AIM - Education and Public Outreach

Clouds in the mesosphere have similarities in their appearance to tropospheric clouds but are typically described as being silvery blue. Their most common distinguishing feature is that they can be seen at night time when the Sun is below but not far from the horizon. This is why these clouds are often called "noctilucent" or night shining clouds (NLCs). The reason they are visible is because they are so high, near 83km (50 miles), that even though the Sun is no longer shining on the surface of the Earth, it is still shining on these high clouds. The water vapor available in the mesosphere is reduced by a factor of more than 100,000 compared to the troposphere, so this is a very difficult location for clouds to form in. The required temperatures for ice formation (at the pressure of the mesosphere) are less than -120C. It is a subject of debate as to what the nucleation site is for NLCs, but the most common theory is that NLCs form on meteoritic dust left from passing comets and collected as the Earth travels though their orbits. It is known that a layer of this dust exists at NLC altitudes, but it is not well known how many dust particles exist there or how variable their number are.