How Do Clouds Form? Cloud Ingredients and Formation

By Rachelle Oblack

We all know what clouds are—visible collections of tiny water droplets (or ice crystals if it's cold enough) that live high in the atmosphere above the Earth's surface. But do you know how a cloud forms?

In able for a cloud to form, several ingredients must be in place:

• water
• cooling air temperature
• a surface to form on (nuclei)

Once these ingredients are in place, they follow this process to form a cloud:

Step 1: Change Water Vapor into Liquid Water

Although we can't see it, the first ingredient -- water -- is always present in the atmosphere as water vapor (a gas). But in order to grow a cloud, we need to get the water vapor from a gas to its liquid form.

Clouds begin to form when a parcel of air rises from the surface up into the atmosphere. (Air does this in a number of ways, including being lifted up mountainsides, lifted up weather fronts, and being pushed together by converging air masses.) As the parcel ascends, it passes through lower and lower pressure levels (since pressure decreases with height). Recall that air tends to move from higher to lower pressure areas, so as the parcel travels into lower pressure areas, the air inside of it pushes outward, causing it to expand. It takes heat energy for this expansion to take place, and so the air parcel cools a bit. The further upward the air parcel travels, the more it cools. Cool air can't hold as much water vapor as warm air, so when its temperature cools down to the dew point temperature, the water vapor inside of the parcel becomes saturated (its relative humidity equals 100%) and ​condenses into droplets of liquid water.

But by themselves, water molecules are too small to stick together and form cloud droplets. They need a larger, flatter surface on which they can collect.

Step 2: Give Water Something to Sit on (Nuclei)

In able for water droplets to form cloud droplets, they must have something—some surface—to condense on. Those "somethings" are tiny particles known as aerosols or condensation nuclei.

Just like the nucleus is the core or center of a cell in biology, cloud nuclei, are the centers of cloud droplets, and it is from this that they take their name. (That's right, every cloud has a speck of dirt, dust, or salt at its center!)

Cloud nuclei are solid particles like dust, pollen, dirt, smoke (from forest fires, car exhaust, volcanoes, and coal-burning furnaces, etc.), and sea salt (from breaking ocean waves) that are suspended in the air thanks to Mother Nature and us humans who put them there. Other particles in the atmosphere, including bacteria, can also play a role in serving as condensation nuclei. While we usually think of them as pollutants, they serve a key role in growing clouds because they're hygroscopic—they attract water molecules.

Step 3: A Cloud is Born!

It is at this point—when water vapor condenses and settles onto condensation nuclei—that clouds form and become visible. (That's right, every cloud has a speck of dirt, dust, or salt at its center!)

Newly formed clouds will often have crisp, well-defined edges.

The type of cloud and altitude (low, middle, or high) it forms at is determined by the level where an air parcel becomes saturated. This level changes based on things like temperature, dew point temperature, and how fast or slow the parcel cools with increasing elevation, known as "lapse rate."

What Makes Clouds Dissipate?

If clouds form when water vapor cools and condenses, it only makes sense that they dissipate when the opposite happens—that is, when the air warms and evaporates. How does this happen? Because the atmosphere is always in motion, drier air follows behind the rising air so that both condensation and evaporation continually occur. When there's more evaporation taking place than condensation, the cloud will return once again become invisible moisture.

Now that you know how clouds form in the atmosphere, learn to simulate cloud formation by making a cloud in a bottle.

原文传送门 THOUGHTCO