The diverse reasons for a destructive deluge

A large mural dominates the foyer of the Dublin headquarters of the Met Eireann

A large mural dominates the foyer of the Dublin headquarters of the Met Eireann. Its inscription reads: "Is mairg a baitear in am an anfa, mar tagann an ghrian i ndiaidh na fearthainne" - "Pity him who perishes at the time of the flood, because after the rain-storm comes the sun."

At present, to judge by news reports, it seems as if it could apply throughout the entire northern hemisphere.

Our flooding in Ireland is relatively easy to explain. In recent days, the anti-cyclone which gave such pleasant conditions while it lasted has been replaced by a large slow-moving area of low pressure, an ideal breeding ground for the large cumulonimbus clouds that give heavy showers or thunderstorms.

The high temperatures at this time of year mean that the atmosphere sucks up vast quantities of moisture by evaporation from the ocean or other areas of open water, so that a cumulonimbus, sufficiently well developed to provide a shower, has the potential to produce enormous quantities of rain.

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The fact that there is little wind, either at ground level or in the upper atmosphere, means these showers, rather than being blown rapidly along, remain for long periods discharging their relentless deluge over the one unfortunate spot.

The floods throughout most of Asia, on the other hand, stem from the perennial seasonal monsoon, albeit in more active mode than usual.

During the winter, a large anticyclone over Siberia provides most of the continent with dry north-easterly winds, but in spring the sun moves northward and shines straight down on southern Asia.

As the air over the continent responds to the sun's heat, it expands and becomes lighter and a low pressure area develops over the sub-continent.

With the development of this low pressure area in late May or early June, warm moist air from the Indian ocean, moving anticlockwise around the low, streams in over India from the south-west; the warm humid air is forced to rise as it crosses the gently sloping terrain towards the Himalayan Mountains, and heavy showers, thunderstorms and torrential rain appear.

Sometimes, especially as at present when the monsoon rains are exceptionally heavy, the land, baked hard as concrete by the early summer sun, cannot absorb these rivers from the sky, and the super-abundant waters sweep away and drown hundreds of people in their muddy tides.

In Korea, the flooding has been exacerbated by an unwelcome visit from Typhoon Olga, exactly the same phenomenon as the familiar North Atlantic hurricane, usually associated with fierce destructive winds, but commonly bringing a deluge of torrential rains as well.