Weekend weather blog: What is fog?

TWIN FALLS, Idaho (KMVT/KSVT) — You may have woken up Saturday morning to low visibility. Obviously much of the valley saw fog Friday night through Saturday morning, but why just then. Also, why didn’t higher altitudes see the same fog? First off, it looks like it’s something we’re going to have to get used to over the next few days – a pretty similar pattern should develop in the region for much of the week.

If you remember from last week’s post, we talked about how the snow on the ground traps cold air trapped in the valley – both behind the mountains and below the warmer temperatures above. This warm air residing above the warm air above is a key factor in keeping the region fairly foggy. However, this warm air has remained above the cooler air for most of the past week. Why didn’t we see that fog then?

Much of this has to do with how dry the air above the surface is. Above this block of text you will see two rather complicated graphics. This article won’t go very in depth on what these maps do, but they basically show us how temperature and humidity change with height in the atmosphere. Notice the two wavy white lines on each graph.

The left chart is from Saturday (the day we had fog) and the right chart is from Thursday (a day we had no fog). The two wavy white lines represent temperature and dew point. The farther apart they are, the less humid the air overall. Notice how Saturday the air at the surface (the bottom of the graph) was much wetter than Thursday.

When the temperature and the dew point are very close, the air will not be able to hold much more moisture. This will force this moisture to condense into water droplets, i.e. mist. Another reason for the increased fog was the more pronounced inversion (or air warming with height) on Saturday compared to Thursday.

Ultimately, it’s safe to assume that when the temperature and dew point are close (or the same), in addition to a calm pattern, fog is likely.

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