The daily average high in Bangkok in April was 40.1 Celsius (104.2 Fahrenheit), the Meteorological Department says, prompting warnings from authorities for residents to be alert for heat-related ailments.
Critics say the heat has been exacerbated by poor urban planning in the fast-growing city of 12 million people- from a thinning of trees by city workers, often to accommodate electrical power lines, to heat-trapping building designs and a small number of parks.
"It is a factor," Prawit Jampanya, director of the Central Weather Forecast division at the Meteorological Department, said, referring to the lack of green spaces in trapping Bangkok's mercury-pumping heat.
"Having trees does help to relieve poor air quality and urban heat traps," he said.
Though a tropical city, Bangkok has fewer trees and green spaces in proportion to its population than other Asian cities.
An Asian Green City Index of 22 cities released last year by the Economist Intelligence Unit put Bangkok's green spaces at 3 square metres per person in the metropolitan area.
That is well below the index average of 39 square metres and contrasts with Singapore, 1,430 km (890 miles) to the south, which has 66 square metres.
Singapore, with 5 million people, is the world's second most densely populated country after Monaco, but it has made great efforts to expand green areas even as it has urbanised.
Its Urban Redevelopment Authority noted in 2010 its green cover had increased since 1986 as parks and recreation areas were created, even as the population grew nearly 70 per cent.
Kuala Lumpur, capital of Malaysia, is another victim of rapid development and poor planning, but it has 44 square metres of greenery per person and the government says it wants to plant another 30,000 trees this year to help.
LACK OF PLANNING
In the Philippines, 13 million people are sweltering in the congested capital, Manila. High-rise condominiums and commercial buildings have sprouted up and the population is growing fast.
April 30 was the warmest day of the year at 36.6 Celsius, a bit lower than the all-time high of 38.6 Celsius on May 7, 1915.
In 2009, Typhoon Ketsana brought massive rainfall and floods into Manila, killing nearly 250 people. That highlighted the impact of climate change plus a lack of urban planning.
"We can't do anything to expand green areas. What we are trying to prevent is informal settlers occupying the remaining green spaces," said Francis Tolentino, chairman of Metro Manila Development Authority.
Urban planning in Bangkok can seem arbitrary - from chronic congestion on main roads to obstructed or non-existent sidewalks, and poorly enforced zoning laws that allow homes and apartment buildings next to office towers and shopping malls.
Authorities hope to bring some order to the city with a new urban plan that takes effect from May next year.
Chalermwat Tantasavasdi, associate dean at the Faculty of Architecture and Planning at Thammasat University, says Bangkok's heat is made worse by outdated building designs that lack the proper insulation needed to keep buildings cool, leading to a rise in energy consumption.
The heat coincides with drought in 50 out of Thailand's 77 provinces, plus an increase in man-made and natural fires, just months after the worst floods in more than 50 years, which devastated seven big industrial estates in the centre.
The country's biggest industrial complex, Map Ta Phut, is in the east and escaped the flooding, but a lack of rainfall is beginning to worry its 147 businesses.
A local reservoir is only half-full and the director of the estate, Pratheep Aeng-Chuan, said officials would meet the Royal Irrigation Department and the provincial Department of Disaster Prevention and Mitigation on May 14 to discuss the situation.