Doc #18 — NZ Climate Baseline Data

COMPUTED DATA: CLIMATE NORMALS AND NUCLEAR WINTER ADJUSTMENTS

View the Climate Data Tables → — Monthly normals for 14 NZ locations, nuclear winter temperature adjustments, growing season impacts, and wind data.

View the generation script → — Python source code and data sources (NIWA CliFlo).

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RECOMMENDED ACTIONS

Immediate (Days 1–7) — Phase 1

1. Download and archive complete NIWA CliFlo data for all principal climate stations. Store on at least three independent physical media. The CliFlo database contains continuous daily records for hundreds of stations; the summary normals printed here are derived from these records, but the raw data enables finer-grained analysis.²
2. Print this document in at least 50 copies for distribution to regional agricultural offices, civil defence centres, and the national coordination authority.
3. Print expanded regional cards (Section 7) for each region — single laminated A4 sheets suitable for field use.

Short-term (Days 7–30) — Phase 1

4. Establish a climate observation network using existing MetService and NIWA automatic weather stations (AWS). Where AWS units fail, revert to manual Stevenson screen readings at schools and community centres. Manual readings are less precise than AWS (typically twice-daily observations vs. continuous logging, no automated wind or humidity sensing, and observer error rates of 0.5–1.0°C), but they are adequate for detecting the multi-degree departures expected under nuclear winter. Standardise observation protocols to enable comparison against baseline data.
5. Begin monthly departure-from-normal reporting — compare observed conditions against the tables in this document and report to the national coordination authority.

Medium-term (Months 1–12) — Phase 1–2

6. Integrate climate departure data into agricultural planning. The departure tables (Section 6) provide the framework for estimating pasture growth reduction, crop viability windows, and frost risk under nuclear winter.

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1. DATA SOURCES AND LIMITATIONS

1.1 Source data

New Zealand’s climate record is maintained by the National Institute of Water and Atmospheric Research (NIWA) through the Climate Database (CliFlo), which holds data from approximately 6,500 climate stations, of which roughly 600 are currently active.³ The MetService operates complementary stations, primarily at airports and major population centres. The data in this document uses NIWA’s 1991–2020 normal period, the most recent standard 30-year averaging period as recommended by the World Meteorological Organisation (WMO).⁴

1.2 Station selection

For each region, a single representative station is selected — typically the principal town or city. This simplification is necessary for a printed document of manageable length. Users should recognise that climate varies significantly within each region: altitude, aspect, distance from the coast, and local topography produce large variations. Canterbury Plains stations, for instance, record significantly different conditions from Banks Peninsula or the foothills. The selected stations represent lowland conditions where most population and agriculture are concentrated.

1.3 Limitations

These are 30-year means. Actual year-to-year variability is significant — a “normal” January in Christchurch might range from 15°C to 20°C mean temperature across different years. The baseline gives the central tendency, not the envelope. For nuclear winter assessment, the key question is whether observed conditions fall outside the normal range of variability, not whether they match the mean exactly.

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2. REGIONAL CLIMATE SUMMARY

New Zealand spans latitudes 34°S (Northland) to 47°S (Southland), a 13-degree range comparable to the distance from the Mediterranean coast of France to Denmark in the Northern Hemisphere. Combined with the maritime climate, the Southern Alps rain shadow, and significant altitude variation, this produces a wide diversity of regional climates.⁵

The fourteen regions are grouped below by broad climate zone:

Northern (warm, humid): Northland, Auckland, Waikato, Bay of Plenty Eastern (warm-dry summers, moderate winters): Gisborne, Hawke’s Bay Central-western (maritime, windy): Taranaki, Manawatū-Whanganui, Wellington Upper South (sheltered, dry east): Nelson/Marlborough, Canterbury Western South (wet, mild): West Coast Southern (cool, variable): Otago, Southland

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3. SAMPLE DATA TABLES — FOUR CONTRASTING REGIONS

The following tables present monthly climate normals for four stations chosen to represent the range of NZ conditions: Auckland (warm, northern maritime), Wellington (windy, central), Christchurch (Canterbury — dry, continental for NZ, frost-prone), and Invercargill (Southland — cool, wet, southern).⁶

