Weather in Texas: The Essential Handbook

By George W. Bomar

Only in Texas could a snowstorm pelt the Panhandle at the very moment abrasive dust is scouring the Permian Basin while searing heat is wilting the Winter Garden region in the south. The state’s large size and central location within North America subject it to a great variety of weather occurrences. Texas state meteorologist George W. Bomar has been observing Texas weather for nearly half a century, and in Weather in Texas, he provides the essential guide to all of the state’s weather phenomena.

Writing in lively layman’s language, Bomar fully explains both how the weather works and how Texans can prepare for and stay safe during extreme weather events. He describes the forces that shape Texas weather from season to season, including the influence of tropical cyclones, frontal boundaries, El Niño, and the polar jet stream. Bomar puts specific weather events in historical context, using a ranking system to illustrate how recent droughts, snowstorms, hurricanes, flash floods, and tornadoes compare with those of previous generations. He also includes comprehensive tabulations of weather data for every area of Texas, quantifying what constitutes “normal” weather, as well as the extreme limits of variables such as low and high temperatures, rain days, snow accumulations, and earliest and latest freezes. With everything from the latest science on climate change and weather modification to dramatic stories about landmark weather events, Weather in Texas is a must-have reference for all Texans..

• Language: English
• Publisher: Open Road Integrated Media
• Release date: Dec 1, 2017
• ISBN: 9781477315026

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Weather in Texas

The Essential Handbook

THIRD EDITION

George W. Bomar

UNIVERSITY OF TEXAS PRESS

AUSTIN

Support for this book comes from an endowment for environmental studies made possible by generous contributions from Richard C. Bartlett, Susan Aspinall Block, and the National Endowment for the Humanities.

Copyright © 1983, 1995, 2017 by the University of Texas Press

All rights reserved

Third edition, 2017

Portions of this text were previously published under the titles Texas Weather (1983) and Texas Weather: Second Edition, Revised (1995).

Photographs that appear on the pages indicated below are credited as follows: page iv, © Wyman Meinzer; page vi, © Kenny Braun; page viii, © Kenny Braun; page xiv, Photograph by Susan Cobb; page 22, National Weather Service Collection; page 36, © Kenny Braun; page 78, Photograph by Shawn Newsom; page 132, Texas Department of Transportation; page 152, © Kenny Braun; page 194, Photograph by GalgenTX; and page 214, © Kenny Braun.

Requests for permission to reproduce material from this work should be sent to:

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University of Texas Press

P.O. Box 7819

Austin, TX 78713-7819

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Library of Congress Cataloging-in-Publication Data

Names: Bomar, George W., author.

Title: Weather in Texas: the essential handbook / George W. Bomar.

Other titles: Texas weather

Description: Third edition. | Austin: University of Texas Press, 2017. | Portions of this text were previously published under the titles Texas Weather (1983) and Texas Weather: Second Edition, Revised (1995). | Includes index.

Identifiers: LCCN 2016048673

ISBN 978-1-4773-1329-9 (pbk.: alk. paper)

ISBN 978-1-4773-1501-9 (library e-book)

ISBN 978-1-4773-1502-6 (non-library e-book)

Subjects: LCSH: Texas—Climate.

Classification: LCC QC984.T4 W67 2017 | DDC 551.69764—dc23

LC record available at https://lccn.loc.gov/2016048673

doi:10.7560/313299

In memory of E. L. Bomar, my grandfather and my inspiration to be a student of Texas weather, and dedicated to all Texas volunteer weather observers, storm trackers, and first responders who help us cope with the effects of bad weather.

