Technical note: some points of detail on adjustments for Wellington
This page provides technical explanations for some aspects of the explanation of why we need to make adjustments when combining data from multiple sites in Wellington
Offset between Wellington Airport and Kelburn
The average offset of +0.79°C between Wellington Airport and Kelburn is calculated over the full period of overlap, 1962-2004, excluding years with missing data at the airport.
Offset between Thorndon and Kelburn
Daily weather observations were made at both Thorndon and Kelburn for just 29 days, covering 3–31 December 1927. These Kelburn data for the overlap month with Thorndon have now been added to NIWA’s Climate Database. Such a short period is not acceptable for determining the climatological differences between sites. However, we note that the average daily mean temperature for December 1927 shows 14.7ºC at Thorndon (31 days) and 13.7ºC at Kelburn (29 days only), so Kelburn was 1.0ºC colder than Thorndon that month. If the first two days of December are removed from the Thorndon average (so exactly the same 29-day period is compared), then the Thorndon average increases to 14.9ºC and the Kelburn-Thorndon site difference to 1.2ºC.
The offset of 0.8°C used in the webpage illustration agrees with what we would expect for the altitude difference between the sites, based on the free atmosphere lapse rate of 0.65°C per 100 metres.
In practice, the adjustment is calculated by comparing overlapping temperature records for other sites across the 1927/28 site change.
The altitude effect is an a priori reason for expecting Kelburn to be colder than Thorndon, but there is no straightforward theoretical way of calculating the actual difference along a sloping land surface in the same way as there is for the free atmosphere.
In fact, over a much broader spatial scale, the lapse rate along a land surface in New Zealand tends to average around 0.5°C/100m (Norton, 1985).
This would equate to Kelburn being colder than Thorndon by 0.6°C; the larger calculated difference of 0.8°C being used in the “seven-station” series therefore suggests other local influences such as exposure or aspect may also be affecting the sites in question.
Norton, D.A., 1985. A multivariate technique for estimating New Zealand temperature normals. Weather and Climate, 5: 64-74.