Are systems in
the real world either isothermal or adiabatic?
Real processes are rarely completely adiabatic or isothermal - but often
approximate to one or the other depending on the time scales and space scales
involved.
Extremely rapid events when there is insufficient time for much heat transfer
to occur can approximate to adiabatic
behaviour - as can the behaviour of very large volumes of gas (eg. moving
up or down through the atmosphere) where conductivity is low over the distances
involved.
Conversely, for very slow processes or those happening on a small scale,
heat conduction is likely to be sufficient to give an approximation to isothermal
behaviour.
links:
The adiabatic atmosphere