The moon of the rainy season is an useful tool to judge the rainfall - provided the clouds do not hinder sighting the Moon.
One can find a halo around it during this season. The halo may be seen close to it or farther form it with a dark patch in between. The halo might be thick and bright or might just be vague and less glowing.
There are some meteorological information contained in these according to Brihad Samhita.
The halo is known as Parivesha. (The Upa-graha of moon also is known as Parivesha)
If the parivesha of the moon is close to moon - as though bordering the disc, then it means it is raining in places far away from the place where the moon is observed like this.
The thicker, the brighter and bigger the parivesha is, then the rainfall also is heavier. The broader it is (starting from the disc) it means the rainfall is wide spread.
If the parivesha is away from the moon, that means there are rains in nearby areas. The farther away the parivesha, the closer the area of rainfall. If the wind direction and clouds are surging, it also means rains will come to the place of observation.
This kind of knowledge of parivesha of the moon was common in those days.
That helped them in planning agricultural operations and also travel plans in particular.
Halos around the sun and the moon can be seen in winter months until Maagha (Maasi) when there will be dew-fall. If seen so in those months, they are not indicative of rainfall in the immediate place. They are indicative of good rainfall after 6 and a half lunar months. That means the rainy season of the coming year will be bountiful.
There may be times when two hazy suns will be seen - due to atmospheric conditions. If seen so, then that is the most unfortunate thing. It means the arrival of deluge - heavy, incessant and torrential rains destroying everything on the ground.