WXforum.net
Weather Software => Station Software Development => Topic started by: iisfaq on June 11, 2011, 01:43:17 AM
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From WikiPedia http://en.wikipedia.org/wiki/Fog#cite_note-4 and other online sites
Fog can be considered a cloud at ground level. The processes forming it, however, are usually different from those that form clouds. Like clouds, fog is made up of condensed water droplets which is the result of the air being cooled to the point (actually, the dewpoint) where it can no longer hold all of the water vapor it contains.
Fog forms when the difference between temperature and dew point is generally less than 2.5 °C or 4 F.
The difference between Fog & Mist is that Fog reduces visibility to less than 1 km (5/8 statute mile) whereas Mist reduces visibilty to 1km or more.
How do we determine visibility?
I guess based on this as a starting point we could have:
function bool IsFoggy(double TemperatureInF, double DewPointInF)
{
if (TemperatureInF > DewPointInF)
if (TemperatureInF - DewPointInF <= 4)
return true;
else if (DewPointInF - TemperatureInF <= 4)
return true;
else
return false;
}
Is this just too simple or is it about right? should we also be looking at humidity?
Also from WikiPedia
For aviation purposes in the UK, a visibility of less than 2 km but greater than 999 m is considered to be mist if the relative humidity is 95% or greater - below 95% haze is reported
Does humidity also affect fog? I found a reference to humidity being 100% for fog?
chris
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No fog outside tonight
TemperatureInC = 5.86c
DewPointInC = 5.83c
Difference = 0.03c
Humidity = 96%
Baro = 1013hpa
There is dew on the ground/car roof.
Does the temperature also need to be below a specific level?
I have a trial version of Weather Display and it has these options under Universal Setup / Weather Icon Setup / Fog
FogIcon Temperature Threashold of 8C
FogIcon Humidity Must be 100%
Low Wind Speed Threashold of 5kts.
So based on that all I need to do is wait until humidity hits 100% and then maybe fog?
Chris
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Do you always get Mist prior to Fog?
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What about wind too? Usually is calm or still when it is foggy outside. It can be misty and windy too.
Dave
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We get fog when the three parameters Tdp, Twb and Tdb "coincide, " ie: Tdp = Twb = Tdp
Thanks
I will have to find a formula to Caclulate WetBulb without actually having a Wet Bulb Sensor.
Chris
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Hi iisfaq,
Do you have now a calculator for this?
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Since fog is observed vs calculated perhaps you could take a different approach and calculate the probability of fog. That is, based on your calculation of current conditions, you could indicate that fog is unlikely/likely or the probability of fog is none/low/mod/high...etc. Just a thought...
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http://www.skystef.be/calculator-fog.htm
http://tinyurl.com/h25b78y
:twisted:
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http://www.skystef.be/calculator-fog.htm
is Tmin forecasted?
and where can I possibly get the forecast values of cloud amount (oktas) and geostrophic wind speed (kn) , for instance, here in the Philippines?
This is very interesting.