Hong Kong Issues Record Late Heat Warning as October Temperatures Reach 36°C

Late October brings summer heat

Hong Kong issued its latest Very Hot Weather Warning of the year on record at 6.45am on Wednesday, October 21, as temperatures reached 36°C in Yuen Long by lunchtime and 33.8°C at the Hong Kong Observatory headquarters in Tsim Sha Tsui.

The warning came two days later in the calendar than the previous record, October 19, 2024. The headquarters temperature also surpassed the previous October 21 record of 31.5°C, set in 2024, by 2.3°C.

Amos Tai Pui-kuen, professor and deputy chair of the Department of Earth and Environmental Sciences at the Chinese University of Hong Kong, attributed the unusual heat to three overlapping influences: sinking air associated with a distant typhoon and an anticyclone, changes in tropical circulation linked to El Niño, and the higher temperature baseline created by global warming.

The immediate forecast offered limited relief. The Observatory expected daytime highs above 30°C through the following Wednesday, while Tai said very hot conditions could extend into November and warned of a warmer winter.

How hot did different districts become?

Yuen Long's reading of 36°C was 2.2°C above the maximum recorded at the Observatory headquarters. Before the afternoon heat developed, forecasters had expected parts of the New Territories to be two to three degrees hotter than urban areas.

The Observatory forecast an urban maximum of about 33°C and a minimum of around 26°C. Its Very Hot Weather Warning uses a heat index threshold and is generally issued when temperatures are expected to reach or exceed 33°C.

The warning and the temperature measurements describe different parts of the event. The warning alerts residents to dangerous heat conditions, while readings from individual weather stations show how much temperatures vary across the city. On Wednesday, people in Yuen Long experienced substantially hotter conditions than those recorded at the headquarters.

Why sinking air made Hong Kong hotter

Super Typhoon Champi was about 1,040 kilometres east northeast of Manila at 8am, moving across the western North Pacific towards waters east of Taiwan. Tai said air descending around the distant storm contributed to Hong Kong's heat, alongside an anticyclone aloft.

An anticyclone is a circulation associated with higher air pressure. When air sinks, it becomes compressed and warms. Descending air also makes it harder for clouds and rain to develop, leaving more sunshine to heat the ground.

Weak winds added to the effect by allowing heat to build locally. Hong Kong could therefore experience hotter weather from circulation around a typhoon even while the storm remained far away.

The Hong Kong Observatory described the conditions over the wider region in its Wednesday weather statement:

Winds are weak over the coast of Guangdong. Meanwhile, under the influence of an anticyclone aloft, the weather is generally fine and very hot over Guangdong.

Less cloud, strong sunshine and limited air movement worked together. The same conditions also helped explain the forecast for mainly fine weather and dry afternoons through the rest of the week and into the weekend.

Where El Niño fits into the explanation

Tai identified what he described as a Super El Niño as another contributor. El Niño is a climate pattern involving unusually warm surface water in the central and eastern equatorial Pacific Ocean. Changes there can alter winds and rainfall far beyond the Pacific.

In Tai's explanation, the altered tropical circulation produced an unusual anticyclonic pattern over the South China Sea and the Philippines. This suppressed convection, the upward movement of warm air that helps form clouds and showers.

The circulation also weakened the northeast monsoon, which normally brings cooler air towards southern China during autumn. With less cooling air arriving, local heat could accumulate more easily under clear skies.

Tai said El Niño can also shift typhoon tracks farther north, away from Hong Kong. At the same time, unusually warm seas can supply more heat and moisture to a storm that approaches the region. A storm's route and its strength depend on several conditions, so the two effects can occur together: a changed track across the ocean and more energy available from warm water.

Why warmer seas and global warming matter

Global warming supplied the third part of Tai's explanation. Hong Kong now experiences short periods of unusual weather against a warmer starting point than in the past. When sinking air, sunshine and weaker cooling winds occur together, temperatures can climb above historical averages more readily.

Berkeley Earth estimates that Hong Kong has warmed by 1.7°C since the Industrial Revolution. That figure describes a long period of warming, while Wednesday's 33.8°C reading describes the maximum temperature at one station on one day.

