Wind Chill Factoring by Simon Phillips
Whilst we are in the grips of winter here in the UK, I thought it wise to provide some useful information on wind and how this affects the temperatures. Cold temperature on its own is not too bad with the correct gear, what makes it potentially dangerous and feel much much colder is due to strong winds.
As yacht skippers, delivering yachts all over the world, we want wind for the effective movement of our boat. We all know about true wind and apparent wind. This becomes very important when calculating wind chill.
The air we feel on a cold day which blows across us, makes the temperature feel much less than it actually is.
Some theory behind this – As convection from a warm surface heats the air around it, an insulating boundary layer of warm air forms against the surface. Moving air disrupts this boundary layer allowing for cooler air to replace the warm air against the surface. The faster the wind speed, the more readily the surface cools.
The effect of wind chill is increasing the rate of heat loss and the reduction of any warmer objects to the ambient temperature more quickly.
How to calculate wind chill
We need the air temperature in Fahrenheit and the wind speed in mph.
If you need to convert Centigrade to Fahrenheit, we do this by using this method – told by my Grandfather long ago!
9/5 x Temp +32 = Temp in F
eg If we have a temperature of 20C, the F equivalent will be 68 degrees F.
9/5 x 20 + 32 = 68
Wind speed – how to convert knots into mph;
1.1507794 × Wind in kts = Wind in mph
eg If our wind speed is 18kts then,
1.1507794 x 18kts = 20.71 mph
We can probably use 1 decimal place here!
Then, the Wind Chill can be calculated using this formula:
WindChill = 35.74 + (0.6215 × T) – (35.75 x Wind 0.16) + (0.4275 x T x Wind) 0.16 Factor.
If this all sounds like too much, I have put this into a chart for you!
Apparent windspeed is across the top and Temperature in Centigrade ranges from +20 at the top to -20 at the bottom.
It can be seen that even with an air temperature of +12 degrees, it is still possible to obtain a below freezing wind chill with 40 knots of wind! Cooling very rapidly from there on. If you are wet, then the evaporation will start to reduce these temperatures even further. Stay in the Caribbean!!
Written and compiled by Simon Phillips
Seaway Yacht Delivery