Snow star crystal 1.5mm diameter grown on an electric needle: Photo Ken Libbrecht. The author: Dr. Donald Perkins examinimg the hail-pad at the weather station.

Llansadwrn (Anglesey) Weather

Ice Precipitation: Snow, types of hail and how to make a hailometer

Logo: Llansadwrn Weather - Melin Llynnon, Ynys Môn.

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Ice, frozen (solid) water, can appear in several ways in the atmosphere. As ice precipitation on the ground we see 2 main forms, snow and hail. Snow is composed of ice crystals that aggregate, in clouds, to form snow flakes. Hail is an aggregate of ice particles of which different types can be identified. Wilson Bentley (1865 - 1931) photographing ice crystals. Courtesy Jericho Historical Society. Interest in ice crystals began in 1611 when Johannes Kepler published a short work 'On the Six-Cornered Snowflake'. He wondered why snow crystals showed a six-fold symmetry. He thought that the hexagonal packing of spheres had something to do with the morphology of snow crystals. He could not pursue this investigation as he would have needed X-ray crystallography that was not invented for another 300 years. In 1635 a mathematician René Descates published a description of snow crystals he had made by eye. But it was not until Robert Hooke in 1665, using an early microscope, made and published a volume of crystal drawings that their complexity and symmetry was revealed.

Probably the first to combine the use of a microscope and photography was a farmer from Vermont. Wilson Bentley (1865 - 1931) spent much of his winter lifetime finding crystals and taking over 5000 photographs. Some of these were published, just before he died, in Snow Crystals containing some 2000 images 1. In 1954 Ukichiro Nakaya, a nuclear physicist, identified and catalogued many of the different types of frozen precipitation. He also grew snow crystals in the laboratory, describing the morphology of crystals grown under different controlled conditions, to gain an understanding of snow crystal formation. Ken Libbrecht has done much to further the interest in ice crystals in his website 2.

Forms of ice precipitation are recorded at most weather stations. But it is also something you can do without the need of any expensive equipment, so read on.

Snow (ice precipitation)

Snow in Llansadwrn and Snowdonia on 26 February 2004. The weather station under snow on 28 Dec 2000. Snow flakes are familiar to most people. For a day of snow it is only necessary to observe a single flake; it would be extremely rare just to see one, but you can often see just a few in during or, more likely, just after showers of snow pellets (see below). Ice crystal aggregations falling from clouds normally melt before they reach the ground and arrive as rain. But when the freezing level is close to the ground, say below 1000 ft (300 m), they do not melt and fall as snow. Usually the air temperature in a Stevenson screen needs to be below about 3C, it would be very rare to fall when above 4C. Sometimes snow arrives in a mixture with rain, this is called sleet in Britain (in the United States for example sleet is ice pellet precipitation). Snow flakes can vary in size from a few millimetres to 2 to 3 centimetres. A good place to observe falling snowflakes is inside a parked car on a cold windscreen (not travelling on the road of course). Close-up photographs can be taken in this way but the temperature has to be really low. In Britain the snowflakes are usually melting when they reach the ground and their structure largely spoiled.

Snow is recorded as lying snow when 50% of more of the ground is covered with snow. No account is taken of bare patches under trees, roads, rivers, lakes or rock outcrops. At climate weather stations particular note is made of any snow at the observation hour oft 0900 GMT. So you may see in station records 3 days of lying snow in February; this means it was lying at more than 50% cover at 0900 GMT. If it snows later in the day and melts before next morning it does not count; if it lasts until 0900 GMT next day it does.

Snow depth is recorded, also at 0900 GMT, where it is lying evenly in several places and the mean taken. Snow depths are reported in centimetres. Each day that snow remains on the ground the depth is recorded. As fallen snow compacts, thaws or freeze dries (sublimates), ice changing directly to water vapour without melting first) measurement of the lying snow is no indication of that fallen in the past 24 hours. In countries where a lot more snow falls than in Britain, and particularly over several days, additional amounts of snow are measured on a snow board. I use one myself. At each observation the snow board is cleared of snow; the next measurement on the board gives the depth fallen since the last observation.

Definition of rates of snow fall


Different types of hail (ice precipitation)

Sun pillar above setting sun at 1823 GMT on 21 March 2003: Photo © D Perkins 2003. Hail can simply be recorded as one of two types based on size. Hail of 5 mm, or more, is large hail, if less than 5 mm small hail. But it is much more interesting to identify and classify the different types of hail.

A day of hail is a day (midnight to midnight GMT in the UK) when ice precipitation (except for snow or sleet) is observed falling at the weather station.

