Wednesday, December 2, 2015

Bloody Cold in Some Spots

With generally calm, clear, dry weather in place over northern Utah, overnight minimum temperatures are getting way down there in some spots.

Cache Valley: Lewiston -1ºF, Evans Farm 0ºF, Logan Airport 3ºF
Bear River Valley: Randolph -13ºF, Pixley -14ºF
Rush Valley: Faust 0ºF
Uinta Basin: Duschesne 1ºF, Maeser 3ºF

Meanwhile in the friendly confines of the Salt Lake Valley, we hit minima of 22ºF at the University of Utah and 19ºF at the airport.

The Cache Valley, Bear River Valley, Rush Valley, and Uinta Basin are well known for low minimum temperatures.  These are basins or valleys that are largely enclosed, resulting in very light nighttime winds during periods of weak flow.  The lack of flow and associated turbulence reduces the mixing of air near the surface at night, allowing temperatures near the basin or valley floors to drop very low.  Note that this has nothing to do with "cold air flowing to lower elevations" as is commonly claimed.  Studies show repeatedly that shortly after sunset the flow in these very cold low-elevation areas typically becomes quite weak, which limits mixing and allows the air near the surface to cool dramatically.

In contrast, the Salt Lake Valley is more open, enabling a steady nocturnal drainage flow that helps keep the surface layer a bit more mixed and warmer.  The urban heat island helps as well.

There is one other factor presently at play in the Cache Valley, Bear River Valley, and the Uinta Basin: Snow cover.  Yesterday's modis image shows extensive snow cover in these areas, in contrast to the Salt Lake Valley, which has only spotty coverage.

Snow affects the surface energy balance in several ways.  During day it reflects more sunlight back to space than bare ground (in most instances).  At night, however, it reduces the flux of heat from the ground to the atmosphere, which allows the air near the surface to cool more than over bare ground (in most instances).

Ultimately, the presence of snow leads to stronger inversions within the valleys and basins of Utah. We're fortunate to be snow free in the Salt Lake Valley at the moment.


  1. You cannot forget Peter Sinks in a conversation like this!

  2. -37 at Peter Sinks on the 29th. Rush Valley is crazy cold when they get deep snow cover on the ground, similar to Randolph.

  3. Where do you get the MODIS images and how frequently are they available? A quick search of the U site and the NASA MODIS site didnt reveal what I was expecting. (This one in particular is interesting due to highlighting/contrasting the lesser mountain ranges against the basins and valleys as a result of the snow elevation.)

    1. The images I use around Utah are from the Great Salt Lake Basin subset and directly available at This is also useful: