Weather –

October 2009, Vol. 64, N

o. 10

269

The wrong kind of snow?Donald PerkinsLlansadwrn, Anglesey

The phrase ‘the wrong kind of snow’ first

gained notoriety during severe weather in

1991 when British Rail’s Director of Opera-

tions said that it was having problems with

the type of snow. Snowfall being relatively

rare in the UK has perhaps led to a media

and public view that anything white fall-

ing from clouds must be snow. The Inuit of

the Arctic regions have 16 words describing

snow.

But what of hail? Ice precipitation has

many forms and distinction must be made

between these, as in ‘wintry showers’ that

frequent hilly and coastal areas. Sometimes,

following heavy showers of hail, we see the

headline ‘traffic disrupted by snowdrifts’.

In July 2008, residents of Sydney, Australia

were disappointed when their first snowfall

since 1836 turned out to be soft hail (snow

pellets). What is going on? This explanation

is based on my observations of ice precipi-

tation in Snowdonia and Anglesey (Perkins,

2005).

Snow is ice precipitation in the form of

‘star-like’ crystals that grow in clouds – a

process that begins with the freezing of a

water droplet and ends with a snowflake.

Snowflakes reaching the ground vary in

size from a few millimetres to perhaps two

or three centimetres. Sleet, in the UK and

Europe, is a mixture of rain and melting

snow, but confusingly is defined differently

in the USA.

Other forms of ice precipitation are all hail

(Met Office, 2004); classification depends

on whether or not the observer is, to use a

botanical analogy, a ‘splitter’ or a ‘lumper’ of

species. Lumped together, hail can be sim-

ply called small (up to 5mm in diameter), or

large (5mm or more – Met Office (MO) Code

5) that can damage vegetation, glasshouses

and vehicles, and could injure people and

livestock. TORRO (2009) has developed

a hailstone scale 0–10 based on size and

intensity criteria, with useful comparisons

if the observer is caught without a ruler:

5–9mm diameter hail (Code 0) is described

as ‘pea’ size, whereas, for example, 41–50mm

diameter hail (Code 5) is ‘golf-ball’ size.

Large hailstones, accretions of clear and/

or opaque ice like the rings of an onion,

usually fall as separate pieces, but may be

agglomerated into irregular shapes.

It is small hail, particularly anything snow-

like, and a classification that involves split-

ting into types, that leads to confusion. In

December 2007, BBC weather forecasters

used the term ‘snow grains’ and were accused

in the media of using unnecessary jargon

and confusing the public. Easily missed,

snow grains are small (usually <1mm, MO

Code 2) hard, white and opaque fairly flat or

elongated ice crystals. They do not break up

on impact, they do not fall in showers, but

usually from stratiform clouds. Even smaller,

very rare in the UK, are ice prisms (MO Code

1), also known as ‘diamond dust’. These are

unbranched ice crystals in the form of nee-

dles, columns or plates, and usually fall from

a cloudless sky and glitter in sunshine; they

are sometimes associated with halos.

More common in the UK are other types

of small hail: ice pellets and snow pellets.

In the past, small hail falling from cumulus

or cumulonimbus clouds has been distin-

guished from ice pellets falling from alto-

stratus or nimbostratus clouds. For practical

purposes observers today do not try to

make the distinction. Ice pellets (formerly

called ‘grains of ice’) occur very frequently;

they are hard, transparent or translucent

pellets of ice (<5mm, MO Code 4), spherical

or irregular, often showing a layered struc-

ture, more rarely conical in shape. These

nearly always fall with rain, do not break

up, and make a loud sound on impact. The

mixture of rain and ice pellets in the USA is

known as ‘sleet’.

Snow pellets (formerly called ‘soft hail’,

known elsewhere as ‘graupel’) occur fre-

quently in the UK, in convective showers,

especially in coastal areas prior to snow or

mixed in flurries with snow. They are white,

opaque, of low density and easily compress-

ible, but often bounce on impact without

breaking up. Close examination using a

hand lens would usually reveal a conical

shape even in the smallest, resembling the

shape of NASA’s Apollo space re-entry vehi-

cle. Sometimes rounded, they may also be

coated loosely with adhering ice crystals.

Usually between 1 and 5mm (MO Code 3),

7mm is not uncommon and larger pellets

may be found – a bonus, as they may then

be classified as ‘large hail’. So don’t be disap-

pointed if your snow turns out to be hail.

ReferencesMet Office. 2004. SPOT-ON Observers Guide: Present weather Vol. 2 3/04. Met Office: Exeter.

Perkins D. 2005. Ice precipitation: Snow, types of hail and how to make a hailom-eter. http://llansadwrn-wx.co.uk/ice/pre-cipitation.html [Accessed 20 July 2009.]

TORRO. 2009. Hail scale. Tornado and Storm Research Organisation: Warrington, UK. http://www.torro.org.uk/site/hscale.php [Accessed 20 July 2009.]

Correspondence to: Donald Perkins,Gadlys Lodge, Llansadwrn, Anglesey, LL59 5SE, UK.

Email: weather@llansadwrn-wx.co.uk

© Royal Meteorological Society, 2009

DOI: 10.1002/wea.493