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- The summer polar mesopause is the coldest region of the Earth's atmosphere, reaching temperatures as low as -140°C. It is sufficiently cold for noctilucent ('night shining') clouds to form in summer, at altitudes around 83 km.
Noctilucent clouds can only be seen when the sun is shining on them (at ~83 km) and not on the lower atmosphere, i.e. when the sun is between 6 and 16 degrees below the horizon.
They are a summer, polar phenomena but because of the restrictive viewing conditions they are most commonly observed at latitudes between 55 and 65 degrees.
Noctilucent clouds were first reported in 1885 when they were independently observed in Germany and Russia. This was two years after the volcanic explosion of Krakatoa in the Straits of Java.
One hypothesis is that the initial observation of noctilucent clouds was related to an increase in the number of observers of the twilight skies attracted by the spectacular displays resulting from the globally distributed volcanic debris of Krakatoa. Alternatively, water vapour injected into the upper atmosphere by the volcano ultimately reached the cold, dry upper mesophere.
Subsequent observations have proved that noctilucent clouds are not solely related to volcanic activity, and their volcanic association is now scientifically contentious. It has been alternatively claimed that the appearance of noctilucent clouds is the earliest evidence of anthropogenic climate change.
Noctilucent cloud observations from north-west Europe over the last 30 years show an increasing trend in the number of nights on which the clouds are observed each summer season, superimposed on a decadal variability that appears to be solar-cycle related.
Competing anthropogenic explanations for this increasing occurrence of noctilucent clouds focus on either excessive greenhouse cooling of the middle atmosphere or increased water vapour linked to increased methane release associated principally with intensive farming activities.
They have been observed thousands of times in the northern hemisphere, but less than 100 observations have been reported from the southern hemisphere. It has not been resolved if this is due to inter-hemispheric differences (temperature &/or water vapour) in the atmosphere at these altitudes, or the lack of observers and poorer observing conditions in southern latitudes.
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| Abstract from DBPedia | Noctilucent clouds, or night shining clouds, are tenuous cloud-like phenomena in the upper atmosphere of Earth. When viewed from space, they are called polar mesospheric clouds (PMCs), detectable as a diffuse scattering layer of water ice crystals near the summer polar mesopause. They consist of ice crystals and from the ground are only visible during astronomical twilight. Noctilucent roughly means "night shining" in Latin. They are most often observed during the summer months from latitudes between ±50° and ±70°. Too faint to be seen in daylight, they are visible only when the observer and the lower layers of the atmosphere are in Earth's shadow, but while these very high clouds are still in sunlight. Recent studies suggest that increased atmospheric methane emissions produce additional water vapor once the methane molecules reach the mesosphere – creating, or reinforcing existing noctilucent clouds. They are the highest clouds in Earth's atmosphere, located in the mesosphere at altitudes of around 76 to 85 km (249,000 to 279,000 ft).夜光雲（やこううん, 英: noctilucent clouds, NLC）は中間圏にできる特殊な雲で、日の出前や日没後に観測される気象現象である。地球上で最も高い高度に発生する雲と言われる。夏になっている北・南どちらかの半球の高緯度地域で多く発生する。極中間圏雲 (polar mesospheric clouds, PMC) とも呼ぶ。 (Source: http://dbpedia.org/resource/Noctilucent_cloud) |
