Precipitation: The Ways Water Falls from the Sky

Precipitation. Some find it an intimidatingly long word, but it simply means any particle of water (be it liquid or solid) that originates in the atmosphere and falls to the ground. In meteorology, an even fancier term that means the same thing is hydrometeor.

There are only so many forms water can take, and because of this, only a limited number of precipitation types. The main types include: 

Rain

Splashing Water Drops On Road
Shivani Anand / EyeEm / Getty Images

Rain is comprised of liquid water droplets, known as raindrops. 

Rain is unique because it's one of the few precipitation types that can occur during any season. As long as air temperatures are above freezing (32°F), rain will fall. 

Snow

heavy snowfall park bench
Sungmoon Han/EyeEm/Getty Images

While we think of snow and ice as two different things, snow is actually millions of tiny ice crystals that collect and form into flakes, which we know as snowflakes.

In order for snow to fall outside your window, air temperatures at the ground and well above the surface must stay below freezing (32°F). It can be slightly above freezing at some pockets and still snow as long as they aren't substantially above the freezing mark and stay above it for very long, or else the snowflakes will melt.      

Graupel

graupel on flower
Graupel appears white like snow, but more ragged than hailstones. hazel proudlove/E+/Getty Images

If supercooled water droplets freeze onto falling snowflakes, you get what's called "graupel." When this happens, the snow crystal loses it's identifiable six-sided shape and instead becomes a clump of snow and ice.

Graupel, (also known as "snow pellets" or "soft hail") looks white like snow. If pressed between your fingers, it will usually crush and break apart into granules. When it falls, it bounces like sleet does.

Sleet

Runningonbrains via Wikimedia Commons

If a snowflake partially melts, but then refreezes, you get sleet.

In other words, sleet forms when a thin layer of above-freezing air is sandwiched in-between a deep layer of sub-freezing air high up in the atmosphere and another down at low levels. In such a scenario, the precipitation starts out as snow, falls into a layer of warmer air at mid-levels and partially melts, reenters subfreezing air, and refreezes while falling in it towards the ground.  

Sleet is small and round, which is why it is sometimes referred to as "ice pellets." It makes an unmistakable sound when hitting and bouncing off of the ground and your house.

Hail

hailstones deck
Westend61/Getty Images

Often confused with sleet, is hail, which is 100% ice but is not necessarily a wintertime event. It usually falls only during thunderstorms.

Hail is smooth, rounded (although parts of it can be flat or have spikes), and can be anywhere from pea-sized to as large as a baseball.

Although hail is ice, it is more of a threat to damaging property and vegetation than it is to causing slick travel conditions.

Freezing Rain

freezing rain icicles
Accumulating freezing rain is a major cause of ice storms. Joanna Cepuchowicz/EyeEm/Getty Images

Freezing rain forms similarly to sleet, except the icecream sandwich is layer of warm air at mid-levels is deep. Precipitation either starts out as snow or supercooled raindrops, but becomes all rain in the warm layer.  While freezing air may hug the ground, it is such a thin layer that the raindrops don't have enough time to freeze into sleet before reaching the ground. Instead, they freeze when they strike objects on the ground whose surface temperatures are 32 °F or colder.

If you think the "rain" in freezing rain makes this winter weather event somewhat harmless, think again! Some of the most disastrous winter storms and ice storms are due primarily to freezing rain. That's because when freezing rain falls, it covers trees, roadways, and everything else on the ground with a smooth, clear coating of ice or "glaze," which can make for hazardous travel. Ice accumulations can also weigh down tree branches and power lines, causing damage from downed trees and also widespread power outages.

Activity: Make it Rain or Snow

To test your understanding of how air temperatures overhead govern what kind of winter precipitation will fall on the ground, head over to NOAA and NASA's SciJinks precipitation simulator. Can you make it snow or sleet?