What is the water vapor concentration at the tropopause?

At the tropopause, the water vapor concentration is typically quite low, generally ranging from about 0.1% to 0.4% by volume. This decrease occurs because the temperature drops with altitude, leading to lower evaporation rates and reduced moisture retention. The exact concentration can vary depending on geographic location, season, and weather conditions. Overall, the tropopause acts as a barrier, limiting the upward movement of water vapor from the lower atmosphere.