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Precipitation Formation Processes

A note regarding how various types of precipitation are formed, with an eye to use by observers trying to distinguish one type of hydrometeor from another.



0. Outline:
This note is intended to amplify the rather 'bald' descriptions attached to the various precipitation types elsewhere; what I have attempted is to describe, using generally accepted current ideas, how we arrive at a drizzle droplet, or why a snow pellet is what it is!

However, cloud physics is a highly specialised field of research and readers are urged to bear this in mind - the text below is an attempt to capture the important elements and thus may not fully describe the processes involved with sufficient scientific rigour.


1. Classification of precipitating clouds:
In very broad terms, precipitating clouds can be classed in one of two ways:

  • By temperature: then clouds can be classed as either 'warm' (temperature throughout above 0degC), or 'cold' (mix of ice particles and water droplets, the latter being 'super-cooled', i.e. they are liquid, but have a temperature below the melting-point of ice).
  • By stability: then clouds can be classed as either 'layer' type - formed in a stably stratified environment, e.g. altostratus, stratus, or 'heap (or cumuliform)' type - formed where instability is released in some way, e.g. cumulus, cumulonimbus.

 

Both types of classification will be mentioned below. The primary division for the purpose of this note though will be that due to temperature structure throughout the precipitating cloud.


2. Warm clouds.
In 'warm' clouds, precipitation formation depends upon the individual tiny cloud droplets (which of themselves are too small to fall through cloud-environment updraughts) growing by collision / coalescence, such that they eventually become heavy enough to overcome the upward currents and fall from the base of the cloud. [ It is of some interest here to note that just because droplets collide, they do not necessarily 'join together' - see specialised texts for more on this. ]

In stratus (St) and stratocumulus (Sc) clouds, which are fairly shallow, precipitation is not common unless turbulence within the cloud causes enhanced collision and capture / coalescence between droplets of varying size - even then the air below the cloud must be humid enough to allow rain or drizzle to reach the ground without evaporation. The cloud-water density within the cloud must also be high, or evaporation within the cloud will offset the growth of large, precipitating elements.
Sometimes, particularly within Sc over the sea, instability causes enhanced vertical motions (and therefore increased chance of growth by collision / coalescence), and rain is more readily achieved.

However, the classic 'warm cloud' precipitation belongs to the tropical convection group; here vigorous upward currents allow huge quantities of water droplets to grow to a large size quite quickly (by collision & coalescence / wake-capture of drops with varying fall-speeds), before finally overcoming the net upward motion and rushing to earth as a torrential (or 'tropical') rain-storm. This plume of earthward-headed larger drops will also gather (or 'sweep up') other drops on the way in an accelerating (or 'chain-reaction' ) fashion, adding to the high surface rainfall totals.


3. Cold clouds.
In 'cold' clouds, the fact that ice crystals (albeit in fairly low concentration) and water droplets (with temperature below 0degC) can co-exist is the key factor in the precipitation process. Water droplets are usually dominant (at temperatures warmer than about -20degC) but the ice crystals are more efficient centres of growth from the vapour stage; this follows from the difference between the value of saturation vapour pressure over an ice surface, when compared with a water surface. In these 'mixed' clouds, the air is close to being saturated with respect to liquid water, but is super-saturated (an unstable phase) with respect to ice. Consequently, in mixed clouds, ice crystals grow from the vapour phase much more rapidly than do the nearby droplets. This is usually known as the Bergeron - Findeisen process.
[ In fact, the idea could be ascribed as easily to Alfred Wegener who suggested this process as early as 1911; the theory was refined by Bergeron (~1933) and Findeisen (~1938).]

The ice crystals that grow in this way may themselves fall from the cloud as precipitation but in very small quantities & in isolated elements. Usually though, some further growth is required from one (or both) of the following:
(i) the enhanced crystal might grow through aggregation ('joining-together') with other such crystals (snow), or . . .

(ii) coalesce with water droplets in the riming process (graupel formation), which will lead to hail of varying sizes, densities and composition.

