Water around region a prime source for foggy days

January 12, 2014 

Fog blankets the South Sound last fall. The many bodies of water in the area are a prime source for the moisture that helps create fog.


Did you know you can walk on a cloud without taking a trip to the sky? Here in Tacoma, fog is a huge part of our weather system, but fog is simply a cloud that appears close to the ground instead of up in the sky. When you head outside and there is fog on the ground you can truly say, “I’m walking on the clouds.”

Water in our atmosphere constantly evaporates and condenses through the clouds, an important part in our planet’s water cycle. Even when the skies are a clear blue without any clouds in sight, water is still there in the form of vapor and small droplets that cannot be seen.

Clouds form when water droplets mix with particles of dust, salt and smoke. As the droplets grow in size, clouds develop.

Temperature plays a big part in cloud formation because the lower the temperature, the more energy it takes for molecules to separate from each other. As the molecules increase in density, the water vapor in the air are converted into liquid.

Formation of clouds full of moisture is crucial for the water cycle as they precipitate and give out much needed water in the form of rain. In the water cycle, condensation is the exact opposite of evaporation which takes water away from the surface of the Earth.

You also might have heard the term “dew point.” That occurs when the air can no longer hold the water vapor it contains. When this occurs in overhead clouds, we experience rainfall or snow depending on the temperature.

So, what causes fog?

Fog is a ground level cloud. When condensation occurs at the Earth’s surface, instead of in the sky, fog forms. Fog is a phenomenon caused when air that contains a lot of water vapor comes in contact with a colder surface, such as earth, which causes it to cool down to its dew point.

Fog consists of millions of condensed water droplets that form when air is cooled to its dew point. Fog can appear over many areas, but is most often formed near creeks, rivers and other bodies of water. Because our area has so many bodies of water around it, the Northwest is a perfect region for fog.

It’s hard to believe, but there are many types of fog, including advection, radiation, sea, valley and freezing fog. The two most common are advection fog and radiation fog.

Advection fog is caused by moist warm air moving over a cool surface like a body of water and is common along the Pacific Coast. Radiation fog, or ground fog, is formed when there are clear warm skies with low winds. The fog is caused when there is a rapid loss of heat from the Earth’s surface and is most commonly found in valleys.

Ever wonder where the fog disappears to as the day goes on?

When the sun starts to peek through the clouds, the fog we wake up to begins to evaporate; the thicker the fog, the longer it takes to dissipate.


1. Tape a strip of black paper three-fourths of the way around the mid-section of a one gallon glass jar.

2. Fill a one-gallon ziptop bag with ice.

3. Fill one-third of the jar with warm water. Add food coloring, if available.

4. Light a match and hold it over the jar opening. After a few seconds, drop the match into the jar and immediately cover the top of the jar with the bag of ice.

As the warm water heats the layer of air that it touches, water will evaporate into the air forming water vapor. The air containing water vapor rises and cools as it comes in contact with the air is cooled by the ice. When the water molecules cool, they slow down and stick together more readily. The particles of smoke act as nuclei for “bunches” of water molecules to collect on. And voila, you are an official cloud-maker.



Visit a local park with a lake or stream to safely experience the feeling of walking in the clouds on a foggy morning. Notice how you can feel the weight of the air against your skin from the heavy water vapor in the air. If you have the time, as the sun begins to shine you can watch as the fog vanishes before your eyes.

To locate a park near you with a water source, where fog is more likely to form, go to metroparkstacoma.org/ parks-with-water.

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