Sunday, November 30, 2014

The Wild Daily Temperature Swings Of Tahoe

A friend in Portland told me that he has often noticed that the weather report for Tahoe shows large temperature differences between the daily highs and the nighttime lows. I've noticed it too, but I never really paid attention to it. For those of us in Tahoe, we just love the way summer nights cool down into the upper 30s even when the daytime highs sometimes get into the upper 80s. The winter swings are not so dramatic, but they are still significant.

With the fall nights getting down into the teens, I was reminded of my friend's comment. Here's a screen shot of the National Weather Service website from last month. Three days in a row, we had a 50-degree temperature variation from high to low.

The question is, why do some places (like Tahoe) have such large temperature swings and others (like Portland) do not?

So I did some research. What I learned is that it's all about humidity. 

Water vapor is a Greenhouse gas. Which means, it keeps heat it by letting sun rays pass through during the day to warm up the earth. Then at night, when the earth is radiating its heat back into space in the form of infrared radiation (a substantially different wavelength than much of sunlight), water vapor bounces that radiation back to earth. So water vapor is relatively transparent to much of sunlight but relatively opaque to infrared radiation.

The bottom line is that moist climates don't lose anywhere near as much heat during the night. Dry climates like deserts lose a great deal of heat during the night because there is almost no water vapor in the atmosphere to bounce that infrared radiation back to earth where it came from.

You may be thinking, "But Tahoe isn't a desert." It's true that most years Tahoe gets a lot of precipitation, mostly in the form of snow. But what's interesting is that Tahoe's precipitation comes in storms, or waves of storms off the Pacific. We rarely get moist air masses that just hang out and drizzle on us. Our storms come in fast and move out in a day or two. During those times, we have high humidity and thus we have small daily temperature swings. 

But when we don't have storms, we get the same dry climate as the high deserts of Western Nevada.

It's a great climate. We get storms maybe once a week during the winter, and the rest of the time, summer and winter, we have dry air which brings us our amazing sunshine.

It also brings us wild daily temperature swings, warm sunny days with cool nights.

Those of us who live here think it's perfect.

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