Saturday, May 5, 2012

Precip Type in Dry Air

Friday afternoon and evening brought convective showers to Fairbanks-land. However, with a dry and still cool-ish airmass in place, the form of the (light) precipitation that did occur was perhaps not as might be expected. There were a few reports of light rain in the Fairbanks bowl. However, snow was reported near the Chatanika River on the Elliott Highway, below 800 feet elevation. Here on Keystone ridge, it snowed between 330pm and 4pm, with temperatures no lower than 38F. Unsurprisingly, light snow was also reported on Wickersham Dome, elevation 2600 feet.

The graphic shows a plot of the 00Z May 05 Fairbanks upper sounding (heights are in geopotential meters: for practical purposes, just think of the y-axis as in meters). The freezing level was over 3000 feet above the ground. However, the elevation of the "wet-bulb zero" (the elevation that if you saturate the air the temperature would be at freezing) was only 1400 feet above the ground. In at least some of the precipitation shafts, the air was not saturated, allowing snow to fall perhaps as much as 2500 feet below the nominal freezing level.


  1. Thats a good illustration of why snow can fall at lower elevations than the free air freezing level due to dry air. One thing we have been taught to look at for snow levels is lapse rates for prospective convection that can locally drive snow levels lower. Is that the same phenomena?
    Matthew K.

  2. Hi Matthew,

    I think the idea is to use lapse rate as a proxy for sub-cloud saturation. So if the lapse rate is dry adiabatic, the height of the wet-bulb zero will be well below the freezing level, while if it's moist adiabatic, then the two will be functionally the same. This has been a bit of a contentious issue at work lately, which was the original motivation for the graphic.


  3. Hmm, I guess I hadn't thought about it that way. That makes sense. If you are ever interested western region has a snow level tool that acts like a serp tool, but displays model data as well. It also shows the lapse rates averaged over the area which is what made me think of it. It's useful here where we deal with snow levels nearly all winter long.

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