// Definition
The temperature at the surface of cannabis leaves, which differs from ambient room temperature and is critical for transpiration, VPD, and resin quality. HPS lights heat leaf surfaces from above while LED heat dissipates from the ballast, requiring different environmental management strategies.
// From the Episode
Jenn Doe (SPEAKER_01) gave a detailed explanation of why LED rooms differ from HPS rooms: with LEDs, heat dissipates off the ballast/heatsink rather than radiating down onto leaves, so leaves don't transpire the same way, leading to different humidity dynamics. She noted LEDs don't actually run cooler — they just distribute heat differently. Thomas/GML (SPEAKER_08) confirmed that matching leaf surface temperature from a successful HPS room is more important than the common advice to simply run 5 degrees warmer. Fletch added that the lower leaf surface temp from LEDs means higher humidity, which can hurt terpene retention.