a Bad Moon Rising?

Perigee is the name astronomers give to the closest earth-moon distance for the month. On 6 May it is the May perigee. It will coincide with full moon.

Some astronomers have coined the phrase 'supermoon' for when full or new moon occurs with perigee.

This time the moon will swing in 356,953 kms from earth, and will look extra-big and extra-bright as it rises at sunset. On the day of any full moon the moon rises above the horizon at the exact second that the sun disappears beyond the horizon, which in Auckland on 6 May will be 5.24pm.

Not only does this month's perigee coincide with full moon but this perigee will be the nearest to earth of any this year, as the distance of the moon's close approach varies.

The last closest was 20 March 2011 and the next time it will be as close will be January 2014.

This month's full moon is due to be about 16 percent brighter than average.

In contrast, later this year on 29 Nov, the full moon will coincide with apogee, the moon's farthest approach, offering a particularly small and dim full moon.

There is always reason to be careful around all perigees, as they exaggerate whatever else is going on in weather.

Closest perigees are even more reason to be vigilant.

The closest perigee for last year in March brought the Japanese tsunami earthquake and a 7-intensity earthquake to Canterbury.

Tidal variation around the world is always particularly high and low when the moon is close because the moon exerts over 40% more tidal force than during the next apogee 13.6 days later.

Kingtides are primarily kingtides in the land, and large earthquakes occur with double the frequency in the week of any perigee.

Last year's perigees all brought the next largest earthquake to Christchurch.

Sea mammals have already started beaching in Peru which portends earthquake activity in that region.

This round of higher tides may bring risk of seismic activity to NZ.

The kingtides span 6th-9th and the concurrence of significant planetary alignments that include Sun Venus and Mercury should produce extra sunspot activity which disturbs earth's electromagnetic field enough to incur earthquake activity.

Watch for the barometer to sharply rise for that is an indicator of pressure release imminent in the ground.

With the North Island already starting to twitch, May could be an active month for shakes, with the need to be watchful around 5th, 11th, 15th-18th, 20th-21st, and 27th-30th.

Perigees bring high winds because the extra gravitational pull on the atmosphere created by the moon's proximity creates turbulence.

Around the world we may be reading of extreme weather events in the media. In Australia the only rain is expected along the North, Central and South Coast of Queensland and along the south coast of Western Australia, also the North Coast of Northern Territories, the extremities of the continent.

But some extreme heat is likely in South Australia and heavy frosts in the Southern Tablelands. Gales are expected in Tasmania and a cyclone is likely to form in Western Australia 's northwest.

A few days after the perigee, snow should arrive in Tasmania and Brisbane should get thunderstorms. Ireland is expected to be rain-free on the 6th-10th but wet 1st-5th.

The USA can expect rain and snow in the western States and rain with thunderstorms in the east, with winds generated by the perigee bringing severe thunderstorms capable of inflicting damage.

When full moon and perigee occur together on the same day, the few days on either side is usually a dry period and this should be the case for most of NZ.

In our hemisphere this time the 6 May Libra moon goes well below the horizon when it sets, which by virtue of the extra low air-tide will create much cooler overnight temperatures.

With the moon's southernmost position occurring two days after perigee the likelihood for NZ is of heavy frosts, particularly in the far south and in some places it may resemble snow.

However significant and widespread snow is not expected in NZ until the last week in May.

Of course none of these may happen, but the potential is there. Longrange predictions are nothing more than warnings due to potentials, and it is up to the reader to take note or ignore.

The reason we have metservices is for these warnings to be issued, based on patterns that have occurred before.

At the very least boats should be secured or brought to sheltered havens during a close perigee, because coastal gales and higher tides have a habit of breaking moorings.

As always the perigee full moon is good news for diver fishermen, because the brief interval of extra low tides provide more accessibility to shellfish.

Ken Ring is a longrange forecaster and author of the Predict Weather Almanacs for NZ (Random House publishers). For more information go to www.predictweather.com