Imagine for a second it’s a 95-degree day — it’s easy if you try.
You dump out a chest full of ice onto the ground.
That ice is going to melt, of course, but not all at once. Depending on how much there is and how thick the cubes are, it may last several minutes on the ground as ice, even though it’s 95 degrees.
That is the beginning of understanding how snow can sometimes accumulate when it’s not quite as cold as the freezing mark and it’s been downright warm just hours before.
Parts of Cardinal News’ Southwest and Southside Virginia coverage area have had this experience twice during the past week, on Thursday and again Monday, as temperatures plunged from warm to cold very quickly with rain changing to snow from west to east and higher elevations to lower ones.
Most of our region had at least some sleet and/or snow on Thursday, the day after many locations recorded midsummerlike temperatures in the 80s and South Boston scraped 90 (no snow reported there officially, but a trace of snow after 89 degrees at Danville).
Based on some research I did a few years ago during my time writing Weather Journal for The Roanoke Times, I believe this was the first time many locations in our region have had accumulating snow the day after highs in the 80s, going back over a century. A 70s-to-snow overnight shift on March 30, 2003, appears to be one of the closest.
Monday’s snow was mostly confined to areas along and west of the Blue Ridge, and while temperatures were “only” in the 60s instead of the 80s, they plummeted to near freezing in less than four hours.
“It won’t stick” is one of the most over-uttered phrases when there is a chance of snow and the ground seems to be warm.
If it snows hard enough, long enough, when it is just barely cold enough, or even when it seems to be just a little too warm, it will eventually accumulate.
Snow never truly “melts on contact” when it hits a warm surface, though it can seem like it in the scale of human time. There is a rate of melt, which may be tenths or hundredths of a second if the surface is particularly warm, but it’s not instantaneous.
That ice dumped out on a hot day is melting, but it doesn’t disappear instantly. It drips into liquid water at whatever the melt rate is.
The trick for snow accumulation when the air temperature and/or surfaces are slightly above the freezing mark is that the rate of snowfall must exceed the rate of melt to the point that the melting snow forms a slushy layer underneath on which additional snow atop it begins to accumulate.
The snow at the base continues to melt, but the snow on top is collecting on the partially melted snow and not the warm surface. This is how wet snow begins to accumulate.
If the snowfall intensity drops or the temperature warms slightly, melting can increase and begin to erode the snow that has already accumulated. We’ve all seen that at some point, when a heavy snow becomes a light snow, or the temperature warms from 32 or 33 to 34 or 35, and much of what had collected melts or evaporates away.
But often in situations with heavier snow, colder air deepens, and the lift aloft continues to be strong, so the heavier snow continues to fall with a slightly colder temperature.
This is how it can collect inches of wet snow even if the temperature is right at the 32-degree mark or even a degree or two above — and even if it has been in the 60s three hours before like Monday or in the 80s the day before like Thursday.
It is true that various surfaces hold in prior heat at varying levels, concrete and asphalt holding in more of it than most soils and grass.
Cold rain preceding a change to snow can help chill the ground temperature and make it more ready for the snow to come.
Sleet is particularly helpful in helping later snow accumulate, as it is a denser form of ice that doesn’t melt as readily even with temperatures several degrees above freezing. Many though not all locations in our region had some sleet between rain and snow during the two recent sudden wintry episodes.
This gets us back to the ice dump on a hot day.
Sleet is more like those cubes of ice than the fragile ice crystals of snow, melting away much more slowly. Many of us saw that in evidence in January and February with how long it took 3-6 inches of mostly sleet compared to how quickly a similar depth of snow would have melted with sunny days in the 40s and 50s.
Even more like the ice dump on a hot day, there is a hailstorm somewhere every now and then that dumps many inches of hail, or even a couple of feet, sometimes drifting like snow. It’s often in the 70s or even warmer when that hail is falling, yet it doesn’t melt away immediately — though it is continuously melting.
If it hails long enough and hard enough, if it sleets hard enough and long enough, if it snows hard enough and long enough, it can accumulate even when the temperatures of air or ground might seem to prohibit it.
Journalist Kevin Myatt has been writing about weather for 20 years. His weekly column, appearing on Wednesdays, is sponsored by Oakey’s, a family-run, locally-owned funeral home with locations throughout the Roanoke Valley.
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