Noctilucent clouds (also known as polar mesospheric clouds) are rare bright cloudlike atmospheric phenomena visible in a deep twilight (the name means roughly "night shining"). They are most commonly observed in the summer months at latitudes between 50° and 60° (north and south). Saturns atmosphere is made up of hydorgen, helium and methane ... Twilight in Denmark, just after sunset Twilight is the time before sunrise and after sunset when sunlight scattered in the upper atmosphere illuminates the lower atmosphere and the surface of the earth. ... Latitude, sometimes denoted by the Greek letter φ, gives the location of a place on Earth north or south of the Equator. ...

Noctilucent clouds over Lake Saimaa.

They are the highest clouds in the Earth's atmosphere, being located in the mesosphere at altitudes of around 85 kilometres, and are visible only when illuminated by sunlight from below the horizon while the ground and lower layers of the atmosphere are in the Earth's shadow; otherwise they are too faint to be seen. Noctilucent clouds are not fully understood, as under most understood meteorological concepts, clouds generally are not able to reach such high altitudes, especially under such thin air pressures. Noctilucent clouds over Lake Saimaa. ... Noctilucent clouds over Lake Saimaa. ... A Picture of Saimaa Saimaa, or Saimen in Swedish, is a lake in southeastern Finland. ... Cumulus of fair weather A cloud is a visible mass of condensation droplets or ice crystals suspended in the atmosphere above the surface of the Earth or another planetary body. ... Layers of Atmosphere (NOAA) The Earths atmosphere is a layer of gases surrounding the planet Earth and retained by the Earths gravity. ... The mesosphere (from the Greek words mesos = middle and sphaira = ball) is the layer of the Earths atmosphere that is directly above the stratosphere and directly below the thermosphere. ... A kilometre (American spelling: kilometer) (symbol: km) is a unit of length equal to 1000 metres (from the Greek words khilia = thousand and metro = count/measure). ... Satellite image of Hurricane Hugo Meteorology is the scientific study of the atmosphere that focuses on weather processes and forecasting. ... Altitude is the elevation of an object from a known level or datum, called zero level. ...

As an explanation, it was once proposed that they were composed of volcanic or meteoric dust, but they are now known to be primarily composed of water ice (confirmed by UARS). They appear to be a relatively recent phenomenon--they were first reported in 1885, shortly after the eruption of Krakatoa--and it has been suggested that they may be related to climate change. Recent evidence also suggests that most noctilucent clouds today are created by water exhaust from manned spaceflight, such as the Space Shuttle. Diamond Head, a well-known backdrop to Waikiki in Hawaii, is an ash cone that solidified into tuff Volcanic ash is the term for very fine rock and mineral particles less than 2 mm in diameter that are ejected from a volcanic vent. ... A burst of meteors A meteor is the visible path of a meteoroid that enters the Earths (or another bodys) atmosphere, commonly called a shooting star or falling star. ... A natural, 4 tonne, block of ice on a beach in Iceland Icicles Ice is the solid form of water. ... The Upper Atmosphere Research Satellite (UARS) is an orbital observatory whose mission is to study the Earths atmosphere, particularly the protective ozone layer. ... 1885 is a common year starting on Thursday. ... Krakatoa (Indonesian name: Krakatau) is a volcano near the Indonesian island of Rakata in the Sunda Strait. ... Variations in CO2, temperature and dust from the Vostok ice core over the last 400 000 years The term climate change is used to refer to changes in the Earths global climate or regional climates. ... The Space Shuttle Columbia seconds after engine ignition, 1981 (NASA). ...

Noctilucent clouds can be studied from the ground, from space, and in situ by sounding rockets; they are too high to be reached by weather balloons. The AIM satellite mission, scheduled for launch in 2006, is dedicated to research into noctilucent clouds. A sounding rocket, sometimes called an elevator research rocket, is an instrument-carrying suborbital rocket designed to take measurements and perform scientific experiments during its flight. ... Rawinsonde weather balloon just after launch. ... A satellite is any object that orbits another object (which is known as its primary). ... 2006 (MMVI) is a common year starting on Sunday of the Gregorian calendar. ...

See also

Noctilucent clouds.

{{Commons2| Image File history File links Noctilucent_pp_big. ... Image File history File links Noctilucent_pp_big. ...

External links

• AIM satellite mission: http://aim.hamptonu.edu/
• Gary Thomas's page at U. Colorado: http://lasp.colorado.edu/noctilucent_clouds/
• http://www.nlcnet.co.uk/
• http://www.meteo.helsinki.fi/~tpnousia/nlcgal/nlcgal.html
• Noctilucent Clouds in the Classroom: http://nlc.gats-inc.com/
• Solar Occultation for Ice Experiment (SOFIE): http://sofiedata.org
• NASA site explaining how noctilucent clouds may be created by space shuttles.
• Southern Noctilucent Clouds observed at Punta Arenas, Chile: http://ona.fi.umag.cl/~rmonreal/nlc02.html