WEATHER FOLKLORE: "When dew is on the grass, rain will never come to pass"
APPLICABILITY: Maximum application is in the morning time in the cool season.
ANALYSIS AND INTERPRETATION: Dew is seen on the grass in the morning. Dew forms from the temperature falling to the dewpoint overnight. Once the temperature drops to the dewpoint, any further cooling will produce dew. This situation is most likely to occur on a clear night. On a clear night the earth is able to emit radiation into space and thus cool much more effectively as compared to a cloudy night.
With dew on the grass it usually means the sky is clear. During the day, the clear skies will continue and thus it will be a sunny day with no rain.
We should assume that "rain will never come to pass" means for the daylight hours after the dew forms.
PITFALLS: Weather systems can move in during the daylight hours. Weather can change rapidly as you know. Also, this lore does not work well for air mass thunderstorms. It is often stable in the morning with dew but by afternoon the moisture and instability contribute to afternoon thunderstorms.