A number of factors affect the daily count, including daily fluctuations in temperature, wind conditions, humidity and precipitation, and of course the biology of the grasses themselves.
Most grasses flower in late Spring and early Summer. During the flowering season, weather conditions such as wind and humidity will also affect how much grass pollen is in the air.
Melbourne's worst grass pollen days are in November, when hot northerly winds bring pollen into the city from pastures in the surrounding countryside. Southerly winds, by contrast, are cooler and more humid and bring mainly pollen-free air into Melbourne from the ocean.