Contents

PREFACE

1. Our Ocean of Air

2. Eyes to the Skies

3. Weather’s Change Agents

4. The Hardships of Summer

5. Winter’s One-Two Punch

6. Cascading Rains

7. Nature’s Rain Engines

8. Cyclones from the Sea

9. Whirlwind at Its Worst

10. Airfield in Motion

11. Harvesting the Skies

APPENDIXES

ACKNOWLEDGMENTS

WEATHER TERMINOLOGY

SUGGESTED READING

INDEX

Preface

Few subjects evoke as many tall tales and jokes as the weather, especially the varieties that legitimize Texas as one of the more interesting places on Earth to visit—or live. In many respects, Texas is truly a land of contrasts, and exceedingly often the extent of the state’s diversity is punctuated by extreme and abrupt changes in its weather. Every winter ushers in bursts of biting cold from the Arctic Circle, occasionally in the form of blinding blizzards or paralyzing ice storms. Spring is reliably tumultuous, raking communities seemingly at random with tornadoes, vicious hailstorms, and overwhelming floods. Although notorious for its monotonous heat and humidity, summer is often rich with surprises, interrupting a debilitating drought with a water-laden tropical cyclone or letting in a dose of momentarily refreshing Canadian air. The autumn, while heartily welcomed for the relief it affords from heat and drought, can be exceptionally capricious with its propensity to spawn flash floods and premature frosts. Every year is unique, and in any year the fate of some Texans is diametrically opposite to the good fortunes of others. More the rule than the exception is the prevalence of drought in one region while other sectors have more rainwater than they can capture and hold. It is no stretch to say, on most any early-spring or late-autumn day, a weather observer can observe a snowstorm pelting the Panhandle at the very moment abrasive dust is scouring the Permian Basin and searing heat is wilting the Winter Garden region in the south. For every region of Texas, the utter dominance of drought one year is no assurance that rising floodwaters will not be a concern in the year that follows.

Texas weather is also a target for an inordinate amount of praise, and blame, that folks habitually assign for the favors and misfortunes they experience at the hands of Mother Nature. We boast about the intermittent balminess of our winters when folks elsewhere complain about the ever-present snow cover and lack of sunny skies in northern climes. We excuse having to endure weeks of torrid temperatures and absence of rain clouds in summer by asserting how the mildness of winters more than compensates for the momentary discomfort. Likewise, the weather absorbs much of the blame for what often goes wrong. Whereas some of that blame is justifiable, a lot of the time the unfavorable outcome has more to do with our carelessness or lack of understanding on how to cope when adverse weather happens.

This book attempts to capitalize on our unending quest to know more of what is the most prudent course to take when we face challenging weather. Knowledge often begets confidence, so the more we understand the capabilities and limitations of various weather threats, the less fearful we tend to be. In this edition, an enlarged portion of most chapters is devoted to the precautions and proactive steps the reader can take to ensure a most favorable outcome once the weather threat has passed. While some long-held notions about the weather are debunked, new ideas on how best to cope have evolved to help protect humans—and our cherished pets—as well as prized possessions.

Great strides have been made in recent years in understanding how the weather affects the populace—and marked improvements have occurred in the abilities of weather observers and specialists to warn the public when danger is imminent. It is today a well-worn joke that forecasting is the one occupation for which predictions of future weather can be wrong half the time while the prognosticator gets to keep his or her job. The fact is, weather forecasting in the twenty-first century is more precise than ever—and the accuracy of those forecasts now extends well beyond the 24- to 48-hour time frame with which previous generations were familiar. This volume seeks to inform the reader on what we know about various weather threats, why they happen, and the extent of their effect on life in Texas. Its contents are geared to meet the needs of those working in fields such as engineering, building and highway construction, and disaster preparedness. The material also will serve to enlighten readers whose work, and hobbies, center upon farming, ranching, gardening, outdoor sports, and recreation.

While this book can serve as a text for students of Texas weather, it is designed to address the needs of an audience with wide-ranging interests. Consequently, in-depth treatment of physical concepts underlying particular weather phenomena, including mathematical and theoretical considerations, will be left to other books. Many readers will discover some chapters to be of greater interest and utility than others. Some readers will search, unsuccessfully, for mention of a particular weather event that made a lasting imprint on them. Though the book is devoted to describing a plethora of weather episodes affecting a significant portion of the populace over many years, far more incidents have to be left unmentioned because of constraints on space. The federal government maintains an elaborate file of storm events that can be accessed to fill in details on specific weather occurrences, no matter how localized they may have been at the time.

