Precipitation

Precipitation is an important part of weather and nature. In meteorology, it means any form of water that falls from the sky because it gets heavy enough. This can be rain, snow, hail, or smaller drops called drizzle. These forms of precipitation happen when tiny water droplets in clouds grow big enough to fall down to the Earth.

Precipitation is a key part of the water cycle, helping to bring fresh water to the planet. This water comes from oceans and lakes and is very important for plants, animals, and people.

Sometimes, special weather conditions cause interesting types of precipitation. For example, when warm air meets cold air near lakes, it can create heavy snowfall called lake-effect snowfall. Even other planets can have precipitation — Saturn's moon Titan has methane that falls like light rain. Understanding precipitation helps us know more about our weather and climate.

Types

Precipitation is any form of water that falls from clouds to the ground. It can come in different forms depending on the temperature in the atmosphere. The main types include rain, drizzle, snow, and ice pellets. Rain happens when water droplets become heavy enough to fall. When it’s very cold, the water can freeze and fall as snow or ice pellets.

Rain and snow can sometimes mix, creating a slushy kind of precipitation. Scientists measure how much rain falls using special tools called rain gauges, usually recording the amount in millimeters. For snow, they measure the depth in centimeters and sometimes melt it to see how much water it would make if it were rain.

Air becomes saturated

The dew point is the temperature at which air becomes full of water vapor and forms clouds. This usually happens when water vapor clings to tiny particles like dust or salt. There are several ways air can cool to this temperature. One way is when air rises and spreads out. This can happen when warm air rises by itself, moves with big air patterns, or is pushed up by mountains (orographic lift). Air can also cool when it touches a colder surface, when it loses heat into space, or when water evaporates into it.

Water vapor gets into the air in many ways. Wind can push moist air together. Clouds can drop water that doesn’t reach the ground. Sunny days can evaporate water from oceans and lakes. Plants can release water into the air. Warm air can pick up moisture as it moves over cooler water. Mountains can also push air upward, helping clouds form.

Forms of precipitation

Main article: Water cycle

Precipitation happens when tiny water drops in clouds grow big enough to fall to the ground. Rain happens when small drops called cloud drops join together to make larger drops. These drops become heavy enough to fall through the air. Raindrops can be from about 5 to 20 millimeters wide.

Other types of precipitation include ice pellets, which are small balls of ice that bounce when they hit the ground. Hail is made when water drops freeze inside storm clouds and can grow quite large. Snow forms when tiny water drops freeze and stick together, making beautiful, unique snowflakes. Each snowflake looks different because it grows in its own special way as it falls.

Causes

Main article: Weather fronts

Precipitation happens when air goes up and gets cooler. This makes water vapor turn into clouds. The clouds then drop water as rain, snow, or other types.

One way this happens is with weather fronts. Weather fronts are where warm air and cold air meet. When air is pushed up slowly along these fronts, it cools down. This creates steady rain or snow. This kind of precipitation often happens around tropical cyclones and mid-latitude storms.

Another way precipitation forms is through convection. Convection happens when warm, moist air rises quickly from the ground. This makes tall clouds. These clouds can bring short, heavy rain showers. Sometimes hail or graupel can fall too. Convection is common in tropical areas and near fronts where warm and cold air meet.

Mountains can also change how precipitation happens. When moist air is pushed up over a mountain, it cools. This causes rain or snow to fall on the side of the mountain the wind is coming from. The other side of the mountain often stays drier. The air coming down on this side warms up and holds less moisture. This creates areas called rain shadows.

Measurement

We measure rain or snow with a rain gauge. Rain gauges can be made of plastic or metal. When it rains, water fills a small tube inside the gauge. Extra water flows into a larger tube. By looking at how much water is collected, we can measure the precipitation. In winter, rain gauges can also collect snow or freezing rain.

When we cannot use rain gauges, like over the ocean or in faraway places, we use satellites. These satellites have special tools to look at clouds. They help us learn about how much rain or snow is falling, even where we cannot put gauges on the ground.

Main articles: Rain gauge, Disdrometer, and Snow gauge

Return period

See also: 100-year flood

The return period tells us how often a certain type of weather event, like a heavy rainstorm, might happen. For example, a "1 in 10 year storm" is a big rainstorm that usually happens only once every ten years. It’s rare but possible in any year. An even rarer storm is called a "1 in 100 year storm," which might only happen once in a hundred years. Even though these storms are very rare, it’s still possible to have more than one big storm in the same year.

Uneven pattern of precipitation

In many places, most of the rain or snow that falls each year happens on just a few days. For example, about half of all the rain or snow may fall during the 12 days with the most rain or snow. This shows how unevenly precipitation can be spread through the year.

Role in Köppen climate classification

Main article: Köppen climate classification

Rain forests get a lot of rain, usually more than 1,750 mm (69 inches) each year. Tropical savannas, found in places like Africa and Australia, have less rain, about 750 to 1,270 mm (30 to 50 inches) a year. Humid subtropical areas, like parts of the southeastern United States, have rain in both winter and summer, often from big storms. Oceanic climates, such as those along west coasts, have rain throughout the year. The Mediterranean climate, found in places around the Mediterranean Sea and parts of California, has dry summers and wet winters.

Effect on agriculture

Precipitation, especially rain, is very important for agriculture because all plants need water to live. Rain is the best way to water plants, but too much or too little rain can hurt crops. Drought can damage or kill crops and make soil wash away. Too much rain can cause harmful fungus to grow on plants. Different plants need different amounts of water; for example, cacti need only small amounts of water to live.

In areas that have wet and dry seasons, the soil can lose nutrients and erosion can increase during the wet season. Animals in these areas have found ways to survive the wetter conditions. The dry season can lead to food shortages because crops are not ready to harvest, and in developing countries, people may have trouble getting enough food until the first harvest in the wet season.

Climate change

See also: Effects of climate change on the water cycle and Extreme weather

When the Earth gets warmer, more water turns into vapor from oceans and lakes. This can bring more rain and snow to some places.

Some areas, like parts of North America and Europe, have gotten wetter since 1900. But other places, like the Sahel in Africa and some parts of Asia, have become drier.

We have seen more heavy rainstorms in many places in the last century. Some tropical areas now face more droughts. In the United States, there has been more precipitation, especially in the eastern and southern parts. Hawaii, however, has seen less rain.

Changes due to urban heat island

See also: Urban heat island

The urban heat island makes cities warmer than nearby areas. This warmth can create more clouds and lead to extra rain and thunderstorm activity. Because of this, cities can sometimes have more rain than the areas around them.

Forecasting

Main articles: Probability of precipitation and Quantitative precipitation forecast

The Quantitative Precipitation Forecast (QPF) tells us how much rain or other types of precipitation might fall in a certain time and area. Forecasts are usually given for specific times during the day. Forecasters study the shape of the land and past weather to help make better predictions. They also use computer models and radar images to estimate how much rain will fall, and they check these estimates against real rain measurements.