Final answer:
Valley fog may not lift during the daytime if the temperature stays below the dew point, there is no wind to disperse it, the sun's strength is insufficient for evaporation, or the valley's geography traps the fog.
Step-by-step explanation:
When valley fog forms and does not lift during the daytime, there are several possible reasons. One reason might be that the temperature remains below the dew point, preventing water vapor from condensing out of the air. Because of the release of latent heat during condensation, when the dew point is reached, further temperature drops are prevented, keeping the fog stable. Another possibility is that there is no wind to disperse the fog, causing it to persist in the valley area. Furthermore, if the sun is not strong enough due to low angles or cloud cover, it might not provide enough energy to evaporate the fog. Additionally, the geography of the valley or low-lying area can naturally trap the fog, acting as a barrier against the dispersal of fog, due to the surrounding higher elevations.