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In this lesson, you will learn how water molecules turn into precipitation. Also, you will discover how varying conditions can lead to different types of precipitation.

What Is Precipitation?

It’s a gorgeous sunny day, and you’ve just settled down by the pool. There’s not a cloud in the bright blue sky, when suddenly the sky grows a bit darker.

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You look up to see a cloud blocking the sun! Not only that, you hear the rumbling of thunder. As the pitter patter of rain begins, you are forced to go inside.Rain is a form of precipitation, or water that falls from the sky to the surface of the earth. Precipitation may be in liquid form, like rain, or in solid form, like snow. It’s easy to get disgruntled by rain or snow on a day when you’d rather be outside getting some sunshine.

But the phenomenon of precipitation is actually quite cool–literally!To better understand how precipitation forms, let’s imagine that we are up in the atmosphere and only about 10 micrometers large. Just in case you don’t remember, one micrometer is one one-thousandth of a millimeter. Though you may not see them, you are surrounded by countess gaseous water molecules, known as water vapor.

When the air becomes saturated with water vapor, meaning there is more water vapor than the air can hold, clouds will form. This usually happens at high altitudes, but it can happen at lower altitudes. Ever been in a fog?The state of being saturated is similar to being in a really crowded room; every available space is occupied.

Once the room is too crowded, you and other people might be forced to leave. In the case of cloud formation, water vapor is forced to condense into water droplets or ice crystals. Collectively, all these droplets and ice crystals make up a cloud.A cloud is a region of atmosphere where moisture condenses, forming tiny water droplets or grains of ice.

As you’re looking around, you see water droplets that range from one to ten times your size. All of the droplets inside the rain cloud are falling, but movement is so slow that air currents are strong enough to push them back up. In our cloud, we can also see small grains of ice.

Their individual shapes vary; however, all bear semblance to a hexagonal structure.Water vapor molecules have condensed onto condensation nuclei, which are minuscule solid or liquid particles floating around in the atmosphere. The tiny water droplets we see collect other surrounding water molecules.

This basic process is known as growth by condensation. Once the weight of the droplet is too much for the atmosphere to support it, it will fall from the sky.

Growth in Warm Clouds

Let’s say we’re suspended in a cloud that’s warm–well, relatively warm. To be a warm cloud, temperatures just need to be above freezing.

Inside our warm cloud, there are tiny water droplets. These droplets are constantly colliding with each other, and sometimes, they stick together after colliding. This process of collision and coalescence leads to the growth of rain drops.

Droplets can grow to a million times their original size through collision and coalescence! Once the drops are too big, they will fall from the cloud. On the way down, these drops will collect other droplets. If drops become too big, however, resistance from air will cause them to break down into smaller drops.If drops pass through hot or dry air, they will evaporate before hitting the ground. If the air is above freezing, the drops will fall as rain. But, if the air is below freezing, the raindrops will freeze into sleet.

Growth in Cool and Cold Clouds

When temperatures drop below freezing, our surroundings change a bit.

Inside a cool or cold cloud, both ice crystals and super cooled water can exist. Super cooled water is water that is below freezing but is still in a liquid state. Ice crystals are tiny bits of frozen water.

Under these conditions, precipitation may form through the Bergeron process. When I think of this process, I think Brrrrgeron because it involves super-cooled water and ice crystals. In the Bergeron process, super tiny ice crystals attract water vapor to themselves. The water vapor particles bond to the ice crystals, leading to a drop in surrounding vapor pressure.

This in turn, leads to the evaporation of the surrounding water droplets–transforming them from their liquid state into water vapor. These vapor particles stick to the growing ice crystals. From our perspective in miniature, we see hexagonal shapes slowing forming around us.Ice crystals in a cloud will collide with one another. At temperatures of -5 degrees Celsius or above, the ice crystals are slick or sticky enough that collisions lead to crystal growth in a process known as aggregation. Continuous aggregation will produce a familiar phenomenon–snowflakes.

Like growing raindrops, snowflakes will eventually get too heavy and fall from the cloud.Snowflakes descending through cold air (below freezing) will fall as powdery snow. Snowflakes falling through warmer air (approximately freezing) will aggregate more into clumps. If the air is above freezing, the snow will turn into rain.Occasionally we might observe a phenomenon called riming or accretion, which happens when droplets of water freeze to the surface of ice crystals. Instead of the nice hexagonal shapes we observed during the Bergeron process, we see clumping; the end product of this is often graupel, a form of precipitation that looks like miniature snowballs.

Lesson Summary

Let’s review all the vocabulary we’ve learned in this lesson.

  • Precipitation is water that falls from the sky to the surface of the earth.
  • Water vapor consists of gaseous water molecules. When the air becomes saturated with water vapor, clouds will form.
  • A cloud is a region of atmosphere where moisture condenses, forming tiny water droplets or grains of ice.
  • Condensation nuclei are minuscule solid or liquid particles floating around in the atmosphere.
  • Growth by condensation happens as water vapor condenses onto water molecules.
  • Collision and coalescence occurs in warm clouds when water droplets crash into each other and stick.
  • Super cooled water is water that is below freezing but is still in a liquid state.
  • The Bergeron process occurs when tiny ice crystals attract water vapor to themselves.
  • At temperatures of -5 degrees Celsius or above, the ice crystals are slick or sticky enough that collisions lend to crystal growth in a process known as aggregation.
  • Riming or accretion happens when droplets of water freeze to the surface of ice crystals.

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