Fog… the word is associated with everything from lethargy to romance to suspense. Fog sets the tone in the most iconic movie scenes: the final goodbye in Casablanca, Jack Nicholson stumbling through a snowy hedge maze, Mr. Darcy striding across the fields at dawn. What creates this magical mist, and why is it so ephemeral? Let’s check out the science of fog.
The air that surrounds us consists of molecules in gaseous form, and although we breathe these molecules in and out and use them to fill balloons and fly kites, we can’t see them. Some of the many types of molecules that make up air are gaseous water molecules. When warm air is rapidly cooled, the water molecules lose energy and slow down. Some of the water molecules slow down sufficiently to condense around tiny dirt particles in the air, much like the “sweat” that condenses on a soda can in the summer. This produces tiny droplets of liquid water suspended in the air. This visible cloud of liquid water is fog, and this is what inspires our poems and cinematic art. If the air temperature rises or the humidity level falls, the tiny liquid drops of water in the air become gaseous, and the fog disappears.
Fog occurs when warm, humid air is cooled. When does this happen? One scenario is at the end of the day, when the ground cools as it radiates heat absorbed through the day. If the cushion of air close to the ground is humid, as it cools, the water vapor will slow and condense, producing “radiation” fog.
“Advection” fog forms when warm and cold air currents flow past each other. This happens at the Gulf coast when northern air currents as well as cold water from rivers work together to cool the humid Gulf coastal waters.
“Valley” fog forms along mountain slopes as air cools as it rises. The gaseous water slows and condenses, and the steep incline prevents the dense air from escaping.
Fog serves as a source of fresh water for people such as the Ecuadorian Galte community, who live at altitudes over 11,000 feet high up in the Andes Mountains. In 2014, engineers installed fog catchers, which are giant screens that catch water droplets as fog rolls in. The water collects on the mesh and drips into reservoirs below the screens. For every square yard of fog-catching screen, the Galte people collect up to 5 gallons of water, which they use to irrigate corn.
The next time you find yourself walking or driving through a cloud of fog, think about the amazing mix of air temperature and humidity that is happening at that moment to produce a delicate meteorological display, so quiet and beautiful. And keep an eye out for Mr. Darcy!
Citation: CARRERA-VILLACRÉS, David & Robalino, Iveth & Rodriguez Espinosa, Fabian & Sandoval Erazo, Washington & Hidalgo, Deysi & Toulkeridis, Theofilos. (2017). An Innovative Fog Catcher System Applied in the Andean Communities of Ecuador. Transactions of the ASABE. 60. 1917-1923. 10.13031/trans.12368.