The final precipitation element is largely dependent upon the temperature, size & concentration of the water droplets involved - and in many cases, melting below the cloud base will lead to rain, rather than snow, soft-hail etc. All of these are dealt with below.


4. Types of 'solid' precipitation.

  • Ice crystals growing from the vapour state (via the Bergeron-Findeisen process - see above), can assume a wide variety of shapes (often known as 'habits'). The two basic types are plate-like or prism-like. It appears from experimentation that the final habit is a function of temperature. Because the temperature within a cloud containing growing crystals will vary by many degrees, the final habit will be quite complex, hence the wide variety observed. Any surface precipitation formed by this process is very light, and may not even affect the visibility or state of ground (e.g. ice needles).
  • In such a mixed cloud, ice particles will themselves grow by colliding with supercooled droplets, which then freeze onto them. This process is referred to as growth by riming and it leads to a wide variety of structures depending upon the shape (or habit) of the original crystal, and the density & arrangement of the rime. It is often difficult to determine the orginal shape of the crystal and in such cases the result is called graupel - this is a common form of embryonic precipitation element in all clouds above the 0degC level, including many in summer & in the tropics.
  • Hailstones are an extreme phase in the riming process: they form in vigorous convective clouds which have high liquid water content. The final form of the hail will depend upon many factors: if a stone collects droplets too quickly, its surface temperature rises to 0degC, and some of the water will remain unfrozen. The stone then becomes covered with a layer of liquid water and this may become incorporated into the growing stone, forming 'spongy hail'. If the stone is cycled through parts of the cloud with droplets of differing temperature, then the nature of the ice 'skins' formed will change: small droplets with temperatures well below zero will freeze quickly, trapping air, and leading to opaque or 'cloudy' ice; larger droplets with temperatures not far below zero (as above), will often lead to a layer of 'clear' ice.

5. Detailed notes on observed surface precipitation.
Listing (based on the ordering in the "Observer's Handbook"): the listing for Code 4680 are subject to change.

 Name:  RAIN (& RAIN SHOWERS)
 Alternatives:  (none)
 SYNOP / SHIP
(4677 ww):
(FM 12/13-VII)
 21, 25, 58, 59, 60, 61, 62, 63, 64, 65, 80, 81, 82, 91, 92, 95, 97
 SYNOP
(4680 wawa):
(FM 12-IX)
 21, 23, 40, 41, 42, 43, 44, 57, 58, 60, 61, 62, 63, 80, 81, 82, 83, 84, 92, 95
 METAR (w'w'):  RA, SHRA (& variants / combinations)
 Beaufort letter:  r, pr (and variants for intensity, continuity)
 Symbol:  image of rain symbol
 Standard description & additional notes: precipitation of liquid water particles, either in the form of drops of more than 0.5mm diameter or of smaller widely scattered drops. The droplets should make an impact on a still (or near-still) water surface such as a puddle, pond, lake etc.
 Clouds producing precipitation: Stratocumulus (Sc), Altostratus (As), Nimbostratus (Ns), Altocumulus (Ac), Cumulus (Cu), Cumulonimbus (Cb).
 Physical processes: In mid & high latitudes the Bergeron-Findeisen process (see elsewhere) is important, as ice crystals grow, aggregate into snow-flakes, which upon melting fall as rain at the surface. However, other mechanisms produce rain at the surface, chief amongst them being the formation of 'graupel' or 'snow pellets' which form by riming within a deep cloud with strong vertical motion (convective types), and then outside of deep winter airflow, these elements melt before reaching the surface to give the rain. Another process involves rapid collision / coalescence of unstable water droplets in warm clouds - those with the temperature wholly above freezing-point: this is primarily a tropical phenomenon.
 Name:  FREEZING RAIN
 Alternatives:  (Glaze, Glazed Ice, "Ice storm", Rain-ice)
 SYNOP / SHIP
(4677 ww):
(FM 12/13-VII)
 24, 66, 67
 SYNOP
(4680 wawa):
(FM 12-IX)
 21, 25, 40, 41, 42, 43, 44, 47, 48, 64, 65, 66, 80
 METAR (w'w'):  FZRA (and variants for intensity)
 Beaufort letter:  (no distinction from rain - see above)
 Symbol:  image of freezing rain symbol
 Standard description & additional notes: rain the drops of which freeze on impact with the ground or with objects on the earth's surface or with aircraft in flight. (NB: with changes to the METAR coding, this is now reported with the air temperature below 0degC, whether or not ice is actually deposited.)
 Clouds producing precipitation: (as for rain - see above)
 Physical processes: The original precipitation is produced as for rain (see above), but at some stage, the droplets, having been at a temperature just above zero, encounter a near-surface sub-zero layer which brings the temperature of the liquid droplets back down close to or below 0degC. If these then encounter objects with temperatures below zero, then glazed ice is the result. Of particular importance to low-level aircraft operations, for public-service safety forecasts (ice on roads, pavements etc.), and for advice to utilities having overhead transmission networks.
 
