How do I find out when the morning fog will be the thickest?

September 27, 2008 6:35 PM

Date for thickest morning fog?

During this time (late September / early-mid October) nearby parks and farms (in West Coast Canada), there is always a thick sheet of fog in the early mornings. I want to know / estimate which upcoming days the fog will be the thickest. I'm not sure what temperature and what other factors come into play (ex: weather and humidity levels during the night before, rain).

Any fog / weather experts wanna give me some pointers?

posted by querty to science & nature (3 comments total)

The presence of ground moisture, very weak winds, and strong radiational cooling with clear skies are going to be the main contributors to a foggy morning. That far out I would look at the charts here and go day by day to see how it looks. Sep 29 - October 1 look plausible for fog. October 2-9 looks bad for the fog you want since a big weather system will be mucking up the coast. October 10 looks good. This all may change somewhat since model accuracy is not too good beyond 5-10 days.
posted by crapmatic at 8:14 PM on September 27, 2008

it's pretty difficult to accurately forecast fog beyond 24 hrs out (accurately meaning timing and intensity, as opposed to just presence of). climatology reports (almanacs) can give you an average number of days per month, but that's about it (this was the best I could find for Canada, and doesn't provide fog/visibility statistics).

here's a rundown of the ideal conditions for fog so you can maybe make your own forecast, or at least sanity-check someone else's:

1. clear skies overnight - without the insulating effect of a layer of clouds, the earth can radiate heat out into space, helping to form a temperature inversion, usually about 1000ft thick. a temperature inversion is when the temp at the surface is colder than the temp at, say, 1k ft. cloudy skies overnight will inhibit fog formation.

2. light winds - optimum winds for fog formation are 5-8 knots. calm winds don't provide enough stirring action to help the fog bank build, while too much wind will tear it apart.

3. dew-point depression should be less than 5C. the dew-point depression is the difference between the temperature and the dew-point temperature. both are usually provided on any weather observation that you would pull up online (here's one source). 5C is pretty much the limit. for really heavy fog it will typically be zero (meaning that temperature = dew-point temperature). so if tomorrow morning's forecast minimum temperature is within 5C of this morning's minimum dew-point temperature (provided no fronts have passed in the interim) then this is a good indicator for fog tomorrow morning.

4. terrain effects - you mentioned parks and farms, and this is a good lead-in to terrain effects (there are a lot of 'em). first is the urban heat-island effect, which can inhibit fog formation, and helps explain foggier rural areas. another terrain effect is lakes/rivers/ponds that provide a moisture source. keep wind direction in mind with this one, as it will determine which way a fog bank spreads from it source. next is mountains/valleys. obviously fog likes to form in valleys, primarily because cold air sinks. but also keep in mind that fog will persist longest on westward-facing slopes, or in a field with tall trees along its eastern side which can shelter a fog bank after sunrise. i saved the ocean (i guess that's a terrain effect) for last. sea fog is typically pretty easy for forecasters to see coming, and depending on how close you are to the coast, may not be an issue.

(i should caveat this by stating that all my forecasting experience has been in central-southeast US)
posted by bilgepump at 8:31 PM on September 27, 2008 [3 favorites has favorites]

If your fog is advection fog from the shore, I would expect it to be increased by an inland low and/or a light onshore wind, in addition to the factors bilgepump mentions.
posted by exogenous at 11:04 PM on September 27, 2008