The present study aimed to identify the best day time to evaluate the relative chlorophyll index in maize and soybean crops as a function of weather conditions. The experiments were conducted in the 2018/2019 harvest, in January and February, in the experimental area of the State University of Goiás, Ipameri University Unit, in Ipameri-GO. A completely randomized design with three replications and 40 plants per plot was used. The measurements of relative chlorophyll index at 7:00, 10:00, 13:00,

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... 16:00, and 19:00, at the maize stage R1 (silking) and soy stage R2 (full bloom), were evaluated. The replications consisted of alternating days over seven days. The relative chlorophyll index, leaf temperature, air temperature, and relative humidity, and light incidence were evaluated. The data were organized, and regression analysis was performed for all variables. The statistical analysis was processed using the Sisvar statistical analysis software. The maize shows higher values of relative chlorophyll index at times with the highest temperatures. For soybean, the relative chlorophyll index must be measured at temperatures close to 30 ºC. Studies with more species of C3 metabolisms, such as soybeans, should be carried out to standardize an adequate time to read the relative chlorophyll index.