On Oct 9, 5:19*am, T8 wrote:
On Oct 9, 12:55*am, Tim Taylor wrote:





On Oct 8, 8:52*pm, Eric Greenwell wrote:

Jim Logajan wrote:
" wrote:
The sun shines down through the atmosphere, hits things on the ground,
the things on the ground get hot, and then the hot things on the
ground heat the air?

Sun light passing through the air does not heat the air at all? *Heats
the air some but not much?

If the air is transparent to solar energy, how is *the air able to
pick up so much energy from hot objects on the ground?

Try reading the content of this web page for some basic answers to all your
questions:

http://www.ucar.edu/learn/1_1_1.htm

It's a good article by a good source, but robably more than he wants to
know, as it's not directly concerned with thermal production.

The thermals we like so much use air heated primarily by conduction from
the aforementioned hot objects, but I believe some "micro" (my term)
convection near the ground is important for making the warm layer near
the ground thicker than conduction alone would provide.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

Bill,

Back to the basics, heat is transfered in general by three methods
conduction, convection and radiation (http://www.mansfieldct.org/
schools/MMS/staff/hand/convcondrad.htm).

Radiation is not radioactive but transfer from a warm body (the sun
in
our case) to a cooler body (the earth) by waves. *As you stated the
air is mostly transparent to the waves so most of the energy passes
through and hits the surface below. *As you stated some of the energy
is absorbed by the atmosphere (air, water vapor, dust, smog, etc),
some is scattered (bent so it goes off at an angle and and some is
reflected back to space.

The radiative energy that reaches the earth is transfered into heat
or
water vapor generally. *The warmed earth then transfers the heat to
the air above it by a combination of conduction (the act of
transferring energy from one molecule to the next by vibrational
energy and direct contact) or convection (the transfer of energy from
the flow of molecules over a warm surface).

In general thermals are generated by a combination of conduction and
convection and you have probably noticed that the best soaring days
generally have at least a slight wind to help induce more heat
transfer by causing more convection rather than by just conduction
alone. *The heat transfer from the earth to the air is driven by the
driving force of the difference in temperature between the air and
the
ground. *If there is not airflow the layer of air above the ground
quickly heats up and nearly matches the ground temperature so the
heat
transfer slows down. *Having a little convection as Eric said helps
mix the air up and keeps cooler air near the hotter ground to allow
greater amount of heat to be transfered to the air and therefore more
energy that can by turned into thermals.

Tim, Eric... think you guys need to check your sources.

Principle thermal input to lower atmosphere is long wave infrared from
the earth. *Over 50% iirc, but don't have an authoritative ref handy.

Conduction in air is notoriously poor....

regards,
Evan Ludeman / T8

Evan,

While the long wave does have a greater input back into the total
atmosphere and to radiation back into space I think in the boundary
layer conduction and convection are still the primary sources of heat
transfer. Since the air is primarily oxygen and nitrogen that are
both opaque to long waves as well it it mostly the other sources that
generate thermals.

From: http://www.auf.asn.au/meteorology/se...ic_temperature

1.7.4 Tropospheric transport of surface heating and cooling

The means by which surface heating or cooling is transported to the
lower troposphere a

by conduction — air molecules coming into contact with the heated (or
cooled) surface are themselves heated (or cooled) and have the same
effect on adjacent molecules; thus an air layer only a few centimetres
thick becomes less (or more) dense than the air above

by convective mixing — occurs when the heated air layer tries to rise
and the denser layer above tries to sink. Thus small turbulent eddies
build and the heated layer expands from a few centimetres to a layer
hundreds, or thousands, of feet deep depending on the intensity of
solar heating; refer to section 3.3.1. Convective mixing is more
important than mechanical mixing for heating air, and is usually
dominant during daylight hours. In hot, dry areas of Australia the
convective mixing layer can extend beyond 10 000 feet

by mechanical mixing — where wind flow creates frictional turbulence;
refer to section 3.3.2. Mechanical mixing dominates nocturnally when
surface cooling and conduction create a cooler, denser layer above the
surface — thus stopping convective mixing. If there is no wind
mechanical mixing cannot occur, refer to section 3.4.

The term (planetary) boundary layer is used to describe the lowest
layer of the atmosphere, roughly 1000 to 6000 feet thick, in which the
influence of surface friction on air motion is important. It is also
referred to as the friction layer or the mixed layer. The boundary
layer will equate with the mechanical mixing layer if the air is
stable and with the convective mixing layer if the air is unstable.
The term surface boundary layer or surface layer is applied to the
thin layer immediately adjacent to the surface, and part of the
planetary boundary layer. Within this layer the friction effects are
more or less constant throughout, rather than decreasing with height,
and the effects of daytime heating and night-time cooling are at a
maximum. The layer is roughly 50 feet deep, and varies with conditions.