Nighttime negatives due to icing of ventilation on 2021/05/02 to 03 (night), 2021/05/03 (morning), 2021/05/05 to 06 (night), 2021/05/07 (morning), 2021/05/07 to 08 (night), 2021/05/13 to 14 (night), 2021/05/14 to 15 (night), 2021/05/15 to 16 (night), 2021/05/16 to 17 (night), 2021/05/17 to 18 (night), 2021/05/18 to 19 (night), 2021/05/19 to 20 (night), 2021/05/20 to 21 (night), 2021/05/21 to 22 (night), 2021/05/22 to 23 (night), 2021/05/24 to 25 (night), 2021/05/27 to 28 (night), 2021/05/29 to 30 (night)