He has just announced that "the humidity is about 50 % right now," so his younger sister takes the hygrometer outside, then comes back and announces the result--Lee's estimate is very accurate.
「現在濕度大約 50 %，」李瑞河說完，一旁「三姑」（李瑞河的三妹）立即拿著濕度計往外走，不一會兒功夫，就聽到她走進來公布結果──李瑞河預估的一度不差。