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University of Dundee

Why did it go from summer to winter in just a few days?

Meteorologist Karl Mehlhorn of the Irish Weather Network has the scientific explanation for why we went from t-shirts to thermals in the past week.

THE LAST WEEK in March and now the first week of April, we have witnessed some extraordinary weather across Ireland in the space of just a few days.

The warm and fine spell of weather was noteworthy due to the number of records broken at several long-term climatological stations for high temperatures around the country. The highest temperature of the warm and sunny spell was 22.2C in Belmullet, Co Mayo. Considering that the highest temperature recorded for 2011 was 26.0C in Athy, Co Kildare and in Drumconnick, Cavan on 3 June, these high temperatures – which ranged widely across parts of the south, the west and north – were very impressive for the month of March.

The table below shows the records broken at the climate sites and by how much they were exceeded:

(Table compiled by Niall from IWN. More info here).

The change

The gradual change occurred as the high pressure which brought us the fine settled weather drawing up the warm continental winds had now drifted across the country and out into the Atlantic Ocean. A cool airflow was now coming in from the north west with cloud and patchy rain moving down over the country last weekend.

Instead of the flood gates opening to the Atlantic weather fronts – which would bring mild and wet weather – another high pressure cell built to the north of Ireland. This, in turn, blocked the Atlantic weather systems from crossing the country. What tends to happen when there is a high pressure system is that all the low pressure systems which bring unsettled weather are deflected around the periphery or the outside of these high pressure cells where the winds move in a clockwise direction.

The changeover to the very cold and wintry weather was due to the high pressure to the north of Ireland now slowly being nudged more to the west and allowing low pressure to move south in the northerly airstream around the outside of the high. With the weather fronts moving ahead of the centre of the low pressure, a spell of wet weather was experienced across the country.

The snow

Then, as the cold front moved south, a polar airmass was introduced – the cold air quickly dug in behind the weather front leading to a significant drop in temperature. The winds blew in from a northeasterly direction – they got up to gale force across Leinster and a significant windchill was felt everywhere. Showers merged in the Irish Sea and fed in along eastern coastal counties – turning to snow over higher ground where snow started to settle.

Wintry showers also moved into Northern Ireland and across parts of Co Mayo with a dusting over higher ground here. With the low pressure moving slowly down the spine of the UK, the showers continued to feed into the east and particularly the south east corner of the country right through the night. Mostly of rain or sleet by the immediate coast, these where falling more wintry inland where a dusting of snow occurred at lower levels with totals of 5 to 10cm over higher ground in Wicklow.

Some of the Irish Weather Network member stations in Co Kildare, which had reported temperatures of 20C some six days previous, woke yesterday to 1 to 2cm of snow this morning from showers that moved in from the Irish Sea overnight on Tuesday.

This kind of weather can be typical of April and it is not uncommon to see snowfall at this time of year. What makes this event stand out, is how quickly it followed the summer-like conditions of a few days previous.

Look at this side-by-side satellite image of Ireland and the UK of the recent fine weather and cold polar conditions. In the picture on the right you can see the veil of cloud over our south coast – that is the cold front which moved south with the showery speckled cloud in over the rest of the country.

(Image courtesy of University of Dundee)

Karl Mehlhorn is a meteorologist who runs the Irish Weather Network. You can check them out on Facebook, follow them on Twitter and sign up for regular forecasts. The site has a live weather map of Ireland which is available here.

If you’re interested in owning a weather station yourself, they’ve written this handy guide.

In pictures: Did somebody say ‘snow’?>

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