September 10, 2012

Change is in the air

By Eric D'Aleo, Naturalist
Photos by Eric D'Aleo

Can you feel it? The air has changed over the last few weeks from the hot, humid days of summer to the warm days and cool nights of autumn. I first noticed it one early morning in mid-August as the cool air drifted down from the open window and flowed to the floor where I was stretching. I shivered at the unexpected coolness on my skin. While I finished my exercises my mind started ruminating on what this change meant and one of the first things I thought of was fog.

I’ve always been excited by fog, even as a child, because it makes me perceive the world differently. I remember hiking through early morning fog on exposed mountain ridges or driving through fog on the highway when I was younger. I often felt transported to another world or as if some strange, white, amorphous alien had landed on earth. Even now I’m fascinated watching fog form as its transparent, ethereal arms of white haze slowly stretch out across a lake surface or reach up from a swamp, crawling silently across the ground like some ghostly creature, enveloping everything , or something from a book or movie foreshadowing some danger.

Fog often forms at night, so calm, cold weather, along with reduced visibility and damp, close air evokes feelings of solitude, isolation, contemplation, loneliness, and fear. The world seems hushed and silent; shapes pass in and out of sight like wraiths. The sudden unexpected sound of the slap of a beaver’s tail can startle you or the gnarled, twisted tree that looked harmless earlier in the day suddenly appears ominous and threatening. Humans are visual creatures relying more on sight than sound and fog often limits both of these senses, which causes us to feel disoriented and unsure.

To a meteorologist, fog is just a very low stratus cloud that often touches the ground. It’s made of tiny water droplets light enough to remain suspended in air. These water droplets reflect the ambient light that’s present in many different directions, giving fog its familiar white color. In order for fog to form, the air needs to be saturated with water. This happens when air reaches its dew point, the temperature at which water vapor in the air will condense, or moisture evaporates into the air, increasing its water content.

Autumn is the time of year of when conditions are most favorable for fog formation with long, cool, clear, calm nights. The chance of fog forming increases as the night progresses since the air usually cools off and reaches its lowest temperature around dawn. This is also when the air is most likely to reach its dew point and condense the water vapor into fog. Areas of low elevation, such as river valleys, are places where fog tends to form and rivers may contribute moisture to the air, increasing the humidity and the likelihood of fog.

There are various types of fog based on how they are formed but there some types that most people are familiar with. Radiation fog, also known as ground fog, forms when heat absorbed by the earth’s surface is radiated into space at night. When this process occurs under clear skies with calm winds, a layer of moist air develops near the ground. Once the moisture level reaches 100 percent relative humidity, a stationary fog of 3 to 100 feet in thickness may form. Radiation fog may also form as a low stratus cloud under a temperature inversion, which causes the cloud to gradually expand and descend to the ground. This type of fog is commonly formed in valleys with the air along the upper slopes of mountains and ridges cooling after sunset and draining down to the valleys below. As the air in the valley continues to cool due to radiational cooling it becomes saturated and forms fog. The fog in the valley can be very dense, making driving hazardous due to reduced visibility but it often dissipates quickly after sunrise as the suns warmth starts to evaporate it. This I can attest to experiencing on many autumn days when I drive through the Baker River Valley in the morning on my way to work.

Advection fog can be confused with radiation fog but forms as moist warm air moves over a cool surface, like an ocean, causing condensation of the moist air to occur. This causes the fog to move horizontally along the ground making it different from the more stationary radiation fog. A fog that comes in off the sea is an advection fog because oceans don’t radiate heat like land and never cool down enough to produce radiation fog. However, this type of fog may also form when moist ocean air drifts over a cold inland area, usually at night, when the land temperature drops due to radiational cooling.

A third type of fog known as evaporation fog is commonly formed over lakes, ponds, pools, and even hot tubs. This fog forms when cold air moves over warm water. When the cool air mixes with the warm moist air over the water, the moist air cools until it reaches 100 percent relative humidity and fog forms. This type of fog takes on the appearance of wisps of smoke rising off the water’s surface. It does not get very thick and usually does not rise above 30 feet in height, but having watched it rise and twist in light morning breezes on Squam Lake, it is still quite striking.

Keep a lookout in your travels over the next few weeks to see where and when fog forms in the areas around your home. Who knows maybe you’ll feel the change in the air as well.

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