Glaciology

Glacier reports are full of figures, and German hangs each one on a small preposition that decides what the figure means. Um gives the change: die Zunge ist um vierzig Meter zurückgeschmolzen says the tongue has melted back by forty metres. Auf gives the point the change ended at: die Gleichgewichtslinie ist auf dreitausend Meter gestiegen says the line now lies at three thousand metres, wherever it started from. Swap the two and a measured loss becomes an altitude, or the other way round. An English reader who knows um mainly as “around” meets a second trap here, because a single article brings that sense back. Um die vierzig Meter is roughly forty metres, an estimate; um vierzig Meter without the article is the surveyed difference. The pair works the same way with vorstoßen, sinken, zunehmen and every other verb of change, so read the preposition before the number.