According to Ayurveda, a year is divided into two periods or kalas : adaan and visarga. Aaadan kala is also called utarayana (northern solstice). During this period the sun and wind become strong and take away the cooling properties of the earth. It starts from mid January to mid July. Ritus (seasons) included are Sisra (cold and dewy season); Vasanta (spring season) and Grisma ritu (summer season).