Pressure Levels MET

High surface pressures always bulge upwards at the surface; low pressures always bulge downwards at the surface.

What happens aloft depends on the core temperature. If the core is warm, the "column" or air in the centre will expand, making it taller that the columns at the edge of the system. If the core is cold, the "column" of air in the centre will compress, making it shorter than the columns at the edge of the system. The effect of this is that a warm high bulges upwards at all levels; a cold low bulges downwards at all levels, and a cold high and warm low may bulge up, down or be flat aloft. This is summarised below.