Visible Effects of A Strong Ridge
Summertime heat waves like this one are generally caused by a strong “ridge” in the atmosphere. A ridge is simply a large region of abnormally hot air throughout several thousand feet in the atmosphere. (Click here for the difference between a ridge and a trough.) Ridges do two main things to the sensible weather: make it hot and make it dry.
That big circle over the Central US is the center of the ridge at 500 millibars (almost 20,000 feet above the ground). The other contours that are packed tightly from California to North Dakota and then east dipping down across New England show where the jet stream can be found. Ridges keep the jet stream to the north, and since the jet is the over-all dividing line between colder air to the north and hotter air to the south, you can say a ridge will generally prevent the southward movement of cooler air.
Underneath the ridge, the weather is usually dry and hot due to a sinking motion in the atmosphere called subsidence. On a day when the air is rising, a plume of smoke will rise high into the atmosphere and be dispersed by stronger winds aloft. When there is subsidence (like today under this big ridge), smoke plumes don’t rise too far above the ground, and they tend to linger like this one viewed from the west side of Redstone Arsenal this morning:
(From WHNT.com – Large Plume of Smoke Coming from Arsenal; Officials Confirm Missile Test)
Subsidence squashes rising air associated with convective air currents that would normally produce afternoon thunderstorms, so we don’t get the cooling effects from the clouds or rain. Subsidence is also a warming process, so it makes the air even hotter by moving it from the middle of the atmosphere down toward the surface through a dynamic process called adiabatic warming. Think of it this way: diabatic temperature changes occur due to heating from solar energy in the afternoon and cooling when that solar energy goes away at night. Adiabatic temperature changes have nothing to do with daylight and darkness; it is a physical process that warms air by increasing the pressure on it!