Hi,

I have searched both the wider Internet and this group's FAQ but am still
struggling with what appears to be a basic concept. Can anyone enlighten me?

Information on dew point is widely available, and from that I understand
that, broadly speaking, when the temperature and dew point are the same,
moisture will be precipitated and form mist or fog (or, I guess, dew). I
have a formula to calculate dew point given relative humidity and
temperature, and my calculated figures match published figures. However (and
here's my problem) as the temperature drops over night, so the calculated
dew point drops. So for example, at a constant relative Humidity of say 45%,
as the temperature drops, you get something like this:

Air temp 25 degC = dew point 12.2 degC
Air temp 20 degC = dew point 7.7 degC
Air temp 15 degC = dew point 3.2 degC
Air temp 10 degC = dew point -1.4 degC
Air temp 5 degC = dew point -5.9 degC

So, my current understanding that "when the temperature reaches the dew
point mist will form" clearly cannot be true since the temperature NEVER
reaches the dew point.

Can someone please tell me where I'm going wrong? Essentially, I want to be
able to predict mist in the morning from conditions the previous night (all
other things being equal). As things stand at the moment, I can't even
predict mist right now from conditions right now!

Thank you

Gordon

On Thu, 2 Sep 2004 09:38:23 +0100, Gordon Dooley wrote in

snip
However (and
here's my problem) as the temperature drops over night, so the calculated
dew point drops. So for example, at a constant relative Humidity of say 45%,
as the temperature drops, you get something like this:

Air temp 25 degC = dew point 12.2 degC
Air temp 20 degC = dew point 7.7 degC
Air temp 15 degC = dew point 3.2 degC
Air temp 10 degC = dew point -1.4 degC
Air temp 5 degC = dew point -5.9 degC

So, my current understanding that "when the temperature reaches the dew
point mist will form" clearly cannot be true since the temperature NEVER
reaches the dew point.
snip

I think you are omitting the fact that Relative Humidity is going to
*increase* during the night, until it approaches 100% and dew/fog will form
and the two temperatures will be the same.


--
Mike 55.13°N 6.69°W Coleraine posted to uk.sci.weather 02/09/2004 08:48:19 UTC

Mike Tullett wrote:
On Thu, 2 Sep 2004 09:38:23 +0100, Gordon Dooley wrote in

snip

However (and
here's my problem) as the temperature drops over night, so the calculated
dew point drops. So for example, at a constant relative Humidity of say 45%,
as the temperature drops, you get something like this:

Air temp 25 degC = dew point 12.2 degC
Air temp 20 degC = dew point 7.7 degC
Air temp 15 degC = dew point 3.2 degC
Air temp 10 degC = dew point -1.4 degC
Air temp 5 degC = dew point -5.9 degC

So, my current understanding that "when the temperature reaches the dew
point mist will form" clearly cannot be true since the temperature NEVER
reaches the dew point.

snip

I think you are omitting the fact that Relative Humidity is going to
*increase* during the night, until it approaches 100% and dew/fog will form
and the two temperatures will be the same.


Hello Gordon

As I understand, dew point temps generally stay pretty much the same or
vary little in an airmass, though air temps vary a greater amount.

So at night as the air temps fall, the dew point temp stay about the
same, so eventually the air temps become the same as the dew point temp.
Consequently the air is completely saturated, and mist, dew or fog form.

You can see for yourself how dew point and air temps vary by looking at
some weather reports at http://weather.noaa.gov/weather/GB_cc.html, if
the report is still there look at Bournemouth airport, you'll seen that
during the night the air point temp falls until it's the same as the air
temp and fog forms.

HTH
Joe
Wolverhampton

"Gordon Dooley" wrote in message
...
Hi,

I have searched both the wider Internet and this group's FAQ but am still
struggling with what appears to be a basic concept. Can anyone enlighten
me?

