The science of subsidence, nature’s pressure cooker

A much-needed break from July heat has taken place in recent days across Northwest Missouri and Northeast Kansas. After a 6-day streak of 90 plus degree high temperatures in St. Joseph and surrounding areas; highs have been in the upper 70s since Wednesday, July 22. That’s about 10 to 12 degrees below average for this time of year. 

Paired with less oppressive humidity at times, the cool-off has truly been a breath of fresh air. Cloud coverage and passing showers have also brought a reprieve from a 9-day dry stretch that accompanied the summer sizzle, at least in the greater St. Joseph area. 

One could say it’s like mother nature turned off the oven, or more accurately, the pressure cooker. That’s because a large dome of pressurized air in the mid to upper atmosphere is the primary cause of summer heat waves. Also called a ridge of high pressure, these masses of air swirl in a clockwise direction, often covering entire regions of the United States. 

In a process called subsidence, the air within an upper-level ridge sinks, compressing and warming as it does so. As pressure builds, temperatures rise. Just like a pressure cooker. Subsidence within an upper-level ridge also suppresses cloud development, allowing more radiant energy from the sun to make it to the surface. 

This week’s brief cool-off was able to take place as the latest ridge of heat shifted southwest, away from the Mid-Missouri River Valley. With the drop in temperatures also came several waves of low pressure, which promoted rising air, cloud development and eventually periods of scattered to widespread rainfall.

Just as quickly as it dissipated, sweltering summer heat will soon be returning to the Central Plains and much of the Midwest. A reinvigorated dome of high pressure will overspread the region this weekend, and stick around for much of next week. Afternoon 90s will be the norm once again through the final days of July, no surprise for what’s widely known to be the hottest month of the year.