ELMIRA, N.Y. (WETM)- Early this morning we see widespread dense valley fog. It also didn’t burn off until close to noon, but why did it form and why did it take so long to dissipate.

Early Wednesday morning, high pressure built in and allowed skies to clear. The areas calm winds and clear skies, valleys especially, saw the formation of dense radiation fog with low visibility. The fog did not burn off until around the early afternoon; the reason for that is the lower sun angle around this time of year takes longer for the fog to burn off.

The type of fog we saw forms when there is a deep layer of moisture present at the surface; we had this because we had seen rainfall in the last 24 hours. The two other things needed are clear skies and calm winds. When there are clear skies the longwave and shortwave radiation is radiated back into space. As the moist air at the surface begins to cool it will cause the humidity to reach 100 percent and fog will form. Calm winds which are less than 15 miles per hour are needed because if the wind is stronger it will allow moist air and dry air to mix which does not support fog formation.

Valley fog is a type of radiation fog and it is one we commonly see. Valley fog forms when the warm air in the hilltops begins to cool and sink down to the floor. As the air continues to cool it will become saturated and fog will form.