June 2026 heatwave diary

At first I wanted to call it a "2026 heatwave"  but how can I assure in early July it will be the only such intense and abnormal hot spell this summer?

Tuesday, 16 June 2026
Judging by average temperature for the first half of the month, so far this is the coolest June since 2014. with hindsight, it turned out to be over 2 Celsius degrees warmer than 1991-2020 average in Warsaw.

Saturday, 20 June 2026 – Sunday, 21 June 2026
The first two days this year with highs above +30C, the latter humid and stormy. Around that time long-term forecasts hint at serious heatwave incident. At first they seem to be a model error, but with each next update, the heat dome prediction solidifies.

Thursday, 25 June 2026
Here it begins. Night-time low of +15C means at dawn it's still pleasant and one can keep the dwelling cool. Inside: between +21C and +24C. Temperature tops at +31C, so no tragedy.

Friday, 26 June 2026
The night is not that chilly, no colder than +18C, afternoon high of "mere" +31C, which is relatively bearable vs. what lies ahead. Inside: between +22C and +24C, humidity 60%

Saturday, 27 June 2026
I return home from an open-air party around midnight. At that time temperature in Powsin on the verge of Las Kabacki stood at +21C, while at the same time in Ursynów it declined to only +24C. Urban heat island proven existent. The last night when pre-dawn temperature dropped below +19C. In the morning temperature shoots up to climb to awful +34.7C, nearing the June maximum temperature record of +35.3C set on 26 June 2019, which is bound to be broken tomorrow. A country-wide June maximum temperature record for Poland has been beaten. Weather station in Słubice reported +38.5C, vs. previous record of +38.3C set also on 26 June 2019 in Radzyń. Inside: between +20C (air humidity above 70%) and +25C (air humidity just below 60%, not nice).

Sentence to be pondered upon: "Our houses have been built for a climate which no longer exists." For decades the primary concern was how to keep the dwellings warm during the colder half of the year, while growing frequency, intensity and duration of heat waves necessitate also keeping dwellings cool when outside it's well above +30C.

Sunday, 28 June 2026
First tropical night this year, with low of +20.4C in Warsaw. In western Poland minimum temperatures measured do not drop below +25C. From early morning the temperature soars. At 9:00 it reaches +30C, at noon it climbs to +35.7C, setting a new short-lasting heat record for the capital of Poland. Late in the afternoon a country-wide record (+40.2C measured on 29 July 1921 in Prószków near Opole) is shattered. The weather station in Słubice officially reports a high of +40.5C. Warsaw's highest ever temperature of +37.0C measured on 8 August 2013 is rendered outdated with a new readout of ghastly +38.1C. Inside: between +25C (air humidity 60%) to +26C (air humidity 55%). Awful, but cool compared to the oven outdoors.

The sentence to be pondered upon (continued). Air conditioning is a quick fix, lets one function properly, but has its dispersed cost. Prevalent in the US, seldom installed in private dwellings in Europe. With the current electricity output from solar panels it can be powered with nearly green energy during the day. This is the theory. In recent days around midday no more than 60% of energy output in Poland came from PV, while in the evenings demand for energy in Poland was satisfied in over 80% from conventional units, i.e. burning fossil fuels. Bear in mind AC removes the effect, not the cause. Walls give off heat after the sunset and you need to run AC overnight too to keep a dwelling cool and unless the electricity comes from a battery storage charged up during the day, you need conventional energy to power the AC. Besides, AC blows out hot air outside a cooled off building, thus increasing temperature to its neighbours and intensifying the urban heat island effect. I believe primary investments in thermal modernisations should involve proper insulation and protection from heat, not to let it trespass into a building and use of AC to augment it, when construction measures (thick insulation on walls and roofs, triple-glazed windows, external blind rollers, trees giving shade, ripping off paved urban areas) are insufficient. My take on duplicity of architects has been laid out here and I uphold it. I believe a balanced take is presented here.

