Weather: Precipitation

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Learn about humidity and the many forms of precipitation.

Humidity

Humidity is the amount of water vapour in the air. Dew, frost and that ‘muggy feeling’ on hot days are all signs of water vapour in the air. Humidity is an important part of any habitat. It determines which plants and animals can thrive there.

People are also affected by humidity. When we sweat, the sweat evaporates from our skin. This is what keeps us cool on a hot day. But when it is humid, our sweat evaporates slowly. This is because the air around our bodies is already filled with water vapour. If the humidity is high enough, people can develop heat stroke.

Sometimes people use air conditioners to cool the air inside buildings. But air conditioners also lower the humidity. They have elements that cool the air around them. When water vapour hits these elements, it condensates into liquid water. This removes it from the air. 

Microorganisms like bacteria and mould can grow when it’s too humid indoors. Some of these microorganisms can be bad for people’s health. This is why many people use dehumidifiers to remove moisture from the air. 

Precipitation

Precipitation is an important part of the Earth’s water cycle. Drizzle, rain, snow, sleet and hail are all different kinds of precipitation. All precipitation starts as water vapour. This condenses into droplets that fall from the sky. Some droplets stay liquid. Other droplets freeze.

Rain

In the clouds, small water droplets bump into each other to form larger droplets. Eventually, these become so heavy that they fall to Earth. When the air is unstable, more and more droplets collide, resulting in bigger raindrops. Raindrops can measure from 0.1 mm all the way to 9 mm. That’s a pretty big raindrop! 

Freezing rain can happen when the temperature near the ground is close to 0°C, but the air above is warmer. The rain does not have enough time to freeze as it falls, but it turns to ice the instant it lands.

Late summer storm/Orage de fin d'été
Late summer rainstorm (Source: Malene Thyssen [CC SA 3.0] via Wikimedia Commons).

Snow 

Sometimes water droplets in clouds freeze and form ice crystals. One kind of ice crystal is the snow crystal. Snow crystals form when droplets become supercooled. Then they form a six-sided crystal lattice. These snow crystals bump into water droplets in the clouds and grow bigger and bigger. Just like raindrops, when snowflakes get heavy enough, they fall to Earth. They might collide and stick to other snowflakes on the way down. Sometimes, around −2 °C, snowflakes can be 3-dimensional

Did you know?

Snowflakes are normally flat.

You may have heard that no two snowflakes are alike. But how can you prove this? Wilson Alwyn Bentley photographed thousands of snowflakes using a microscope. He didn’t find two identical ones. But his photographs helped scientists understand different snowflake types.

Photos of snowflakes/Le photographe Wilson Alwyn Bentley
Photographs of snowflakes taken by Wilson Bentley in 1902 (Source: Wilson Bentley [Public domain] via Wikimedia Commons).

 

Hail and Sleet

A hailstone/Un grêlon
A large hailstone (Source: National Severe Storms Laboratory (NSSL) Collection [Public domain] via Wikimedia Commons).

Hailstones or hail forms when ice crystals fall to the Earth on warm days. Warm air rises inside clouds. It pushes water droplets up into the colder air above the clouds. This happens over and over again. During this time, the ice crystals gather together and grow. These ice chunks drop to Earth once they get heavy enough to fall through the warm air. Hailstones are larger than 5 mm across. And lots of people report hailstones about the size of golf balls! 

Ice pellets are also known as sleet. They are small, almost see-through, balls of ice. They are usually smaller than hail. Sometimes you can see ice pellets bounce when they hit the ground. And they don’t usually freeze into a solid layer of ice.

Ice pellets form when there is a layer of warm air between two layers of cold air. Snowflakes form in the cold top layer. Then they melt a bit as they fall through the warmer layer. Then they freeze again when they pass through the lower layer and hit the ground.

 

 

References


Bradač, M. (2015). The science of snowflakes.

Cenedese, C., Mason, B. and others. (2020, March 21) Climate - Types of precipitation. Encyclopedia Britannica.

National Oceanic and Atmospheric Administration. Freezing Rain and Sleet. Weather.gov

Rutledge, K., Ramroop, T. et al. (2011, January 21) Hail. National Geographic

United States Geological Survey. The Fundamentals of the Water Cycle.

YouTube. National Geographic.(2011). Humidity.

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