Fog or dew forms when the temperature-dew point spread decreases to what value, causing the air to become saturated?

Fog or dew forms when the temperature-dew point spread decreases to zero degrees, indicating that the air has reached its saturation point. At this point, the air can no longer hold all the moisture present as water vapor, leading to condensation. This saturation is crucial for the formation of dew, as it condenses on surfaces, and for fog, which occurs when this moist air is suspended in the atmosphere, reducing visibility.

The significance of the temperature-dew point spread being zero is that it reflects the conditions necessary for moisture to transition from a gaseous state in the air to liquid droplets, which is essential for both phenomena. The other values mentioned—five degrees, two degrees, and one degree—would still indicate that the air is not fully saturated, hence fog or dew would not form under those conditions.