Gone with the wind
How is pollen dispersed? Birch pollen, which is small, light and produced in astronomical quantities, is designed to be airborne: this type of pollen is referred to as anemophilous. While anemophilous pollen is dispersed randomly, certain adaptations maximize the efficiency of this process. Some pollen grains – those of a hazel tree, for example – are firmly attached to the flower, and released only on sufficiently windy days. And pollen is usually freed only on dry days, as rain would wash it straight to the ground. All the same, though, most pollen grains (between 60% and 95.5%) land within 100 metres of their source.
This is also why chronic allergies are particularly bad on dry, windy spring days. A... tchoo!