Atmospheric Science Web Page Photo Archives

 

Kelvin-Helmholtz Waves above Denali (Photo by K. Sassen)

Kelvin Helmholtz Waves (KH waves) form in a stably stratified atmosphere in the presence of vertical wind shear.

 

Noctilucent Clouds (Photo by K. Sassen)

Noctilucent clouds (NLC) are clouds that literally shine at night. We who live in Alaska are lucky to have the chance to see them, because they only occur in high latitudes where the summer upper atmosphere gets unusually cold. They form at incredible heights, at about 80 km (50 miles) above the Earths surface, which is ten times higher than the usual wispy cirrus clouds we regularly see. The place they form is called the mesopause, a very frigid layer where what little water vapor that is up there can freeze, maybe on meteoritic dust, to form tiny ice crystals. These pictures of NLC were taken at 230 in the morning on 5 August 2004, from near Cleary Summit to get a good view of the northern horizon. If you want to see a rare noctilucent cloud, you need to keep in mind that in Fairbanks there is only a brief period when you can see them, and that is mainly in August when the mesopause is cold enough! You can only see NLC when the sun is below the horizon and the sky is turning dark. Only then can you see the dim sunlight that is reflected off the tiny ice crystals. If you look carefully at the pictures, you can see some stars shining! So, look to the northern horizon a few hours after sunset and you may be lucky enough to see the elusive NLC.