Clouds are organized masses of tiny water droplets and/or ice crystals. Many clouds exist in our atmosphere no more than a few minutes, while others persist for many hours or days (fog). The 2 very basic types of clouds are cumuliform (heaps) and stratiform (layers). 3 altitude categories are typically defined, from observation that typical cloud types tend to form in one of them. High clouds contain the prefix cirro, which derives from the Latin word cirrus - a curl of hair. Middle clouds contain the prefix alto, from the Latin word altus, meaning high (but evidently, not as high as cirriform clouds). Other clouds are considered as low clouds. Combining the 2 basic types with the 3 altitude categories, cloud types are :

  Altitude                   Typical
  Category    Type       Altitude (1000 ft)
   High     Cirrus (Ci)            16-32
            Cirrocumulus (Cc)      18-31
            Cirrostratus (Cs)      15-25
   Middle   Altocumulus (Ac)        8-19
            Altostratus (As)        8-18
   Low      Cumulus (Cu)            3-9
            Cumulus Congestus (Cg)  2-8
            Towering Cumulus (Ct)   2-8
            Cumulonimbus (Cb)       1-7
            Stratocumulus (Sc)      2-7
            Nimbus (Nb)             1-10
            Fractus (Fr)            1-7
            Stratus (St)            0-6

Cloud formation is quite complicated. Clouds form at locations of rising air. Ascension rate varies from several centimeters/sec in broad cloud layers such as altostratus, to tens of meters/sec in massive cumulonimbus towers. Such motion typically cools air because environmental lapse rate (temperature decrease with increasing height) is less than temperature decrease of an unsaturated ascending air parcel (because of expansion). Because less water vapor can exist as temperature decreases, relative humidity increases. Once relative humidity becomes 100 % (approximately - see below), condensation (phase change of water vapor to liquid water) begins, causing cloud droplet formation (after which temperature decrease of ascending air is less because of heating from condensation). At the center of each cloud droplet is at least one speck of 'dirt' - sea salt, smoke particles, clay, fungus, pollen...some sort of condensation nucleus (a few of which are unnatural substances). If no condensation nuclei were present, relative humidities of much greater than 100 % would be necessary for cloud droplets to spontaneously form. Sufficient nuclei exist though, such that condensation occurs when relative humidity becomes very slightly greater than 100 % (condensation occurs on some hygroscopic nuclei when relative humidity is significantly < 100 % - largely responsible for haze). Cloud droplet diameters are a fractions of a micrometer (mm - millionths of a meter), growing to about 20 mm as diffusion of water vapor to them occurs. (A human hair is approximately 90 mm thick, so cloud droplets are visible - especially directing a bright light on them during a foggy night.) After a sufficient number of clouds droplets form, a cloud exists, composed of billions of droplets of various sizes. Because their sizes vary, air drag on them does. Thus, they slowly fall at different rates, causing largest droplets to collect smaller ones, a process called collision-coalescence. Actually, fall is relative - not very strong updrafts or turbulence is sufficient for moving them upward - in a cloud you would see quite a variety of these moving up & down along with the horizontal wind, among a general descending motion. Cloud droplets fall only a few tens of meters per hour, and tend to evaporate when falling thru drier air below cloud base (height at which condensation began). Thus, cloud base height often appears fixed :

When enough collisions occur, cloud drops of diameters of approximately 10 to 100 mm form. When cloud drops become large, they quickly form drizzle-sized drops as large as 1 millimeter (mm = 1000 mm) diameter, largest drops scavenging smaller ones, then raindrops of approximately 1-5 mm diameter. Thus, rain !

Ice crystal formation in clouds is very similar. Because water only freezes during 0 °C (32 °F) temperature if a surface exists for such to occur, supercooled water (water with temperature < 0 °C) is common in clouds. Cloud droplets tend to form at temperatures to as low as -15 °C, sometimes as low as -60 °C in tropical cumulonimbus ! That is typically true with soluble condensation nuclei. Specific nuclei (insoluble) are very efficient for ice crystal formation, some during temperatures as high -4 °C, but typically not unless temperature is @ most -10 °C. Such nuclei include silver iodide and lead iodide (often used for weather modification), ferrous oxide, and volcanic materials. Thus, many clouds contain mixtures of ice crystals and water droplets, which is important regarding precipitation processes because ice crystals grow quicker via diffusion than water droplets (lower saturation vapor pressure) and water droplets fall much quicker than ice crystals, freezing to them very effectively. Such is referred to as the "Bergeron process", and is responsible for much of the precipitation in our atmosphere.

Thus, cirriform (except some cirrocumulus) and some altostratus and altocumulus clouds consist of ice crystals, where air tends to be cold. Others contain water droplets or water/ice mixtures. You may notice that cumulonimbus clouds have very well-defined edges (and sometimes bases), indicating water droplets, but poorly-defined tops, indicating mainly ice crystals. The anvil-shaped tops (thunderheads) are cirriform clouds being blown off the tops of cumulonimbus clouds by very strong winds near the tropopause.

Preferred areas of formation for each cloud type exist, in relation to weather systems. A typical Low which might be seen on a weather map is shown, with cloud types often associated with it :

I refrain from drawing continuous cloud masses associated with fronts as is often depicted because very seldom does such an idealized situation occur, but clouds specified tend to form where indicated. Relatively warm air gradually ascends at broad areas ahead of warm fronts, causing middle and high altitude clouds there, and warm air locally rapidly ascends at areas (sometimes continuously) ahead of cold fronts (more commonly, ahead of upper air trofs, thus not always along or exactly parallel to a surface front), causing low cumuliform clouds and showery precipitation.

So much for the basics regarding clouds and their formation. I plan to discuss cloudiness forecasting next week !

Text and embedded images are copyright of Joseph Bartlo, though may be used with proper crediting.