The Intergovernmental Panel on Climate Change has found that human activity has increased the frequency and intensity of heatwaves since the 1950s. Greenhouse gases, including carbon dioxide, trap heat in the atmosphere and raise the planet's surface temperature.

Tai also pointed to the slower temperature response associated with warmer seas. Water gains and loses heat more slowly than land, allowing the ocean to retain warmth as the seasons change. That helps explain why cooler autumn air may arrive while surrounding waters continue to release stored heat.

What comes next in the forecast?

The Observatory expected a surge of the northeast monsoon to reach coastal areas on Wednesday night. Light winds were forecast to give way to moderate northeasterly winds, with the possibility of isolated showers.

The Observatory explained the approaching change in its weather statement:

Besides, pressure is rising over northern Guangdong. A surge of the northeast monsoon is expected to reach the coastal areas tonight.

Daytime conditions were still forecast to remain mainly fine and hot through the week and weekend, with highs above 30°C continuing until the following Wednesday. Residents could therefore feel stronger winds without seeing an immediate return to cool autumn afternoons.

Friday, October 23, brings Frost's Descent, the final autumn solar term in the traditional Chinese calendar. The name marks a seasonal point rather than a promise of frost in Hong Kong. If temperatures reached 33°C, the day would exceed the previous record of 31°C for that solar term, set in 2006, by two degrees.

The key dates and measurements are:

• October 19, 2024: the previous latest date for a Very Hot Weather Warning.
• October 21, 2024: the previous record for that calendar date was 31.5°C.
• Wednesday, October 21: the warning was issued at 6.45am; temperatures later reached 33.8°C at headquarters and 36°C in Yuen Long.
• Wednesday night: a northeast monsoon surge was expected to reach the coast.
• Friday, October 23: Frost's Descent could bring another temperature record if the forecast heat persisted.
• The following Wednesday: daytime highs were forecast to remain above 30°C through this point.

A longer season of dangerous heat

Hong Kong's heat records show that Wednesday's warning arrived amid a much broader increase in hot weather. Observatory data put the annual average number of Very Hot Days at 13.4 during the 2000s, compared with more than 50 a year since 2020.

The Observatory's extreme weather series also provides a longer-term comparison: the annual average number of Very Hot Days rose from 2.2 in 1884–1913 to 24.8 in 1996–2025.

Using those figures, the recent annual count is more than 3.7 times the average in the 2000s. Residents now face many more days when outdoor work, exercise and routine journeys take place in intense heat.

The annual number of hours covered by heat warnings has also tripled since 2000. The increase affects both the frequency of hot days and the amount of time people spend under warnings.

The Observatory urged the public to guard against heatstroke during Wednesday's conditions. Heatstroke occurs when the body can no longer control its temperature and requires urgent medical attention. Strong sunshine and high temperatures increase the burden on people working or spending long periods outside.

Could the warmth continue through winter?

Tai said very hot weather remained possible in November because periods of high temperatures were lasting longer. He expected the influence of the El Niño pattern he described to persist until the following spring, contributing to a warmer winter.

No numerical winter temperature forecast was given. Tai's assessment pointed towards unusual seasonal warmth, with the possibility of an exceptionally warm winter, while acknowledging that daily weather is difficult to predict precisely.

For Hong Kong residents, the immediate concern was more concrete: several further days above 30°C, dry daytime conditions and the possibility of another record at Frost's Descent. The expected monsoon surge offered a change in wind conditions, but the forecast still kept summerlike heat in place well into late October.

Key Points

• The Observatory issued its latest Very Hot Weather Warning on record at 6.45am on Wednesday, October 21.
• Yuen Long reached 36°C, while Observatory headquarters recorded 33.8°C.
• CUHK professor Amos Tai attributed the heat to sinking air, circulation changes linked to El Niño and global warming.
• A northeast monsoon surge was expected on Wednesday night, but daytime highs above 30°C were forecast through the following Wednesday.
• Temperatures of 33°C on October 23 would surpass the previous Frost's Descent record of 31°C.
• Hong Kong has recorded more than 50 Very Hot Days annually since 2020, compared with an annual average of 13.4 in the 2000s.
• Tai said very hot weather could extend into November and expected a warmer winter.