When the upper airmass is descendig, it is pressing the lower
layers against the ground - the pressure on the surface is high.
There is adiabatic heating in the compressed air. Additionally,
the lowest layer of the air which is in contact with the ground
is cooled by the cold ground. Cold air is near the surface,
ower it lays a relatively warmer air. This layers does not mix
very well.
Cold air is heavier, so there is something like an ocean of cold air
and lighter air ower it. This is called inversion. There is only very
little vertical movement = stable atmosphere. Simply there is no force
which would make the heavy particle of cold air climb upwards into
the lighter air and vice versa.
Dust, smoke, different particles can not climb trough the
inversion layer - the visibility under the inversion is
negatively affected. Over the inversion layer the visibility
can be good.
The descending air is pushed to the sides. There is a flow OUT of the
centre of the high. This dampens the rotating movement of the air
around the centre of the high - coriolis force. The winds are low.
In contrary, in the atmospheric low there is a movement TO the
centre - coriolis force accelerates the rotation around the centre
of the low - stronger winds, the dust and smoke can dissipate better.