When should a Dispatcher be alert for fog formation?

A Dispatcher should be particularly alert for fog formation when the temperature and dew point spread is 3 degrees Celsius or less. This condition indicates that the air is likely near saturation and can easily become foggy. When the temperature approaches the dew point, the moisture content in the air becomes sufficient for condensation to occur, leading to visibility reduction in the form of fog.

Humidity reaching 100% can indicate the potential for fog, but it does not always guarantee that fog will form, as other atmospheric conditions are also significant. Similarly, heavy rainfall can lead to fog, but it is accompanied by other factors such as temperature and humidity that determine if fog actually develops afterward. Calm winds can contribute to stable conditions that support fog, but the critical factor that reliably predicts fog formation is the narrow temperature-dew point spread, making this choice the best answer.