By Geoff Wadge
When we have a very large volcanic eruption (as for example Mt Pinatubo in 1991) it gets global news coverage, not least because of its potential to perturb the climate. Whether smaller eruptions get widely noticed usually depends on whether spectacular images are available or there is some human interest story involved (“My EasyJet flight’s been cancelled …”). Most eruptions (there are one or two new ones each week) draw little attention, except locally if the activity is hazardous.
But some new research suggests volcanoes of all types can have largely unrecognised influences on – how much it rains. The first comes from our old friend Pinatubo, and its neighbouring volcanoes in the Philippines. They lie upwind (to the west) of Manilla, about 80 km distance. During the south-west monsoon season (June to September) of 2012 and 2013 Manilla suffered paralysing flooding following extreme rainfall of about 300 mm/day. Observations and modelling by Lagmay and colleagues showed that these 1-2 km high conical volcanoes acted as intensifers, by up to one third, of the downwind rainfall over the city.
Figure 1. Volcanoes as rain magnets?
Whilst the classic conical shape turns out to be a good rainfall intensifier, what about that other obvious characteristic of volcanoes – heat? Large explosive eruptions are often accompanied by spectacular thunderstorms and heavy convective rainfall. But what about a much more modest, hot but not explosive surface on a volcano? The answer according to a study by Poulidis and colleagues is that it takes a temperature of about 20-40 °C above ambient over an area of about 1 km2 (for example as you might get with a lava flow) to trigger a vigorous convective plume and create a localised rainstorm with rainfall rates above 10 mm per hour. Low wind speeds and a weak inversion layer favour such storm development.
I have spent many a sleepless night next to an erupting volcano not being able to decide if I am listening to volcanic explosions or thunder. Now I know why.
Lagmay, A.M.F., Bagtasa, G., Crisologo, I.A., Racoma, B.A.B., David, C.P.C., 2015. Volcanoes magnify Metro Manila’s southwest monsoon rains and lethal floods. Frontiers in Earth Science, 2/36.
Poulidis, A.P. Renfrew, I.A., Matthews A.J., 2015. Thermally induced convective circulation and precipitation over an isolated volcano. Journal of Atmospheric Sciences. Doi.org/10.1175/JAS-D-14-0327.1.