Radiation fog is the most common type of fog. It is formed when heat from the surface radiates back to space at night, cooling air near the surface to saturation and producing fog. To get radiation fog, you need clear skies and cooling, moisture in the air (a wet ground really helps), and light winds. Light winds help mix this cool, saturated air to higher levels in the atmosphere, creating a deeper layer of fog.
Most fog reaches a few hundred meters up, but sometimes radiation fog doesn’t even hit above the knee. If winds are calm, there is no mixing of the atmosphere and the saturated air at the immediate surface is stagnant, leaving a very thin layer of fog at the ground. This type of fog is called ground fog, and it is a specific type of radiation fog.
Radiation fog is the most common type of fog for most places in the Pacific Northwest. October, with its rapidly shortening days and occasional storms to moisten the ground, is the foggiest month for most places west of the Cascades, with fog commonly forming in calm periods between storms. From November-January, persistent storminess decreases the amount of foggy days west of the Cascade crest, but a strong inversion and “cold pool” develops over the Columbia Basin, helping trap moisture and cold air near the surface and creating ideal conditions for radiational fog to form.
As we transition into spring, we see longer days and generally do not have the large high pressure systems needed for fog formation. Seattle gets little fog over the summer because of the relatively dry air and warm temperatures there.
Summer is the foggiest time of the year for the Pacific Coast, but they experience a different type of fog: advection fog!
Advection fog forms when relatively warm, moist air “advects” (moves) over a cooler surface. The surface cools the air to the dewpoint, resulting in fog. Advection fog is common over coastal regions where the ocean is significantly cooler than the surrounding land. The Pacific Coast during the summer is a prime example, and San Francisco is famous for its summer advection fog.
Upslope fog forms when moist air rises up terrain and condenses into clouds. Upslope fog is common on mountain slopes and passes. Interstate 90 will often be perfectly clear leading up to Snoqualmie Pass, but as soon as you get to the pass, you are shrouded in fog and are left wondering how it is possible that there were bluebird skies just a couple miles away!
Steam fog forms when cold air moves over warm water… kind of like advection fog in reverse. It is much more shallow and less dense than radiational, advection, or upslope fog. Although steam fog is rare in the Pacific Northwest, it can be seen during our occasional winter arctic outbreak. Look for it over bodies of water during our next cold snap!
Frontal fog is generally associated with warm fronts and forms when precipitation evaporates into the atmosphere. Not only does this increase the moisture content of the surrounding atmosphere – it cools the atmosphere due to a process known as evaporative cooling. As this continues to occur, the temperature decreases and the dewpoint rises, eventually converging at the “wet bulb” temperature. If additional moisture is added to the air after the wet bulb temperature has been reached, frontal fog results.
Freezing fog is a subtype of other types of fog – most often radiation fog or upslope fog. It occurs when the temperature is below freezing but the cloud droplets remain in a liquid state, rendering them supercooled.
Freezing fog is by far the most dangerous type of fog. It can quickly glaze a thin layer of black ice over roadways and runways, turning them into an ice rink within a matter of seconds, When temperatures are below freezing and fog rolls in, it’s best to stop driving and wait for the road to clear up. It could very well save your life.
If temperatures are below -20 degrees Fahrenheit, fog is often composed of ice crystals instead of supercooled water droplets. This type of fog is called ice fog and is less hazardous than freezing fog due to the fact that the ice crystals do not glaze surfaces with a layer of ice.
Next time you see fog, see if you can determine what type of fog it is!
Written by Charlie Phillips – charlie.weathertogether.net. Last updated 12/1/2017