Scientists in the School of Botany take daily measurements of the pollen levels in Melbourne, which are combined with the weather forecast to produce a pollen forecast.

The count is given as a qualitative assessment, on a scale from low to extremely high, and as actual values of the number of grass pollen grains per cubic meter of air/total number of all pollen types. For example, 30/105 means there were 30 grass pollen grains and 105 pollen grains of all types per cubic meter of air in the preceding 24-hr period.

Factors that affect the count, include daily fluctuations in temperature, wind conditions, humidity and precipitation, and of course the biology of the plants themselves.  Many plants flower in the morning so concentrations of airborne pollen are usually highest between 5 a.m. and 10 a.m.

Weather conditions also affect pollen levels. The biggest factors affecting pollen counts are wind, and humidity.  Melbourne's worst pollen days are characterised by hot northerly winds that bring pollen in to the city from pastures in the surrounding countryside.  When the air is humid, such as during or after it rains, pollen that is small, light and dry and easily spread by wind, becomes heavy with moisture and can't travel as far.  

For more information, please check out The Melbourne University School of Botany's frequently asked questions