One of the tell-tale signs that we are fast approaching autumn in the Twin Tiers is the almost nightly occurrence of fog in many of the river valleys throughout the region. Over the past couple of weeks we have seen fog in parts of the area almost every single morning, and in general we can expect this continue for much of the next six to eight weeks. So what causes the fog to form, and why is it typically restricted to the river valleys? In this blog entry I'll attempt to explain the basics of fog formation, and I'll also touch upon the different types of fog that are common in our region during the late summer and fall.
The fog that we often see at this time of year is what meteorologists refer to as radiation fog. It forms as a direct result of radiational cooling, a process during which much of the heat that builds up at the earth's surface during the daytime radiates skyward in the absence of cloud cover. As the air temperature drops its capacity to hold water vapor decreases, which in turn causes the relative humidity to rise. Eventually the air temperature drops to the dewpoint temperature at which point saturation (100% relative humidity) has been reached.
When the air becomes saturated any water vapor contained within it quickly condenses into millions of tiny liquid water droplets, visible to the naked eye as fog. The fog can be reinforced if a layer of dry air is present just above the earth's surface, which causes evaporation and additional cooling at the top of the fog layer. Vegetation and warm bodies of water can also assist in the development of fog by further increasing the relative humidity at the surface through a process known as evapotranspiration, which is why fog often forms so readily throughout many of our local river valleys. One factor that can restrict the formation of radiation fog is wind, as this often causes milder air from above the earth's surface to "mix down" and prevent saturation from ever being achieved. The combination of longer nights and relatively light winds at this time of year is the primary reason why fog is so common.
Another type of fog that is commonly observed in our area is advection fog, which is produced when warm moist air advects (is transported by wind) over a much colder land surface. The air is then cooled from beneath to its point of saturation, causing fog to form. Advection fog tends to be much more widespread than radiation fog, and we most often see it during the late winter and early spring when warm moist air moves over a cold snowpack. Advection fog is also commonly observed in coastal communities during the late spring and early summer when warm humid air is advected over the chilly nearshore waters.