The photo above shows radiation fog overspreading Alhandra, Portugal on the River Tagus, about 12 mi (20 km) north of Lisbon. Only the steeple and top of Alhandra’s central church (Igreja de Alhandra) can be easily detected. Photographer António Ferreira Pinto; Antonio’s Facebook Web site
On clear calm mornings, fog often forms over the lower Tagus Valley during the cooler months of the year if the air is sufficiently moist. The photo was taken approximately 8:00 a.m. February 26, 2012.
Radiation fog or ground fog results when the surface temperature rapidly falls due to loss of infrared energy at night by radiative cooling. Since no shortwave (visible energy) is available at night to offset the infrared loses into the clear sky, the surface and air immediately above is cooled to the dew point. Once the air is saturated, fog envelops the surface.