[--MAIN HOME--] [--ALL HABYHINTS--] [--FACEBOOK PAGE--] CHARACTERISTICS OF LOW-LYING FOG METEOROLOGIST JEFF HABY Often fog forms first in low-lying areas, especially radiation fog (with light wind speeds). This happens for two primary reasons. First, the low-lying areas tend to have more moisture. Streams are located in low-lying areas and the soil moisture is also higher in low-lying areas. Vegetation may concentrate in the low-lying areas due to a better access to water. More vegetation leads to more evapotranspiration in the low-lying areas. Second, cooled air sinks to the lowest elevation. Cool air is denser than warm air and will thus, under gravity, pool into the lowest elevation it can find. Since cool air does not have to evaporate as much moisture as warm air to reach saturation, fog will form in the coolest air first. The cooler air will sink into the low-lying areas. This combined with more moisture in the low-lying areas will lead to fog forming first and being most dense in the low-lying regions, especially where the soil moisture and vegetation is highest. Again, this applies best to fog that develops on a clear night with light winds (radiation fog).