Yann Dufournet

Dec 022009

Polar Mesospheric Clouds PMCs (also known as noctilucent clouds) exist at a height of about 76 to 85 kilometers. They are transient, upper atmospheric phenomena observed usually in the summer months at high latitudes (greater than 50 degrees)
 of both the Northern and Southern Hemispheres. PMCs are made of crystals of water ice formed at about −120 °C, although their formation mechanism are still not really understood nowadays. For more information, I let you refer to the wikipedia website : http://en.wikipedia.org/wiki/Noctilucent_cloud.

This image was acquired at an altitude of just over 200 miles in the pre-dawn hours of July 22, 2008 as the International Space Station was passing over western Mongolia in central Asia (credit NASA).
PMCs have been increasing over the past 30 years… sign of climate changes?
Nov 262009

This week, we come back to a mountain region (Mont-blanc in the Alps) to present a typical orographic cloud: the altocumulus lenticularis (two in this image):

These lenticular clouds often appear in the lee of the mountains due to the presence of gravity waves produced by moist air flowing over the mountain crests.
Nov 192009

The below image is a GOES-12 infrared imagery showing the location of the intertropical Convergence zone (ITCZ – red line).

In this area, the air from the subtropical regions of both hemispheres converges, leading to strong upward motion up to 15 km and divergence uloft (part of Hadley circulation). This zone is often associated with well developped cumulonimbus clouds with great vertical extension and thunderstorms. Many of the world’s rainforest are associated with these climatological low pressure system.

Nov 092009

Sometimes clouds are produced in an unusual manner….

Such cloud is most probably composed of water droplets created by the sudden drop of air pressure around the rocket

(1-X Rocket Lifts Off from NASA)

Aug 242009

  So I will start with a first picture from Australia (credit Mick Petroff).



Explanation: What causes these long, strange clouds? No one is sure. A rare type of cloud known as a Morning Glory Cloud can stretch up to 1000 km long and occur up to 2 km high. Although similar roll clouds have been seen at specific places across the world, the ones over Burketown, Australia occur predicably every spring. Long, horizontal, circulating tubes of air might form when flowing, moist, cooling air encounters an inversion layer, an atmospheric layer where air temperature atypically increases with height. These tubes and surrounding air could cause dangerous tubulences for airplanes when clear.