Wednesday, February 17, 2010

Sunny Skies are Good For Olympic Snow


The forecasts for the next four or five days at the Olympic venues is for lots of blue skies and sun. No snow. None.

Believe it or not, this is very good for maintaining snow at Whistler and Cypress Mountains! The worst of the snow melt is over. The games will be ok.

So why is sun and lack of clouds and rain a good thing?

It turns out that the very worst enemy of snow is what they had a few days ago:

Strong winds, low clouds and fog, heavy rain, warm air.

It sounds like the plagues of Pharaoh.

And you know what was probably worst of all? The fog.

Warm air is obviously bad...it causes snow to melt. Strong winds and warm air are even worse, since the wind continuously brings more warm air to melt the surface. But believe or not, warm air is not an efficient melter of snow. It can do the job, but it takes a lot of time. In the business we call the effects of warm air sensible heating.

Now warm rain is not great in the melting department either. Warm rain can bring some heat to the snowpack, and if it refreezes it releases the latent heat of fusion into
the snow (about 80 calories per gram). Yes, heat is released when water freezes...the reach orchardists east of the Cascades spray water on their trees when the temperatures drop below freezing during critical periods.



But you want real melting? If the air is moist, and foggy/cloudy air is nice and moist, the water vapor can condense on the snow and that releases the latent heat of condensation...which is HUGE....around 600 cal per gram. Mega heat source.

And one more thing, clouds and fog intercept and remit infrared radiation leaving the surface, preventing cooling to space.

So the conditions earlier this week at Cypress Mt and lower Whistler were as bad as it gets: strong winds, warm temperatures, heavy rain, clouds to stop the radiation from escaping, fog and humid air for maximum latent heat release.

Today. Lighter winds, cooler air, no clouds or fog. Drier air. No warm rain. Snow reflects much of the solar radiation that falls on it, but is a very good emitter in the infrared. In short, they are in a much better place for maintaining the snow.

3 comments:

zephyr said...

Ahhh. nothing like waking up to a great explantion of exothermic reactions. :)

April said...

The best news could be no snow and a better weather.Too hard to go through the hard and tough winter days.

cold

fpm said...

Minor technicality: heat of vaporization is approx 540cal/gm, not 800cal/gm (still huge).

[Great blog, many thanks!]