. In the right conditions, a lens-shaped cloud forms at the crest of each wave. Lenticular clouds are very cool.”

Each genus is divided into several species. These are smaller groups with similar features. Clouds’ shapes or inner structures define their species. Species are broken down by variety. That’s based on how much light shines through them or how they’re arranged in the sky.

“The species and varieties really describe the character of the cloud,” says Cohn. For example, take the species and variety altocumulus castellanus. “It has turrets like a castle,” says Cohn. There’s also altocumulus lenticularis. This cloud forms downwind of mountains. “When the wind flows over mountain ranges, air gets lifted,” says Cohn. “Then, as it continues downwind of the mountain, the air bobs up and down as a wave. In the right conditions, a lens-shaped cloud forms at the crest of each wave. Lenticular clouds are very cool.”

The cloud atlas updates include a brand-new species of cloud called volutus (see Fascinating Formations). It’s a long, tube-shaped cloud that appears to roll across the sky. “It’s very rare to name a new cloud species,” says Cohn. “But people realized that the cloud they were seeing didn’t really fit the classification they had always used.”

The changes also include several new features, which better describe cloud varieties. Along with asperitas, there’s also fluctus (Latin for wave, or billow), which looks like a surfer’s wave on top of a cloud. Cavum (for cavity) marks a hole that looks like it’s been punched into a thin cloud. Murus, cauda, and flumen (for wall, tail, and flowing) are associated with stormy cumulonimbus clouds and indicate severe weather conditions.

Another important addition to the atlas is the recognition of clouds created by special conditions. Examples are cataractagenitus—caused by the spray from a waterfall—and silvagenitus—from the misty evaporation from forest canopies. Flammagenitus (for flame) is a cloud created by a forest fire or volcanic activity. Human-made cloud features—homogenitus—are recognized too. “A prime example is aircraft condensation trails, or contrails,” Cohn says. As winds cause contrails to spread, they become known as homomutatus.

The cloud atlas updates include a brand-new species of cloud called volutus (see Fascinating Formations). This long, tube-shaped cloud seems to roll across the sky. “It’s very rare to name a new cloud species,” says Cohn. “But people realized that the cloud they were seeing didn’t really fit the classification they had always used.”

The changes also include several new features. These describe cloud varieties better. Besides asperitas, there’s fluctus. That’s a Latin word for wave, or billow. This feature looks like a surfer’s wave on the topside of a cloud. Cavum means “cavity.” It names a hole that looks like it’s been punched into a thin cloud. Murus, cauda, and flumen are words for wall, tail, and flowing. They’re linked with stormy cumulonimbus clouds, and they mark severe weather conditions.

There’s another important change. Now the atlas lists clouds created by special conditions. One example is cataractagenitus, caused by the spray from a waterfall. Another is silvagenitus. This cloud comes from the misty evaporation from forest canopies. A forest fire or active volcano creates the cloud called flammagenitus. The atlas also includes human-made cloud features, called homogenitus. “A prime example is aircraft condensation trails, or contrails,” Cohn says. Winds cause contrails to spread, and they become known as homomutatus.

When making weather predictions, today’s meteorologists rely on observations from high-tech equipment like radar and computer models. But they can still learn by simply looking up.

“Clouds do a lot besides floating there in the sky,” says Brian Jackson, a meteorologist with the National Weather Service in Washington, D.C. “We can learn a lot from their shapes, types, and movements. Bubbly, cotton-ball-shaped clouds tell us that the atmosphere is unstable. Low, gray, flat clouds tell us that there is a large-scale rising motion in the atmosphere, and there is most likely a front nearby. High, thin, and feathery clouds signify the early approach of a storm system or that an area of strong upper-level winds is nearby.”

Plus, looking at clouds is fun. And it doesn’t take an expert to find something special. “People from all over the world helped us create the new atlas by sending in their cloud pictures,” says Cohn. “We used the best examples to illustrate each cloud and feature. Everyone can participate and make a difference in observing nature.”

Today’s meteorologists rely on data from high-tech equipment to make weather predictions. For instance, they use radar and computer models. But they can still learn by just looking up.

“Clouds do a lot besides floating there in the sky,” says Brian Jackson. He’s a meteorologist with the National Weather Service in Washington, D.C. “We can learn a lot from their shapes, types, and movements. Bubbly, cotton-ball shaped clouds tell us that the atmosphere is unstable. Low, gray, flat clouds tell us that there is a large-scale rising motion in the atmosphere, and there is most likely a front nearby. High, thin, and feathery clouds signify the early approach of a storm system or that an area of strong upper-level winds is nearby.”

Looking at clouds is also fun. And it doesn’t take an expert to find something special. “People from all over the world helped us create the new atlas by sending in their cloud pictures,” says Cohn. “We used the best examples to illustrate each cloud and feature. Everyone can participate and make a difference in observing nature.”