The Development of Warm Core Lows and Cold Core Highs at the Surface

Thermal Low


The diagrams above apply to any situation in which there is a differentially warming air column "next" to a differentially cooling air column. All you need to know to understand these diagrams is the hypsometric relation (heating an air coluumn between two isobars increases its thickness and vice versa) and the hydrostatic law (which becomes violated when you disturb vertical pressure gradients.

Some examples include air columns at the poles relative to that at the equator, air columns over hot (cold) continents next to air columns over cold (hot) oceans, air columns over islands or subcontinental areas that are hotter than the surrounding oceans during the day, and colder at night. As long as heating (as an example) continues, at the macroscale and synoptic scales, even at latitudes at which Coriolis parameter is significant, air motion will have a significant ageostrophic component (across the isobars) and the above diagram explains a portion of the General Circulation of our atmosphere as well as for some synoptic scale systems like hurricanes.

At the meso and microscale and at any latitude at which Coriolis parameter is not significant (such as any latitude between 5 degrees north and 5 degrees south latitude), the above diagrams work nearly perfectly. In all these cases, the pressure patterns that we observe on weather maps at different elevations will correspond generally to the model above.

The SURFACE pressure systems that develop are called, in the warmed air columns, warm core/thermal lows and, in the chilled air columns, cold core/thermal highs. Here's a side by side comparison of a thermal low (Hurricane Katrina) and a dynamic low (the wave cyclone of January 8, 2008) with the 1000-500 mb thicknesses colorized for their respective temperature values. The large thermal low pressure area over Asia during the summer which reverses to a high pressure area during the winter is due to the seasonal temperature swings over Asia, and results in what is popularly known as the seasonally reversing wind system over Asia called the monsoon.