The term precipitation is commonly associated with rainfall. In some regions, it encompasses snow, hail, and sleet. However, there are many more types of precipitation. Some are quite dangerous, and others are prettier than snowfall.
If you knew what kind of precipitation you are witnessing at any particular time, you would know to immediately seek safety or sit on your porch to enjoy the phenomenon. In this article, we look at the types of precipitation and how they are formed. Look out for some meteorological fun facts that we share along the way.
Related Read: Rain Quotes.
Any liquid or solid water that forms in the atmosphere and falls to the earth’s surface is precipitation. The ground temperature determines if precipitation will fall as liquid, frozen solid, or something in between.
Precipitation is a crucial part of the water cycle as it ensures that water returns to the earth after evapotranspiration. Without precipitation, our freshwater resources would dry up. There would be severe droughts and food shortages. The earth would also be unbearably hot as precipitation is one of the ways the planet cools itself.
Usually, water is measured in volume, but modern rain gauges measure precipitation by height. An average of 39 inches of water falls to the earth as annual precipitation. However, precipitation is not evenly distributed across the globe. Some places receive as little as 0.1 inches of precipitation per year, while others get as much as 900 inches yearly3.
Mawsynram, in the northeast part of India, receives the most precipitation compared to any other place on earth. Its average annual rainfall is 494 inches. The place with the lowest rainfall in history is Arica, a city in northern Chile. It had a yearly precipitation of 0.02 inches for 43 years.
Besides the rain gauge, other instruments for measuring precipitation include the snow gauge and the disdrometer.
If the Earth's surface was uniform globally, we would expect the same amount of precipitation everywhere. However, the natural topography, wind currents, and the distribution of land and seas determine how much and what type of precipitation falls where.
The wind carries warm, moist air across the land. Typically, the air flows upwards, and the warm air rises, cools, and returns as precipitation. So humidity affects precipitation.
In Mediterranean climates, summers are dry, and winters are wet. These areas receive most of their precipitation in winter. In contrast, humid continental climates have dry winters and wet summers. These climates receive the most precipitation in summer.
As part of the water cycle, vapor builds up in local air (atmosphere) and cools to form clouds. These clouds are about 4 to 12 miles above the ground in the area of the atmosphere, specifically referred to as the troposphere.
Interestingly, although all clouds contain water vapor, not all clouds precipitate. The clouds may be too small or short-lived.
In addition to water, clouds contain microscopic bits of salt, dust particles, solid pollutants, and other airborne particles. These materials form the cloud condensation nuclei (CCN).
Precipitation forms in two ways. The first is when water vapor condenses around a nucleus. We call this process coalescence. It occurs in warm clouds with more cloud droplets than ice crystals.
The wind pushes the droplets to collide and become larger and heavier. When the droplets become too heavy for the air to keep afloat, it rains. Typically, coalescence only produces liquid precipitation.
The other way precipitation occurs is called the Bergeron process. This happens when moist air cools high up in the troposphere beyond the dew point. Tiny ice nuclei form and attract supercooled droplets to form flakes.
The Bergeron process can result in liquid or frozen precipitation, depending on the temperature from the cloud to the ground.
For all precipitation types, warm air must rise, cool, and condense. The categories of precipitation are based on how the air rises. Understanding precipitation categories helps weather scientists to predict and prepare for severe weather.
On hot days, intense sunshine heats up air close to the ground. The warm air rises and cools, and the water vapor it carries condenses to form cumulus clouds. When the clouds become saturated, they precipitate.
Convectional precipitation is typified by extremes in severe weather–from hot sunny weather to heavy rainfall with thunderclouds. It brings short, localized, sometimes intermittent showers. Convectional rainfall is typical in the world's northern hemisphere and equatorial regions.
Orographic precipitation occurs near a mountain range. Warm, moist air blows in from the sea and is forced to rise upward when it meets a high mountain, causing an orographic lift. The air mass cools beyond the dew point and forms clouds with ice crystals.
After some time, the clouds precipitate, usually on the side of the mountain facing the sea. We call it the windward side. The other side is the leeward side, which gets less precipitation. Orographic rainfall occurs in the Sierra Nevada and Andes mountain ranges.
