各位大神，偶是新手，想请教大家一个问题，万分感谢！

现有一田间气象站记录的实时小气候，因为没半小时记录一次数据，现需要计算平均值换成逐日数据，第3到8列，都是每48个数值求一次平均数，如1-48,49-96，······数据例如下，

date                 time     ta      RH       MS       par     TS    Am

2013/04/18 0:00:00, 2.88, 48.72, 2.5, 10.266, 15.14, 2.89

2013/04/18 0:30:00, 2.25, 49.658, 2.5, 8.800, 14.70, 2.32

2013/04/18 1:00:00, 1.81, 50.127, 2.427, 10.266, 14.32, 2.06

2013/04/18 1:30:00, 1.46, 51.26, 2.427, 8.800, 13.93, 1.80

2013/04/18 2:00:00, 1.03, 52.393, 2.427, 8.800, 13.61, 1.38

2013/04/18 2:30:00, 0.65, 53.721, 2.427, 10.266, 13.27, 0.95

2013/04/18 3:00:00, 0.41, 56.027, 2.355, 8.800, 12.93, 0.74

2013/04/18 3:30:00, 0.14, 56.652, 2.355, 8.800, 12.63, 0.45

2013/04/18 4:00:00, -0.26, 57.824, 2.355, 8.800, 12.34, -0.02

2013/04/18 4:30:00, -0.46, 54.542, 2.355, 8.800, 12.02, -0.17

2013/04/18 5:00:00, -0.67, 53.135, 2.355, 8.800, 11.72, -0.28

2013/04/18 5:30:00, -0.88, 47.235, 2.355, 8.800, 11.46, -0.45

2013/04/18 6:00:00, -1.04, 44.969, 2.282, 10.266, 11.19, -0.50

2013/04/18 6:30:00, -1.24, 43.953, 2.282, 14.666, 10.95, -0.72

2013/04/18 7:00:00, -1.28, 41.062, 2.282, 111.464, 10.74, -0.67

2013/04/18 7:30:00, -1.20, 37.233, 2.282, 202.395, 10.53, -0.55

2013/04/18 8:00:00, -1.02, 34.615, 2.209, 209.728, 10.34, -0.34

2013/04/18 8:30:00, -0.55, 33.326, 2.209, 520.654, 10.19, 0.90

2013/04/18 9:00:00, 0.65, 30.552, 2.209, 931.311, 10.14, 3.48

2013/04/18 9:30:00, 2.14, 27.387, 2.209, 1083.841, 10.29, 6.09

2013/04/18 10:00:00, 2.99, 25.433, 2.282, 1240.770, 10.66, 7.38

2013/04/18 10:30:00, 4.01, 24.808, 2.355, 1333.168, 11.11, 8.55

2013/04/18 11:00:00, 5.04, 24.574, 2.355, 1429.966, 11.63, 9.45

2013/04/18 11:30:00, 6.30, 25.355, 2.355, 1487.164, 12.18, 10.57

2013/04/18 12:00:00, 7.22, 27.113, 2.427, 1559.029, 12.76, 11.45

2013/04/18 12:30:00, 8.26, 27.543, 2.427, 1579.562, 13.30, 12.42

2013/04/18 13:00:00, 9.06, 26.644, 2.427, 1561.963, 13.79, 13.44

2013/04/18 13:30:00, 9.76, 27.035, 2.5, 1161.572, 14.32, 13.98

2013/04/18 14:00:00, 10.55, 27.231, 2.573, 1158.639, 14.77, 14.87

2013/04/18 14:30:00, 11.24, 27.113, 2.573, 1359.567, 15.14, 15.48

2013/04/18 15:00:00, 11.93, 28.872, 2.573, 1170.372, 15.50, 15.99

2013/04/18 15:30:00, 12.04, 29.458, 2.573, 827.180, 15.85, 15.55

2013/04/18 16:00:00, 12.20, 30.356, 2.573, 813.980, 16.01, 15.50

2013/04/18 16:30:00, 11.87, 31.528, 2.573, 720.116, 16.08, 14.51

2013/04/18 17:00:00, 11.80, 32.466, 2.573, 423.856, 16.06, 14.10

2013/04/18 17:30:00, 11.37, 33.482, 2.573, 300.659, 15.94, 13.14

2013/04/18 18:00:00, 10.93, 34.732, 2.5, 102.664, 15.73, 12.04

2013/04/18 18:30:00, 10.55, 35.123, 2.5, 137.863, 15.45, 11.40

2013/04/18 19:00:00, 10.36, 35.201, 2.427, 17.600, 15.14, 10.84

2013/04/18 19:30:00, 10.10, 34.771, 2.427, 10.266, 14.84, 10.41

2013/04/18 20:00:00, 9.85, 33.013, 2.427, 10.266, 14.55, 10.10

2013/04/18 20:30:00, 9.78, 32.544, 2.427, 10.266, 14.29, 9.91

2013/04/18 21:00:00, 9.70, 32.115, 2.355, 10.266, 14.04, 9.82

2013/04/18 21:30:00, 9.62, 33.326, 2.355, 10.266, 13.84, 9.71

2013/04/18 22:00:00, 9.40, 33.482, 2.355, 10.266, 13.61, 9.47

2013/04/18 22:30:00, 9.15, 39.968, 2.282, 10.266, 13.43, 9.26

2013/04/18 23:00:00, 8.45, 57.082, 2.282, 10.266, 13.30, 8.78

2013/04/18 23:30:00, 7.83, 60.207, 2.282, 10.266, 13.14, 7.88

2013/04/19 0:00:00, 6.63, 68.842, 2.209, 13.200, 12.96, 5.89

2013/04/19 0:30:00, 5.16, 81.463, 2.5, 8.800, 12.67, 4.64

2013/04/19 1:00:00, 2.72, 95.568, 3.662, 10.266, 10.40, 2.91

2013/04/19 1:30:00, 2.16, 102.796, 9.402, 10.266, 9.13, 2.50

2013/04/19 2:00:00, 1.98, 104.71, 10.419, 10.266, 9.13, 2.32

2013/04/19 2:30:00, 2.12, 106.781, 10.637, 10.266, 8.94, 2.48

2013/04/19 3:00:00, 2.20, 108.188, 10.637, 10.266, 8.71, 2.53