Why visibility suddenly to 1 hex?

[vionville]

The weather.dat is like this. The date is 10, Jun, and the visibility started in 3 hexes then drop to 2 then to 1? Although programmed weather is selected and the data said it should vary from 3 to 5.

10 6 1989 80 20 0 0 0 3 5
11 6 1989 100 0 0 0 0 3 4
12 6 1989 100 0 0 0 0 4 5
13 6 1989 100 0 0 0 0 4 5
14 6 1989 100 0 0 0 0 4 5
15 6 1989 80 20 0 0 0 2 4
16 6 1989 100 0 0 0 0 3 4
17 6 1989 100 0 0 0 0 2 3
18 6 1989 100 0 0 0 0 2 3
19 6 1989 70 30 0 0 0 1 2
20 6 1989 70 30 0 0 0 1 3
21 6 1989 60 40 0 0 0 2 3
22 6 1989 30 60 10 0 0 3 4
23 6 1989 40 60 0 0 0 4 5
24 6 1989 80 20 0 0 0 3 4
25 6 1989 100 0 0 0 0 4 5
26 6 1989 90 10 0 0 0 3 4
27 6 1989 95 5 0 0 0 4 5
28 6 1989 100 0 0 0 0 2 4
29 6 1989 100 0 0 0 0 2 3
30 6 1989 90 10 0 0 0 2 3
1 7 1989 80 20 0 0 0 2 3
2 7 1989 100 0 0 0 0 2 3
3 7 1989 100 0 0 0 0 3 4