Low clouds, those that form below 6,500 above ground level (agl) often reduce visibility making VFR flight impossible. Fog is one type of low cloud, and I am going to discuss fog in a little more detail. Fog typically forms when the air temperature near the ground is at or near the dew point. The dew point is the temperature at which the air can hold no more moisture. I found out recently that there are five, no six types of fogs. For those of you who are familiar with discussions of fog, I am using the six types of fogs as referenced in the glossary (under F) of weather terms from the Mt. Holly office of the National Weather Service.
I'll start with the sixth type of fog, ice fog. Ice fog forms in very cold temperatures, e.g. 25 to 30 degrees (F) below zero. It forms in conditions similar to radiation fog (below). Because it forms in such cold climates, ice fog is generally not a factor in aerial wildland fire fighting.
Radiation fog frequently forms at night when the ground is still warm and moist radiates against cold air near the surface. It frequently forms in mountain valleys. As the sun rises and the temperature increases, the radiation fog burns off.
Advection fog forms somewhere else, usually over water when warm air moves over the cooler surface. Wind moves the fog over land.
Sea fog occurs when a very cold and dry airmass moves over warmer water. As the water evaporates, it rises forming fog. Sea fog is then moved along by the wind. Sea fog is sometimes referred to as sea smoke or steam fog.
After a heavy rain, often near a warm front, precipitation or rain induced fog will form. In this case, the atmosphere is saturated with moisture and little if any cooling is required. G, a former air tanker pilot and certified flight instructor offers this observation about precipitation induced fog:
These are the "water dogs" seen rising out of the tree tops after a heavy rain. They are frequently mistaken as smoke by over zealous look-outs in the forest.The final type of fog is upslope fog. It forms when a moist airmass is forced uphill by wind and the up sloping topography found in areas such as hills and mountain ranges. As the air expands and is forced uphill into the cooler air upslope, the airmass cools to the dew point forming fog.
Why are these different types of fog important to aviators? G explains:
The reason you might want to know the six reasons fog will form is so that you can predict it and decide if its potential will affect the safety of your flight. Fog is fog, but it forms in very predictable circumstances. For example, "It rained like hell this morning, but it is clear and calm this evening. I think I'll go up for a sunset flight and just have some fun." That little scenario is setting you up to where you might not be able to land back at your home port because it fogs in while you are up enjoying the sunset. Did you bring along enough gas to get to a safe place to land? That alternate airport might be a long way away.
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Note (added July 2012): I'd like to thank a couple of my tanker pilot friends whom I corresponded with as I was preparing the articles in this series for their time and help. Here is a complete list of articles in this series with hyperlinks:
1. Introduction (Jan 4)
2. VFR, I can see (Jan 6)
3. IFR, I can't see but I have instruments (Jan 8)
4. Fog (Jan 11)
5. Inversions (Jan 12)
6. Flying fires: VFR or IFR? (Jan 13)
7. Reflections (Jan 18)
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