Clouds above the troposphere

B. Geerts and E. Linacre


Clouds are found almost exclusively in the troposphere. The stratosphere is very dry, because vertical transfer is limited by the high stability and because any transfer would have to occur through the tropopause, which is so cold that the saturation vapour pressure is negligibly small. Yet on occasion thin veils of clouds are observed above the tropopause. Presumably these clouds consist largely of ice, although their exact composition is not known.

Nacreous clouds, also known as mother-of-pearl clouds, are stratospheric; they occur between 15 and 30 km. Large volcanic eruptions emit dust particles in the lower stratosphere. These may combine with ice to produce nacreous clouds. In fact, in the year following Mt Pinatubo eruption in 1991, many nacreous clouds where spotted by airline pilots flying in the twilight. Polar stratospheric clouds occur at about 20 km, where the air is at -80 C during the Antarctic winter. Their presence is a factor in the formation of the Antarctic ozone hole (Note 1.E)

Nacreous clouds are normally too thin to be visible from the ground (Fig 1). They are most common in two situations. One situation is when strong winds (and winds increasing with height) cross a long mountain ridge, such as the Rocky Mountains in North America. The resulting high-amplitude gravity waves may propagate into the stratosphere. Nacreous clouds there are the stratospheric equivalent to lee wave clouds (Ac lenticularis) in the troposphere. Large thunderstorm systems (mesoscale convective systems, MCSs) may also trigger nacreous clouds. The residual kinetic energy of convective updrafts within MCSs repeatedly poke through the tropopause. The combined effect of these overshooting tops is to bulge the tropopause upwards, and also to diffuse some water vapour in the lower stratosphere. The combination of water vapour transfer and uplift may result in a nacreous cloud above the remnants of an MCS.


Fig 1. A nacreous cloud (photo by Hans Nillson in Sweden)



Noctilucent clouds (Fig 2) occur in the upper mesosphere, at about 80 km. Their name derives from the fact that they can be seen from the ground when the Sun is 7-10 degrees below the horizon and only reflects off these very high clouds (1). It arises from the water vapour released upon oxidation of methane. The recent observed increase of such clouds is related to increased atmospheric concentrations of methane, a greenhouse gas. Noctilucent clouds are most common in the summer in polar regions. At this time the mesospheric lapse rate is close to neutral, and this makes uplift easier.

Fig 2. A noctilucent cloud (photo by Pekka Parviainen in Finland)


(1) The Noctilucent Cloud observers' homepage.