Exhibit 2-3

Note: Based on 1961-1990 precipitation data from the cooperative weather observing network of the National Weather Service.² Intermediate contours (white lines) are indicated at 2 inch intervals. Throughout the data-sparse Trans-Pecos region, countours reflect higher uncertainty than in other parts of the state.

View Exhibit 2-3: Average Annual Precipitation, in Text Format.

Temperature

An assessment of the state’s renewable energy resources must also take into account how energy in the form of heat is expressed as air temperature, the distribution of which is influenced to a very large degree by the amount of solar radiation reaching the surface. This quantity of energy is of no small value owing to the fact that Texas covers a broad range of latitude (26°N in the extreme south to 36°N in the northern fringe) and, hence, is on the equatorial side of the mid-latitude regions. But its subtropical latitude is only one of the controlling factors related to the way solar radiation is used. Another is the influence of the Gulf of Mexico, which is best evidenced by the prevailing winds that blow from the sea surface inland for much of the year. Cold-air outbreaks in winter are quickly moderated once they reach Texas because they are readily mixed with air previously supplied from the Gulf.

Unlike mean annual precipitation, mean annual temperature varies not so much from east to west as, quite consistently, from north to south (latitudinally). The coldest temperatures anywhere in Texas are observed in the extreme northern sector of the High Plains, which also features the lowest mean annual temperatures anywhere in the state (Exhibit 2-5). Conversely, the highest (warmest) mean annual temperatures occur in Southern Texas along the Rio Grande, from Eagle Pass to Falcon Reservoir. In some years, the hottest temperatures of summer are observed in this region. (Average maximum daily temperature is shown in Exhibit 2-6; minimum average daily temperature appears in Exhibit 2-7.) In winter (January), the coldest mean minimum temperatures (in the low 20s), are observed in the High Plains (at Amarillo, for example), while in the summer (July), hottest mean daytime readings (in the upper 90s), are measured in the area along the Red River (at locations such as Dallas and Wichita Falls) (Exhibit 2-4).

In spite of Texas’ proximity to the Gulf of Mexico, day-to-night (diurnal) variations in temperature across the state are appreciable in all but parts of the state’s coastal plain. On most days the moisture content of the lower atmosphere is sufficiently dry that, with the setting of the sun, air temperatures drop steadily. Mean annual diurnal temperature variations of 30°F or more are observed in much of Texas west of the Pecos River where the air is exceptionally dry, while along the upper Texas coastline (most notably, Galveston Island) more than ample moisture much of the time keeps extreme minimum and maximum temperatures from varying more than 15°F (Exhibit 2-8).

Occasionally, and particularly in winter, the air may be so laden with moisture that the diurnal range in temperature is only a few degrees. A blanket of clouds excludes much of the incoming solar radiation, thereby preventing the temperature from rising substantially above morning minimum values, and the same cloud cover can restrict outgoing heat energy at night so that minimum temperatures do not fall appreciably. The preponderance of cloud cover explains why, in a typical winter month, the range in diurnal temperature readings in the coastal plain is markedly less than that in the higher elevations in West Texas, where the air is much drier and the sky is usually cloud-free (Figure 2-5b). While the average diurnal variation is as little as 15-20° in that strip of coastal terrain within 20-30 miles of the coastline, the variation can be as little as 5° or less on as many as a half-dozen days in each of the months of December, January, and February. In the summer, because the air is warmer (and hence capable of holding more moisture), the diurnal temperature variation is not quite so small in the coastal plain (Figure 2-5c).

Wind Energy

The product of differences in atmospheric pressure between locations (pressure gradient), the strength and ubiquitous nature of the wind offers substantial promise as a source of renewable energy. While the wind is not merely a horizontal flow of air (its myriad of circular-moving eddies provides both upward and downward-moving pulses of energy), it is the lateral component of air motion that is of primary interest to energy providers and consumers. Though it has been poorly documented, the action of these individual eddies and the sum total of the vertical movement of the air at a particular place over a period of time may prove to be an additional and appreciable source of energy.

A southerly wind—or some component of it (southwesterly or southeasterly)—is the predominant wind condition in Texas for much of the year. In most sections of the state, the average wind speed varies between 7 and 15 miles per hour (Exhibit 2-8). A southerly wind is especially dominant in summer, when wind shifts induced by advancing cool fronts are much less common. In the southern half of Texas, where cool fronts often do not extend, a southerly wind is present some 90 percent of the time. In the north, northerly winds do blow on occasion, but southerly winds are observed at least 80 percent of the time. By contrast, the frequent intrusion of polar air in winter ensures a northerly wind about half of the time in much of Texas. Northerly winds are far from uncommon in both spring and autumn, though southerly flow remains dominant during those two seasons.

Direct measurements of incoming solar energy reflect a maximum in semi-arid West Texas that is coincident with the occurrence of the summer solstice (June 21) (Exhibit 2-15). The rainy season west of the Pecos River usually does not get underway until some weeks after the solstice, and the onset of an almost daily occurrence of significant thunderstorm development sometime in July brings about a rather sharp diminution of normal insolation.

In the east, especially in the coastal plain, a seasonal rainfall maximum in the late spring coincides with a relative minimum in normal insolation at locations such as Houston, Corpus Christi, and Brownsville. This is followed by a respite in thunderstorm frequency in early summer, when normal insolation increases appreciably. However, insolation drops proportionally in late July or early August with a tropical-cyclone season increasing the frequency of daytime showers and thunderstorms along the coastline and tens of miles inland.

Regrettably, while it is a key element in the spectrum of renewable energy resources in Texas, solar radiation may be the most poorly quantified, and hence least understood. This is due to the lack of an extensive observation network in Texas that detects sunshine. Where sensors are deployed, most have operating characteristics that are hardly uniform from one location to another. Solar energy is a resource marked by great variability over short distances, owing to cloud or turbidity conditions that are highly erratic. For the most part, reliable sunshine data are available for only a few of the largest metropolitan areas of the state. This means that vast areas of the High Plains, Low Rolling Plains, Edwards Plateau, Trans-Pecos, and Southern Texas are not well represented by existing data on sunshine availability. Even the more densely populated regions of northeastern and East Texas are lacking in good-quality sunshine data. That is all the more reason why the need for expanded coverage of radiation sensors and better standardization of instrument usage should be recognized and addressed. Nonetheless, the reliable data that do exist corroborate the fact that Texas is well endowed with this resource.

Summary

Texas, by virtue of its proximity to a surface energy source (the Gulf of Mexico) and its strategic position beneath a potent stream of energy aloft in the atmosphere (the subtropical and polar jets), is rich in renewable energy resources. The degree of abundance of each climate-related resource can be attributed to the intensity of solar insolation and by the gradient of that insolation from place to place across the state. After all, it is the disparity in incoming solar energy, from season to season and from locale to locale, that dictates the temperature gradients observed from west to east and from north to south across the Lone Star State. These temperature gradients ultimately determine the pressure gradients and the fluctuations in wind associated with them. The differential in pressure, in turn, determines the origin of air masses that migrate into and out of Texas with a striking degree of regularity throughout much of the year. How the weather behaves from year to year, in relation to this intricate energy budget, provides us with some measure of just how energy-rich the state really is.

References