- Tishtrya or Tir is the name of the Yazad who presides over stars in general and the star Sirius in the constellation of Canis Major in particular. This Yazad also presides over rain. The process of rainfall is described in Tir Yasht and the Vendidad.
- In the Tir Yasht, it is allegorically described that Tishtrya in the form of a white horse battles Apaosha the demon of drought, who appears in the form of a black horse. Finally Tishtrya overcomes Apaosha and brings rains for the people of the earth.
- The Vendidad offers a scientific explanation for the rains. It states that rain occurs on account of the evaporation of water from the seas and rivers, which forms into clouds between the earth and the atmospheric planes. The clouds are scattered with the help of the wind (Govad Yazad).
- The Farvardin Yasht informs us that the Fravashis (Asho Farohars) help in the distribution of the rainfall and dispatch the clouds to places which are needy and deserving of rains.