Complex sentence: Dalton regularly holidayed in the Lake District where his study of meteorology involved a lot of mountain climbing: until the advent of aeroplanes and weather balloons, the only way to make measurements of temperature and humidity at altitude was to climb a mountain.
Split into simpler sentences: Dalton regularly holidayed in the Lake District where his study of meteorology involved mountain climbing. Until the advent of aeroplanes and weather balloons, the only way to make measurements of temperature and humidity at altitude was to climb a mountain.