Monday, 29 June 2026
With a minimum temperature of +24.7C last night was the warmest ever in Warsaw. I slept with windows closed, opened them after waking up at 4:00 a.m., to lock up the flat at 6:30 a.m. and head for air-conditioned office. Outside around 7:00 a.m. it was already +27C. To my surprise, underground stations and trains have not been warmed up horridly, I would hazard a guess it was around +25C inside. At Rondo ONZ station, despite early hour, droves of people were rushing to offices to find corporate shelters from heat. I admit it felt nice to cut off the awful heat, with today's high of +36.0C. No storms nor heavy rain have haunted Warsaw. Inside: temperature slowly increases to +27C (humidity: 60%), no longer comfortable, not yet tragic. Today administration of the building I live in issued a permission to install external blind rollers in windows of my bedroom and kitchen, both facing north-eastern side. In previous years, they were unnecessary, as the south-western living room ones did their job well enough. During the current heatwave I could swelter at 6:00 a.m. in my bedroom.

On both 28 June and 29 June daily temperature in Warsaw averaged out +30.6C, beating the previous record of +29.3C set on 8 August 2015.

Tuesday, 30 June 2026
The third tropical night in row. With temperature reaching a low of +21C it brings a minimum relief and lets the dwelling cool down a bit. Around 6:00 a.m. a short shower comes over Warsaw. It gives some little respite to the dried-up soil and leaves the air humid. Afterwards the heat is less tolerable, despite lower absolute temperature (max. +32C). Temperature creeps up above +30C around 2:00 p.m. which looks luxuriously cold. Inside: between +24C (humidity: 70%) and +27C (humidity: almost 60%).

Wednesday, 1 July 2026
The fourth tropical night in row, with low of +21C. In the morning outside it feels much better than inside, where neighbours' walls heat up mine. The afternoon is sultry, with high of +32C and much vapour in the air. Late evening brings a thunderstorm which puts the period of intense heat to a long-awaited end. Chasing away the warmth accumulated in buildings will take a while. This is formally the last day of the heatwave, with tomorrow expected to fall overnight to +17C and climb to +27C in the afternoon. Next days are about to be cold (in July terms) and I keep fingers crossed for that forecast.

The heatwave formally lasted 7 days and was not as awful as 15 days in a row with maximum temperature above +28C in August 2015. Until early July we have had so far 9 days this year with day-time highs above +30C, vs. long-term average of 9.5 days per year (1991 – 2020 benchmark period) and maximum number of such days of 24 in a year (2015 and 2024).

The latest heat in Warsaw ever

As I am a weather and climate statistics freak, knowing nearly by heart the history of weather records in Warsaw and having written 14 winter timelines here, each weather event which modifies weather stats for Warsaw should not pass unnoticed on PES.

With a delay, I note the maximum temperature record for July was broken on 3 July 2025, when temperature peaked at +36.5C, beating the reading of +35.9C dated 31 July 1994.

Last Sunday, on 21 September 2025 day-time high in Warsaw reached +30.8C. It was the first incidence of heat (defined as maximum temperature in excess of +30C) in the third decade of September ever. Moreover, the latest heat record, until last Sunday set on 14 September 1951 was broken by 7 days. Those are stats for Warsaw stand-alone. Country-wide, the latest incidence of heat was reported on 27 September 2012 (Jarczew, +30.8C, in most Polish cities temperature on that day reached above +26C).

The record Warsaw will be difficult to be beaten quickly, as such inflows of hot air are infrequent (we had it on 1 January 2023 when temperature peaked at +18.9C). Lucky us, it haunted us in the second half of September, not in mid-July as in high summer temperature would be likely to get close to +40C.

A matter of short time will probably be a first incidence of heat in October in Poland. So far the record is +29.3C measured on 3 October 2023 in Legnica.

Warmth spells in months other than summer ones are welcome, but they are a part of a broader picture whose element is a drought ongoing for a decade. Deficiency of rainfall has lasted two months now and forecasts for October, full of sunshine and no consensus about above-average temperatures, give little hope for making up for it.

