Nacreous and Noctilucent clouds form not in the part of the atmosphere in which we live, but much higher up, in the stratosphere and mesosphere. Both are rare clouds and can only be seen, briefly, at sunrise and sunset when all the other lower clouds are in shadow.
Nacreous clouds
The lowest of these clouds is nacreous, which normally forms between 10 and 20 miles above the Earth (compared with ice cloud cirrus at around 6 miles). They don't appear to move and are probably as a result of rapid uplift of air over a mountain range.
There is some speculation that they form more readily after large volcanic eruptions...
There is some speculation that they form more readily after large volcanic eruptions as the resulting dust particles combine with the ice crystals. It appears that the temperature has to be below minus 80 degrees Celsius, so these clouds are seen mainly from Scotland northwards during the northern winter.
They are also seen in high latitudes in the southern hemisphere winter with some particularly good displays over the Antarctic. These clouds are commonly known as 'mother of pearl clouds' due to the beautiful colours, and are at their most brilliant when the sun is some 5 to 10 degrees below the horizon.
Noctilucent clouds
Noctilucent clouds are the highest clouds in the sky at a height of about 50 miles, which puts them at the top of the mesosphere. These can only be seen during the summer between about 50 and 65 degrees north and south. At any higher latitude it doesn't get sufficiently dark enough to see them.
These clouds look very similar to high cirrus but are bluish or silver in colour and, like the nacreous clouds, are illuminated when the sun is below the horizon, in this case some 6 to 12 degrees.
...most scientists now believe that they are made of water ice.
There has been a lot of speculation on what they are made up of and how they are formed, but most scientists now believe that they are made of water ice. But how do these ice particles exist in what should be the warmest part of our atmosphere, and how do they get there in the first place? The answer is probably gravity waves.
The jetstream or mountains that transfer the lower level momentum upwards often form these waves. These gravity waves can change the local temperature fields, as well as transport water vapour from below, and can actually produce some of the lowest temperatures in the atmosphere for a short period of time. Although, of course, temperature and heat has a totally different concept in this very rarified atmosphere than we experience at the surface of the Earth.
Another reason why observing the weather can be so much fun. It's not just looking for the obvious or the common sights, but keeping an eye out, after dark, for those illusive but beautiful clouds that look down on all but the satellites. |
