Determining Ice Thickness and Ice Safety
Disclaimer: This information if based on the ice which forms in Sweden. Snow and ice conditions can be very different in other parts of the world, never assume this set of specifics is enough by it's own! Seek local knowledge and stay safe!
I did my military service in northern Sweden with the Arctic Rangers, and we frequently passed over frozen lakes and bogs. Part of the winter training included determining ice safety, both for passage on skis as well as with "band-wagons", which is a compact non-armoured people and equipment carrier vehicle on rubber-tracks or cars.
Driving on ice is a common pastime in Sweden, heaps of car manufacturers flock to the areas around Arvidsjaur and Boden each winter to test-drive their cars in real proper conditions, including on frozen lakes etc.
When ice-diving, obviously you need surface support, and not everyone should be expected to be waiting around in a drysuit ready to break through the ice around the designated entry hole(s). So, how do you determine whether any of this is safe?
First, a couple of basics:
- ice is often thinner and weaker at the immediate shore-line than it is 2-3 meters out from shore, this is due to waves and warmer water often nibbling away at the shoreline ice
- in most cases, the center of a lake freezes last, so the ice there might be substantially thinner!
- once water has frozen over, the thickness is gained by more ice forming and adding to the underside of the ice. Snow on top does not compact down to ice, unless we are talking about glaciers which take years or decades to add thickness from the top!
- as a rule of thumb, for every degree celsius below zero and 24 hour time period, the ice will gain 2.5mm in thickness. At minus 4 degrees, that equates to about a centimeter of thickness gained per day - but only if there is not snow on top of the ice! 2cm of snow reduces the ice formation by 90%!
- i.e., snow insulates extremely well! Even as little as 5cm on top of the ice may prevent the ice from growing any thicker at all!
- if, shortly after the ice formed, 20cm of snow cover the ice, there will hardly be any more ice "growth" (in thickness) no matter how cold it gets or for how long!
Determining the thickness:
- look at the ice surface - does it look like it is at least 6cm's thick? If so, step onto it and walk out about 2-3 meters, provided it seems stable, is not making strange noises and appears to be the same thickness.
- drill a hole with an ice-drill, until just before you go through - empty the hole of the slush and look at the ice - does it look dark, almost like clear glass? Take off your glove and crush the slush you emptied from the hole - if it feels dry, then you've got great ice!
- you should mark the hole with a twig so other's know about the hole, and continue in your direction of travel, repeat the drilling procedure several times every couple of meters.
What thickness is required for what? (based on solid, winter ice - not spring ice when all sorts of weirdness* can happen to it)
- 5 to 6 cm of thickness is usually safe for slow walking
- 10 cm or more: walking, skies, ice skating, light motorbike - 15cm is the official minimum thickness for ice skating races in Sweden
- 13 to 15cm or more: snow mobile, motorbike, ATV
- 25cm or more: normal car up to about 1700 to 1800 kg, stick to less than 80 km/h and keep a good distance (50-100m) between cars
- 30cm or more: light truck, small tractor, go faster with a normal car
- 40cm or more: large tractors, normal/large trucks
- 80cm or more: a tank (seriously!) or even a plane to land on ice!
*in spring, the higher sun angle and the melting ice can sometimes create a hollow space between several layers of ice - by breaking through the first layer you might even find you don't go through the second - but this could break your legs instead... and sometimes "spike ice" is formed. This results in tiny spikes between two more solid layers of ice. Think of it as a nail cushion used by fakirs - when spaced very close together, this can support a human body - but when the spikes become thinner (due to thawing) and more widely spaced, the top layer of ice may no longer support the weight. People who fall through such ice often get bad cuts from these spikes, and getting out of it can be a nightmare, even with the special ice spike tools - hence never go out on ice by yourself, and make sure you both have a rescue line.
Recommended safety equipment:
- Ice spikes (special handgrips with metal spikes on the bottom - worn around the neck with a protective cap over the spikes
- rescue line - a line you can throw to a helper on the ice, or to someone who went through the ice - do not throw it AT the person, throw it behind or at the side, and pull it back so the person can grip it. The Hansalinan is the classic Swedish rescue line. Similar to what is used by canoeists and kayak paddlers.
- A backback with waterproof lining or bags inside will help tremendously to get out of the water
- a whistle to attract help and attention (never, ever venture out by yourself!)
What happens if you break through?
- Two things happen in the first 5 to 30 seconds: you will feel much, much colder than you ever imagined - and your lungs will refuse to work at first, this is what often leads to the panic of people experiencing it for the first time
- KEEP CALM for 5 to 10 seconds, and you will find you can breathe normally very quickly
- Your body heat will allow you to work fine for two to three minutes. Two minutes is actually a LONG time to get out of the ice, so take it easy.
- turn around so you face the direction you came from - evidently that direction has ice strong enough to carry your weight!
- take off your gloves, throw the rescue line if you have it handy, then use your ice spikes to dig into the ice (expect the first few stabs to break off more ice) until you find it carries your weight, drag yourself out of the hole like a seal coming onto land - that's it - retreat to safe ice, change into dry clothes, have a cup of hot tea/chocolate and enjoy the scenics.
