A COL forms when the high and low pattern arranges in such a way as to cause a convergence or divergence of isotherms. The pattern arrangement of highs and lows includes 2 juxtaposed highs and 2 juxtaposed lows. In this article, the COL that produces a convergence of isotherms and frontogenesis will be examined. WAA from the south merges with CAA from the north. This thermal advection strengthens the isotherm gradient within the COL and causes a front to develop. For frontogenesis to occur, the southern low will be in the SW quad. The diagram below shows the high and low quadrants in order for the production of a COL and frontogenesis.

In the south, the low in the SW quad and the high in the SE quad funnel air toward the north between the high and low. The southern sector of a COL is the WAA sector. In the north, the low in the NE quad and the high in the NW quad funnel air toward the south between the high and low. The northern sector of a COL is the CAA sector. The flow pattern within a COL is shown in the diagram below.

Isotherms placed within the COL pattern will merge together over time. The diagram below shows this process.

In the real atmosphere, rarely will each high and low have an equal wind flow pattern around it. The WAA from the south may be stronger than the CAA from the north or vice versa. The highs and lows may not be perfectly in their quadrants either. When examining the models, you will come across a COL more often than you might think. A common COL set up in the U.S. is a low near Texas, a high in the SE US, a low in the Northeast, and a high in the northern Great Plains. With this set up, a front will gradually develop in the central U.S.

Haby sez: A COL pattern is a set of juxtaposed highs and lows. If a low is in the SW quad of the COL, frontogenesis will occur. Frontal formation and movement depends on the strength of each of the lows and highs and the preexisting temperature gradient in the COL pattern.