The probability density function of settling velocity was experimentally measured in a field study. A settling velocity chamber was used, in which long exposure digital pictures were taken and the size of the streaks and exposure time were used to determine the settling velocity of individual pollen grains. A strongly bi-modal distribution was observed indicating that pollen grains were either wet or dry, with settling velocities of 0.29m/s and 0.20m/s respectively. The results indicated that the wet and dry pollen fractions varied throughout the morning, shifting from mostly wet in the early morning, to mostly dry around noon. Simultaneous measurements of meteorological variables suggest that this effect was related to reductions in the ambient relative humidity and increases in vapor pressure deficit.
An analytical two-dimensional dispersion model was used to characterize the effects of the settling velocity distributions on corn pollen dispersion. On average, the model indicated that dry pollen grains travel considerably farther from the source than wet ones. This suggests that pollen grains released later in the morning are more effective in cross-pollinating corn fields than pollen released earlier in the day (assuming they remain viable).