### Author Topic: What is Fog and how to calculate if it is foggy?  (Read 11666 times)

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#### iisfaq

• Christchurch Weather Live
• Senior Contributor
• Posts: 209
##### What is Fog and how to calculate if it is foggy?
« on: June 11, 2011, 01:43:17 AM »
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

#### iisfaq

• Christchurch Weather Live
• Senior Contributor
• Posts: 209
##### Re: What is Fog and how to calculate if it is foggy?
« Reply #1 on: June 11, 2011, 06:57:57 AM »
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

« Last Edit: June 11, 2011, 07:04:51 AM by iisfaq »

#### iisfaq

• Christchurch Weather Live
• Senior Contributor
• Posts: 209
##### Re: What is Fog and how to calculate if it is foggy?
« Reply #2 on: June 11, 2011, 07:05:51 AM »
Do you always get Mist prior to Fog?

#### 130dB

• Member
• Posts: 10
##### Re: What is Fog and how to calculate if it is foggy?
« Reply #3 on: June 26, 2011, 05:35:49 PM »
What about wind too? Usually is calm or still when it is foggy outside. It can be misty and windy too.

Dave

#### iisfaq

• Christchurch Weather Live
• Senior Contributor
• Posts: 209
##### Re: What is Fog and how to calculate if it is foggy?
« Reply #4 on: July 24, 2011, 07:50:46 PM »
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

#### dean.martin

• Senior Member
• Posts: 74
##### Re: What is Fog and how to calculate if it is foggy?
« Reply #5 on: July 04, 2016, 04:53:54 AM »
Hi iisfaq,

Do you have now a calculator for this?

#### floodcaster

• Forecaster
• Posts: 582
##### Re: What is Fog and how to calculate if it is foggy?
« Reply #6 on: July 04, 2016, 08:10:43 AM »
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...
Bill

#### Cutty Sark Sailor

• WxElement panel
• Forecaster
• Posts: 3023
##### Re: What is Fog and how to calculate if it is foggy?
« Reply #7 on: July 04, 2016, 09:53:20 AM »

#### dean.martin

• Senior Member
• Posts: 74
##### Re: What is Fog and how to calculate if it is foggy?
« Reply #8 on: July 04, 2016, 09:59:12 PM »

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.