With temperatures as low as 15°F, it seems unusual that parts of the south saw sleet and freezing rain instead of snow on Monday. Wintry precipitation isn't always determined by the temperature at the ground. Here's an explainer on how sleet and freezing rain can turn a beautiful snowfall into an icy death match.
The atmosphere is so much more than what we see at the ground. You could see light winds and 65°F outside your front door, but three thousand feet above you, the wind could rip at 70 MPH with temperatures down below freezing. The temperature, moisture, and wind through a vertical section of the atmosphere are the focus of conversation during severe weather season, as the speed at which the atmosphere cools with height (called a lapse rate) is key to thunderstorm development. However, considering the vertical profile of the atmosphere is just as important during the winter months.
The vertical profile of the atmosphere is crucial to winter weather, and more specifically, determining what kind of wintry precipitation a location sees during a storm. Warm air is less dense than cold air, so when a warm airmass approaches the latter, it rides up and over the colder airmass, allowing a vertical profile that's cold at the surface, dramatically warmer a few thousand feet above the ground, and then it gets cold again above it.
The terribly-drawn graphic above illustrates the general profile of the atmosphere during an "all of the above" frozen mess like we saw on Monday. The amount of warm air aloft is key to determining who will see snow, sleet, freezing rain, or a regular, cold rain. In the graphic above, the blue background corresponds to air temperatures below freezing, while the red background depicts air temperatures above freezing.
In order to understand the vertical profile of the atmosphere, you have to look at at a tool called a SKEW-T chart, or the type of chart used to plot temperature, dew point, and wind data collected by the sensors attached to weather balloons. The above image is a model-simulated SKEW-T chart, showing what the model thinks the vertical profile of the atmosphere will look like for a certain place at a certain time.
On this particular chart, altitude is measured in thousands of feet by the horizontal white lines. Temperatures are noted (in Celsius) by the dark blue lines that stretch from bottom-left to top-right. The temperature of the atmosphere is traced in red, and the dew point of the atmosphere is traced in green. When the temperature and dew point lines are close together, the atmosphere is moist, and when they're far apart, it indicates a dry layer.
Snow is both the easiest and the hardest precipitation type to come across during the winter months. It's abundant in the frigid north, while it's exceptionally hard for any to fall in the south. Snow forms when three atmospheric factors—moisture, lifting, and sufficient cold—are able to come together at the same time. When temperatures are at the "sweet spot" of -12°C to -18°C, the atmosphere is primed for the development of dendritic snowflakes. Water vapor condenses and freezes around a fine particle suspended in the air (like dust, sand, smoke, or pollution), allowing additional moisture to deposit onto the frozen droplet and grow the system of intricate branches that we're so familiar with.
Understandably, snowflakes are very fragile and they require a sub-freezing atmosphere in order to remain intact from cloud to ground. Snowflakes can survive with surface temperatures slightly above freezing, but the layer of warm air must be confined to the area immediately above the surface, or else the flake will melt and it'll fall as a sloppy mix of rain and snow.
The above SKEW-T chart shows an all-snow temperature profile from Charleston, West Virginia on Monday afternoon. The entire atmosphere was well below freezing from top to bottom, allowing flakes to reach the ground intact. The city saw about seven inches of snow from the storm.
Sleet, also called ice pellets, is essentially a frozen raindrop. Sleet forms when a snowflake falls into a shallow layer of warm air a few thousand feet above the surface, allowing the snowflake to begin to melt. Due to the shallow nature of the layer (which is only one or two degrees above freezing), only the outer edges of the snowflake have a chance to melt before it re-enters the sub-freezing air near the ground.
Once the partially-melted snowflake enters the sub-freezing air, it begins to refreeze around the tiny ice crystal that remains in the heart of the snowflake. The droplet completely freezes by the time it reaches the ground, striking the surface as an ice pellet. Sleet is loud and bouncy, and looks very similar to snow when it begins to accumulate. The major difference between accumulated sleet and accumulated snow is that the former tends to freeze into a solid, thick sheet of ice after a while. A frozen sheet of accumulated sleet is extremely hard (and in some cases impossible) to shovel or plow once it's had the opportunity to harden.
I took the above image during Monday's storm, showing about one inch of sleet on top of two inches of snow on the roof of a vehicle. Below is a zoomed-in view showing the sleet in better detail. It looks like tiny droplets of water, because that's what it is—it's just frozen.
Here's a model-generated SKEW-T chart showing a temperature profile favorable to the development of sleet, from Charlotte, North Carolina during Monday's storm.
The layer of above-freezing air in the atmosphere was extremely shallow—only a few hundred feet—but it was enough to partially melt the snowflakes enough to allow them to refreeze and reach the ground as sleet. Charlotte saw a period of accumulating sleet before precipitation changed over to freezing rain. Elsewhere in North Carolina, some areas that were stuck on the far northern edge of the warm air intrusion saw a good bit of sleet mixing in with the snow.
Freezing rain is arguably the most dangerous type of wintry precipitation, as it can cause extensive damage and it's nearly impossible to drive or walk on the glaze of ice it leaves behind. It is rain that freezes on contact with any exposed surfaces, such as trees, power lines, roads, cars, sidewalks, and anything else that's outside and below freezing.
Unlike sleet, which requires a shallow layer of warm air a few thousand feet up, freezing rain requires a deep layer of warm air that's several degrees above freezing (34°+ F). This deep pocket of warmth above the surface allows the snowflake to completely melt, leaving no ice crystals remaining in the raindrop. As there are no impurities in the raindrop, it cannot freeze, so the temperature of the water droplet falls below freezing, or becomes "supercooled."
These supercooled raindrops will fall to the surface and refreeze once an impurity is introduced. In the case of freezing rain, this "impurity" is exposed surfaces like trees, power lines, roads, and sidewalks. The droplet freezes almost instantly when it touches a surface, leaving behind the thick glaze of ice we're familiar with.
Above is a model SKEW-T chart from southern Tennessee during the height of the ice storm on Monday afternoon. Snow fell into a layer of warm air that was in the upper-30s for a few thousand feet above ground level, allowing the flakes to completely melt before entering the sub-freezing air for the last one thousand feet of their descent. As surface temperatures were hovering around 30 degrees, the raindrops froze on contact. Ice accretions of up to one-half of an inch were reported across Arkansas and Tennessee, with significant accretions reported as far east as eastern North Carolina near Raleigh.
Unfortunately, winter storms that form late in the season will start to feature this sloppy mix of precipitation more often as the atmosphere begins to warm as spring and summer approach. The only good news is that it means that warmer weather is on its way...some day.