World Meteorological Day - 23.03.2017 – Understanding Clouds

            World meteorological day is celebrated all across the world by the member state’s meteorological organizations every year on 23rd of March. It is an annual event being commemorated yearly by the almost 191 meteorological organization members worldwide as well as the worldwide meteorological communities using a particular chosen theme of the year in order to commemorate the establishment of the World Meteorological Organization to keep constant watch on the weather and climate for the better life and future. The celebration of the day was started on 23rd of March 1961. It was on this day in 1950 World Meteorological Organization came into being.

            WMO is an intergovernmental organization established for meteorology. It originated from the “International Meteorological Organization” which was established by the International Meteorological Congress at Vienna, Austria in the year 1873. Whereas, World Meteorological Organization was first established on 23rd of March in the year 1950 and had became the specialized agency of the United Nations to operate the weather and climate from one place as well as performing the operational hydrology including all the related geophysical sciences. The headquarters of the World Meteorological Organization is in the Geneva, Switzerland. Every year the day is celebrated with a central theme in focus. The theme of the world meteorological day celebration 2017 is “Understanding Clouds”.

            When most people think of a cloud they know what it is, a white puffy thing in the sky. But when pushed to be precise it is more difficult. Formally speaking a cloud is an aerosol that is a suspension of one phase of matter in another. The glossary of the American Meteorological Society defines an aerosol as follows: A colloidal system in which the dispersed phase is composed of either solid or liquid particles, and in which the dispersion medium is some gas, usually air. A characteristic of an aerosol is that it is disperse. It is not one thing, but many things, or many repetitions of the same thing, dispersed in space. This makes the boundary of the aerosol, or a cloud, somewhat difficult to define objectively and precisely. But clouds are a special type of aerosol, so special in fact that we rarely speak of a cloud as being an aerosol. Clouds are a type of aerosol that comes into being when the atmosphere becomes supersaturated with respect to water, and cloud particles grow rapidly and become visibly apparent in a way that the aerosol in a sub saturated environment rarely is. This difference encourages the tendency to associate the atmospheric aerosol with only the smallest particles, traditionally those under 1 µ m, and the growing particles that are found in water saturated environments as clouds. This distinction encourages one to speak of clouds as singular, compact, entities in contrast to the disperse atmospheric aerosol. However a cloud’s origin as a component of the atmospheric aerosol lingers in attempts to define “a cloud” objectively.

            The ability of the particles that constitute a cloud to grow in a supersaturated environment leads to a sequence of events that can grow particles sufficiently large to efficiently scatter light, and eventually so large as to efficiently precipitate from the atmosphere. Cloud particles are hydrometeors, and a subset of these form precipitation. The study of how cloud particles come into being, how their distribution affects the transfer of radiant energy, and how they transform themselves into precipitation is the subject of cloud physics.

            On scales of a few nanometers, one finds freshly nucleated aerosol particles, while hail stones have been documented to grow to sizes of tens of centimeters in diameter. Thus particles in the atmosphere span a range of sizes of as much as eight orders of magnitude and the mass of atmospheric particulate matter spans a range of scales that is more than twenty orders of magnitude. Small particles sediment with a terminal velocity that is proportional to their diameter squared, hence a factor of ten in diameter means a factor of one hundred in the time it takes a particle to settle and fall out of the sky. Very small particles effectively never fall from the sky, and are only removed by collisions with larger particles, or because they grow by other means to sizes large enough to effectively fall from the sky. While large particles are rare, as once form they rapidly precipitate to the surface. Clouds also get their meaning because we can see them, or we feel their presence through their emission of radiation which keeps the ground from cooling on a cloudy night. Hence an important part of what makes a cloud is its radiative properties, its propensity to scatter visible radiation and absorb and emit infrared radiation. The scattering of visible radiation depends both on the amount of suspended water mass, and the size of the suspended particles, while the efficacy of clouds in absorbing and emitting infrared radiation depends primarily on the suspended water mass. While the suspended water mass, sometimes called the liquid water path, is a cloud macroscopic parameter, largely controlled by dynamical processes, the characteristic drop size is a microphysical parameter and can be strongly influenced by cloud microphysical processes.

            Outside of poetic motivations there are principally two reasons why we are interested in clouds: 1. Clouds couple to the water cycle because they are the vessels in which precipitation develops. 2. Clouds couple to the radiative balance because they interact strongly with both short and long-wave radiation. The importance of the water cycle, and the role clouds play in it should be self evident. Although here it is important to say that the role microphysical processes play in the water cycle is less clear. Cloud physics owes much of its origins to attempts, dating to the middle part of the last century, to artificially influence precipitation formation and weather. The basic idea was that by altering cloud microphysical processes it might be possible to make clouds rain more, or less effectively, thereby bringing needed rainfall to dry regions, or perhaps limiting the negative impacts of severe whether. However, the link between cloud microphysical processes and rainfall have been difficult to establish, in part because if the atmosphere is determined to precipitate it manages to find some microphysical pathway to do so. Hence the importance of cloud microphysical relative to cloud macrophysical processes has proven difficult to establish in any general sense. Radiation, as mentioned earlier is also an important reason for studying clouds. Clouds reflect significant amounts of solar radiation. As much as 50 Wm−2 on an annual and global average. This is a large number, more than a factor of ten larger than the radiative forcing associated with a doubling of CO2 concentrations in the atmosphere. This tendency of clouds to reflect solar radiation warms the planet, and is called the albedo effect. or the shortwave cloud radiative effect, or sometimes simply “shortwave cloud forcing.” This strong tendency of clouds to cool the surface is partially compensated by their greenhouse effect. By absorbing thermal radiation emitted at high temperatures (characteristic of the surface) and re-emitting it at colder temperatures (characteristic of the clouds) the net amount of thermal radiation emitted to space is reduced, thus acting to reduce the planets ability to cool itself. This is a warming, or greenhouse, effect, but can also be called the longwave cloud radiative effect, or longwave cloud radiative forcing. Globally and annually averaged the effect is about 25 Wm−2 , thereby offsetting by about half the effect of cooling due to the cloud albedo effect. Ironically modern interest in cloud physics is also influenced by a desire to artificially modify clouds so as to control the Earth’s radiation balance. Indeed a number of schemes have been proposed whereby global warming associated with increased concentrations of greenhouse gases might be offset through the deliberate modification of cloud optical properties. Whether this is feasible, let alone sensible remains an area of vigorous scientific debate.

Fig 1: Formation of clouds

Fig 2: Sizes of Cloud Condensation nuclei, cloud droplet and raindrop

Fig 3: Drizzle, raindrop, hail formation