Smog

 

Smog

Although freezing rain, thunderstorms, and tornadoes are significant weather events, one of the most important things impacted by an inversion layer is smog. This is the brownish-gray haze that covers many of the world’s largest cities and is a result of dust, auto exhaust, and industrial manufacturing.

Smog is impacted by the inversion layer because it is, in essence, capped when the warm air mass moves over an area. This happens because the warmer air layer sits over a city and prevents the normal mixing of cooler, denser air.

The air instead becomes still and, over time, the lack of mixing causes pollutants to become trapped under the inversion, developing significant amounts of smog.

During severe inversions that last over long periods, smog can cover entire metropolitan areas and cause respiratory problems for the inhabitants.

In December 1952 such an inversion occurred in London. Because of the cold December weather, Londoners began to burn more coal, which increased air pollution in the city. Since the inversion was present over the city, these pollutants became trapped and increased London’s air pollution. The result was the Great Smog of 1952 that was blamed for thousands of deaths.

Like London, Mexico City has also experienced problems with smog that have been exacerbated by the presence of an inversion layer. This city is infamous for its poor air quality, but these conditions are worsened when warm subtropical high-pressure systems move over the city and trap air in the Valley of Mexico.

When these pressure systems trap the valley’s air, pollutants are also trapped and intense smog develops. Since 2000, Mexico's government has developed a plan aimed at reducing ozone and particulates released into the air over the city.

London’s Great Smog and Mexico’s similar problems are extreme examples of smog being impacted by the presence of an inversion layer. This is a problem all over the world, though, and cities like Los Angeles, Mumbai, Santiago, and Tehran frequently experience intense smog when an inversion layer develops over them.

Because of this, many of these cities and others are working to reduce their air pollution. To make the most of these changes and to reduce smog in the presence of a temperature inversion, it’s important to first understand all aspects of this phenomenon, making it an important component of the study of meteorology, a significant subfield within geography.

Autumn Air

 

Autumn Air

Like winter, autumn's crisp, clean smell is partially thanks to the drop in air temperature which suppresses strong odors. But another contributor is autumn's hallmark symbol; its foliage.

Although leaf peepers are disappointed when fall's brilliant crimsons and golds fade to grayish-brown, this is when leaves take on their sweetest smell. During the autumn season, a tree's cells begin the process of sealing off its leaves in preparation for winter. (During winter, temperatures are too cold, sunlight too dim, and water too scarce and susceptible to freezing to support growth.) A corky barrier is formed between each branch and each leaf stem. This cellular membrane blocks the flow of nutrients into the leaf. As leaves are sealed off from the rest of the tree and lose moisture and nutrients they begin drying out and are further dried by autumn's sun and lower humidity. When they fall to the ground, they begin to decay; that is, they're broken down into essential nutrients. Also, when leaves are brown it means they're carbon-rich. The dry, decomposition process gives off a mildly sweet, almost floral-like aroma. 

Wondering why the leaves in your yard don't smell as sweet in other seasons? It's largely because they're full of moisture and are nitrogen-rich. An abundance of moisture, nitrogen, and improper aeration generates pungent, rather than sweet, odors. 

Tornadoes' Terrible Sulphur Scent

Most of us are familiar with the sound a tornado makes, but what about its accompanying smell? According to a number of storm chasers, including the late Tim Samaras, the air sometimes smells of a mix of sulfur and burning wood (like a freshly lit match) during a tornado. Researchers haven't determined why this is a recurring smell with observers. It could be from broken natural gas or sewage lines, but no one knows for sure.  

In addition to sulfur, others report the smell of fresh-cut grass during a tornado, likely as a result of tornado debris tearing tree limbs and leaves, and of the storm itself uprooting trees and turf.

Which smell you get depends on how close you are to the tornado, how strong of a twister it is, and what objects it destroys.   

Eau de Exhaust 

Temperature inversions are another weather phenomenon linked to atmospheric odors, but rather than trigger a certain smell, they exacerbate odors that are already airborne.

