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.