Interest in the vagaries of Texas weather is keen and abiding, in part because of the potential of a type of weather to do for us as well as to us. This interest is acknowledged at the beginning of most chapters by an anecdotal description of some of Texas’s most calamitous events in history. The intent is not so much to frighten as it is to cultivate an awareness of potential hazards that history warns are repeatable—so we can take the necessary steps to remain out of the line of fire when something similar recurs in our neighborhood in the future. It is true that lightning does strike twice in the same place.

Unusual features of the weather leave indelible imprints on our hard drives, so in the chapters a concerted effort is made to include more than a few of the most unforgettable incidents. The voluminous cache of weather data that is the National Climatic Data Center was accessed to create various tables featuring events, and periods of time, so that the reader might have a better perspective of how those events rank in the historical narrative.

The introductory chapter is geared to identifying the framework within which our weather functions. Weather is a key manifestation of the ways our Earth interacts with the sun, the supplier of the energy that fuels the engine that is our atmosphere. Chapter 2 gives a history of how Texans have paid attention to the capricious nature of that atmosphere, including the way the National Weather Service has transformed its way of measuring, predicting, and warning of the more deleterious elements of the weather. There is an appreciable percentage of the populace with an affinity for keeping account of the weather, so the reader will find guidance on how to maintain a personal weather station. Chapter 3 takes a measure on the large-scale movers and shakers of our weather, including the jet stream and frontal boundaries that are the catalysts for the changes we expect in every season of the year. A nod is given as well to the role that El Niño, and his counterpart, La Niña, play in skewing our weather away from normal in most years. Chapters 4 and 5 discuss the excessive heat and chill, respectively, that create hardship for many Texans in some years.

Beginning with chapter 6, the focus of the book shifts to specific weather phenomena, outbreaks of which warrant close attention and a readiness to react. Too much rain over short intervals spawns floods, often with precious little time to take refuge—though the National Weather Service is more adept than ever at alerting the public to imminent danger. In chapter 7, the thunderstorm is appreciated for its life-giving rains, though recipients of those showers must be wary of what too often accompanies the deluges: flash floods, lightning, and hail qualify the stormy weather as a bittersweet experience. For several months during the warmest part of the year the threat of a tropical cyclone, most notably a named storm as serious as a hurricane, looms for coastal residents, as illustrated in chapter 8. Yet hurricanes of a different sort—from the eastern Pacific that careen into the Mexican coastline—supply semi-arid West Texas with the bulk of its rainfall in some years. A much more localized swirling of the wind, the tornado, is presented in chapter 9 as perhaps the most feared of all atmospheric eruptions. Although the likelihood of a twister hitting a specific target is remote at best, the tornado’s legacy of occasional bizarre fallout is enough to foster a disproportionate angst in some people. A wind of far less velocity is the focus of chapter 10, the kind of air movement that can bring relief to some (sea breeze) while annoying others (dust storm, dust devil).

With so much to read and talk about, it can no longer be said that no one ever does anything about the weather. To be sure, efforts to change our weather misfortunes are feeble—and some would assert strictly wishful thinking. Yet for several decades now, a well-orchestrated effort has been underway to extract more water out of the typically inefficient thunderstorm, as chapter 11 describes. There is evidence that working with Mother Nature has a payoff, though quantifying the benefit remains elusive. Even with a growing confidence that small steps can help us nullify the ravages of drought, we are sure to see, for years to come, much of our energy still being devoted to coping with the distasteful aspects of the weather, if not avoiding them altogether.

1.

Our Ocean of Air

The very instant you took your first breath, you locked in an unbreakable bond with Earth’s atmosphere. The immediate and involuntary interaction you had with your environment activated your lungs and fomented other biological adjustments within you that ensured you would irrevocably be changed—and bound inextricably to that atmosphere. The ocean of air in which you were immersed would now be an allegiant companion, with whom you would have a give-and-take relationship. In an imperceptibly small way, your presence in the world would forever alter the atmosphere as well. At this very moment, as you read these words, you are making a donation to the water supply in the air around you. This involuntary offering comes in the form of moisture evaporating from the pores of your skin and water emitted into the air with each breath you take. The approximate one quart of water you supply each day, when combined with exhalations from others around you, feeds nature’s hydrologic cycle, which ensures that water will return in a week’s time in the form of precipitation—to be consumed by creatures like you. To be sure, because the total global precipitation is some one quadrillion times (1015) more than any one individual’s contribution, your effect is infinitesimally minute. Nonetheless, those 1,000 molecules of water you supply daily serve as fodder for the cloud cover you observe, possibly even the rain shower or dusting of snow that occurred a fortnight ago. Perhaps months before you were born, your parents were advised that your arrival would change their world, and the instant you were born, you began to do precisely that—in more ways than one.

