Wind conditions will also play a major role in terms of the flow into and out of openings, and thus the nature of the neutral planes in the various openings in a structure. The effect of external and internal pressure on the smoke flow through a vent can be seen by comparing the smoke issuing from the roof vents in Figure 26. On the left, the wind impinging on the opening has the effect of raising the ambient pressure at the opening, thus retarding the flow of smoke from the opening and essentially holding the hot gases within the structure. Virtually the same opening can be observed in the image on the right. However, a build-up of pressure is created within the structure, and this is sufficient to drive the flow of smoke out of the opening-even though the wind is still blowing towards the opening. The danger of having pressure built up in the structure is also illustrated in the image on the right. In this image, flames and smoke can be observed along the front edge of the roof, suggesting that the interior pressure has driven flames up into the attic/roof area.

Figure 26: Effect of wind on the neutral plane