- How does precipitation form?
- Can hail be dangerous?
- How is the amount of precipitation measured?
Why does it rain?
Inside the cloud, the small droplets are constantly moving around. Water droplets and ice crystals are so small and light that it's as if they were hanging in the air. That's why not every cloud produces rain. Rain will begin when the droplets collide with each other and become so heavy that they will no longer stay in the air. Rain, snow, slush, or hail falling to the ground is called precipitation. Precipitation is any form of water that falls to Earth’s surface from the clouds.
When the temperature is below freezing in the clouds, the water vapour becomes ice crystals. When they join, snowflakes will form. The longer the ice crystals move inside the cloud, the larger the snowflakes will fall to the ground. Often, however, the snowflakes begin to melt as they fall due to the air near the ground being warmer than it is several kilometres higher. When this happens, we get sleet, or rain.
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Precipitation does not always fall from the cloud. When the air cools down near the ground, the water vapour begins to condense, and fog is formed. Fog is more frequent in the late evenings because of the cold air at night. Fog is sometimes called the 'creeping cloud'. Therefore, if you've ever been in a fog, you can say that you have walked through a cloud. When wet air comes into contact with the cool ground, water vapour condenses and creates dew.
When the air temperature drops below zero, ice crystals may form from the water vapour. Frost or rime ice occurs. On a clear autumn morning, the land is often covered with frost. On a cold winter day, rime ice can be seen on windows in patterns sometimes called ice flowers. Sometimes rime ice can be seen as a sugar-like powder on cars, trees, and other plants. When you breathe on the window in frosty weather, rime ice appears quickly.
On a sunny day, the air heats upabove the dark soil on a field than above the light sand on a beach. Warm air starts to . Rising air. Cold air can hold water vapour than warm air. Water vapour condenses intoand clouds are formed. Ice crystals form in the clouds when the temperature isfreezing. Dew occurs when the temperature at night is0 degrees. In the autumn morning, when the daytime degrees in the positives have been replaced by degrees in the negatives at night, we can often seeon the ground.
Think!
- When is it more likely that fog will occur: in the evening, or at noon? Justify your answer.
Bonus: Hail and freezing rain
It usually hails in the warm season and when strong northerly winds bring a lot of cold air. There are cold temperatures in the upper part of the hail cloud, and the water droplets freeze and fall. However, in the lower part of the cloud, the temperature is above zero, and the ice particles begin to melt. If the air currents move upwards in the cloud, the ice particle may move back upwards, where a new layer of ice will grow on it. When the hailstones become too heavy, usually about the size of a pea, they fall to the ground. In a very thick cloud, the hailstones can grow as large as a chicken egg.
Another unique type of precipitation is freezing rain. It can occur in late autumn, winter, or early spring when the weather gets warmer after the cold season. It occurs when raindrops fall through a layer of cold air and freeze. Freezing rain also occurs when snow falls through a layer of warm air and melts and then re-enters the layer of cold air.
Think!
- Why does it sometimes sleet in winter and hail in summer?
One slope of a coastal mountain usually gets more precipitation than the other
The air masses coming from the sea are humid. Mountains get in the way of the air movement and force it to rise. The rising air cools, and the water vapour in it condenses. Clouds form, and it starts to rain on one slope of the mountain. The other slope of the mountain doesn't get as much precipitation since the air is already less humid when it reaches the other side of the mountain.
Think!
- Where in the world does it rain the most? You can look up the answer online.
- What is the average annual precipitation in your home area?
- It always rains more inland than on the coast.
- It usually rains more on the coastal slope of the mountain.
- Terrain doesn't affect the amount of precipitation.
- Humidity affects the amount of precipitation.
Precipitation measurement
The amount of precipitation, i.e. the thickness of the water layer that has fallen to the ground, is measured with a precipitation gauge. Precipitation falls through the funnel into a large container. From there, it is poured into a beaker, and the amount of precipitation is determined. The amount of solid precipitation is measured after it has melted in a warm room. Based on the daily measurement results, precipitation is calculated per day, per month, or per year. For this purpose, the measured precipitation amounts are added together. Modern measuring tools automatically record precipitation, and the data goes directly to the computer database.
- Rainmeter
- Precipitation gauge
- Water gauge
Bonus: The driest and wettest places in the world
Precipitation is measured in millimetres. The driest places in the world are the deserts. The annual rainfall there is less than 250 mm. The driest place is the Atacama Desert in South America. It usually rains there about once every ten years. There was no major rainfall in the Atacama Desert from 1570 to 1971. Over 400 years! The last rain was in the spring of 2017, and it made the whole desert bloom.
In the wettest areas, where it rains about 2000 mm a year, lush rainforests grow. The highest precipitation is measured in Cherrapunj, South Asia, on the slopes of the Himalayan Mountains. The average amount of rainfall there is 23 meters a year!
Important terms
- precipitation – any form of water that falls to Earth's surface due to condensation of water vapour
- fog – a layer of water droplets or ice crystals suspended in the air near the ground that reduces visibility
- dew – the tiny water droplets formed on the ground by the contact of moist air with the cooled ground
- frost – a thin layer of ice crystals formed on the ground
- rime ice – the snow-like precipitation that can be seen as ice flowers on a window or as sugar-like crumbs and crystals on cars, trees and plants
I now know that…
The amount of water vapour in the air is characterized by humidity. It starts to rain when the water droplets in the cloud join together and become so heavy that they can no longer stay in the air. Rain, snow, slush, or hail falling to the ground is called precipitation.
