Early Morning Fog
September 28, 2001
The above photo shows a fog-shrouded tree with the Sun just beginning to break through the foggy morning air. September is one of the foggiest months across much of the US. At this time of year, the air is still humid, and the longer nights of early fall often allow the air to cool to the dewpoint temperature - the temperature at which moisture in the air condenses. If the night is relatively clear and the wind is calm, some of the Earth's surface heat will radiate to space (radiation cooling), causing temperatures near the ground to drop. Fog will form if the air temperature reaches the dewpoint temperature.