Spring brings foggy mornings

By: Brandon Orr
By: Brandon Orr

Meteorologist Brandon Orr discusses the ingredients that come together to create dense Spring fog and tricky travel in tonight's weather blog.

 Patchy dense fog settled across eastern Kentucky Sunday morning. Some locations reported visibility near zero at times (visibility map from Sunday at 8am below). More patchy fog, although not as dense, is expected Monday morning as well.

So how do we know this will happen? There are several ingredients meteorologists use to forecast fog.  Fog develops when the air becomes saturated, which means the air is holding as much water vapor as it can. When this happens, the invisible water vapor condenses into tiny water droplets that make up the fog, which is really just a cloud over the ground. Ask an elementary school student about this. They will be excited to explain the condensation part of the water cycle!

Now to some of the things we look for when forecasting fog:

  • Clear Skies:  There is a temperature that the air must reach to saturate and produce fog. This is the dew point temperature. The closer the temperature is to the dew point, the higher the humidity. So if they are equal, humidity is 100%. To get the temperature lower, clear skies at night allow the temperature to drop more rapidly, thus getting it closer to the dew point.
  • Light Winds: Light winds prevent warmer air from mixing down from above. This can surely ruin a fog forecast if the winds are too strong.
  • Wet Ground: The thunderstorms on Saturday kept the ground moist. This moisture evaporates, bringing more water vapor to the air.

Fog dissipates once the sun rises and starts warming the ground, which in turn warms the air from the ground, up. That is why the fog typically dissipates or “lifts” from the ground into stratus clouds above. This image is from our UPike weather camera over downtown Pikeville Sunday morning, which shows this nicely. Two hours later, temperatures warmed up enough from the dew point to dissipate the fog and there was full sunshine into the afternoon.

Hopefully you were able to learn a thing or two about one of many weather features we forecast every day. Enjoy your week!

-Meteorologist Brandon Orr (Find me on Facebook!)

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