NIWA: Combining Multiple Sites' Temperature Data

Published: Fri 27 Nov 2009 05:16 PM
Combining Temperature Data from Multiple Sites in Wellington
In Wellington early temperature measurements were made at a site in Thorndon, but the site was relocated in 1928 to Kelburn. The Kelburn site is colder because it is about 120m higher than the Thorndon site. The process of combining data from various Wellington sites is illustrated below.

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Annual temperatures at three Wellington sites: Thorndon (3 metres above sea level), Kelburn (125 m asl), and Wellington Airport (4 m asl), as recorded in the NIWA Climate Database.
• Clearly, the temperature at Wellington depends on exactly where it is measured – Wellington Airport is consistently higher than Kelburn, but both sites fluctuate up and down together from year to year.
• Thus, if one measurement station is closed (or data missing for a period), it is acceptable to replace it with another nearby site provided an adjustment is made to the average temperature difference between the sites.
How do climatologists make such adjustments?

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Annual temperatures at the same three sites, but with Thorndon and Wellington Airport adjusted to match the Kelburn record.
• Where there is an overlap in time between two records (such as Wellington Airport and Kelburn), it is a simple matter to calculate the average offset and adjust one site relative to the other.
• Wellington Airport is +0.79°C warmer than Kelburn, which matches well with measurements in many parts of the world for how rapidly temperature decreases with altitude.
• Thorndon (closed 31 Dec 1927) has no overlap with Kelburn (opened 1 Jan 1928). For the purpose of illustration, we have applied the same offset to Thorndon as was calculated for the Airport.
• The final “adjusted” temperature curve is used to draw inferences about Wellington temperature change over the 20th century. The records must be adjusted for the change to a different Wellington location.

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