The symbiotic relationship you maintain with your atmosphere explains, only in part, why no other item of human interest is so much the subject of more insipid conversation on street corners and in coffee shops than the vagaries of Texas weather. Because of its diversity, severity, and—all too often—its unpredictability, Texas’s brand of weather produces a disproportionate share of disappointments, both minor and major. It is the object of hilarious jokes and hyperbolic claims. With few exceptions, it is the focal point of intense and immediate daily interest.

The innumerable gradations of heat and cold, of drought and downpours, of wind and calm, do far more, however, than serve as topics of popular conversation. They mold and shape the citizenry to fit the environment; the never-ending skirmishes between competing masses of polar and tropical air hold the populace in their embrace. Of course, the extent of influence is not nearly as acute now as in the past when nearly all our ancestors lived directly off the land, fishing, hunting, caring for their herds, and literally raising cane. Today we live in predominantly centrally heated or cooled environments. Still, our dependency on the demeanor of the atmosphere is never more clearly understood than when the absence of rain shrinks the water supply to alarming levels or when too much heat or cold inhibits the production of food and fiber. We take solace in the fact that the fortitude and vitality that allowed our ancestors to persevere in the midst of storms, flood, and drought remain as the inspiration we will need to endure what many experts say is a more foreboding environment that awaits us because of climate change.

The oft-expressed assertion that if you don’t like the weather right now, just wait a moment and it will change is not unique to Texas. Folks in other parts of the United States make the identical claim. The truth is, Texas weather, particularly in summertime, often becomes downright monotonous. If you hardly can stand the heat and high humidity that evidences the shift from spring to summer, hunker down—or make plans to visit Colorado. Summer’s heat becomes entrenched around the solstice in late June, and only a haphazard tropical cyclone is likely to afford any relief for weeks and weeks. Get beyond the wearisome uniformity of summer, and you just might thrive amidst the heterogeneity of nature’s often irrational ways. It matters not that you live in the vast expanses of the Trans Pecos, the undulating prairies of the Low Rolling Plains, or the dense thickets of East Texas, the one constant about Texas weather is its mutability (fig. 1.1). The weather’s proclivity to be different from one week to the next—if not from day to day—is occasionally punctuated by the occurrence of a dramatic event that earns inclusion in the almanac of Texas weather extremes (appendix 1 is a list of Texas weather extremes).

FIG. 1.1 Climatologically, Texas is segmented into ten climatic divisions, which are referred to often in this text. 1. High Plains; 2. Low Rolling Plains; 3. North Central; 4. East; 5. Trans Pecos; 6. Edwards Plateau; 7. South Central; 8. Upper Coast; 9. Southern; 10. Lower Valley. Source: Illinois State Water Survey

The same reversals in weather fortunes afflict most other regions of the country just as they confound Texans. After all, a change in the weather most often stems from the migration of air masses born thousands of miles to the north and west. What makes the shifting vagaries of weather so palpable—and popular—to residents of the Lone Star State is the state’s location in the midlatitudes of North America and its multiplying concentrations of people, whose numbers make the Lone Star State today the second-most populous in the nation. The sequence of events in one September a few decades ago illustrates how vulnerable the state remains to the whims of Mother Nature.

