A Masterton father and daughter are looking skyward after a MetService weather balloon plummeted more than 69,000ft (21km) from the stratosphere to land on their roof in an "extremely rare" fall to earth.

Roy Merriman and daughter Joanne Bilton were clearing away lunchtime dishes on Wednesday when they heard a thump from the southwest corner of the roof at their Norris Way home, Masterton.

"We thought it was a duck landing on the roof. But what we found was a burst balloon, a transmitter, and this long string that went right over our roof and into our backyard," Mr Merriman said.

On the small rectangular transmitter, known as a radiosonde, was an explanation regarding the battery-powered scientific instrument "carried aloft by balloon" to transmit pressure, temperature and humidity data by radio signal to the Meteorological Service.


The pair called the MetService phone number that came with the device. They received more detailed information about the apparatus, which they were told to pass on to a school for use as a science lesson.

Mr Merriman said the apparatus would probably be offered to nearby Hadlow School, with any information the MetService sent about it and the weather-balloon system.

MetService spokesman Steve Knowles said seven balloons were released each day from four sites in a network running in a north to south alignment at Raoul Island in the Kermedecs, Whenuapai in Auckland, Paraparaumu, and Invercargill.

The balloon which landed on the Masterton home had ascended to well within the stratosphere on Wednesday and risen to a height of 69,000ft before bursting, he said: "That's twice the altitude at which your average international jet aircraft flies."

The hydrogen-filled balloon was released at Paraparaumu on Wednesday morning, he said, from where two balloons were released each day, morning and night.

He said each launch cost about $250 and the remains of the devices rarely came down in residential areas.

"It's not very often at all that one lands on someone's roof, in fact that's extremely rare and the first case I've ever heard of.

"They do sometimes turn up in a paddock and farmers find them when they're feeding out. But in a residential area -- that's very, very rare."

The balloons, about 1m in diameter when fully inflated, could rise to about 100,000ft and expanded to about 5m in diameter before bursting as the surrounding air pressure increased during the ascent. Most fell into the sea and weren't recovered.

The balloon's biodegradable radar target measured about 40cm by 40cm and was made from cardboard and aluminium foil. It enabled radar tracking and slowed the descent to prevent property damage or injury "in the even more rare possibility that it landed on somebody".

The data was used for New Zealand weather prediction and also was sent to meteorological services around the world and supercomputers tracking global weather patterns.

"From that data, weather can be predicted out to about 10 days," Mr Knowles said.

Data recorded from the balloon recovered in Masterton included the maximum height reached of 69,000ft, coldest temperature of -62.9 C at 39,370ft, and strongest wind of 70km/h from the west at 26,000ft.