The hay fever season moves into its second phase in June, as tree pollen levels decline and pollen from grasses starts to rise. How much pollen you actually experience depends on the weather; clouds and rain are generally good news for hay fever sufferers, but under some conditions they can be catastrophic.
Plants release pollen on dry, sunny days, when the particles can rise on thermal updraughts and be carried by light winds. In general, the more warm weather, the more pollen there is. Clouds mean less pollen is released, and rainfall washes pollen out of the air, with prolonged rain having more of an effect.
But intense summer thunderstorms can actually increase pollen levels. This is because the strong winds stir up pollen, and thunderstorms typically come after a period of dry weather during which pollen accumulates on the ground. Moisture can rupture dry grass pollen grains, producing clouds of tiny, allergy triggering fragments. And thunder showers are usually brief and have large droplets that do little to clean the air.
The net result is that summer thunderstorms can produce waves of “thunderstorm asthma” or “thunderstorm hay fever”, which have been recorded in London and elsewhere. Fungal spores may also be a factor, turning the summer air into a rich cocktail of allergens.
