3) Rain on Snow in the Alps

Rainy Days

Furthering yesterday's post, I read a paper from 2016 on the topic on rain-on-snow (ROS) events. Although the paper didn't specifically talk about skiing, a ROS event can have dramatic effects on increasing flooding further lower down the mountains. 

ROS events are events where precipitation falls as rain on to snow, which melts it due to the higher temperature of the water droplets than the snow. The resulting floods can have significant impacts on the downslope areas by dramatically increasing surface runoff, potentially damaging the economy by flooding settlements and businesses.

The paper (published 2016) found that in a valley in the Swiss Alps, ROS events increased surface runoff by 3 times (compared to a rain-only event) due to the amount of snow the rain melted when it fell, and had the potential to increase the levels of the river surveyed by 130mm. 

These ROS events are caused by rising temperatures. Usually, any precipitation at a certain high altitude would fall as snow, resulting in permanent snow cover throughout the winter. However, as temperatures increase, so does the height at which rain falls instead of snow. This can result in large scale floods, as well as reducing the thickness of the snow coverage. 

Although warmer temperatures do mean that the permanent snow line is rising higher in altitude (which would, in theory, decrease the levels of runoff coming from ROS events), the warmer temperatures also increase precipitation at higher altitudes, which results in an overall increase of runoff. So, unfortunately, the threat of ROS events grows as the climate warms.

Next Time

My personal experience of skiing is largely different to what this blog has so far described. In my next post I’ll be discussing the 2017/18 season and the stark contrast it’s been to the previous 20 or so years.