Sentence:  Since the mid-1980s, when Indonesia first began to clear its bountiful forests on an industrial scale in favour of lucrative palm-oil plantations, “haze” has become an almost annual <WSD>occurrence</WSD> in South-East Asia. The cheapest way to clear logged woodland is to burn it, producing an acrid cloud of foul white smoke that, carried by the wind, can cover hundreds, or even thousands, of square miles. 
Meaning of the word marked with <WSD> tags:  An actual instance when a situation occurs; an event or happening.