III. Weather Phenomena

7. Fog

Fog is formed as extremely small and dense water drops which are invisible to the naked eye. It is suspended in the air near the ground. According to the definition by the World Meteorological Organization, only when the visibility is less then 1 kilometer is it considered to be fog.

People are often confused about fog and clouds. To use the most common comparison, a cloud appears in the sky and fog near the ground, so you could call fog a cloud on the ground. However, the reason why fog forms is different.

Fog formation is quite complex; we can divide fog by its formation pattern into radiation fog, advection fog, frontal fog, vapor fog and upslope fog, and so on. The most common types of fog in Taiwan are radiation and advection fog; they usually appear in winter and spring.

Radiation fog mostly appears in winter. On a clear night when the breeze is weak, the air on the ground is fairly stable, and when there is enough water vapor, the air in the lower sky will be cooled and condense into tiny water drops as a result of fast heat dissipation of the ground. These water drops are suspended in the air near the ground and form fog. This type of fog dissipates after sunrise when the ground temperature rises and the air returns to an unsaturated status. Many people think radiation fog predicts good weather. In fact it is because of good weather that radiation fog is formed, so when the fog disappears the weather will certainly be good.

The formation of advection fog is related to the horizontal movement of air flow. When warm and wet air travels past cooler sea or land surfaces, the lower part of that air will be cooled and condensed into fog. Provided with adequate wind direction and wind speed this fog can exist for a substantially long period of time once formed. This type of fog will dissipate only when the wind stops or changes its direction, thereby cutting the fog off from its source of wet warm air.

Frontal fog appears near a front. It is formed when cold air near the ground is condensed into fog as it meets with raindrops traveling down from the cloud.

The physical processes behind radiation fog and advection fog can also compliment one another to create advection-radiation fog. During the daytime in late winter and early spring, western Taiwan regions will often be affected by sea breeze; warm and wet air enters from the sea (advection). The advection-radiation fog will easily be formed by radiation cooling after nightfall. When this type of fog appears, it will reduce the visibility to 100 meters or lower. This will create grave danger to traffic safety. Regions along the highway, such as Linkou and Sanyi, often experience dense fog situations. Preliminary investigation has shown two major reasons: first, formation by advection-radiation and second, due to the elevated state of the region (terrace). Due to the geology of the region, when low clouds move into lower parts of the mountains they will become fog. In other words, this fog is in fact the cloud we see hover around the middle of a mountain.