dew point?

A dew point above about 50 F indicates that storms are
possible and above 60 F that sever storms are likely due the
available supply of latent heat.
A close spread indicates that fog is possible, also a dew
point around 60 with a air temp of 90 is a good indicator of
carb ice conditions.



--
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.


"Peter R." wrote in message
...
| wrote:
|
| Where does a pilot use the dew-point?
|
| A close temperature/dewpoint spread (less than 5 degree
Celsius difference
| between the two) can indicate the presence or possibility
of fog.
|
| --
| Peter

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.
|
|
|
|
|
|
|
|
|

On a clear night, the ground cools by radiation (IR) and the
air is cooled by conduction to the ground. Inversions are
common. If there is an overcast, the IR cooling doesn't
happen and the temperature will not drop as far.


--
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.


"Robert M. Gary" wrote in message
oups.com...
| When I flew the J-3 in Sacramento I would often take off
in the butt
| cold morning only to find it quite warm at altitude of 500
feet.
|
| -Robert
|

Jim Macklin wrote:

On a clear night, the ground cools by radiation (IR) and the
air is cooled by conduction to the ground. Inversions are
common.

Late one very clear night last year, I was flying back to Syracuse, NY, and
the temperature at 10,000 feet was around -5 Celsius.

As I started to descend for the approach, the cabin of the aircraft got
noticeably colder (I didn't have the heat on). When I retrieved the ATIS
for the airport, the temperature on the ground was a -25 Celsius. This
was one of the more extreme examples of an inversion I have experienced.


--
Peter

I think you mean.... "using the rule of thumb that the height (in feet) of
the bases of convective cloud is approximately 400 times the
temperature-dewpoint spread at the surface (in degrees Celsius)"

Yes, that's it... not so much the lapse rate as the conversion rate.

Jose
--
Money: what you need when you run out of brains.
for Email, make the obvious change in the address.

conversion rate.
convergence rate.

Jose
--
Money: what you need when you run out of brains.
for Email, make the obvious change in the address.