The process that causes ice to form a thin layer over a lake is freezing. When the air temperature drops below the freezing point of water (32°F or 0°C), the surface water cools and eventually freezes to form ice crystals. These crystals intertwine and grow, creating a thin, solid ice layer on the lake's surface. Other processes like sublimation (solid turning directly to gas), evaporating (liquid to gas), and condensation (gas to liquid) do not directly cause the initial formation of ice on the lake surface. Freezing is the key process responsible for ice layer formation over lakes.