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Small changes in atmospheric pressure can start a landslide in certain weather conditions. Understanding why will help us to assess which slopes are at most risk of failing.

Just over a decade ago, there was huge surprise when it was discovered that the intermittent sliding of the Slumgullion landslide in the San Juan Mountains of south-west Colorado was . We knew that landslides are initiated by earthquakes or heavy rains, but the Slumgullion findings raised the possibility that changes in atmospheric pressure could be setting hillsides on the move in other places too.

Now, at the University of Rennes in France and his colleagues have fed weather and landslide data from Taiwan 鈥� where steep hillsides and typhoons are a recipe for frequent landslides 鈥� into a landslide model. They found that typhoon events prime hillsides by bringing heavy rain and increasing the water pressure in the pores between grains in the sediment, but the timing and triggering of landslides depends on the weather over the preceding months.

The model showed that heavy rain after a dry spell pushes the water table up suddenly, causing a large and rapid change in pore pressure and triggering an immediate landslide.

A typhoon falling onto already saturated ground, however, didn鈥檛 alter pore pressure enough to trigger a slide. A subsequent change in atmospheric pressure 鈥� as the eye of the storm passes over, for example 鈥� was then enough to set things in motion. 鈥淭he atmospheric effect will only provide the last push when rainfall can鈥檛 infiltrate any more, or when it is faster and affects the slope before the rainfall does,鈥� says Pelascini.

The findings help to explain why some landslide events don鈥檛 occur until hours or even days after heavy rain has fallen. They also explain why Typhoon Morakot in 2009 鈥� Taiwan鈥檚 deadliest typhoon in recorded history 鈥� was so catastrophic. The dry period before the typhoon meant the water table was low and resulted in a large and very sudden change in pore pressure, which instantaneously triggered multiple landslides.