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Northern Ireland weather

THURSDAY APRIL 30, 2015

Low pressure systems in the northeast have been affecting countries to the northeast of Ireland, bringing cooler temperatures and unusually wintry conditions. From the point of view of the lunar forecasting method, the moon has been averagely closer to earth (perigee) since 19 February, the day the moon, in combination with new moon phase was the second closest to earth for the whole year. On 20 March perigee was again in new moon phase and was now the third closest for the year. Perigee always exaggerates the effect of phase and the extent of the airtide being either ‘in’ or ‘out’, and a closer perigee brings more influence. This year the more powerful perigees have been in the late winter.

New moon alone is typically a cold breeder because the air tide is diminished when the new moon, being a day moon is absent from the sky over night. This reduction in air density lessens the insulation of the lower portion of the atmosphere and in winter the cooler and heavier overnight air is enabled to come closer to the ground, causing ground frosts, fog and overnight snows, with less thickness of the air preventing this process.

The closer perigees this winter combining with new moons have been the cause of the persisting winter weather in northern counties, because polar air masses have been dragged from the north a week after each perigeal new moon. The moons’ northernmost points (called declinations) have been on 27 February and 27 March, each one a week after perigee. Perigee brings winds and the northern declination brings a lowering of temperatures. (In contrast, when the moon is at southern declination temperatures tend to rise).

This year the moon increases its distance from earth in months after February. The perigee of 17 April was the seventh closest to earth and can be considered of relatively diminished power. On 15 May the moon is tenth closest for the year, and although moving further from earth, the new moon of 18 May is near the northernmost moon day of 21 May. It means the potential for more cold temperatures, but with lessened potential for strong gusts, because as the moon moves averagely away the winds averagely lessen. It is why February to mid March may have been the windiest part of the year. During June and July the moon occupies a more southern position alongside perigees, and combines with full moon phase. This full moon/perigee combination is an engine for warmer summer conditions.

The moon has an 18.6 year max-min declination range, and 2015 is the year of the next minimum declination, with the last in 1997. Minimum declination has the effect of slowing ocean currents, with the result that the Gulf Stream has been averagely slower this winter. This tends to prevent milder water from the south from reaching the northern part of Ireland. Quieter currents in slower Gulf Stream years bring less tidal action and prevents the normal mixing of deeper cooler currents with surface water warmed by the sun. Lesser currents produce weaker air flows, which in turn diminish the distribution of cold from the poles and warmth from the equator, with the result that the polar regions can grow even colder. This may explain the recent increases in ice thickness in both poles. Also, in minimum declination years, winter systems and summery spells are slower to both develop and leave.


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