Distance travelled ~ 465'586'300 km: day 181
Noctilucent clouds are a summertime phenomenon which were first observed in around 1885. Noctiluscent [ the name means night shining in Latin ] are high wipsy clouds made of tiny crystals of water ice up to 100 nanometers in diameter. They are the highest clouds in the Earth's atmosphere, occurring in the mesosphere at altitudes of around 76 to 85 kilometers (47 to 53 mi).
Clouds in the Earth's lower atmosphere form in a process called nucleation when water gathers on dust particles, but Noctiluscent clouds also form directly from water vapour as well as forming around on dust particles. It is unclear where the dust or water in the mesosphere comes from but it's thought that the particles may be from dust from micrometeors, although some scientists think dust from volcanoes may also be involved. The source of the water is equally unclear as the mesosphere contains very little moisture - approximately one hundred millionth that of air from the Sahara desert but it's possible that the water comes from lower in the atmosphere or from chemical reactions in the upper atmosphere. This water vapour freezes directly into ice crystal to form the clouds in the thin upper atmosphere when temperatures drop to about -120 °C (-184 °F).
For many years Noctiluscent clouds were a very rare sighting, but over the past 20 years they have become more common. Originally confined to the higher latitudes they are increasingly observed in lower latitudes nearer the equator. So why are they becoming more common and reaching lower latitudes?
Nasa's AIM satellite mission (Aeronomy of Ice in the Mesosphere) which launched in 2007 was set up to study the Noctilucent clouds and to answer these questions.
Dr James Russell at the University of Hampton explains to me some of the findings of the AIM mission (James Russell III of Hampton University, Hampton, Va. Is AIM's principal investigator):
"Noctilucent clouds are the highest cloud in the Earth's atmosphere forming in the mesosphere at high altitudes (approximately 76 to 85 kilometers, or 47 to 53 miles). It seems odd that they are a summer time phenomenon when they feed off extremely cold temperatures, however as heat warms the air near the ground, the air rises. As it rises, it also expands since atmospheric pressure decreases with height ( temperatures in the mesosphere down past a freezing -210º F (-134 ºC).
We are still unsure exactly why they are increasing in lower latitudes or showing up brighter, they are like a geophysical light bulb, you go from no clouds to full formed clouds in days. This may be due to a sudden change in temperature at the altitude that these clouds are formed. They form in an atmosphere with 100 times lower pressure than at earth surfaces.
During the summer season the temperature stays very low at the poles. For a long time we thought the increase in frequency was a result of temperature decrease but now our research is leaning more to water vapour. Increase in water vapour increases the frequency of clouds. The primary reason for more water vapour at higher altitudes is methane which we are most likely responsible for.
Our research still has far to go however. We have been at solar minimum whilst the AIM mission has been out. Heating is different and dynamics is different so we need to continue our research for a full solar cycle."
The mission has now been extended until 2014 and Dr Russell thinks that the additional research may show a link between frequency in Noctilucent clouds and human activity and that this data may prove helpful to climate scientists investigating climate change.