Fog detection?

Skip to first unread message

František Slimařík

Aug 31, 2021, 3:35:07 PMAug 31
to weewx-user

Hi all,

I got question for hardcore meteorologists here. Is it possible to detect fog based on classic meassured values like temperature, humidity, preasure, etc?
Logically I would say fog appears when dew point equals current temeperature but I guess it will not be so easy. Fogs didn't appear here this year in my locality so I am waiting for autumn to start with observations.

Thanks for hints


Aug 31, 2021, 4:19:25 PMAug 31

I wonder what the official textbook answer might be, if there is one.

I for one have seen pockets of fog over roads and in low lying areas where micro climate is very close to each other but clearly particulate or radiative cooling or something makes fog form and 40 feet away there isn't any (yet).

How much particulate matter make fog more likely, like clouds to some degree?

I know that some programs (like Weather Display) will indicate FOG in the conditions visual, but it isn't there, frequently.  I think it says is is possible, but not a sure thing.

Vaisala makes a gizmo that I think for aviation is called RVR (Runway Visual Range) that uses a light source and a receiver mounted at an angle to each other yet only a few feet at most apart, and it does a pretty good job of telling if the conditions are hazy but not sure if there is a light or heavy fog output in its data stream.

Great question.  I hope others with experience or training can chime in.

You received this message because you are subscribed to the Google Groups "weewx-user" group.
To unsubscribe from this group and stop receiving emails from it, send an email to
To view this discussion on the web visit

Greg Troxel

Aug 31, 2021, 4:19:37 PMAug 31
to František Slimařík, weewx-user
Good question, but my impression is that it is really difficult to
measure humidity above 95%. I more or less have the impression that
fog happens when temp/dewpoint are about the same and the temperature

I think you need a fog sensor that actually measures impaired light
transmission. Surely there is a norm for measuring that already. I
need one too, right after I build a seismograph.

VE4PER / Andy

Aug 31, 2021, 4:36:58 PMAug 31
to weewx-user

per request

-------- Forwarded Message --------
Subject: Re: [weewx-user] Fog detection?
Date: Tue, 31 Aug 2021 15:26:46 -0500
From: VE4PER / Andy <>
To: Greg Troxel <>

I think the sensor is referred to as transmissometer; usually in pro unattended observer stations that is used and also a similar ceilometer for cloud heights

Doug Jenkins

Aug 31, 2021, 5:59:06 PMAug 31
I wonder if you take cloud base and the humidity values to determine if you have fog. If the cloud base is low and you have high humidity, then one could speculate that fog is imminent.

Any meteorologists can weigh in?

You received this message because you are subscribed to the Google Groups "weewx-user" group.
To unsubscribe from this group and stop receiving emails from it, send an email to

Nate Bargmann

Aug 31, 2021, 6:02:58 PMAug 31
I'm certainly no meteorologist!

It seems we've had more foggy mornings this summer than in years past.
Humidity has certainly been high with only a handful of stretches with
more than a day below 50% relative humidity.

Many times it seems as though warm air over cooler moist ground is
necessary for its formation. I don't think this is a temperature
inversion, as such, but they also seem to sometimes be quite localized
and are related to some interesting effects.

I along with other radio amateurs and possibly Greg find the effects of
these weather phenomena on VHF and UHF radio propagation interesting.

- Nate

"The optimist proclaims that we live in the best of all
possible worlds. The pessimist fears this is true."
GPG fingerprint: 82D6 4F6B 0E67 CD41 F689 BBA6 FB2C 5130 D55A 8819


Aug 31, 2021, 6:29:22 PMAug 31
to weewx-user
In general, the possibility of fog developing is when the dewpoint spread is less then  5 degrees F.


Sep 3, 2021, 7:51:16 AMSep 3
to weewx-user
I'm using Weather Display with one of my stations and use the built in function to set the fog/haze conditions. It works very good.
Fog conditions:
* T < 10°C
* 95% >= humidity
* low wind or no wind
* solar radiation low

* T < 13°C
* 94% >= humidity
* low wind, low solar

I might have slightly adapted humidity threshold, I don't remember.

sreda, 01. september 2021 ob 00:29:22 UTC+2 je oseba napisala:

František Slimařík

Sep 3, 2021, 9:06:21 AMSep 3
to weewx-user

just like I mentioned in the start, it will not be so easy to predict fog. In case of my location I had following conditions this morning between 5-6AM and no fog was visible. Just condensation on the cars etc.

Temperature between 6-7°C
Humidity 97%
no wind
no solar radiation
also difference between temperature and dew point was just 0,4°C

Dne pátek 3. září 2021 v 13:51:16 UTC+2 uživatel mihec napsal:

p q

Sep 3, 2021, 11:10:52 AMSep 3
I think you need a way to measure light attenuation between two points. 

Around here, (SF Bay Area), summer fog comes with cold dry wind off the ocean/bay mixing with warm damp air over land. It's fog on the hill tops and low clouds elsewhere. Winter fog (tule fog) is extremely patchy with conditions essentially identical at one spot where it's very foggy and another a mile away where it's clear. Not sure which kind of fog you're trying to detect.

In either case I think you need something that actually detects the change in visibility. I think a light source and a detector a meter or two away would do it.

You received this message because you are subscribed to the Google Groups "weewx-user" group.
To unsubscribe from this group and stop receiving emails from it, send an email to

Peter Quinn

Les Niles

Sep 3, 2021, 11:58:28 AMSep 3
Parameters like that can predict that fog is likely, but to actually identify fog you need to observe that the visibility is less than 1 km. (Visibility between 1 and 5 km is haze or mist.) The automated weather observation systems commonly installed at airports and other locations include a visibility sensor that measures how much light is scattered in a small volume of air. The principle is pretty simple but I haven’t seen any low-cost consumer version. 


On Sep 3, 2021, at 6:06 AM, František Slimařík <> wrote:

Reply all
Reply to author
0 new messages