The location of Texas with relation to the North American continent, the warm Gulf of Mexico, and the not-far-distant Pacific Ocean guarantees a constant exchange of settled and unstable weather. The state's varied physiography, from the forests of the east and the Coastal Plain in the south to the elevated plateaus and basins in the north and west, also brings a wide variety of weather on almost any day of the year. Because of its expansive and topographically diverse nature, Texas offers continental, marine, and mountain-type climates. West of the Caprock on the High Plains, a continental climate, marked by cold winters and low humidity, predominates. In the Trans-Pecos, a form of mountain climate is found. The eastern two-thirds of Texas, on the other hand, has a humid, subtropical climate that is occasionally interrupted by intrusions of cold air from the north. Though variations in climate across Texas are considerable, they are nonetheless gradual.
Precipitation is not evenly distributed over the state, and variations in precipitation at any one locale from year to year are apt to be pronounced. The mean annual precipitation varies from a statewide maximum of 59.20 inches at Orange, in the lower Sabine River valley of East Texas, to a minimum of 7.82 inches at El Paso, at the western tip of the state. The mean annual rainfall distribution correlates roughly with longitude and varies little from north to south across Texas. Generally, annual precipitation decreases about an inch for each fifteen-mile displacement from east to west. The Trans-Pecos is the driest region in the state, with an average annual regionwide precipitation of 11.65 inches, while the Upper Coast (45.93 inches) and East Texas (44.02 inches) are the wettest. At most locations rainfall for any single month will vary appreciably from the norm. Likewise, the number of days with precipitation usually is significantly abnormal. Moreover, the number of "rain days" follows the general trend of rainfall totals in that seasonal frequencies of rain days are lowest when rainfall totals are lowest. The mean number of days in January with at least .1 inch of precipitation varies from seven in East Texas to one or fewer in the Trans-Pecos; in July rain days normally are as numerous in the mountainous Trans-Pecos as in East Texas and along the upper coast. Particularly in the western half of Texas, one or two rainstorms often account for nearly all of a month's rainfall. The wet season does not occur at the same time of year in all parts of Texas. Intense and prolific thunderstorms, often moving in "squall lines," roam much of Texas in the late spring; Central, North, and East Texas receive their maximum rainfall in May. The warmest time of year is also the wettest for the High Plains and Trans-Pecos; nearly three-fourths of the total annual precipitation in these regions occurs from May to October. Tropical weather disturbances ensure that the late summer and early autumn are the two wettest periods for the part of Texas within 100 miles of the Gulf.
On the other hand, winter is the driest time of the year in nearly all of Texas. The exception is East Texas, where rainfall typically is the least substantial in July and August. December or January is normally the driest month on the High and Low Rolling Plains, as well as on the Edwards Plateau. The dry season peaks somewhat later farther east in north central and south central Texas, while on the coastal plains March is the driest month. Early spring (March-April) is normally very dry in the Trans-Pecos; in fact, in this semiarid region, rainless spells often last several weeks at a time, and two or even three months can elapse without significant rain. Since much of the annual rainfall occurs quickly, excessive runoff often leads to flooding. The broad, flat valleys in the eastern half of Texas, where mean annual rainfall exceeds twenty-five to thirty inches, sustain comparatively slow runoff. When rain is heavy, these valleys store vast amounts of water before slowly releasing it into the streams. The resulting flat-crested, slow-moving flood in the lower basins causes protracted periods of inundation. By contrast, in the western half of Texas, where ground and tree cover is sparse and stream slopes are typically quite steep, high-intensity rains produce rapid runoff that frequently leads to flash flooding. The area along the Balcones Escarpment (from Austin south to San Antonio, then west to Del Rio) is one of the nation's three most flash-flood-prone regions.
Snowfall occurs at least once every winter in the northern half of Texas, although accumulations rarely are substantial except in the High Plains. Snow is not uncommon in the mountainous areas of the Trans-Pecos, though heavy snows (five inches or more) come only once every two or three winters. More often than not, snow falling in the southern half of the state melts and does not stick to the surface; snow stays on the ground only once or twice in every decade. Snowfall rarely is observed before early November and hardly ever occurs after mid-April. Where it is not uncommon, snow is almost always heaviest in either January or February. Mean seasonal snowfall is 15–18 inches in the Texas Panhandle and 4–8 inches elsewhere in the High and Low Rolling Plains.
