Dense Fog Advisory Through 8:00 This Morning

Fog begins to form when water vapor condenses into tiny liquid water droplets suspended in the air. Six examples of ways that water vapor is added to the air are by wind convergence into areas of upward motion;^[6] precipitation or virga falling from above;^[7] daytime heating evaporating water from the surface of oceans, water bodies, or wet land;^[8]transpiration from plants;^[9] cool or dry air moving over warmer water;^[10] and lifting air over mountains.^[11] Water vapor normally begins to condense on condensation nuclei such as dust, ice, and salt in order to form clouds.^[12][13] Fog, like its elevated cousin stratus, is a stable cloud deck which tends to form when a cool, stable air mass is trapped underneath a warm air mass.^[14]

Fog normally occurs at a relative humidity near 100%.^[15] This occurs from either added moisture in the air, or falling ambient air temperature.^[15] However, fog can form at lower humidities, and can sometimes fail to form with relative humidity at 100%. At 100% relative humidity, the air cannot hold additional moisture, thus, the air will become supersaturated if additional moisture is added.