Evaporative Cooler Theory
Evaporative coolers require low relative humidity in the atmosphere to operate because they work by drawing outside air through water-saturated pads. The action of evaporation from the wet pads cools the air, but without low ambient humidity, the water will not evaporate well. Evaporative coolers can cool the air up to 30 degrees F when the humidity is only 20 percent, but when humidity climbs to 50 percent, the air will only be about 10 degrees cooler than the outside air.
Climate of Central Texas
Central Texas has what's known as a humid subtropical climate. That means it doesn't get very cold or icy in the winter, it does get hot in the summer, and the area gets considerable rain. Except for drier West Texas and the Panhandle, all of Texas is humid subtropical, and that means relative humidity levels are too high for evaporative coolers.
Refrigerated A/C Is Needed for Central Texas
If you are in central Texas, and you want your home or business to be consistently cool through the summer heat, you really have to use a refrigerated air-conditioning system. These systems work well in even high humidity.