Monday, September 21, 2015

Huge Daily Temperature Swings Hit the Region

Imagine frigid frosty temperatures in the low 20s in the morning and then a few hours later the mercury reads near 80F, suitable for a dip in an outdoor pool.  Amazingly enough, 50-60F air temperature swings are not rare around our region during the late summer and autumn, particularly in what I will call the frost region of the Northwest:  valleys in the high plateau of eastern Oregon.  And before I get done with you, Crow Flat will be a place you will not forget easily.

Let's start by looking at the minimum surface air temperatures this morning around the region (see map).  A large range, with lots of stations dropped into the lower 50s and 40s.   And the 17F on the Olympic Peninsula is clear wrong.   But look at central eastern Oregon, there are temperatures in the 30s and 20s!

You can see those low temps more clearly in this blow-up map.  The  Bear Valley region is particularly cold.  Bears like cold temperatures, I understand.

The coldest location is the Crow Flat site with 22F.  Here is the temperature plot there for the past week at that location..  Yesterday had a 60F temperature range....amazing.   That could crack come concrete!

 The Crow Flat site is shown below

 Nice open location above natural vegetation at an elevation of around 5000 ft.  Importantly, it is in a hollow or low spot, with cold air tending to settle into such valley locations (see topo map)

So with clear skies last night and much longer nights this time of the year, the ground is able to radiate heat to space effectively and for a long time.  The ground cools the air above.  The radiational cooling is enhanced by the very dry air over that region (dew point was about 20F).  Moist air can act as a radiational blanket--that is why deserts have big diurnal (daily) temperature swings.  Particularly cool air pools into hollows and low areas, such as Crow Flat in the Bear Valley region.

On the other hand, the sun is still fairly strong this time of the year and the air temperatures aloft remain warm, so with clear skies the surface heats rapidly during the day.  Look at the temperature traces above.....the air warmed abruptly during the AM.   A list of the temperatures and dew point this morning also show this.  The air warmed from 26F to 60F in two hours, between 8:07 and 10:07 AM. That would wake you up.

Very few people live in the Crow Flat area of Oregon, which is just fine with its other residents, who like peace and solitude.


Richard Fuhr said...

The National Geographic, in a recent special issue, made an interesting mathematical error related to high daily temperature variations. They said that temperatures in the Gobi Desert could vary by as much as 95F (35C) in one day. While it is true that 95F = 35C it is NOT true that a VARIATION of 95F is equivalent to a VARIATION of 35C.

Christina Wilsdon said...

Re Richard Fuhr above: could you explain a bit more? Maybe I just need more coffee this morning :) but I am not a math wizard and it's been many years since algebra, thanks!

Jim said...

Back in the day when the weather was locally reported on the radio, we'd always heard temp predictions followed by "and lows to 30 in the cranberry bogs". Being a cranberry farmer it was fun to know that the cool air would settle in the swales between the dunes (where cranberry bogs are typically) and those low bowl like areas would be several degrees cooler than surrounding areas. A wonderful local weather flavor.

Meredith said...

I find that this is the most annoying time of year for figuring out layering for bicycle commmuting. It's beautiful and generally drier, but I'm constantly shifting the number of layers needed (jacket and baselayer in the morning morning, halfway to work, jacket is too hot, baselayer too hot for afternoon....repeat)

Rich Seymour said...

re: Christina ...

A 35 degree Celsius swing is only 63 degrees Farenheit.
(5 degrees C is equal to 9 degrees F, so (35/5)*9=63)

The Nat Geo error was to simply look at a thermometer, which adds in the 32F shift to account for Celsius having Zero at what F calls "32".
Yes, on a wall thermometer, 35C is the same as 95 F.
But in terms of *change*, it's only that 63 degree shift.

Richard Fuhr said...

To reply to Christina Wilsdon's comment, here is a bit more about the Fahrenheit/Celsius issue. Now 32 degrees F is equivalent to 0 degrees C and 212 degrees F corresponds to 100 degrees C. The general formula is C = (5/9)*(F - 32). So using that formula, 68F equals 20C and, in the National Geographic example, 95F equals 35C. But the National Geographic was using the formula in an inappropriate context. In the present discussion we are talking about a change of temperature (between day and night) not a particular temperature reading. A change of one degree Fahrenheit is a change of only 5/9 degree Celsius. Therefore a change of 95 degrees Fahrenheit is a change of (5/9)*(95) degrees Celsius, which is about 52.78 degrees Celsius. Another way of thinking about this is to consider a change in temperature between day and night of 32F. We would of course not then say that the temperature changed 0C, and the actual change would turn out to be (5/9)*(32)C.

Richard Fuhr said...

Here is a link to the relevant portion of the article in National Graphic in which they claimed that a change of 95F was equivalent to a change of 35C.

Christina Wilsdon said...

Thanks, Richard, that makes total sense...I will print out and keep this distinction in mind when I write or edit material that flirts with temperatures and temperature variation! It's an easy mistake to make as one can readily reach for the formula for conversion without thinking it out.