Wednesday, October 19, 2011

Temperature Differences Between Airport And Rillito Wash

Jim asks wrt previous post:

What caused the 16 degree difference between your house and airport? Are you wind-sheltered? Amazing there could be up to a 26 degree difference between the airport and the bottom of Rillito Wash!
My answers - First, our back yard is not wind-sheltered by obstacles. My max/min thermometer is situated in a "desert" area - rocks and sand, native vegatation, no watered yard anywhere nearby, and is at a height of only 4 ft above the surface. The largest differences in minimum temperatures between here at house and TUS have been 20-25F. Note that maximum temperatures are not much different. Lowest temperature I've measured here during past 13 winters has been 9F. The map below shows our location ("A") - note that it's pretty "brown" in this whole area. The wash is only about a long city block directly to the east of the house.

Second - the difference in elevation between TUS and the Rillito Wash is about 300 ft msl. Our backyard is probably 30 to 40 feet higher than the wash where it crosses under the Campbell Avenue bridge. So, the wash is often considerably colder at sunrise than here at the house. The airport location is prone to wind during the night, so nocturnal mixing often keeps temperatures quite warm down there, relative to many other locations witin the City (we are within the Tucson city limits here).

Third, cold air drainage into the Rillito Wash is very substantial, as is very effective local radiation within and near the wash. Low specific heat surfaces dominate down there. The terrain gradient just on the north side of the wash is substantial between the wash and Ina/Sunrise. So cold air that develops during the early night seeps down the smaller washers into the Rillito. Cold air develops quickly enough on clear, dry nights to effectively provide a stable cushion of cold air that often keeps the wash calm all night.

So, low temperatures up here near the wash are usually cooler than those at TUS, regardless of time of year. The really big differences occur on nights with strong radiational cooling and enough synoptic pressure gradient to keep winds blowing some during the night at TUS.

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