One of the most obvious features of pollen allergy is its seasonal nature; people have allergy symptoms only when the pollen grains to which they are allergic are in the air. Each plant has a pollinating period that is more or less the same from year to year. Exactly when a plant starts to pollinate seems to depend on the relative length of night and day (and therefore, on geographical location) rather than on the weather. On the other hand, weather conditions during pollination can affect the amount of pollen produced and distributed in a speci?c year. Thus, in the Northern Hemisphere, the farther north you go, the later the start of the pollinating period and the later the start of the pollen allergy season.
A pollen count, familiar to many people from local weather reports, is a measure of how much pollen is in the air. This count represents the concentration of all the pollen (or of one particular type, like ragweed) in the air in a certain area at a specific time. It is shown in grains of pollen per square meter of air collected over 24 hours. Pollen counts tend to be the highest early in the morning on warm, dry, breezy days and lowest during chilly, wet periods. Although the pollen count is an approximate measure that changes, it is useful as a general guide for when it may be wise to stay indoors and avoid contact with the pollen.