Clouds cover a lot of Earth, storing water in air. Even in dry places like deserts, you find clouds. Clouds are made of tiny ice crystals or small water droplets. The droplets form when invisible water vapor finds enough tiny particles like smoke or pollen to help them start growing. Clouds can travel and stretch for thousands of miles. There are two main types of clouds. Clouds that are wide and stretched out are called stratus clouds. They are stratified. Clouds that bubble upward and may grow very tall are called cumulus clouds. They accumulate in one spot. Each type of clouds is found at different heights, ranging from low, to middle, to high. Middle level clouds start with alto. The highest clouds start with cirro. There are also many combinations of clouds, and variations in cloud types. Meteorologists around the world continue to use a cloud classification system that was proposed by Luke Howard, in the 1800s.
Stratus clouds can be made of water droplets or ice crystals, depending on how cold the air is above the ground. Middle-level stratus clouds are altostratus, and high-level stratus clouds are cirrostratus. Regular stratus clouds are lower clouds that are flat, stretched out, and gray. They can cover the whole sky and hide the sun or moon. Stratus clouds can make rain or snow. Sometimes, they stay for more than a day. If they linger for a long time, you might get a lot of rain or a lot of snow. They might also make very light rain called drizzle. In the winter, stratus clouds can make light snow called flurries. When they block the sun, stratus clouds keep you from getting warm. At night, stratus clouds will keep an area from getting as cold as it would get on a clear night.
Cumulus clouds can be made of water droplets or ice crystals, depending on how cold the air is above the ground. Middle-level cumulus clouds are altocumulus, and high-level cumulus clouds are cirrocumulus. Regular cumulus clouds are fluffy and puffy, and they may grow tall. They look like cotton balls as they bubble up into the air. Small cumulus clouds with lots of space in between them tell us the weather will be nice. The larger and taller cumulus clouds make rain showers, or snow showers if it is cold enough. The showers usually don't last very long. When cumulus clouds make lightning, thunder, and heavy rain they are cumulonimbus. The cumulonimbus is the tallest single cloud. Sometimes it can reach more than 10 miles high. It's the cloud that makes thunderstorms. When you see a cumulonimbus cloud you have to make sure that you are safe from lightning, and from the heavy rain. Cumulonimbus clouds can make a lot of rain in one spot to start a flood.
Cirrus clouds are very high, thin clouds. The air is so cold where they are that cirrus clouds are made of tiny ice crystals, not water droplets. Cirrus clouds are thin. You can see the sun or the moon right through them. They can be more than 5 miles above the ground- that's higher than where most airplanes fly. Cirrus clouds are gentle, soft, and feathery. They are not thick enough to make precipitation so when you see them you know that the weather is calm. Cirrus clouds that totally blanket the sky are cirrostratus. Cirrus clouds with tiny bubbles are cirrocumulus.
Fog is a special type of stratus cloud. Fog is a very low cloud that sits on the ground or on water. We find thick fog in places where there is a lot of water on the ground, or in rivers or lakes. Fog is also seen where there is moisture in the air near the ground. That means if you drive up a mountainside into a stratus cloud, once you are in it, it’s called fog. Fog forms when moisture is added to the air, or when the air cools down a lot. We can see fog on clear, humid nights when the wind is calm. Fog can develop in valleys, and over snow. There are also many lakes, rivers, and oceans where fog forms over the water and then the wind pushes it onto land. When the sun is strong and heats up the air or when wind mixes the air, the fog disappears, but the moisture is still floating in the air. Some fog is only a few feet off of the ground while other fog can rise as high as a tall building. When fog is thick it makes travel dangerous because drivers can’t see very far, and pilots can’t safely find airport runways.
An airplane vapor trail forms as a byproduct of burning fuel, in combustion. Most of what comes out of an aircraft exhaust is carbon but there is also water vapor and smoke particles. High in the atmosphere, the air temperature is far below freezing. It is so cold that adding moisture to air, can quickly raise relative humidity to 100%, forming a cloud made of ice crystals. This condensation forms in twin tubes. The tubes are always there as invisible vortexes created by airplane wings. You often see a short trail of vapor behind an airplane, known as a vapor trail or condensation trail. The relative humidity at flight level does not always follow the same pattern as relative humidity at the ground. That means that some days, vapor trails are very short. Other days, when moist air is aloft, the trails are very long and last for a long time. They are common in high-traffic flight corridors around the world, in any season. When the condensation clouds linger and spread, they become cirrus clouds. Another type of cloud formed by airplanes is one that is brief, behind very fast jets often flying close to the ground. It's called a vapor cone.
Ship tracks are not the actual track or wake of ships in the water. They are trails of clouds, caused by moisture in the exhaust of ships. Ship engines burn fuel to release carbon, water droplets, and tiny particles in smoke that allow water vapor to quickly condense into a cloud, and persist as a trail or track, behind the ship. We see ship tracks when looking across the planet, using a satellite. Most of us have noticed something similar, looking up, into the sky- airplane vapor trails or condensation trails. Why are ship tracks curved, when most of the ships are moving in straight lines? It’s the winds around highs and lows that change the shape and location of ship tracks after they form. Because the tracks last for such a long time, the wind pushes them into different shapes. This video shows how it happens. Ship tracks are an unintentional byproduct of human activity, that end up mixing with nature.
