Sheetali pranayama is a breathing technique to reduce heat in the body.  It is often referred to as the “Cooling Breath”.  Using the cooling tendencies of the tongue, sheetali pranayama only allows cool, relaxed breaths to enter the body.  These breaths have the effect of lowering the temperature of the body, as well as cooling the mind.  People with disorders resulting from excess heat in the body/mind, such as high blood pressure, acidity, or ulcers will benefit greatly from the daily practice of this pranayama.  Remember, since this technique is very cooling to the body, it is preferrable to practice it in warm weather compared to cold weather.

For further reading, check out these links:
http://yogashaastra.blogspot.com/2008/05/beat-heat-with-sheetali-sheetkari.html
http://www.healthandyoga.com/HTML/news/sheetali.html
http://living.oneindia.in/yoga-spirituality/yoga/sheetali-surya-bhedan-pranayama.html

–Rishi