NZ Winter Notes 2012

By gravitational pull during the course of any day the moon changes the height of the air.  The ocean is connected to the air just above it, which means 75% of the surface of the planet is composed of an air/water interface. Just as a bulge of water follows the moon during the transit between moonrise and moonset, so does a bulge of air. But this daily fluctuating atmospheric tide is factored out by meteorological weather models that wish only to compare atmospheric pressures at sealevel. It is known though, that weather balloons float higher at new and full moon because of a king tide in the air exactly concurrent with the king tide in the ocean. The effect of the air-tide is to change, at different times of the day, the extent of insulation the earth gets from the daytime searing heat of the sun or the night-time freezing cold from space.

For instance over summer full moon through last quarter period the moon rises in the evening, causing the air height to expand. When earlier in the day the moon was absent from the sky the air height was lower, allowing more of the sun's heat to reach the ground without too much air thickness in the way. This means that the summer-full-moon's sun's heat can be more intense than at a time such as new moon when the daytime moon above the horizon holds beneath it a greater amount of air, thereby insulating the ground from the sun. It is why burn times in summer are nearly always during full Moon -last quarter phase. 

As for the sea, the air tide at any moment is also monitored by how far away in that moment the moon is from earth. Once every 27.3-days the moon comes closer to earth, called perigee by astronomers. 13.6 days later it is furthest away for the month, called apogee. When closer, the moon contributes 20% more to air height and adds to a decrease in air height 12 hrs later when the moon sets. Perigee exaggerates whatever is around, because its effect is to magnify the air tide. If it is summer the perigee will bring hotter conditions, but if winter then more frosts and lower temperature minimums.

In any year perigee associates with new moon for 5 months followed by a changeover month, then 5 months with full moon. Perigees cause kingtides, and so for half the year we get full moon kingtides and the other half new moon kingtides, repeating over the same months again every 4.4 years. For 2012, full moon perigee kingtides run between February-July, followed by the new moon perigee kingtides from September through next February. It means most of this year's winter is controlled by full moon perigees.

Whereas days are the 'heat engine' of summer, it is nights that engineer the cold of winter. It matters not which country you live, a hotter summer is always one with summer months combining perigee+full moon, and a colder winter is when the winter months combine perigee with new moon. The latter causes air height to be lower overnight, enabling the cold of space to come closer to ground. The heat from the winter sun during daylight isn't enough to warm everything up again and the next night brings another round of coolness. Older skiers look for perigee new moon winters because such seasons bring heavy night snowfalls.

Unfortunately for skiers in the southern hemisphere, May of this year through to August will see full moons accompany perigees. This will bring heat waves and droughts to some parts of the northern hemisphere because those are their requirements for a hotter summer. But for the southern half of the world it means that the cold of winter during full moon nights will not be as cold because the moon is in the high sky when the perigee is about, rather than out of the sky as it would be if it was new moon which would enable colder air from space to descend as snow. Therefore winter nights and temperature minimums should be milder this winter, at least until September when new moons begin to accompany perigees again.

By October and November new moon perigees again rule, bringing a cooler late spring. The last half of October should bring precipitation for the country, which means unwanted late snow for the south and the chances of floods in the north, because perigeal spring new moons also create large cool low-latitude anticyclones and their strong moist easterlies that sweep rain in large amounts over eastern coasts. This wintry weather will be too late for any NZ ski field to capitalise on.

Overall for winter the relatively warm winter nights will make for milder days. The really good long snow seasons will not return before 2016. This season should be similar to some of the early 2000 years, the early part of the 1990s and some of the later 1970s, which were years of similarity of moon declinations.

If you are a skier you may have to plan carefully. The snow will come, then rain should wash it away, only to be followed again by snow. The South Island may be more easily able to run snow machines for longer periods than the North Island. Some skifields may be lucky to have 10 days of southerlies all winter, whereas the more southerly fields can expect between 20-30. Nevertheless the coldest may be at the Chateau this year, where around -9C may be reached in the second week of July.

Winds are important when working out which snowfields will get the best snow.  Whakapapa can get good snow from just about all winds, including northerly, while Turoa gets best snow from wind coming from the south.  Treble Cone needs westerly or north-westerly winds and gets virtually nothing from southerlies. Mt Hutt needs southerlies or south-easterlies, also easterlies if they are cold enough.  Coronet Peak needs southerlies or south-westerlies, although not many southerlies carry precipitation. If Treble Cone is getting snow, Coronet probably isn't.  Ohau needs southerlies and westerlies. From south-easterlies the Southern lakes just seem to get cloud.  Cadrona gets snow from south-westerlies and southerlies, but little or none from the west or north-west. This season should see southeasterlies in good measure, which, being subtropical, can carry warmth. In turn this creates precipitation, rain in the north and snow in the south.

This winter, for the Chateau, expect more westerly systems than easterly systems in May, June and July; and more winds from the east during August. For Ohakune, there should be more westerly systems than easterly systems in May and July, but with rather calm conditions in June and August. For Mt Hutt, there may be westerly systems as well as easterly systems in May, then systems more westerly in June and July, followed by many easterlies in August. For Queenstown, expect cold southerlies and westerlies in May, easterlies in June, and southerlies and southwesterlies in July. August in Queenstown brings easterlies in the first half switching to southwesterlies in the second half. Finally for Wanaka, expect westerlies and northwesterlies in May, westerlies in June and July, and westerlies and northwesterlies through August.

Reports for the whole season, covering each day and enabling better trip planning, are now available online for Chateau, Ohakune, Craigieburn, Mt Hutt, Wanaka and Queenstown
http://www.predictweather.co.nz/ArticleShow.aspx?ID=388&type=home

For further enquiries email ken@predictweather.com