Many plants release pollen early in the morning, usually not long after dawn. So near the source plants, the highest count will be early in the morning. Because pollen travels when it is warm, dry and breezy, pollen counts often peak around noon in urban areas. What time of day the pollen count is highest in an area also depends on what time of the year it is and what plants are growing. Temperature, wind, humidity and rain can also affect the pollen count.
Once a year pollinators
Some plants only pollinate once a year. For many plants, this is in the spring. Plants that pollinate once a year tend to be more common in places where there are four distinct seasons and plants go dormant for the winter.
In more temperate climates where many plants do not need to go dormant for the winter to survive, the plants can pollinate all year or nearly all year long.
Time of day
Plants can pollinate at any time of day. Many plants pollinate in the morning, but some pollinate in the evening, some in the middle of the night and some all the time.
Sudden temperature drops can lower the pollen count.
Strong wind can carry pollen long distances and might bring more pollen into an area.
Humidity and rain
High humidity makes pollen damp and heavy so it is less likely to be airborne. Rain can "wash" out pollen from the air.