Condensation: Understanding the nitty-gritty about fog

KOIN 6 Weather Kids

A layer of fog over downtown Portland on November 5, 2019. (KOIN)

PORTLAND, Ore. (KOIN) – Have you heard of Casper, the Friendly Ghost? Well, today we will be learning about fog, the friendly cloud!

It is the cloud that we are in touch with the most because it dominates space at the ground. Fog sure does bring eeriness to our scenery too. You know those mornings when you look out the house window and all you see is gray and you know the fog has arrived.

However, fog isn’t always friendly, since it can cause trouble for our travel routine. Fog can reduce visibility making it very hard to see beyond a short distance. There are mornings where fog can reduce visibility so you can’t even see a few feet in front of you!

Weekend Meteorologist Joseph Dames

With all that in mind, fog doesn’t always get the respect of other clouds. Fog is sometimes not even considered to be a cloud because the base touches the ground. Although the makeup of fog is that of other clouds, it can be considered a hazard that is measured by visibility.

What exactly is this ground cloud? Fog by definition is water droplets suspended in the atmosphere. Fog is situated at the ground and impacts visibility.


When the KOIN 6 Weather Team is forecasting fog in the Willamette Valley, it means we are noticing a set of parameters that will likely give a helping hand to fog development. You can find the list in the photo above, but the parameters we are analyzing include:

wind speed – wind direction – moisture content – overnight sky conditions – temperature

Fog forms when water vapor condenses to liquid droplets. That is when the temperature and the dewpoint reach a point of similar value. For example, the temperature outside is 45 degrees and the dewpoint is equally 45 degrees. We learned about the different phases of water and the process between changing from one state to another in this weather kids lesson. That lesson will help clarify the process of condensation.

You may not have known that there are multiple forms of fog! Some forms of fogs are more prevalent than others, because of water sources or valleys. There are several different types of fog:

ground fog – advection fog – precipitation fog – valley fog – freezing fog – upslope fog

The photo above is a representation of ground fog. That is a common variety of fog and the likely candidate when we see fog develop during the morning. This fog forms when all the energy from the sun from the day decides to bail (overnight cooling). It is that process that allows the temperature to meet up with the dewpoint. That fog can be even more dense if it had recently rained and created a moist ground. The higher the dewpoint, the easier it will be for fog to form because the temperature will not need to be cooled as dramatically. Lastly, you need the wind to be light. The stronger the wind, the harder it will be for fog to form because it will mix in dry air!

Way to go, you’ve finished this lesson! Take a look at some of the photos below and some additional graphics about fog.

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