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Perigee 15 May

THURSDAY MAY 14, 2015

It is a perigee tomorrow. There was heavy rain and flooding in Wellington today. What is a perigee? When any two bodies in space orbit each other they are never equidistant. There is a closer orbit point (perigee) and a furthest away position (apogee). When the moon comes closer, on its 27-day perigee beat, the increased lunar gravitational effect on the sea causes stronger currents, gale force winds above, greater swells, higher tides, flooding if there is heavy rain and rougher sailing conditions.

I first became aware of perigees when in the 1970s I noticed that there was usually a storm around whenever there was a kingtide. The moon’s closer proximity exaggerates the height of the daily air tide as well. Cyclones are more numerous and of greater magnitude when perigee sits astride the equatorial latitude, as during the Katrina/Monica years of 2004-2006. This is because the curvature of the earth is greatest at the equator and when perigee is equatorial earth-moon distances are less. And today (14 May 2015) the moon rose over the equator.

The Wahine disaster in April 1968 occurred within four days of a full moon in perigee. The tide of the air, which manifests in a daily change in air-height, also increases at perigee time. Weather conditions are exaggerated as a result. Around perigee a passing shower may build to a storm event, or a dry patch can produce hotter than usual temperatures. Old mariners' manuals always warned about perigees, and the annual Nautical Almanac produced by the NZ coastguard listed all perigees and apogees for the year because storms and waterspouts were well known to accompany higher and rougher tides, hazardous to sailors entering channels.

Perigees cause currents to strengthen, and this causes above-surface winds to increase. These exacerbated windy conditions create lower pressures which bring cloud, and beneath the cloud warmth which heats the air which then induces evaporation, causing rain. The wind then blows rain across land. This can happen suddenly, and even the best meteorologists do not see it coming. On Wednesday (Severe Weather Warning.; Ref: OMS13-25959) the NZ Metservice forecast heavy rain for Taranaki, with no mention of rain for Wellington. However, by this morning, the actual day of the rain, NZ Metservice realised that heavy rain was hitting Wellington and put out a warning (Severe Weather Warning; Ref: OMS13-55370).

Leading up to today's perigee, many earthquakes have struck NZ, including an above-4mag near Christchurch on Tuesday. Yesterday, the day before perigee, another 7.4mag hit Nepal. As well, a 6.8mag shook Japan. Tornadoes have swept a destructive path across several US states including Texas. One hit Mt Maunganui this evening. We might recall that in Christchurch during 2010-11 each largest earthquake was within a few days of perigee if not right on the day. These were on 4 September 2010, and in 2011; 22 February, 20 March and 13 June. The 1931 Napier earthquake also occurred on perigee day.

There are many examples of terrible perigee events in NZ. On 3 June 1886, two days before perigee, Lake Tarwera erupted, inducing the destruction of the famous Pink and White terraces, regarded as a wonder of the world. Within a week they were completely destroyed. A close perigee occurred on 28 February, 2006 and within two days widespread strong to gale southerly winds arrived in Canterbury. 82km/h was the highest wind gust ever recorded at some stations, easing by the 4th. On 10 April, 1968, 4 days before perigee, Cyclone Giselle brought flooding and destructive winds to many parts of New Zealand and the inter-island ferry Wahine sank in Wellington Harbour, with the loss of 51 lives.  Cyclone Nagris struck Burma on the exact day of perigee of 6 May 2008.

Of course not all perigees bring extreme events, and not all extreme events occur on perigees. But perigees are to be watched, for they cause the kingtides, the cyclones and the earthquakes, due to higher tidal action in solid land, in the sea and in the air. When rivers are full the rain cannot get away, and so banks up, causing flooding if there is rain about. We are only a few days from the next kingtide - close enough for tides to be already higher than normal. And as a result there is still earthquake risk for NZ over the next few days

Update (24 hours after this article posted):

We can predict through astrology/astronomy where the moon will be in years to come, just as we do for the tides, so we can predict the timing of potential extreme weather. Perigees are always bad news, and severe weather can usually be guaranteed. But as the NZ Metservice has shown, despite expensive technology - including a $42 million computer forecasting system called Fitzroy - where the destruction strikes is seldom possible except on the day itself. However, most mariners and farmers know about perigees, and they and the people of Christchurch know that perigees are not to be trusted. The next perigee day is 10 June. 


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