Fog: What is it and what causes it?

Weather News

Courtesy NOAA

SIOUX CITY, Iowa (KCAU) – Driving to work in parts of Siouxland Thursday morning was like driving through thick split pea soup.

The fog was so thick that visibility was reduced to zero miles of visibility to around five miles in clear visibility.

The National Weather Service even issued a dense fog advisory for portions of southern Siouxland, which is where the fog was mostly sticking around.

Fog is typically seen after the sun has set, overnight, or in the early morning hours before the sun rises or while the sun is rising. Fog typically peaks shortly after sunrise according to the National Oceanic and Atmospheric Administration (NOAA).

But why was that fog so thick and what creates fog?

Fog is just droplets of water suspended in the air at the surface of the Earth, like a cloud for the ground, but it can also be dangerous when it reduces visibility.

Fog forms from a few different reasons but the main reason behind fog formation deals with current temperatures and dew points.

When the surface temperature decreases overnight, there’s a chance it can be equal to or within a couple degrees of the dew point temperature.

Dew point is the temperature that air needs to be cooled to at constant pressure for it to reach a relative humidity of 100%. The higher the dew point rises, the greater the amount of water vapor, or moisture, there is in the atmosphere.

When dew point and current temperatures are within a few degrees of each other or the same temperature the more humidity there is in the atmosphere, meaning the more sticky it feels outside.

When they are close or the same, it also creates fog, which is typically seen overnight into the early morning hours, dissipating as the sun continues to rise.

Fog typically forms when:

  • The surface temperatures decrease to the dew point temperature, warm moist air comes in contact with colder surfaces.
  • The dew point increases to the temperature through evaporation, air moves over a moist surface like a lake, ocean, rain-soaked ground, or other bodies of water.
  • A combination of both.

There are several different types of fog (five different types):

  • Radiation fog
  • Advection fog
  • Upslope fog
  • Rain-induced fog
  • Steam fog

The fog Sioux City and portions of Siouxland received early Thursday morning was considered radiation fog. This type of fog forms when the ground cools at night due to radiational cooling. This happens when conditions include clear skies and light surface winds.

With radiational fog, it is often patchy, shallow, and low in areas, meaning that it can be random patches of fog for a few feet and then clear a few feet later. The lowest visibility with this type of fog occurs right around sunrise and it typically dissipates around mid-morning.

Fog dissipates because of daytime heating. As the sun rises, it warms up the surface by conduction and it becomes saturated, which is why it typically dissipates around mid-morning. By the mid-morning hours, the sun has been up for a few hours and has been heating up the surface throughout it rising.

As the sun continued to rise in Siouxland Thursday morning, the fog slowly started dissipating, gradually increasing visibility.

When driving in fog, it is important to remember a few things the NOAA recommends:

  • Fog reduces visibility so it’s best to slow down and allow for some extra time to get to where you need to go.
  • Always drive with your headlights on so other vehicles can see you.
  • Driving with the brights on (high beams) only makes the fog look thicker, which further reduces visibility.
  • Leave extra distance between you and the car in front of you.
  • When visibility is drastically reduced, it’s best to follow the lines of the road with your eyes to help you stay in your lane.
  • If you do not feel comfortable driving in thick fog, slow down, and turn your hazards on so others see you and pull off at a safe location like a parking lot or local business.
  • If there is no place to pull off like a parking lot of local business, pull off the road as far as possible, turn the car off, including all your lights except the hazards, set the emergency brake, and take your foot off the brake pedal to make sure the tail lights are not light up.

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