Under normal circumstances, air temperature decreases as you move from the ground up. However, under an inversion, this is reversed and air near the ground cools faster than that a few hundred feet above it. This setup of relatively warm air overlying cooler air means the atmosphere is in a stable configuration, which, in turn, means there are little wind and mixing of air. As the air sits motionless and stagnant, exhaust, smoke, and other pollutants build up near the surface and hang in the air we breathe. If you've ever been under an air quality alert in summer, an inversion (and the presence of high pressure domed over the region) is likely the cause. 

Similarly, fog can sometimes hold a light smoky smell. If gasses or dirt particles are suspended in the air and weather conditions are right for moisture to condense onto them, these pollutants essentially dissolve into the water droplets and are suspended in the air for your nose to breathe them in. (Such an event is different from smog, which is a dry "cloud" of smoke that hangs in the air like a thick fog.) 

Your Nose vs. Your Forecast 

While being able to smell the weather may mean your olfactory system is as acute as they come, take care not to depend solely on your sense of smell when sensing your weather risk. When it comes to forecasting approaching weather, meteorologists are still a nose above the rest. 

 

The Smell of Rain

Many people claim they can "smell a storm coming" (meaning they can sense when bad luck is headed their way), but did you know that this weather expression also has literal meaning?

It's true, there are some kinds of weather that actually do produce a unique smell, and we're not just talking the smell of flowers in spring. Based on personal accounts, here are some of the weather's recurring aromas, plus, the scientific reason behind them. 

When Rainstorms Wet Dry Earth

Rainfall is one of nature's most soothing sounds, but it's also behind one of the weather's most pleasing smells. Described as an "earthy" scent, petrichor is the aroma that arises when raindrops fall onto the dry soil. But, contrary to belief, it isn't the rainwater that you're smelling.

During dry spells, certain plants secrete oils that become attached to the soil, rocks, and pavement surfaces. When it rains, the falling water disturbs these molecules and the oils are released into the air along with another soil inhabitant; a naturally occurring chemical called geosmin that's produced by fungi-like bacteria. 

Had a recent rainstorm, but didn't have the lingering petrichor afterward? How strong the scent will depend on several things, including how long it's been since the last rainfall and rainfall intensity. The longer the geosmin and plant oils are allowed to accumulate during periods of dry weather, the stronger the scent will be. Also, the lighter the rain shower, the stronger the petrichor scent, since lighter rains allow more time for the ground's scent-carrying aerosols to float. (Heavier rains keep them from rising up as much into the air, which means less smell.)    

Chlorinated Clashes of Lightning

If you've ever experienced a lightning strike that's too-close-for-comfort or stood outdoors just before or after a thunderstorm, you may have caught a whiff of another rain-related scent; ozone (O3).

The word "ozone" comes from the Greek ozein meaning "to smell," and is a nod to ozone's strong odor, which is described as a cross between chlorine and burning chemicals. The smell doesn't come from the thunderstorm itself, but rather, the storm's lightning. As a bolt of lightning travels through the atmosphere, its electrical charge splits air's nitrogen (N2) and oxygen (O2) molecules apart into separate atoms. Some of the lone nitrogen and oxygen atoms recombine to form nitrous oxide (N2O), while the leftover oxygen atom combines with an oxygen molecule in the surrounding air to produce ozone. Once created, a storm's downdrafts can carry the ozone from higher altitudes to nose level, which is why you'll sometimes experience this smell before it starts storming or after the storm has passed.  

Unscented Snow

Despite some people's claims that they can smell snow, scientists aren't entirely convinced.

According to olfactory scientists like Pamela Dalton of Philadelphia's Monell Chemical Senses Center, the "smell of cold and snow" isn't so much about a particular smell, it's more about the absence of smells, as well as the nose's ability to sense that air is cold and moist enough for the weather to possibly turn snowy.

"We're not as sensitive to odors in winter... and odors aren't as available to be smelled," Dalton says.

Not only do smells not waft as easily when air is cold, but our noses don't work as well. The "smelling" receptors within our noses bury themselves more deeply within our nose, likely as a protective response against the colder, drier air. However, when cold air becomes more humid (as it does before a snowstorm), the sense of smell would sharpen ever so slightly. It's possible that we humans link this small change in smell to an oncoming snowstorm and hence, why we say we can "smell" snow.