By mid summer, a ridge of high pressure develops over the south-central United States. Within this dome, the air sinks, compresses and heats up and the sinking air stifles both wind development and cloud formation. As a result, excessive heat develops across the region; this year, the dome has persisted through much of July and has yet to abate in early August, leading to record heat on land and sea (water temperatures of the northern Gulf of Mexico have reached 90 degrees F).
Along the edge of the dome, winds flow clockwise, pulling in Gulf moisture across South Texas and Mexico; a seasonal low over the deserts of southern California and Arizona also pulls in moisture from the Sea of Cortez and the combined effects of these systems create the Southwest Monsoon, producing heavy rains and thunderstorms across Arizona, New Mexico and Colorado. On the north side of the dome, Pacific fronts and their component storms are swept across the Northern Plains and Upper Midwest, bringing copious precipitation to that area. To the east, any moisture pulled in from the Atlantic cannot penetrate the atmospheric ridge and storms are shunted to the southeast.
While swifts and cicadas seem oblivious to the conditions, severe heat poses a significant stress for most plants and animals, including humans. Though man evolved in the tropics and is better equipped to handle heat than cold, heat waves kill far more humans in modern, urban areas than do periods of cold and snow. With heat indices pushing 120F across the Gulf Coast and Southern Plains, those of us under the dome look to the northwest for relief; only a dip in the jet stream, now ensconced along the Canadian border, will nudge this oppressive ridge to the east.