Climate-friendly buildings

Summer 2025 by many has been perceived as cool and wet. Sadly, when such observations were shared in public, they fuelled claims global warming is a hoax, despite numerous figures giving lie to flat-earth believers and other wackos. In fact, in Warsaw this summer was thermally normal and dry (benchmark period: 1991 – 2020).

June 2025 with mean temperature of +18.5C and total precipitation of 47.7 millimetres was slightly warmer and drier than long-term average (respectively: +17.7C and 63.9 millimetres).

July 2025 with mean temperature of ++20.0C and total precipitation of 53.8 millimetres was normal and markedly drier than long-term average (respectively: +19.7C and 82.2 millimetres).

August 2025 with mean temperature of +19.3C and total precipitation of 9.2 millimetres was slightly warmer and extremely dry in comparison to long-term average (respectively: +17.7C and 60.6 millimetres).

The first week of September brought temperatures a few degrees above long-term averages and little rainfall. The summer-like weather is foreseen to continue at least for the next two weeks.

The total number of hot days (defined as those with maximum temperature above +30C) has reached 11 so far (the last one on Friday, 5 September) and in line with weather forecasts it stands a little chance to still go up. It is conceivable, as in the past the capital of Poland saw three incidences of heat in second decades of September in Warsaw: 14 September 1951, 11 September 2012 and 13 September 2023. On top we have had so far 8 days with highs between +28C and +30C, which formally do not meet the definition of a "hot day".

Not everyone is fond of such weather. I belong to those whose bodies don't feel well in high summer and hence I have highly appreciated moderate weather, milder than in recent years.

Hot summers in Poland are still colder than what residents of southern Europe had to endure decades ago, when air-conditioning was not widespread. Since central heating was also a missing installation in most dwellings, walls were thick and windows were tiny. Window shutters gave shelter both from the heat and the cold. Roofed terraces did not let sunrays reach windows in when the sun was shining high. The price to pay were dark interiors.

I last recalled how old-style architecture protected from heat, when I entered a several-century old monastery in Święty Krzyż. With +29C and full sunshine outside, the edifice gave great shelter from heat, despite no air-condition inside.

On Friday I strolled around the centre of Warsaw and stared at modern skyscrapers. They all had walls made entirely from glass, with each storey having windows from floor to ceiling. I realise window panes these days need to meet stringent energy efficiency requirements, but glass will never be as energy-efficient as a 40-centimetre-thick brick wall. All those modern buildings can boast of fancy eco-certificates, while their architecture boosts their energy demand. The same applies to modern premium properties, with large windows being a housing equivalent of SUVs in motoring.

No energy is fully green. Generating electricity from solar panels involves carbon footprint 95% lower than from burning black coal, while for wind turbines it is 99% lower. The calculations take into account total life cycle of specific installations (source: ChatGPT).

In the office building where I work, radiators blowing in hot air and air-conditioning blowing in cold air were working at full blast all summer round. No matter how green the energy is, such waste of energy in a building which boasts of being energy-efficient is unacceptable. Same as a defunct fire alarm which has failed to inform of two small fires in the underground garage this year...

Old, energy-inefficient buildings are torn down and new, energy-efficient ones are constructed on the same plots. Has anyone calculated how many years of lower energy usage it takes to make up for carbon footprint in the demolition and subsequent construction? ChatGPT needed 4 minutes to come up with an answer: 30 years for a typical energy-efficient building, 15 years for an ultra-energy-efficient building. I have not verified it, but if it is true, business and money matters much more than actual care for environment. Much better for the planet would be to modernise existing buildings.

No matter how green the electricity that powers air-conditioning is, detrimental effects of prevalent aircon in city centres persist. Masses if hot air are blown outside several buildings and along with excessive concrete areas, exacerbate the urban heat island effect, making living conditions in city centres unbearable, especially for elderly residents.