Relative humidity relative to ice (RH_i) is a key variable in the formation of cirrus clouds and contrails. This study shows that the properties of the probability density function of RH_i differ between the tropics and higher latitudes. In line with RH_i and temperature variability, aircraft are likely to produce more contrails with bioethanol and liquid hydrogen as fuel. The impact of this fuel change decreases with decreasing pressure levels but increases from high latitudes to the tropics.