The Reason For The Drier Season
While the summers and winters in Alabama usually feature weekly rain events, we often observe dry stretches in early fall that can cause droughts and wide temperature fluctuations.
The culprits? Limited atmospheric forcing and the reduction of moisture.
The longer days of intense heat and abundant moisture during the summer supplies the necessary energy for the development of showers and thunderstorms. Under the Sun, saturated pockets of air can heat more rapidly than the air surrounding it, which decreases their density. The heated air pockets are then forced to rise through the cool atmosphere, condensing water vapor from the surface inside into liquid droplets. The air parcels can continue rising and creating water droplets until they reach the top of the troposphere, forming dispersed showers and thunderstorms that can persist for hours.
As the heat dies down, frontal boundaries take over to produce more infrequent but widespread rain in the winter. Low pressure systems associated with the polar jet stream dive farther south into the U.S., causing the warm, moist air over the Southeast to be replaced by extensive waves of cold, dry air from the North. As the air behind the cold front moves in, the less dense air ahead is wedged up into the upper atmosphere where the moisture it holds condenses to create broad lines of showers and thunderstorms.
However, this transition between atmospheric forces doesn’t occur overnight. While we see the summer’s extreme heat start to fade mid-September, strong cold fronts usually don’t arrive until the last half of the fall season. This creates the dry stretches often seen in October as the encroaching dry air from the North slowly pushes the Southeast’s moisture out of the region instead of up into the atmosphere. As a result, locations in the Southeast can go weeks without substantial rain, drying out plants and crops, as well as increasing the risk of fires.
But the gradual depletion of our moisture grants us one benefit: relief from the summer’s oppressive heat. Humidity levels during the summer generally remain high throughout the season’s days and nights. While the ground releases the Sun’s energy it absorbed back into space after sunset, thick water vapor at the surface can hold heat more easily, keeping the air at the surface warm throughout the night. On dry and clear nights in the fall, surface heat escapes more freely instead of being reflected, allowing temperatures to sharply dip.
While a couple cold fronts provided two significant temperature drops this past month, they left us in desperate need of rainfall as their energy and moisture fizzled out before reaching the Wiregrass. However, relief from our extensive drought conditions can be seen in the coming weeks as winds become more southerly, supplying the Southeast with new moisture as the polar jet stream slides closer and closer.
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