Snow

Primary reference(s)

WMO, 2017. International Cloud Atlas Manual on the Observation of Clouds and Other Meteors: Snow. World Meteorological Organization (WMO). Accessed 13 October 2020.

Additional scientific description

The form, size and concentration of ice crystals differ considerably according to the temperature and supersaturation at which they develop. A fall of snow usually includes various types of snow crystals and almost all types of crystal may be observed during a single fall of snow. Small droplets of frozen water are often attached to snow crystals. If present in great numbers, these can obscure the crystalline structure of the snow. At temperatures above about -5°C, the crystals generally clump to form snowflakes (WMO, 2017).

The National Oceanic and Atmospheric Administration (NOAA) National Severe Storms Laboratory reports the occurrence of various type of snow hazard (NOAA, 2019):

• Snow flurries: Light snow falling for short durations. No accumulation or light dusting is all that is expected.
• Snow showers: Snow falling at varying intensities for brief periods. Some accumulation is possible.
• Snow squalls: Brief, intense snow showers accompanied by strong, gusty winds. Accumulation may be significant. In North America, snow squalls are best known in the Great Lakes Region of Canada.
• Blowing snow: Wind-driven snow that reduces visibility and causes significant drifting. Blowing snow may be snow that is falling and/or loose snow on the ground picked up by the wind.
• Blizzards: Winds over 35 mph with snow and blowing snow, reducing visibility to 1/4 mile or less for at least 3 hours.

Metrics and numeric limits

Not available.

Examples of drivers, outcomes and risk management

The UK Natural Hazards Partnership has developed a short science informed guide on snow and ice that addresses transport, critical infrastructure, environmental contamination and health (UK NHP, 2013).

Road transport: The main effect of snow and ice on roads is to reduce adhesion of the surface, resulting in loss of control and collisions. On hills, loss of traction may result in vehicles being unable to progress uphill. Blockage of the road leads to disruption according to the traffic density. Road operators attempt to maintain adhesion of the surface by application of salt and grit, both prior to and during ice and snow formation. However, heavy snow at temperatures well below 0°C requires very frequent treatment which can be impossible to deliver to congested roads (UK NHP, 2013).

Rail transport: Trains are affected by loss of adhesion, and this is treated at vulnerable locations by application of sand. However, the main impacts of snow and ice on rail transport are freezing of points and loss of electrical connection between electric trains and power supply from catenaries or especially from the third rail (UK NHP, 2013).

Airports and aircraft: The lift surfaces (mainly the wings) of aircraft must be ice free for safe take-off, so any aircraft that has accumulated ice or snow while on the ground must be de-iced before take-off. This is a time-consuming procedure which can result in delays. Large civil aircraft do not generally accumulate much ice during flight and heaters keep the critical areas free. However, other aircraft flying at low levels are vulnerable to icing and must follow defined procedures to maintain safe flight. The biggest delays at airports are usually caused by snow that impedes progress along taxiways and aprons. Compared with runways, these cover a vast area and are difficult to maintain in a clear state (UK NHP, 2013).

Energy supply and telecommunications: Snow and ice accumulate on transmission cables, trees, masts and pylons, especially when the temperature is close to freezing. The additional weight can lead to sagging of cables and collapse of trees and masts. Collapse of pylons is exceptional in the UK, but has occurred in other countries in these conditions. Ice on electricity cables can affect transmission. The most frequent impacts are from falling trees that pull-down local transmission cables with them (UK NHP, 2013).

Water Supply: Low temperatures can lead to freezing and bursting of water supply pipes in the ground or in buildings, leading to loss of supply, freezing of the escaped water on surrounding roads or other surfaces, and water damage to buildings and their contents, including potential impacts of electrical short-circuits through the water, including electrical fires (UK NHP, 2013).

Environmental impact: The main effects of snow and ice on the environment are: the polluting potential from treatments used to prevent and remove ice from roads, vehicles and buildings; the polluting potential from accumulations of slurry and milk on farms due to road disruption; disposal of salt, grit and oil contaminated snow into rivers; and release of polluting materials as a result of freezing pipes etc. at industrial processes (UK NHP, 2013).

Health and health services: Health impacts of snow and ice may be divided into two categories. Cold temperatures usually coexist with snow and ice and have separate and more significant health effects.

• Health: The direct impacts are mainly broken bones and lacerations resulting from falls on ice covered pavements and roads. The indirect impacts are injuries resulting from accidents incurred from the impact of snow and ice on travel leading to road traffic accidents, etc. Loss of mains water supplies due to frozen pipes which burst can also be a health risk, particularly for vulnerable individuals (such as those dependent on renal dialysis). Loss of power supplies during and after a snowstorm also impacts on health, including obtaining health care supplies and continuing medical treatment for chronic illness in the community such as dialysis. Injuries/infections sustained while clearing snow after a snowstorm (UK NHP, 2013).
• Health services: Impacts on health services include increases in hospital admissions for acute injury and trauma from slips and falls and other ice- and snow-related incidents and acute presentation of chronic illnesses due to loss of essential services such as power, water and transport. This occurs on the background of increased demand for health services in winter, due to circulating infections such as influenza and the impact of cold temperatures on health (UK NHP, 2013).

The snowfall alert by Canada is an example of a national alerting parameter (Government of Canada, 2019).