Happy Monday and welcome to another edition of Beyond The Forecast!
The warmer-than-average temperatures we have to start this year definitely make it feel comfortable. With these nice temperatures, we could get more fog than you would expect in January. Differences in heat and recent rainfall are two factors that greatly increase the chance of fog.
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There are four main types of fog common in the United States:
- Radiation Fog
- Precipitation Fog
- Advection Fog
- Upslope Fog
When you hear the name Radiation Fog, you might think of a high schooler being bitten by a cloud in a science lab, but unlike Peter Parker, it comes from a completely natural process. Any object that gives off heat is technically radiating; a great example of this is the Earth itself. Heat from the sun warms up whatever it touches, and the Earth is able to hold on to some of that heat and then give it off as heat radiation.
Warmer temperatures near the surface (especially when there is leftover moisture from rain) help raise the dewpoint temperature. As temperatures drop through the night and reach the dewpoint, water vapor condenses just like it does when clouds form in the sky.
Radiation fog is common in valleys or near bodies of water, where temperatures are generally warmer overnight than open ground. Winds must be calm for radiation fog to form; if winds are strong, temperatures mix more and make it harder to cool to the dewpoint.
Precipitation fog forms when relatively warm rain falls through cold air close to the ground. The colder air allows the rain to evaporate. That evaporation uses up some of the heat in the atmosphere.
The then cooler temperatures let the water molecules condense again, but instead of heavy raindrops that fall down, these small drops are light enough to stay suspended in the air.
Advection fog is all in the name: advection is the process of moving an atmospheric trait from one location to another. Warm air coming up from the south is called warm air advection, and it’s the advection of moisture into a dry and cool region that allows this fog to form.
The cold air close to the ground cools the incoming warm air that’s carrying moisture. Once the air advecting in reaches the dewpoint, we get fog.
Upslope fog is similar to advection fog, but the main difference is a change in elevation. Warm, moist air travels towards a slope and then starts to cool as it rises. This is a very similar system to the one that creates clouds in the sky but aided by the natural change in height a hill or mountain provides.
As temperatures drop they get closer to the point where the water vapor in the air can condense. This fog is the kind that obscures mountain tops when there is plenty of moisture near the ground to carry.
With more rain coming this week, we could very well have reduced visibility through the middle of the week. Keep up to date on what’s headed our way; you can download our weather app and get Meteorologist Chris Michaels’ latest updates online.
You can always get specific forecast details for your zone, whether it’s the Roanoke Valley, the Lynchburg area, the New River Valley or elsewhere around Southwest and Central Virginia, anytime at WSLS.com/weather. Know your zone!
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-- Marshall Downing