Seeing is an important index to evaluate the quality of an astronomical site. To estimate seeing at the Muztagh-Ata site with height and time quantitatively, the European Centre for Medium-Range Weather Forecasts reanalysis database (ERA5) is used. Seeing calculated from ERA5 is compared consistently with the Differential Image Motion Monitor seeing at the height of 12 m. Results show that seeing decays exponentially with height at the Muztagh-Ata site. Seeing decays the fastest in fall in 2021 and most slowly with height in summer. The seeing condition is better in fall than in summer. The median value of seeing at 12 m is 0.89 arcsec, the maximum value is 1.21 arcsec in August and the minimum is 0.66 arcsec in October. The median value of seeing at 12 m is 0.72 arcsec in the nighttime and 1.08 arcsec in the daytime. Seeing is a combination of annual and about biannual variations with the same phase as temperature and wind speed indicating that seeing variation with time is influenced by temperature and wind speed. The Richardson number Ri is used to analyze the atmospheric stability and the variations of seeing are consistent with Ri between layers. These quantitative results can provide an important reference for a telescopic observation strategy.

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