BAA / RMetS Joint Meeting, 2004 November 27

Tropospheric Clouds

Moving down in the atmosphere from the high altitudes of the previous two talks, Dr Wilson recalled that a previous speaker had already remarked that "one man's signal is another's noise". For amateur astronomers, this was certainly true of the present talk. Before starting, he wished to credit those who had contributed to the RMetS Cloudbank, an online image library from which many of his images were borrowed, and which was freely available for all to browse.³

The speaker first explained that clouds were all composed either of water droplets, as in the case of cumulus or stratus, or ice droplets in the case of higher altitude clouds such as cirrus and altostratus. The simplest explanation for their existence was that the atmosphere contained water vapour, but could only contain a certain amount before becoming saturated, whereupon it condensed. The exact concentration corresponding to saturation depended upon the physical conditions: for example, warmer air could hold more water before reaching saturation. Clouds formed whenever water-laden air exceeded its saturation point, when it became favourable for the vapour to undergo a phase change to form water droplets or ice crystals. It was already clear that one mechanism by which this might come about in an air mass was cooling: as its temperature lowered, the saturation water-content lowered, so that previously unsaturated air might condense into clouds.

But the speaker added that it had been known since the 18th century that the rate at which air could hold more water with rising temperature itself rose with temperature. In other words, a profile of the saturation water-content against temperature was not a straight line, but it curved upwards. A very familiar demonstration of this was the rapid condensation of exhaled breath on a cold day. As the moist breath mixed with surrounding air, both its temperature and water content would average towards that of the surroundings, traversing a straight line across a water concentration vs. temperature plot. Given that the exhaled air did not start out saturated at its initial warm temperature, and that the surrounding air was similarly unsaturated, the two ends of this straight line lay below the saturation limit. Yet as the exhaled air travelled along the line, it was observed to condense – a sign that it had exceeded saturation – demonstrating the saturation limit to be curved. This itself was of profound importance in cloud formation: the mixing of two bodies of air at different temperatures and with different water contents could lead to cloud formation, even if neither was itself initially saturated.

Dr Wilson went on to discuss various other mechanisms by which clouds might form. The fog seen on the floors of Martian valleys, for example, was caused by the radiative cooling of the surface during the night-time, and the subsequent cooling of the layers of air directly above them. By contrast, the sea-smoke seen in Arctic regions – not strictly smoke, but rather fog – resulted from the mixing of cold dry air with warmer moist sea air. As with an exhaled breath, the mixing of these unsaturated air masses caused condensation.

However, one of the single most important mechanisms was the rising of air masses. In this case, as the altitude of the air increased, its pressure dropped to equal that of its surroundings, and it expanded. But because the air evolved adiabatically – without heat input – this expansion also caused it to cool. It turned out there were many mechanisms by which vertical air motions might be induced, for example by convection, or the pushing of air masses over the windward side of mountains.

As well as these orographic considerations, cloud formation also varied with latitude as a result of the uneven heating of the Earth. Pressure was found to decrease more rapidly with altitude in cold regions, generating horizontal pressure gradients at high altitudes, and in turn air flows toward cold regions. To balance this out, counter-flows were required at low altitudes toward warm regions, as well as vertical flows to connect the two and complete the circulation cycle. This effect culminated in falling air in the cold polar-regions, and rising air at the equator, with roughly three circulation cells in between, each spanning 30° latitude. The speaker remarked that the size of each cell was limited by the rotation of the Earth, the consequent Coriolis forces drawing each flow out of the latitudinal direction, and into the longitudinal direction. Whilst the details were complex, the rich latitudinal banding of the Jovian winds provided illustration of this: on a larger planet, the rotational forces were more significant, and each circulation cell was consequently constrained to a smaller range in latitude.

To close, the speaker discussed where might be the best site for a telescope. Broadly speaking clouds formed where air was rising, and so latitudes 30° N/S, where the prevailing flow was downwards, were best. However, this was not universally true: the Canary Islands were at this latitude, but were very often cloudy because of boundary layers where moist air mixed with dry. But the speaker remarked that at high altitudes, above these mixing layers, the skies were generally good, hence the choice of location of so many professional observatories on the mountains of Tenerife and La Palma. In addition, the Sahara region was also poor for another reason: because of dust storms. Further complication arose in the selection of sites, because thus far the talk had only been concerned with transparency: by contrast, seeing conditions depended upon the mixing of air masses of differing temperatures and refractive indexes in the jet stream, bending rays of light. Atmospheric turbulence was therefore also a matter of concern, quite aside from avoiding cloud.

Following the applause, a member asked, given that water vapour was always in the atmosphere, why it only prevented the passage of light when condensed. The speaker replied that vapour was composed of particles much smaller than the wavelength of light, and thus which had little effect upon it. By contrast, condensed water droplets were significantly larger than this wavelength. Prof Collier then proceeded to welcome Mr John Nayler, a former physics teacher with a long interest in amateur astronomy, and author of the recent book Out of the Blue on his present subject.