Glacier FarmMedia – Precipitation is a vital aspect of Earth’s climate and plays a significant role in shaping our environment. From gentle drizzles to heavy snowfall, precipitation comes in various forms, each with unique characteristics and formation conditions. This week we’ll delve into different types of precipitation and the atmospheric conditions necessary for their creation.
In the last issue I explained how the two main types of precipitation form in our atmosphere. In clouds that are above 0 C, water vapour condenses onto condensation nuclei, forming cloud droplets. These droplets collide and coalesce into larger and larger droplets until they get too heavy for the prevailing updraft to keep them in the air. Then they fall as rain.
In clouds below 0 C, water droplets remain liquid at temperatures well below freezing and are known as super-cooled droplets. This combines with a different process, known as the Bergeron process, to produce snow.
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If there is just enough water vapour in the air to keep a super-cooled water droplet from evaporating, there is more than enough water vapour in the air for an ice crystal to grow larger. This is because the saturation vapour pressure over ice is lower than that over water.
Ice crystals will attract water vapour more readily than water droplets, so snowflakes can grow quickly.
As for other types of precipitation, nearly all our precipitation starts as snow and the temperature profile of the atmosphere determines what we see on the ground. I’ve included a graphic to illustrate this information.
Rain
Rain occurs when liquid water droplets fall from the sky. For rain to hit the ground, there must be a sufficiently deep layer of above-freezing air between the ground and the source of the precipitation. Remember, rain often starts off as snow, which melts as it passes through warm air above the ground.
Snow
We know snow consists of ice crystals that form in clouds when temperatures are below freezing. The snowflakes fall when they become too large to remain suspended in the air. As the flakes fall, they remain in sub-freezing temperatures the whole way down.
Sleet or ice pellets
This type of precipitation can often be misidentified as small hail. Hail forms in cumulonimbus clouds or thunderstorms. Ice pellets form when there are alternating layers of freezing and warm air.
There must be an elevated layer of above-freezing air that causes any snow falling to melt and turn into a water drop. The drop then encounters a layer of freezing air and as it passes through it, the droplet freezes forming an ice pellet.
That layer could be thin but very cold, or not that cold but thick. Either way, it needs conditions that allow the droplet time to freeze. At the surface, the air may or may not be below freezing.
Hail
Hail is distinct from other forms of frozen precipitation that make it to the ground, as it occurs during severe thunderstorms. Hailstones are large ice balls that form when updrafts in a thunderstorm carry raindrops upward into extremely cold regions of the storm.
Within the storm, the raindrops freeze into layers, creating hailstones. Strong updrafts are necessary for hail formation, and the size of hailstones is often indicative of storm intensity.
Graupel or snow pellets
These form in cold, convective clouds. Due to the convective nature of snow pellet formation, we usually only see this type of precipitation in fall or spring. For snow pellets to form, the falling snow needs to encounter a layer that is warm, but not warm enough to totally melt the snow.
The partially-melted snowflake then falls into a layer of sub-freezing air and the liquid around the outer edge of the snowflake freezes. This creates a thin shell of ice around a core of snow. You can tell the difference between snow pellets and ice pellets by squeezing them; the snow pellets will break. Snow pellets are also much lighter than ice pellets, sometimes resembling Styrofoam.
Freezing rain
This occurs when supercooled raindrops fall and then immediately freeze on contact with cold surfaces. It requires a shallow layer of sub-freezing air at or near the surface and warmer air aloft.
When the raindrops fall through the sub-freezing layer and reach the surface, they freeze on contact, creating a layer of ice. For freezing rain to occur, the falling raindrop must be at or below the freezing point. This allows it to freeze immediately upon contact with a cold surface.
It’s important to understand the atmospheric temperature profile for each type of precipitation. It helps us picture what is going on in the air above us. The atmosphere is three-dimensional and there are unique conditions for each type of precipitation.
Hopefully this will allow you to cut forecasters a little slack when they try to forecast freezing rain or ice pellets. Better yet, give them credit when they get it right. It is not easy to figure out how the atmospheric temperature profiles will play out.
