Essays

Moseying: Exploring the Natural World

A 20 percent chance of rain with an isolated thunderstorm on the Stockton Plateau
May 2, 2007

In the live oak over my head mourning doves and white-winged doves came and went to their nestling- filled stick platforms. It was the first time I had seen the two species nesting in one tree. I do not know how often it happens, or if I had discovered something “new.” Mourning dove young, like roadrunners, have dark skin under their feathers. Is this an indication that both species were originally found in colder climates? I love being outdoors – I find lots of things to wonder about, to contemplate. The out of doors is the best school in the world!

Earlier I had noticed clouds beginning to swell. Small puffy clouds became gray-bottomed while the tops built higher. As I sat watching the doves, the clouds began to coalesce. Two of the swelling puffy clouds had joined and begun to form the classic anvil shape of a nascent thunderstorm. The mesa to my southwest was brightly lit under the dark clouds. The clouds seemed to be moving in typical west Texas fashion – from southwest to northeast. A cool wind began to blow from the cloud. I might get hit with a little rain, but I was not going to move, not yet.

With thunderheads in the vicinity, errant updrafts form. To the north in the bottom of the wide valley here on Stockton Plateau I could see a dust devil form and move towards the growing cloud. Another formed, further up the valley, and then another. Most dust devils are only fifty feet tall and last a minute or two. The first dust devil kept growing – to a hundred feet, then even higher. After it held together at least two minutes I began timing it, and it kept spinning for over seven minutes. It changed color as it passed from the clay bottom of the valley to the limestone gravel and silt slopes closer to the mesa.

For years I have believed that dust devils carry plant seeds. The parachute seeds of daisies, of course, but also the fine dust sized seeds of lovegrasses could be lifted to the top of a dust devil, but even the heavier seeds of bristlegrass might tumble for a number of yards before being spit out of the whirling winds. As the dust devil neared the small patch of escarpment live oaks where I sat I noticed a cloud of small insects swarming in a mating lek. Were the insects ant or termite alates? Or were they a swarm of the small wasps that form the diverse population of bizarre galls on oak leaves and twigs?

Weak flying insects that specialize in one species of plant, like the gall wasps might not normally be able to move from one grove of oaks to another, especially if the groves were several miles apart, as they were in the valley where I sat. To ensure gene flow from one population of the wasps to the next the dust devils must play a part, I decided. The dust devil whipped by, and the insect swarm was gone, but not a hair on my head was ruffled. Straight-line winds can move insects, but provide no lift, as the dust devils could. The more lift, the further the insects might travel.

The roll of thunder became almost continuous. The anvil cloud had reached upper level winds and its trailing clouds completely obscured the sky. Within a few minutes the thunder became physical, loud and abrupt enough to stimulate “neural reactions”, i.e. flinching. Strong gusts of wind moving toward the storm began whipping the live oak branches. A few strings of rain began to curtain the slope to the southwest, but within another minute, the mesa disappeared from sight. Serious rain! I believed I was about to get wet, and being near the tallest trees in the visible part of the valley, I began to worry about lightning. “Outdoor school” is very real – the threat of death from lightning is part of the lesson

The grove of oaks was along the confluence of a side canyon of the valley and was along a low cliff that sheltered the trees from late afternoon sun and northwesterly winds. I moved to the cliff and found a place where an overhang might protect me from both rain and lightning. A few large drops managed to penetrate the gaps between the oaks and splatter me before I reached cover. When I hunkered down on a boulder I could see that part of the curtain of rain had changed from the gray of the rain to the eggshell of hail. The few drops hitting the oaks near me had begun falling over the mesa but had been carried by the convection cell’s cycle of winds to randomly fall a quarter mile from the pounding rain.

The combinations of clouds, wind, thunder, and lightning created a symphony – exhilarating and emotionally moving. The closer to the storm, and the bigger the storm, the more a storm makes a person react. The rain continued to pound the upper part of the valley. I realized I no longer saw any of the Turkey Vultures or Red-tailed Hawks that had been lazily soaring fifteen minutes earlier. Cliff Swallows began swooping low in front of me, catching the aerial fauna pushed down by the lowering air pressure and swirling winds. After three or four circuits between the rain and my sanctuary they peeled off, almost in formation, headed to their mud “horno” nests on another cliff further up the side canyon.

Usually thunderstorms zip along at 20 to 40 miles an hour, tracking along the edge of a frontal boundary. This storm was not of weather front origin, however, but of supersaturated air and tremendous heat. Gulf moisture, pushed northwest by a low-pressure system in the gulf, had met the dry heat of the Trans-Pecos. For 30 minutes the upper valley remained obscured by the torrential rain. The storm did not move. No more than the occasional spattering of a handful of drops ever fell on the oaks.

When the thunderstorm collapsed and I began to see patches of blue sky to the west, I ventured out to the valley floor where I spent the next hour walking along the edge of the flash flood the storm sent down the valley, observing depositional and erosional forces at work, but that is another story.