Weather: 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 cooling elements that cool the air around them. When water vapour hits these elements, it condenses 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

Raindrops start when water vapour condenses on tiny dust particles. Then, these small water droplets bump into each other to form larger droplets. Eventually, they become so heavy that they begin falling to Earth, gathering more water as they go. 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.

Snow

Sometimes water droplets in clouds freeze around dust or pollen particles 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! 

You may have heard that no two snowflakes are alike. This is very difficult to prove. Snow crystals form in many different shapes. They are also sensitive to temperature changes. Each one falls from a cloud at a different time, changing shape as it encounters different temperatures along the way. 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.

Did you know?

Scientists think snowflakes can be sorted into eight general categories, 39 intermediate categories, and as many as 121 elementary categories! That means there are up to 10¹⁵⁸ possible shapes!

Hail and Sleet

Hail is when 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 collide with more water droplets and grow. These ice chunks, or hailstones, drop to Earth once they are heavy enough to fall through the warm air. Hailstones are larger than 5 mm across. And there have been hailstones the size of golf balls and baseballs!

Did you know?

The largest hailstone ever measured was between 18.8 cm and 23.6 cm in diameter!

Ice pellets are also called 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. 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.