dew point?

Live for one winter in Sacramento and you won't ask that question. Near
sunrise most pilot have ATIS continuously tuned monitoring the dew
point second by second. If the spread is right, the moment the first
bit of sun touches the earth the fog forms, sometimes going from CAVU
to 1/2 vis in just minutes.

-Robert

Why does sun touching the earth form fog? I've seen it happen, but I've
never understood why.
"Robert M. Gary" wrote in message
ups.com...
Live for one winter in Sacramento and you won't ask that question. Near
sunrise most pilot have ATIS continuously tuned monitoring the dew
point second by second. If the spread is right, the moment the first
bit of sun touches the earth the fog forms, sometimes going from CAVU
to 1/2 vis in just minutes.

-Robert

Why does sun touching the earth form fog? I've seen it happen, but I've
never understood why.

I"m sure if I get this wrong someone on the list will correct me. In
Sacramento we receive two types of fog...
1) Advection Fog. This gets blown up from the Bay Area and usually
forms late at night and will often stay around until mid-day.
2) Radition Fog. This is the stuff that forms out of the blue. I
believe what happens is that the ground is already very moist but the
moisture is held in the ground. Once the sun hits it the water on the
ground warms releasing the moisture into the cool calm air. Sometimes
you'll drive by a field and see the field totally engulfed in fog while
the road you are on is clear. Unfortunately airports seem to be
composed of a lot of grass and dirt areas. Perhaps if you paved the
entire airport boundry you'd have less fog. I know some airports have
used helicopter to just fly around and blow the fog around.This fog
forms very, very fast at dawn. However, I've never been able to predict
it. Sometimes given the same temp/dew points and winds fog will form,
other times it will not. I'm not sure what all the factors are.

It causes some vertical motion which stirs the air a little,
casing the fog to form.



"Steve Foley" wrote in message
news:EkoNf.1255$v34.1168@trndny02...
| Why does sun touching the earth form fog? I've seen it
happen, but I've
| never understood why.
| "Robert M. Gary" wrote in message
|
ups.com...
| Live for one winter in Sacramento and you won't ask that
question. Near
| sunrise most pilot have ATIS continuously tuned
monitoring the dew
| point second by second. If the spread is right, the
moment the first
| bit of sun touches the earth the fog forms, sometimes
going from CAVU
| to 1/2 vis in just minutes.
|
| -Robert
|
|
|

"Jim Macklin" wrote in message
news:%mqNf.107786$QW2.7653@dukeread08...
It causes some vertical motion which stirs the air a little,
casing the fog to form.





Yes!

The very coldest air at that time of morning is right at the surface of the
earth. As it starts to mix with air above it, it cools a whole layer of 50
or a hundred feet or so, instead of just the thin surface layer. If the
average moisture content through that layer is such that the cooling drops
the temperature down to the average dew-point, bingo.

If you watch weather reports carefully, you will find that in those
clear-morning situations, the lowest temperature occurs *after* sunrise,
when you would have expected that warming is started. But when this mixing
starts, that coldest air at the surface (as part of the slight turbulence)
reaches up to the 1 metre level where the "official" thermometers are kept.

If you check a weather observing site which observes "grass" temperatures,
such as an experimental farm or the like, you will see just how great this
temperature difference between the grass level and the official 1 meter
level can be at sunrise... exceeding 5 degrees Celsius in clear-sky cases.

That is also why it takes just a slight breeze to get thick
fog. No wind at all, no fog, stronger winds push the cloud
bases up a few hundred feet and IFR becomes possible when
ILS minimums are reached.


--
James H. Macklin
ATP,CFI,A&P

--
The people think the Constitution protects their rights;
But government sees it as an obstacle to be overcome.
some support
http://www.usdoj.gov/olc/secondamendment2.htm
See http://www.fija.org/ more about your rights and duties.


"Icebound" wrote in
message ...
|
| "Jim Macklin" wrote
in message
| news:%mqNf.107786$QW2.7653@dukeread08...
| It causes some vertical motion which stirs the air a
little,
| casing the fog to form.
|
|
|
|
|
| Yes!
|
| The very coldest air at that time of morning is right at
the surface of the
| earth. As it starts to mix with air above it, it cools a
whole layer of 50
| or a hundred feet or so, instead of just the thin surface
layer. If the
| average moisture content through that layer is such that
the cooling drops
| the temperature down to the average dew-point, bingo.
|
| If you watch weather reports carefully, you will find that
in those
| clear-morning situations, the lowest temperature occurs
*after* sunrise,
| when you would have expected that warming is started. But
when this mixing
| starts, that coldest air at the surface (as part of the
slight turbulence)
| reaches up to the 1 metre level where the "official"
thermometers are kept.
|
| If you check a weather observing site which observes
"grass" temperatures,
| such as an experimental farm or the like, you will see
just how great this
| temperature difference between the grass level and the
official 1 meter
| level can be at sunrise... exceeding 5 degrees Celsius in
clear-sky cases.
|
|
|
|
|
|
|
|
|

wrote in message
oups.com...
HI,

Where does a pilot use the dew-point? Why is the volmet transmiting
this info?

Thanks
frank


The dew point is also used to forecast the expected overnight low
temperature. This will be used most by pilots who may be concerned about
their gardens and whether they need to be covered to protect from frost.

Happy landings,