Why is it raining instead of snowing when the temperature is below freezing? It’s a common question during the cooler months during a messy winter storm.
Freezing rain and ice pellets are underrated hazards. It takes just the right setup to turn a gentle snowfall into a dangerous and disruptive ice storm. Here’s a look at how these precipitation types form.
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Precipitation exists on a spectrum
The difference between precipitation types is critical to the impacts you’ll experience from a storm. Winter weather can arrive as rain, snow, ice, or a mix of all at once.
Ice pellets are frozen raindrops.
They’re hard beads of ice that plunk out of the sky with a satisfying tinking sound. Known as “sleet” south of the border, ice pellets can accumulate like snow. A blanket of ice pellets has a nasty habit of freezing into a solid sheet of ice that’s very hard to remove once it’s solidified.
Freezing rain is liquid rain that freezes on contact with any exposed surfaces.
Ice accretion from freezing rain leaves a crust of ice on roads, vehicles, trees, and power lines. A prolonged spell of freezing rain can lead to extensive tree damage and power outages.
Freezing drizzle is very light freezing rain—almost a mist—that’s very efficient at leaving behind a glaze of ice on exposed surfaces.
Melting snow leads to icy headaches
Almost all precipitation that falls during the cold season starts life high in the sky as snow. Ice crystals grow in the clouds where water vapour deposits onto impurities such as dust or pollutants.
If these newborn snowflakes fall into warm air beneath the clouds, they’ll melt into droplets and fall as plain old rain.
Temperatures aren’t uniform throughout the atmosphere, though, and that’s where things get tricky. The bottom of the atmosphere can host many layers of warmer and colder air stacked on top of each other.
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Cold air is dense and tends to hug the ground. If winds blow lighter warm air into the region, that warm air can rise up and over the cold air like a cap. Meteorologists sometimes refer to this as a “wedge” of warm air—and it’s key to the formation of freezing rain and ice pellets.
If a snowflake falls through a shallow layer of warmer air aloft and partially melts, it leaves behind a couple of ice crystals within the water droplet. These ice crystals act like a nucleus that allows the droplet to refreeze into an ice pellet as it falls to the ground.
A deep layer of warm air, though, can completely melt the snowflake and leave behind no trace of ice crystals. The resulting raindrop is pure water. As it falls into the freezing air near the ground, this raindrop can’t refreeze on its own. The supercooled water droplet remains liquid as it falls until it instantly freezes on contact with any exposed surfaces.
It’s possible to see all of these precipitation types during a dynamic winter storm as powerful winds churn up the atmosphere. Snow often gives way to ice or a cold rain, and sometimes rain or ice can transition into a burst of snow.
The temperature at the surface is just part of the equation when winter weather is in the forecast. There’s often more going on above our heads than meets the eye.