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Sunny but cold – how wind chill works

Today is a mostly sunny day across New Zealand but that southerly has a sting to it. 

So what makes a sunny, dry, winter’s day so bitterly cold sometimes?   It’s because of the wind.

The “Feels Like” temperature measures air temperature + humidity in summer and air temperature + wind in winter…to calculate what the air actually ‘feels like’ on your skin when you’re outdoors in the elements.

Here are some examples:

Many places across New Zealand today will at least reach the 10 degree mark.  Add a gentle southerly breeze (say walking speed) and it will feel more like 8.  Add a slightly stronger southerly wind (say jogging speed) and it will feel more like 5 or 6.  That’s a typical set up for Auckland.

However most places that use wind chill calculators really only use them for temperatures 4 or 5 degrees and lower.   

Lets say Wellington has a strong cold southerly change arrive.  The air temperature is 4 degrees and the southerly is blowing at 10km/h.  The wind chill is now 1.5 degrees.  Quite often in winter Wellington can have a bitterly cold southerly change with winds around 50km/h.  So if it was 4 degrees with a 50km/h wind the wind chill (what the air/wind feels like on your skin) is -3.

Quite often in Christchurch when a southerly has arrived and it’s borderline snowing the wind chill can be extreme.  Lets say it’s 1 degree with a gale southerly (62km/h).  The wind chill factor is -8.

And the other day in exposed parts of northern Southland it was -8 (the air temperature, not the wind chill).    Add a fairly breezy southerly to that (20km/h) and the wind chill is -15.  Add a strong southerly (40km/h) and it’s -18.  Add a gale southerly and it’s minus 20. 

These sorts of temperatures, -20 for example, are frequent wind chill temperatures in places like Toronto and Chicago each winter.   Some parts of Canada can reach -30 degrees and that’s withOUT the wind.    So, even in winter, we really are sub-tropical to some degree!


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