A Wind To Make The Blood Run Cold: More Weather-Eye Wisdom

We’re still celebrating the late Brendan McWilliams and Illustrated Weather Eye, our beautiful new collection of his wonderful Irish Times column. Back in November 2005, he looked at why the wind really bites when the weather turns cold...

‘The air bites shrewdly,’ if I might borrow Hamlet’s way of putting it. Or if it doesn’t yet where you are, it will do so over the next few days if the forecasts are to be believed. The rationale behind these predictions of ‘a nipping and an eager air’ can be clearly understood by glancing at the weather map, and recalling that, for practical purposes, the wind can be regarded as flowing straight along the isobars.

From this, it will be clear that the air moving southwards towards Ireland for the next few days will have originated somewhere near Spitzbergen in the Arctic, and will have passed over the frozen wastes between Iceland and the north of Norway. Needless to say, even by the time it reaches us, this air will still be very icy, cold and raw.

But another factor makes matters even worse. You will recall that not only does the wind flow along the isobars with low pressure to the left, its speed is also inversely proportional to the distance apart of the isobars themselves; the closer they are together, the stronger the wind. And if you watch successive weather charts for the next few days, either in your Irish Times or on the television, you will notice that as time passes, the isobars squeeze closer and closer together. In other words, it will not just be cold, it will be very windy too; and when low temperatures and strong winds come together, wind chill plays a significant part in how we feel.

Low temperatures in themselves are not particularly unpleasant when the wind is calm. We can feel relatively comfortable because we are encapsulated in an envelope of stationary air whose inner-most layers adapt to the body temperature, insulating it effectively from the cold, surrounding atmosphere. But even the lightest breeze will disturb and blow away this protective, invisible cocoon, and then the cold wind quickly carries away whatever heat our bodies may produce. In general, for a given combination of clothing and air temperature, the higher the wind speed, the colder any individual will feel.

It is for this reason that a wind chill factor is sometimes included in the weather forecast. The wind chill equivalent temperature, to give it its full name, is not an actual temperature at all, but a conceptual assessment of the influence of the wind on the perceived temperature; it is a measure of discomfort, rather than a reading from any real or imaginary thermometer.

The notion is popular because we have all from time to time felt chilled by a stiff breeze when standing in the cold. But the figure should not be taken too literally; the extent to which we feel cold or otherwise involves many other factors, including how much clothing we have on, our age and physical condition, and whether we are physically active at the time or not.

24 November 2005