To the dismay of those living on the plains and prairies of West Texas worn down by seemingly interminable heat, summer refused to take a back seat as the autumnal equinox approached. Afternoon temperatures soared yet again to near 100°F (38°C) as the cotton farmer adjusted the controls of his irrigation system and the oil-field worker inspected one in an array of seesawing pumps. Within hours, however, a recognizable omen appeared on the far northern horizon that signaled an abrupt and drastic alteration in the weather was in the offing. A bank of ominous dark clouds spread rapidly southward, yielding flashes of lightning, rumbles of thunder, and dashes of rain. The key to those watching was the sky was changing complexion to the north, a telltale indicator that the door to the Arctic Circle was about to open. Sure enough, all of a sudden the unfamiliar breath of Old Man Winter was whipping through the High Plains and across the Caprock. Readings that had lofted to triple digits plummeted to the 50s as the sun dropped below the western horizon. Winds picked up in speed as the evening grew longer, and a persistent overcast supplied welcome—if only intermittent—bursts of light rain throughout the night. For the next three days, a chilly northeast wind retained temperatures in the 50s even at midday. The abrupt and marked change in the weather was due to none other than a bona fide blue norther.

As delectable as the abrupt reversal in the weather pattern was to plainspeople ready for summer to be discarded into the dust bin of history, the hemisphere’s weather engine had more surprises in store. Once the wind veered from the northeast back into the southwest, temperatures shot skyward again. The 3-day spell of chilly, damp, and windy weather, then supplanted by a rapid warm-up, placed excessive stress on tens of thousands of acres of burgeoning cotton. This time around, the culprit was not the usual untimely pounding by pea-to-marble-size hail. Rather, a war between frigid and simmering air masses that originated in such diverse areas of the globe as the North Pole and the Chihuahuan Desert was responsible for a loss of $40–50 million wrought on a cotton crop that is perennially the treasure of a 25-county area of the southern and central High Plains.

Our Weather’s Point of Origin

Like every other part of the world, Texas weather begins with activity on the sun, which supplies the planet with the energy needed to drive weather systems that shape our climate from year to year. While the Earth is dwarfed by the sun (a third of a million Earths would fit inside it), the source of our energy is itself a yellow dwarf star, one of many millions that adorn our universe. It is, by far, the closest star to Earth, the next nearest being Alpha Centauri, over 250,000 times farther away from Earth than the sun. The amount of matter used by the sun to generate light and heat continuously is gargantuan—some 4.4 million tons each second! That activity reveals the sun has a surface temperature of about 10,800° Fahrenheit (6,000° Celsius), which pales in relation to the estimated temperature at the core—a mind-boggling 25,000° Fahrenheit (14,000° Celsius)! The engine that is the sun is constantly in a state of violent flux: large flares of energy intermittently eject from the sun’s surface far into outer space. At other times, and sometimes coincident with flare activity, darker, relatively cooler areas called sunspots wax and wane in cycles that last around 11 years each.

Strictly speaking, we survive—and thrive—in one gigantic greenhouse whose transparent ceiling encircles the globe at an altitude of 8–10 miles (13–16 kilometers). Within this gaseous envelope, a mixture of gases protects all living things from the deep cold and the lethal radiation of outer space. The biosphere in which we function is far from self-sustaining, however. Rather, life on Earth is at the mercy of a colossal, incandescent cauldron of gas known as the sun. Without heat energy from the sun, life on this planet could be maintained for only a few fleeting moments. But our atmosphere gets credit as well. Without its capacity for transforming and distributing energy from the sun around the globe, life as we know it could not flourish.

An Atmosphere Gassed for Action

The relatively thin atmosphere enveloping Earth is made up of a uniform mixture of permanent gases known as dry air, which contains varying amounts of other materials, such as water vapor and organic and inorganic impurities. Four gases—nitrogen (78 percent by volume), oxygen (21 percent), argon (1 percent), and carbon dioxide (0.03 percent)—account for more than 99 percent of the pure, invisible, and odorless dry air that we depend upon for survival. Oxygen is the most crucial gas for the sustenance of animal life, whereas carbon dioxide is vital for the plant world. However, carbon dioxide is also of monumental climatic significance mainly because it effectively, and selectively, absorbs appreciable amounts of radiation emitted by Earth that would otherwise be lost to space. In the absence of this capacity to capture warmth, nighttime temperatures would be markedly lower. Of greatest importance to our weather and climate, however, is the presence in the atmosphere of water vapor, also an invisible and odorless gas that is highly variable in amount but usually accounts for about 3 or 4 percent of the total volume of air. Its value to us far outweighs its percentage contribution to the total volume of air, for it not only provides the ingredients for clouds and precipitation but also absorbs certain types of solar and terrestrial radiation. Water vapor also possesses the unique characteristic of being able to change its state from solid to liquid to gas while remaining an integral component of the atmosphere. It is while water vapor is undergoing a transition—from vapor to liquid to form clouds, for example—that it serves as a major source of atmospheric energy.

