New insights into mysterious ice clouds


According to the IPCC, clouds remain one of the greatest unsolved mysteries in Earth’s climate. Now a new measuring instrument promises to unravel these complex formations – from a weather satellite.

Montage of the meteorological satellite MetOp-SG in space.
Prof. Stefan Bühler
Cirrus clouds play a decisive role for climatic events.

On fair days, I can study the subject of my research just by looking out my office window: cirrus clouds, which look like feathers painted on the sky with delicate brushstrokes. They can be found at altitudes of six to fourteen kilometers, are composed of tiny ice crystals, and play a decisive part in the climate system. But we’re only beginning to understand just how they actually work.

To learn more about cirrus clouds, I have developed a measuring instrument that can peer inside ice clouds from the vantage point of a satellite. For the past 13 years, I’ve been trying to convince the European Space Agency (ESA) of the instrument’s value – and now I’ve finally succeeded: in 2022 the “ICI” (Ice Cloud Imager) will be launched into orbit together with the weather satellite MetOp-SG, where it will observe cirrus and other ice clouds for the next twenty years from 800 kilometers above the surface.

Clouds have an enormous influence on our climate. For one thing, they reflect part of the sun’s radiation back into space; for another, they capture part of the heat produced on Earth, which would otherwise dissipate into space. As such, they help shape the temperature on our planet.

That being said, not all clouds are created equal. Low-hanging clouds composed of water droplets reflect more short-wave solar radiation and let the Earth’s long-wave radiation pass through them – we can often feel their cooling effect firsthand. In contrast, cirrus and other clouds made of ice have very different physical properties: they let in more solar radiation and keep more heat near the Earth’s surface; as such, they have more of a warming effect.

The ICI will measure how much of the radiation coming from the surface a given ice cloud allows to pass through. To date, researchers have only been able to gauge this radiation in certain segments of the electromagnetic spectrum: one third of it is released into space before it can be analyzed. The ICI will penetrate this unexplored area and record radiation in the “sub-millimeter range,” i.e., at wavelengths of between 0.5 and 1.5 millimeters.

Thanks to various tests, we know that such measurements can be taken with the sensors available today. For example, together with British researchers we mounted a prototype of the instrument on an airplane and had the craft fly over ice clouds. We’re very satisfied with the results; though there’s still no guarantee that the ICI will work just as smoothly in space, but they’re a promising start.
The new data this approach yields will allow us to determine the makeup of a given cloud: how much ice it contains, how large the individual crystals are, and whether there are also water droplets. We need this type of information to refine our climate simulations, which can currently only portray clouds in a very rudimentary form: that’s true for today’s clouds, and even more so for the clouds of tomorrow, which will be affected by climate change. But we don’t yet know if there will be more or fewer ice clouds in the future, if their distribution will change, or if their composition will be altered. Thanks to the ICI we’ll be able to make more accurate predictions – and to follow the first changes “live.”

This content was first published as a guest article in the newspaper Hamburger Abendblatt in April 2018.

Prof. Stefan Bühler is a member of Universität Hamburg’s Center for Earth System Research and Sustainability (CEN) and Managing Director of its Meteorological Institute.  

Go to whole Abendblatt-series.