 Name:  DRIZZLE
 Alternatives:  (none, apart from dialect e.g. 'mizzle', though in North America it may be called 'mist')
 SYNOP / SHIP
(4677 ww):
(FM 12/13-VII)
 20, 50, 51, 52, 53, 54, 55, 58, 59
 SYNOP
(4680 wawa):
(FM 12-IX)
 21, 22, 40, 41, 42, 43, 44, 50, 51, 52, 53, 57, 58, 80
 METAR (w'w'):  DZ (and variants for intensity & combinations)
 Beaufort letter:  d (and variants for intensity & continuity)
 Symbol:  image of drizzle symbol
 Standard description & additional notes: fairly uniform precipitation composed exclusively of fine drops of water, less than 0.5 mm diameter, very close to one another. The effect of their individual impact on (still or near-still) water surfaces is imperceptible - i.e. it does not produce 'ripples' etc. Unless a drizzle-producing event is long-lasting (many hours), then this type is unlikely to produce quantities sufficient to trigger automatic rain-gauge 'trips'. As it is also not picked up by standard weather radar imagery, human observation is essential to detect such precipitation. Visibility is usually moderate or poor in association with a drizzle event.
 Clouds producing precipitation: Stratus (St): Drizzle falls from a fairly continuous and dense layer of stratus, usually with a low base, sometime touching the ground (fog / hill fog). Stratocumulus (Sc), may also produce drizzle under certain circumstances [ high relative humidity below the cloud ], though usually Stratus is present & it may be unclear whether it is truly the Sc or the St that is producing the precipitation.
 Physical processes: Coalescence of the very small cloud droplets to such a size (albeit still very small) that they can just leave the cloud against any weak upcurrents that are involved in the cloud-formation process. These St & Sc clouds are essentially a 'warm' cloud type (i.e. presence or absence of ice crystals are not a factor in production of preciptiation) and the clouds must have a high liquid water content to precipitate, or evaporation will offset the precipitation-forming process. The relative humidity below the cloud base must also be high (well above 90%) or again, evaporation will come into play. And, although vertical (upward) motion must be small (or the elements would not fall from the cloud), there must be a reasonable amount of turbulence through the cloud-layer to lead to efficient collision & coalescence. The cloud must also be reasonably thick - various studies suggest somewhere between 400 & 600 metres (or roughly more than 1300 ft), but this is highly dependent upon such things as condensation nuclei present, humidity content, updraught strength etc.
 Name:  FREEZING DRIZZLE
 Alternatives:  (Rime, Rime-ice)
 SYNOP / SHIP
(4677 ww):
(FM 12/13-VII)
 24, 56, 57
 SYNOP
(4680 wawa):
(FM 12-IX)
 21, 25, 40, 41, 42, 43, 44, 47, 48, 50, 54, 55, 56, 80
 METAR (w'w'):  FZDZ (and variants for intensity)
 Beaufort letter:  (no distinction from drizzle - see above)
 Symbol:  image of freezing drizzle symbol
 Standard description & additional notes: drizzle the drops of which freeze on impact with the ground or with objects on the earth's surface or with aircraft in flight. (NB: with changes to the METAR coding, this is now reported with the air temperature below 0degC, whether or not ice is actually deposited.)
 Clouds producing precipitation: (as for drizzle - see above)
 Physical processes: The original precipitation is produced as for drizzle (see above), but at some stage, the droplets, having been at a temperature just above zero, encounter a near-surface sub-zero layer which brings the temperature of the liquid droplets back down near to or below 0degC. If these then encounter objects with temperatures below zero, then glazed ice is the result. Of particular importance to low-level aircraft operations, for public-service safety forecasts (ice on roads, pavements etc.), and for advice to utilities having overhead transmission networks - however, as the droplet size and intensity of fall are usually small, these do not pose the same problems as for freezing rain (above).
 Name:  SLEET
 Alternatives:  (Rain and snow mixed, 'slushy' or wet snow etc.)
 SYNOP / SHIP
(4677 ww):
(FM 12/13-VII)
 23, 26, 68, 69, 83, 84, 93, 94, 95, 97
 SYNOP
(4680 wawa):
(FM 12-IX)
 21, 24, 40, 41, 42, 45, 46, 67, 68, 80, 92, 95
 METAR (w'w'):  RASN (and variants for combinations & intensity)
 Beaufort letter:  rs (and variants for intensity & continuity)
 Symbol:  image of rain and snow symbol
 Standard description & additional notes: precipitation of rain and snow mixed, or of partially melted snow.
[ On an historical note: the word 'sleet' has a completely different meaning in the United States: where it relates to Ice Pellets (q.v.), but in the British Isles, sleet is reserved for a mix of rain and snow, or for melting snow. The word 'sleet' can be traced, through variants, back to at least the 13th century. Because of the potential confusion, meteorologists try to avoid the use of the word - not very successfully! ]
 Clouds producing precipitation: Altostratus (As), Altocumulus (Ac), Nimbostratus (Ns), Stratocumulus (Sc), Cumulus (Cu), Cumulonimbus (Cb).
 Physical processes: as for rain and snow, but at some point, either the snow element partially melts, or the combination of melting and evaporation in the layers near the surface allow rain and snow to penetrate to the surface together.
 
