Kelly asked

> Jon Anderson referred to our recent bout of freezing rain as a "silver thaw".
> This is the second time I have heard this term in the last week. What a
> curious name. Where does the "thaw" come from? I could better understand
> "silver rain" but the "thaw" part seems to have little to do with the
> phenomenon.

Freezing rain, in 99.9% of the cases, occurs when warm air ahead of an
approaching warm front overruns a shallow wedge of retreating cold air at the
surface (because the cold air is denser than the warm air, it travels much
more slowly than the warm air aloft). Precipitation starts out as snow above
the freezing level (close to 10,000 feet during the recent Washington episode),
and then changes to rain below the freezing level. The rain then freezes on
contact with the still frozen ground. If the cold air at the surface is deep
enough, the rain will freeze while still in the air and will reach the ground
as ice pellets ("sleet" to most people) which are essentially frozen rain
drops. Thus, the "classic" progression of precipitation when coming out of a
cold snap is snow-->sleet-->freezing rain-->rain. The time spent in each
category depends on the speed of retreat of the cold air. The term "silver
thaw" encapsulates two features of this progression: the coating of ice
which accrues on surface structures during the freezing rain stage ("silver");
and the subsequent thawing as the freezing rain changes to rain.

- Jack





Jack Bowling
Prince George, BC
jcbowling at mindlink.bc.ca