Garden Diary - February 2022

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Freezing Rain
Friday, 25 February 2022

Yesterday evening's weather report advised there would be freezing rain.

That's different from sleet. It's a glaze event or a silver thaw, resulting
from gentle rains occurring at temperatures just below freezing.

Conditions are similar to sleet, but with a major difference. It begins with a layer of above-freezing air above a layer of sub-freezing temperatures closer to the surface. Frozen precipitation melts to rain while falling into the warm air layer, and then begins to refreeze in the cold layer below. If the precipitation refreezes while still in the air, it will land on the ground as sleet.

When the layer of freezing air is so thin that raindrops don't have enough time to freeze solid before reaching the ground the liquid droplets can continue to fall without freezing, passing through the cold air just above the surface. However this thin layer of air then cools the rain to a temperature below freezing.

While the drops themselves do not freeze, a phenomenon called supercooling takes place. When the supercooled drops strike ground or anything else below 32 degrees Fahrenheit such as power lines or tree branches, a layer of ice accumulates as the cold water drips off, forming a slowly thickening film of ice. Thus, freezing rain. Just one quarter of an inch of ice accumulation can add about 500 pounds of weight to the span between poles of a power line. We lost power for about 12 hours, until 7:15 p.m. Caused by freezing rain? Perhaps.

I'm not sure about this precipitation on the driveway. It is not freezing rain but it might be graupel, also known as soft hail or tapioca snow, which is defined as "precipitation consisting of white, opaque, approximately round (sometimes conical) ice particles having a snowlike structure, and from about 2 mm to less than 5 mm in diameter." What's on the driveway are clearly discrete units, like sand on a beach. The ripple patterned area is due, I believe, to water dripping off the garage roof and carrying the particles along its path.

Rain froze on the cupped, upside down bells of the rain chain.

It froze on the horizontal wires of a trellis made from metal fencing.

As it does, traditionally, freezing rain froze on the branches of shrubs.

And turned the forest into a fairyland decked with silver thaw.

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