Sky High

PROBLEM: In the troposphere, temperature decreases as height increases, so the height of a volcanic plume is typically determined by using infrared-based satellites to measure the temperature at the top of the plume, and then comparing that to a reference vertical temperature profile. But if a volcanic eruption pushes the plume into the stratosphere, this method becomes inaccurate because the ozone layer’s absorption of solar radiation causes the temperature in the stratosphere to increase with height.

SOLUTION: The parallax effect is the apparent difference in an object’s position when it is viewed from varying lines of sight. In a recent study published in Science, a research team utilized this effect to measure the plume from the massive eruption of the Hunga Tonga–Hunga Ha'apai volcano in January 2022. The area where that volcano is located is monitored by three geostationary weather satellites, and they recorded images of the rising plume every 10 minutes during the eruption. The researchers applied the parallax effect to those images and determined that the plume rose to an altitude of 57 kilometers (35.4 miles), far higher than any volcanic plume ever recorded. In fact, the plume is the first to be directly observed to have reached the mesosphere (which begins about 50 kilometers above Earth’s surface). “It's an extraordinary result, as we have never seen a cloud of any type this tall before,” says lead author Simon Proud of the University of Oxford and the National Center for Earth Observation. “Furthermore, the ability to estimate the height in the way we did (using the parallax method) is only possible now that we have good satellite coverage. It wouldn't have been possible a decade or so ago.” The researchers hope to develop an automated system that utilizes the parallax effect to calculate plume heights. The previous highest recorded plume—during the 1991 eruption of Mount Pinatubo in the Philippines—reached a height of 40 kilometers (24.85 miles). [Source: University of Oxford]

* For more content from the Bulletin of the American Meteorological Society, please click here.