3.1 Auckland (Māngere / Auckland Airport, 6 m asl)

Month	Mean Min (°C)	Mean Max (°C)	Mean (°C)	Rain (mm)	Rain days	Frost days	Sun (hrs)	Wind (km/h)	RH (%)
Jan	16.0	23.8	19.9	72	8	0	228	16	76
Feb	16.3	24.0	20.2	68	8	0	199	15	77
Mar	14.9	22.5	18.7	87	9	0	189	15	78
Apr	12.5	19.8	16.2	100	11	0	162	14	81
May	10.2	17.0	13.6	112	13	0	139	14	83
Jun	8.1	14.6	11.4	126	15	0.2	115	15	84
Jul	7.2	13.9	10.6	132	16	0.5	120	15	83
Aug	7.9	14.5	11.2	111	14	0.3	137	15	81
Sep	9.5	16.2	12.9	97	13	0	157	16	79
Oct	11.1	17.9	14.5	90	12	0	178	16	78
Nov	12.7	19.8	16.3	82	10	0	197	16	76
Dec	14.7	22.0	18.4	82	9	0	217	16	76
Year	11.8	18.8	15.3	1159	138	1	2038	15	79

3.2 Wellington (Kelburn, 125 m asl)

Month	Mean Min (°C)	Mean Max (°C)	Mean (°C)	Rain (mm)	Rain days	Frost days	Sun (hrs)	Wind (km/h)	RH (%)
Jan	13.5	20.2	16.9	78	8	0	246	22	74
Feb	13.7	20.4	17.1	64	7	0	217	21	75
Mar	12.5	18.9	15.7	81	9	0	193	20	77
Apr	10.6	16.4	13.5	97	10	0	155	19	80
May	8.7	14.0	11.4	107	12	0.3	130	19	82
Jun	6.9	11.8	9.4	117	13	1.2	105	20	83
Jul	6.1	11.1	8.6	121	14	2.0	108	20	82
Aug	6.5	11.5	9.0	113	13	1.5	122	21	81
Sep	7.8	13.0	10.4	96	11	0.4	149	21	80
Oct	9.3	14.7	12.0	102	12	0	174	22	78
Nov	10.7	16.5	13.6	84	10	0	199	22	76
Dec	12.4	18.8	15.6	82	9	0	221	22	75
Year	9.9	15.6	12.8	1142	128	5.4	2019	21	79

3.3 Christchurch (Airport, 37 m asl)

Month	Mean Min (°C)	Mean Max (°C)	Mean (°C)	Rain (mm)	Rain days	Frost days	Sun (hrs)	Wind (km/h)	RH (%)
Jan	12.2	22.5	17.4	42	6	0	228	15	68
Feb	11.8	22.0	16.9	40	6	0	195	14	71
Mar	10.1	20.1	15.1	48	7	0.3	175	14	74
Apr	7.0	17.0	12.0	52	7	2.5	149	13	78
May	4.1	13.5	8.8	56	8	7.5	124	13	82
Jun	1.6	10.6	6.1	58	8	13.0	101	13	84
Jul	1.0	10.1	5.6	56	8	15.0	109	12	84
Aug	2.0	11.4	6.7	56	8	11.5	130	14	81
Sep	4.1	14.2	9.2	43	7	5.5	157	15	76
Oct	6.1	16.5	11.3	48	8	2.0	182	16	73
Nov	8.2	18.7	13.5	46	7	0.5	200	16	70
Dec	10.7	20.8	15.8	48	7	0	214	15	69
Year	6.6	16.5	11.5	593	87	57.8	1964	14	76

3.4 Invercargill (Airport, 2 m asl)

Month	Mean Min (°C)	Mean Max (°C)	Mean (°C)	Rain (mm)	Rain days	Frost days	Sun (hrs)	Wind (km/h)	RH (%)
Jan	9.8	18.7	14.3	88	11	0	189	15	75
Feb	9.7	18.4	14.1	76	10	0	160	14	77
Mar	8.2	16.7	12.5	80	11	0.5	141	14	79
Apr	5.8	13.8	9.8	82	11	3.0	111	13	83
May	3.5	11.1	7.3	90	13	8.5	87	13	85
Jun	1.4	8.5	5.0	76	12	14.0	67	13	86
Jul	0.7	8.0	4.4	72	12	16.5	74	13	85
Aug	1.7	9.3	5.5	78	12	12.0	95	14	83
Sep	3.6	11.6	7.6	80	11	6.5	120	15	80
Oct	5.4	13.5	9.5	88	12	3.0	145	15	78
Nov	7.0	15.4	11.2	82	11	0.5	164	15	76
Dec	8.6	17.1	12.9	86	11	0	170	15	76
Year	5.5	13.5	9.5	978	137	64.5	1523	14	80

