This paper investigates the relative humidity with respect to ice (RHi) in the upper troposphere and lower stratosphere (UTLS). This is done by evaluating measurements of commercial airplanes of the MOZAIC and IAGOS program. The probability density functions of RHi is shown for different regions (high-latitude, mid-latitude and tropics). Also PDFs are split into clear and cloudy conditions. Additionally, using this data the influence of different fuel types on short-lived and persistent contrails are examined. 

This is a carefully conducted study and well written. It fits very well into the scope of ACP. Therefore, I recommend publication with minor revisions.

Comments

• Figure 5: As the regional differences here are sometimes quite significant (differences in dry mode), it would be interesting to know how much these results differ seasonally.
• Section 3.4: 
• Even if reference is made to the literature on the seasonality of ISSRs, I would also find a seasonal view or statement helpful for the results of Figures 7 and 8. 
• In the explanation of the different contrail frequencies, it is mentioned that this is partly due to the ratio of EI_h2o to Q. What other reasons are there for behaviour?
• Could you briefly discuss how the results relate to climate projections. What is the expected impact of the different fuel types in a warming climate based on your results?

Minor

• P2L49 The last sentence in the paragraph sounds a bit tacked on
• Table 3: There is a word missing in the caption.
• P14L271 remove the “by” between “essentially” and “undersaturated”
• P18: Values for combustion heat Q should be in MJ /kg instead of J / k
• P19L325 missing dash between “non” and “persistent”
• P19L326 remove “and”

Dear authors, 

please find my comments in the attached document.