Related Clues:

• Absence of turbulence
• Period of inactivity
• Tranquilize
• Storm antecedent
• Hard to fluster
• Far from flustered
• Imperturbable
• Sedated
• Not nervous
• It may precede a storm
• Halcyon
• Pre-storm status
• Storm preceder, it's said
• Storm preceder, supposedly
• Serenity found in 17-, 26-, 42- and 54-Across