Noctilucent clouds

These two kinds of clouds will be discussed separately. They are only visible after sunset, and both are formed at extreme heights- the stratospheric nacreous clouds at about 25 km and the noctilucent clouds even up to 80 km. The latter occur in the mesosphere. All other clouds are tropospheric and therefore can never extend higher than 12 km above the Earth's surface.

The appearance of noctilucent clouds is often preceded by an exceptionally bright yellowish twilight in the direction of the already set sun when the sky is very clear. Then these undulating silky luminous clouds appear against the dark sky. Because of their great height they are lit by the sun for a long time after sunset. This finishes when the sun is more than 15º below the horizon. Their colour is bluish silver and they are usually low above the horizon in the same section of the sky as the set sun. They are typical summer clouds and are virtually only observable from May to August. Apparently, only in those months does the vertical structure of the upper atmosphere allow their formation.

They occur most frequently in northern latitudes; at latitude 52º N they, are visible on about 25% of the clear nights of July. The more beautiful complexes among them are of course rarer. Because of their great tenuity these clouds are never visible at day.

The light of this form of clouds is Tangentially Polarized just like that of other clouds. Their polarization is, however, much stronger and may even reach a degree of polarization of as much as 96% at 90º from the sun. This is considerably stronger than that of blue sky. It is, nevertheless, not easy to see this polarization because of the very low light intensity of these mysterious clouds. Moreover, we hardly ever see them at angular distance, greater than 50º from the sun (in which case the degree of polarization is still about 50%). The polarization of the light from the sky, however hardly affects the situation because it is already so dark that the sky is emitting very little light. This kind of observation is best taken at as great an angular distance from the sun as possible; the nearer one looks at the sun the lower the degree of polarization.

Unfortunately, the brilliant nacreous clouds do not occur in many countries. They appear on the lee of long mountain chains, when a strong wind is blowing from a direction perpendicular to the chain. In Norway and at the Antarctic these clouds are regularly seen; and in east Scotland they sometimes appear. They can be observed most beautifully when the sun has gone down but is still able to shine upon the stratosphere; then they appear at about 20º from the sun. Their riot of colour is characteristic of these clouds. They indeed show 'mother-of- pearl' shades. Their light polarized, and according to Minnaert the colours change when the clouds are looked at through a polarizer and the filter is rotated. Such behaviour is to be expected when the cloud particles are about 0.8µm in size. In this case, polarization of the scattered light is already considerable at a small angular distance from the sun and depends strongly on colour and angular distance. Here also the dominant direction of polarization of the clouds is probably tangential with respect to the sun. On a journey to Norway, one should not lose the opportunity to try to see this unique phenomenon. The also colourful iridescent clouds, which can be seen anywhere, are unpolarized, and bear no relation to nacreous clouds.