Sentence:  The point where occluding starts is the <WSD>triple point</WSD>, because there are three air masses involved: that ahead of the occlusion, the warm air aloft, and the air behind the occlusion, or where the cold, warm and occluded fronts meet. 
Meaning of the word marked with <WSD> tags:  The intersection of an occluded front, cold front, and warm front.