DISCUSSION: In order for cloud droplets to form, they must have something to condense onto in our atmosphere (e.g., grain of dust, sea salt, etc.) Thus, it is thought that aircraft, for example, can aid cloud formation by emitting particles in their exhaust in addition to water vapor. However, scientists in Finland conducted a study where they found a different way for aircraft to potentially enhance precipitation processes. The picture above (image credit: Michael Bryant-Mode) is a dramatic illustration of the potential interaction between aircraft and clouds.
We have to first understand some basic ideas about precipitation formation before understanding the results of the study. When cloud droplets or cloud ice crystals first form, they are too small to fall as precipitation. In a pure liquid or pure frozen cloud, bigger droplets/crystals fall faster than smaller ones, collide with and stick to the smaller droplets/crystals, and eventually become large enough to fall as precipitation. In our atmosphere, water often doesn't freeze at 32 degrees Fahrenheit, but can exist in liquid form down to -40 degrees Fahrenheit (i.e., supercooled water). Thus, clouds can and often do contain a mixture of ice and liquid water. In this situation, the ice grows at the expense of the liquid, and this growth process is often much quicker than if the cloud was pure ice or pure liquid.
Imagine there is a cloud of supercooled liquid water (no ice) through which an aircraft flies. As the plane's wings and/or propeller moves through the air, the pressure and density of the air change such that temperature rapidly drops in a small area. This temperature decrease can result in the freezing of some of the supercooled water which can then trigger the accelerated ice growth process described above. These large ice crystals can then fall faster, collide with other smaller ice crystals, and grow even faster. Thus, even if aircraft produced no exhaust, the study from the Finish scientists indicated that 6-14 times more precipitation could be produced over a small area than if no aircraft flew through the cloud.
This is an example of another way that human activity can potentially influence the weather, perhaps in a way that we haven't thought of before.
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© 2019 Meteorologist Dr. Ken Leppert II