The leaves are changing colour, and there’s a chill in the air; autumn is here, which means winter is just around the corner. Just like the changing seasons, the types of weather and terms we use also begin to change—shifting away from thunderstorms and moving toward snowstorms.
However, these colder months aren’t just all about fluffy, blowing snow; there are a slew of meteorological terms and phrases used to describe everything from the type of ice we see form to the type of storm that’s brewing over the continent.
To those in the meteorological field, these terms and phrases are about as clear as ice, but to others they may not even sound like real terms.
With that said, here are just a few of the many terms you may hear in your fall and winter forecasts:
Frazil ice
You may have encountered frazil ice before, thinking it was something else.
Frazil ice comes from the formation of ice crystals in flowing, supercooled water. Since the water is flowing, the ice crystals can’t form one solid sheet, so instead it becomes more ‘slush-like’, with the ice crystals oriented in random directions.
WATCH: This stream disappears as you watch and frigid weather is to blame
Don’t get too frazzled, though; there is an important difference between frazil ice and slush.
Slush forms from snow and ice being warmed to just above freezing—to the point where it starts to melt, but not completely. Essentially, it’s the inverse of frazil ice in that it’s ice crystals slowly returning to a liquid state, whereas frazil ice is formed from water crystallization into ice.
Frazil ice can become problematic for municipalities when temperatures drop as it can form inside water pipes and mains, causing them to break.
Additionally, frazil ice in rivers can contribute to flooding by accumulating and blocking water flow.
Frost quake
Imagine this: after a mild and wet winter, suddenly an Arctic vortex descends over your region and freezes everything overnight. You wake up in the night to a boom, and your house begins to shake slightly, but you’re not in a seismically active area, so what caused this mysterious quake?
That would be a frost quake, also known as an ice quake or cryoseism. It’s important to note that a frost quake is not the same as an earthquake, as it is not the result of tectonic activity.
A frost quake is seismic activity resulting from the sudden freezing of water deep in the saturated ground.
Water expands when it freezes, and when it freezes deep in the saturated ground, the ground will begin to crack and suddenly burst from the sudden expansion.
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Graupel
Graupel is another name for snow pellets. Smaller than hail but denser than snow, graupel is formed from ice crystals high in a cloud falling and colliding with supercooled water, freezing into small spheres that fall to the ground.
While you may have heard graupel referred to as ‘soft hail’, it is, in fact, not hail. For starters, hailstones must be at least 5 mm in diameter to be considered hailstones. Hail is also formed from strong updrafts in powerful storms, which lift water droplets to the freezing level in a storm cloud until it becomes so heavy that it falls to the earth.
Graupel is much smaller than hail, being about the same size as a snowflake, and is not formed through updrafts.
These small pellets have also been referred to as ‘tapioca snow’ due to their tapioca pudding-like appearance when they accumulate!
WATCH: What is graupel? We break down this often confused precipitation type
Thundersnow
While summer may be the season for thunderstorms, it is possible to see the odd storm or two develop in the fall and winter as well.
When this happens, we see the precipitation fall as snow, as is expected in the winter, but lightning suddenly lights up the sky and a rumble of thunder stirs the air.
Thundersnow happens when a mass of unstable, frigid air sits above a rising mass of comparatively warmer air, such as the air over large, ice-free bodies of water, or in some of the strongest winter storms. While the air inside the cloud and at the surface are still cold enough for snow development, the contrast in temperatures between the upper and surface levels is great enough to fuel lightning development and, thus, thunder. Thundersnow tends to occur in the area of a storm where you have the greatest instability in the storm.
Lightning from thundersnow will appear brighter in the sky due to the light being reflected off the ice crystals and snow in the clouds. Thunder from thundersnow will still be loud if you’re near the lightning, but the sound won’t carry far distances like it does in the summer because the snow acts as a sound insulator.
WATCH: Rare thundersnow caught on camera in Oshawa, Ontario
Weather bomb
The phrase ‘weather bomb’ may sound made up and, quite frankly, exaggerated, but we can assure you it is very real and should be taken seriously.
The phrase comes from the term ‘bombogenesis’, which is when the centre of a low-pressure system rapidly decreases in pressure over a short amount of time. If a system’s minimum pressure falls by at least 24 mb in under 24-hours, it is considered a weather bomb or ‘bomb cyclone’.
We see this happen more often in the colder months due to the contrasting clash between cold continental air and warm tropical air along North America’s coasts. The clash in temperatures fuels large and powerful storms to develop, most often off the East Coast as Nor’easters.
The rapid drop in pressure with these systems also results in a rapid increase in the storm’s intensity, posing a greater threat to those in the storm’s path. Precipitation and winds can rapidly change in intensity, becoming dangerous in mere hours.