Case Study: Cold Air Advection vs. Downsloping

An interesting struggle for control of surface temperatures took place at the Middletown Airport in Harrisburg, Pennsylvania, on the afternoon of June 27, 2003. The contest pitted cold-air advection following in the wake of a cold front against solar heating and compressional warming associated with downsloping northwesterly winds.

Just in case you're on the edge of your seat in anxious expectation of knowing the eventual outcome, here's what happened. About 18Z, a cold front went through the Middletown Airport. In its wake, cold-air advection arrived in southeast Pennsylvania. Now you might think that the temperature began to decrease at Harrisburg as cold-air advection arrived, but one look at the meteogram (shown below) from the Middletown Airport (MDT) should prove to you that the temperature increased after 18Z. Just to be sure you have your meteogram bearings, the trace of temperature with time is marked in red on the topmost rectangular graph and the third "tic" mark from your left marks 18Z.

A cold front moved into eastern Pennsylvania around 18Z on June 27, 2003 (left). Weak cold-air advection followed immediately in the front's wake, but stronger cold-air advection arrived soon thereafter (right).

The meteogram also shows that the sun was out (focus on the station model icons above the second rectangular graph). So solar heating took some of the starch out of the cooling power of the cold-air advection. Moreover, the winds blew from the northwest in the wake of the cold front (again look at the station model icons above the second rectangular graph). Referring to the topographical map of Pennsylvania, you'll note that Harrisburg lies to the east of the mountains. So the northwesterly winds "downsloped" into Harrisburg. And that means that the air is compressionally warmed during descent.

The meteogram for Middletown Airport near Harrisburg, Pennsylvania from around 12Z on June 27, 2003, to 11Z on June 28. After a cold front passed Middletown around 18Z (2 P.M. local time), the temperature increased as solar heating and compressional warming associated with downsloping northwesterly winds offset weak cold-air advection. Within a few hours, cold-air advection got stronger, causing the temperature to decrease.

So the moral of my story is that the heating power of the sun, coupled with compressional warming associated with downsloping northwesterly winds, offset the cooling power of cold-air advection for a while (the temperature increased). But, eventually, stronger cold-air advection arrived and, within a couple of hours, the temperature began to fall (as shown on the meteogram). Thus, cold-air advection eventually got the upper hand over solar heating and compressional warming.

It was one heck of a fight!