The cold season is upon us in the northern hemisphere, and millions of people in the central United States got an early dose of wintry weather during the Thanksgiving holiday, when a large and slow moving storm dropped a combination of rain, snow, sleet and freezing rain, creating icy driving conditions from Texas to Minnesota. But do you know the difference between the different types of wintry precipitation? We explain them all below:
Snow
Some love snow, while others downright hate it. But what’s not to love about the artistic form of supercooled cloud droplets bonding together in individually unique formations? Ok, there is the shoveling, and the hazardous roads, and the slipping and sliding. It can be a love/hate relationship.
Meteorologically, snow formation is a complex process where flakes begin to take shape at almost microscopic sizes, before sometimes growing up to 1 or 2 inches in diameter as they fall to the ground. Along with having enough moisture present, the other key ingredient for snow to form is cold air, and not just at the ground where we are. Forecasters have to look at the atmosphere three dimensionally in order to determine precipitation type (Figure 1), and for snow, it’s important for the entire column of air from at (or near*) the surface through the cloud producing the snowflakes to be at or below freezing (32°F).
Figure 1 (Source: NOAA): Atmospheric temperature profile in a snow scenario
The specific temperature and air pressure will also determine what type of snow we get on the ground. For example, if the air temperature where snowflakes form is between approximately -5°F and 15°F, the flakes tend to be the big, fluffy type. But if the temperature is closer to freezing, the flakes are usually smaller and heavier. These details can be important, especially for ski resorts and road crews that have to plow the snow.
*Note: It is possible to have snow when the temperature at the ground is a few degrees above freezing, though it’s important that the air just above the surface and the rest of the way up is below freezing, otherwise the flakes will melt into raindrops before they reach the ground. This scenario is sometimes referred to as a rain/snow mix, since the precipitation can be a combination of the two.
Freezing Rain
Freezing rain is a travel nightmare, turning roads into virtual ice rinks. But it is often strangely beautiful, as ice formations on trees can look alien and bizarre. We marvel (or curse) at it so much, because freezing rain is a fairly rare occurrence when compared to how often it snows.
For freezing rain to occur, the vertical temperature of the atmosphere must be very specific (Figure 2), hence why it’s so rare. Precipitation that starts as snow must fall through a layer of above freezing air for long enough to melt and turn into rain. At the same time, below freezing air must be present at and just above the surface. However, because the cold layer near the surface is shallow, the rain does not have enough time to freeze once again, instead remaining in a liquid state. But as soon as the liquid drop hits a cold surface (roads, grass, trees, power lines, cars) it will freeze, creating a coat of ice. It only takes a thin coating to wreak havoc on roads, while major ice storms caused by prolonged periods of freezing rain can not only cripple travel, but also knock out power and topple trees due to the excessive weight (like the picture up top).
Figure 2 (Source: NOAA): Atmospheric temperature profile in a freezing rain scenario
Sleet
Similar to freezing rain, a sleet situation still requires a warm, above freezing layer above a cold layer. The difference is that the warm layer tends to be shallower compared to a freezing rain scenario, causing snowflakes to only partially melt (Figure 3). The partially melted drops then freeze into tiny ice pellets as they fall through the cold, subfreezing layer near the surface. While not as disruptive as freezing rain, sleet can make driving and other travel conditions more difficult than even snow. Tires in good shape can gain traction on snow. But with sleet, the surface becomes slick with the icy slush, leaving little friction for cars to grip the road.
