This study investigates the number concentrations of accumulation mode (0.3–1 µm) and coarse mode (1–10 µm) particles in the eastern Mediterranean urban environment with and without precipitation and dew formation. The results show that the number concentrations of both the accumulation mode and coarse mode particles decreased (ratio ~0.7 and ~0.36, respectively) during precipitation. In the analysis we assumed that dew formation occurs in Amman during nighttime when the relative humidity (RH) was higher than 80% and the difference between the temperature and dew point temperature (i.e., T – DP) was less than 2.5°C. In general, the accumulation mode particle number concentration increased with increasing relative humidity. On average, the accumulation mode particle number concentration doubled during the dew formation. On the contrary, the coarse mode particle number concentration during the dew formation was 30% less than that during no dew formation. The increased number concentration of accumulation mode particles during the dew formation conditions is most likely due to enhanced water vapor condensation on existing ultrafine aerosols (diameter < 0.1 µm) enabling their growth to detectable sizes reaching the accumulation mode particle size. The decreased number concentration of coarse mode particles during the dew formation could be due to enhanced wet deposition.