Windless means completely without wind, perfectly still and calm. On a windless day, flags hang limp against their poles, leaves don't rustle, and smoke from a chimney rises straight up into the sky instead of drifting sideways.

Sailors dread windless conditions because their boats can sit almost motionless on flat water, unable to move forward. Historic sailing ships could be stranded for days or even weeks in windless parts of the ocean, waiting desperately for a breeze. Pilots of small aircraft also pay attention to windless conditions: taking off and landing can feel different without a headwind to help lift the plane.

But windless weather isn't always unwelcome. A windless summer evening makes it easier to light a campfire or set up an umbrella at the beach. Windless winter days feel warmer than windy ones at the same temperature, since there's no breeze to steal heat from your skin. If you've ever tried to measure something outside, like the height of a model rocket, you know how much easier it is on a windless day, when everything stays put instead of blowing around.