Noctilucent Clouds

Some unexpected demands have impeded my progress on the "transgenic
drug" series.  I've written part of what I planned to post
today.  But I want to work on it some more.  



In the meantime, here is a picture of noctilucent clouds, from href="http://antwrp.gsfc.nasa.gov/apod/ap060718.html">NASA's
APOD site.  We are all relieved that the space
shuttle Discovery has landed safely.  It turns out that the
space shuttle may be one reason that these unusual cloud formations
occur.

As explained by NASA:

Noctilucent Clouds Over Sweden

Credit: href="mailto:%20clearskies%20at%20bostream%20dot%20nu">P-M
Hedén

Explanation:
Sometimes it's night on the ground but day in the air. As the Earth
rotates to eclipse the Sun, href="http://antwrp.gsfc.nasa.gov/apod/ap060712.html">sunset
rises up from the ground. Therefore, at sunset on the ground, sunlight
still shines on href="http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/cld/home.rxml">clouds
above. Under usual circumstances, a href="http://antwrp.gsfc.nasa.gov/apod/ap980526.html">pretty
sunset might be visible, but unusual href="http://lasp.colorado.edu/noctilucent_clouds/">noctilucent
clouds float so high up they can be seen well after dark. href="http://www.clearskies.se/NLC%2016.htm">Pictured above
last month, a network of href="http://science.nasa.gov/headlines/y2003/19feb_nlc.htm">noctilucent
clouds cast a colorful but eerie glow after dusk near href="http://en.wikipedia.org/wiki/Vallentuna">Vallentuna,
Sweden.
Although noctilucent
clouds are thought to be composed of small ice-coated
particles, much
remains unknown about them. href="http://www.nrl.navy.mil/pressRelease.php?Y=2003&R=35-03r">Recent
evidence indicates that at least some href="http://www.gsfc.nasa.gov/topstory/2003/0522shuttleshine.html">noctilucent
clouds result from freezing water exhaust from href="http://antwrp.gsfc.nasa.gov/apod/ap021023.html">Space
Shuttles.