(MGS), Mars Odyssey (later shifted to 7AM-7PM local times; Smith, 2019a), and Mars Reconnaissance Orbiter (MRO). Until 2014, when MAVEN (Mars Atmosphere and Volatile EvolutioN) and the Mars Orbiter Mission (MOM) arrived at Mars, Mars Express (MEX) was the only mission in a non-sun-synchronous orbit. The ExoMars Trace Gas Orbiter (TGO), which arrived in 2016 and began science operations in 2018, is also in a nonsun-synchronous orbit.

Given this observational bias, few authors have researched the daily evolution of water ice clouds. Hunt et al. (1980) studied the daily evolution of several orographic clouds based on Viking images, and Akabane et al. (2002) used ground based telescopic observations (see also Glenar et al., 2003). Other authors (e.g. Wilson et al., 2007) took advantage of the night side observations of afternoon sun-synchronous missions to analyze the behavior of clouds during the night. More recently, Smith et al. (2019a) compared THEMIS observations from before and after the shift in the local time of the Mars Odyssey sun-synchronous orbit. Afternoon sun-synchronous orbiters can only be used to study variations within a few hours around the afternoon, given the narrow range of different local times covered by their field of view (Wolff et al., 2019; Benson et al., 2003,2006). Landers and rovers are also suitable observers of the diurnal variations (Kloos et al., 2018; Sutton et al., 1978). More observations from non-sun-synchronous orbiters (e.g. Giuranna et al., 2019) are necessary to achieve a full comprehension of the diurnal variability of the martian climate.
We report the observation of a previously observed but unnoticed phenomenon that is the formation of an extremely Elongated Cloud extending to the west from the Arsia Mons volcano (Figure 1). We first noticed this phenomenon in 2018 (MY34), as a result of the better observational conditions favoured by MEX and MAVEN orbits. We then found this phenomenon in archived images from previous years, always around the same season, in the Ls range 220º-320º, which included the Southern solstice (Ls 270º) and the perihelion (Ls 251º). This Arsia Mons Elongated Cloud (AMEC) develops in the early morning and dissipates before afternoon; this is why it was seldom observed by the instruments on missions in afternoon sun-synchronous orbits, such as the Mars Color Imager (MARCI) on MRO or Mars Orbiter Camera (MOC) on MGS.
The occurrence of the AMEC in this particular season is likely related to a specific coincidence of atmospheric conditions such as winds, temperatures, pressure, and water vapor content in the region. Since the cloud seems to occurevery MY, we are interested in the interannual variability of the phenomenon, and its possible value as a proxy for the general state of the atmosphere in different MYs (Sánchez-Lavega et al., 2018b). We note that 2018 observations of the AMEC were after the Martian Global Dust Storm of 2018 (GDS 2018; Sánchez-Lavega et al., 2019, Guzewich et al., 2019), and Global Dust Storms are the most conspicuous Martian events that exhibit interannual variability. In fact, Benson et al. (2006), based on afternoon observations, reported lower cloud activity over Arsia Mons following the 2001 GDS, compared to the previous MY.