 Name:  SNOW (& SNOW SHOWERS)
 Alternatives:  (none)
 SYNOP / SHIP
(4677 ww):
(FM 12/13-VII)
 22, 26, 70, 71, 72, 73, 74, 75, 85, 86, 93, 94, 95, 97
 SYNOP
(4680 wawa):
(FM 12-IX)
 21, 24, 40, 41, 42, 45, 46, 70, 71, 72, 73, 80, 85, 86, 87, 92, 95
 METAR (w'w'):  SN, SHSN (and variants for combinations & intensity)
 Beaufort letter:  s, ps (and variants for intensity & continuity)
 Symbol:  image of snow symbol
 Standard description & additional notes: precipitation of ice crystals, most of which are branched (sometimes star-shaped), and often having a light, feathery structure. The branched crystals are sometimes mixed with unbranched crystals. At temperatures higher than about -5degC, the ice crystals are generally agglomerated into snowflakes [ water coating aids adhesion of individual flakes ]. Small flakes, up to 4 or 5 mm in diameter, especially those occurring at the beginning of a snowfall in very cold weather, often show a six-rayed star-like structure of great beauty. Larger flakes usually consist of tangled aggregates of such crystals, so that the geometrical structure ceases to be evident.
 Clouds producing precipitation: Altostratus (As), Altocumulus (Ac), Nimbostratus (Ns), Stratocumulus (Sc), Cumulus (Cu), Cumulonimbus (Cb).
 Physical processes: Deposition of vapour to ice (on ice nucleus - sublimation). Individual crystals grow / aggregate with others within the cloud, then fall through updraughts to precipitate to the surface. In the range -4degC to 0degC, most crystals carry a thin film of super-cooled water & aggregate easily: this is why large collections of flakes are lumped together at these sort of (surface) temperatures. Conversely, at much lower temperatures (but with no definite values involved), the crystals are 'dry' (i.e., they do not carry this water coating), and individual crystals fall.
 
