After sunset these past few days we were offered a spectacle quite unusual at our latitudes. Although the sun had already retreated below the horizon, silvery cloud structures became visible, so-called “noctilucent clouds”.
Noctilucent clouds are not conventional clouds and thus of considerable interest to scientists. They are visible ambassadors of the climate change that is under way. Noctilucent clouds form at about 90 kilometres altitude. At this elevation they are illuminated from below by the sun long after it has set.
Very low temperatures, typically below –100°C, are necessary for these clouds to form. Such low temperatures can be found at altitudes around 90 kilometres at high geographic latitudes -- Earth's coldest location. The fact that we can now see these clouds at our relatively moderate latitudes means that also here it must have become very cold overhead.
EOC measures the temperature at this altitude every night with special equipment. And indeed, the GRIPS instrument (Ground based Infrared P-branch Spectrometer) measured temperatures down to –110°C at 90 kilometres altitude over Oberpfaffenhofen during the night from 05 to 06 July – good conditions for the formation of noctilucent clouds.
Climate change is leading to the continuous warming of Earth's surface. But at 90 kilometres altitude the opposite takes place. There, the atmosphere is continuously cooling. And so it is reasonable to assume that with time the conditions for the formation of such noctilucent clouds will also be met more often at moderate geographic latitudes. During the solstice, which occurs in June/July, temperatures at about 90 kilometres altitude drop to the lowest values of the year -- so the seasonal change is exactly opposite to that on the surface.
Measurements of the GRIPS instrument at 90 kilometres altitude over Oberpfaffenhofen