There are many reasons why no count is available at various times. Changes in temperature, wind conditions, humidity, or precipitation can affect the counts greatly. A sudden temperature drop lowers the pollen count significantly. Certain pollens are seasonal. Trees are dominant in the spring, grasses occur in late spring and early summer, and weeds grow from late summer until the first hard frost.
Pollens are small, light, and dry so they are easily spread by wind.
The distance the pollen travel can depend on whether the wind is strong or calm that day. When the air is humid, pollen becomes damp and heavy with moisture keeping it still and on the ground. When the pollen is wet, it becomes heavy with moisture keeping it on the ground. Generally tree, grass, and weed seasons are similar every year in the same location. However, the intensity can differ depending on the current weather, the previous year's weather, and other environmental factors.