Pollen, or "flower sperm," is a fine powder made up of microspores produced by male plants. It carries the male reproductive cells (gametes) of both conifers and flowering seed plants. A plant is pollinated when the pollen travels from the stamen (the male part of the plant) to the pistil (the female part of the plant). A plant might self-pollinate or the pollen might travel to another plant of the same species either on the wind or by sticking to birds and insects that land on the plants. Pollination allows the plant to reproduce by creating new seeds.
An allergy to pollen is called hay fever. Pollen that comes from brightly colored flowers and that travels with birds and insects usually doesn't trigger allergies. The anemophilous plants, those whose lightweight pollen is dispersed by the wind, are much more allergenic. Ragweed causes 75 percent of all pollen allergies. Weeds generally pollinate in the late summer and in the fall. Trees pollinate in the late winter and spring, and grasses pollinate in the late spring and summer. Plants release the most pollen in the early morning, just after dawn, but it takes time for the pollen to travel away from the direct vicinity of the plants and toward you.
Your local weather station probably reports the daily pollen count (how much pollen is in a cubic meter of air). They calculate this using a formula based on the amount of pollen collected on a rod covered with a greasy substance that spun in the air for a 24-hour period. If you know that the pollen count is particularly high, especially during the pollination season of the type of plant you are allergic to, you can take steps to keep your allergies from flaring up. You should try to stay indoors, especially during midday, which is when pollen tends to peak in urban areas, and when the weather is warm and dry (the best weather for pollen to travel in). Also keep your windows closed and set your air conditioning to "recirculate" so it won't bring in air from outside.