6  Climate and Weather

Modified

December 6, 2024

6.1 Climate

Climate parameters affect soil development directly (e.g. freeze-thaw cycles) and indirectly (e.g. by their influence on vegetation community composition and structure).

6.1.1 Global

The Köppen-Geiger climate classification (Köppen 1936) is the most well-known and accepted global system, and has been periodically updated (e.g. (Kottek et al. 2006; Beck et al. 2018; Beck et al. 2023)). Current maps and projections under various climate change scenarios are available from https://www.gloh2o.org/koppen/.

In and around New Zealand, the Köppen regions that currently occur are listed in Table 6.1.

Table 6.1: Köppen climate regions of New Zealand and its neighbours
Code Name Locations
Af Tropical, rainforest Pacific Islands e.g. Tonga, Samoa
Am Tropical, monsoon Northwest coast of Fijian islands
Bsk Arid, steppe, cold Central Otago, Melbourne, Northwest Victoria
Cfa Temperate, no dry season, hot summer Coastal New South Wales and southern Queensland
Cfb Temperate, no dry season, warm summer Majority of New Zealand; Chatham Islands; coastal southeastern Australia; Tasmania
Cfc Temperate, no dry season, cold summer Small areas of North Island e.g. upper Ruahines; lower mountains in South Island e.g. Peel Range, Nelson
Dfb Cold, no dry season, warm summer Sheltered pockets in the Southern Alps
Dfc Cold, no dry season, cold summer Lower slopes of major volcanoes and the Southern Alps
ET Polar, tundra Upper slopes of major volcanoes and the Southern Alps
EF Polar, frost Antarctica

Since most work programs will take place within one zone, regional climate rarely needs to be noted in the field.

Where a study area appears to deviate from a regional map, the extent and nature of the deviation should be discussed in project reporting.

6.1.2 Regional

Various climate classifications specific to New Zealand have been produced (Robertson 1956; Garr and Fitzharris 1991; Sturman et al. 1999).

NIWA have more recently published a useful regional summary classification, although it is not accompanied by spatial data. The classification is briefly reproduced in Table 6.2 below. As with the global classifications, these don’t need to be recorded in the field.

More detailed discussions of each region’s climate are published in NIWA’s Regional Climatologies.

Table 6.2: NIWA climate regions of New Zealand
Code Name Locations
NiN Northern New Zealand Auckland, Tauranga, and regions north
NiC Central North Island Hamilton, Taupō, Rotorua
NiW South-West North Island New Plymouth, Wanganui, Palmerston North, Wellington
NiE Eastern North Island Gisborne, Napier, Masterton
SiN Northern South Island Nelson, Blenheim
SiW Western South Island Westport, Hokitika, Milford Sound
SiE Eastern South Island Kaikoura, Christchurch, Timaru
SiI Inland South Island Lake Tekapo, Queenstown, Alexandra, Manapouri
SiM Mountainous South Island Mount Cook and other alpine areas
SiS Southern New Zealand Dunedin, Invercargill
Chi Chatham Islands Chatham Islands
Ant Antarctica Scott Base

6.2 Specific measurements

Some climate parameters are more directly related to soils than others, mainly through their influence on vegetation community and evapotranspiration. Spatial estimates of these parameters have been published in the NZ Environmental Data Stack (McCarthy et al. 2021), available from the MWLR Datastore. Estimates can also be constructed from local records, where these are available.

6.2.1 Rainfall

Soil moisture availability strongly influences chemical weathering in soils (Dixon et al. 2016), and in many parts of the landscape the main source of soil moisture is rainfall.

  • Total annual rainfall: Some soils are unlikely to form outside a particular total annual rainfall range, e.g. Allophanic soils are uncommon where total annual rainfall is less than 1000 mm (Hewitt 2010).
  • Rainfall seasonality: Calculated as the mean of monthly rainfall totals, divided by their standard deviation. More pronounced seasonality implies stronger soil wetting and drying cycles, which in turn influences weathering processes.
  • An alternate measurement of seasonality involves calculating the ratio of rainfall in the warmest versus the coolest three months. In New Zealand this is usually (Dec + Jan + Feb)/(June + Jul + Aug).

6.2.2 Temperature

  • Mean annual temperature: The mean temperature recorded year-round.
  • Mean summer temperature: The mean temperature recorded over the warmest quarter of the year. In New Zealand, this is usually December-January-February. Mean summer temperature was a key parameter used by Singers and Rogers (2014) to classify NZ terrestrial ecosystems.
  • Growing Degree Days (GDD): GDD measures the total number of °C above a set minimum temperature (usually 5, 10, or 16°C). Average daily temperature (max - min/2) is used in the calculation, and where temperature is below the set minimum, 0 is recorded. Plant species have specific GDD requirements that help determine their distribution.
  • Frost Days (FD): Frost days are days where the temperature drops below 0 °C, measured 1.2 m above the land surface.

6.2.3 Insolation

The amount of solar energy received by a site, modified by terrain and cloud cover, can strongly affect plant growth and soil weathering rates.

6.2.4 Prevailing winds

The prevailing wind direction, its usual strength and seasonality all affect evapotranspiration rates and even the physical development of growing plants.

6.3 Weather

Contemporary weather conditions have not necessarily had a strong influence on soil formation, except perhaps in young and/or rapidly developing soils. These parameters are more relevant for agricultural suitability assessment and for interpreting soil profile descriptions.

6.3.1 Current weather

Record the weather at the time of observation using the codes in Table 6.3, plus temperature in degrees Celsius. These are not always essential, but can affect colour recording and some field tests.

e.g. SU 21 for a perfect day in the field.

Table 6.3: Current weather conditions, Schoeneberger et al. (2012), 1-1
Code Name
SU Sunny
PC Partly cloudy
OV Overcast
RA Rain
SL Sleet
SN Snow

6.3.2 Recent rainfall

Record recent rainfall using the codes in Table 6.4. This is optional, but can provide valuable context when recording surface cover (Section 11.3), the presence of surface water at a site (Section 11.4), detailed erosion/deposition descriptions (Section 11.7, Section 11.8) and soil moisture status (Section 12.6).

Table 6.4: Recent rainfall, adapted from FAO (2006), Table 2
Code Name
NM No rain in the last month
NW No rain in the last week
ND No rain in the last 24 hours
RD Rain but no heavy rain in the last 24 hours
RH Heavy rain for some days or excessive rain in the last 24 hours
RE Extremely rainy or snow melting
UK Unknown/uncertain