High pressure areas form when an airmass cools over a cool land or ocean surface. The cooling causes the air layer to shrink, becoming slightly thinner. This shrinkage then causes surrounding air in the upper troposphere to fill up the extra space. The added weight of the extra air causes higher pressure at the surface. The higher pressure air at the surface then tries to flow outward toward lower pressure, but as it does, the rotation of the Earth turns the wind to the right, resulting in the clockwise wind flow around the high pressure (in the Northern flows in the opposite direction in the Southern Hemisphere).