Eternal Astro Physics, Sanatana Jyotis Shaastra,  has many branches. One of them is Astro Meteorology, which  is an integration of Meteorology, an atmospheric science and Astrology, the Wisdom of the Nine Revolving Heavens. The Solar transits of the constellations are studied  and each solar transit was found to have some characteristics. The Kerala mathematicians, after years of Experimentation, Observation and Hypothesis,  came up with the Kerala Njauttuvela Calendar. Intuitive Sciences involve the sixth element  of Intuition also.                                                        

According to Physics, the land gets heated faster than the Sea and cool winds from the Bengal of Bengal pick up moisture and pour over India, during Thula Varsha. The reverse happens during Edava Pathi  ( Half sidereal Taurus, that is when Sun enters the 15th degree of Edava or Taurus ) cool winds from the Indian Ocean pick up moisture and precipitate  over India ! 

This astronomical phenemenon was known to the Indian mathematicians who accurately devised the Calendar and the two Monsoons, Kala Varsha and Thula Varsha ! 

 

    Degrees        Event

Kala Varsha -   From May 29 to Sep 16

Thula Varsha - From Sep 17 to Dec 26

In Kerala, only three major seasons are experienced. They are more or less related to the revolution of the Earth or the Sun's longitude.

Shaithya, Winter -When Solar Longitude is in between 240 and 300. ( Dhanus, Makara )

Ushna, Summer - When Sun's longitude is in between 300 and 45 ( Kumbha, Meena, Mesha and 1/2 Vrishabha )

Varsha, Rainy Season - When Sun's long is 45 - 240 degrees,.( Half Vrishabha to Vrischika )

The Farmer's Almanac