The agronomy of hydrogen and its compounds is discussed in a simplified model intended to represent a diurnal and global average. Vertical transport by eddy and molecular diffusion is included for the major components H2O, H and H2. The flow at 100 km is found to be dominated by H2, which is converted to H in the thermosphere and escapes. The flux is insensitive to the details of the chemistry and to large variations in the assumed eddy coefficients. Slightly above the homopause at 100 km, the H2 flux is close to its “limiting” value, which is proportional to the H2 mixing ratio. This mixing ratio, in terms of total H atoms, is slightly less than that of H2O, H2 and CH4 at 30 km. Any variation of the latter therefore varies the escape flux in proportion. The model reproduces the observed escape flux fairly well, along with other observations of OH and H in the mesosphere. Reduction of the input mixing ratio by a factor of 2 would improve the agreement. It is suggested that the H2O mixing ratio at... Abstract The agronomy of hydrogen and its compounds is discussed in a simplified model intended to represent a diurnal and global average. Vertical transport by eddy and molecular diffusion is included for the major components H2O, H and H2. The flow at 100 km is found to be dominated by H2, which is converted to H in the thermosphere and escapes. The flux is insensitive to the details of the chemistry and to large variations in the assumed eddy coefficients. Slightly above the homopause at 100 km, the H2 flux is close to its “limiting” value, which is proportional to the H2 mixing ratio. This mixing ratio, in terms of total H atoms, is slightly less than that of H2O, H2 and CH4 at 30 km. Any variation of the latter therefore varies the escape flux in proportion. The model reproduces the observed escape flux fairly well, along with other observations of OH and H in the mesosphere. Reduction of the input mixing ratio by a factor of 2 would improve the agreement. It is suggested that the H2O mixing ratio at...