Related Clues:

• Something in the air
• Contrail's makeup
• Mist
• Steam, e.g.
• Humidifier output
• Steam
• Kind of trail or lock
• Cloudy diffused matter
• Contrail content
• Exhaust, perhaps
• Water in the air
• Humidifier emission
• Steam, for example
• Type of trail
• Smoke or fog