mm wrote:
In high school physics, the three methods of heat dispersal were
presented as conduction, radiation, and convection?

Is that all there is?

That's about it. At least that's what's still being taught at all
levels, these days.

Is diffusion a fourth or is it subsumed by
convection?

Depends what you actually mean by diffusion. If you're talking about
the ability of a gas to spread out to fill it's container, then yes,
you're talking convection.

I could be wrong but:
Conduction seems to be limited to within a solid, or from the surface
of a solid to that part of a liquid or gas in contact with the solid.

Conduction can occur in any medium. Remember it's just the direct
transfer of heat energy through the media.

ICBWB:
Convection seems to be limited to liquids and gases.

In normal, everyday situations, yes. But geologists talk about
convection in rock in the earth's mantle, albeit very hot rock, but
still technically a solid at the pressures considered.


And ICBWB: radiation seems to be limited to from a solid or maybe a
liquid through a gas to another solid or maybe a liquid.

Well, radiation can be generated in any medium and absorbed by any
medium, regardless of state.

As to convection, it was always described and seems to be limited to
broad currents, such as hot air rising and cold air sinking, but is
that all that happens?

Basically. Convection is the transfer of heat by a moving medium, such
as air.

In, say, a room with moderate cooling in the
summer or moderate heating in the winter, while in general the hot air
rises, doesn't the random motion of some of the hot air cause it to go
downward and to mix with the cooler air below it?

Statistically yes, but on average, warm air will rise and cooler air
will fall. It is thermodynamically possible for the very opposite to
occur, but the probability of that happening is astronomically small.

Is this radiation?

Radiation is light, UV, IR, etc.

Is it still convection?

In your above example, it is practically all convection, since the air
is moving and carrying the heat with it. There could also be some
conduction occuring, as energy is directly transferred through the air
between hot and cold regions,.and perhaps some radiation if sunlight is
shining through a window or there is a light turned on.