Thermofront

How the Thermofront Works

Pick a number of days—say 7. Now look at every 7-day stretch during the summer and ask: what was the lowest temperature during that stretch? The thermofront finds the 7-day stretch where that low was as high as possible. That is the warmest week of summer, measured by how cool it got at its coolest.

The Pareto frontier

At k = 1 the thermofront is simply the highest single-day low of the season. As k grows, the window must include more nights, inevitably catching a cooler one, so the curve drops. The thermofront traces every point where you cannot find a longer hot stretch without accepting a lower trough—the classic shape of a Pareto frontier.

Tropical nights

A tropical night is one where the overnight low stays at or above 20 °C (68 °F). Where the thermofront crosses that threshold, you can read off how many consecutive tropical nights occurred during the season.

Reading the chart

Each line is one season (Oct–Sep). The bold line is the current season, updating as new nights are recorded. Hover to see the exact date range of the warmest k-day window. A small ★ marks a value that is the warmest in the displayed history.

The dashed “Last k days” overlay shows the minimum daily low over the most recent k days. It is always at or below the current season’s thermofront, since the thermofront cherry-picks the best window while “last k” is anchored to today.

Daily lows only

All computations use daily low temperatures—one value per day, representing the coolest temperature recorded that day (usually overnight). Hourly data is not used; the Open-Meteo historical archive provides the daily minimum.

Seasons & data

A season runs from October 1 through September 30. The current season’s curve updates throughout the year as new days are added. When two equally warm streaks exist, the most recent one is shown—so an ongoing heat wave always appears.

Daily low temperatures come from the Open-Meteo historical archive.

Include current season

The current season (Oct–Sep) is often incomplete—summer may not have arrived yet, so its thermofront only reflects data so far. Uncheck this box to hide the current season and compare only complete historical seasons against each other. When unchecked, the bold “current” line disappears and records are computed across historical seasons only.

Include today so far

By default, data ends at yesterday—the most recent day with a final, complete daily low. Check this box to append today’s observed low so far, computed from hourly temperatures recorded up to the most recent hour. Because the day is still in progress, today’s low may drop further; the value updates each time you reload. This affects both the season curves and the “Last k days” overlay.