Note: Falls of any type of hail, including snow grains or snow pellets, should not be recorded as a day of snow (British Met Office confirmed). You may see both hail and snow on the same day (frequently snow pellets and snow are seen here) in which case both are recorded.

+ Refers to Llansadwrn data.


Key for identification


Use this key to help identify the types

  1. Are any pieces of ice >= 5 mm diameter?

  2. Yes: It is large hail: Go to 2 or
    No: Go to 3

  3. If hail is <10 mm then is [MO code 5]

  4. If 10-19 mm then large hail [MO code 6]
    If >= 20 mm then large hail [MO code 7]

  5. Hail is < 5 mm. Is hail hard, and transparent or translucent, and spherical or irregularly shaped (rarely conical) and sometimes multilayered?

  6. Yes: then they are ice pellets (small hail) [MO code 4],. or
    No: Go to 4

  7. Is the hail compressible, white and opaque (usually conical shaped)?

  8. Yes: then they are snow pellets (soft hail) [MO code 3]. If >5 mm then class as hail, but note in observations that they were snow pellets, or
    No: Go to 5

  9. Is the hail generally < 1 mm, white and opaque and fairly flat or elongated?
  10. Yes: then they are snow grains [MO code 2], or
    No: Go to 6

  11. Is the hail very small ice crystals in the form of needles, columns or plates often appearing suspended in the air?

  12. Yes: then they are ice prisms [MO code 1]. Can appear in sunshine, so look out for haloes.
Snow pellets 1-7 mm diameter faintly marked the hailometer.

Measuring 7 mm hailstones with calipers. Click for larger.
Large hail 25 mm that fell in Wokingham on 25 August 2005. Click for larger.



Hailstones can start melting straight away. So always measure quickly (best with with calipers) or compare with something (pen, pencil or coin) that you can measure accurately later. Some people like to compare the size with familiar objects such as peas, marbles or eggs. I don't like that approach very much, after all peas, marbles and eggs all come in different sizes, but it is better than nothing. The hailstones could be a record so take a photograph and, if possible, put some in the freezer!

Note: When reporting hailstones, the higher numbered code always takes precedence, if more than one category is observed, or there are different falls during the day. Always make a note of the type and time that it fell because it is of great interest.

Definition of rates of hail fall

Make your own hailometer (hail-pad)

Assembliy the hail-pad. A very useful device to record hail falls can easily be made out of a discarded plastic tub, or carton, and aluminium foil. Choose a strong round tub ( about 15 cm diameter) with a tight fitting lid. Carefully remove the centre of the lid using scissors leaving the rim in one piece. Put a few stones (or thin layer of cement) in the bottom of the carton, to prevent it blowing away in a strong wind! Place a sheet of aluminium foil over the carton (I find the cheapest narrow rolls (20 m x 200 mm) of 'saver' kitchen foil suitable), taking care not to crumple it, and fix in position using the rim of the lid. The foil should be as smooth and tight as possible. Expose the hailometer outside in your chosen position. Hailometer set up at the weather station. I have mine set up on a small plinth inside a larger plastic plant pot. It sits 5 cm below the outer rim with 2-3 cm space around. The reason? It helps to prevent birds perching on it, perforating and fouling the surface. They perch on the flower pot rim. Examine the pad at least daily, or as often as you like but remember daily records run midnight to midnight GMT. Hint: Small snow grains do not usually mark the pad. Snow pellets, even large ones, being of small mass make only a light impression. Ice pellets make a definite mark related to their size. Large hailstones can penetrate the pad. Make a record of the days that you detect hail. If you keep the marked foils you can compare the marks you find with previous records and build up a comprehensive record at your 'hail station'. I would be pleased to hear about any observations you make.

Marks made on hail-pad by 3-4 mm ice pellets. Click for larger.

Hailstones 16 mm diameter punched holes in the hail-pad on 26 Dec 2004. Click for larger.

Code for reporting hailstones


Weather stations record and can report hail according to this code:


Hailstone Code


UK Met Office 4




Ice prisms



Snow grains



Snow pellets



Ice pellets



Hail (diameter 5 - 9 mm)

2 - 3


Hail (diameter 10 - 19 mm)

4 - 7


Hail (diameter 20 mm, or more)


1: Wilson A. Bentley The Snowflake Man

2: Ken Libbrecht Snow and ice crystals

3: TORRO Tornado and Storm Research Organisation TORRO.

4: UK Met Office The Meteorological Office Met Office

These pages are designed and written by Donald Perkins, Copyright © 1998 - 2009

Page dated 20 February 2005

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