Friday, August 1, 2008
In Praise Of Pea Soup
When you feel the cool dampness of fog starting to roll in, pull out your camera and get ready for a rare opportunity
Nothing can add a sense of moment, mystery and beauty to a landscape like those beautiful wisps of water vapor we know as fog. It’s an elusive and hard-to-predict condition, and most of us consider ourselves lucky when we stumble upon it. Yet, like most things, the more you know about it, the less mysterious it becomes, and the better your chances are of getting “lucky.” So let’s look at “the pea soup,” why and when it happens and some of the pitfalls of trying to photograph in it.
What’s The Soup Of The Day? It’s important to realize that there are at least three major types of fog: radiation, advection and upslope. Radiation fog is my personal favorite because it occurs during times of calm winds and clear skies, when the temperature and dew point get close to one another.
Dew point, according to www.freedictionary.com, is defined as: “The temperature at which the water vapor contained in a volume of air at a given atmospheric pressure reaches saturation.” The dew point varies, depending on how much water vapor the air contains, with humid air having a higher dew point than dry air. When large droplets of condensation form, they’re deposited onto surfaces as dew. When smaller droplets form, they remain suspended in the air as mist or fog—lovely, lovely fog.
Radiation fog occurs because the clear skies allow the land to radiate heat freely, cooling the air near the surface to the dew point and causing fog to appear. This usually happens in the early morning or evening, when the light is sweet. Radiation fog is usually patchy; that’s to say it doesn’t blanket huge areas evenly. In morning conditions, as the sun rises, the temperature rises and burns off the fog. So you get those great conditions with shafts of sunlight penetrating the fog, which looks especially good in backlight.
I’ve run into radiation fog most often in the fall and the spring. When I was looking to shoot aerials for my coffee-table book about Bucks County, Pennsylvania, I watched the dew point closely until a clear night when it looked like the temperature and the dew point would meet up. Sure enough, there was wispy fog over the Delaware River and among the stands of beautiful fall foliage in the valleys. In Tuscany, Italy, the springs are humid and clear, and there’s often a morning fog that’s well worth skipping your morning cappuccino to photograph. (You always can get your caffeine later, after the fog has burned off!)
Advection fog occurs most often near the coast, when moist air moves in from over the water to over the land. The moist air condenses into fog when cooled by the land. This is the type of fog that occurs off the Maine coast and other seaside areas. Tough to predict, you need to know wind direction and surface temperatures along the shores, water and land. A high relative humidity also is a necessity.
I never got the hang of predicting this condition myself, but fortunately, the weatherpersons are really good at it, especially along those coasts where it’s a common occurrence. If you hear of 100 percent humidity in a nearby area and there are winds moving in your direction, you’re likely to experience a nice dose of advection fog that can last for days. It’s thicker and more persistent than radiation fog, but can be dramatic.
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