Jan 09

An exerpt from Dr. Spencer’s latest post where he answers the question of how that Arctic air gets so cold:

As of this writing (January 9), it looks like the coldest temperatures in the Lower 48 are yet to come, as the coldest airmass over northwest Canada finds its way down into the central and eastern U.S. starting around next Wednesday (January 14) or so. Gee, where is global warming when you really need it?

The ’scientific consensus’ is that these frigid air masses are the ones that should warm the most with manmade global warming. The reasoning goes that since they contain very little water vapor (Earth’s main greenhouse gas), the warming effect of the extra carbon dioxide should be proportionately greater there.

But what causes these air masses to get so cold in the first place? Well, little or no sunlight is the most direct reason, which means they radiatively cool to outer space without any solar heating to offset that infrared cooling.

But what limits how cold they can get? Why do these temperatures seldom fall below -60 or -70 deg. F….temperatures reached fairly early in the winter, but which then level off? The answer is mostly related to the water vapor content of the air.

There is an interesting issue of causation involved with these cold and dry air masses. Contrary to what some meteorologists think, the air doesn’t become dry because of the cold. If that was the case, the air would become continuously saturated with clouds and fog as it keeps cooling, rather than clear and relatively dry as is observed.

No, rather than being dry because it is cold, the air instead becomes cold because it is dry. And the reason the air is so dry is because it has been slowly sinking from high in the atmosphere, where there is very little water vapor. And why is THAT air so dry? Because precipitation processes have removed the water vapor as relatively warmer and moister air ascends in low pressure areas — snowstorms — which move around the periphery of the high pressure zones that are created by the strong cooling.

So, ultimately, it is precipitation processes in regions remote from these cold high pressure areas that mostly determine how cold surface temperatures will get. And since we have little understanding of how these precipitation processes in the upper atmosphere might change with ‘global warming’, there is (in my mind) more uncertainty about water vapor feedbacks than the IPCC has led us to believe.

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