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Weather has a critical impact on observations at optical and infrared wavelengths, and is a key factor in assessing the performance and productivity of future facilities. As a reference for comparison, we present an analysis of cloud cover, 200-hPa wind speed, precipitable water vapour, vertical wind velocity, and aerosol index, for 15 observatory sites around theworld. Our study employs ERA-40 and Total Ozone Mapping Spectrometer climatological data covering a 45-yr time period. We have examined this data set for long-term trends. Several sites are found to show significant long-term changes in some parameters, possibly due to climate change. Four sites show significant increases in cloud cover (99 per cent confidence level). Four sites show changes in 200-hPa wind speed. Eight sites show increases or decreases in precipitable water vapour. The largest changes are found for Antarctic sites, which have clear trends of increasing cloud cover and water vapour. These results should help to characterize the relative astronomical potential of the sites and highlight the influence of both seasonal variations and long-term trends.