 Name:  SNOW PELLETS
 Alternatives:  (Soft hail, Graupel, 'Tapioca snow')
 SYNOP / SHIP
(4677 ww):
(FM 12/13-VII)
 27, 87, 88, 93, 94, 96, 99
 SYNOP
(4680 wawa):
(FM 12-IX)
 21, 40, 41, 42, 45, 46, 80, 89, 93, 96
 METAR (w'w'):  GS (and variants for combinations and intensity)
 Beaufort letter:  ph
 Symbol:  image of small hail symbol
 Standard description & additional notes: precipitation of white and opaque ['cloudy'] grains of ice. These grains are spherical** ['rounded'] or sometimes conical** ['drawn to a point']; their diameter is about 2 to 5 mm. ( they may also be observed lumped into irregular 'graupel' coagulations. The grains are brittle and easily crushed (hence 'soft' hail); when they fall on hard ground, they bounce and often break up (which helps to distinguish them from snow grains) - but of course, on grass surfaces, they are often preserved intact. Precipitation of snow pellets generally occurs in showers, together with precipitation of snowflakes or raindrops, when surface temperatures are not far from 0degC. In other words, to contrast them from other (similar) types, they are regarded as a 'cold-weather' phenomenon.
[ ** whether graupel is rounded or pointed appears to depend upon how the element falls: if it rotates on the way down, then growth is fairly even, and a spherical object is achieved; if it comes straight down, the element is drawn-out to a 'tail' with the 'head' pointing downwards. ]
 Clouds producing precipitation: Cumulus (Cu), Cumulonimbus (Cb) [perhaps Stratocumulus over/downwind sea/coasts (Sc).]
 Physical processes: Either snowflake intercepting supercooled water drops (in which case, on careful examination, the original snow element may be seen) or cloud-ice particle (grown by vapour sublimation) intercepting supercooled water drops which freeze on contact & collect more super-cooled drops - both essentially a heavy 'rime-ice' process within the cloud. Growth is therefore due to collision & coalescence, in this instance with relatively small droplets; the opaque character is due to the trapping of air within the ice as it forms on the initial particle, when the liquid water content of the parent cloud is small. Areas downwind of relatively warm seas in winter / early spring are favoured, e.g. Northern Ireland, NW Wales.
 
 Name:  SNOW GRAINS
 Alternatives:  (Granular snow)
 SYNOP / SHIP
(4677 ww):
(FM 12/13-VII)
 20, 77
 SYNOP
(4680 wawa):
(FM 12-IX)
 21, 22, 40, 41, 42, 45, 46, 77, 80
 METAR (w'w'):  SG (and variants for intensity)
 Beaufort letter:  sh (and variants for intensity & continuity)
 Symbol:  image of snow grain symbol
 Standard description & additional notes: precipitation of very small white and opaque ('cloudy') grains of ice. They resemble snow pellets in external appearance, but are fairly flat or elongated; their diameter is generally less than 1 mm. When the grains hit hard ground, they do not bounce or shatter (c.f. snow pellets - in fact, snow grains in my experience in the British Isles usually just 'float' to the surface in a rather lazy fashion). They usually fall in very small quantities, mostly from stratus or fog, and never in the form of a shower. [ An observer I used to work with called these 'sago', which is not a bad description!]
 Clouds producing precipitation: Stratus (St), Stratocumulus [low/deep](Sc), [perhaps Fog].
 Physical processes: These elements are the cold-weather equivalent of drizzle. They form in / fall from shallow clouds only. Cloud particles (ice & water drops) aggregate / coalesce and manage to fall through any weak updraughts. Variation of the rime-ice process (with very small supercooled water drops involved). Vertical motions are small, therefore the resultant precipitation is small in size and light in accumulation. The physical structure can be quite variable: from the AMS Glossary ... "very fine, simple ice crystals; tiny, complex snow crystals; small, compact bundles of rime; and particles with a rime core and a fine glaze coating". The cloud type, and the fact that they usually fall in very small quantities distinguish them from snow pellets - the latter are primarily a convective type, these fall in highly stable air mass situations.
 
