Fog



Fog in Appalachian Mountain Valleys (NOAA)


Fog is a cloud on or close to the ground and like all clouds there are a number of items which must be present for the cloud to form.

For any cloud to form the relative humidity much reach 100%. And the two ways the relative humidity can be increased are by adding moisture to the air and by cooling the air.

Adding moisture has little to do with the fog we had this morning or the fog I expect tomorrow morning because those fogs are all about cooling the air.

Water can be added to  the air and form fog when air passes over a large body of water  like the ocean or the Great Lakes  or very wet soil or rain falls through the air. The water that is evaporated in each of these situations increases the relative humidity and when it reaches 100% condensation takes place and the formerly invisible gaseous becomes very small visible liquid drops.

If the temperature is below freezing the small liquid drops freeze on contact with tree limbs, wires, your windshield and deck railings and form a milky ice called rime. The fog is then called freezing fog.


When air cools the molecules in the air, including water vapor, slow down and eventually some slow enough so that when they collide or pass just close enough to each other the forces of attraction between molecules are stronger than the energy of motion.  When two water vapor molecules "stick" together we call it condensation and the formerly independent water vapor molecules are now part of a tiny water drop. The are now liquid water molecules.

A relative humidity of 100% means all the available energy has been used to do the work of evaporation and no more evaporation can take place and as air cools further condensation must take place.

The only differences between water vapor molecules and liquid water molecules is how fast they move and water vapor molecules are independent of others because they move too fast to stick together.  Liquid molecules stick together in drops because they have less energy.

So on clear, cool, calm nights the molecules slow as the temperature drops. When the molecules start to stick together we call it condensation. Eventually enough stick so a visible drop has formed and when trillions and trillions of drops are floating in the air we call it fog.

To be complete water condenses easily on substances that are said to be hygroscopic or water attracting. Water will not spontaneously condense in very clean air when the relative humidity reaches 100% but will quickly if bits of dust, sea salt, nitrate fertilizer etc. that are hygroscopic are present and there is plenty of all of these in the air over the tristate.  Yes even microscopic bits of sea salt are easy to find high in the atmosphere here.