One feature NOT mentioned in our weather forecasts is wind CHILL.   

According to the figures kept by David Walsh of the NZ Mountain Safety Council, in a 20 year period there have been 64 hypothermia deaths in New Zealand.  This is on a par with river crossings (71 deaths) and about twice the number of deaths from avalanches (34) or skiing (30).

We are warm-blooded mammals and our metabolism needs to maintain our blood heat at around 37 Celsius to keep us going.  This makes skin heat 33 Celsius — it isn’t often that the environment we live in is this warm. Usually the air that surrounds us is cooler than 33 Celsius and stops us from overheating. We have evolved techniques and adapted our clothing so that we are most comfortable when the air around us is about 15 to 25 Celsius.  Once the air temperature gets below 10 Celsius our metabolism has to do more to cope.  For any given temperature (below blood heat) an increase in wind speed helps to drain heat from our body faster (the faster the air arrives and departs the more heat it takes away).  Wind chill is the term given to this factor – the extra chill that we sense when the wind increases.

A plant does NOT experience wind chill – it simply takes on the temperature of the air touching it.  Sadly, we make hopeless thermometers.  Our skin senses cooling from the wind and warming from sunlight, but air temperature DOES NOT respond to either of these influences directly. Human feelings of hot and cold and responses to hot and cold stress are very difficult to measure.  To some extent they vary with your physical make up (age, height, weight, health and fitness), your metabolism and its ability to cope with stress,  your acclimatisation, what you are wearing, what you are doing, and even with your mood or what you had for breakfast or how well you slept.  These feelings affect your susceptibility to hypothermia but can’t be measured using an instrument.  The cooling sensation that we feel on our skin CAN be related to air temperature and wind speed.  This wind chill formula does NOT cover wet conditions or the extra loss-of-heat to the night sky, but these factors exacerbate the cooling sensation.

There has been a wind chill index around since 1940, based on research done in Antarctica by Paul Siple and Charles Passel.  They measured the time it took for a vial of water originally at blood heat to freeze when placed in various winds.  This is a poor replica of the metabolism of a living human and so this wind chill index overestimated the cooling power of the wind and over the years people tended to ignore its predictions.  However, wind chill exists; and all that was needed was a more accurate measurement of these sensations.  In 2001 Canada came to the rescue.  A team of research scientists and weather specialists in Toronto developed a new wind chill index that was adopted by North America in late 2001. 

The new index is based on results of trials conducted on actual human subjects, and can accurately measure conditions that will cause frostbite and hypothermia in humans.  It is based on loss of heat from the face – the part of the body that is most exposed to severe winter weather.  Volunteers were exposed to a variety of temperatures and wind speeds inside a refrigerated wind tunnel.  They were dressed in winter clothing, with only their faces exposed directly to the cold.  To simulate other factors affecting heat loss, they also walked on treadmills and were tested with both dry and wet faces. 

The index likens the way your skin feels to the temperature on a calm day.   However, since the wind chill index represents the sensation of cold on your skin, it is not an actual temperature, so some will preface its reading with the words “feels like” and may drop the “degrees”.  For example, “Today the temperature is 6°C, and the wind chill feels like zero.”  Findings indicate that exposed skin can freeze in less than two minutes at a wind chill index of -60.  The risk of frostbite begins at -25, with prolonged exposure.  In New Zealand these conditions would be limited to the alpine regions within a storm.  However it can be useful to know that if you drive a motorbike at 50km/hr and the early morning temperature is five degrees then your face will feel as cold as the calm air of a freezer set to -2.  Also you will need to slow down to less than 30 km/hr to stop that freezing sensation. 

Wind Chill Index (2001) 

For example, if the air temperature is five degrees C (top row) and the wind speed (or your speed through the air) is 50 km/hr (left column) then your wind chill is -2, and this combination gives the same cooling sensation as being in calm air in a freezer set to -2.  This table is more accurate than the wind chill tables used in the twentieth century.

5 km/h4-2-7-13-19
10 km/h3-3-9-15-21
15 km/h2-4-11-17-23
20 km/h1-5-12-18-24
25 km/h1-6-12-19-25
30 km/h0-6-13-20-26
35 km/h0-7-14-20-27
40 km/h-1-7-14-21-27
45 km/h-1-8-15-21-28
50 km/h-2-8-15-22-29
55 km/h-2-8-15-22-29
60 km/h-2-9-16-23-30
65 km/h-2-9-16-23-30
70 km/h-2-9-16-23-30
75 km/h-3-10-17-24-31
80 km/h-3-10-17-24-31

For the mathematically inclined the best fit formula that has been decided upon for the Canadian 2001 Wind chill is 13.12 + 0.6215T-11.37*V0.16 + 0.3965*T*V0.16 where T is the air temperature in degrees Celsius, and V is wind speed (as measured at the standard height of 10m above ground level or as mentioned in the weather forecast) in kilometres per hour.

Signs of Hypothermia 

Shivering is the first and only recognisable sign that you are having problems keeping warm.  This is called mild hypothermia and this is the time to get out of the open or to seek warmth and shelter.  Continued exposure starts to cool the blood,  To combat this, the body stops shivering and starts to close down – restricting blood flow to just the inner core.  This is moderate hypothermia. The brain slows down and denies the problem.  The victim becomes clumsy and feels drowsy and unmotivated and finds it hard to think.  Unless the victim is with someone who manages to take avoiding action, survival chances are slim.

What to Do 

If you encounter someone with moderate hypothermia, AVOID doing anything such as massage that would take chilled blood back to the heart and brain.  Instead, you should get them out of the wind and warm them slowly – HUG THEM or wrap something around them – go two to a sleeping bag.  If skin temperature on the outer limbs drops to about zero the victim experiences frostbite.  If the inner core blood temperature drops below 30C the victim’s brain ceases functioning causing unconsciousness, then failure of breathing, then failure of circulation and finally death.  To stop this, keep them awake. 

Bob McDavitt retired from the MetService in 2012. FMC thanks Bob for his valued contribution as the Bulletin weather columnist over many years. This column was originally published in the August 2004 FMC Bulletin