 Name:  ICE PELLETS
 Alternatives:  (Ice Grains, Ice Pellets type [a], Grains of Ice (British only), Sleet (US only))
 SYNOP / SHIP
(4677 ww):
(FM 12/13-VII)
 23, 79
 SYNOP
(4680 wawa):
(FM 12-IX)
 21, 40, 41, 42, 45, 46, 74, 75, 76, 80
 METAR (w'w'):  PL (and variants for intensity)
 Beaufort letter:  h (and variants for intensity)
 Symbol:  image of a pellet symbol
 Standard description & additional notes: precipitation of transparent [ clearly see through ], or translucent [ see vaguely through ] pellets of ice, which are spherical [ rounded ] or irregular (only rarely conical [ with a distinct point ]), and which have a diameter of 5 mm or less. The pellets of ice usually bounce when hitting hard ground and make a sound of impact. Usually ice pellets are not easily crushable. On close inspection, the structure of the original pellet or flake is not evident. [To distinguish them from small hail (below), this type falls from layer cloud.]
[ On an historical note: the word 'sleet' is in common usage in the United States for this type of precipitation (since at least the latter part of the 19th century), but in Europe (more particularly in the British Isles), sleet is reserved for a mix of rain and snow, or for melting snow. The word 'sleet' can be traced, through variants, back to at least the 13th century. Because of the potential confusion, meteorologists try to avoid the use of the word - not very successfully! ]

[ Additional confusion was caused (in my humble view) by the no doubt well-meaning attempt to clarify the above via the WMO declaring that there would be two forms of 'Ice Pellets' ... type [a] and type [b]. As far as I can work out, this was promulgated sometime between 1951 (the founding year of the World Meteorological Organisation) and 1956 (when the WMO International Cloud Atlas was published, and probably around 1954/55). What were Grains of Ice (British) and Sleet or Ice Pellets (US) were assigned to the nomenclature: "Ice Pellets type [a]" & what were widely known as small hail were re-named "Ice Pellets type [b]": yet the formation, originating cloud type and indeed appearance of the two types appear to be quite distinct. The use of 'type a/b' gained much ground in the 1960's, but appears to have fallen out of favour now - probably for the best!]
 Clouds producing precipitation: Altostratus (As), Nimbostratus (Ns).
 Physical processes: These are regarded as frozen raindrops or largely melted and refrozen snowflakes. The freezing process usually takes place near the earth's surface, after the precipitation elements have descended through a layer where the temperature is below 0degC, which in turn must be deep enough to cool the droplet below 0degC. The freezing process proceeds from the outer skin of the droplet inwards. These precipitation falls are indicative of "freezing rain" temperature structure which may accompany this phenomenon, and are therefore important for aviation forecasting & for advice to utilities with overhead transmission systems.
 
