What is a teletherm? The summer teletherm is the
calendar date of the year’s highest daily high temperature; the
winter teletherm is the date of the lowest daily low.
This chart tracks when those extreme days fall each year over 75 years.
If the red dots drift later, summers are peaking later.
If the blue dots shift, the coldest day is moving too.
Dashed lines show the linear trend.
The concept comes from
Dodds et al. (2016).
More details…
About Teletherms
For each calendar year, the summer teletherm is the
date on which the daily high temperature was highest, and the
winter teletherm is the date on which the daily low
temperature was lowest. These are, in the words of
Dodds et al.,
“the statistically hottest and coldest days of the year”—an
intuitive and powerful measure of local climate.
Reading the chart
The x-axis shows years (a 75-year span). The y-axis shows the day
of the year, labeled by month.
Red dots mark the warmest day
each year (highest daily high);
blue dots mark the coldest
(lowest daily low). Dashed lines show the linear trend over time.
If a trend line slopes upward, that extreme is shifting later in the
year; downward means earlier. Hover over any point to see the exact
date and temperature.
What drift means
Systematic shifts in when the hottest or coldest day occurs can
signal changes in seasonal patterns—longer or shorter summers,
changes in jet stream behavior, or the breakdown of historically
stable seasonal timing. Dodds et al. found clear and sometimes
dramatic shifts across U.S. stations over 160 years.
Tie-breaking
When multiple days in a year share the same extreme temperature,
the latest occurrence is shown. This means, for example, that a
late-season heat wave takes precedence over an earlier one of the
same intensity.
Data source
Daily high and low temperatures come from the
Open-Meteo
historical archive. Only fully completed years are included.
Reference
Dodds, P.S., Mitchell, L., Reagan, A.J., & Danforth, C.M. (2016).
“Tracking Climate Change through the Spatiotemporal Dynamics of
the Teletherms, the Statistically Hottest and Coldest Days of the
Year.” PLOS ONE, 11(5), e0154184.
doi:10.1371/journal.pone.0154184