Final answer:
To determine the likelihood of fog formation, a remote pilot should monitor the temperature and dew point spread since close values can lead to fog when relative humidity is high. Barometric pressure and wind conditions offer less direct indicators of fog formation.
Step-by-step explanation:
The best way for a remote pilot to determine the likelihood of local fog formation is to monitor the temperature / dew point spread. Fog is more likely to form when the temperature is near the dew point. The smaller the difference between the temperature and the dew point, the greater the possibility of fog formation. Monitoring wind conditions is less effective because fog can form even with some wind present, and it tends to dissipate fog rather than form it. While barometric pressure is related to general weather patterns, it is less directly related to the formation of fog compared to the temperature and dew point relationship.
Since relative humidity tells us how much water vapor is in the air compared to the maximum possible, a high relative humidity, particularly at 100% when it reaches the dew point, can lead to the formation of fog. This is because at the dew point, the air is saturated and water droplets can condense out of the air. Thus, conditions with high relative humidity and a small temperature/dew point spread are conducive to fog formation.