Proposal of a new equation to estimate globe temperature in an urban park environment.

  • Okada, M.,Kusaka, H., 2013: Proposal of a new equation to estimate globe temperature in an urban park environment. J. Agric. Meteol.,69(1),23-32. 2013/03/01 .

黒球温度は湿球黒球温度WBGTを構成する要素の1つであり、放射による暑さを意味している。黒球温度の計測を安定的かつ長期的に行うことは困難であるため、その他の気象要素からその温度を推定する推定式がこれまで考えられてきた。既存の推定式が公園緑地に適応できるかを調べるために、つくば市内の3カ所の公園にて黒球温度を含めた気象観測を行った。黒球の熱収支式から黒球温度が全天日射量に対して曲線的に変化することが分かった。公園観測の結果においてもそのような傾向が確認された。既存の推定式から推定された黒球温度を公園観測の結果と比較した結果、既存の推定式には系統的な誤差が含まれることが分かった。この誤差は推定式の係数を観測値にフィッティングし直しても取り除くことができなかった。そこで、黒球の熱収支に基づいた推定式の新しい形を開発した。この推定式から得られた黒球温度は全天日射量に対する曲線変化を正しく表現でき、風速の変化にも左右されないことが示された。

Globe temperature is one element of the heat stress index, the Wet Bulb Globe Temperature, which is used to evaluate how radiation adds to thermal discomfort in the workplace. As the measurement of globe temperature is not standardized, empirical equations were introduced to estimate the globe temperature from weather factors, including air temperature, solar radiation and wind speed. As it was not known whether these equations were applicable in an urban park environment with vegetation, we observed the globe temperature using a set of instruments in three parks. The observation along with the heat balance analysis of the globe revealed that the globe temperature depended curvilinearly on solar radiation and that wind speed influenced this dependence. We compared two previously proposed empirical equations to the observed globe temperature and found both equations had systematic estimation errors. Although the errors were reduced by fitting the equations to the observed data and reevaluating their numerical constants, the equations still had shortcomings, as one did not consider wind speed and the other included a discontinuity. We therefore derived a new equation based on the heat balance equations of the globe with its numerical constants experimentally determined. This equation was able to predict the curvilinear dependence of the globe temperature on global solar radiation without any discontinuity, and it also showed the globe temperature response to wind speed.

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