Relative humidity is a measure of the amount of water vapor in the air compared to the amount needed for saturation at a specific temperature. Dew point is the temperature at which air becomes saturated and condensation forms. Dew point is a better measure of moisture content because relative humidity can decrease as temperature increases even if dew point remains the same. Fog is likely when the air temperature and dew point temperature are the same, as cooling air to the dew point causes saturation because cooler air has less energy to overcome liquid bonds between water molecules.
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