Lenticular cloud and protected “dark” sky over Flagstaff

by Joe Archive on April 16, 2008

a onblur=”try {parent.deselectBloggerImageGracefully();} catch(e) {}” href=”http://apod.nasa.gov/apod/image/0804/flagstaffsky_usno.jpg”img style=”margin: 0pt 10px 10px 0pt; float: left; cursor: pointer; width: 320px;” src=”http://apod.nasa.gov/apod/image/0804/flagstaffsky_usno_big.jpg” alt=”” border=”0″ //abr /p b/b/pblockquoteb Explanation: /b This sky is protected. Yesterday marked the 50 year anniversary of the a href=”http://flagstaffdarkskies.org/”first lighting ordinance/a ever enacted, which restricted a href=”http://en.wikipedia.org/wiki/Searchlight”searchlight/a advertisements from sweeping the night skies above a href=”http://en.wikipedia.org/wiki/Flagstaff%2C_Arizona”Flagstaff/a, a href=”http://en.wikipedia.org/wiki/Arizona”Arizona/a, a href=”http://en.wikipedia.org/wiki/United_States_of_America”USA/a. Flagstaff now enjoys the status of being the first a href=”http://www.flagstaffdarkskies.org/idsc.htm”International Dark Sky City/a, and maintains a lighting code that limits lights from polluting this a href=”http://apod.nasa.gov/apod/ap070508.html”majestic nighttime view/a. The current dark skies over a href=”http://www.flagstaff.az.gov/”Flagstaff/a not only enable local astronomers to a href=”http://www.chrismadden.co.uk/moon/micro.html”decode the universe/a but allow local a href=”http://apod.nasa.gov/apod/ap040808.html”sky enthusiasts/a to see and enjoy a tapestry contemplated previously by every human generation. The above image, pointing just east of north, was taken two weeks ago at 3 am from Fort Valley, only 10 kilometers from central Flagstaff. Visible in the above spectacular panorama are the a href=”http://en.wikipedia.org/wiki/San_Francisco_Peaks”San Francisco Peaks/a caped by a a href=”http://www.crystalinks.com/lenticular.html”lenticular cloud/a. Far in the distance, the a href=”http://apod.nasa.gov/apod/ap070930.html”plane/a of the a href=”http://apod.nasa.gov/apod/ap070814.html”Milky Way Galaxy/a arcs diagonally from the lower left to the upper right, highlighted by the constellations of a href=”http://en.wikipedia.org/wiki/Cassiopeia_%28constellation%29″Cassiopeia/a, a href=”http://www.hawastsoc.org/deepsky/cep/index.html”Cepheus/a, and a href=”http://www.astronomical.org/portal/modules/wfsection/article.php?articleid=31″Cygnus/a. On the far right, the a href=”http://apod.nasa.gov/apod/ap000501.html”North America Nebula/a is visible just under the very bright star a href=”http://www.astro.uiuc.edu/%7Ekaler/sow/deneb.html”Deneb/a./blockquote p/pp /p script type=”text/javascript” digg_url = ‘http://apod.nasa.gov/apod/ap080416.html’; digg_skin = ‘compact’; /scriptI am more interested in the lenticular (lens-shaped) cloud over the San Francisco Peaks in the image. Lenticular clouds are caused by air moving across the peaks – as the air approaches the peak, it rises, following the contours of the terrain. As the air rises, it expands with the decrease in pressure. This expansion leads to cooling. If the air cools sufficiently, it can become saturated, leading to rapid condensation of the moisture in the air, forming the clouds.br /br /As the air goes over the peak and descends, the air is compressed and heats back up. As the temperature increases, the parcel ceases being saturated and the moisture evaporates.br /br /The cloud appears to be stationary, while the air moving over the peak is flowing quite rapidly – the moisture is quickly condensing in parcels of air as they reach the level where the air saturates, and the cloud persists in the parcel until it exits the region of condensation, usually a very short time afterwards – The cloud is not a long lived collection of droplets, rather it is an area in the atmosphere where moisture is (briefly) condensed.br /br /The cloud is flattened because it relies upon a large vertical displacement of relatively moist air. Air further above the terrain is both displaced less (so it cools less) and is usually drier (so it would have to be cooled more to condense).br /br /br /a href=”http://apod.nasa.gov/apod/ap080416.html”Astronomy Picture of the Day/a

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