Information on dew point is widely available, and from that I understand
that, broadly speaking, when the temperature and dew point are the same,
moisture will be precipitated and form mist or fog (or, I guess, dew). I
have a formula to calculate dew point given relative humidity and
temperature, and my calculated figures match published figures. However
(and
here's my problem) as the temperature drops over night, so the calculated
dew point drops. So for example, at a constant relative Humidity of say
45%,
as the temperature drops, you get something like this:

Air temp 25 degC = dew point 12.2 degC
Air temp 20 degC = dew point 7.7 degC
Air temp 15 degC = dew point 3.2 degC
Air temp 10 degC = dew point -1.4 degC
Air temp 5 degC = dew point -5.9 degC

So, my current understanding that "when the temperature reaches the dew
point mist will form" clearly cannot be true since the temperature NEVER
reaches the dew point.

Can someone please tell me where I'm going wrong? Essentially, I want to
be
able to predict mist in the morning from conditions the previous night
(all
other things being equal). As things stand at the moment, I can't even
predict mist right now from conditions right now!


If you consider some air in isolation, then the dew point would be constant
with change of temperature, unlike in your rather extreme example. Dew point
is a convenient measure of how much water vapour there is and if you do not
change that, then the dew point remains the same.

However in the real world, particularly near the surface, other factors are
involved.

During the day, water evaporated from lakes, rivers, damp soil and last, but
not least there is evapotranspiration from plants. All these effects are
dependent on other factors (wind speed, sunshine, temperature etc), but
usually result in a net input of water vapour into the air and consequently
there will be a rise in dew point temperature. There are of course some
occasions when drying of the air will occur by day.

Overnight, surfaces cool due to radiation and are usually cooler than the
surrounding air and consequently water will condense on the cooler surfaces,
a loss of water vapour from the air and a subsequent lowering of the dew
point. Also, evapotransiration from plants all but stops overnight, and many
plants have leaves which have evolved to be particularly effective at
causing water to condense upon them. Generally, it also becomes cooler and
less windy overnight, which decreases the rate of evaporation from lakes,
rivers, damp soil etc.

Even when the temperature and dew point do coincide, mist/fog are not
automatically the result, under some conditions, condensation continues onto
surfaces rather than in the air, a situation known as wet air.

However (and
here's my problem) as the temperature drops over night, so the calculated
dew point drops. So for example, at a constant relative Humidity of say 45%,
as the temperature drops, you get something like this:

You have the wrong end of the stick there.

At 25 deg C and an RH of 45% you have effectively got a measure of the
amount of water vapour in your parcel of air. It is 45% saturated AT THE
GIVEN TEMPERATURE.

Cool or heat this parcel of air (without adding or removing water) and
you still have the same amount of water in the parcel, but the RH has
changed due to warm air being able to hold more water before it is
saturated than colder air.

Now, if you have a parcel of air at 25 deg C, and its RH dictates that
the dew point is 12.2 deg C, then you cool it to, say 16deg C, the RH
increases but the dew point will still be 12.2 deg C. Hit 12.2 (actually
a hairs breadth below it) and condensation will occur as there is now
more water in the parcel than its temperature will allow it to hold.

I may stand corrected on this as it is a very long time since I learned
this stuff.

--
Murray McGregor

"Steve" wrote in message
...
snip
Overnight, surfaces cool due to radiation and are usually cooler than
the
surrounding air and consequently water will condense on the cooler
surfaces,
a loss of water vapour from the air and a subsequent lowering of the
dew
point.

.... I believe, subject to correction, that it was for this reason (that
the dew point decreases on a bog-standard low-wind/clear-sky radiation
night) that the concept of the 'Fog - point temperature ' was conceived
(empirically). Modern mesoscale models will attempt to calculate
physically the temperature and humidity profiles in the lowest few
metres of the atmosphere to come up with conditions producing fog (plus
wind flow etc.).
Before these (and it is only in the last 10-or-so years that they have
been good enough to do the job), we relied upon the empirical
calculation of Tf (fog point) either graphically from radio-sonde
ascents (Saunders) or using algorithms (e.g. Craddock and Pritchard)
based on 12Z or later temperature & dew points at any particular
station.
The latter did not find much favour, though I used it (and developed
some simple BASIC routines to calculate same), and as with all such, if
the data in was good, the results were often quite good.

