March and April are the months of the "Burning Season" in northern Thailand. The rainy season ends in November, and the land is dry, reservoirs are at their lowest points of the year. A thick haze covers much of the northern countryside and many people wear masks because of the pollutants in the air.

Farmers are preparing their fields for planting, which starts in May. Traditionally, they prepare their fields by burning out the stubble from last years' crops. It's cheaper than plowing it under and they use the ash as a fertilizer.

But pollution generated by the smoke sends people to local hospitals with breathing ailments and covers everything with a fine layer of soot. In some cases, the smoke is so thick commercial airports have to close or restrict operations. The Thai government it trying to prohibit burning but it's been a difficult campaign to win. Farmers here have been burning out their fields for thousands of years and see no reason to stop now.