By Amy Sawitta Lefevre

BANGKOK (Reuters) - Five months after the worst floods in half a century, the Thai capital is facing a near record heat wave with temperatures at three-decade highs, stoking debate over the often chaotic urban planning in one of Asia's hottest and largest cities.

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 relatively 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 meters per person in the metropolitan area.

That is well below the index average of 39 square meters and contrasts with Singapore, a fellow Southeast Asian tropical city 1,430 km (890 miles) to the south, which has 66 square meters of green space per person.

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dean at the Faculty of Architecture and Planning at Thammasat University, says Bangkok's heat is made worse by outdated building designs that lack the proper insulation needed to keep buildings cool, leading to a rise in energy consumption.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.

Businesses report surging sales of air conditioners, sun-screen and other cooling products.

Mistine Cosmetics Thailand, for example, saw sales of sunscreen products, lotions and creams jump 14 percent in April compared with the same period last year, says the company's marketing planner, Cholacha Subeuong.

Humans aren't the only ones suffering.

"Because of the heat, we have had to put in place cooling measures for the animals," says Waraporn Gunton at Bangkok's Dusit Zoo.

Measures have included mixing ice with animal food and watering some animals down with sprinklers.

(Editing by Jason Szep and Robert Birsel)