10 LTST (Local True Solar Time) are included. Positive observations (i.e. with the AMEC present) are indicated by the background colour. Flat colored areas indicate continuous observations, and striped areas infer the probable minimum duration of the phenomenon. Single positive observations are indicated by a small striped area. The actual duration of the phenomenon is very likely longer than shown here for most MYs, as few suitable observations are available before MY34, and they are all positive. This figure will be discussed again in section 7 as part of the interannual comparison. Refer to tables S1-S6 in the supporting material for further details.]

[Figure: Figure 3. Examples of images from different intruments.  Panels a to e show cylindrical projections of images displaying the AMEC at Arsia Mons. (a) VMC observation. 2018-10-22, MY 34, Ls 274º, LTST 7.8. (VMC observation 180371). (b) OMEGA observation. 2018-09-17, MY 34, Ls 252º, LTST 7.5.  (MEX orbit 18612). (c) IUVS observation from apocenter. 2018-09-07, MY 34, Ls 245º, LTST 8.9. (MAVEN orbit 7673). (d) HRSC observation. 2018-09-21, MY 34, Ls 254º, LTST 8.5. (MEX orbit 18627). (e) MCC observation. 2014-12-13, MY 32, Ls 252º, LTST 6.7. (MCC product MCC_MRD_20141213T134802312_D_GDS).]

The largest set of observations corresponds to MY 34, spanning the orbital longitude range from Ls 223º to Ls 296º. This set comprises of about 75 observations obtained with the instruments VMC, IUVS, HRSC, and OMEGA, together with some MARCI observations. The use of this set of instruments has allowed us to follow the evolution of the phenomenon in MY34, both throughout the season and at a broad range of local times. There are far fewer observations from other MYs. Examples of images obtained by the various instruments used here are shown in Figure 3. Details on how the images from different instruments were processed and measured can be found in the supporting information. In general, we consider that a detection is positive if we find a morning cloud near the volcano or an elongated  cloud west to the volcano, even if the volcano is not present in the image.
We used the geometry package Elkano (Hernández-Bernal et al., 2019, see for example Fig. 1) for the navigation of VMC, Viking/VIS, and MCC images, and for geometrical calculations in general. When necessary, spacecraft geometric information was extracted from SPICE (Acton 1996; Acton et al., 2018), with SPICE kernels provided by NASA NAIF in the case of MAVEN, MRO and Viking, and ESA SPICE Service in the case of MEX (Costa, 2018). In the case of MCC, we used kernels distributed by ISRO, and gaps in these kernels were covered through geometric information provided in the labels of MCC publicly available products. MARCI images were navigated and projected with the aid of the ISIS3 software package (Edwards, 1987). We projected the images onto cylindrical maps after correction of their luminosity (for uniformity near the terminator) before making measurements (see details in supporting material).
For topographic calculations (precise geometry calculations, and shadow measurements) we used Mars Orbiter Laser Altimeter (MOLA) data (Smith et al., 2001), and for topographic maps we use the topography produced by Fergason et al. (2018) from MOLA and HRSC data. All altitudes in this paper are given relative to the areoid as defined in MOLA data. For time calculations, we used algorithms and definitions described by Allison (1997) and Allison & McEwen (2000), and the enumeration of MYs proposed by Clancy et al. (2000). All Local True Solar Times (LTST) mentioned in this paper refer to the position of the volcanic rim of Arsia Mons volcano (239.5º E, 9.2º S), unless otherwise indicated.
Linear regressions shown in figures 6, 7 and 12 are calculated using orthogonal distance regression, which takes into account error in both variables.