The Fog Blog
It's a calm, clear summer night. You're looking outside your window to the giant field nearby and see what looks like a cloud right on top of the ground. That's right, you are look at fog. A few months later, fog returns again the day after a snowstorm, as temperatures soared into the 50s. Although both instances created fog, or clouds forming near the ground, they were created by two difference mechanisms. Fog can last anywhere from a few minutes to even a few days. In some cases, the fog is so pronounced, it becomes ingrained in the local culture. In this blog, we will explore the development of these different types of fog.
Let's go back to the most recent scenario we laid out. Fog returned again the day after a snowstorm, as temperatures rose into the 50s. Is there a correlation? It turns out that the answer is yes. This called advection fog. To dive into the process of advection fog, let's first define what advection actually means. Advection is the usual horizontal movement of a mass of fluid1. So in an advection fog scenario, warm air slides (horizontally) above the cold air at the surface. As it does so, the cold air then chills the warm air mass to the dew point temperature, saturating the parcel of air. The moisture then condenses out of the cool air and forms fog! It doesn't need snow cover, though, for it to form. One iconic example is the San Francisco Fog during the summer. In this case, the clash of hot air within in the inland valleys, which can reach 100°F+ and the cool, California Pacific current are the players. When the cool flow turns onshore toward San Fran, as is often the case with the semi-permanent North Pacific high pressure, the advection process takes place, creating the well known fog.
(Advection fog spreading over the Golden Gate Bridge in San Francisco, CA)
While not as icocnic as the San Francisco advection fog, radiational fog can frequently occur, as described previously. To have radiational fog, there ideally needs to be:
- An afternoon with a small difference between the air temperature and the dew point, also known as a dew point depression.
- A saturated ground, to help maintain the low dew point depression.
- Clear evening clear skes with calm winds to help cool the air temperature. It is also know by the term radiational cooling.
- A surface inversion, or an increase of temperatures with height.
The more ingredients we have, the better the potential for radiational fog to develop. Radiational fog usually occurs on a localized scale. On a night that has radiational fog, it will more likely develop in rural areas first, away from the warmer urban heat island.
(Radiational fog on April 6th, 2015. Notice how the urban heat island of New York City does not have fog.)
If you want to try to predict fog yourself, we'll share a handy trick we learned in school. On a calm, clear night, find the highest dew point the day. If the actual nighttime air temperature is forecasted to drop a few degrees below that dew point, then radiational fog is favored to develop!
1 - http://www.merriam-webster.com/dictionary/advection