As a native Floridian, I am mesmerized by snow. The world covered in a blanket of white seems so pure and beautiful. Now that I’ve experienced four winters’ worth of snow, I understand why some people hate it. It’s cold. It’s messy. It impedes travel. Still, I can’t help being transfixed by it.
When my family first moved north, we naively called anything white and cold falling from the sky “snow”. It didn’t take long for us to acquire new words for our vocabulary: black ice, sleet, freezing rain, and even freezing drizzle. I quickly became curious about what causes the different types of winter precipitation.
What Causes the Different Types of Winter Precipitation?
All types of winter precipitation (including rain, snow, sleet, and freezing rain) start out as snow. Whether the precipitation reaches the ground as rain, snow, sleet, or freezing rain all depends on the temperature of the air as the water falls to earth.
If the air through which the precipitation falls is colder than freezing, it will fall to the ground as snow. For snow to reach the ground, the entire air column from cloud to ground must be at a temperature at or below freezing.
Those of us who have lived with snow recognize that not all snow is created equal. Snow can be “powdery” or “wet”. Powdery snow (also called dry snow) is less dense and easier to shovel (so I’m told). Dry snow contains less water than wet snow. In general, 5 inches of dry snow will melt to only half an inch of water (The Difference Between Wet and Powdery Snow). Those of us who enjoy building snowmen or having snowball fights recognize that dry snow isn’t optimal for these activities because it just won’t “stick”.
Wet snow is snow that has passed through a shallow layer of warmer air just above the ground. The snow begins to melt in the warmer air but doesn’t melt completely. Water vapor in the warmer layer of air binds to the snow, adding more water. Additionally, snowflakes can stick to one another as they fall, resulting in fat, thick flakes. In general, 5 inches of wet snow will melt to one inch of water. Wet snow makes the best snowmen and snowballs, but is burdensome to shovel. It also accumulates on the ground faster than dry snow.
Rain and Freezing Rain
If the air between the cloud and the ground is warmer than freezing, the snow will melt into rain. Freezing rain occurs when rain passes through a shallow layer of air above the ground that is at or below freezing. The rain doesn’t have enough time to freeze in the air, but will freeze as soon as it hits the ground or any surface. Freezing rain is dangerous for many reasons. It can quickly produce a layer of ice on roads and other surfaces, making travel difficult. It can also form thick layers of ice on trees and power lines, adding weight and causing them to break.
Sleet is formed when the precipitation melts as it passes through an area of warm air, then freezes again as it falls through an area of cold air. In contrast to freezing rain (which falls to the ground as rain), sleet falls as ice pellets. Sleet pellets are hard and one can often hear them hitting the ground or the window as they are falling.
But What About Graupel?
When my friend told me there was a fifth type of winter precipitation called graupel, I thought she was pulling my leg. She wasn’t.
The National Weather Service defines graupel as small pellets of ice created when super-cooled water droplets coat, or rime, a snowflake. In simpler terms, graupel forms when snowflakes are covered with a layer of ice. Like sleet, graupel falls to the ground as pellets. But while sleet looks like pellets of ice, graupel is white and opaque. (To me, graupel looks like the stuffing out of a beanbag. Incredibly scientific, huh?). While sleet is hard, graupel is soft and easily falls apart when touched.
This short video gives a recap of the different types of winter precipitation.
Did you know that snowflakes are really ice crystals? The process of snowflake formation is amazing. Here is a video that describes the chemistry of snow and how snowflakes form.
Here is a beautiful video of snowflakes forming.
While you may never “love” the snow (or other forms of winter precipitation), I hope you now have a better understanding of the amazing science behind how each form is made.