A very simple model of atmospheric transmittance uses as its input regularly observed and archived measures of upper air humidity. The five model parameters were determined empirically to optimize the agreement with radiation measured at six sites in the northwestern contiguous U.S. in summer and winter 1990. The model uses the relative humidity at 500 hPa, the relative humidity at either 950 hPa or 800 hPa depending on the altitude of the station, and the precipitable water. Although the model does not use cloud observations explicitly, it is partitioned into a low-humidity stage and a high-humidity stage with markedly different dependences on relative humidity. In the low-humidity stage the transmittance is approximated as the product of factors for dry air and for water vapor. In the high-humidity stage the transmittance is the product of the transmittances of each of the two layers used; the product is a strongly decreasing function of relative humidity, as a representation of the effect of cloud. A split-sample test using measurements for spring and autumn indicated that the model parameters were not badly distorted by conditions unique to the summer and winter seasons. In terms of the average daily global shortwave radiation received at the surface, the rms errors are 31 W m~(-2) in spring, 33 in summer, 30 in autumn, and 18 in winter. The corresponding relative errors are 0.25, 0.17, 0.20, and 0.31. Because of high-frequency temporal variation in the model residuals, the error declines to about half of the daily error for 7-day averages and to about a quarter of the daily error for 30-day averages.
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机译：一个非常简单的大气透射率模型将定期观察和存档的高层空气湿度测量值用作输入。凭经验确定了五个模型参数，以优化与1990年夏季和冬季在美国西北部连续六个站点测得的辐射的一致性。该模型使用500 hPa的相对湿度，950 hPa或800 hPa的相对湿度，具体取决于车站的高度和可沉淀的水。尽管该模型未明确使用云观测，但将其分为低湿度阶段和高湿度阶段，它们对相对湿度的依赖性明显不同。在低湿度阶段，透射率近似为干燥空气和水蒸气因素的乘积。在高湿度阶段，透射率是所用两层中每层的透射率的乘积；该产品是相对湿度的强烈降低功能，代表了云的影响。使用春季和秋季的测量结果进行的抽样调查显示，模型参数不受夏季和冬季特有条件的严重扭曲。从地表平均每天收到的全球短波辐射来看，春季的均方根误差为31 W m〜（-2），夏季为33 Wm〜（-2），秋季为30，冬季为18。相应的相对误差为0.25、0.17、0.20和0.31。由于模型残差的高频时间变化，对于7天平均值，该误差下降到每日误差的一半左右，对于30天平均值，该误差下降到每日误差的四分之一。
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