 Name:  SMALL HAIL
 Alternatives:  (Ice Pellets [b], Graupel)
 SYNOP / SHIP
(4677 ww):
(FM 12/13-VII)
 27, 87, 88, 93, 94, 96, 99
 SYNOP
(4680 wawa):
(FM 12-IX)
 21, 40, 41, 42, 45, 46, 80, 89, 93, 96
 METAR (w'w'):  GS (and variants for combination), SHPL (see below)
 Beaufort letter:  ph
 Symbol:  image of small hail symbol
 Standard description & additional notes: precipitation of transparent [ clearly see through ], or translucent [ see vaguely through ] pellets of ice, which are spherical [ rounded ] or irregular, rarely conical [ with a distinct point ], and which have a diameter of 5 mm or less: they have a glazed appearance & on close inspection (with a magnifying glass), the structure is still evident. The pellets of ice usually bounce when hitting hard ground and make a sound on impact. At first glance, these might be confused with snow pellets (see above), but small hail is not easily crushable, and regarded as belonging to classically 'thundery' weather (a spring / summer-time phenomenon, and you should be able to at least see 'vaguely' through these (unlike with snow pellets, which are highly - white particles). Also, to distinguish them from ice pellets (above), this type falls from heap or cumuliform cloud.
[ Most observers will code the METAR present weather as SHGS (or TSGS etc). However, the code form does allow for SHPL (i.e. shower of ice pellets), which could be used when it is clear that convection is involved.]
 Clouds producing precipitation: Cb (cumulonimbus), vigorous / deep Cu (cumulus congestus)
 Physical processes: Snow pellets/Graupel/soft hail (the core) encased in a thin layer of ice which has formed from the freezing either of droplets intercepted by the pellets or of water resulting from the partial melting of the pellets. Perhaps best regarded as hail 'thrown-out' of a Cb 'factory' before it has had time to grow, or the hail-formation process is not vigorous or the Cb is decaying.
[ Confusion was caused (in my humble view) by the no doubt well-meaning attempt to clarify the difference between various forms of 'ice pellets' via the WMO declaring that there would be two forms ... type [a] and type [b]. As far as I can work out, this was promulgated sometime between 1951 (the founding year of the World Meteorological Organisation) and 1956 (when the WMO International Cloud Atlas was published, and probably around 1954/55). What were Grains of Ice (British) and Sleet or Ice Pellets (US) were assigned to the nomenclature: "Ice Pellets type [a]" & what were widely known as small hail were re-named "Ice Pellets type [b]": yet the formation, originating cloud type and indeed appearance of the two types appear to be quite distinct. The use of 'type a/b' gained much ground in the 1960's, but appears to have fallen out of favour now - probably for the best!]
 
 Name:  HAIL
 Alternatives:  (none)
 SYNOP / SHIP
(4677 ww):
(FM 12/13-VII)
 27, 89, 90, 93, 94, 96, 99
 SYNOP
(4680 wawa):
(FM 12-IX)
 21, 40, 41, 42, 45, 46, 80, 89, 93, 96
 METAR (w'w'):  GR (and variants for combination)
 Beaufort letter:  ph (and variants for intensity)
 Symbol:  image of a hail symbol
 Standard description & additional notes: precipitation of small balls or pieces of ice (hailstones) with a diameter ranging from 5 to 50 mm or sometimes more, falling either separately or agglomerated into irregular lumps. Hailstones are composed, almost exclusively, of transparent [see 'vaguely' through] ice, or of a series of transparent layers of ice, at least 1 mm in thickness, alternating with translucent ['cloudy'] layers. Hail falls are generally observed during heavy / violent thunderstorms. [ for practical observing, if solid precipitation falls, and it might otherwise be identified as snow pellets, graupel etc., if the diameter is =>  5 mm, then such elements will be called 'hail' by default.]
 Clouds producing precipitation: Cumulonimbus (Cb).
 Physical processes: A cloud ice crystal acts as a nucleus, which is carried vertically large distances - encounters (collides with) varying sizes of super-cooled water droplets - this is known as 'riming'; alternate clear and opaque ice shells may form, before the hailstone becomes heavy enough to fall out of the Cb. The standard 'hailstone' is regarded as an extreme form of the riming process in a cloud of vigorous convective motion (strong updraughts) and having high liquid water content (as a proportion of the total cloud mass).
If, however, a hailstone collects (and grows from) super-cooled liquid droplets at too great a rate, the latent-heat release (water > ice) may raise the surface temperature of the stone to 0degC, and some of the water will then remain liquid: the hailstone is said to 'grow wet'. Some of this water is shed (to take part in growth of other crystals / ice particles), whilst the remainder precipitates as 'spongy' hail.
Hailstones collected & cut in half can exhibit clues to the formation process; they are often seen to be made up of dark and light layers of ice in alternating fashion. The dark layers are 'cloudy' [or opaque] ice, containing a high proportion of small air bubbles, and the lighter layers are 'clear' [ transparent or translucent] ice, and are effectively bubble-free. Clear ice is more likely when the hailstone is 'growing wet', i.e. the freezing process is effectively 'slowed' to allow air to be expelled. Cloudy ice implies a rapid freezing process, where air bubbles are trapped.
In spring & summer, there are usually large quantities of super-cooled water drops - large frozen centres - and hail as defined here is most likely. In winter (and like situations), there are smaller quantities of small super-cooled water droplets, and large (or 'standard') hail less likely, and small / soft hail perhaps more prominent.
 
