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How do clouds form?

We have all spent time looking up at the clouds in the sky, pointing out the ones that look like rabbits or unicorns, or wondering if the darker ones are about to bring some rain. It is not often, though, that we wonder about how those clouds formed in the first place. Here is an explanation for how clouds form, so you can think about it the next time you are gazing up at the sky on a lazy summer afternoon.

What is a cloud, exactly?

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The first question to ask when wondering about how clouds are formed is "what exactly is a cloud?" It's not cotton candy or a puff of smoke as you might have imagined when you were a child—it is something a bit more scientific than that. A cloud, in the technical sense, is a big bunch of water droplets and ice crystals that have collected in one spot. These droplets or crystals are extremely tiny and extremely light, which is why they are able to hang suspended in the air above us.

How do clouds form?

The first step in the formation of a cloud is evaporation. Water vapor enters the air through the evaporation of water from oceans, rivers, lakes, or other bodies of water. The gas that forms from this evaporation rises up into the atmosphere and is cooled. When this cooling process occurs, the air can't hold as much moisture as it did at first. The pressure in the atmosphere is also much lower, making it even more difficult for the air to hold water. So, the vapor changes forms and becomes small droplets of water or ice crystals, which then join together to form a visible cloud.

Clouds formed from heat

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The steps described in the last section represent the most basic explanation for how a cloud is formed, but there are other ways they can form, including through a heating process. Sometimes when the sun heats the ground, it causes the air just above the surface to heat up as well. This warm air is lighter and not as dense as the other air around it, so it rises upward. As it rises, it expands due to the pressure difference and becomes cooler. At this point, the cooler air can no longer hold all the water, and it starts to condense into a cloud as described previously. Clouds formed from this heating and cooling process are called cumulonimbus, stratocumulus, and cumulus clouds.

Other ways clouds are formed

Stratus clouds and lenticular clouds are formed when wind blows against a mountain range or other high, hard terrain and is pushed upward. As it is pushed upwards, the air is forced to cool down, changing its form as it becomes a cloud. Clouds can also form due to low pressure areas pushing air upwards, forcing it, again, to go through the cooling process that makes it condense.

Fronts and storms

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A weather front is when two large masses of air run into each other at the surface of the Earth and cause air to rise up higher into the atmosphere. With a warm front, in which a mass of warm air slides overtop a mass of cold air, the warm air gets pushed upwards and forms a wide variety of cloud types, including the ones that bring rain. In a cold front, heavy, cold air masses push the warm air mass upward, forming cumulus and cumulonimbus clouds, the latter of which can produce thunderstorms.