As the Northern Hemisphere transitions from summer to fall, weather conditions become more favorable for the development of late night and early morning valley fog. With the lengthening of nights, the atmosphere has increasing amounts of time for the air temperature to cool down to the dew point temperature which will allow water vapor to condense into fog. As air cools, it sinks. So, air that is cooling will sink into valleys which will help to accelerate the rate in which the air temperatures cools to the dew point temperature in lower elevations.
Valley fog is most common on a clear, calm night during the late night and early morning hours. Rivers and streams that flow in the base of valleys can also enhance the fog potential because of the relatively warm water of these bodies of water. If the air temperature around a river or stream cools down to or below the temperature of the water, an instance source of condensation and water vapor becomes available for the production of fog. Visibility as a result of valley fog can drop from unrestricted across higher terrain down to near zero miles in just a few hundred feet! Valley fog can also make for some picturesque scenery during the morning hours as the sun is rising.
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