We present monthly statistics for a two-year meteorological site-testing campaign at the two candidate sites for the location of the GTC. The windroses presented here are a fairly complete representation of the wind regimes prevailing at the lower site and indicate that at night the predominant wind direction for the site is from the NE, whereas during the daytime the wind-vector pattern shows a substantial north-westerly anabatic component. Apart from the expected small differences between the sites due to their difference in altitude (110 m), the only significant difference, in terms of local meteorology, is a greater wind-direction dispersion at the higher site due to its more complicated orographical profile. Statistically, any differences between the sites arising from instabilities in the height of the inversion layer are counterbalanced by occasional irruptions of warm, humid air from the caldera. Hence neither site is particularly privileged over the other with respect to cloud cover, and the ``caldera effect'' does not seem to play a significant role over the course of a year.