What constitutes Earth’s atmosphere in the lowest 50–60 miles (80–97 kilometers) remains rather constant. The concentration of ozone (O3) increases with altitude to a maximum 15 miles (24 kilometers) above the surface. Ozone is for us a bulwark, an important regulator of the types and amounts of solar energy that reach the land and water surfaces of Earth. It shields terrestrial life from the lethal effects of ultraviolet radiation emanating from the sun. Hundreds of dust particles per cubic inch fill the atmosphere and play an important role in the formation of clouds and precipitation by acting as nuclei upon which atmospheric moisture collects to form droplets. It is the presence of these myriads of submicroscopic dust particles, along with certain molecules of gas, that give us the blue color of the sky and the brilliant red hues of sunsets by selectively scattering the sun’s rays. Though some of this dust is washed to Earth’s surface by the rainfall it helps to generate, the atmosphere’s supply of dust particles constantly is being replenished. Of increasing concern these days is the massive introduction of many impurities, especially those that result from the burning of fossil fuels, which are decidedly harmful to humanity. Because of the state’s blossoming population—and a concomitant explosive growth of industrial activity and use of automobiles—atmospheric pollution has become a principal concern for Texans living in the state’s biggest cities.

A Layered Look

The Texas atmosphere manifests four fairly distinct layers that are differentiated mainly on the basis of how temperature varies with elevation. The layer adjacent to Earth’s surface, and the sphere in which virtually all of humanity operates (except for astronauts living in the space station), is the troposphere. Extending to about 8–9 miles (13–14 kilometers) above the ground, it is the domain within which variations in the weather are most pronounced. This is so because the troposphere contains about three-fourths of the atmosphere’s total mass and practically all of its water vapor (and clouds). Throughout the troposphere, the upper limit of which is called the tropopause, temperature for the most part on most days decreases with increasing height.

In contrast to the troposphere, the stratosphere exhibits very little, if any, change in temperature with increasing height. Relatively warm temperatures may be found near the top of this layer, resulting from the concentration of ozone, the highly efficient absorber of solar energy. Above the top of the stratosphere, at an altitude of about 16 miles (26 kilometers), is the mesosphere, where temperature increases and then decreases with greater elevation. It is in the upper limit of the mesosphere, at an altitude of about 50 miles, that most meteors burn and disintegrate. At the top of the heap, the layer from 50 to 300 miles high (80–480 kilometers), known as the ionosphere, has a pivotal role to play as well. Particles that make up this slice of the atmosphere reflect certain radio waves, allowing humans to communicate with one another. While changes in the density and composition of the upper layers of Earth’s atmosphere conceivably affect the weather near Earth’s surface, our greatest concern is with the behavior of the lowest layer of the atmosphere, for the troposphere is the sphere of our weather.

Checks and Balances

An incessant and immense stream of energy from the sun enters this envelope of air we call our atmosphere. Energy gained from the visible segment of solar radiation (also known as insolation) supplies the fuel necessary for the multitude of machinations that make up Earth’s weather and climate. Because Earth, in the short term, is not warming up or cooling off substantially, it must return about as much energy to space as it receives from the sun. However, some parts of Earth’s atmosphere—the tropics and subtropics—collect more solar energy than they give back to space, whereas other parts—mainly the Arctic and Antarctic Circles—give off much more than they receive directly from the sun. This continual transfer of heat energy, both horizontally and vertically, keeps the whole mechanism known as the atmosphere in balance. It is this perpetual exchange that sets up a host of temperature variations.