If you are interested, I can try and find the BASIC routine I wrote and
send it along.

Martin.

Martin Rowley wrote in message ...

"Steve" wrote in message
...
snip
Overnight, surfaces cool due to radiation and are usually cooler than
the
surrounding air and consequently water will condense on the cooler
surfaces,
a loss of water vapour from the air and a subsequent lowering of the
dew
point.

... I believe, subject to correction, that it was for this reason (that
the dew point decreases on a bog-standard low-wind/clear-sky radiation
night) that the concept of the 'Fog - point temperature ' was conceived
(empirically). Modern mesoscale models will attempt to calculate
physically the temperature and humidity profiles in the lowest few
metres of the atmosphere to come up with conditions producing fog (plus
wind flow etc.).
Before these (and it is only in the last 10-or-so years that they have
been good enough to do the job), we relied upon the empirical
calculation of Tf (fog point) either graphically from radio-sonde
ascents (Saunders) or using algorithms (e.g. Craddock and Pritchard)
based on 12Z or later temperature & dew points at any particular
station.
The latter did not find much favour, though I used it (and developed
some simple BASIC routines to calculate same), and as with all such, if
the data in was good, the results were often quite good.

If you are interested, I can try and find the BASIC routine I wrote and
send it along.


Nothing much to add except that I like the "potential dewpoint" method of
Saunders, then I would as I worked for him at Manby in 1971 !

Also please do not ever forget that fog can be extremely localised and will
depend critically on wind flow, stability, turbulence, availability of moisture
near rivers, lakes etc. Use the above, but also get to know the climatology of
your local area. Walk or drive round on foggy mornings and just watch and learn.
You will soon get to know the "favoured spots" which will tend to form fog in
the most marginal of conditions. Also satellites are getting better at detecting
fog now, especially the new Meteosat which has many more channels. You should
also read up on the subject, especially the work of Findlater, Brown et al who
discovered that you do need *some* wind to get fog (circa 1 knot IIRC), flat
calm nights will not "stir" the air enough and all the moisture drops out onto
the ground. Bit loose terminology but you get the idea.

Cheers,

Will.
--
" A cup is most useful when empty "
-------------------------------------------------------------------------------
A COL BH site in East Dartmoor at Haytor, Devon 310m asl (1017 feet).

mailto:
www: http://www.lyneside.demon.co.uk

DISCLAIMER - All views and opinions expressed by myself are personal
and do not necessarily represent those of my employer.
-------------------------------------------------------------------------------

Thanks very much to everyone who has contributed to answering my question.
Your very thoughtful and helpful responses have clarified where my thinking
was flawed and pointed me in the right direction. Having been part of many
discussions in other newsgroups, may I also say that it was an unexpected
pleasure to be answered in such a polite and non-patronising fashion.

Thanks again

Gordon

"Gordon Dooley" wrote in message
...
Hi,

I have searched both the wider Internet and this group's FAQ but am still
struggling with what appears to be a basic concept. Can anyone enlighten
me?

Information on dew point is widely available, and from that I understand
that, broadly speaking, when the temperature and dew point are the same,
moisture will be precipitated and form mist or fog (or, I guess, dew). I
have a formula to calculate dew point given relative humidity and
temperature, and my calculated figures match published figures. However
(and
here's my problem) as the temperature drops over night, so the calculated
dew point drops. So for example, at a constant relative Humidity of say
45%,
as the temperature drops, you get something like this:

Air temp 25 degC = dew point 12.2 degC
Air temp 20 degC = dew point 7.7 degC
Air temp 15 degC = dew point 3.2 degC
Air temp 10 degC = dew point -1.4 degC
Air temp 5 degC = dew point -5.9 degC

So, my current understanding that "when the temperature reaches the dew
point mist will form" clearly cannot be true since the temperature NEVER
reaches the dew point.

Can someone please tell me where I'm going wrong? Essentially, I want to
be
able to predict mist in the morning from conditions the previous night
(all
other things being equal). As things stand at the moment, I can't even
predict mist right now from conditions right now!

Thank you

Gordon