Frost and warm days

  • Image, Frost and warm days.

    The number of frost and warm days changes from year to year in response to climate variation, such as the warming pattern induced by El Niño. Climate models project we may experience fewer cold and more warm extremes in the future. Changes in the number of frost and warm days can affect agriculture, recreation, and our behaviour, for example, what we do to keep safe on icy roads or whether to use air conditioning to keep cool.

    We classified Frost and warm days as a national indicator.

    Key findings

    Image, Increasing trend.   Increasing trend for warm days.

    Image, Indeterminate trend.   Indeterminate trend for frost days.

    Between 1972 and 2016, frost days decreased and warm days increased at some sites across New Zealand.

    At the 95 percent confidence level:

    • Frost days (minimum recorded temperature below 0 degrees Celsius) at 30 sites across New Zealand between 1972 and 2016:
      • decreased at 10 sites
      • increased at 1 site
      • had indeterminate trends at 19 sites
      • showed no trend for the whole country, or for either of the North or South islands when all the sites were combined.

    • Warm days (maximum recorded temperature above 25 degrees Celsius) at 30 sites New Zealand for growing season years between 1972/3 and 2015/6:
      • increased at 8 sites
      • decreased at 1 site
      • had indeterminate trends at 21 sites
      • increased across New Zealand and the North Island when all sites were combined; however, no trend could be determined for the South Island.

    Figure 1

    Trend for frost days 1972–2016 and warm days 1972–2015 for 30 stations – interactive map 

    Map, Trends for frost days, 1972–2016 and warm days, 1972–2015, for 30 stations.

                                                                                                                                                                                                    Source: NIWA
    Note: The data record is shorter for four sites (start dates in brackets): Kerikeri (1982), Masterton (1993), Taupo (1976), and Whangaparaoa (2000).
    For frost days we used calendar years. For warm days we used growing seasons (1 July–30 June of the following year), defined in the map by the start year. For example, the year 1972 in the map represents the growing season 1 July 1972–30 June 1973.
    Figure 2
    Note: The number of frost days are averaged across all sites.
    For frost days we used calendar years.
    Data are from 26 sites that had data from January 1972 to December 2016: Auckland, Blenheim, Christchurch, Dannevirke, Dunedin, Gisborne, Gore, Hamilton, Hokitika, Invercargill, Lake Tekapo, Milford Sound, Napier, Nelson, New Plymouth, Queenstown, Reefton, Rotorua, Tara Hills, Taumarunui, Tauranga, Timaru, Waiouru, Wellington, Whanganui, and Whangarei.
    Figure 3
    Note: The number of warm days are averaged across all sites.
    For warm days we used growing seasons (1 July–30 June of the following year). For example, the period 1972/73 in the graph represents the growing season 1 July 1972–30 June 1973.
    Data are from 26 sites that had data from January 1972 to December 2016: Auckland, Blenheim, Christchurch, Dannevirke, Dunedin, Gisborne, Gore, Hamilton, Hokitika, Invercargill, Lake Tekapo, Milford Sound, Napier, Nelson, New Plymouth, Queenstown, Reefton, Rotorua, Tara Hills, Taumarunui, Tauranga, Timaru, Waiouru, Wellington, Whanganui, and Whangarei.

    Definition and methodology

    A frost day is when the minimum temperature recorded is below 0 degrees Celsius. It refers to a temperature measured in an instrument screen 1.2m above the ground rather than a ‘ground frost’. We define a warm day as having a maximum recorded temperature above 25 degrees Celsius. The threshold of 25 degrees Celsius is chosen to represent days where action might be taken to keep cool (eg turn air conditioning on).

    For frost days we used calendar years. For warm days we used growing seasons (1 July 1–30 June of the following year).

    NIWA collected the data, using World Meteorological Organization standards, at its climate stations around New Zealand. The data are archived in the National Climate Database, which is ISO 9001:2008 accredited.

    Of the 30 sites, annual data are available for 26 sites from January 1972 to December 2016, while Kerikeri has data from 1982, Masterton from 1993, Taupo from 1976, and Whangaparaoa from 2000. Missing data were infilled where records were not available for all days (Macara & Tait, 2015).

    Criteria used to select the 30 regionally representative climate stations are that the station must currently be open and be likely to remain open for the foreseeable future; have a long record of reliable, good-quality data; be located near a large city (eg at an airport site) so is representative of the climate where many people in the region live. One station per region is to be selected, but if deemed necessary, two or three stations may be selected to represent a large region.

    The Intergovernmental Panel on Climate Change noted (for Australia and New Zealand combined) a “significant trend since 1950: cool extremes have become rarer and warm extremes more frequent and intense” (Reisinger et al, 2014).

    Data quality

    We classified Frost and warm days as a national indicator.

    Relevance

      This national indicator is a direct measure of the ‘Climate’ topic.

    Accuracy

     Image, High accuracy. The accuracy of the data source is of high quality.

    See Data quality information for more detail.

    References

    Macara, G & Tait, A (2015). Infilling of missing climate data for the 2015 Environmental Synthesis Report: Temperature, rainfall and wind. Client report no WLG2015-33. Prepared for the Ministry for the Environment, 37p. Wellington: NIWA.

    Reisinger, A, Kitching, RL, Chiew, F, Hughes, L, Newton, PCD, Schuster, SS, …Whetton, P (2014).  Australasia (PDF, 5MB). In Field, CB, Barros, VR, Dokken, DJ, Mach, KJ, Mastrandrea, MD, Bilir, TE, … White, LL (Eds), Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part B: Regional Aspects. Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (pp 1371–1438). Retrieved from http://www.ipcc.ch.

    Archived pages

    See Frost and warm days (archived October 2017).

    Updated 30 January 2018

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