An empirical study of near-surface air temperature time lags and delay function during the longest total solar eclipse of the 21st century at Tianhuangping (Zhejiang), China, under cloudy conditions

Among the different ways that the solar heating of the Earth's surface can be interrupted, the most impressive is by a solar eclipse. While the solar radiation drops rapidly, the near-surface air temperature responds more gradually, typically reaching a minimum several minutes after the total phase of the eclipse; therefore, the response of the terrestrial temperature is not instantaneous: there is a lag. Sometimes, when clouds are present, this lag disappears leading to an unexpected reverse effect to be studied in this paper, as occurred during the cloudy and longest total solar eclipse of this century in China. Although during a solar eclipse, changes in near-surface air temperature typically lags behind changes in solar radiation, observations sometimes show that under cloudy skies we note the extent of cooling during the final partial phases prior to totality, which we will call a pre-minimum effect. That was the case found during the longest total solar eclipse of the 21st century, which we observed from Tianhuangping (Zhejiang) on 22 July 2009. We attempt to analyze mathematically this opposite lag through a (tentative) “delay function”, derived using our own measurements from this eclipse at three different heights above the ground. We describe how this lag changes with time. We use two methods: (1) the solar radiation-instantaneous temperature method takes the solar radiation model and the obscuration function into account; (2) the geometrical occultation function method, which only uses the occultation function used by others. Results show that under cloudy skies the first performs better than the second. The delay function has been applied to derive the delayed empirical near-surface air temperature profile that would have been the case in a hypothetical clear sky. Results also show that the fall in air temperature would have instrumentally been imperceptible or undetectable over heights of 15 m above the ground approximately.