One technique they're experimenting with is so-called "dew-point cooling," which relies on the natural energy produced by evaporating water, rather than fossil fuels.
The NWS Web site features a handy on-line calculator for determining the heat index based on a combination of either temperature and relative humidity or temperature and dewpoint.
Like all clouds, it forms when the air reaches its dewpoint, which is the temperature to which an air mass must be cooled for the water vapor in it to condense into liquid droplets.