original: Rainfall comes only 2 - 3 times a year, never exceeding 20 mm.
simplifications: Rainfall comes only 2 - 3 times a year and doesn't exceed 20 mm. it only rains 2-3 per year, but never more than 20mm. It rains only 2-3 times a year. Rainfall comes only 2 - 3 times a year. It never exceeds 20 mm. Rain falls only 2 - 3 times a year. It is never more than 20 mm. Rain is only 2 - 3 times a year. It is never more than 20 mm. It only rains 2-3 times a year. Rain is 20 mm or less, 2 - 3 times a year. Rainfall is only 2-3 times a year, have not ever exceeding 20 mm. Rainfall comes only 2 - 3 times a year.