Q for a Met expert...

[fernytickles]

On more than one occasion, the ATIS has declared the temp/dewpoint to be the same. Digging deep into the very small storage areas in my brain, I can only recall that a close, or same, temp/dewpoint would mean saturated air, therefore visible moisture.

On each of these occasions, it has been totally clear, little or no visible moisture, but humid on arrival. For instance, this morning, it was 23'C/23'C, 7nm, Few at 7,500

Can anyone explain this to me (in words of less than one syllable, please.....)

Thank you.

[quartic]

Yes
In F it meant something in C it means less. 1 degree C is too wide a margin.

[Deano777]

Also just because Temp/Dewpoint is the same it doesn't mean there is the moisture in the air to give you saturation resulting in fog/mist, and as has already been said ½ a ° out is too wide a margin too.

[Chris Scott]

Temp "+23C" could be +23·4C; Dew Point "+23C" could be +22·5C.

But even if the two are truly identical, I'm not sure that the water droplets forming (by the condensation) necessarily have to be big enough to impede visibility. Isn't that why the visibility in some cloud/fog is better than in others? But I'm not a Met man.

Deano777, I don't think that you can have a measurable dew point – by definition – unless some water vapour is present in the atmosphere. [It would presumably be -273C if the air contained no water vapour.]

[quartic]

I was at Yellowknife in northern Canada. Temp was minus 30 something and dew point was the same. (F or C not an issue as minus 40 the same) Sky clear great day. Ground crew started the external power unit that was admittedly not up to current green standards. Much condensation and smoke ensued and the visibility went from unlimited to 50m in two minutes. We laughed and went for a coffee.

Yes Dew point and temp can be the same and in very cold air with no 'bits' in it (sorry forgot the pedants term) and - no prob - add the bits -whammee.

That technical enough?

[Deano777]

Chris agreed

I'm not saying, nor did I say no moisture present, I'm saying no appreciable moisture to cause fog, afterall we all know there is always some water vapour present.

[Flash2001]

I have seen a clear morning with the air saturated in which one aircraft took off and within 2 minutes or so the field was fogged in. In southern Ontario there is a region in which fog forms almost instantly. I think in these cases the dewpoint is actually higher than the temperature but fog doesn't form until there is a disturbance or some nucleii to form centres of condensation. I am also sure that the lost precision between F and C is important as other contributors have said.

After an excellent landing you can use the aircraft again!

[selfin]

The temp/dp values falling at xx.5 are not both rounded up. I forget which is rounded down but suspect it is the temperature. Note in Mike Wickson's met book that it's not impossible therefore to see dp higher than temp, iirc. Reference on request.

[A Very Civil Pilot]

no 'bits' in it (sorry forgot the pedants term)

Condensation nucleii. If the air is 'clear' then no mist will form. Fill it full of carbon particles, such a exhaust from the GPU, then the air has something on which to condense.

It is also the same with freezing nucleii. Saturated air temp of -15 with nothing in it, no ice. Stick in an aircraft and there's something nice and big on which to freeze.

[Agaricus bisporus]

And this situation (Equal/close Temp and Dp) should start to ring loud bells with flashing red lights in the minds of N European pilots seeing them in the Met brief.

This is the fine edge of forecasting, and beyond what is realistically achieveable. (To be fair, predicting the minutrest change in Temp, Pressure or Dew point must be almost impossible, for a tiny change in only one of these can make a T6K Ovc 0700 TAF turn into 0050 /// at minutes notice).

Thus under such a forecast, disbelieve any wx prediction as "fact" and plan your fuel and diversion accordingly.

Or you'll get caught one day...