What causes smelly fall mornings?

COLUMBUS Ohio ( WCMH ) — Many of us have been enjoying beautiful weather the past few weeks. Some, however, have noticed and complained about the smell in the mornings.

While there could be a few explanations of what is causing the smell in certain areas, we can thank a couple of natural processes that make that smell more noticeable this time of the year, especially in the mornings.

Decaying leaves can add to the smell

This time of the year, we are seeing more and more leaves hitting the ground. Those leaves are starting to decay, which can cause a musty smell, especially if the ground is full of dew or otherwise wet. On cool, moist nights, this decaying smell will become stronger to our noses. If there is little to no wind at night, the smell can become even more apparent.

How does the weather help trap the smell?

With the warm afternoons and the cool and longer nights, we are getting larger temperature swings each day.

During the day, the lowest level of the atmosphere where we live heats up to very comfortable levels. While we’re enjoying the warm, the rate of leaf decay is also accelerating.

The beautiful weather pattern in parts of the country has also been dominated by high pressure aloft, which indicates sinking air. This gives us the dry, tranquil, and sunny pattern associated with the warm days.

At night, the warm air slowly rises, as does the heat collected by buildings, the ground, and other items. With the general pattern of sinking air with high pressure aloft, this creates a temperature inversion. This means there is an area of the atmosphere that is no longer cooling with height as it normally does.

The image above from the Storm Prediction Center at NOAA from Thursday morning shows the two temperature inversions. The first is near the surface, at about 600 feet above the ground, while the other inversion is roughly 4,000 feet above the ground. You can see these in the purple circle, where the red line makes a jog to the right.

Temperature inversions happen often and are not uncommon at all, but during this time of the year, we tend to see more of these close to the ground with our longer, cooler nights, and our warmer afternoons. Going back to the high pressure, when in direct influence, we typically do not have much wind either. Winds will help to mix the air more efficiently and will cut down on the likelihood of an inversion forming.

Inversions can cause the air below them to basically be trapped and not mixed well. So think about smog out west being trapped by an inversion, pollution, a shallow fog layer, or in this case, a smelly layer. The inversion will cause the air in that pocket to essentially recirculate.

As the sun comes up in the morning and begins to warm the ground temps, we will start to see winds picking up during the day, and the temperature inversions near the ground will go away.

As the sun starts to go down, as the air near the surface starts to cool faster than the air above it, the process can begin again, leaving us with a smelly fall morning.

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