单位：美元/千立方英尺
| 年份 | 一月 | 二月 | 三月 | 四月 | 五月 | 六月 | 七月 | 八月 | 九月 | 十月 | 十一月 | 十二月 |
| 1989 | 1.72 | 1.88 | 1.81 | 1.73 | 1.76 | 1.81 | 1.75 | 1.77 | 1.77 | 1.78 | 1.91 | 1.99 |
| 1990 | 2.12 | 2.1 | 1.99 | 1.82 | 1.79 | 1.79 | 1.83 | 1.78 | 1.8 | 1.84 | 2.12 | 2.13 |
| 1991 | 2.11 | 1.92 | 1.86 | 1.79 | 1.74 | 1.72 | 1.68 | 1.6 | 1.67 | 1.8 | 1.91 | 1.98 |
| 1992 | 1.98 | 1.68 | 1.65 | 1.63 | 1.66 | 1.77 | 1.7 | 1.83 | 1.96 | 2.22 | 1.96 | 2.13 |
| 1993 | 2.07 | 1.89 | 1.94 | 2.03 | 2.17 | 1.98 | 1.92 | 1.97 | 2.09 | 1.93 | 2.07 | 2.27 |
| 1994 | 2.11 | 2.24 | 2.21 | 1.96 | 1.94 | 1.77 | 1.82 | 1.76 | 1.66 | 1.54 | 1.71 | 1.72 |
| 1995 | 1.6 | 1.46 | 1.4 | 1.37 | 1.46 | 1.47 | 1.4 | 1.34 | 1.43 | 1.48 | 1.61 | 1.8 |
| 1996 | 2.09 | 1.95 | 1.92 | 1.87 | 1.72 | 1.7 | 1.84 | 1.8 | 1.62 | 1.71 | 2.26 | 3 |
| 1997 | 3.26 | 2.52 | 1.72 | 1.67 | 1.83 | 1.88 | 1.84 | 1.83 | 2.01 | 2.33 | 2.72 | 2.19 |
| 1998 | 2.08 | 1.94 | 1.98 | 2.04 | 2.02 | 1.88 | 1.97 | 1.82 | 1.68 | 1.93 | 2.12 | 2.16 |
| 1999 | 2.03 | 1.93 | 1.8 | 1.85 | 2.17 | 2.13 | 2.18 | 2.39 | 2.63 | 2.5 | 2.85 | 2.34 |
| 2000 | 2.44 | 2.6 | 2.61 | 2.86 | 3.05 | 3.87 | 3.94 | 3.62 | 4.15 | 5.16 | 4.86 | 7.35 |
| 2001 | 9.47 | 6.45 | 5.42 | 5.35 | 4.95 | 3.94 | 3.17 | 3.14 | 2.63 | 2.14 | 2.95 | 2.68 |
| 2002 | 2.73 | 2.32 | 2.61 | 3.27 | 3.25 | 3.08 | 2.84 | 2.7 | 3.06 | 3.56 | 4.04 | 4.2 |
| 2003 | 4.89 | 5.71 | 7.61 | 5 | 5.04 | 5.53 | 5.11 | 4.54 | 4.67 | 4.48 | 4.51 | 5.08 |
| 2004 | 5.98 | 5.67 | 5.15 | 5.21 | 5.63 | 6.06 | 5.75 | 5.62 | 4.96 | 5.38 | 6.98 | 6.96 |
| 2005 | 6.3 | 6.14 | 6.24 | 6.92 | 6.58 | 6.35 | 6.82 | 7.5 | 9.96 | 11.99 | 11.31 | 10.91 |
| 2006 | 10.11 | 7.96 | 7 | 6.73 | 6.53 | 5.83 | 5.95 | 6.74 | 5.83 | 4.88 | 7.24 | 7.6 |
| 2007 | 6.5 | 7.39 | 7.45 | 7.24 | 7.45 | 7.32 | 6.56 | 6 | 5.49 | 6.35 | 7.21 | 7.51 |
| 2008 | 7.66 | 8.34 | 9.03 | 9.61 | 10.8 | 11.71 | 11.78 | 8.67 | 7.54 | 6.87 | 6.54 | 6.74 |
| 2009 | 6.18 | 5.21 | 4.34 | 3.67 | 3.48 | 3.51 | 3.5 | 3.29 | 2.91 | 3.92 | 4.4 | 5.23 |
| 2010 | 5.95 | 5.62 | 4.87 | 4.13 | 4.13 | 4.23 | 4.4 | 4.13 | 3.76 | 3.83 | 3.96 | 4.56 |
| 2011 | 4.69 | 4.5 | 4.25 | 4.35 | 4.32 | 4.61 | 4.39 |
