Air humidity at 99% in dense fog

Here in the UK (Cambridge) we have been shrouded in dense fog all day. My Tempest is reading 100% humidity, but my Air is doggedly stuck at 99%. Is this a slight calibration issue, or does the RH not actually need to be 100% for fog to form?

I had the same issue. the air only can reach 99%. The tempest can do 100%.

I feel they can tweak this but they don’t.

I’m glad you posted. I was going to open a ticket. My Tempest is often hitting 100% in the morning. Seems software should keep it below 100%. Mine is obviously not the only one doing it.

I’m no expert, but why do you think your Tempest shouldn’t be reading 100%?

Peter: I am far less an expert than you are :slight_smile: I just assumed that: no fog, no mist, and beautiful clear sky would indicate less than 100% humidity.

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As with so many things regarding weather, the real answer is “it depends”. In general, in the absence of an air mass change (like a frontal passage, cold air advection, etc.) the highest relative humidity will occur in the morning since the morning air temperature drops close to the dewpoint temperature. So, the RH being close to 100% is not a big surprise. It is also not uncommon for the highest RH to occur very close to the surface and drop off rather quickly with height, especially if you are experiencing a “nocturnal inversion” … something that is very common in the morning, especially on clear calm nights. During a nocturnal inversion, the temperature rises pretty rapidly with height and the dew point drops very quickly with height as well. If this is what is happening, your Tempest may be mounted close enough to the surface to be reporting 100% but, the lack of fog/mist/clouds could be due to that rapid drop in moisture combined with the rapid rise in temperature.


…or as we say in FL… just wait 5 minutes, it’ll change…

Thank you very much for the wonderful explanation! I learn something valuable from this community every day.