Summer of Simmer: Going For the Gold

PM Update: It's 92° at 3 pm, so the Washington summer average temperature moves up to 81.3°.

Original post:
As noted in the previous post, the summer of 2010 reached the halfway point well on its way to being one of the hottest since Washington weather history began in 1871. The chart on the right shows that it would be quite feasible to set an all-time summer heat record.

Each line on the chart represents the cumulative average summer temperature from June 1 through the given date on the x-axis. The temperatures are shown for 2010, the current hottest summer of 1980, and the two runners-up, 1991 and 2002. The lower green curve, labeled "Normal", is the 30-year climatological average from 1971-2000. The observed data so far this year is shown as a solid line, and the dotted line is a projection for the rest of the summer. Note that this is called a "projection", not a forecast. It includes the official National Weather Service forecast for today through the 25th, posted at noon today, followed by the climatological average from July 26 through August 31. Using this relatively conservative projection, the final average for summer 2010 would end up being 80.1°, edging out the previous record by 0.1°.

Here are a few points to consider about the chart. An average, of course, is simply an arithmetic calculation based on the sum of a group of numbers, so order has no effect. On the other hand, once a certain subset of the numbers are already known, each additional number will have a smaller effect on the final result. This means that the lines start jumping around a lot with the first few points in June, but they begin to settle down as more points are added moving to the right.

Interestingly, all 4 of the years shown started off well above average in early June, then declined substantially later in June before rising again to approach their final values. In 1980, the average declined so much that it was below normal for most of 2 weeks from June 13 to the 27th. In fact, although all 3 months were above normal, June was only slightly above, and August was the warmest month, setting the record for that month.

Although 2010 started off a little slower than the other years, it had much less of a dip in late June, going on to set the record for the month. It has continued to steam on (literally) through July to reach the current value of 81.2°, which is likely to rise by another 0.1° if today's high goes up by 1° and the morning low of 77° is not exceeded on the down side.

This year's trend could be reversed, of course, by a cool August, although that's not in the current long-range forecast. Notice how 2002 was in the lead in late August until a cool snap knocked high temperatures into the low and mid 70s for the last 4 days. Barring such a reversal, however, this should be an interesting race for the next 6 weeks; the chart will be updated through the summer as conditions warrant. It's also an instructive example of how, hour by hour, day by day, week by week, month by month, season by season, weather becomes climate.

Image (click to enlarge): Washington, DC average summer temperatures to date for 2010, 1980, 1991, 2002, and climatological period 1971-2000; CapitalClimate chart from National Weather Service data, background photo from Kevin Ambrose