The response of three Egyptian cotton cultivars to three measurements (maximum, minimum and average) of three climatic variables (air temperature, soil temperature and relative humidity) was studied. Significant and negative correlation coefficients between lint yield of Giza 89 cultivar and mean of daily maximum air temperature were noticed over five periods. The five periods were middle of June, beginning of July, middle of August, last of August and last of September. For Giza 83 and Giza 85 cultivars, correlation coefficient between lint yield and mean of daily maximum air temperature, at last of September, was negative and significant. The correlation coefficient between lint yield of Giza 89 and average of daily minimum relative humidity was significant and positive over all periods except the three intervals of May and last of July.

The contribution percentage of maximum air temperature to lint yield variance revealed that Giza 85 cultivar showed the highest response during the last third of August. Giza 83, which is adapted to high temperature, showed low response to the maximum air temperature during the beginning of April. Meanwhile, it showed the highest response to the minimum air temperature during beginning of May, last of June and last of July. For maximum soil temperature, at 20 cm. depth, Giza 85 showed the highest response during last of April, last of May and beginning of July. Giza 83 showed the highest response to the minimum soil temperature during beginning of May, beginning of August and middle of September. Giza 89 showed the highest response to the minimum relative humidity during beginning of April, middle of June, beginning of July, middle of August and middle of September.