Column density at the beacon centre (35°N and central longitude) as a function of time. The column densities are integrated from ~100 mbar towards the top of the atmosphere. The cross point in January 2011 is derived from the background water field of Cavalié et al. (2019) at 35° N. Darker coloured points relate to the fitting of the observations with a constant water mole fraction in the beacon. Light coloured points refer to the fitted water column density after subtracting the pre-storm column of January 2011, and therefore show the additional column density coming from a combination of vertical winds and sublimation. The red/orange points correspond to the water maps at 66.44 μm and the dark blue/light blue to those at 67.09 μm.