Focus on Radiation Fog |
 Radiation fog (Photo Credit: M. Ritter) |
Radiation fog is one of the most common types of fog. Radiation fog forms at night when there are few clouds in the sky and the air is still. At night, the radiation balance of the surface is dominated by outgoing terrestrial (earth) longwave radiation. The lack of clouds enables much of the radiant energy to escape to space. As the surface loses energy it is cooled. The air in contact with the surface then begins to cool leaving the air aloft somewhat warmer. The cooling surface air reaches its dew point temperature and condensation begins to occur. Soon a cloud near the surface forms. A radiation fog can be seen in the photograph above. The picture was taken at about 7:30 am with sunrise being about an hour earlier. Moving from right to left notice the bottom of the fog. It appears higher on the right and lowers to the left of the picture. The structure of the fog is related to the surface beneath. On the right of the picture is an asphalt parking lot. To the left is a grassed playing field. The dark surface of the parking lot has absorbed incoming solar radiation, and is heating the air above. The added heat "burns off" the fog and causing it to rise. The grassed playing field remains cooler and hence a lower fog deck. |