Here is the trap in every “scorching temperatures” headline: heat is not a number, it’s a comparison. Thirty degrees in Delhi in June is an unremarkable Tuesday. Thirty degrees in Reykjavík would be the hottest day in Icelandic history. The same reading is banal in one place and historic in another — which means a temperature only becomes information when you know what’s normal for that exact place at that exact time of year.
The baseline is everything
Meteorologists formalise “normal” as a climatological baseline: average the recorded temperatures for this calendar window across many past years. EarthPulse computes one for any point on the planet — we take a ±5-day window around today’s date across the last ten years of the Open-Meteo archive, roughly 110 historical days, and average their highs and lows. Today’s forecast against that baseline gives the anomaly: “+6.4° above the seasonal norm” is a sentence with meaning anywhere on Earth, from Phoenix to Svalbard.
When does hot become a heatwave?
There’s no single global definition, but most national weather services agree on the shape: temperatures well above the local seasonal baseline, sustained for several consecutive days. The UK Met Office, for instance, requires a location-specific threshold to be met for at least three days running. The persistence matters because that’s what drives harm — bodies, buildings and power grids can absorb one hot afternoon, but nights that never cool are what turn heat into a public-health event. It’s also why the most dangerous statistic in a heatwave is often the overnight minimum, not the daily maximum.
Humidity, the silent multiplier
Human bodies shed heat by evaporating sweat, and humid air takes that tool away. A dry 35° can be more survivable than a humid 31° — which is what “feels-like” temperature attempts to capture by folding humidity and wind into the reading. When the wet-bulb temperature (the reading a thermometer gives wrapped in wet cloth) approaches the low 30s, evaporative cooling stops working entirely; sustained exposure becomes dangerous even for healthy people in the shade with water.
This relative view of heat is exactly what the Heat Watch panel on the EarthPulse globe does continuously: it scans forty world cities against their own ten-year norms and surfaces the largest anomalies — not the highest temperatures. Kuwait City at 44° might not make the list on a day when Stockholm at 31° tops it. That inversion is the entire point.