The Long Foggy Night - WNCT

The Long Foggy Night

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GREENVILLE, N.C. -

Have you ever heard the expression that fog is just a cloud on the ground, well I’m going to give you a brief description of what weather processes it takes to form this ground cloud.

Fog is formed when air is cooled below its saturation point (dew point) or evaporation and mixing (moist air mixes with relatively dry air). Fog is maintained by continual cooling or by evaporation and mixing of vapor in the air (See Figure 7). There are several types of fog, but we’ll focus on two of them here.

Radiation fog or ground fog

Radiation and conduction (the transfer of heat caused by temperature differences) are the primary means for cooling nighttime air near the ground. It forms best on clear nights when a shallow layer of moist air near the ground is overlain by drier air. Since the moist air is shallow, it does not absorb much of the earth’s outgoing infrared radiation. The ground therefore cools rapidly and so do the air directly above it and a surface inversion (where un-naturally cold temperatures are closer to the ground) forms. The moist lower layer (chilled rapidly by the cooler ground) quickly becomes saturated and fog forms, thus the longer the night, the longer the time of cooling and the greater the likelihood of fog. Hence, radiation fogs are most common over land in late fall and winter. (See Figure 1)

Another factor promoting the fog is a light breeze of less than 5 knots. Although radiation fog may form in calm air, slight air movement brings more of the moist air in direct contact with the cold ground and the transfer of heat occurs more rapidly. The ingredients of clear skies and light winds are associated with large high pressure areas. So when a high becomes stagnant over an area then radiation fog can occur over man consecutive nights. Radiation Fog forms upward from the ground as night progresses and is usually deepest around sunrise (See Figure 2)

Fog often “burns” off with daytime heating/sun. The sun warms the ground, causing the air temperature in contact with the ground to increase. The warm air rises and mixes with the foggy air above, which increases the temp of the foggy air. In the slightly warmer air, some of the fog evaporates, allowing the sun to penetrate the ground, which produces more heating and soon the fog disappears.

Advection Fog:

Warm moist air moving over a cold surface forms advection (transfer of heat) fog. For advection fog to form, the surface must be sufficiently cooler than the air above so that the transfer of heat from air to surface will cool the air to its dew point and produce fog. (See Figure 5)

In the Pacific, surface water near the coast is much colder than surface water farther offshore. Warm moist air from the Pacific is carried by westerly winds over the cold coastal waters. Advection fog always involves the movement of air, unlike Radiation Fog.  (See Figures 8 and 9)

(In the U.S. fog is defined when visibility is restricted to 6 miles or less and the spread between air temp and dew point is 5 degrees Fahrenheit or less. When visibility is less than one-quarter of a mile, fog is considered dense.) See Figure 10. In fact, dense fog was to blame for a 100 car pile-up on a bridge in The United Kingdom back in September. (See Figure 6)

Hopefully, you can avoid an accident due to fog by heeding one of the following safety tips:

Safety tips for fog

Safety tip #1: Drive with low beams on—high beams actually reflect back to you and impair your visibility even more

Safety tip #2: Reduce your speed-fog gives you the illusion that you’re driving slowly when you’re actually speeding

Safety tip #3: Listen for traffic you cannot see-open your windows a little bit to hear

Safety tip #4: Use wipers and defrosters to help visibility

So the next time you see fog as you head out in the morning, you’ll know exactly how that forms and around the time it will begin to dissipate. Storm Team 9 is keeping you weather savvy and you can always stay up-to-date with your forecast by going to www.wnct.com/weather.

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