What is meant by the dew point and what might prevent a cloud from forming even if the temperature of the air is below the dew point?

The dew point is the temperature at which the air becomes saturated with moisture, leading to condensation. If the air temperature is below the dew point and no cloud forms, it could be due to a lack of condensation nuclei - particles in the air on which water vapor can condense. Without these particles, the process of cloud formation may be inhibited even when conditions are otherwise conducive.