Graupel - What On Earth Is It?

Over the past few days there have been many people wondering what this term "graupel" is, writes TMT's's Fergal Tierney.  

Graupel is simply a snowflake which has a layer of ice, or rime, around its surface. It may look like hail, but on closer inspection you will see it is less dense, and may fall apart on handling. Graupel is the German term for it, but it is also known as soft or small hail, and will appear as GS in METARs (routine aviation weather reports from airports).

METAR from Dublin Airport, 1530, November 30th, reporting a recent graupel shower - EIDW 301530Z 06013G24KT 8000 -SHSN FEW004 SCT020CB BKN050 01/M02 Q1018 RESHGS TEMPO 9999 10290095=

In the atmosphere, water can exist in liqiud form, even at temperatures down to -40°C. This is known as supercooled liquid. If there is some condensation nucleus present - a snowflake, for example - this supercooled water will instantly freeze on its surface and form a layer of rime ice over it. This will transform the snowflake from its normal "fluffy" appearance to a particle more similar to a hailstone. This is what has been falling in many parts of the east over this cold spell, and has led to the widespread curiosity around it.

So why have we got so much graupel? As I explained in my article on Lake-effect Showers last week, the warm Irish Sea is generating enormous updrafts throughout the cumulonimbus clouds, and these are dragging snowflakes from lower levels into mid-upper levels, where the supercooled liquid is present. Here they get coated and hence more dense, and cannot be supported by the updrafts anymore, thus falling to earth as the famous graupel particles. In most cases, graupel will occur at the leading edge of the snow shower, with more snowflakes or hail falling as core of the shower passes over.

So what's the difference between hail and graupel? Hail is formed by a slightly different process. Supercooled liquid accretes around a condensation nucleus, growing in size as more and more rime freezes around the particle as it bounces up and down in the cloud's updrafts. Eventually it becomes too heavy for the updrafts and falls to earth. If you cut open a hail particle you will see it has layers, like an onion. Graupel will have a snowflake, but it will probably fall apart when you try to cut it!