On Tuesday night, many were surprised to see their relatively clear night skies obscured by what appeared to be low-hanging clouds.
Visibility across parts of interior and western Trinidad dropped as low as a few hundred meters as dense fog developed across the land.
Fog was reported in parts of Rio Claro, Tableland, Princes Town, San Fernando, Siparia, New Grant, Arima, Penal, Fyzabad, Mayaro, and Gasparillo as winds remained near-calm and moisture levels were high due to the passage of Tropical Wave 17.
While dense fog is uncommon, fog in Trinidad is a fairly regular occurrence, especially during nights with high moisture and relatively calm winds.
How does fog form?
There are many types of fog and different ways it develops, but in Trinidad and Tobago, the most common (and expected) type is radiational 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.
In the valleys of southern, central, and northern Trinidad, 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 and generally forms during the Dry Season when daytime temperatures are warm while nighttime temperatures are quite cool. This type of fog tends to dissipate very quickly once the sun comes up and starts to evaporate the fog layer.
Fog or Mist?
According to the World Meteorological Organization, the term “fog” is used when microscopic droplets reduce horizontal visibility at the Earth’s surface to less than 1 km, while the term “mist” is used when the droplets do not reduce horizontal visibility to less than one kilometer (1,000 meters). In practice, mist is considered synonymous with “light fog.”
Reduction in visibility depends on the structure of the fog and especially on the number, density, and size distribution of the droplets. This structure may vary a great deal in time and space. The air in “fog” usually feels damp, moist, or wet. When illuminated, individual fog droplets are frequently visible to the naked eye and are seen to be moving in a somewhat turbulent manner.
“Fog” forms a whitish veil that covers the landscape, while “mist” generally forms a fairly thin, greyish veil. When mixed with dust or smoke, fog may take on a faint coloration.
