Pine cones react to changes in humidity. Warm, dry weather is the best for seed dispersal for pine trees, letting their winged seeds float farther on the wind. When the weather is dry, the outside surface of each of the pine cone's scales contracts more than the inner surface. This causes the scale to bend outwards, opening the cone so the seeds can fall out. In humid weather, the outside layer expands more than the inside layer, bending the scale inwards to close the cone.

This change lets us use pine cones as hygrometers to measure the humidity.