Written by Meteorologist Ross Marsden
Lately, during this spell of fine sunny weather, there have been a lot of comments about the air temperatures we are showing on our web site, where they are measured and opinions about them. In this Blog post I will explain the reasons for the location of our weather stations. I will also offer some help about how you can get some use from the information they provide.
The longest established weather stations are located in municipal gardens, and key places around the coast. In municipal gardens because in the late 1800s and early 1900s, meteorology was a new science and organisations like the Royal Society wanted to establish temperature and rainfall records in places that would be safe from needing to close or move. Around the coast because the weather-sensitive activity of high value at the time was coastal shipping. Lighthouse keepers and harbour authorities reported the weather conditions on a routine basis.
The rise of aviation
During WW2, aviation became important. We needed to accommodate a rapidly growing Air Force and train the air crews. Meteorological staff or Control Tower staff made hourly weather reports, or more often as required. After WW2 there was a steep rise in the popularity of air travel and most of the airfields that were air bases used for pilot training were re-purposed to become airports and new ones were built; for example Rongotai and Mangere.
Napier Airport, NZNR, view to the east
Aerodromes often (but not always) provide ideal weather observing sites with wide open spaces that satisfy World Meteorological Organisation exposure criteria very well. The trouble though, is that while they're perfect for meteorology, temperatures measured there can be very different from what people 'feel' and increasingly measure with low-cost weather stations around their towns and homes. More about this later.
During the 1980s the Ministry of Transport decided to de-man the lighthouses, but weather reports were still required, so that began the development of automated weather observations. There have been several generations of AWS (Automatic Weather Station) over the years, and that continued as the airport stations were de-manned of weather observers. Airport AWS are now quite sophisticated in that, as well as the usual meteorological elements, they can detect up to 3 cloud layers (up to the cirrus level), measure horizontal visibility and even detect what kind of precipitation is falling – hail, snow, rain, showers, and the presence of mist or fog. Increasingly they are including webcams.
Locations for stations
Land transport and its safety concerns have led to the location of a class of weather stations for that purpose – Desert Road Summit, Arthurs Pass, Rimutaka Hill Summit are examples. The observing network continues to grow by trying to fill in the gaps between the coastal stations and the airfields. Some stations are located in strategic places to solve a particular weather monitoring or forecasting problem – Golden Valley, Galatea, Flat Hills, Culverden, Fairlie, Roxburgh, Birchwood, Slipper Island and Le Bons Bay are all examples of this.
Whangaparaoa, looking south
Good AWS observing sites are fairly hard to find. They need to be secure from vandalism. They need to have good unobstructed exposure for wind, rain and temperature. Access to power and communications needs to be reliable. Most urban and suburban areas are not suitable because of lack of security, or because of obstructions or poor and unrepresentative exposure for wind, rain and temperature. Large parks and sports fields are the best prospects. Ideally the station should be located in a flat grassed area with nearest obstruction 10 times its height away from the enclosure. Schools are probably worth considering. Lumsden AWS is located in the local high school grounds.
Lumsden, looking south
Aside from exposure considerations of the site, the instruments – particularly the thermometers – have to be housed in a screen. The old style screen (see the Whangaparaoa photo) is a wooden cabinet, painted flat white with louvred sides, mounted 1.25 metres above the ground. This shades the thermometers from the direct sun, and provides ventilation. Modern modular automatic weather stations have a smaller plastic “screen” to shade and ventilate the instruments. The combination of the site environment and the screen makes meteorological observations comparable (as far as possible) across the network, as well as being representative of the wider vicinity. In some cities, the Council has provided a weather station (sometimes only a thermometer) in the central city area, and we welcome this to provide temperature information where people are going about their day to day activities. But it is essential that the thermometer gives an accurate and representative reading of the temperature of the air. Dunedin City (located at the Octagon) is a good example of this. In other places there is a choice of weather stations. In Wellington there is an AWS at the Airport, one at Kelburn, in Lower Hutt, Wainuiomata, and Mana Island. The temperature at the Airport site is influenced by the proximity to the sea, particularly when there is a sea breeze. But it has wind exposure quite representative of many parts of the region. Kelburn is located 127 metres above sea level and because of that will, on average, be about 0.8 degrees Celsius cooler than the temperature at sea level. And there will be other influences at other places involving the local environment and characteristics of the larger scale air ventilation.
Adapting nearby data to your location
On Monday 28 January 2013, there was a light southerly wind flow over the region. The Airport was reporting temperatures around 18 to 20 degrees in the afternoon while Lower Hutt was reporting 23 to 24 degrees, and Kelburn had 24 to 25 degrees. Mana Island was reporting 17 to 18 degrees, and Wainuiomata was baking in 26 to 28 degrees. All of these are representative of their vicinity and nearby places that gave similar geography and relationship with the sea. We show temperatures from these stations more or less in relation to their immediate suburban areas, and most of the time they will be satisfactory. Other times, when conditions are a bit unusual (height of summer, little wind not of the prevailing direction, no cloud), there are going to be significant variations around the region. We want people to make allowance for this when they are consulting our website (or mobile apps) for temperature information in relation to what they may be experiencing in their particular spot. For Christchurch there is a choice of weather stations. Sometimes Christchurch Airport is more representative of the Eastern suburbs than New Brighton Pier. We expect people to interpret the data on our website when checking out the observations in their location.
More recently, there have been some exciting developments with weather observations. Really keen weather watchers will already be aware of web sites such as wunderground.com and the Met Office UK's Weather Observations Website that collect privateer weather station data and display it in a common format. In December 2014 MetService teamed up with Met Office UK to launch Your Weather on metservice.com - so now New Zealanders have a truly real-time view of weather conditions across the country, combining the latest observations from MetService automatic weather stations, weather cameras and traffic cameras along with observations from weather stations owned by members of the public and other groups.