Feature article

Wind chill equivalent temperature

The number of days per week of high or extreme wind chill risk for the years 1972–73 to 2007–08, at Napier.

Loss of body heat from young animals soon after birth is asignificant cause of lamb mortality on New Zealand farms eachspring. Key drivers of heat loss are the ambient air temperature,wind, which increases convective heat loss, and skin wetness,which causes evaporative heat loss.

Weather models can provide estimates of wind chill risk on different time scales, and provide some warning of high risk periods. Farmers know that low air temperature, wet days, and windy conditions can be a fatal mix for new-born animals.Moving stock to sites that are sheltered from rain and wind,for example under trees, can considerably reduce exposure towind chill.

The wind chill equivalent temperature is a term for the effective temperature to which animals are exposed, given adverse weather conditions. For example, an ambient air temperature in still conditions of 10 °C is a relatively comfortable situation for a dry, well-fed lamb. If the wind speed subsequently rose to 9 m/s (a ‘fresh’ breeze), the lamb would experience an apparent fall in temperature to 0 °C, i.e., a wind chill equivalent temperature of 0 °C. A stronger breeze could depress the temperature by a further 3–4 °C, which could prove fatal,especially if the lamb was wet.

Wind chill risk at Napier

The figure right compares the number of days per week of high or extreme wind chill risk (see the legend below) for the years 1972–73 to 2007–08, at Napier. (For simplicity, a week here has been normalised to a quarter of a month). As is expected, the risk decreases as spring advances, with typically lower risk by October compared to July and August. Autumn,as a lambing option, is noticeably benign compared to spring.

The data in the figure suggest that the current season,2007–08 (top line of the figure), has been less severe for lambing than usual, with just one week in July and two in August with 3–4 days of high or extreme risk (green segments).During September the number of risk days per week has been two or less (grey shading).

Note that methods of estimating wind chill vary – changing weather data criteria may yield different results.

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