Chapter of the Rains in the Year
Definition
The Chapter of the Rains is Masha'allah's mundane technique for forecasting a year's rainfall: whether it will be abundant or scarce, and whether more will fall early or late. It works from the annual Aries ingress and from the Sun's entry into Scorpio, then narrows to quarterly ingress charts, monthly lunation charts, and per-day positions of the Moon. The reading turns on the rain-indicating planets, the rain-prohibiting planets, and the four rainy signs, judged against the moisture of the signs they occupy.
In Tradition
In this branch of Arabic mundane astrology, planets and signs are sorted into wet and dry, and rainfall is forecast by weighing which dominate the relevant chart. The Moon and Venus are the strongest rain-indicators, Mercury also indicates rain, while the Sun, Mars, and Saturn prohibit it, Saturn excepted in a rainy sign. The four rainy signs are Scorpio, Cancer, Aquarius, and Pisces, read against the lord of the year.
In Practice
First check the season opener: when the Sun enters Scorpio, look to Venus. If she rises in the east before dawn, expect little rain early in the year and more late. Then cast the Aries ingress and take the lord of the bound of the rising degree as lord of the year for rains; a watery connection between that lord and Venus or Mercury promises more rain. For quarterly detail, cast ingress charts as the Sun enters Libra and Capricorn and judge the placements of Venus, Mercury, the Moon, and Mars. For monthly rain, cast a chart at each Sun-Moon conjunction or opposition and watch for the Moon joined to Venus or Mercury. For a single day, note the Moon entering an angle while connected to Mercury or Venus in a water sign. Throughout, count the Moon and Venus in rainy signs as the strongest promise of rain, and treat Mars or Saturn dominating dry signs as drought, cloud, or wind instead.
Historical Origin
The technique is set out in Masha'allah ibn Athari's Chapter of the Rains in the Year (sections 1-5), a distinct meteorological-horary work surviving alongside his On the Roots of Revolutions and On Rains. It is preserved in Benjamin N. Dykes's English translation in Works of Sahl & Masha'allah (2008); the related On Rains reached Latin Europe through the translator named in the manuscripts as Amagro Drogone.