You want to find out if a certain plant grows best in a hot, warm, or cool climate. What variables should you control? What variables will you test?
Sounds like a question from a university/school project….
First you need identical plants in identical growing systems.
Provide more heat to one of the identical plants from a normal household lamp, and a bowl of open water in this hot environment for humidity. Then use a fan for cool, drier environment on the other plant.
Ensure you measure the temp and humidty levels you have created. You could now tell how different plants performed. If lettuce, you could measure wet and dry weights.
Hope that helps – Scott