Winter Hazards

The shortest day may have passed (21 December), but the coldest months on average are still to come and can bring with them some potential risks. However, with knowledge comes power, so we hope that after you read this article, you’ll be fully equipped to face winter weather head on!

Temperature

Whilst Christmas films can have us believing that cold weather is quite romantic, containing visions of cosying up in blankets in front of a fire with a mug of hot chocolate, the reality is that it can make us feel a bit miserable at times and have a real impact on our health. In addition, if there is wind blowing, the effective temperature you experience will be even lower than the measured value. - this is known as wind chill.

• If you spend too long outside in sub-zero temperatures without appropriate clothing, then frostbite (freezing of the skin and underlying tissues) is a significant risk. For mountaineers and others spending prolonged periods in extreme conditions, no amount of clothing can protect them completely; finding shelter may be the only option.
• Hypothermia (cooling of the core body temperature to below 35˚C) can set in, even at temperatures well above freezing point. If it is windy as well, air temperatures of up to 10˚C can initiate it, whilst a sedentary lifestyle indoors and a temperature lower than 18˚C can also put you at risk. So, wrap up warm and keep active where possible.

Snow

Most rainfall in the UK will have started out high in the atmosphere as ice crystals which then melt as they fall into air below that has a temperature above 0˚C. However, when the surface air temperature drops below 4˚C, there is an increasing chance that they will reach the ground as snowflakes. These will then melt if that ground is above freezing point, but if not, snow will begin to accumulate.

• Both public and private transport can be significantly impacted by lying snow and the slippery conditions and hidden hazards under snow also increase the risk of injury to those on foot.
• In heavy snow, visibility is seriously reduced, often to below 1000m, with risks to traffic on the roads, ships at sea and aircraft at airports.
• If there is a strong wind, blizzard conditions can reduce visibility to below 400m and make it so difficult to see the horizon and surroundings that disorientation results. This is known as a whiteout and can be life-threatening. Snow already on the ground can be blown into drifts, blocking roads and railway lines, even after the snowfall itself has stopped.

Only travel if necessary and make sure to take care when out and about in snow. Alternatively, revert to traditional transport in these conditions and enjoy a bit of sledging!

Freezing Rain

This can produce some of the most challenging situations, because it often produces large areas of ice which can be hard to see.

• If a prolonged cold spell has resulted in very cold ground temperatures, then any rain which subsequently falls onto it will freeze on impact.
• This will occur when warmer air is pushing across areas with very cold ground temperatures, so that the ice crystals (mentioned in the previous section) melt as they fall through the lowest few thousand feet of the atmosphere.
• Because freezing rain can cover a wide area of ground all at once, this phenomenon produces large expanses of clear, almost invisible ice, which turn hard surfaces into ice rinks. This makes roads, footpaths, railway lines and airport runways into frictionless surfaces. If you have grass verges, this may be the safest walking option.
• Even as air temperature rises, it can take many hours for all the ice to melt, prolonging the problems.
• The same process occurs whenever falling rain lands on a cold object. Items become covered in thick, heavy layers of ice, which can then damage them.

Ice

Any hard surface left wet after a period of rain or subject to runoff from adjacent fields, may then turn icy as the temperature falls overnight.

• The formation of ice depends upon the ground cooling sufficiently.
• This will happen if the sky is clear, allowing the land to lose its heat by radiation.
• Cloud cover, especially low clouds, absorb this radiation and then re-radiate it back to earth, effectively preventing the ground from cooling. Therefore, it is most unlikely to find ice forming in overcast conditions.
• Areas shaded from the sunshine may remain slippery throughout the following day.
• It is the patchy nature of the ice that can make it particularly dangerous when it appears unexpectedly.

Hoar Frost

When air containing sufficient water-vapour meets a surface or object with a temperature below freezing, ice crystals are deposited onto it as hoar frost.

• Amongst other things, the object can be a twig, a fence post, blades of grass, footpaths or a car windscreen. Whilst it can make for some amazing photography, it is worth following advice on protecting your car windscreen and de-icing sensibly.
• Ground frost occurs when air that is within a centimetre or so of the ground is cooled by contact with the cold surface. Note that the air temperature is measured around 1.25m above the surface and may well be a few degrees above freezing point.
• The temperature underground is generally higher than the ground surface temperature and solid surfaces such as asphalt tend to be in better thermal contact with deeper levels than vegetation is. Therefore, roads and pavements, warmed from below, can be free from frost when neighbouring fields are covered in it.

Rime Ice

In a similar way to freezing rain, rime forms when tiny super-cooled water droplets are blown by wind into contact with surfaces that are below 0˚C, where they freeze. In the United States, the more severe occurrences are known as ice storms.

• The ice tends to accumulate on the windward side of the object.
• If the freezing occurs quickly, a soft rime, which looks superficially similar to hoar frost is found. Slower freezing means clearer and heavier ice deposits.
• Since water droplets rather than water vapour are involved, this process occurs readily in freezing fog.
• Power and telephone lines can become so overburdened with rime ice, that they and their supporting structures can be badly damaged.
• This mechanism can also be responsible for aircraft suffering airframe icing in cloud. Most airports and aeroplanes now have de-icing and anti-icing systems in order to counter this risk.

Fog

Fog occurs when air is cooled, and water vapour condenses to form tiny suspended droplets. There are five different mechanisms of cooling. The most commonly seen inland at low levels is when the ground radiates heat to space overnight and cools the (almost still) air in close proximity to it. This produces radiation fog. Where there are valleys, the cooled air, being denser, will tend to sink, and valley fog may form. If moist air is forced to rise over high ground, it will cool and may form hill fog. The fourth type occurs where warm moist air is cooled as it blows over an already cold surface; advection fog. The rarest type around UK coasts is evaporation fog or Sea Smoke, which is formed when moist plumes of air rise by convection over a relatively warm sea surface to meet a very cold air flow just above.

• Visibility is reduced by the droplets, and when it is only possible to make out objects less than 1km away, it is described as fog.
• For visibilities greater than 1km, the term mist is used.
• For public forecasting, the UK Met Office generally only refer to fog when the visibility is less than 180m.
• Since the air temperature overland can vary markedly, fog formation can be very patchy. In such situations, the risks posed by suddenly finding that visibility has reduced dramatically can be significant, especially on the roads.

So, now you know more about the main features of winter weather and the hazards it can bring, you can be fully prepared for the coming months and able to enjoy getting outside and experiencing the beauty that the cold temperatures also provide. We look forward to seeing your photos of it in our Weather Photographer of the Year competition later this year too!