Scientists explain why linden trees appear to 'smoke'
While walking outside in the evening, you may notice an unusual sight, as if linden trees were smoking. Nature scientists Arne Sellin and Olavi Kurina explain what is happening.
Based on visual observation, the phenomenon is most likely caused by small swarms of insects hovering above linden trees, not by any atmospheric, optical or tree‑related physiological process.
Blooming lindens attract huge numbers of insects in summer. Their rich nectar and strong scent draw bees, hoverflies, midges, mosquitoes and other hymenopteran and dipteran insects.
However, this is probably not about feeding. Many small insects, especially nonbiting midges, form upright swarming columns for mating. These swarms consist of male insects trying to attract females.
Most likely these are swarming nonbiting midges (Chironomidae), whose columns can hover several meters above a landscape marker such as a tree crown. Adult midges do not bite; they feed only on nectar and other sugar‑rich liquids, and many species do not feed at all as adults.
Footage shows dark formations that remain in place for many seconds and appear as several narrow, slightly wavering upright "smoke columns" above the linden crown. This shape results from dense clusters of small insects, where individual insects cannot be distinguished from a distance and their rapid movement blends into a blurry column because of camera exposure and low image resolution. The fact that there are several separate columns also fits insect behavior.
In smaller swarms, a single male has a better chance of attracting a female, so very large swarms usually do not form. Swarming, which typically occurs at dusk, is an important evolutionary mechanism for midges and is essential for mating.
Less likely explanations do not match the phenomenon well. Warm‑air shimmer often seen in landscapes occurs because air layers of different temperatures have different densities and therefore different refractive indices. As warm air moves and mixes with cooler air, light refraction constantly changes, making distant objects appear to shimmer. Warm‑air shimmer distorts the background but cannot form narrow dark columns.
Volatile organic compounds (VOCs) can also be ruled out, even though lindens emit significant amounts of them (mainly isoprene and various monoterpenes) on warm summer days. VOCs are invisible, colorless gases. Even though a large tree can produce them in high quantities under favorable conditions, they quickly disperse in open air to extremely low concentrations that cannot scatter visible light enough to become visually noticeable.
Rising air currents form above tree crowns warmed by the sun — this is convective air movement. Air itself is invisible, but dust or other fine particles in the air can make rising currents visible. Early summer also brings plenty of pollen, but linden pollen is insect‑borne and its grains are relatively large and sticky, so they do not travel on convective air currents the way wind‑borne pollen does.
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Editor: Sandra Saar, Argo Ideon
