at 8.5 LTST. In the following hours, the cloud becomes thinner and more turbulent, evaporating whilst new thin clouds grow locally over Arsia Mons, as observed in the afternoon in previous MYs (Wang & Ingersoll, 2002; Benson et al., 2003; 2006).
Due to the fact that the AMEC phenomenon occurs at the Martian sunrise, we have few observations that show the details of the head formation in high resolution. One of the best images of the earlier head morphology was obtained by Viking Orbiter 2 in MY12 (Ls 308°)

(Fig. 5 in Hunt and Pickersgill, 1984). The image, obtained at LTST 6.6, shows a high dynamical richness in the head, and another image (Fig. 6 in the same reference) at Ls 314º shows the plume growing some hundred kilometers to the west of Arsia Mons at 7.6 LTST, in good agreement with what we have observed in recent years.
The pattern traced by the AMEC during its development and expansion typically exhibited a curved shape with its concavity pointing toward the north (Figs. 6 and 12) , as driven by the ambient zonal and meridional winds. We found evidence suggesting seasonal changes and interannual variations, but we also observed strong sol-to-sol variations. Thus, the tail length and orientation are a good proxy to characterize the ambient wind field structure and its variability.
With these data in hand, the scenario we propose is that the cloud head formation takes place when moist air is forced to ascend in updrafts along the volcano slopes, and the head forms when water ice condenses due to low temperatures at higher altitudes. We suggest that after the head formation, high altitude westward winds expand and drag the cloud to the west. The vertical ascent of air parcels into the Martian mesosphere driven by anabatic winds along the slopes of the giant Mars volcanoes has been studied by means of mesoscale models (Rafkin et al., 2002; 2017; Michaels et al., 2006). These studies show that water vapour may be transported up to altitudes higher than 40km (Michaels et al., 2006). On the other hand, recent works have shown that the hygropause can reach up to 90 km during a GDS (Heavens et al, 2018). In a second paper, we will present a full theoretical study of the AMEC phenomenon based on its orographic nature and numerical models.
Observations in the afternoon show that Arsia Mons is the only location on Mars where water ice clouds are usually found in the season of occurrence of the AMEC (Benson et al. 2003, 2006; Wang & Ingersoll 2002). The AMEC takes place in the early morning and thus it was not part of the mentioned studies. Previous works have studied the water vapor concentration on the slopes of the Tharsis giant volcanoes (Maltagliati et al. 2008; Titov et al., 1994). This topic together with other aspects regarding the AMEC formation will be discussed in the second part of this work.
The AMEC phenomenon develops during the season of maximum insolation at the volcano (daily average at the Top of the Atmosphere TOA ranges from  200 to 225 Wm-2; see Fig. 3 in Sánchez-Lavega et al. 2019) with the perihelion occurring at Ls 251° and the southern solstice at Ls 270°. The AMEC season is also the  period when dust injection in the atmosphere and 28 the development of dust storms on Mars take place (Kahre et al., 2017;