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Is it Fog or a Cloud?

Is it Fog or a Cloud?

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I work at snowbird (8000' above sea) as a night time resort shuttle driver. When the visibility is low with out snow, my passengers always make a remark and say, "it sure is foggy". I live in Sandy and I know it's blue skies down there. I call this low visibility style moisture in the mountains, clouds. My question is.... when clouds reach the snowbird and Alta village, do you call it fog or clouds? My only explanation to people is, when you are down in the SLC Valley, you don't point up at the mountains covered in clouds and say, "look at the fog up in the mountains". The only time I can remember about fog in the mountains is in the Sierra Nevada mountains on I-80 well above Reno and closer to Truckee. There is a area there that has signs and warns for toole fog and I have often seen it. I don't think I've ever seen toole fog in Little Cottonwood Canyon. Am I on the right track of opinion? If so, why can some mountains have fog and others can't? Some of my friends say, "when clouds reach the ground, it's fog". I think this is a false statement. Please clear this all up.

Thanks, Brett


A great question Brett. After a little research, I can't find any info on said 'toole fog'. As a meteorologist, I have never heard of such, but I'll run it by some more senior members of our team this afternoon. There are different types of fog including advection fog, radiational fog, upslope fog and steam fog.

The low visibility at night where you are is probably a fog. The American Meteorological Society glossary defines fog as "water droplets suspended in the atmosphere in the vicinity of the earth's surface that affect visibility". Fog is different from clouds since the base of the fog is at the surface and clouds are higher up. However, knowing what goes on at the ski area, when clouds are thick and snowing on us and you can't see anything, you certainly are in it and not just fog.

You can also tell what's happening if it was clear until the sun went down and surface fog begins to develop. Clearly, that isn't a cloud, it's fog.

The difference in definitions on the cloud are that a cloud is defined as "A visible aggregate of minute water droplets and/or ice particles in the atmosphere above the earth's surface". So it's the surface factor that makes the difference.

Saying when a cloud reaches the ground it's fog isn't a totally wrong statement, it's a very fine line between the two. Clouds usually form in the free atmosphere vs. Fog which will form by interaction with the moisture at the surface of the earth.

There are various reasons why some mountains would have fog on them while another wouldn't at the same time of night. You can have orographic clouds or fog (upslope) which forms when the air is rising over the windward side of the mountain. So depending on which face of the mountain you are on, you can have a cloud on one side and not the other.

Another reason would be that even though say Alta and Solitude are relatively close together, the wind conditions could be different. If the wind was calm at Alta and the conditions were right for radiation fog, you could have fog there. But if the winds were gusty for some reason at Solitude, then the conditions wouldn't be right for fog there. There are a lot of fine variables the go to make fog.

In the springtime we get a ton of advection fog which wouldn't shock me if you are seeing a lot of that this time of year in April. Advection fog is nasty and can get very thick and if you are over a snow covered area that is large (like a ski slope) it can go over the whole thing! Advaction fog forms when warm air moves over cold ground. The warm air is cooled to its dewpoint and fog is formed. If you're ever in Vermont in the springtime at night this fog is some really bad news on highways.

In short, if it's at the surface it's fog, if it's above it, it's a cloud. When cloud is at the surface it's not completely wrong to call it fog but just remember, they form in different ways.

Answered by KSL Meteorologist Dina Freedman.

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