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Treebeard's Stumper Answer
15 January 1999

In a Fog

DMS students have a science assignment to get outside and do some star watching this week, so I'm not surprised that it's been foggy most nights. That's Murphy's Law. Fog doesn't look like much from the inside, but it can be very beautiful from above, like a swirling, fluffy blanket pulled up tight over the valley. I drive from my home in the mountains down into the fog every morning on my way to school. It usually starts suddenly, like a wall. Fog and clouds are just water vapor, so why do they have such sharp edges?

Why does fog have such a sharp edge? It helps to ask another question: When the morning fog burns off, where does it go? The water vapor is still there, but we can no longer see it. There's always some moisture in the air. It only becomes visible when conditions of humidity, temperature, and pressure are such that it condenses into tiny water droplets that scatter sunlight. The sharp and often beautiful surfaces of fog banks and clouds are actually a complex boundary of changing atmospheric conditions, like an equation made visible.

Notes:

Here's a nice GOES-8 satellite image I found on a NASA FTP site that shows the sharp outlines of fog filling the mountain valleys (with some higher clouds above):

Fog-filled valleys image

Humidity doesn't usually have a sharp edge, but dew point does. Graybear puts it succinctly:

Fog occurs when temperature, pressure, and humidity conditions conspire. Inside the cloud/fog the conditions are present in proper proportion, outside they are not.
Graybear also recognized the landscape in the photo:
The fog-filled valleys in the aerial photograph are the northern reaches of the Chesapeake Bay, which also reaches the Shenandoah Valley where I live, about 400 miles from me. Also visible are the finger lakes of New York, and two of the Great Lakes.

Sometimes there is a definite edge to the humidity. In the summer, Santa Barbara is typically covered by a dense fog bank that moves in from the ocean at night and backs up against the east-west wall of the Santa Ynez Mountains. This results in a temperature inversion, with cool marine air below and warm inland air above. (Air is usually cooler at higher elevations.) I drive from Santa Barbara to my home in the mountains in dense fog until I break through the upper edge, and it's suddenly clear, hot, and dry. The temperature can jump 30 degrees at the edge of the fog, and the humidity drops very low.

Watching the clouds is like watching a flame, it's irresistable. When clouds move across the sky, I've often wondered whether it's the actual water droplets that move or just the conditions under which they form. Are the clouds continually reinventing themselves along their edges?

I'm sure this is true for the clouds that sometimes form on our mountain tops, especially when a storm is approaching. (This is a good indicator of approaching rain, along with a south wind.) These orographic clouds form when pressure and temperature drop as moist air ascends the mountain wall and water droplets form. The wind keeps blowing over the mountains, but the cloud cap sits in place on the summit. I'm sure these clouds are a standing wave, like a wave over a rock in a river that stays in one place even though new water keeps flowing through it.

Orographic clouds image

While researching this stumper, I found the answer to another phenomenon that's long puzzled me. When storms approach the Santa Ynez Valley, I usually see a cloud cap on the mountains. I also see long bands of low dark clouds that run parallel to the mountain range, often with blue sky in between. The above picture shows why. Here's the explanation given by the UIUC Weather World 2010 site:

Upon reaching the mountain top, the air is heavier than the environment and will sink down the other side, warming as it descends. Once the air returns to its original height, it has the same buoyancy as the surrounding air. However, the air does not stop immediately because it still has momentum carrying it downward. With continued descent, the air becomes warmer than the surroundings and begins to accelerate back upward towards its original height (beginning the cycle again). It is during the upper-most ascent phase of this cycle that clouds develop. In regions where air is descending, skies are clear.

Here are a few Web sites with more information about fog and clouds:

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Copyright © 1999 by Marc Kummel / mkummel@rain.org