Fall is getting off to a slow start this year. It gets cool for a couple of days, then it warms right back up (not that that’s a bad thing!). As the cool days outnumber the warm, though, you’ll begin to notice a lot more noise at night and in the morning, and it’s all thanks to the weather.
Whether you think about it obsessively or with passing curiosity, the weather affects our everyday lives in more ways than you can imagine. Its effects range from the obvious—do I need a coat?—to the subtle, such as how we perceive our surroundings.
The chilly days gearing up to sweep most of the Northern Hemisphere will often bring about a phenomenon known as a “temperature inversion,” which is where a layer of warm air develops above a layer of cooler air.
Here’s an example of one of these inversions using a simulated SKEW-T chart, which traces the temperature and dew point data collected by weather balloons. The isotherms—lines of constant temperature—are the dark blue lines skewed from bottom-left to top-right. The red trace shows the temperature through the atmosphere. The altitude is shown by the gray numbers on the left, in thousands of feet.
In this example, the surface temperature is about 11.7°C/53°F. As you get a little higher off the ground, the air cools to 10.2°C/50°F. However, between 1,000 feet and 1,900 feet above ground level, the temperature skyrockets to 18.9°C/66°F before dropping off again with altitude.
That 800-foot layer of warmer temperatures is a classic (and extreme) example of a temperature inversion.
The majority of the inversions we experience around this time of the year are the result of radiative cooling, or the ground cooling off faster than the atmosphere above it, thereby cooling the air near the surface through conduction. (Radiative cooling is also the cause of so many foggy fall nights, and why fog can sit just a few feet above an open field like some freaky effect in a horror movie.)
These inversions prevent air below them from rising up through the atmosphere—this is why places in valleys (like Salt Lake City) can choke on pollution for weeks during the fall and winter. The AP image at the top of this post shows smoke rising and hitting an inversion in Pocatello, Idaho, back in December 2005.
If you’re not a hardcore weather geek, you’ve probably heard the nerds on The Weather Channel or your local news mention these features during the spring as a capping inversion, or a “cap” for short. Caps play a significant role in severe weather outbreaks; if the temperature below the cap is able to grow warmer than the inversion above it, it can lead to violent updrafts and the rapid development of powerful thunderstorms.
Inversions are also the reason we have ice storms. Warm air that gets trapped between two layers of sub-freezing air can melt snowflakes into sleet or freezing rain, turning a winter wonderland into an icy deathmatch for anyone caught beneath it.
Thankfully, though, most inversions are harmless.
What does this have to do with noise? When there’s a strong temperature inversion present, it can affect sound much like an echo in a big, empty room. The density difference between the cool air near the surface and the warmer air above the ground can reflect sound waves back toward the ground, allowing localized noises to be heard over a much greater distance.
The most common example of this auditory nightmare is highway and airport traffic. If you live a few miles from an interstate or busy airport, you can sometimes hear the traffic when the wind blows the right way. However, when there’s an inversion overhead, the noise can be even louder than when the wind alone is carrying it to your location.
Inversions are responsible for all sorts of strange noises. The constant stream of helicopter tours in New York City can make residents’ lives even more miserable on cold days. The whirring, banging, clanging, and grinding from manufacturing plants and construction sites can reach for miles around when a strong inversion is in place. Thunder that occurs in the vicinity of an inversion layer can travel dozens of miles, startling (and confusing) residents enjoying otherwise calm weather conditions.
Folks who live near military bases and quarries are very familiar with the phenomenon; the shockwaves from munitions testing and blasting can reflect off of an inversion, rattling (and sometimes shattering) windows in nearby towns. The MythBusters team had to replace a bunch of windows in Esparto, California, back in 2009 after the shockwave from one of their experiments bounced off of an inversion and blew through the adjacent community.
As our nights grow longer and temperatures fall off a cliff, you can blame the weather if your normally quiet surroundings are interrupted for a couple of months.