Possible Clues:

• Lack of clarity
• "Purple ___" (rock classic)
• Clearly it cannot be seen through in the heat
• 'Purple ___' (song played at Woodstock)
• 'Mist, light fog (4)'
• Fog or smog
• Harass, in a way
• Mist or shimmer due to heat
• Smoky state
• Part of a summer forecast