Sunday, September 14, 2014

Huge daily temperature range hits the Northwest

Often during the late summer and early fall, a huge daily temperature range is observed at Northwest stations, particularly those away from the water.

Here is western Washington for example, stations like Olympia have risen into the 80s during day, but drop in the mid-30s at night.   Here is the proof!

And some locations in eastern Oregon have dropped below freezing!

Why have we seen such large daily temperature ranges the past few days?  Why is this season a favored one for such behavior?

Before I answer that, let me show you how cold it has gotten at night.  Here are the minimum temperatures on Saturday morning over western WA.  30s in Olympia and at inland stations around SW Washington.  Lots of 40s over land, but 50s near the water.  Some 60s southwest of the Olympics where downslope warming occurred,

Want even colder?   Here are the temps over eastern Oregon....lots of places got below freezing, particularly in low spots of the high plateau of the region.  A few locations dropped into the mid 20s.

But then the sun comes up and temperatures soared until clear skies.

Why have the minimum temps overnight got so cold?

First, the skies are clear or nearly clear.....and clear skies allow effective emission of infrared radiation to space, which produces cooling at and near the surface.

Second, nights are longer now and that allows greater opportunity for nighttime cooling.

Third, the air became much drier during recent days.   To show this, here is the plot of dew point (a good measure of water vapor) at the surface at Seattle Tacoma Airport.   A major drop from the

previous mid-fifties to the 20s and 30s.    The lower the dew point, the drier the air.    Drier air absorbs less infrared radiation leaving the surface, allow more cooling to occur.  This is because water vapor is a potent greenhouse gas, a gas that absorbs and emits actively in the infrared. Water vapor absorbs some of the infrared radiation leaving the planet and emits some of it back down to the surface.  So the drier air promotes cooling.

By the way, dry air aloft also explains why deserts often have large daily temperature ranges. And I hear that such large daily temperature swings produce excellent wine grapes....another reason why the dry areas of eastern WA give us such excellent wines.

There are some other lesser effects. For example, the ground is relatively dry and that lessens the vertical conductivity of heat from the warmer subsoils during the night.  But that is a secondary.

So enjoy the warm days, but nights cool enough for comfortable sleep.  Today and Monday will continue the nice weather.

So enjoy the cool temperatures at nigth.

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