Bella Vista in fog: Clouds drop from the sky

BELLA VISTA — After a two-week period of bitter cold — including sub-zero temperatures, snow and ice in Bella Vista — Jan. 23 staggered in with a fog, bringing temperatures above 40 degrees.

Fog and mist/rain continued to plague Bella Vista for the next three days as temperatures hovered between nighttime lows of 38 degrees and daytime highs of 62 degrees.

A factor in fog formation is how much moisture is in the air. The higher the dew point temperature, the more water vapor is in the air. For fog to form, the air temperature and dew point temperature have to be very close to each other.

According the the National Weather Service, fog forms in these conditions:

RADIATION FOG

This type of fog forms at night under clear skies with calm winds when heat absorbed by the earth’s surface during the day is radiated into space. As the earth’s surface continues to cool, provided a deep enough layer of moist air is present near the ground, the humidity will reach 100% and fog will form.

Radiation fog varies in depth from 3 feet to about 1,000 feet and usually remains stationary. This type of fog can reduce visibility to near zero at times and make driving very hazardous.

Valley fog is a type of radiation fog. When air along ridgetops and the upper slopes of mountains begins to cool after sunset, the air becomes dense and heavy and begins to drain down into the valley floors below. As the air in the valley floor continues to cool due to radiational cooling, the air becomes saturated and fog forms. Valley fog can be very dense at times. This type of fog tends to dissipate very quickly once the sun comes up and starts to evaporate the fog layer.

ADVECTION FOG

Advection fog often looks like radiation fog and is also the result of condensation. However, the condensation in this case is caused not by a reduction in surface temperature, but rather by the horizontal movement of warm moist air over a cold surface, such as warm moist air flowing over snow. Advection fog can sometimes be distinguished from radiation fog by its horizontal motion along the ground.

EVAPORATION OR MIXING FOG

This type of fog forms when sufficient water vapor is added to the air by evaporation and the moist air mixes with cooler, relatively drier air. The two common types are steam fog and frontal fog. Steam fog forms when cold air moves over warm water. When the cool air mixes with the warm moist air over the water, the moist air cools until its humidity reaches 100% and fog forms. This type of fog takes on the appearance of wisps of smoke rising off the surface of the water.

The other type of evaporation fog is known as frontal fog. This type of fog forms when warm raindrops evaporate into a cooler drier layer of air near the ground. Once enough rain has evaporated into the layer of cool surface, the humidity of this air reaches 100% and fog forms.

Whatever the reason for the prolonged fog coming and going and hovering over Bella Vista for four days, it was at times like moving through a cloud that had dropped from the sky.

(NWA Democrat-Gazette/Terri O’Byrne) Most days allow Lake Windsor residents to their see neighbors’ homes across the lake. Not so on Jan. 23 as fog drifted in and out of the lake valley throughout the day, offering rare backyard privacy.

(NWA Democrat-Gazette/Terri O’Byrne) The top of Wandsworth Drive in Bella Vista disappeared in a shroud of fog on Jan. 23.

(NWA Democrat-Gazette/Terri O’Byrne) Lake Norwood’s shoreline eerily disappears close to the dam Jan. 23 in a fog that smattered itself throughout Bella Vista.