For cyclonic precipitation to occur, tropical warm air must come in contact with cool air from the polar region. The two large air masses don't mix; the warmer air is forced to rise above the cold air mass.
Therefore, the warm air gets high enough into the atmosphere to cool. Then, its vapor condenses into stratus clouds, precipitating above the boundary between the two air masses.
Cyclonic rainfall, or frontal rainfall, is typical in Britain and Ireland. It is usually the kind of persistent rain with a grey, overcast sky.
Whether a region gets liquid or frozen precipitation depends on the formation method and temperatures as it falls. Sometimes, different types of precipitation fall at the same time.
Rainfall is the most common type of precipitation. It occurs in almost every part of the world. It is precipitation that falls to the earth as water droplets.
Rain often starts as frozen water that melts as it encounters warmer temperatures on its way down. Other times, it forms as cloud droplets collide, merge, and expand. Raindrops measure over 0.02 inches in diameter, with 100 to 1,000 drops falling per cubic meter.
There are different types of rain based on how it is generated. We have convectional, relief, and frontal rainfall. Excessive rainfall can destroy crops and cause mudslides and extreme flooding.
Fun fact: The shape of a raindrop is spherical, not the teardrop shape that illustrations show.
Related read: How does rain form?
Another way to classify rainfall types is by the size of the water droplets. Drizzles have tiny water droplets less than 0.02 inches in diameter and more than 1,000 drops falling per cubic meter. Wind currents quickly push the fine, uniform drops of a drizzle, and their direction can be observed.
Drizzles fall from stratus clouds closer to the ground–day 1,500ft or less. That's why areas with hills are likely to experience more drizzles.
Most people consider drizzle light rain because it is less intense than regular rainfall. However, a drizzle can sometimes present more danger than heavy rainfall. Drizzles tend to be foggy, and poor visibility leads to travel accidents.
This precipitation begins as supercooled water droplets and falls to the ground through a sub-freezing air mass. It doesn't require a warm air sandwich like freezing rain does.
Due to the warm air, the snow melts entirely, and the layer of freezing air on the ground is too thin to allow it to harden. But it makes the snow supercooled, so it freezes on contact with any surface at or below 32°F.
Freezing rain is one of the most dangerous types of rainfall. It forms a solid sheet of clear ice that's smooth and slippery. A heavy coating of freezing rain can cause tree branches to collapse and power lines to fall.
This precipitation begins as supercooled water droplets and falls through a sub-freezing air mass all the way to the ground. Notice that it doesn't require a warm air sandwich like freezing rain does.
It forms in clouds close to the ground like a drizzle, with droplets less than 5 mm. Like freezing rain, the droplets freeze in contact with unprotected surfaces.
Freezing drizzle produces a thin, slick glaze that is hard to detect, especially on the roads. Therefore, it's essential to be extra careful when driving or walking during or immediately after a freezing drizzle. Even aircraft can be compromised by freezing drizzle.
Snowfall is a common type of precipitation in places with cold temperatures due to distance from the equator and high elevation. It develops when water vapor immediately freezes into ice crystals without liquifying first. This happens when atmospheric temperatures are below freezing at about -40°F to 32°F.
As they fall, the ice crystals attach to each other, forming snowflakes. Each snowflake has an intricately beautiful and unique pattern determined by the cloud's temperature and humidity.
When snow falls, temperatures from the cloud base to the ground are typically below freezing. Warmer temperatures will melt the ice crystals before it reaches the ground.
Sleet only occurs in winter. To people in the U.K., sleet is rain and snow mixed together. The United States National Weather Service describes it a bit differently.
Sleet forms when falling snow partially melts as it passes through a thin pocket of warm air. Then, they enter another layer of freezing air closer to the ground and quickly refreeze into icy pellets. That's why people also call sleet ice pellets.
Sleet ice pellets are usually the size of a pea. They are clear like ice cubes, not white like snow. The pellets bounce off surfaces and may even provide traction for vehicles on wet roads.
Hailstorms can be very destructive as weighty ice pebbles fall violently from the sky. Getting hit by hail is like being pelted with small pebbles with great force. Hailstones are typically the size of pennies to golf balls, but they can get incredibly larger.