While much of the sun’s energy is absorbed (one-half by the Earth’s crust, either directly or indirectly, and about one-fifth by the atmosphere), the leftovers are reflected back into space by clouds and Earth’s surface—or they get scattered elsewhere in the atmosphere. Heat energy is transferred between the Earth’s surface and the air—or from one portion of the atmosphere to another—by one of three processes: (a) convection, (b) conduction, or (c) radiation. The processes of condensation and evaporation of water also bring about exchanges of heat energy between various surfaces and the atmosphere. These various methods of transferring heat energy maintain a fairly stable global-heat budget over relatively long periods of time. The amount of insolation received at any point on Earth depends upon such factors as the output of energy from the sun, the distance between Earth and the sun, the angle at which the sun’s rays strike Earth’s surface, the duration of the daylight period, and the constituency of the atmosphere itself. The varying angle at which the sun’s rays hit Earth’s surface is responsible for the unequal distribution of solar energy over this planet, which in turn determines the seasons of the year and the degree of variability of our weather. Table 1.1 shows the orientation of the arc that the sun traces across the sky for each season of the year. At the start of summer, because Brownsville is at latitude 25°54' N (or a little more than 2° from 23.5°, the Tropic of Cancer), the sun at noon over the city is virtually directly overhead (87.5°), while 785 miles (1,264 kilometers) to the north in Amarillo, the sun is 12° to the south from overhead. By contrast, with winter beginning just days before Christmas, the path the sun takes from sunrise to sunset is only one-third (31°) of the distance above the horizon toward vertical. Even in Brownsville, shadows are long because at midday, the sun is almost equidistant (41°) between the horizon and vertical. These stark differentials in the sun’s orientation between summer and winter explain why the two seasons are so drastically different temperature-wise—from one end of Texas to the other.

The huge disparity in temperatures from summer to winter is also attributable to the amount of time the sun’s rays are reaching the surface. In summer, it is the nearly vertical position the sun takes as it traverses the sky, combined with the amount of time its rays are penetrating the atmosphere, that makes the season a torrid one. As shown in table 1.2, on a cloud-free early summer day, the sun shines for more than 14-1/2 hours in Amarillo, nearly an hour more than in Brownsville. On the other hand, at the start of winter, daylight lasts for only 9-3/4 hours in the Panhandle, almost an hour less than in the state’s southern tip. No wonder, then, that the average annual temperature increases almost linearly (and latitudinally) from the northern tip of Texas to the state’s southern extremity (fig. 1.2).

TABLE 1.1

Declination of the Sun

FIG. 1.2 Average annual temperature (°F) across Texas. Source: Illinois State Water Survey

The Atmosphere’s Autograph

The best measure of this uneven distribution of incoming radiation from the sun is the temperature. The word temperature is a relative term indicating the degree of molecular activity, or simply the measure of warmth—or lack of same—of a substance. As the molecules move more rapidly, the temperature climbs. To measure the degree of coldness or hotness, an arbitrary scale is used. In the United States, temperature is commonly expressed in degrees Fahrenheit (°F); the boiling point of water at sea level is 212°F and the freezing point is 32°F. Another temperature scale, one that is used in nearly every other country, is Celsius (formerly known as Centigrade). In this system of units, the boiling point is 100°C and the freezing point is 0°C. One scale may be converted to the other using the following formulas:

°F = 32° + 9/5°C

°C = (°F − 32°) x 5/9

Values of temperature throughout this text are expressed in degrees Fahrenheit, with degrees Celsius included parenthetically (except in the appendixes). A few decades ago, an attempt was made to wean the United States from the use of English units such as Fahrenheit and miles, but to no avail. Somehow, a temperature of 40°C is not nearly as enchanting as 104°F!

