This short communication calculates (for some lower bounds) the residence timescale of an atmosphere for Venus, Earth, Mars, and Titan. The authors use the comparability of the residence timescale and the KH timescale to argue that both have the same meaning, i.e., both express the time needed to return to equilibrium after a thermal perturbation. This article can be considered for publication after addressing some concerns listed below.

Major comments

1. I am Not sure I am convinced that the residence timescale is equivalent to the time needed to return to equilibrium after a thermal perturbation. This claim is from the comparability between the residence timescale and the KH timescale for the Sun. However, this can be a mere coincidence. By definition, the residence timescale is the time that takes, in this case, energy, from entering the system until it goes out. Although it can be, this is not necessary the time to return to equilibrium, other processes, such as radiative, transport, and turbulence, can act to make the equilibration either faster or slower.
2. Lines 22-35: The transition to the Sun is unclear and not motivated. After reading Osacar et al. 2020, it became clear that it is for claiming the residence time as an equilibration timescale. You should give a motivating sentence before this discussion and emphasise the point you want to say.
3. Line 71: "The longest of these scales corresponds to the residence time as computed in Section 3." - Is this statement correct? Can you show or reference this?
4. The radiative timescale discussion is important but feels detached from the rest of the manuscript. You should give a connecting sentence at the beginning of the discussion and a summarising sentence at the end.
5. The parameters you chose are motivated; however, they are not unique. First, the radiative timescale is manifested in our daily living. Consider Earth's seasonal cycle, there is a time-lag of about a month between solstice and the warmest day of the year, resulting from the radiative timescale. Also, Mitchell and Lora (2016) estimate 200 Earth years for the tropospheric radiative timescale of Titan. This radiative timescale is about five times longer than the residence timescale you have calculated. Also, note that there they consider the effect of the atmospheric opacity.

Minor comments

1. Line 21: "Harte (1988) uses this concept to estimate the anomalous temperature in urban heat islands." - Adding this line is a bit confusing. Consider elaborating on this point or remove that line.
2. Line 48: "It is important to remark that S is much bigger than the sum K + L. For example, for the Earth" - This is probably true, but on other planets, like Mars (or Pluto and Triton) where the atmosphere is thin, or Titan, where it is cold, but still has strong winds, these terms might be comparable. So maybe not be so decisive.
3. Line 72: "The residence time of energy in a planetary atmosphere characterises the planet, and is computed in a model independent way." - This sentence is unclear. What do you mean by characterises the planet? In what form?

This short paper presents some simple estimates for what is described as the residence timescale for energy in a planetary atmosphere, as applied to the atmospheres of Venus, Mars and Titan. The calculations are relatively crude, “back of the envelope” estimates based on data derived from the published literature with insufficient detailed explanation or discussion/critique of how accurate or appropriate these are for the purpose described. The motivation for the calculations is also not well developed and the authors seem unaware of the considerable literature on energy storage and transfer in planetary atmospheres. 

Although this is mentioned in Section 4, how does the proposed timescale differ from the well known radiative relaxation timescale in atmospheric physics (e.g. see J. T. Houghton “The Physics of Atmospheres” Chapter 2 - which is similar to the timescale in Wells 2012)? Such timescales have been computed for many years for all three planets in question as well as for the Earth - e.g. see P Gierasch & R Goody, A study of the thermal and dynamical structure of the Martian lower atmosphere, Plan. Space Sci., 16, 615-646 (1968) for Mars; Pollack JB, Young RE (1975) Calculations of the radiative and dynamical state of the Venus atmosphere. J Atmos Sci 32:1025–1037 for Venus; F. M. Flasar, R. E. Samuelson & B. J. Conrath Titan's atmosphere: temperature and dynamics, Nature, 292, 693-698 (1981) for Titan. For Earth’s climate, energetic adjustment timescales have been computed using more sophisticated models - e.g. see T. W. Cronin & K. A. Emanuel, The climate time scale in the approach to radiative‐convective equilibrium, JAMES, 5, 843-849 (2013), which takes into account the adjustment timescale for the surface as well as the atmosphere - which seems more appropriate when comparing with the Kelvin-Helmholtz timescale for the Sun. These may not be computing quite the same quantities as what the authors have in mind here, but why not compare them quantitatively with the residence timescale computed here?

Without a clearer motivation for what is the purpose and significance of the residence timescale it is completely unclear how useful these new estimates are likely to be. I also mention some more detailed comments below. But without a significant clarification of the purpose of the calculations and a proper discussion of the novelty and significance of the new timescale with respect to the considerable existing literature, I cannot recommend publication in its present form.

Detailed comments:

P.2 Eq (7) - This assumes a simple integration with height, but atmospheres also vary in structure horizontally. Won’t this make a difference?

Section 2 - By focusing on E or S as the main measures of energy you focus on essentially the dry static energy, which is dominated by internal energy. But much of this energy will be unchanged by internal dynamical adjustments and would be unlikely to vary unless the global thermal perturbation was to be fairly cataclysmic. Why is this the most significant quantity to calculate?

Table 2 - It is mentioned that most of these figures for fluxes originate from the Trenberth diagrams published by Read et al. (2016). But the fluxes quoted appear to represent either the upward or downward IR fluxes between the atmosphere and surface. Would it not be more meaningful to compute the net flux entering or leaving the atmosphere? For Venus this would look more like 22 W/m^2 at the surface. The corresponding figure for Mars would be nearer 26 W/m^2 and 0.26 W/m^2 for Titan, based on the information in Read et al. (2016). These figures definitely need more explanation and justification.