Nowhere is this more apparent than in Utah's West Desert. Check out the observations from 1347 UTC (0747 MDT) this morning. It was an incredible 55ºF over the salt playa along I-80, but as cold as 38ºF over the sagebrush region of Dugway Proving Ground to the southeast.
Zooming into the Dugway Proving Ground you can really see the intensity of the inversions that formed overnight. Temperatures on the valley floor (~1310 m) range from 38–52ºF. On the summit of Camel Back Mountain (just below the center of the image, 1547 m), it was 65ºF.
This variability is related to the formation of nocturnal cold pools due to radiational cooling at night. The coldest temperatures are often found in those low elevation areas where the flow is weakest as the lack of turbulence and mixing enables the radiational energy loss to be concentrated in a shallow layer. Areas with stronger flows, such as near gaps in the terrain (e.g., the 52ºF observation above is just west of a gap and the flow was 15 knots compared with 5 nots at the other sites), see more mixing and higher temperatures. The coldest temperatures on nights like this are typically in basins where the cold air is trapped and the air becomes calm.
The warm nighttime temperatures over the salt playa are a bit of an anomaly. Based on topography alone we would expect to find the coldest temperatures along I-80, but it turns out that the thermal characteristics of the playa are markedly different than that of the surrounding sagebrush region. We will talk about this in a future post.
I have heard that parts of the Sahara Desert can drop from 120F in the afternoon to the 40s/50s at night. I used to be puzzled about this, since there are many other desert areas (like Phoenix) that don't cool below the 90s at night after a 120-degree day. Not to mention Death Valley's low of 107 this summer. However, much of the Sahara Desert is loose sand (in contrast to city heat islands like Phoenix, and other desert areas with heat-conducting surface material). Loose sand, like fresh snow, cools very quickly at night. So it is really amazing how much difference the terrain surface can make.
ReplyDeleteI'd be interested to hear more about diurnal temperature variations at the same location. Seems like 20-30 degrees is pretty typical around SLC but I think I've seen quite a bit more in the desert.
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