Sunday, September 16, 2012

Big Diurnal Temperature Ranges

If you want to see huge swings between maximum and minimum temperatures, September is probably your month.  There's still a decent amount of incoming solar radiation during the day, the soils are dry so much of this energy goes into heating rather than evaporation and transpiration, and the nights are moderately long.

This enables us all to enjoy warm days and cool nights, but where is the difference between the minimum and maximum temperatures, which meteorologists call the diurnal temperature range, the largest in northern Utah?

The best place to look are in basins that are either fully enclosed or only allow cold air to leak out through very constricted channels.  One such place is the Rush Valley, which is located southwest of Salt Lake City and the Oquirrh Mountains.

Overnight minimum temperatures on 16 Sep 2012.  Ignore misleading
caption at upper left!
Despite its name, the Rush Valley is really an enclosed basin.  Passes to the north to the Tooele Valley, east to the Utah Valley, and to the south are all higher than the valley floor, where temperatures plummet on clear nights.  Last night, the minimum temperature at the Clover observing site plummeted to 36ºF, much cooler than sites to the south that are 100–200 m higher (the slightest variations in elevation matter in conditions like these).  Over the past two days, the diurnal temperature range at Clover has been almost 50ºF.

That's impressive, but we usually get data from another site just to the south called Pony Express Marker that gets a few degrees colder than Clover at night.  Unfortunately, that data is not coming in today.

Those with a keen eye might notice in the top image that a site in the Heber Valley bottomed out at 33ºF.  The Heber Valley is not a basin, but it is fairly large in volume with cold air only able to exit via Provo Canyon, which is fairly narrow.  The diurnal temperature range at the Heber airport the past two days has also been around 50ºF.

Finally, we have the Peter Sinks, limestone sinkholes located in the Bear River Range east of Logan, which holds Utah's all-time minimum temperature record of -69ºF.  As can be seen in the image below, the temperature at the bottom of the sink last night bottomed out at about 22ºF, compared to only 48ºF on the rim of the sink just to the north.

Overnight minimum temperatures on 16 Sep 2012.  Ignore misleading
caption at upper left!
These sites are less than a kilometer apart, with a difference in elevation of only 100 m!  We did a field program in the Peter Sinks several years ago and I froze my butt off but good on a mountain bike descent to the bottom of the sinkhole one morning.  From shorts to hypothermia in less than a minute!

It looks like the diurnal temperature range in the Peter Sinks has actually been somewhat smaller (~40ºF) than found in the Rush and Heber Valleys the past two days.  I'm a bit surprised it isn't larger as 50+ºF temperature ranges have been documented under similar conditions.  It could be the observing site is not quite in the sweet spot.


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