cold-weather hat; arctic cap; watch cap
A warm hat designed for cold climates; often a knitted or insulated cap that covers the ears.
A cold-weather hat is essential for winter mountain climbing.
He pulled his arctic cap down low and went outside.
耳当て are earmuffs that cover only the ears, whereas 防寒帽 is a full hat that covers the head and often the ears.
Compound of 防寒 (ぼうかん, 'protection against cold') and 帽 (ぼう, 'hat').