SIOUX CITY, Iowa (KCAU) — It’s that time of the year again! You start to hear your local meteorologists forecast snow, or a wintry mix. The terms, sleet and graupel are thrown around, as is freezing rain.
But just what are all these different types of wintry weather that wreak havoc on your commute to work and have you longing for the warm summer months again?
We’ll start with snow. The most basic, common form of wintry weather. The fluffy, white, frozen precipitation. Some love it. Some dread it. This winter staple occurs when all layers, from the cloud where the delicate snowflakes form, all the down to us here at the ground, are at or below freezing, or 32° Fahrenheit.
However, you can get different textures of snow, based on just how cold surface temperatures are. For example, if the surface is pretty close to the 32° mark, say between 29-32°, you’ll get a heavier, wetter brand of snow, great for building snowmen and having snowball fights. But as surface temperatures are colder, moisture content decreases, since colder air holds less moisture, so the colder the air is, the drier and fluffier the snow will be. Especially since in general, temperatures decrease with height, so snowflakes will run into even cooler, drier air on the way from the cloud to the surface. So if you’ve got say, a surface temperature in the teens, for example, 15°, and you’re seeing snow, it’ll be a lighter, fluffier snowfall and may even produce more accumulation due to its fluffiness than a heavier, more moist snow. Snows in colder air are better for making snow angels as opposed to making snowman, since the snow won’t stick together and hold as well due to its lower moisture content. These lighter, fluffier snows also tend to blow around much easier and thus create more issues with visibility and make it harder to keep roads and surfaces clear of snow.
Now, moving on from snow, there are a couple other types of precipitation that can from during the winter.
The first of these is sleet. Those little pellets of ice that are commonly mistaken for hail. One major difference between the two though is that hail forms only as a result of convective processes, meaning it only occurs in association with thunderstorms and their updrafts. Sleet on the other hand, begins in the cold upper layers of the atmosphere as a snowflake before encountering a shallow warm layer of air on its way down to the surface. That warm layer allows the snowflake to partially melt as it travels through the above freezing layer. The partially melted snowflake then refreezes again as it moves through a sub-freezing layer from just below the warm layer all the way to the surface. The resultant precipitation is small balls of ice referred to sleet as opposed to a snowflake.
The other common type of winter precipitation is freezing rain. This is also one of the most dangerous types of winter precipitation. Freezing rain, like sleet, begins as snow in the very upper layers. As it travels towards the surface though, unlike with sleet, the snowflake encounters a deep layer of warm air aloft (above the surface level). This allows the snowflake to completely melt and fall as liquid all the way to a shallow layer of colder, at or below freezing temperatures. Due to such a shallow layer of colder air, the water droplet has no time to refreeze and thus falls as rain. However, when it reaches the freezing/sub-freezing surface, the liquid freezes on contact with things at the surface level like roads, trees, sidewalks and cars.
As the rain freezes on contact, a layer of ice is produced, creating very slick conditions, especially on roads where black ice is common in these scenarios. Roads can quickly become solid sheets of ice and become impossible to drive on. Sidewalks also become hazardous as they become coated in thin layers of ice. Another aspect of this is that pre-treating is often difficult or nearly impossible with freezing rain events due to freezing rain developing near sharp temperature gradients where there is oftentimes periods of liquid rain ahead of the onset of freezing rain which washes away any attempts at pre-treating of surfaces. And once freezing rain begins, it’s extremely treacherous even for salt trucks to get out and treat roads.
Most often, with winter storms there is some type of transition from rain to snow, leading to the formation of sleet and freezing rain ahead of the onset of snowfall. However, for meteorologists, these scenarios are very complex and oftentimes difficult to forecast ahead of time precisely what type of precipitation may fall and when. This is because even just a single degree can mean the difference between rain and freezing rain, freezing rain and sleet, or rain and snow or snow and freezing rain and/or sleet.
Finally, a fourth and slightly less common type of winter precipitation, commonly mistaken for sleet and hail, is something called graupel. Graupel is soft, wet pellets. The snowflakes that collect supercooled water (water that remains liquid even below 32°) droplets. As the snowflake moves through the cloud towards the surface, these water droplets collect and freeze all over the snowflake, creating a little snowball, kind of like how when you roll a ball of snow for a snowman down a snowy hill towards the bottom, it collects more snow and grows bigger and bigger until you eventually reach the bottom of the hill. These little snowballs eventually fall to the surface, resembling sleet, but are much softer than their icy counterpart. Graupel is also more fragile than sleet, easily breaking apart in comparison.
Graupel is usually most common when there is very cold air aloft but temperatures at the surface are above freezing, common conditions in late winter and early spring.