| Jun | Feb | Mar | Apr | May | June | July | Aug | Sep | Oct | Nov | Dec | |
| 1970-1979 | 37.1 | 29.7 | 40.8 | 43.3 | −2.9 | −33.4 | −48.0 | −51.4 | −40.8 | 5.2 | 6.8 | 33.3 |
| 1980-1989 | 18.7 | 26.7 | 12.5 | 8.6 | −11.7 | −47.3 | −54.3 | −50.0 | −39.8 | −20.8 | 26.4 | 9.4 |
| 1990-1999 | 12.7 | 20.8 | 29.7 | 10.5 | −22.2 | −37.7 | −53.3 | −53.3 | −31.3 | −19.8 | 4.1 | 29.1 |
| 2000-2016 | 5.2 | 14.6 | 18.0 | −0.3 | −18.1 | −44.1 | −52.3 | −48.9 | −3.5 | 36.8 | 14.1 | 21.3 |
| N=Average(D) | 18.6 | 23.0 | 25.2 | 15.5 | −13.7 | −40.6 | −51.9 | −50.9 | −28.8 | 0.3 | 12.8 | 23.3 |