Posted by Dean Sauskojus on December 31, 2009 at 2:33 AM
Is it possible to predict fog with good success? What do you go by, just the dew point and temp being close to the same? Thanks.
Posted by Bill Wen on December 31, 2009 at 7:41 AM
Very good question. I'd like to know as well.
Bill
Posted by David Taylor on December 31, 2009 at 10:22 AM
From Wikipedia:
Fog forms when the difference between temperature and dew point is generally less than 2.5 °C or 4 °F.
Fog normally occurs at a relative humidity near 100%.[2] This can be achieved by either adding moisture to the air or dropping the ambient air temperature.[2] Fog can form at lower humidities, and fog can sometimes not form with relative humidity at 100%. A reading of 100% relative humidity means that the air can hold no additional moisture; the air will become supersaturated if additional moisture is added.
You can read more about fog here: http://en.wikipedia.org/wiki/Fog
We recently had over a week of "Valley Fog" caused by a temperature inversion, up here in the Anchorage Bowl in Alaska. The possibilities were amazing! http://sixtyonenorth.com/wp/2009/12/foggy-daze/
Statistically, dew point and temperature are the key, but anecdotally, depending on where you live, an equally good predictor is a very clear cold night or morning after a relatively rainy, cloudy, or warmer day. I'm sure the temps and dew points are affected by this weather shift, but for me, just knowing the forecast is a fairly good barometer (pun intended).
Posted by Frank Field on December 31, 2009 at 6:05 PM
The National Weather Service is a very good source of such forecasts. NWS has a series of regional offices that handle local forecasts. The the San Francisco Bay Area, the forecast is available at http://www.wrh.noaa.gov/mtr/. Click on a city and get the forecast. Once you have a city forecast, best place to look is the "Forecast Discussion" also known as the "Area Forecast Discussion (AFD)" which you should find as a link off the city forecast page. The AFD contains a section discussing weather as related to aviation. Fog is a big issue (try landing a light airplane in fog) for aviation and thus will always show up in the aviation forecast if there is a chance of fog.
Other commenters on this thread are correct: fog forms when the temperature drops to the dew point.
Happy New Year!
Posted by Frank Field on December 31, 2009 at 6:18 PM
The National Weather Service is a very good source of such forecasts. NWS has a series of regional offices that handle local forecasts. The the San Francisco Bay Area, the forecast is available at http://www.wrh.noaa.gov/mtr/. Click on a city and get the forecast. Once you have a city forecast, best place to look is the "Forecast Discussion" also known as the "Area Forecast Discussion (AFD)" which you should find as a link off the city forecast page. The AFD contains a section discussing weather as related to aviation. Fog is a big issue (try landing a light airplane in fog) for aviation and thus will always show up in the aviation forecast if there is a chance of fog.
Other commenters on this thread are correct: fog forms when the temperature drops to the dew point.
Happy New Year!
Posted by Stan Rose on December 31, 2009 at 7:36 PM
Dean, Im an NWS forecaster, and could write a long article on this subject. First, it depends what sort of fog you are predicting--depending on where you live there are different weather patterns that are going to be favorable for fog. For example, when i lived on the N CA coast, fog was pretty reliable for long stretches of time on the coast (especially in the summer), and the trick was not so much predicitng it, but predicting how dense, deep, and long lived it was. Other sources include 'radiational fog', alluded to by Marylin--on a clear calm night it can cool to the dewpoint as a strong surface-based temperature inversion develops. A similar process occurs in mountain valleys where cold air pools in the valley overnight. You can also get fog through mixing of different airmasses, and/or following precipitation. The subject is really too detailed to cover here in this forum--if you have any specific questions feel free to email me. I think Frank gave some valuable advice--the Aviation discussion in the Area Forecast Discussion is a good place to look, as is the weather forecast for your site, and if you can find it there are also graphical forecasts of surface relative humidity.
Posted by Don Miller on December 31, 2009 at 9:20 PM
I'll give you a shorter answer.
No wind with high dew point will usual give some photographic fog in the early morning. I have felt wind in fog, but it's uncommon enough that I notice.
Great points. The most important is that there are different types of fog.When you're around bodies of water, watch for periods when air temp is really close to water temp. We get it all the time living here on the ocean, but I frequently see the same thing early or late on rivers in the Southwest. It's actually really neat when rising fog marks the course of a river. Hard to photograph without a little elevation to shoot from, but terrific to experience nonetheless.
Discussion
10 repliesBill
Fog forms when the difference between temperature and dew point is generally less than 2.5 °C or 4 °F.
Fog normally occurs at a relative humidity near 100%.[2] This can be achieved by either adding moisture to the air or dropping the ambient air temperature.[2] Fog can form at lower humidities, and fog can sometimes not form with relative humidity at 100%. A reading of 100% relative humidity means that the air can hold no additional moisture; the air will become supersaturated if additional moisture is added.
You can read more about fog here:
http://en.wikipedia.org/wiki/Fog
We recently had over a week of "Valley Fog" caused by a temperature inversion, up here in the Anchorage Bowl in Alaska. The possibilities were amazing!
http://sixtyonenorth.com/wp/2009/12/foggy-daze/
Other commenters on this thread are correct: fog forms when the temperature drops to the dew point.
Happy New Year!
Other commenters on this thread are correct: fog forms when the temperature drops to the dew point.
Happy New Year!
No wind with high dew point will usual give some photographic fog in the early morning. I have felt wind in fog, but it's uncommon enough that I notice.