  We report a previously unnoticed annually repeating phenomenon consisting of
the daily formation of an extremely elongated cloud extending as far as 1800 km
westward from Arsia Mons. It takes place in the Solar Longitude (Ls) range of
~220-320, around the Southern solstice. We study this Arsia Mons Elongated
Cloud (AMEC) using images from different orbiters, including ESA Mars Express,
NASA MAVEN, Viking 2, MRO, and ISRO Mars Orbiter Mission (MOM). We study the
AMEC in detail in Martian Year (MY) 34 in terms of Local Time and Ls and find
that it exhibits a very rapid daily cycle: the cloud growth starts before
sunrise on the western slope of the volcano, followed by a westward expansion
that lasts 2.5 hours with a velocity of around 170 m/s in the mesosphere (~45
km over the areoid). The cloud formation then ceases, it detaches from its
formation point, and continues moving westward until it evaporates before the
afternoon, when most sun-synchronous orbiters observe. Moreover we
comparatively study observations from different years (i.e. MYs 29-34) in
search of interannual variations and find that in MY33 the cloud exhibits lower
activity, whilst in MY34 the beginning of its formation was delayed compared to
other years, most likely due to the Global Dust Storm. This phenomenon takes
place in a season known for the general lack of clouds on Mars. In this paper
we focus on observations, and a theoretical interpretation will be the subject
of a separate paper.
