"Martin" wrote in message
...
On Tue, 22 Sep 2009 15:58:55 +0100, "Ophelia"
wrote:


"echinosum" wrote in message
...

'jbm[_2_ Wrote:
;865058']Ground frost
Sometimes the air temperature at night dips to 3 or 4 degrees, but the

forecaster still warns of a 'ground frost' and the need to de-ice your
car
in the morning. This is because the ground can reach freezing while the
air
temperature remains above.

It is the same as the way on a sunny day the sun can heat your car so
that it is so hot you can't touch it, though the air temperature may be
only 20C or so. Likewise, the entire black night sky is a source of
cold (just like the sun is a source of heat) that can cool the ground,
or your car, to below the temperature of the air, though the effect is
typically not as powerful as for the sun. I think typically your car
windscreen frosts over before hoar forms on the grass. One term for
this is "radiative frost", as the cold comes from radiation from the
sky, as opposed to "convective frost", where the cold is communicated
by convection (ie movement) of the air.

Heat is lost by radiation FROM the plant not from the sky.


Reducing radiative frost is why plants with overhanging evergreen
branches can survive the winter much better than those that can "see"
the whole black sky. One of the beneficial effects of being against a
wall, as opposed to an open situation, is that it reduces the amount of
black sky the plant can "see".

Oh my! The things you learn here!!



You have to go elsewhere to learn about the advantages of Stroh Rum in a
cold
climate.

I did, and I still want to know why your lip might be frozen on the car
lock!

"Martin" wrote in message
...
On Tue, 22 Sep 2009 17:19:02 +0100, "Ophelia"
wrote:


"Martin" wrote in message
. ..
On Tue, 22 Sep 2009 15:58:55 +0100, "Ophelia"
wrote:


"echinosum" wrote in message
...

'jbm[_2_ Wrote:
;865058']Ground frost
Sometimes the air temperature at night dips to 3 or 4 degrees, but
the

forecaster still warns of a 'ground frost' and the need to de-ice
your
car
in the morning. This is because the ground can reach freezing while
the
air
temperature remains above.

It is the same as the way on a sunny day the sun can heat your car so
that it is so hot you can't touch it, though the air temperature may
be
only 20C or so. Likewise, the entire black night sky is a source of
cold (just like the sun is a source of heat) that can cool the ground,
or your car, to below the temperature of the air, though the effect is
typically not as powerful as for the sun. I think typically your car
windscreen frosts over before hoar forms on the grass. One term for
this is "radiative frost", as the cold comes from radiation from the
sky, as opposed to "convective frost", where the cold is communicated
by convection (ie movement) of the air.

Heat is lost by radiation FROM the plant not from the sky.


Reducing radiative frost is why plants with overhanging evergreen
branches can survive the winter much better than those that can "see"
the whole black sky. One of the beneficial effects of being against a
wall, as opposed to an open situation, is that it reduces the amount
of
black sky the plant can "see".

Oh my! The things you learn here!!



You have to go elsewhere to learn about the advantages of Stroh Rum in a
cold
climate.

I did, and I still want to know why your lip might be frozen on the car
lock!

I've read it happens when people try to thaw the lock by blowing into it.

In which case, I am very pleased not to have tried that

In message , echinosum
writes

'jbm[_2_ Wrote:
;865058']Ground frost
Sometimes the air temperature at night dips to 3 or 4 degrees, but the

forecaster still warns of a 'ground frost' and the need to de-ice your
car
in the morning. This is because the ground can reach freezing while the
air
temperature remains above.

It is the same as the way on a sunny day the sun can heat your car so
that it is so hot you can't touch it, though the air temperature may be
only 20C or so. Likewise, the entire black night sky is a source of
cold (just like the sun is a source of heat) that can cool the ground,
or your car, to below the temperature of the air, though the effect is
typically not as powerful as for the sun. I think typically your car
windscreen frosts over before hoar forms on the grass. One term for
this is "radiative frost", as the cold comes from radiation from the
sky, as opposed to "convective frost", where the cold is communicated
by convection (ie movement) of the air.

As a former physicist I hear chalk scraping on the blackboard. It's
radiation *to* the sky. The physical process is heat being radiated from
the earth to the sky, not cold being radiated (there's no such think as
radiant cold) from the sky to the earth.

Reducing radiative frost is why plants with overhanging evergreen
branches can survive the winter much better than those that can "see"
the whole black sky. One of the beneficial effects of being against a
wall, as opposed to an open situation, is that it reduces the amount of
black sky the plant can "see".


--
Stewart Robert Hinsley

Yer welcome.

jim

If you like to think of it that way. But I'm entirely happy to talk of source of cold radiating cold, as was my school physics teacher. He demonstrated it with a large block of ice placed at the focus of a parabolic reflector, which becomes a bit complicated if you try to think of it the other way. It is just the same as saying electric current flows from + to -, which is the standard convention. Of course current could be carried by positive particles (as indeed happens when current is carried by postitive ions in an electrolyte), but in practice we mainly use negative ones. The electrons are physically moving in the opposite direction to conventional current, but solid state physicists often talk of "holes" transporting in the opposite direction to the electrons.