Thursday, October 3, 2013

A Lesson on Fog


Whether it's a cancellation, a delay, or botched travel plans, fog can cause all kinds of issues for everyone. 
This week has been one of the foggiest of 2013 so far! But why?


There are a few different types of fog, each with different properties and catalysts. 
Radiation
Advection
Frontal
These are just a few that we deal with on a seasonal basis here in Northwest Ohio. 

Radiation fog may be considered the most common. 
Radiation Fog; Picture Courtesy crh.noaa.gov/
This type of fog typically forms at night or just before the sun comes up when conditions are nearly calm and skies are clear. Everyday, due to something called diurnal heating, warmth is absorbed by the earth's surface from the sun. 

Clear skies at night allow the surface temperature to cool down at a faster rate overnight, so when there aren't any clouds to act as a 'blanket' over the surface, warmth collected through the day is lost more rapidly in the evening.

Humidity levels reach 100%, and POOF: fog. 

That's also why recent rains typically cause fog by the next morning. Higher dew points allow the atmosphere to reach saturation quicker, leading to the development of these pesky low level clouds. 

Advection fog looks like radiation fog, but is caused by warm air moving over cold surfaces.
Advection Fog; Picture Courtesy crh.noaa.gov/

Warm air moving into an area can cause rising motion. Air that rises also reaches its saturation point sooner, causing fog to develop. This is a common occurrence over large bodies of water. Lake Erie, for example, will not 'radiate' heat at the same rate as the surrounding lake shores, thus the surface of the water is cooler. Advection fog forms when warm air drifts over that cold surface and water molecules condense out of the air, creating clouds. 

Finally, Frontal fog. 
Steam Fog; Picture Courtesy crh.noaa.gov/

This type of fog forms when a cooler, but drier layer of air, is brought to saturation through evaporation. This could be from rain drops falling into cool, dry air, or from evaporation coming off the surface of a lake or other body of water (also known as steam fog).

Now, in a situation like this week, school delays (and in some extreme cases, closings) are direct results of a foggy morning. So we'll look for these many factors to help forecast the arrival or development of early fog. 

Once it's developed, we'll often say that the sun will 'burn off' the early morning fog.
I promise, there is no fire involved!
It simply means that the sun's heat will help evaporate the moisture that is keeping the air saturated, and as conditions dry out, the fog lifts or dissolves.