While the distinction between snow and rain is understood by everyone, there are various forms of precipitating ice, which can be confusing.
Sleet is a term that describes tiny, opaque pellets of ice that literally “ping” when the ice grains bounce off windows. Sleet creates a white layer on the ground that crunches as you walk on it.
Freezing rain is precipitation that remains liquid until the drops strike a below-freezing surface, accumulating into a translucent sheet of extremely slick, glaze ice.
(Hail is yet another type of ice that falls from the sky. These are small- to medium-size spherical stones of ice that — ironically — hardly ever occur in Washington during the dead of winter, but rather, during summer thunderstorms.)
Whether we get snow, rain, sleet or freezing rain depends on layers of temperature in the lowest several thousand feet of the atmosphere. Subfreezing air is needed for the solid forms of precipitation to form, i.e. snow and ice, whereas milder temperatures above 32 degrees initiate plain rain.
The cold air is typically funneled over Washington from the north, pumped southward by a strong high-pressure system anchored over New England. The cold air gets trapped against the eastern slopes of the Appalachians, forming a shallow dome of subfreezing air called a cold air dam or wedge. The eastern extent of the subfreezing air often aligns, by coincidence, with the Interstate 95 corridor (and all its major cities) … meaning that Washington and Baltimore frequently sit perilously close to this “freezing line” near the ground.
Meanwhile, mild air is channeled off the warm Atlantic, from the east or southeast, by a developing low-pressure system (and sometimes a coastal storm, called a nor’easter). The mild air, being less dense than colder air, rides up and over the top of the entrenched cold air layer.
What ensues is a layer-cake-like temperature profile, in the form of a mild zone of warm air atop a wedge of cold air near the ground. This is illustrated in the image below.
The image shows a transect through the lower atmosphere, from west to east. The subfreezing air wedge/cold air dam is shown in blue; the milder air wedge is shaded orange. A thick cloud layer atop all this generates precipitation. We will look at four regions, labeled A through D. Washington is arbitrarily placed on the freezing line.
On the western side, close to the mountains (Region A), the air layer is below freezing all the way from the clouds to the ground. Precipitation forms in the clouds as snow, and it remains as snow falling to the surface.
On the eastern side, close to the Chesapeake Bay (Region D), the air layer remains above freezing from the ground upward to the clouds. Precipitation starts as rain, and it remains as plain rain as it falls.
The icy phenomena happen in the middle two regions, involving the overlap of mild and cold air. Here we have to think in three dimensions, as this is where the freezing level slopes upward, away from the surface. A mild air wedge overlies a cold air wedge. In Region B, the cloudy air is mild, so rain forms. But the drops freeze into ice grains as they descend through the cold air, as the cold wedge is quite deep, forming sleet.
In Region C, now the mild air wedge is the deeper of the two, extending from the cloud nearly to ground. Rain forming in the clouds remains liquid until arriving at the surface, where there is a shallow subfreezing air layer. The liquid drops freeze on contact with surfaces, such as roads, cars and tree limbs — creating freezing rain.
This whole setup can shift west or east, depending on a storm’s track. A westward shift can place Washington on the mild side of the freezing line, and the result is just a rainy day. An eastward shift can create an icy mess, and a farther slide eastward brings with it a snowstorm. Very often, with Washington so close to the freezing line, slight incursions of cold vs. warm air during a storm will transition us back and forth among the precipitation types. Hence the forecast of “wintry mix.”
And sometimes, there is very little overlap between warm and cold air, and no wedging but, rather, an abrupt juxtaposition of snow and rain and no ice formation. In this situation, we call the freezing line the “rain-snow line.”
As everyone knows, wintry mix days are miserable and messy and fraught with hazards. A layer of sleet provides a little more road traction than freezing rain and does not stick to trees and power lines. When sleet mixes with snow, snow accumulations are reduced. This scenario is common in D.C.’s southern and eastern suburbs, when a nor’easter is moving up the coast, as the intrusion of warm air from the south is brought closer to the District.
When freezing rain falls for an extended period, accumulations in the quarter-to-half-inch range will cause small tree limbs to snap, accompanied by isolated outages. As ice continues to accumulate above half an inch, prospects for widespread outages rapidly increase, and more than three-quarters of an inch of ice accretion will cripple the region. When this much ice is forecast, prepare for a multiday power outage.
Locations north and west of D.C. are more likely to experience heavy ice storms (some combination of sleet plus freezing rain), being deeper inside the cold air mass that is wedged up against the mountains.
If we are dealing with just a rain-snow mix, those in the transition zone (along the rain-snow line) will experience a heavy, wet snowfall. Here, snowflakes partially melt and stick together, forming large, slushy flakes. The stuff is great for snowballs, but it is backbreaking to shovel, and 5 inches or more of wet snow sticking to trees will snap limbs and knock out the power.
A wintry mix is just a plain misery mix for everyone involved.