TABLE 1.2

Length of Daylight by Season

With the Fahrenheit scale sacrosanct for at least another generation or so, a variety of temperature statistics—in degrees Fahrenheit—is used to describe the personality of a region’s average, or long-term, weather. The most frequently used value is the daily mean, or average, temperature. It is computed by taking the lowest and highest readings for a 24-hour period, summing them, then dividing that sum by two to get the mean. Actually, hourly readings of temperature measured throughout the 24-hour period would serve as a better basis for deriving a daily mean temperature, but the number of locations where the temperature is read only once a day far exceeds the sites where readings are recorded electronically on the hour. Besides, statistical studies have shown that on most days the average of the extremes and the average of hourly observations do not differ significantly. The difference between the minimum and maximum temperatures for the 24-hour period is the diurnal (or daily) range. The vast majority of local weather forecasts identify the diurnal range by providing predictions of overnight low and afternoon high temperatures for a given locale. The average (or mean) monthly temperature is derived by adding the daily means and dividing by the number of days in the month. Average monthly low and high temperatures, when grouped in threes (December through February for winter, March through May for spring, and so on), quantify variations in temperature from one season to another. Though they are of less interest, average annual temperatures, both minimum and maximum, consist of the average values of either mean monthly or seasonal temperatures.

Texas’s patterns of temperature are influenced to a great degree by the amount of insolation reaching the surface, a quantity of considerable import because the range in latitude (from 26°N in the extreme south to 36°N in the northern fringe) places Texas on the equatorial side of the midlatitude regions. But its subtropical latitude is not the only controlling factor related to the receipt of solar radiation. On the majority of days no matter the season, the Gulf of Mexico has a profound bearing upon weather throughout Texas—and especially in the coastal plain—because prevailing winds for much of the year blow from the sea onto the land. Cold spells, the kind ushered by north winds from Canada and the Arctic Circle, usually last no more than a few days at a time near the Texas coastline because of the warming effect of Gulf waters, which becomes pronounced once the wind shifts from the north back into the southeast. Still another, albeit less prominent, influence on the variation of temperature across Texas is the presence of mountain barriers. This insulating effect from mountain ranges is sometimes evident in the Trans Pecos in winter. Cold polar or Arctic air surging southward out of the Great Plains sometimes piles up on the lee side of such ranges as the Guadalupe, Chisos, Davis, Delaware, and Chinati Mountains, sheltering the area from the Big Bend upstream to El Paso from the stiff north winds and plummeting temperatures that accompany cold-air intrusions elsewhere in the state.

The Diurnal Cycle

On the vast majority of days in every season, day-to-night variations in temperature across Texas are almost always appreciable. With the setting of the sun, the amount of incoming solar radiation quickly drops, and the outgoing terrestrial radiation increases markedly. Evaporation plummets and, if Earth’s surface cools sufficiently and the dew point is attainable, condensation in the form of dew or frost occurs. Depending upon the amount of moisture in the air, the difference between daytime high and nighttime low temperatures may be as much as 30°–40°F (17°–22°C). There is a smattering of days in Texas, particularly in winter, when the air may be so laden with moisture that the diurnal range in temperature is only a few degrees. On most days, the temperature will bottom out within about an hour or two of daybreak, the point at which absorbed incoming solar radiation begins to counteract the removal of heat energy by radiative processes that have been going on all night at the Earth’s surface. Though incoming solar radiation ordinarily reaches a maximum at midday, daytime high temperatures usually occur a few hours later, in mid or late afternoon. In the same way a room remains warm for a while after the stove is turned off, this lag is mainly due to the capacity of the atmosphere to store heat.

FIG. 1.3 Average summer morning low temperature (°F). Source: Illinois State Water Survey

FIG. 1.4 Average summer afternoon high temperature (°F). Source: Illinois State Water Survey

The diurnal temperature range is invariably larger on clear days than on cloudy ones because an overcast sky reduces substantially the escape of terrestrial radiation at night. Conversely, with clear skies, a maximum of solar radiation penetrates the atmosphere to Earth’s surface during the day, and at night the outgoing radiation is not impeded by clouds. How much moisture there is in the air also dictates day-to-night temperature fluctuations. Because moisture in the air is most often more abundant in areas along and near the Texas coastline, diurnal ranges in temperature are much smaller than in areas that are more distant from the Gulf. For instance, on a typical summer day, the diurnal temperature range at Galveston is 80°F (27°C) to 86°F (30°C), while up on the High Plains the temperature at Amarillo varies from a low of 65°F (18°C) to a daytime high of