If the temperature/dewpoint spread is small and decreasing, what type of weather is most likely to develop?

When the temperature and dew point are close together and the spread is decreasing, it indicates that the air is becoming more saturated. This situation is conducive to the formation of fog or low clouds since the cooling of the air, whether through radiational cooling at night or when warm, moist air moves over a cooler surface, allows the moisture in the air to condense into tiny water droplets. When the temperature approaches the dew point, the relative humidity can reach levels that result in condensation, leading to visibility reduction due to fog or the formation of stratus clouds at lower altitudes.

Clear skies are less likely in such conditions because significant moisture is present in the air. Similarly, while thunderstorms and freezing precipitation can occur under different atmospheric conditions, they are not typically associated directly with a small and decreasing temperature/dew point spread. Therefore, the combination of close temperature and dew point with a decreasing spread strongly points to the development of fog or low clouds as the resulting weather phenomenon.