The city of San Antonio is located in the south-central portion of Texas on the Balcones escarpment. Northwest of the city, the terrain slopes upward to the Edwards Plateau, and to the southeast it slopes downward to the Gulf Coastal Plains. Soils are blackland clay and silty loam on the Plains and thin limestone soils on the Edwards Plateau. With its location on the NW edge of the Gulf Coastal Plain, San Antonio experiences a modified subtropical climate. Average monthly temperatures range from the 50s in winter to 80s in summer. Elevation varies from near 550 to near and above 1000 feet above sea level.

During winter the area is alternately influeneed by a continental climate, when winds blow from the north and west and by a modified maritime climate, when south and southeast winds blow from the Gulf of Mexico. Mild weather pre-vails during most of the winter. Below freezing temperatures occur on average about 20 days each year. When strong cold fronts do occur, they block any moderating effects from the Gulf of Mexico. The coldest winters overall have come from those winters of frequent cold fronts, accompanied by mostly cloudy weather regimes. The coldest low of record was 0 on January 31, 1949. Although daytime highs are restrained in cloudy winters, overnight lows can be potentially higher, sometimes sparring the area from the much colder minimums that come under clear skies. During sunny winters, the South Texas sun warms daytime temperatures to pleasant levels, while nights are cooler. Sometimes in these regimes, with colder than normal nights, the average temperature can be close to normal, because the daytime sun moves daytime highs to much above normal. Daily temperature variations can be as much as near 40 to 50 degrees. Very warm days occur when dry west winds in a mild airmass allow winter temperatures to climb to spring or summerlike levels, such as 90 December 25, 1955; 89 January 30, 1971; 100 February 21, 1996 and 100 March 6, 1991.

During the summer the climate becomes more tropical like with prevailing south and southeast winds. The moderating effects of the Gulf of Mexico prevent extremely high temperatures; however, summers are usually long and hot with daily maximum temperatures above 90 over 80 percent of the time. In many years summer conditions continue into September and sometimes even to October. The highest of record was 111 September 5, 2000., when dry conditions occurred and the area was blocked off from the moderating south and southeast Gulf winds. Occasionally, cool fronts may move through the area, dropping overnight lows into the 50s and 60s; however, the cooling is brief, and in a day or two the minimum temperatures are back to the 70s. If the drier air with these fronts has lost all of its cool properties, daytime highs have risen above normal, as the moderating affects of the Gulf of Mexico are blocked. Although July and August can have limiting rain, sometimes rather heavy rain events can come in July and August, especially with the remnants of tropical storms or stalled out cool fronts.

San Antonio is situated between a semi-arid area to the west and a much wetter and more humid area to the east. Such a location allows for large variations in monthly and annual precipitation amounts. The normal annual precipitation for San Antonio is about 30 inches, although it may range from near 10 to near 50 inches from one year to another. The extremes vary from 10.11 inches in 1917 to 52.28 inches in 1973. Heavy rain may occur with or without thunder in any season. During some of these events, rain has exceeded 5 inches in several hours and caused flash flooding. A year of normal precipitation is sufficient for the production of most crops, although during the drier years irrigation is essential. On average the heaviest rains fall in May and September, while the driest months, on average, are December through March, and July. Since rainfall is sporadic, the wettest and driest month in any one year may occur in any season and vary widely from year to year. Precipitation from April through September usually occurs as a result of thunderstorms; however, thunderstorms may occur in any month.

Hail of damaging intensity has occurred, although it is not as frequent as it is compared to North Texas or other places in the Great Plains or Rocky Mountain regions. Small hail is frequent with the springtime thunderstorms and has been observed during other seasons. Measurable snow usually occurs only once in three or four years. Snowfall of 2 to 4 inches occurs about every ten years

Since San Antonio is located only 140 miles from the Gulf of Mexico, tropical storms occasionally affect the city with strong winds and heavy rains. One of the fastest winds recorded, 74 mph, occurred as a tropical storm moved inland east of the City in August 1942.

Strong winds also come as a result of microburts from thunderstorms. A microburst during the afternoon of July 10, 1979, caused a wind gust of 77 mph. Strong winds also come from squall lines, and strong cold fronts. On March 27, 1994, a squall line blasted through the city during the early morning hours, causing a wind gust of 104 mph at Randolph AFB, on the northeast side of the city. On May 27, 1997 a squall line swept through the city from the north and northeast, and caused a wind gust of 122 mph at Kelly AFB in a thunderstorm. Although tornadoes are rare, they have occurred, and they have most often been associated with the dissapating tropical storms.

Relative humidity is above 80 percent during the early morning hours most of the year, dropping to near 50 percent in late afternoon

San Antonio receives about 50 percent of possible sunshine during the winter months and more than 70 percent during the summer. Stratus clouds frequently develop at night during all seasons with south and southeast winds, as Gulf moisture is lifted from the coastal plains to the higher terrain over the Balcones escarpment. When these clouds dissipate, partly cloudy to sunny conditions follow by late morning to sometime as late as late afternoon. On some days, these clouds do not dissapate much, hanging in all day, with few or no late afternoon/early evening breaks. In the winter these stratus clouds may be accompanied by fog and drizzle, as south and southeast wind brings Gulf moisture over the top of a cool air dome at the surface. In some years, when very cloudy conditions prevail, even if these clouds break up, mostly cloudy skies linger due to a dense high deck of cirrus, that sometime accompanies an active subtropical jet stream.

Normally the first freezing temperatures occur around December 1st, while the average last freeze occurs in late February. Extreme freeze dates are 32 on October 30, 1917 and 31 April 3, 1987.

In summary, the climate of San Antonio is a moderated sub-tropical climate. Although variations in temperature can be extreme mainly in winter and fall, most of the time they are moderated, mainly by the Gulf breeze and morning clouds. Rainfall variations can be extreme, with some years coming in near 10 to 20 inches of rain, and other years producing near 50 inches of rain. Average yearly rainfall is near 30 inches. The extremes vary from 10.11 inches in 1917 to 52.28 inches in 1973.