The beautiful, crystal clear early fall days that we have recently enjoyed are characteristic of September in southern Wisconsin.
Such days are often quite cool to start, with morning temperatures in the 40s, but often warm by late afternoon when temperatures can soar to the mid- to high 70s.
Such conditions are associated with regions of high pressure either migrating past us or developing over us.
These high pressure regions are characterized by gentle, persistent sinking of the air from the middle troposphere to the surface – most times are rates of only about 150 feet per hour.
The sinking forces the air to warm by compression which reduces the relative humidity, accounting for the deep blue skies.
The clear skies, extending into the overnight hours in such episodes, allow for substantial cooling of the surface by radiation so that the temperature can be quite low by the time the sunrise occurs again, more than 12 hours later.
(Don’t forget that by late September, after the equinox, the night is longer than the day).
The development of regions of high pressure is now well understood to be associated with the passage of upper-tropospheric “ridges” in the flow.
These ridges are regions where the upper-level flow of air is constrained to turn clockwise over horizontal distances of hundreds of miles.
On the eastern edge of such clockwise turning flow regions, the laws of physics compel the air to gently sink and a high-pressure region is either newly created or sustained in that location.
The unsettled weather that accompanies a surface low-pressure region compels us to refer to it as a “storm.”
Though it is not common to refer to high-pressure regions as “storms,” they are, just like their low pressure counterparts, spawned by the passage of upper-tropospheric waves.
In fact, the longest-lived storm in the solar system (as far as we know) is a region of high pressure – Jupiter’s Great Red Spot.