Hail forms within thunderstorm clouds (cumulonimbus clouds). It starts with updrafts lifting water droplets into extremely cold parts of the atmosphere, where they freeze. The frozen water droplets then collide and bond with more liquid droplets. They expand layer after layer.
Depending on the freezing process, a hailstone may be clear or cloudy. When the hailstones become too heavy, or the updraft loses power, hail begins to fall as precipitation.
Fun fact: The largest hailstone recorded in the United States was 8 inches wide and weighed over 1 pound.
Sometimes, frozen precipitation falls as small, soft white balls. Such precipitation is called graupel, snow pellets, soft hail, or hominy snow. Graupel develops when an ice crystal falls through a cloud of supercooled droplets. Then, the droplets instantly freeze onto the crystal through riming.
The process might resemble hail formation, but graupel doesn't require thunderstorms. It is a lot smaller and softer than hail. Graupel pellets are fragile and will disintegrate when touched.
Graupel pellets are opaque white due to instant freezing. Occasionally, they can get as large as 0.24 inches and may be conical or rounded.
Here, precipitation begins as snow or rain from the base of clouds. But it passes through a thick layer of warm, dry air, which causes the precipitation to evaporate before it reaches the ground. Virga is also called phantom rain or a dry storm.
Virga often appears as wispy cloud trails at the base of clouds, which is why people often refer to them as jellyfish clouds.
Sometimes, virga signals oncoming dry microbursts–strong downward drafts of wind caused by rapidly cooling air. A microburst is dangerous to aircraft, creating intense turbulence that makes take-off and landing difficult.
Also called diamond dust, this type of precipitation falls as super tiny individual ice crystals. You can only observe ice crystal precipitation in very cold climates like the polar regions.
It forms when slightly warm, humid air mixes with cold air nearer to the surface. The vapor in the air freezes, turning into crystals with an average diameter of 0.004 inches. The surface air temperature for diamond dust to form is usually less than 14 °F.
Ice crystals often fall from clear skies, hence the name, ‘clear sky precipitation’. This precipitation appears to be floating, and the light from the sun gives it a halo twinkle.
Small hail is a type of precipitation consisting of freezing raindrops, snow pellets, or melted and refrozen snowflakes encased in a layer of solid ice. Small hail occurs in showers from Cumulonimbus clouds2.
Small hail pellets can be clear or translucent. They're typically spherical or conical but can also be irregularly shaped. The pellets usually exceed 0.2 inches in diameter, reaching up to 0.75 inches. They are sort of a middle ground between true hailstones and graupel.
Small hail doesn't break easily like graupel. It bounces off surfaces with noticeable impact. Although it's not as large as true hail, small hail can still cause damage to crops and properties.
All over the world, global warming is interfering with traditional precipitation patterns. Below are some of the effects:
Urban heat islands in urban areas attract heavier rainfall than surrounding areas due to the abundance of warm air. However, there is not enough soil to soak up the rain, so drains overflow and flooding occurs.
In many cities, heavy rains cause more than minor inconveniences. They can lead to severe property destruction and fatal accidents.
It is common knowledge that global warming is causing the ice caps to melt. But more people must be aware that it also increases global precipitation. Heavy rainfall occurs in places already struggling with flood-related disasters.
In places that already do not get much precipitation due to climate conditions, there is a risk of getting even less rain. That means areas with water shortages might be headed for extreme droughts1.
Burning fossil fuels is the key issue driving global warming. Apart from heating the atmosphere, this activity pollutes the air with pollutants that become CCN for precipitation. And when it happens to a large extent, acid rain forms.
Acid rain is damaging to property but hurts our valuable forest resources the most. It also disrupts the ecosystem and can cause health problems for humans.
Precipitation refers to liquid or frozen water that falls from the atmosphere to the earth’s surface. We list 11 types of precipitation in this article, some of which you may have not heard of before.
Jen’s a passionate environmentalist and sustainability expert. With a science degree from Babcock University Jen loves applying her research skills to craft editorial that connects with our global changemaker and readership audiences centered around topics including zero waste, sustainability, climate change, and biodiversity.
Elsewhere Jen’s interests include the role that future technology and data have in helping us solve some of the planet’s biggest challenges.
Fact Checked By:
Isabela Sedano, BEng.