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Background Information Report

Meteorology and Climate

CSSA is located in the south-central part of Texas on the Balcones escarpment. Northwest of the installation, the terrain slopes upward to the Edwards Plateau; to the southeast, the terrain slopes downward to the Gulf Coastal Plains. This results in a modified subtropical climate, predominantly marine during the summer months and continental during the winter months. The resulting weather is characterized by hot summers with daily temperatures above 90°F over 80 percent of the time and mild winters with below-freezing temperatures occurring on an average of only about 20 days per year. The first occurrence of 32°F is in late November and the average last occurrence is in early March. Average annual temperature is 69°F. The highest average daily maximum temperature is 95°F in July, and the lowest average daily minimum temperature is 39°F in January. Temperature extremes for the period of weather records range from 0°F to 108°F.

CSSA is situated between a semi-arid region to the west and the coastal area of heavy precipitation to the east. Average annual rainfall is approximately 29 inches. Precipitation is fairly well distributed throughout the year, with the heaviest amounts occurring in May and September. Approximately 61 percent of the rainfall occurs over the period from April through September and is primarily due to thun­derstorms. Damaging hail seldom occurs, but light hail is common with springtime thunderstorms. Since CSSA is only 140 miles from the Gulf of Mexico, tropical storms occasionally affect the base with strong winds and heavy rains. Measurable snowfall occurs only once every 3 or 4 years.

The highest relative humidity occurs during the early morning hours (0600 hours) and averages about 84 percent over the year. Monthly averages range from 79 to 88 percent. Between 1200 and 1800 hours, relative humidity averages about 53 percent, with monthly averages ranging from 45 to 59 percent.

Northerly winds prevail during most of the winter. Strong northerly winds occasionally occur in conjunction with "northers," cold southward flows produced by an area of high pressure that invades the United States from Canada. Southeasterly winds from the Gulf of Mexico are predominant in the summer but also occur frequently during the winter. The average annual prevailing wind direction is from the southeast, and the average annual wind speed is 9 miles per hour (mph) with monthly averages ranging from 8 mph to 10 mph. The windiest months are typically March and April; September and October have the least wind. Figure 10 is a wind rose showing the frequency distribution of wind speed and wind direction. The wind rose was constructed from 1984-1989 National Weather Service meteoro­logical data recorded at the San Antonio Airport. It should be noted that the wind direction indicated by the wind rose is the direction from which the wind is blowing.

Skies are clear to partly cloudy on average about 225 days per year, or more than 60 percent of the time, and cloudy conditions occur less than 146 days per year, or less than 40 percent of the time. CSSA has more than 70 percent of the possible sunshine during the summer months and about 50 percent during the winter months.