Jerome is performing an experiment for the science fair. He wants to know how long it takes for water to evaporate at varying temperatures. So, he pours exactly 1 mL of water into each of five identically-sized test tubes. Then, he places the test tubes in five different incubators which are set at 20°C, 30°C, 40°C, 50°C, and 60°C. Finally, he measures the amount of time it takes for the water to completely evaporate at each temperature. What is the independent variable in Jerome's experiment?