If there is dew on the grass, it will not rain. “Dew forms when the grass cools down to the dew point (dew point). At night, the grass cools down as heat emanates from the ground. In a clean, dry atmosphere, this heat leaves the atmosphere and rises into space. In a humid atmosphere, water vapor can absorb some of this heat and carry it back to earth. It warms the grass and keeps it moist. This is a sign of a high pressure system and good weather. "