In short, there is nothing ever nice about the ice.
First, a critical distinction: Sleet is the term describing those tiny ice grains that ping on our window panes and accumulate as an opaque, crunchy layer on the ground. Freezing rain describes liquid water drops that fall to frozen ground, then freeze into a clear coating on impact.
Freezing rain will glaze and completely envelop tree branches, causing them to sag as clear cylinders of ice; whereas sleet tends to just bounce off. On the ground, both forms of ice create a slick rink. But the glasslike coating of freezing rain wins when it comes to sending pedestrians wheeling, and cars sliding sideways down the slightest grade.
Sleet and freezing rain develop as a result of a layering of different air masses in the vertical, between the cloud base and ground. These layers take on a triangular wedge shape, as shown in the figure below, with warm air riding up and over a ramp of subfreezing air trapped against the surface. The Appalachian Mountains often trap the dense, icy air layer, known as a cold air dam, while Atlantic winds usher in a milder air mass that glides up and over the top.
On the eastern side of this triangular cold wedge, rain falls through a deep, warm layer beneath the clouds, then into a very shallow subfreezing layer, right at ground level. The drops solidify on contact with cars, roads, sidewalks, trees and utilities as a glaze of clear ice, or freezing rain.
On the western side of this wedge, the layer depths are inverted: The warm layer aloft is quite shallow, the subfreezing layer beneath very deep. Melted snowflakes spend a longer time falling through subfreezing air, allowing them time to freeze into pellets of sleet.
During some storms, we receive a mixture of sloppy wet snow, ice, freezing rain and plain rain — changing back and forth throughout the day. This happens when air temperatures near the ground and lowest few thousand feet hover near the freezing mark.
More uncommonly, ice is the dominant precipitation form, falling for many hours. This is the hallmark of our really bad, crippling ice storms. Below we describe three of these from the past, examining briefly the atmospheric setup, then recounting some of the brutal impacts left in each storm’s icy wake. Let’s take the most recent storm first.
Valentine’s Day, 2007: Ice encases D.C.’s southeastern suburbs
An intense low-pressure system crossed the Southeast United States, drawing mild gulf air northward atop an Arctic air layer situated across the northern Mid-Atlantic. The “overrunning” situation generated a mix of snow, sleet and freezing rain for several hours along the Interstate 95 corridor. The deeper, colder part of the air mass was situated across the District’s northwestern suburbs, where 3 to 5 inches of snow accumulated, mixing with sleet.
To the southeast of the District, the cold air layer was very shallow, such that more freezing rain than sleet fell; a half an inch of ice glaze prevailed across a large area, with a pocket of ¾-inch glaze ice over southern Prince George’s and northern Anne Arundel counties. In that pocket, tree limbs snapped with a vengeance, creating 100,000 power outages. A total of 135,000 outages occurred across the greater region.
“Icy road conditions lead to dozens of car accidents across the region and schools were closed for much of the week,” the National Weather Service reported. “Several restaurants and florists reported reduced Valentine’s Day sales due to the hazardous road conditions.”
Bitter cold air filtered in on Arctic high pressure following the storm, preventing melting of ice, and hampering the efforts of tree and utility crews. A great many endured unheated homes for several days.
Jan. 14-15, 1999: A frozen nightmare on the northwestern side
This event featured a deep wedge of subfreezing air that nosed down the eastern slopes of the Appalachians, as far south as central Virginia. There, a stalled frontal boundary enabled milder air to slide up and over the cold air dam. A prolonged period of freezing rain unfolded in a broad band from Anne Arundel County to Montgomery County.
Only ¼-inch of glaze ice accumulated southeast of D.C., but a half-inch to an inch of ice accretion hammered Montgomery County in Maryland, and many counties across Northern Virginia. In the icy bull's eye, tree damage was phenomenal. During the night of the storm, residents recalled hearing “shotgun-like” snapping of tree limbs, with many limbs crashing down on roofs.
Three-quarters of a million customers lost power, many for days. And just as power was finally restored, a substation in Olney in Montgomery County caught fire, an event that cascaded and shut down eight surrounding substations — plunging an additional 70,000 into darkness and cold.
The Weather Service also reported hundreds of car accidents and slip-and-fall injuries.
Winter 1994: Mega-ice storms area-wide
This winter season was epic, for the number and intensity of ice storms unfolding across the Mid-Atlantic. Between 19 and 23 icy days of precipitation impacted the greater region, creating perhaps the iciest year of the century.
A significant ice storm unfolded Jan. 17, when a stalled front along the Chesapeake Bay allowed mild ocean air to slide over an approaching Arctic air mass from the west. Several days of extremely bitter Arctic chill followed, with icy surfaces becoming concrete-hard. So thick was the ice that crews could not remove it and chemicals failed to work in the extreme cold. The day after the ice storm, Washington observed one of its most frigid days on record, with a high of 8 degrees and low of minus-4.
The highest impact ice event of that winter unfolded Feb. 10-11, with a massive cold air dam that stretched east of the Appalachians from Maine to Georgia. As much as 3 to 4 inches of sleet accumulated across a broad area.
The hardest-hit region from freezing rain stretched across Danville and Fredericksburg, Va., into Southern Maryland. Some counties lost 10 to 20 percent of their tree canopies, according to the Weather Service; many people were without heat and power for one to two weeks. Many utility crews from surrounding states were called in to assist with tree-clearing and restoration efforts.
Storm damage from the Feb. 10-11 storm was sufficient to trigger a presidential disaster declaration, with damages in the vicinity of $100 million, according to the Weather Service.
Ice storms will continue to ravage our region; and as the amount of our built infrastructure increases, losses from these events will climb higher. Experimental tools are available, which combine the depth of ice accretion on trees with wind speed data, to predict the level of utility disruption.
It’s only a matter of time before another ice storm wallops our region, and let’s all hope it will be on the mild end of the spectrum … because the ice is never nice!