Monday, July 13, 2009

The weather with you

Raoul Island has long been an outpost of the Meteorological Service, faithfully gathering all sorts of data that will ultimately assist Karen to make her forecasts for you on the TV1 Weather at 6.56pm.

Many of the instruments in use here are fairly straightforward, so we can record the temperature (max, min and grass), we can note how much rain (and the barometer) has fallen and the nifty little outside prism focuses direct sunlight onto a piece of card so that there is a permanent burnt-on record of sunshine hours too. Most spectacular and trickiest of all them is the weather balloon.
Filling the balloon in the Bombshed
This is a hydrogen-filled latex balloon that carries with it to great altitudes a radiosonde. This is so that the radiosonde will transmit valuable upper air data before the whole thing expands, blows up and falls back into the ocean. The aim is to get the balloon to at least 30 hectopascals before it bursts. A hectopascal is just a different name for a millibar, which is a measure of air pressure. The higher you go, the less air pressure you will encounter. Naturally, there is a bit of competition between the three accredited operators here and each of them is usually pretty keen to get a gold standard release – one that gets to 15 hectopascals, or around 30,000 metres. That’s approximately four Mt Everests high!
Balloon ready for release

So how far does the balloon actually go? The release photo here is one of Chauncy’s efforts on a windy day last week and it disappeared from the computer screen some 300km away.

It’s a reasonably pricey operation. Each set of balloon, batteries and radiosonde transmitter costs around NZ$300 and it’s not unheard of to have to release three in a day before Karen's exacting standards are attained.

Hydrogen being the volatile and highly flammable substance it is, the filling and releasing process is potentially a hazardous operation. Static electricity is the enemy; to reduce the chances of an unwanted spark, the operator must wear a surgical gown with hood and gloves. He must attach an earthing strap to his wrist and his wristwatch must be left outside. While it is filling, the balloon has a fine mist of water sprayed onto it. All of this happens in a little building called the Bombshed, which is as far away from everything else as is practical. Just in case. Operators have a “met buddy” each day, someone who receives on the two-way radio a report that each step in the balloon release process has been completed safely.

Presumably if there is no radio call, the met buddy must go out to the Bombshed and sweep up the ashes. Thankfully, this has never happened here.

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