Climate Change Chart of Washington D.C., USA

Overview of climate changes

Temperature changes

Averaged across calendar months, the difference in monthly mean temperature was +0.74 °C compared with the earlier reference period; this unweighted mean summarizes month-level averages rather than a day-weighted annual figure or daytime highs. The largest monthly increase was in December, at +2.22 °C, while November had the largest decrease, -0.21 °C. These contrasting monthly differences show that the overall mean does not describe every month’s change.

Changes in rainfall

Mean daily rainfall was higher by +0.81 mm/day averaged across calendar months, an unweighted summary of daily amounts by month rather than a monthly rainfall total. The largest monthly increase was in July, with a difference of +1.91 mm/day. By contrast, January recorded the largest decrease, -0.30 mm/day. These monthly contrasts qualify the overall increase: it does not imply that daily rainfall rose in every month.

Changes in sunshine duration

Mean daily sunshine duration had an unweighted, across-calendar-month difference of -0.23 hours/day compared with the earlier period. The largest monthly decrease occurred in February, at -0.74 hours/day, whereas July had the largest increase, +0.47 hours/day. The overall mean therefore summarizes month-level daily hours and should not be read as a change applying uniformly to each month.

Changes in snowfall

Mean daily snowfall increased by +0.24 mm/day averaged across calendar months, an unweighted summary of monthly mean daily amounts rather than monthly totals. The largest monthly increase was in March, with a difference of +1.55 mm/day; the largest decrease was in December, at -0.88 mm/day. Those opposing monthly differences show why the overall average should not be taken to mean snowfall increased in every month.