In situ observations of surface air and dewpoint temperatures and air pressure from over 15 000 weather stations and from ships are used to calculate surface specific (q) and relative ( RH) humidity over the globe (60 degrees S - 75 degrees N) from December 1975 to spring 2005. Seasonal and interannual variations and linear trends are analyzed in relation to observed surface temperature ( T) changes and simulated changes by a coupled climate model [ namely the Parallel Climate Model (PCM)] with realistic forcing. It is found that spatial patterns of long-term mean q are largely controlled by climatological surface temperature, with the largest q of 17 - 19 g kg(-1) in the Tropics and large seasonal variations over northern mid- and high-latitude land. Surface RH has relatively small spatial and interannual variations, with a mean value of 75% - 80% over most oceans in all seasons and 70% - 80% over most land areas except for deserts and high terrain, where RH is 30% - 60%. Nighttime mean RH is 2% - 15% higher than daytime RH over most land areas because of large diurnal temperature variations. The leading EOFs in both q and RH depict long-term trends, while the second EOF of q is related to the El Nino - Southern Oscillation (ENSO). During 1976 - 2004, global changes in surface RH are small ( within 0.6% for absolute values), although decreasing trends of - 0.11% similar to- 0.22% decade(-1) for global oceans are statistically significant. Large RH increases (0.5% - 2.0% decade(-1)) occurred over the central and eastern United States, India, and western China, resulting from large q increases coupled with moderate warming and increases in low clouds over these regions during 1976 - 2004. Statistically very significant increasing trends are found in global and Northern Hemispheric q and T. From 1976 to 2004, annual q ( T) increased by 0.06 g kg(-1) (0.16 degrees C) decade(-1) globally and 0.08 g kg(-1) (0.20 degrees C) decade(-1) in the Northern Hemisphere, while the Southern Hemispheric q trend is positive but statistically insignificant. Over land, the q and T trends are larger at night than during the day. The largest percentage increases in surface q ( similar to 1.5% to 6.0% decade(-1)) occurred over Eurasia where large warming ( similar to 0.2 degrees to 0.7 degrees C decade(-1)) was observed. The q and T trends are found in all seasons over much of Eurasia ( largest in boreal winter) and the Atlantic Ocean. Significant correlation between annual q and T is found over most oceans ( r = 0.6 - 0.9) and most of Eurasia ( r = 0.4 - 0.8), whereas it is insignificant over subtropical land areas. RH - T correlation is weak over most of the globe but is negative over many arid areas. The q - T anomaly relationship is approximately linear so that surface q over the globe, global land, and ocean increases by similar to 4.9%, 4.3%, and 5.7% per 1 degrees C warming, respectively, values that are close to those suggested by the Clausius - Clapeyron equation with a constant RH. The recent q and T trends and the q - T relationship are broadly captured by the PCM; however, the model overestimates volcanic cooling and the trends in the Southern Hemisphere.
