April is the hottest month in the Bangkok calendar, a time of blistering sunbeams and broiling pavements. Relief comes in the form of a southerly wind that sweeps through the city and brings an evening chill to those high-rise balconies that face the right direction. Down by the river, the effect is even more pronounced, and is a blessed relief from the stifling heat.
It's also a great time to fly a kite. Sanam Luang, the park outside the walls of the Grand Palace, is the traditional spot for kite enthusiasts to gather. To the casual passer-by, it's normally the more colourful kites that catch the eye. Red smiley faces, cartoon characters and bold patterns stand out against the hazy blue skies of Bangkok.
But there's another category of kite that is equally impressive: Thailand's traditional chula and pakpao models. Instead of flashy designs, they're made with simple white paper on bamboo frames. These are battle kites that duel in the sky, taking turns to attack. The chula is a pointed star-shaped 'male' and the pakpao is a smaller, diamond-shaped 'female'.
The game is played by competing teams that divide their territory and launch their kites. First, the chula tries to catch the nimble pakpao and drag it back to its territory. Then it's the turn of the pakpao. It uses its tail as a lasso to pull down the chula. Each successful attack scores a point, and the game is fast and furious. At competitions, teams often don traditional Thai costumes - white tunics and silken knickerbockers - that add to the sense of occasion.
The exact origins of the kites are unclear, but they are said to combine some of the features of traditional kites from Malaysia and China. The simple colour scheme reflects the attention paid to aerodynamics and the engineering challenge presented by the large frames. The male and female shapes clearly represent some kind of fertility rite that echoes down through the ages.
One of the biggest boosters of these so-called 'heritage kites' is Ron Spaulding, a Swedish advertising executive who has spent 30 years in Thailand. He helped to organise the country's first international kite festival in 1990. 'Kites are in my heart. I'm always happy when I'm flying kites,' he told the Bangkok Post.
It used to be easy to find kite-flying teams to compete at Sanam Luang, but waning interest among the younger generation is reportedly holding back the sport. Many of the old masters who make and fly the kites are struggling to keep the tradition going in the face of competition from football and other sporting imports. Still, there's support from tourism authorities for the annual competition, and enthusiasts like Mr Spaulding, to keep the flame burning. As long as the wind keeps blowing, the kites will probably keep flying in Bangkok's breezy open spaces.