Tuesday, December 2, 2008

Mountain pass temperatures...

Question of the day:
Why are there two temperatures listed for Stevens Pass? Where are they taken at? I can see 1-2 degree difference in them but sometimes there is 5 or more degrees difference.

Answer:
There are three temperatures displayed on all of the mountain pass pages. The temp in the yellow "Pass Report" box next to the picture of whatever pass you're looking at is what is radioed in to our various Traffic Management Centers from the thermometer on the maintenance supervisor's pickup. That's the air temperature in the shade on the roadside wherever he or she is when they call in the pass report and set the traction restrictions, if any. As a result, the location could vary a mile or more from the summit, depending on the pavement conditions the supervisor has encountered as he/she makes the assessment. That's a call that takes place every 4 hours from November through March. It means that at 1 p.m., you're looking at a temperature that is an hour old. Below the camera image is a blue box labeled "Current Weather". A road weather information station provides that data every few minutes to the web page. The thermometers measure the air temperature and the pavement temperature at the summit of the passes. These are also subject to the influences of any permanent location - sunshine - a bird perching on a sensor or even a vehicle stopped on top of the pavement sensor.

Yesterday, for example, Stevens showed a temperature of 37 for the noon pass report, but by 1 p.m., the current weather box showed an air temperature of 48 and a pavement (surface) temperature of 53. A half hour later, when the sun went back behind the clouds, the temps had dropped to 47 and 51, respectively.

The Blewett pass page showed a similar pattern - 42 degrees in the pass report box and in the current weather box, 52 - air, 42 - surface at 1 p.m.

I checked Snoqualmie, Satus, etc. and Loup Loup (37-37-37) was the only one that showed all three matching, and then only for about fifteen minutes.

You might think we ought to eliminate the temp in the pass report box to eliminate confusion - and we would except that the info in the yellow box is what also appears on several other of our web pages, the 5-1-1 phone system and Highway Advisory Radio's, and NOAA's web site which is what Associated Press uses to provide its member's "news". All of those systems are automated and when conditions do change rapidly, forcing us to change the traction restrictions sooner than the standard "every four hour" schedule, then all of those change, too. It really wouldn't make much sense to update the mountain pass reports every 3 to 10 minutes because the temperature changed a degree or two - and if someone is looking at the pass pages, between the picture, the pass report and the current information, they ought to be able to decide if they want to go or not.

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