Updated March 13, 2018
By Rob Bethell
Clouds are part of the Earth's water cycle. Formed naturally due to the cooling of water vapor within the Earth's atmosphere, clouds are made up of billions of water particles. Clouds take on many shapes and forms, dependent on local weather systems and local terrain. Some of the most common cloud types include cirrus, cumulus and stratus.
Light from the sun hits the surface of the earth. A large part of the solar radiation is absorbed by the ground and gradually heats it up.
Constant heat reaching the surface of the ground causes air to heat up. The heated air becomes lighter, which causes it to rise above the cooler air which lies above it. This process is called convection.
Rising hot air is pushed further upward by wind blowing over terrain such as mountains, or over cliffs onto land from the sea. This process is called Orographic uplift. Wetter areas are generally found near high terrain features, as the air cools at a quicker rate around these areas.
Air is also forced to rise at a weather front. This is due to the differing air masses of the two weather fronts. At cold fronts, cold air is pushed under warm air, forcing it upward and at a warm front, warm moist air is forced up and over the cold air. This process is called convergence or frontal lifting.
Clouds begin to develop in any air mass that becomes saturated. Saturation point is reached when the air reaches its frost point. At this point, air gradually cools, preventing it from rising any further. Water vapor molecules within air begin to clump together.
Water vapor condenses to form cloud droplets or ice crystals. This can be at various heights, which creates a variety of different cloud systems. Clouds contain millions of droplets of water or ice, depending on the temperature, which are suspended in the air.
This Met Office video explores clouds including; how cloud droplets are formed from condensation nuclei and how much a cloud can weigh.
Clouds form when the invisible water vapour in the air condenses into visible water droplets or ice crystals. There is water around us all the time in the form of tiny gas particles, also known as water vapour. There are also tiny particles floating around in the air – such as salt and dust – these are called aerosols. Aerosols make it easier for the water vapour to condense, and once the process starts, eventually bigger water droplets form around the aerosol particles, and these water droplets start to merge with other droplets, forming clouds.
Clouds form when the air is saturated which means it cannot hold any more water vapour, this can happen in two ways:
-
The amount of water in the air has increased – for example through evaporation – to the point that the air cannot hold any more water.
-
The air is cooled to its dew point – the point where condensation occurs – and the air is unable to hold any more water.
The warmer the air is, the more water vapour it can hold. Clouds are usually produced through condensation – as the air rises, it will cool, and reducing the temperature of the air decreases its ability to hold water vapour so that condensation occurs. The height at which the dew point is reached and clouds form is called the condensation level.
Clouds are formed when moist air rises upward. As the air rises, it becomes colder. Eventually the air can't hold all of the water vapor in it, and some of the water vapor condenses to form tiny water droplets. When moist air is cooled at the ground, fog is formed in the same way.
Clouds form at a wide range of altitudes, from near the ground to very high in the atmosphere. The appearance of clouds varies a lot, depending on the motions of the air as the clouds are formed. Other important things to observe about clouds are the percentage of the sky they cover, where they are located in the sky, how much of the sky they cover, and their direction of movement. A good way to find their direction of movement is to stand under a tree branch or an overhang on a building and watch the clouds move relative to that stationary object.
A camera on NASA’s Terra satellite captured this image of clouds over the Southern Indian Ocean. Image credit: NASA/JPL-Caltech
You hang up a wet towel and, when you come back, it’s dry. You set out a bowl of water for your dog and when you look again, the water level in the bowl has dropped even though Woofy has been nowhere near the bowl.
Where did the missing water go? It evaporated. That means some of the liquid water in the towel or bowl changed into an invisible gas called water vapor and drifted away into the atmosphere. (Notice that “evaporated” contains the word “vapor.”)
The same thing is constantly happening with oceans, lakes, rivers, swamps, swimming pools – and everywhere water is in contact with air.
Liquid water changes into a gas when water molecules get extra energy from a heat source such as the Sun or from other water molecules running into them. These energetic molecules then escape from the liquid water in the form of gas. In the process of changing from liquid to gas, the molecules absorb heat, which they carry with them into the atmosphere. That cools the water they leave behind.
Heat causes some of the liquid water – from places like oceans, rivers and swimming pools – to change into an invisible gas called water vapor. This process is called evaporation and it's the start of how clouds are formed. Image credit: NASA/JPL-Caltech/Alex Novati
The air can only hold a certain amount of water vapor, depending on the temperature and weight of the air – or atmospheric pressure – in a given area. The higher the temperature or atmospheric pressure, the more water vapor the air can hold. When a certain volume of air is holding all the water vapor it can hold, it is said to be “saturated.”
What happens if a saturated volume of air cools or the atmospheric pressure drops? The air is no longer able to hold all that water vapor. The excess amount changes from a gas into a liquid or solid (ice). The process of water changing from a gas to a liquid is called "condensation," and when gas changes directly into a solid, it is called "deposition." These two processes are how clouds form.
Condensation happens with the help of tiny particles floating around in the air, such as dust, salt crystals from sea spray, bacteria or even ash from volcanoes. Those particles provide surfaces on which water vapor can change into liquid droplets or ice crystals.
A large accumulation of such droplets or ice crystals is a cloud.
Dust and other particles floating in the air provide surfaces for water vapor to turn into water drops or ice crystals. The tiny drops of water condense on the particles to form cloud droplets. Clouds are made up of a bunch of cloud droplets bundled together with raindrops. Image credit: NASA/JPL-Caltech/Alex Novati
We usually think of clouds as being up in the sky, but when conditions are right, a cloud can form at ground level, too. Then it’s called “fog.” If you’ve ever walked through fog, you’ve walked through a cloud.
Fog can be seen at the top of this image of the 525-foot tall Vehicle Assembly Building at NASA’s Kennedy Space Center in Florida. Image credit: NASA/Ben Smegelsky
Although the basic idea of cloud formation is easy to understand, there is much more to learn. Peek beneath a cloud’s fluffy exterior, and you’ll find a world of complexity. In fact, clouds are considered one of the most challenging aspects of climate science.
That’s because truly understanding clouds requires a deep understanding of the entire atmosphere. Scientists are working to increase their understanding, with the help of instruments such as those on NASA’s Terra, Aqua, Aura, CALIPSO, CloudSat and other satellites that observe different aspects of clouds.
NASA has a fleet of spacecraft that orbit Earth, called satellites. Many of them study clouds, weather, climate and more. Image credit: NASA/JPL-Caltech/Alex Novati
The better we can understand clouds and the atmosphere that creates them, the better we can find out what’s happening to our climate.
Related NASA Missions
Make a cloud in a bottle with this DIY project!