Need help generating new variable to capture 18th observation onwards for each data group

Dear all,
I have generated a new variable (Ambtemp2oC_01) to capture temperature readings above two degrees Celsius for different sensors. Now I need to generate a new variable from Ambtemp2oC_01 to capture the 18th observation onwards for each data blocks (that is all data observations before a blank space) for each sensor. Below is a sample of the data.

Observations	SerialNumber	Ambtemp2oC_01
1	1
2	1
3	1
4	1	6.89
5	1	9.56
6	1	12
7	1	11.94
8	1	12.22
9	1	12.89
10	1	13.61
11	1	14.44
12	1	15.56
13	1	18.78
14	1	17.28
15	1	16.33
16	1	16.67
17	1	17.17
18	1	17.83
19	1	18.44
20	1	18.83
21	1	19.11
22	1	19.33
23	1	19.56
24	1	19.72
25	1	19.83
26	1	19.89
27	1	20
28	1	20.11
29	1	20.22
30	1	20.28
31	1	20.33
32	1	20.39
33	1	20.44
34	1	20.56
35	1	20.67
36	1	20.72
37	1	20.83
38	1	22.72
39	1	23.17
40	1	23.17
41	1	23.11
42	1	23.06
43	1	23.17
44	2	15.94
45	2	15.11
46	2	15.06
47	2	15.06
48	2	15.06
49	2	15
50	2	15
51	2	15
52	2	15
53	2	15.06
54	2	15.06
55	2	15.17
56	2	15.5
57	2	16.11
58	2	16.5
59	2	16.78
60	2	17.11
61	2	17.39
62	2	17.67
63	2	17.94
64	2	18.22
65	2	18.44
66	2	18.72
67	2	18.94
68	2	19.17
69	2	19.39
70	2
71	2
72	2
73	2
74	2	2.06
75	2	2.11
76	2	2.11
77	2	2.17
78	2	2.22
79	2	2.28
80	2	2.33
81	2	2.33
82	2	2.39
83	2	2.44
84	2	2.44
85	2	2.5
86	2	2.56
87	2	2.56
88	2	2.61
89	2	2.67
90	2	2.67
91	2	2.72
92	2	2.78
93	2	2.78
94	2	2.83
95	2	2.83
96	2	2.89
97	2	2.89
98	2	2.94
99	2	2.94
100	2	3
101	2	3
102	2	3.06
103	2	3.06
104	2
105	2
106	2

Thanks for your assistance.
Roland