Weather Analysis: Monthly Averages Can Deceive
April 2013 in New York was rather unique because it was just the third time since 1960 that a month had a mean temperature right at the monthly average, i.e., 0.0 degree difference. The other two months in which this happened were August 1999 and August 1985. However, what set April 2013 apart was the fact that its average high and average low were almost right at the average as well (-0.1/+0.1). And while August 1999's high/low wasn't too far from average (-0.3/+0.3), August 1985's was significantly different (-1.3/+1.3).
April's near bulls-eye inspired me to examine all months that were relatively close to average, temperature-wise, to see how many had highs and lows both with departures from average of the same magnitude, and how many diverged significantly, but neutralized each other (like August 1985). For my purposes I included all months that were either right at the average, or 0.1, 0.2 or 0.3 degree from average (+/-). Since 1960 (thru 2020) there have been 63 months that fit this qualifier (which translates to an occurrence of about once a year).
So, what were my findings? Only six of the 63 months that reported a mean temperature close to average had highs and lows with departures from average that were nearly identical. But perhaps the biggest finding was that March 2009 was actually closer to a 0.0 difference from average than April 2013, as its high temperature was 0.1 below average and its low was right at the average. However, because of rounding, it ended up being 0.1 below average and not at 0.0 as April 2013 was.
| Departure from Average | Spread Between | |||
| High | Low | High/Low | ||
| September | 1989 | -0.2 | -0.2 | 0.0 |
| March | 1980 | +0.1 | +0.1 | 0.0 |
| MARCH | 2009 | -0.1 | 0.0 | 0.1 |
| August | 1990 | -0.2 | -0.1 | 0.1 |
| June | 1995 | +0.2 | +0.3 | 0.1 |
On the other hand, as the chart below shows, there are "average" months that have highs and lows that diverge wildly from their averages, but balanced each other to produce a monthly mean temperature that was "average". The most extreme case was November 1977. Although its mean temperature was 0.1 degree below average, the month's high was 2.5 degrees below average while the low was 2.3 above average. Two months before that, September 1977, exhibited a similar wide divergence.
| Departure from Average | Spread Btwn | ||||
| Mean | High | Low | High/Low | ||
| November | 1977 | -0.1 | -2.5 | +2.3 | 4.8 |
| September | 1977 | -0.2 | -2.1 | +1.8 | 3.9 |
| September | 1996 | -0.2 | -1.9 | +1.6 | 3.5 |
| June | 1996 | -0.2 | -1.8 | +1.4 | 3.2 |
| April | 1983 | -0.1 | -1.7 | +1.4 | 3.1 |
| October | 1998 | +0.1 | -1.4 | +1.5 | 2.9 |
| March | 2007 | -0.3 | +1.1 | -1.6 | 2.7 |
| August | 1985 | 0.0 | -1.3 | +1.3 | 2.6 |
| September | 1982 | -0.1 | -1.3 | +1.2 | 2.5 |
| June | 2006 | -0.2 | -1.3 | +1.0 | 2.3 |
| September | 1960 | -0.3 | -1.4 | +0.9 | 2.3 |
It's interesting to note that in all cases but one, the highs were below average while the lows were above average. This greater propensity for low temperatures to increase is a classic sign of global warming, whereby a more persistent cloud cover, promoted by increased carbon dioxide levels, keeps heat from escaping at night.
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