When can surface temperature be more important than air temperature in dew formation on grass?

Correct answer: When grass surface cools by radiation to dew point

Correct answer: A. Dew is liquid water that forms directly on a cool surface, such as grass. During a clear and calm night, the grass can lose heat by terrestrial radiation and become cooler than the air above it. If the grass surface reaches its dew point, the thin layer of air touching it becomes saturated. Water vapour then condenses into small droplets on the blades. B is wrong because completely dry air contains too little vapour for dew formation. C is wrong because strong midday sunshine usually warms the grass and promotes evaporation rather than dew. D is wrong because very low relative humidity means the air is far from saturation. The key distinction is that dew forms on a surface, so the surface temperature can control the immediate process even when the measured air temperature is slightly higher. Memory cue: cool surface plus moist air plus dew point produces dew.