The correct option is High wind speed.
Explanation
Dew refers to water droplets formed by the condensation of water vapour on relatively cold surfaces of solid objects (such as stones, grass blades, and plant leaves) rather than on nuclei in the air.
For dew to form, the temperature of the object must drop below the dew point of the surrounding air. The ideal conditions for this cooling process are:
- Clear Sky: A clear sky allows terrestrial radiation to escape freely into space, facilitating rapid cooling of the ground and surface objects. Clouds, conversely, trap heat and retard cooling.
- Calm Air: Air must remain in contact with the cold surface long enough to cool down to the dew point. High wind speed causes turbulence and mixes the cold air near the surface with warmer air from above, preventing the necessary drop in temperature. Thus, high wind speed is not an ideal condition.
- High Relative Humidity: If the air contains a high amount of moisture, it reaches the saturation point (dew point) with only a slight drop in temperature.
- Cold and Long Nights: A longer duration of darkness allows for sustained terrestrial radiation, ensuring the surface temperature falls sufficiently.
Key Takeaway: The formation of dew requires conditions that maximize surface cooling and stability: a clear sky, calm air, high relative humidity, and long nights. Strong winds disrupt the cooling layer, hindering dew formation.