High altitude mountaintop observatories provide the opportunity to study aerosol properties in the free troposphere without the added expense and difficulty of making airborne measurements. Climatologies for free tropospheric aerosol radiative properties in cloud-free air, including light scattering, light absorption, light extinction, single scattering albedo, Ångström exponent, hemispheric backscatter fraction and radiative forcing efficiency, from twelve high altitude (2.2–5.1 km) measurement platforms are presented at low relative humidity and at standard temperature and pressure. These climatologies utilize data from ten mountaintop observatories in the 20–50ºN latitude band: Mauna Loa, USA; Lulin Mountain, Taiwan; Nepal Climate Observatory — Pyramid; Izaña, Spain; Mount Waliguan, China; Beo Moussala, Bulgaria; Mount Bachelor, USA; Monte Cimone, Italy; Jungfraujoch, Switzerland; Whistler Mountain, Canada. Results are also included from two multi-year, in-situ aerosol vertical profiling programs: Southern Great Plains, USA and Bondville, USA. The amount of light absorption and scattering observed at these high altitude sites either peaks in the spring or it has a broad spring to summer enhancement. The seasonal variation of the aerosol single scattering albedo, backscatter fraction and Ångström exponent changes from site to site but the timing can be related to aerosol sources and transport processes known to impact the individual sites. The seasonal variation of in-situ aerosol light extinction from these high altitude measurements is in excellent agreement with extinction values derived from CALIPSO lidar measurements. Analysis of the systematic variability among in-situ aerosol properties shows that these relationships can be used to infer aerosol types. In particular, the relationship between single scattering albedo and Ångström exponent can indicate the presence of dust aerosol. Radiative forcing efficiency (RFE = aerosol forcing/aerosol optical depth) is used to assess the importance of single scattering albedo and backscatter fraction on aerosol forcing by eliminating aerosol amount (i.e., aerosol optical depth) from the calculation. Variability in monthly cycles of RFE corresponds with changes in single scattering albedo and hemispheric backscatter fraction. Utilizing site-specific, climatological values of single scattering albedo and backscatter fraction to calculate RFE results in departures from the monthly median values of RFE typically in the range 10–30%. The greatest discrepancy occurs for months with low aerosol loading where the observed variability of single scattering albedo and backscatter fraction is the greatest. At most sites the radiative forcing efficiency at low aerosol loading (light scattering < 10 Mm −1) is slightly less negative (more warming) than at higher aerosol loading. ► We present annual and monthly climatologies of aerosol optical properties at 12 high altitude sites. ► There is an west to east increase in amount of aerosol across northern hemisphere. ► Seasonal values of extinction are consistent with CALIPSO extinction measurements. ► Systematic relationships among aerosol optical properties are related to sources and processing. ► For clean conditions the aerosol particles are darker and smaller and hence, less cooling.