Dewpoint & Night VFR

[Goose]

If you're lucky enough to be flying near civilization, haloes forming around streetlights etc. are a great indicator of deteriorating dewpoint spread and usually means fog could form in the very near future.

[JW Scud]

[quote author=Colonel Sanders link=topic=5330.msg13745#msg13745 date=1483998111]


[quote]What can happen when the dewpoint spread drops to zero?[/quote]

WOXOF.  But not necessarily.  Some mornings, you will see the temp and dewpoint the same (very common with clear skies) and no cloud forms.
[/quote]


I have seen the clear skies with temp and dew point the same. Kind if surprised me. Any explanation as to why no fog?

[Colonel]

This is called physical chemistry.  The phase change
did not occur because condensation of the saturated
or super-saturated heterogeneous mixture did not occur.

Nuclei (eg dust) can cause condensation.  A single jet
taking off can actually fog in an entire airport with no
dewpoint spread - it's happened before.

Any questions about physics or chemistry, just ask
your local TC Inspector.  They have all the answers.

[Barneydhc82]

I can think of one day, winter, at RCAF Namao where a C-47 taxiied out for run-up and within 10 minutes we went from severe clear to WOXOF in ice fog.  All it takes as Andy sez is a bit of junk in the mixture and a bit of a disturbance...FOG!


And many evenings at Blunder Bay with a couple of C-152s doing circuits and...FOG...beautiful fog!



Barney

[Colonel]

This can be inconvenient if you are inbound at the time.

Beware the zero dewpoint spread.  It can be reporting
clear, but unless the sun is out and already hot, it can
turn into instant fog because of one aircraft.

Old-timers know this, but today's pilots do not.