Flora and Fauna News

Time to Bake!

By Michael Plagens
Sonoran Desert Sciences

PHOENIX, Az. -----The Summer Solstice usually coincides with the hottest weather of the year in the Sonoran Desert and for sure the really hot weather is now under way. Temperatures today will be pushing the 45° C mark. The low to high temperature span at this time of year can be phenomenal as well, especially away from the city environments where a spread of 30° C is common. With rapid heating afternoons are often breezy; the searing winds rapidly heating and desiccating the flesh.

Normally as one ascends in altitude the temperature drops at the adiabatic lapse rate of about 1° C per 100 meters. With intense sunshine heating the ground surface, which then heats the air directly above it, an unstable super adiabatic lapse rate is sure to develop. Bubbles of this super-heated air shift about creating the breezes, or may bounce up into the upper atmosphere after colliding with a mountain, creating a short lived cloud. The most distinctive events caused by this unstable heating occur when the lapse rate reaches the auto-convective rate of about 1° per 25 meters: Dust Devils. These vortexes of hot spinning air carry dust and debris upwards like a miniature tornado. Occasionally Dust Devils can grow quite large creating temporary havoc and considerable wind erosion.

This rapid heating occurs because the air is so dry. Humidity in the air would reflect infrared back to space and also serves as a heat pump carrying heat formed at the surface up to higher altitudes. By the first week of July a thermal low that develops over the desert begins to draw in moisture from the tropics. This depresses daytime temperatures by a few degrees, but the increase in humidity more than offsets the difference as far as comfort is concerned.

This most brutal season in the desert selects for those species that are most truly adapted to the hot and dry. Many animals retreat to underground burrows, coming to the surface only briefly during the cooler night time. Plants are often leafless. Two events, which will be reported in coming weeks, are vital for the survival of many desert residents.

Human travelers are advised to be well prepared. Automobiles must be in top mechanical condition and relatives/ friends should be provided with a detailed itinerary. Plenty of drinking water means at least seven liters per day per person.

This dust devil was observed and photographed in the Mojave Desert by Jeff T Alu. Licensed under the terms of the GNU Free Documentation license, Version 1.2.