Graupel, also called soft hail, snow pellets or "Grail," is precipitation that forms when supercooled droplets of water are collected and freeze on a falling snowflake, forming a two- to five-millimetre ball of rime.
It is a type of snow often related to avalanches. Indeed, there are many different types of snow and how they interact can lead to catastrophic circumstances.
Intrinsically, snow is a six-sided crystalline material. It is generated when the kinetic energy of water molecules is insufficient to overcome intermolecular forces. Each water molecule gets stuck in place in an intricate lattice with a hexagonal structure.
But snow comes in the form of beautiful six-sided flakes or hard little ice crystals which are more like pellets. The characteristics of a snowfall are a question of humidity, pressure and temperature during the formation process.
Once the flakes hit the ground, their structure and shape become even more important. When I look out my window, after a snowfall, I see an unbroken winter landscape - a sea of white. However, if I get up close, the interactions of the individual snowflakes leads to clumps through intermolecular forces.
This is something most kids intrinsically understand. Powder snow makes for lousy snowballs and snowmen. Even wet powder snow doesn't always have the cohesion for rolling into a ball. Similar, a blizzard of hard, sharp ice crystals doesn't allow a good snow fort to be built.
To build a good snow sculpture you need wet, semi-flaky stuff. It has to have good internal cohesion so everything stays stuck together but not be so water-laden that it is too heavy to move. Graupel is simply not suitable.
Graupel is fragile enough that it falls apart when touched. It is the white chunky snow that often accompanies ice crystals and looks like tiny pieces of Styrofoam.
In the mountains, the different types of snow can have a significant impact on slope stability. Dry snowflakes layered with graupel and covered by a heavy wet snow could be an accident waiting to happen. This is why back-country enthusiasts are encouraged to dig straight wall pits to check out the structure of the snow layers, particularly in areas prone to avalanche.
There is other science in an avalanche beyond snow structure. The angle of the slope is critical. Below about 25 degrees there is not enough force to start an avalanche. Above 60 degrees, the slope tends to self-clean as it is too steep to hold snow in the first place.
It is only between these limits that an avalanche can start. However, once it gets going, it can travel over shallower and steeper terrain. Indeed, all avalanches can be characterized by three zones: the start, the track and the run-out.
Typically, the starting zone will be above the tree line and near a ridge which will allow for a deep accumulation of snow. Start zones also tend to be on the leeward side away from the wind. The structure of the snow is typically layered as a result of changes in temperature. In essence, a freeze-thaw cycle can result in a layered structure mixing ice and heavy snow layers above and below layers of light, less dense material.
The result is sufficiently unstable that minor triggers can set a slab moving. As the snow moves down the track and into the run-out, scientists have been able to demonstrate that there are two general types of avalanche flows.
Dry avalanches are caused by too much stress in the snowpack. They consist of dry, individual particles of snow that move very quickly and generate a billowing cloud. These sorts of avalanches roar above the surface, reaching speeds as high as 130 km-h although they can pick up speed as they move downhill.
They don't tend to have much mass to them. Rather it is the force of the wind they generate that results in damage. A dry avalanche on Everest after the Nepal earthquake blew people and equipment hundreds of metres. Most of the casualties arose from hikers crashing into objects rather than being buried in snow.
Wet flows, on the other hand, are a result of the ultimate failure in the strength of the snowpack. They move much slower with speeds typically between 15 and 60 km-h. They are more massive and bear a strong resemblance to the sluffing of wet concrete. The shear mass involved can knock down trees and buildings. They are also more prone to burying any victims.
Many scientists study avalanches to both try to predict where they might occur and how to minimize both the loss of life and the damage they cause.
One thing that these scientists know for sure is that graupel is a dangerous component in any snow pack. It can act like tiny ball bearings and get the whole thing rolling.
