Weekend Weather Blog: What’s with the fog?

Some areas in the Heartland will have patchy to dense fog this morning.  (Source: CNews/William...
Some areas in the Heartland will have patchy to dense fog this morning. (Source: CNews/William Foeste)((Source: CNews/William Foeste))
Published: Jan. 15, 2022 at 8:11 PM MST
Email This Link
Share on Pinterest
Share on LinkedIn

TWIN FALLS, Idaho (KMVT/KSVT) — You may have woken up Saturday morning to low visibility. Obviously, much of the valley saw fog Friday night into Saturday morning, but why just at this time. Also, why didn’t the higher elevations see the same fog? First and foremost, it looks like this is something we’re going to have to get used to over the next few days - a fairly similar pattern is expected to develop over the region throughout much of the week.

If you remember last week’s post, we talked about how the snow on the ground is keeping cold air trapped in the valley - both behind the mountains and below warmer temperatures above it. This warm air residing above the warm air above it is a key factor in keeping the region fairly foggy. However, this warm air was remaining over the cooler air for the better part of last week. Why didn’t we see this fog then?

Soundings (Plymouth State Weather Center)
Soundings (Plymouth State Weather Center)(Plymouth State Weather Center)

A big part of this has to do with how much drier the air above the surface has been. Above this block of text, you’ll see two fairly complicated charts. This post won’t go terribly in depth as to what these charts do, but in essence they show us how temperature and moisture change with height in the atmosphere. Notice the two wavy white lines on each chart.

The chart on the left is from Saturday (the day we had fog), and the chart on the right is from Thursday (a day we didn’t have fog). The two wavy white lines represent temperature and dew point. The further these are away from each other, the less moist the overall air is. Notice how, on Saturday, the air at the surface (the bottom of the chart) was much moister than Thursday.

When the temperature and dew point are very close together, air won’t be able to hold much more moisture. This will force this moisture to be condensed out into water droplets, a.k.a, fog. Another reason for the enhanced fog was the more pronounced inversion (or air warming with height) on Saturday compared to Thursday.

In the end, it’s safe to assume that when the temperature and dew point are close together (or he same), on top of a calm pattern, fog is likely.

Copyright 2022 KMVT/KSVT. All rights reserved.