Climate Change Chart of Helena, USA

Overview of climate changes

Temperature changes

Averaged across calendar months, monthly mean temperature shows a difference of +1.86 °C compared with the earlier reference period; this is an unweighted comparison of monthly means, not a day-weighted annual average or a measure of daytime highs. The largest monthly increase was in January, with a difference of +5.16 °C. By contrast, October had the largest monthly decrease, a difference of -0.16 °C. These monthly extremes illustrate that the overall mean difference does not describe every month individually.

Changes in rainfall

Mean daily rainfall, averaged across calendar months, had a difference of -0.05 mm/day; this unweighted mean describes the aggregate of monthly means, not monthly totals or a day-weighted annual amount. The clearest monthly decrease was in June, with a difference of -1.19 mm/day, while August recorded the largest monthly increase, at +0.32 mm/day. Thus, the small overall mean difference does not imply that rainfall shifted in the same direction in every month, and these daily means should not be read as total rainfall for either month.

Changes in sunshine duration

Mean daily sunshine duration averaged across calendar months had a difference of -0.36 hours/day. This is an unweighted comparison of monthly mean daily hours, rather than a measure of total sunshine over a year. The largest monthly increase occurred in June, with a difference of +0.92 hours/day; the largest monthly decrease was in April, at -0.75 hours/day. These contrasting monthly changes help show why the overall mean difference should not be taken as a description of every month.

Changes in snowfall

Averaged across calendar months, mean daily snowfall has a difference of 0.00 mm/day at the displayed precision. That unchanged rounded mean does not establish that snowfall was unchanged in every month. The largest monthly increase was in December, with a difference of +1.69 mm/day, while the largest monthly decrease occurred in May, with a difference of -1.20 mm/day. These are differences in mean daily amounts, not monthly snowfall totals.