Just when you thought you knew everything about winter weather Mother Nature throws out a few oddballs. Here are five winter-weather phenomena you may experience during the course of snow and ice duties.

Thundersnow

Thundersnow is a rare type of thunderstorm that occurs during a winter weather event where snow is the primary precipitation type. Thundersnow forms in localized, heavy bands of snow when warm air quickly climbs toward the cold air above, typically in the “cold sector” or the west/northwest side of a low-pressure system. Instability and friction occur in the atmosphere when these two meet, which causes a lightning strike and then thunder. Thunder during these events is typically quieter than usual because snow acts as a muffler and limits sound’s ability to travel.

Graupel

Graupel is a lesser-known winter precipitation type but is not actually too rare. It looks like small, white pellets and is also referred to as “soft hail.” Graupel forms when supercooled water droplets (water that has remained in liquid form even though temperatures are below freezing) attach to falling snowflakes. The supercooled water droplets freeze onto the snowflake before falling to the ground. Unlike hail and sleet, graupel are soft and easily crushed.

Snow rollers are the tumbleweed of winter weather. Also known as “snow bales”, “snow donuts,” and “wind snowballs,” these unusual cylinders of snow only form in very specific conditions.
NOAA

Snow Rollers

Snow rollers are the tumbleweed of winter weather. Also known as “snow bales”, “snow donuts,” and “wind snowballs,” these unusual cylinders of snow only form in very specific conditions. First and foremost, snow rollers need wide open, flat landscapes to develop with no protruding vegetation or wind blocks such as buildings. Then, there must be an icy layer of compacted snow that is covered with a light dusting of mildly wet snow. The snow cannot be too wet, or it will stick to the ground, but it can’t be so dry that it doesn’t adhere to itself. Next, air temperatures need to warm to 3-5 degrees above freezing, without completely melting the snow. Winds of around 30 mph are needed to create the cylindrical shape.

Bombogenesis

This term has become more widely used by television meteorologists over the past few years, but it’s not anything new. Bombogenesis or a ‘bomb cyclone’ refers to a surface low pressure system that rapidly strengthens in a short period of time. The criteria for bombogenesis is that the central low pressure of the storm must drop at least 24 millibars within 24 hours. A millibar (mb) is a unit of measurement for pressure, with 1013.25 millibars being standard sea level pressure. Anything above 1013.25 is considered high pressure, and anything below is considered low pressure. The lower the pressure, the stronger the storm!

Bombogenesis or a ‘bomb cyclone’ refers to a surface low pressure system that rapidly strengthens in a short period of time. The criteria for bombogenesis is that the central low pressure of the storm must drop at least 24 millibars within 24 hours.
Mary Ellen St. John

Frost Quakes

Frost quakes, or ‘cryoseisms’ are caused by sudden deep-freezing of the ground and are common during the first freeze and first major cold snaps of winter. When saturated soil or rock layers quickly freeze, they expands, and stress builds underneath the ground. Sometimes, the stress becomes too great and is explosively relieved, resulting in loud booms, crashes, and shaking. Cracks on the surface may also appear. Frost quakes are localized events, and you may not feel it if one occurs on your neighbor’s property. The Midwest and northeastern US are most prone to these occurrences.

Beth Carpenter and Joseph “JT” Cooper are founders and meteorologists at Thermodynamic Solutions. They are frequent Snow Magazine contributors.