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4. REMAINING REGIONS — FORMAT SPECIFICATION

The remaining ten regions should be printed using the same table format as Section 3, one page per region. The representative stations are:

Region	Station	Elevation	Key characteristics
Northland	Whangārei (Aero)	27 m	Warmest region, subtropical influence, 1600+ mm rain
Waikato	Hamilton (Ruakura)	40 m	Inland, moderate, frequent fog, 1200 mm rain
Bay of Plenty	Tauranga (Aero)	4 m	Warm, sheltered, moderate rain, high sunshine
Gisborne	Gisborne (Aero)	5 m	Warm-dry, high sunshine, drought-prone summers
Hawke’s Bay	Napier (Nelson Park)	2 m	Warmest-driest east coast, 780 mm rain
Taranaki	New Plymouth	49 m	Maritime, mild, high rainfall (1400+ mm)
Manawatū-Whanganui	Palmerston North (Aero)	46 m	Moderate, windy corridor, 960 mm rain
Nelson/Marlborough	Nelson (Aero)	4 m	Highest sunshine nationally (~2400 hrs), dry, mild
West Coast	Hokitika (Aero)	39 m	Wettest region (2900 mm+), mild, heavy cloud
Otago	Dunedin (Musselburgh)	2 m	Cool, moderate rain, significant inland variation

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5. WHY THIS BASELINE MATTERS

5.1 Measuring nuclear winter departure

Under the recovery scenario, the expected nuclear winter produces mean temperature reductions of approximately 2–8°C across NZ, with variation by season and latitude.⁷ A single observation — “July in Christchurch averaged 1.2°C this year” — is meaningless without a baseline to compare it against. The tables in this document provide that baseline: the July mean in Christchurch is normally 5.6°C, so a reading of 1.2°C represents a departure of -4.4°C — severe, but within the range of nuclear winter models for the Southern Hemisphere.

Without this reference data, agricultural planners, civil defence authorities, and local communities cannot:

• Quantify the severity of the nuclear winter affecting their region
• Determine whether conditions are worsening, stabilising, or improving year-on-year
• Compare their region’s experience against expectations from climate models
• Make informed decisions about crop timing, livestock management, or infrastructure preparation

5.2 Agricultural planning applications

Pasture growth in NZ follows a well-documented temperature relationship with a base temperature of approximately 5°C for ryegrass.⁸ Below this threshold, growth effectively ceases. The baseline data enables direct calculation of thermal growing days lost under nuclear winter:

Example (Christchurch): Under normal conditions, Christchurch mean temperatures exceed the 5°C ryegrass base for 10 months (September–June). Under nuclear winter departures of -3 to -7°C (the Year 2 range from Section 6), the number of months below the 5°C base increases from two (June–July) to four to seven months. Growing season contracts from approximately 300 days to approximately 150–240 days – a 20–50% reduction depending on the severity of cooling in a given year.

This type of calculation, repeated for each region and crop, forms the basis of agricultural adaptation planning in Docs #74 and #75.

5.3 Frost risk assessment

The frost data is particularly important. Under normal conditions, Auckland experiences approximately 1 frost day per year and Christchurch approximately 58. Under nuclear winter cooling, frost frequency increases across all regions. Frost-free growing seasons shorten. Subtropical and warm-temperate crops that currently grow in Northland and Auckland may become unviable. The baseline frost data quantifies the starting point for these assessments.

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6. NUCLEAR WINTER DEPARTURE FRAMEWORK

6.1 Expected temperature departures

Nuclear winter modelling for a NATO-Russia exchange (approximately 150 Tg of soot injection) indicates Southern Hemisphere cooling of 2–8°C, with the following approximate pattern for NZ:^{9 10}

Period	Temperature departure	Sunshine reduction	Notes
Year 1 (months 3–12)	-2 to -5°C	10–30%	Soot arrives ~2–4 months post-exchange; cooling intensifies through Year 1
Year 2	-4 to -8°C	20–40%	Peak nuclear winter. Maximum cooling and reduced sunlight
Year 3	-3 to -6°C	15–30%	Gradual soot removal begins. Conditions still well below normal
Years 4–5	-2 to -4°C	5–20%	Progressive recovery. Still measurably below baseline
Years 6–10	-1 to -2°C	<10%	Approaching normal. Some residual cooling from ocean thermal inertia

Uncertainty: These are modelled estimates, not observations. