Various factors intervene in the release of avalanches. A heavy fall of snow may produce a layer of snow that moves as an immediate avalanche within 3 days of its deposition. The structure of the snow layer is one of the prime factors in avalanche causation; wind and temperature are other decisive factors. Avalanches usually occur during periods of high wind which cause overloading and wind slabs. There is usually a rise in temperature, a factor that normally accompanies heavy precipitations; this temperature rise reduces the cohesion between snow strata but at the same time promotes compaction and consolidation. Temperature rise therefore has a dangerous effect at first, which does not persist except when there is significant melting. On slopes with an incline of over 140% there is no danger of avalanche since the snow cannot accumulate over the long term. The lower limit of dangerous incline is quite low (30%), although avalanche defence construction is not built on slopes of less than 70% incline. Local conditions such as relatively smooth ground surfaces or the presence of convex slopes will increase the avalanche hazard.