Shallow fog is defined by the AMS as the following. “In weather-observing terminology, low-lying fog that does not obstruct horizontal visibility at a level 2 m (6 ft) or more above the surface of the earth. This is, almost invariably, a form of radiation fog.” It is encoded in a METAR via the symbols MIFG. Some great photos of it are available at Everything Looks Better from Above.
One a positive note, it often burns off pretty fast during the day. On a negative note, it can form incredibly fast at night… and worse, it can be really hard to identify at night. In some cases, it may not be apparent until you are in the landing flare, when all of a sudden, visibility goes from near perfect to almost zero, at the worst possible time. In other cases, it can change from shallow fog, to a thick fog bank in a flash.
On another negative note… automated instrument systems often due not detect it, and worse, it usually occurs either right when ATC is going home, or shortly there after. Its interesting to note AMOFSG places it in group D, where sensors are considered difficult, expensive, unproven, or unreliable to utilize. Apart from the technical aspect, their is a political and economic one as well when it comes to sensors. Airliners, corp jets, and many multis engine a/c do not suffer the same adverse affects from MIFG due to their elevated cockpits… ie eye level is almost always above 2 meters, where as the pilots of many ASEL craft are stuck right in the soup. Ultimately, the lack of automation and sensor capabilities, means that if a METAR includes MIFG… it was observed by a human on the field.
Another fun part about MIFG, is that its not very predictable, rather its an issue of probability. Ie, if the dewpoint/temperature spread is wide, its very windy, and the sky is overcast, it is unlikely that any type of radiation fog will form, much less shallow fog. On the other hand, if winds are below 5 knots, temperature/dewpoint spread is under 6 deg F, the sky is clear, and the sun has recently set… it is more likely fog will form, and in many cases, a thick layer of radiation fog is more likely than shallow fog.
To add further insult to injury, shallow fog often randomly forms in agricultural microclimates surrounding a rural airports far from any weather observers. Some crops have much greater moisture retention than others, some have significantly greater total emissivity than others. The net effect is that some airports experienced drastically different radiative cooling properties and others much less so. While such make an accurate prediction of shallow fog difficult, it also meant that if airport A was socked in, more than likely a nearby airport B is clear.
It should also be noted that some airport industrial parks also lend themselves to localized shallow fog development more so than others. Namely decreased dewpoint/temperature spreads due to cooling towers and the like, combined with large surface areas of high emissivity (vacant bituminous parking lots) sets the stage for its development.
With that much gloom and doom… whats a pilot to do?
First, if a METAR indicates MIFG at night, ie it was observed by a human at the field, landing at such a location in a small aircraft is likely to result in a go-around followed by proceeding to ones alternate. In fact, if temperature/dewpoint spread is that narrow, and the winds are light, the dangers of radiation fog forming are very real… and while an approach could be attempted with most MIFG occurrences, a thick fog bank would mean heading direct to ones alternate.
As one who has encountered MIFG at night many a time, the following was my basic procedure when conditions made its formation likely.
Check in with unicom, or any pilots in the area to see what the conditions currently are. Remember, AWOS does not detect it. Granted, often times it forms after everyone has left for the day.
Overfly the field above pattern altitude and look carefully at the runway lights… it they are not bright and clear, it is likely MIFG is present. Then again, I have had the fun of things changing from when I overflew the field to when I was in the landing flare.
If the landing lights appear even the slightest bit obscured or fuzzy, making a low pass is in order. First, such will serve to scare most animals off the runway… and secondly, the proximity to the runway, in addition to the landing light will make its detection easier.
Always assume if conditions are right, MIFG will form, and your visibility just before, or in the flare may go to near zero. Always, always be prepared for a go around and followed by proceding direct to ones alternate.
Ensure the runway lights are on full bright on base… you do not want to be in the flare, encouter MIFG, and at the same time have them shut down or return to low intensity.
The landing light, and strobes should be turned off on short final… there is nothing more disturbing than having the entire cockpit light up in flashing white lights, all the while you are trying to reconfigure the a/c for a go-around.