What makes a foggy day?

Answered by Nicole Peterson, Meteorologist, National Weather Service-Pocatello Office

Fog is simply a cloud, but on the ground. In order for fog to form, there needs to be moisture in the air. Fog typically forms overnight when the air near the ground cools, and disappears in the morning when the sun warms up the air. Usually, winds are light and there are no other clouds in the sky. But fog can form in many different ways – did you know there are 6 different kinds of fog?

Fog is known to form typically in two different ways in Idaho – called Valley Fog and Radiation Fog.

Valley Fog development is a two-step process: Part 1: The ground will cool as heat gathered from the sun’s rays during the previous day is released back into the air. This cooler, denser air at mountain top level will sink into the valley floor and stay there.

Part 2: Moisture is needed in the valley for fog to form. As the moist air cools, the moisture in the air condenses into very small water drops, forming the fog. This is a similar process to how small drops of water form on the outside of a cold glass of water or can of soda pop on a warm day.

Radiation Fog is another common type of fog and forms similarly to Valley Fog without needing the presence of a mountain or a valley. Air near the ground simply cools overnight, aided by clear skies and light winds, and fog forms as the moisture in the air condenses into very small water drops. Fog can sometimes last for days and days! Seattle has gone 13 days with persistent fog before. Imagine that!

Dew Point

The temperature at which water vapor in the air condenses, or turns to liquid, is called the dew point. There is no single dew point, however. It depends on how much water vapor is in the air. If there is a lot of water in the air, the dew point is a high temperature. If the air is dry, the dew point is a low temperature. Once the dew point is reached up in the sky, it causes clouds, rain, snow, and other precipitation. If it happens near the ground, condensation causes dew, frost, or fog.

Check the Dew Point

Materials: Metal or glass cup, bowl of water and ice cubes, thermometer, warm water, syringe

  1. Use the thermometer to note the temperature of the air.
  2. Fill the metal or glass cup one third of the way with water that is about 85° F.  Place the thermometer in the water.
  3. Using the syringe, slowly add small amounts (2 tablespoons of so) of ice water and watch the thermometer.  Continue adding small amounts of ice water until the temperature of the water drops a degree or two each time ice water is added.
  4. Continue adding ice water until condensation begins to appear on the outside of the cup.  You have reached the dew point.
  5. Try the experiment on a day that has different weather–more cloudy, foggy or clear.  Do you have to add more or less water to make condensation appear?

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