Sitali “the cooling breath” is perfect for hot Summer time.
Sit in a comfortable upright, seated position. Spine nice and straight. Hands gently on your knees. You can have your legs folded, or straight in front of you if this is too hard. Keep your pelvis slightly tilted forward, you may sit on a block or bolster to support this gentle pelvic tilt.
We breathe in through a tongue curled like a straw (see left). We breathe out through the nose. If you are unable to curl the tongue you can perform Sitkari instead, where we breathe through the teeth that are lightly held together or sip through your lips like a straw. Inhaling over the tongue, cooling the blood and exhaling through the nose. Equal inhalation and exhalation, 1:1 breath and a minimum of 30 rounds, around 5 minutes to take effect.
This pranayama is a great way to prepare you during the hot summer months for the practice of yoga.
Working much like an evaporative cooling system that cools the air before it hits the lungs, we are impacting the gas exchange with warm gases being exchanged for cool gases. It cleanses the system of excess pita, helping with conditions such as nausea, skin eruptions, eczema, thirst, diarrhoea, fever, hyper acidity, anger, inflammation, menopausal symptoms and other signs of excess heat and toxicity. 
Please refer to this great youtube clip on how to perform this pranayama
 AYA Lecture Notes, D Salerno, 2013