MYSTERY CLOUDS PBL SCENARIO

The highest clouds in the Earth's atmosphere are made of frozen water, or ice crystals, just like some of the clouds that appear in the sky every day. Unlike more common clouds that form up to 5 miles above the surface of the Earth, these clouds are 50 miles high in a layer of the atmosphere called the mesosphere. Also unlike normal clouds, these clouds can only be seen near twilight, when the sun is just below the horizon and the sky is dark. For this reason, they are often called "noctilucent" clouds, or NLCs, because the word noctilucent means "night-shining." Scientists also call these clouds "polar mesospheric clouds", or PMCs for short, because they usually form only at high latitudes near the north and south poles. In recent years, however, several people have reported seeing NLCs at lower latitudes, even as low as 40*N in the continental United States, in Utah and Colorado. Also, NLCs seem to be getting brighter over time. Scientists do not understand why this is happening, and would like to find out. In particular, they wish to determine if these changes are caused by natural variations in the Earth's atmosphere, or if they are influenced by human activities.

A NASA spacecraft scheduled for launch in 2006 should provide some answers. The Aeronomy of Ice in the Mesosphere satellite, or AIM for short, will orbit Earth at an altitude of 550 km. The AIM satellite will have many sensors on board and will take wide angle photos of NLCs, measure the temperature and chemical abundances in the PMCs and surrounding air, and count meteoroids raining down on Earth.

Astronauts onboard the International Space Station have been observing electric blue "noctilucent" clouds from Earth orbit. But we still do not understand how they form. Indeed, although they're lovely, they're also controversial because scientists do not all agree on how they're made or how important they are for understanding our climate. Rather than wait and watch while the AIM satellite is launched, you and your classmates are being asked to investigate the source(s) of these noctilucent or "night-shining" clouds and NLCs potential role, if any, in the global climate.