Main Reason For Development of Surface Pressure Systems:

Divergence- The motion that results in neighboring air parcels at a given level "spreading apart."

Rule: Divergence aloft tends to be associated with surface low pressure area development.

Discussion of Thermal Pressure Systems:

The question arises:  "what causes this upper tropospheric divergence?"  There are two ways, one of which is related to the heating and cooling patterns over the globe.  For example, during the summer the surface of the continent is the site of low pressure.

We can trace this from divergence aloft which can, in turn, be traced to heating of the air columns over the continent which causes the development of upper tropospheric high pressure areas initially.   Since according to our rule of thumb,  at a given elevation air tends to move from high pressure to low pressure, air diverges out of these high pressure areas aloft moving towards the low pressure areas over either ocean, which lowers the pressure on the surface directly underneath the high.  The surface cyclones that are formed in this manner are called THERMAL LOWS.  Similar arguments can be used to explain THERMAL HIGHS which develop at the surface in the chilled air columns.