A cycle that has the "Activate Only on High Temp" option checked will run only if one or both of the following conditions are met:
1. The current temperature, at the start time of the cycle, exceeds the Hot Temperature value that you specified.
2. The forecast high temperature for the day, as of the start time of the cycle, meets or exceeds the Hot Temperature value that you specified.
Generally this option is only checked to provide supplemental irrigation during times when the temperature is hotter than expected.
This option lets you define a "Hot Weather" cycle. The advantage of defining a Hot Weather cycle is that you can generally reduce water application on your main cycles knowing that if the weather turns hot, more water will automatically be applied.