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WEATHER U: Pondering Precipitation

It's clear to see – or not – that freezing rain coated the windows of Nancy Richardson's home in Summerside, P.E.I., last February.  Freezing rain doesn't make a tapping sound or bounce; it falls like rain and freezes on contact.
It's clear to see – or not – that freezing rain coated the windows of Nancy Richardson's home in Summerside, P.E.I., last February. Freezing rain doesn't make a tapping sound or bounce; it falls like rain and freezes on contact. - contributed

In the winter, snow is my preferred type of precipitation.  Growing up, we didn’t get a lot of rain in the winter.  If and when we did, I was not very happy about it: it made a mess of my beloved snow.  

Last night, as a fast-moving system tracked up the Bay of Fundy, a strong south wind pulled warm air into its core, resulting in a smorgasbord of weather. It's going to happen a few more times before we tuck away our winter boots, so now is as good a time as any for a precipitation primer.  

Winter precipitation has as much to do with the temperature in the clouds as it does with the temperature at ground level. In the winter, as a warm front approaches, warm air slides up and over cold, heavy air that can become trapped close to the ground.  When rain falls from the warm air into the cold air, several things can happen.  

Ice pellets: 

If the wedge of cold air above the ground is deep enough, the rain has time to freeze into ice pellets. Ice pellets are small, translucent balls of ice.  They form when the layer of cold air close to the ground extends upward, far enough that raindrops falling from the cloud freeze into little balls of ice before reaching the ground. Ice pellets often bounce when they hit the ground or other solid objects and make a high-pitched "tap" sound when striking objects like jackets, windows and windshields.  

Freezing rain: 

If the layer of cold air is shallow, the rain cools and becomes super-cooled (liquid state below freezing) and freezes on contact when it hits cold objects, like your steps and road surfaces.  Freezing rain is the most dangerous type of precipitation because it looks like rain, falls as a liquid and only freezes when it lands on cold objects.   

Graupel: 

When a so-called "mixed bag of weather" rolls through, I often hear people talk about hail: it's not impossible, but it's unlikely in the winter. Hail has thick uniform layers of ice and usually falls during thunderstorms.   

Graupel is also called snow pellets, or, to confuse matters, soft hail. It forms when supercooled water droplets are collected and freeze on falling snowflakes, forming 2–5 mm balls of rime that look like oblong-shaped snowballs.  Graupel forms in a rapid transition from mild to cold, when snowflakes behind a cold front, encounter super-cooled water droplets in the milder air ahead of it. These droplets collect and freeze on the surface of snowflakes through a process known as accretion. As more water droplets freeze on the snowflake, the original shape of the snowflake changes, resulting in the formation of graupel.  

We’ll likely experience a few of these different types of precipitation before the winter is out. However, I can’t promise that knowing what they are will make the shovelling any easier.  


Cindy Day is the chief meteorologist for SaltWire Network

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