HYGROMETER
any instrument for measuring the water-vapor content of the atmosphere.

Pine cones act as a natural hygrometer, which measures the amount of moisture in
the atmosphere. ... Pine cones open their scales to disperse the seeds when the air is
dry, and close their scales when the air is moist, which keeps the seeds dry.

In dry weather, pine cones open up so gusts of wind can catch and carry the seeds
inside. ... But wet weather will just wash the seeds away. So when it's rainy or
humid, pine cones will close up to protect their seeds. That's why a pine cone
can make a great weather station.