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Baby, it's cold outside. Unless you're living in Los Angeles, where people are so rich that they bought the sun and get to use it as much as they want. For the rest of us, it's cold. And you know what that means? Fewer sound shadows.

I blogged last winter about temperature inversions, which occur in areas where there is not a lot of circulation during the winter. I promise this will get back to sound shadows, but here's the gist of temperature inversions:

As sunlight passes through the layers of our atmosphere and reaches our bright shining faces, it doesn't actually do a lot to heat up the air. Air is a pretty bad conductor of heat, so it takes a while to warm up. Instead, the sun heats up the ground. The ground acts like a hot plate and gradually heats the air right above it. That warm air rises (Sincerely, physics.) and colder air drops down close to the hot plate. Then that warms up, continues the circulation, and pretty soon we've got a pretty toasty little bubble.

BUT when the ground can't heat up very quickly, due to factors like lower levels of sunlight or very cold nights, the little hot plate doesn't warm up and the air near the ground stays cold. The cold air sits near the ground and stays there.

Now normally hot air rises, but the higher up you go, the thinner the air gets. This decrease in pressure means mountain air is usually colder than valley air, even if the sun is shining on both. So it's actually normal to have the air get colder as you go higher in altitute. But when the ground stays cold and no longer heats up the air down low, then you get an inversion: where the air gets warmer as you go up. That means cold days for folks living in the valleys.

The air is like this for pretty much the extent of winter in the arctic, because the sun shines so little there. If you happen to live in a valley or enclosed region, an inversion can cause major problems. Pollutants pumped into the air are not warm enough to rise up and get out, and you don't have the regular circulation. The warm air puts a cap on the region, keeping pollutants trapped. This further clouds the air and keeps even more sunlight from reaching the ground. So it's a self perpetuating cycle. You need a storm system or major change in the weather to knock things back to normal. (The image to the right shows two shots from the same position overlooking the Salt Lake Valley. Guess which one was during an inversion?)

So for the most part, inversions suck. But as it turns out, you might be able to hear things better when there is an inversion.

If you've ever gone camping on a lake you might have noticed that in the daytime you can see a group of campers across the lake, but you can't hear them. However, at night you seem to hear them very well. People often attribute this to the absence of daytime noise, or the sense of quiet at night, but it actually has to do with the temperature of the air.

Sound waves, like light waves, can be refracted, or bent by the medium they pass through. Light passing through a glass of water, for example, can be bent so that the bottom half of a pencil sitting in the glass appears to be in a different location than the top half.

Sound waves and light waves are fundamentally different, but many of the governing principles are the same. The speed of sound through a medium is determined by the temperature of that medium (not so for light, which always travels at the same speed). The warmer the medium, the faster the sound waves can travel. Like taffy - much easier to manipulate when warm.

In the daytime, the warmer areas are near the ground and cooler areas are above it. Portions of the sound wave near the ground move faster, while portions higher up move slower. This causes the portion of the wave near the ground to bend upward. As it does so, it creates a "shadow region" near the ground where the sound wave does not pass. You can see in the image below (c. Dr. Dan Russel of Kettering University) how the acoustic shadow might make it hard to hear someone still within your line of sight.

At night or during an inversion when the temperature is cooler near the ground and rises as you go up, the acoustic shadow is reversed.

Winds can also cause acoustic shadow areas, and there are a few books about how this might have determined the outcomes of Civil War battles (like the number of troops you send might depend on how bad the battle sounds from afar).

Neat as this is, it seems a small compensation that you can hear people's conversations better when there is an inversion. It's pretty much a lot of this: "This weather sucks." "Yeah." "We should probably invest in some cleaner sources of energy before we all get black lung from inhaling all this pollution." "Yeah."

-Agent Utah


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