Newsletter Issue 70 (August 2026)

Registration Link (Deadline: 12 September 2026, Saturday)

As part of Talk Series 2026, the HKMetS will be holding a talk on Aviation Weather! The details are as follows.

Topic: Airport Diary — Weather
Date: 19 September 2026 (Saturday)
Starting Time: 10:00 AM
Visit: HKO Headquarters
Speakers: Sharon Lau (Chairperson of HKMetS), YF Lee (Executive Member of HKMetS)
Language: Cantonese
Maximum participants: 30

The Weather of July 2026 – A gloomy and rainy July with the strike of Noul

Mainly attributing to the rain associated with broad troughs of low pressure, the active southerly airstream and tropical cyclones Maysak and Noul, the weather of Hong Kong was much gloomier and wetter than usual in July 2026. The monthly sunshine duration was 122.2 hours, the lowest on record for July. The monthly mean amount of cloud was 81 percent, one of the second highest on record for July. The monthly rainfall recorded at the Hong Kong Observatory Headquarters was 790.3 millimetres, more than double the July normal of 385.8 millimetres. The monthly mean temperature was 28.7 degrees, close to the normal of 28.9 degrees.

Record-breaking heat and extreme weather continue

Record land and sea surface temperatures, heat, drought, fire and floods define the state of the climate in July and early August. July was either the warmest or second warmest according to NOAA and Copernicus Climate Change Service. Global sea surface temperatures were the highest ever recorded for July. July also saw exceptional wildfire activity across western Europe, in terms of the area burnt and emissions. From late July to early August, China, South Korea and Japan were hit severely by successive rounds of flooding.

El Niño is headed for record-shattering intensity, NOAA says

There is a nearly 70% chance that the building Super El Niño will be the strongest such event on record, according to NOAA. Ocean temperatures are already hotter than normal for this time of year in the Central Equatorial Pacific compared to even the hottest El Niños on record. NOAA forecasters are anticipating that sea surface temperature anomalies in the Niño region will exceed 2.5 degrees Celsius above average, as measured over a three-month timespan. Because of the strength of this El Niño, it is virtually certain that 2027 will set a record for the hottest year on record, and possible that 2026 will as well.

Persistence of Arctic Ocean acidification under negative emissions

Although net negative emissions of carbon dioxide (CO2) are essential to meet climate targets, little is known about how declining atmospheric CO2 levels will affect ocean acidification. By analysing the acidity ([H+]) and corrosivity to aragonite (ΩArag) in eight Earth system models that made simulations under rising then falling CO2 levels, the team identifies the Arctic as a hotspot for delayed reversibility of ocean acidification. The Arctic Ocean experiences not only the greatest acidification but also the most delayed benefits from negative emissions.

Improving tropical cyclone intensity forecasts with a simple AI correction

AI weather models have made remarkable progress in predicting the tracks of tropical cyclones (TCs). However, forecasting their intensity poses a much greater challenge, with models exhibiting a persistent tendency to underestimate storm strength. The team explores a simple way to address this problem. Rather than building a new TC forecasting model from scratch, the team developed a second machine-learning system to learn the errors that the existing Artificial Intelligence Forecasting System (AIFS) tends to make and predicts a correction to that forecast, achieving performance comparable to both leading operational guidance and AI state-of-the-art systems in forecasting maximum wind speed and minimum central pressure.

Machine learning opens new opportunities for global reanalysis

ECMWF scientists are exploring how machine learning models trained directly on Earth system observations can reconstruct the historical state of the atmosphere from observations alone. The scientists evaluated how well the machine-learning reanalysis reproduces the mean structure of the atmosphere and its variability across different timescales. The work opens exciting opportunities for how observation-driven AI prediction could be used to complement future global reanalysis production, supporting faster experimentation and more flexible approaches to generating Earth system datasets.

Physics-based models outperform AI weather forecasts of record-breaking extremes

AI-based models’ ability to extrapolate and reliably forecast unprecedented extreme events remains unclear. The team shows that for record-breaking weather extremes, the physics-based numerical model High RESolution forecast (HRES) from the ECMWF still consistently outperforms state-of-the-art AI models. Their findings underscore the current limitations of AI weather models in extrapolating beyond their training domain and in forecasting the potentially most impactful record-breaking weather events. Further rigorous verification and model development is needed before they can be solely relied upon for high-stakes applications such as early warning systems and disaster management.

Latest Trends in Atmospheric Carbon Dioxide (CO2)

The monthly average Mauna Loa CO2 in July 2026 was 429.12 ppm. The red lines and symbols represent the monthly mean values, centered on the middle of each month. The black lines and symbols represent the same, after correction for the average seasonal cycle. The vertical bars on the black lines show the uncertainty of each monthly mean based on the observed variability of CO2 in different weather systems as they go past the top of Mauna Loa.

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