Flora and Fauna News

Morning Virga --
Evening Thunderstorm

By Michael Plagens
Sonoran Desert Sciences

PHOENIX ----- The summer monsoon, or seasonal wind change, has brought mostly limited amounts of moisture to the deserts of Southwestern Arizona, and a bit more to the mountains of Southeastern Arizona so far this year. Easterly air currents normally bring the moisture up from the Gulf of Mexico, especially through the low pass in the continental divide at Deming, New Mexico.

Just after daybreak several cumulus clouds began to build up in the broad valleys of the Salt and Gila Rivers. These formed as the cooler air that had formed overnight in higher elevations to the east slid down slope and slipped under the hot, humid air that had stagnated over the valleys. With enough lift and cooling the moisture in the air began to condense, releasing its own latent heat, allowing the clouds to billow up even higher. Without additional lifting or much more humid air these cumulus clouds were doomed a short life. The droplets of water coalesced into raindrops and began a journey groundward only to encounter again hot dry conditions. Within a few thousand feet the slight curtains of rain had evaporated completely. Such curtains of evaporating rain are called "virga". Occasionally a brief rainbow will be evident during this frequent summer monsoon event.

Several thunderstorms over the past two weeks have parked themselves above the Eagletail Mountains and the Table Mesa Mountains of Southwestern Arizona. As is usually the case with summer storms these spots got a decent drenching while many others got barely a sprinkle. In those spots that got a deluge mesquites and ironwoods will quickly flush new foliage. Pools of water may bring forth a few daring Spadefoot Toads (Scaphiopus). Among the most ferocious of the insects the rain brings out is the Palo Verde Borer, Prionus. This large beetle can inflict a mild pinch if picked up, but is otherwise harmless. They can fly, and as such are among the bulkiest that are able to do so. The grubs bore tunnels through the underground root system of various desert trees. Mostly, this does no serious harm to the tree. Usually, trees that are heavily infested succumbed first to other environmental stresses.