Temperatures vary considerably among the ten climatic regions of Texas. Few or no areas of Texas escape freezing weather in any winter. On the other hand, the heat of summer is intense everywhere. Whereas precipitation varies longitudinally across Texas, mean annual temperature varies latitudinally. On a year-around basis, readings are the coolest in the extreme north and warmest in the far south. In mid-winter the mean daily minimum temperature varies between the upper teens in the northern periphery of the Panhandle and the low fifties in the lower Rio Grande valley; afternoon highs range from the upper forties in the extreme north to near seventy in the far south. Conversely, summer lows in the Panhandle average in the low sixties and, in the lower Valley, in the middle to upper seventies; daytime highs reach into the low nineties in both regions. All-time temperature extremes in Texas include: -23° F at Tulia (1899) and Seminole (1933) and 120° at Seymour (1936) and Monahans (1994). Extended periods (more than one or two days) of subfreezing highs are rare, even in the far north. However, parts of the Panhandle generally have subfreezing temperatures for many successive winter nights. The mean number of days with freezing temperatures in the northern High Plains is 120. In this region the first autumn freeze ordinarily occurs at the end of October, and the last freeze in spring takes place in mid-April. The "freeze-free" season lengthens with distance north-to-south down the state. The mean number of days with freezes is forty to fifty-five in north central Texas and twenty to twenty-five in south central Texas. In some years the temperature never reaches the freeze level in the Valley. Even when it does, it almost always remains below 32° F for only four to six hours or less, usually around sunrise.
Air-conditioning is desirable in all parts of Texas, as temperatures everywhere climb into the nineties numerous times in July and August. Only on Galveston Island and in the highest elevations of the Trans-Pecos do summer daytime temperatures normally fail to rise above the eighties. Nighttime readings in summer are comfortable in West Texas, where less humid air allows the temperature to drop abruptly into the sixties after sunset. In the eastern half of Texas, however, much more humid air prevents nighttime readings from dropping lower than the middle or low seventies. Heat is usually most intense in the basins of the upper Rio Grande (within and upstream of Big Bend National Park) and in the upper Red River valley, where daytime highs in summer often soar well above 100° F. Triple-digit highs are also very common in South Texas from mid-May until late September.
All of the Texas coastline is subject to the threat of hurricanes and lesser tropical storms during the summer and autumn. Vulnerability reaches a maximum during August and September, the height of the hurricane season in the Gulf of Mexico and Caribbean Sea. Hurricanes strike the Texas coast an average of one every three years. Inland, hurricanes cause damage due to high winds, including tornadoes, and flooding from excessive rainfall. Persons along the coast must also contend with storm tides.
Although tornadoes can occur anytime, most of them materialize during April, May, and June. In a normal year, about 130 tornadoes are sighted in Texas, 30 percent of which occur in May. On average, about 200 people are hurt and a dozen are killed annually by the twisters. Tornadoes are most likely to occur along and south of the Red River between Lubbock and Dallas; they are least likely in the Trans-Pecos. Thunderstorms occur in every month of the year, though least in winter. With an average of sixty thunderstorm days a year, East Texas is most susceptible to the severe localized phenomena fostered by the storm (hail, high winds, flash flooding). The mean annual number of thunderstorm days diminishes from east to west across Texas; the Trans-Pecos has only about forty such days each year. The lower Valley has fewer still (thirty). The peak hail frequency statewide is in May. Most hailstorms are short-lived, however, since the macroscale weather systems (such as squall lines) that generate hail move rapidly. Hailstones are usually largest in the High Plains, where hail the size of tennis balls-even baseballs-is not uncommon in the summer. Sunshine is most abundant in the extreme west, where El Paso receives an average of 80 percent of the total possible sunshine annually. Cloud cover is most prevalent along the coast, especially in the Upper Coast, where the mean annual sunshine amounts to only about 60 percent of possible.
The voluminous records kept by Spanish Texas officials mentioned the weather directly; however, through the years various other sources-diaries, travel accounts, correspondence, newspapers, and medical records of military posts-reveal that the weather in Texas was of concern to early explorers, missionaries, dignitaries, Anglo-American colonists, other immigrants, and temporary residents. By 1855 Texas had twenty-one weather stations reporting data that were published. The stations decreased in number during the Civil War, and by February 9, 1870, when federal weather service officially began, Texas had fourteen weather stations at scattered settlements and military posts. Congress passed an act on October 1, 1890, transferring all meteorological services formerly carried on by the army signal corps to a newly constituted weather bureau under the secretary of agriculture, effective July 1, 1891. Texas had seventy-eight stations in 1890 and 112 in 1900; the number of stations continued to increase until by the early 1970s there were about a thousand weather observers around the state, mostly private citizen volunteers reporting to the federal government, in addition to forty-one major stations and offices operated by the federal weather bureau (which in 1971 became a part of the National Oceanic and Atmospheric Administration). In addition, the United States Air Force air-weather service maintained stations at military installations in Texas. By 1995 the number of stations had been reduced to ten. Offices were maintained at El Paso, Brownsville, San Angelo, Midland, Amarillo, Lubbock, Corpus Christi, New Braunfels, Houston, and Dallas. See also DROUGHTS, PALEOENVIRONMENTS, WEATHER MODIFICATION.