This odd cloud is nicknamed a hole punch cloud because it looks like someone punched a giant hole in it. There are a few things happening here. The layer of bubbly clouds on the outside are altocumulus clouds made of water droplets that are actually below freezing. That’s called supercooled. The cloud in the middle is made of ice crystals that are slowly falling. It is a small cirrus cloud that in this case is also called a fall streak. The hole also has a technical name of cavum, the Latin word for cavity. Hole punch clouds happen when an airplane flies through a layer of altocumulus or cirrocumulus clouds, upsetting the balance so that some of the supercooled water instantly freezes into ice crystals. That starts a chain reaction where more ice forms, as a small cirrus cloud, and eats out a circle in the cloud. The entire cloud can drift dozens or hundreds of miles, slowly changing shape. It’s something that can be seen by a satellite, but you have to look very closely to find. Hole punch clouds can happen naturally, but they are certainly increased by aircraft traffic. When airplanes fly right at the level of supercooled clouds for a while, they may create clearing lines known as ice canals. These show just one way air traffic impacts the environment, in a fascinating way. Read more about hole punch clouds and watch a video showing how they form.
The wind, blowing around and over an island can sometimes leave a wake of no clouds. It’s similar to a wake behind a moving boat, except that the island is not moving. The wind is. A little farther away from the island, swirls of clouds form in the wake, each one as a vortex, that make up a string of swirls. These are called island cloud vortices or cloud vortex streets. They are controlled by the size and shape of the island and how fast the air flows past it. Each vortex does not spin fast enough for your eye to notice but once it forms and is joined by others, the vortices can last for many hours as they slowly drift away from the island. Some of them may be 20, 30 or 40 miles across. They are obvious from space. They are known to scientists as von Kármán Vortices, named after scientist Theodore von Kármán, who is credited with first describing them, in the early 1900s. These vortices are also called eddys. See how they form in this video.
Mammatus clouds might remind you of grapes or upside-down bubbles. They happen often, but not always, around thunderstorms. Mammatus comes from the Latin word mamma. Mammatus clouds are also found with middle and high-level clouds. The pouches or globs of mammatus clouds don’t actually produce severe weather. They form when cloud water droplets or ice crystals sink into dry air, evaporate and cool, to make a downward bubble. When the sun is low, especially when sunlight carries a yellow or orange tint, mammatus clouds really stand out. The sun illuminates the mammatus from the side and then from beneath, creating shadows on the other side. It can be a dramatic display like those in this video.
Asperitas is a feature found in thick clouds. It is random waves seen on the bottom of the clouds. If you think of them upside down, they’ll remind you of large waves on the ocean. They are found on thick clouds that cover large areas, like stratocumulus, and they show you that air is a fluid, constantly changing shape due to wind and turbulence. Watch how asperitas move on video.
Shelf clouds are ominous cloud bands on the forward edge of thunderstorms. They are part of the thunderstorm gust front. The shelf cloud hangs out in front of the storm like a shelf. The technical name for that is arcus. As a gust front nears, winds increase, and the sky may darken. Beneath and behind the shelf cloud, the clouds are chaotic. Lightning increases. This does not necessarily mean it’s a severe thunderstorm, but it does mean it’s a strong thunderstorm. As the gust front passes, you feel cooler downdraft air, and the rain begins. Because the shelf cloud may stick out in front of the storm, it can be directly illuminated by the sun, to make it appear really bright, compared to underneath the thunderstorm. On the edge of the gust front, you’ll often see little fragments of clouds forming and rising straight up. They don’t rotate, but people sometimes confuse them for funnel clouds. They are moisture condensing as air rises. See that in this video. The nickname for them is scud.
Lenticular or lenticularis clouds get their name from being shaped like lenses. They are smooth, curved clouds, typically with sharp edges, that are common around tall mountains, where you often see them standing in place. The shape is found with cumulus clouds at different heights. Lenticular clouds are seen in other places around the planet, where they may also be stationary or drifting with the wind. They may last a long time. When smooth airflow is forced to rise and sink, as it does going over mountains, relative humidity reaches 100% where the air rises, forming a cloud. At sunset, these clouds may stand out, as they are tinted by the sun. Some people say they look like a spaceship hovering over a mountain.
Pileus is a feature found above fast-growing cumulus clouds. It is shaped like a lenticular cloud, but it forms when a rapidly rising cumulus cloud forces moist air just above it to rise and cool so that the vapor condenses as a smooth cloud. The word pileus means something shaped like a cap. The pileus is usually brief, lasting only a few minutes.
Kelvin Helmholtz clouds, named after two people, are also called breaking wave clouds, because they look like waves that are cresting and breaking on the ocean. They are caused by wind shear. The wind near the top of the cloud is a little stronger than the wind near the bottom of the cloud. The stronger wind pulls upward and forward on the cloud.
Scud is the nickname for ragged or shredded cloud fragments that are separated from main clouds. Technically, they are called fractus. They are common in the rising air that feeds into thunderstorms, where the relative humidity falls to 100% in a layer beneath the main cloud. You may notice them seeming to rise, but they don’t rotate. By itself, scud is not dangerous. Scud can also form where rain falls into a layer of air to cause saturation and additional cloud condensation.
A funnel cloud is a tube or funnel-shaped cloud that you may see stretching down from a fast-growing cloud, often a cumulus or cumulonimbus cloud. The clouds in the funnel rotate. If they reach the ground, it would be called a tornado. If they reach water, it would be called a waterspout. Even small cumulus clouds can create tiny funnels. Most of the time, the funnels are brief. Skinny or tiny funnels from small or medium cumulus clouds, on a quiet day, are not the type that are known to create tornadoes.