 Name:  ICE PRISMS
 Alternatives:  (Diamond dust, Ice crystals, Ice needles)
 SYNOP / SHIP
(4677 ww):
(FM 12/13-VII)
 76
 SYNOP
(4680 wawa):
(FM 12-IX)
 11, 21, 40, 41, 42, 45, 46, 80
 METAR (w'w'):  IC (no variants)
 Beaufort letter:  h
 Symbol:  image of diamond dust symbol
 Standard description & additional notes: a fall of unbranched ice crystals in the form of needles, columns or plates, often so tiny that they seem to be suspended in the air. The crystals are visible mainly when they glitter in the sunshine (hence the alternative name "diamond dust"); they may then produce a luminous pillar or other halo phenomena.
 Clouds producing precipitation: these may fall from a cloud (usually stratus [St]) or from fog, or indeed a cloud-less sky.
 Physical processes: Ice prisms are frequent in polar (and other bitterly cold ) regions and occur at very low temperatures and in stable air masses. Updraughts are negligible - therefore allowing individual ice crystals ( grown onto ice nuclei by vapour deposition and enhanced by very light riming) to precipitate.
 
 Name:  ISOLATED STAR-LIKE CRYSTALS
 Alternatives:  (Snow crystals)
 SYNOP / SHIP
(4677 ww):
(FM 12/13-VII)
 78
 SYNOP
(4680 wawa):
(FM 12-IX)
 21, 40, 41, 45, 78, 80
 METAR (w'w'):  SN (light variant only)
 Beaufort letter:  s
 Symbol:  image of snow crystal symbol
 Standard description & additional notes: an extreme form of snow; only found in very cold, still conditions.
 Clouds producing precipitation: various, mainly Stratocumulus (Sc), Altostratus (As).
 Physical processes: Essentially the same process as is involved with snow generation (q.v.), but the crystals so formed do not have other elements in place to allow them to either grow large, or to aggregate into larger snowflakes.
 

6. Alphabetical listing of precipitation types.

Diamond dust
Drizzle
Freezing drizzle
Freezing rain
Glaze
Glazed ice
Grains of ice
Granular snow
Graupel
Hail
Ice crystals
Ice grains
Ice needles
Ice pellets
Ice pellets [a]
Ice pellets [b]
Ice prisms
Ice storm
Isolated star-like crystals
Mizzle
Rain
Rain and snow: mixed
Rain-ice
Rain showers
Rime
Rime-ice
Sleet
Sleet (US)
Small hail
Snow
Snow crystals
Snow grains
Snow pellets
Snow showers
Soft hail
Tapioca snow


7. Diagram showing precipitation formation ideas.
This diagram attempts to show the various processes involved in the production of rain, snow, hail etc. It is based upon (and added to), the diagram published in 'Clouds, rain and rain-making', written by Mason.


8. Bibliography.

  • Atmospheric Science: an Introductory Survey (JM Wallace & PV Hobbs)
  • Clouds, rain and rainmaking (BJ Mason)
  • Meteorological Glossary (Meteorological Office)