What is fog and how does it help?

While driving through fog recently, I tried to recall what fog really is and why it occurs.

There are varying forms of water, such as liquid, solid and gas, but what is fog? It is actually a low cloud of air condensation, which is water that collects as droplets on a cold surface and is formed when our ground temperature is colder than the air temperature.

During the evening and night, the earth becomes colder, but our air temperature might be slightly higher. When the humidity is close to 100 percent, water molecules, called vapor, begin to collect droplets and fog is formed.

Spring, summer and early fall mornings are usually the time fog is most visible. Our overnight temperature is the lowest temperature reached daily, and this is the most common time we notice fog.

Radiation fog, or ground fog occurs at the lowest surface and is released back or radiates into our atmosphere at night. We notice it floating above the grass and can be very thick. This ground fog can reduce the visibility of drivers significantly and can quickly change from thin to thick.

Fog that occurs in valleys and mountains can be beautiful, yet hazardous. The lower the area, the colder it is and more dense fog becomes, especially in river valleys when the surface is colder than the air temperature.

As the sun rises and warms the earth, the fog also “lifts” or “burns off,” two terms heard frequently to indicate the drying of the earth’s surface.

Our winter months and early spring have its share of fog, called advection fog. As the cold temperatures from snow on the ground meets the warmer air, it moves horizontally by wind and covers a much larger area, remaining for a longer period of time. When advection fog condenses, it causes the snow to melt more rapidly, even when the warmth of the sun is not melting it.

On a warm or hot summer day, the ground, sidewalks and streets become very warm and if it begins to rain on this warm or hot surface, the water will evaporate which becomes vapor and rises. This evaporation fog, or steam fog, is named because it resembles steam rising from the surface. This is the exact opposite of the earth’s surface which is cooler than the air temperatures.

In spring, fog can be a good thing in the form of low clouds which hold in heat and reduce chances for frost. It is in spring that we should pay even more attention to fog if we are growing plants for food or enjoyment.

Fog forming early in low areas is a bad sign if you are a gardener or farmer, as these are frost pockets which can cause loss of buds on fruit trees and other frost issues.

An Ohio State University study suggests that there are methods of collecting water, especially in a desert area. Collecting fog is one way for this to be done. An easy experiment is to place two jars outside on a foggy evening, one jar with a cone shaped top (funnel) and one with a cylindrical top. If you have grooved surfaces, that is more efficient, but it is not necessary.

In the morning, gather the jars and compare each to notice which jar has more water. There should be slightly more water in the cone jar. You have just performed a scientific experiment comparing two methods.

To learn more about types of fog and formation, go to: http://go.osu.edu/fogtypes.

Kane Shipka is a Master Gardener Volunteer for The Ohio State University Extension in Mahoning County.


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