The earliest record of lake ice break-up in Sweden is from as early as 1701, when the ice on Torne River at Haparanda melted on May 31st. Since then SMHI has successively extended the ice observation network. By 1900 the network included about 150 sites, and by 1950 it included over 320 sites (Table 6, #2). By 1950, observations had been terminated at only 9 sites. During the following 50 years 72 new sites were added to the network while observations were terminated at 255 sites. The reason for the extensive network during the latter nineteenth century and the early twentieth century was the use of frozen lakes and rivers for transportation, but also the need to know when spring activities, e.g. floating timber, could commence. The ice broke up on Torne River at Haparanda, on average, on May 20th during the eighteenth century, on May 17th during the nineteenth century, and on May 10th during the twentieth century, indicating a long-term trend of earlier lake ice break up.
lake ice formation + break up dates
Swedish Meteorological and Hydrological Institute (SMHI)