What is Fog?Donna L. Long

Here in the Delaware Valley, the last several days were thick with fog. The air had the misty look of a fantasy film. You can imagine hobbits whistling as they stroll along a shady path cushioned by pine needles or hear the galloping of a horse as a handsome elf advances toward you.

But my mystical fantasy imaginings do not match the magical reality of fog.

What is Fog?

Fog is water vapor that hangs in the air and takes different forms. It can become frost, dew, fog or clouds.

Fog is a collection of tiny water droplets that float in the air or near the earth’s surface. It is the water which evaporates from bodies of water such as oceans, bays, lakes, rivers, and marshes. Moisture evaporating from plants and soil adds to the water vapor.

Fog forms when the difference between air temperature and dew point is less than 2.5 °C (4.5 °F). It usually forms when the relative humidity is 100%.

Fog is a Cloud

Fog is basically a cloud that touches the ground. We can walk through something that is usually thousands of miles above us. It is usually a stratus-type cloud.

Fog and clouds are made up of the same material, water vapor. It forms when moist air cools. When the air cools and there is more water vapor in the air than the air can hold, the water vapor changes into small droplets of water, transforming into rain.

Fog Evaporates

Fog disappears as the air temperature rises. This explains why this kind of water vapor is often seen in early morning and “burns off” (evaporates) as the sun climbs higher in the sky, warming the air.

The overcast cloudiness didn’t burn off yesterday. It was just as cloudy at noon as it was at sunrise yesterday morning. I find it disorienting when I can’t see the sun or moon. It is almost as if I can’t tell which way is up.