Interpreting Paekakariki weather station readings
Colin Iles                                                                                                                                                                          Index


Its Saturday morning and for once it looks like it is going to be one of those very rare flyable days in the Wellington region.  So how do you decide whether to fly or not and where to go?  Here are my personal observations, but by no means the last word, and just to confuse there are no doubt many experienced views to the contrary

If you were thinking of flying today you probably checked the weather forecast the night before: looking at the movement of highs, lows and fronts, and particularly the closeness of isobars on the map good indicators of wind strength.  This morning, apart from looking at the sky and assessing wind speed and direction you probably also checked again the weather forecast from the radio, TV teletext; ringing the Control Tower at Wellington or Paraparaumu  airports or obtained a fax.  Near Wellington City you might even observe the Brooklyn wind turbine.  You may do any or all of these but it is highly unlikely you'll make a decision without ringing the Paekakariki weather station unless you intend to fly in the Wairarapa..

Anyway, the first thing to know about the Paekakariki weather station is that it does not read true wind direction.  Don't ask me why, but if it tells you West the true wind direction is northwest.  There is some slight logic in this.  West means straight on to the hill, with both North and South being parallel to the hill.  But if you want to fly anywhere else on the West Coast nearby you'll need to add approximately 45 degrees.  Okay, so it says: West Northwest.  Take this to mean slightly off to the right, and North Northwest to be well off to the right and probably unflyable. Similarly, SW should be flyable.

The first information the weather station gives you is temperature.  Is this relevant?  Surprisingly the answer is yes.  This is because temperature affects the density of the air, the lower the temperature the denser it is and vice versa.  So the lower the temperature the more lift for a given wind speed. Seven or eight kph will likely be sufficient to maintain with the wind straight on and a temperature under 10 degrees Celsius.

Next you are given a series of readings: direction and speed at that moment.  Note any variations.  Major fluctuations in direction are likely to indicate a wind more off to one side, subject to turbulence from shoulders of the hill.  Fluctuations in wind strength could be anything, including thermal activity, but remember that this is only a spot check.  It often pays to make a series of calls to the station to assess more accurately what is happening and what changes are occurring over time.

Press * before the last reading and you get the statistics.  People vary in the value put on these.  I like at least to have the highest reading for the last hour.  This will invariably be a good few km above the spot readings.  This can have several interpretations:  if the wind is angled for instance, this is to be expected due to turbulence, but beware a maximum well above your glider's ability to penetrate (paragliders  not too much above trim speed); it can also be that the maximum was nearly an hour ago and has dropped since.  Again, use all the information to hand to interpret.  Other readings are unreliable and the maximum over the last day and max ever are meaningless as the computer is not set to assess these, even though it gives figures.

Unless you live very close to Paekakariki you'll need to take into consideration the time it takes to get there, and the fact that conditions will almost certainly be different by the time you get to TO.  This is where the weather forecast can be so valuable.  Knowing what is happening in the atmosphere gives you the opportunity to guess ahead.  However, as we know from experience, due to the proximity of mountains and the venturi effect of Cook Straight, forecasts can be notoriously unreliable.  A good example of this occurred in October this year during a forecasted dying southerly.  Pilots found the Wairarapa to be blown out from the south (though some hangies did manage to fly), whilst Paekakariki had a light westerly changing to a moderate northerly (4-10 kph SW changing to 25 - 30 kph NW on the weather station), flyable all afternoon.

The Paekakariki weather station can be very useful in assessing the wind conditions at Pukerua Bay just to the south of Paekakariki.  When the weather station records between NW and NNE, it is good for Pukerua. Remember however that because the wind is blowing along Paekakariki it tends to get compressed and the wind speed at Pukerua will likely be a little stronger.  Marginal at Paekakariki usually means too strong at Pukerua Bay.  If a southerly change is forecast keep a careful watch behind when flying here.  Incidentally, in a southerly, the wind often swings around the headland to flow from the north, up the face and even for a distance over the back.  Don't get caught out  you'll get well and truly trashed. I know!

Using Paekakariki to determine the weather situation at other sites around the rest of the region can be very tricky, but sometimes useful.  Too strong at Paekakariki suggests also too strong at Climie. However, when in the westerly or northerly quarters the wind in the Hutt Valley tends to be lighter than at Paekakariki.  Also Paekakariki can be an indicator of wind that is yet to arrive at Climie.  Bear in mind that Climie is an inland site and in good weather is much more thermic than Paekakariki.  However it also stands higher and can also be in a stronger airflow!  In good thermic conditions Climie can come on with warm air rising up the face even when the prevailing wind is lightly over the back.

Southerlies at Paekakariki are usually an indicator of southerlies over the whole region including the Wairarapa, but more often the weather over there is totally different.  For example Climie can be blown out when there is not a breath of wind just over the hill. Beware however the sudden arrival of a Norwester in the Wairarapa when the wind is strong at Paekakariki

Because winds can vary so much around the region, convergences can be quite common.  Always use all your senses to assess the conditions, particularly observation of the clouds.  In good summer conditions sudden changes can occur in the Wairarapa with the arrival of the sea breeze, usually dampening down thermic activity.  Sea breezes also affect the Hutt Valley to a lesser degree, but have very little affect at Paekakariki.

After all this, there remains the golden rule: just because you think it is not flyable at this moment does not mean that it will remain unflyable by the time you get there or a little later.  Use your judgment, experience, local knowledge and official sources.  If you want to fly, take a punt and be there if/when it comes on.  There may only be a brief window, but at least you will have a flight  and learn something too no doubt!  Living in a mountainous region such as ours, weather forecasts can be a long way off!  In the final analysis it is the conditions at the site when you are there that determine whether or not you will fly.

I have left till last a web site I find extremely useful both for current and forecasted weather:
http://www.metvuw.com/?current/wafs/wafs.html

Colin


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