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)~-~40 AUTOMATIC HYDROLOGICAL OBSERVATIONS(ii) A radioactive source of low activity only is required (of the order of 30millcuries) which only emerges from the snow for a few seconds per day andfor about 10 em;(iii)There is no upper limit to the depth of snow which can be measured, otherthan that set by the height of the columns.Figure 3.8 shows the various stages of the snow cover during the winter of1967-68 from 1 November to 15 May on Lac Blanc, Alpe d'Huez, as observed by a telemeteringdevice at Grenoble. More than 200 tele-soundings were made. Only a partof them is shown in the figure.De,;....1'\>~ '\Depth , ~ KWater eq valent......;- ~/~+--- - -1'rt~ L_~ h) i'l ~!~ ~ ~ "- "- "- "- ~ "- "J "J ,"- .'\: .'\: '\:-~ ~:n~ lb •• " .,"cl ""d ~ N :'"• Jan -t1ry"- ._ . Ilfem er,,February • I Match. . , -. pitl-~ :~~[. ~:-_-t~Figure 3.8 - Telemetering of the snow cover obtained by snow-gaugewith horizontal beam at Lac Blanc (winter 1967-1968)The succession in time of profiles, enables the packing of the various layersto be followed. In particular, contrasts of density are seen ~to persist in somecases for several months. This provides important information on the transformationof the snow cover and the risk af avalanches.Thus the device provides complete information which may be of interest to allcategories of users, not only hydrologists but also foresters, the winter sports industryand technicians connected with snow clearance and avalanches.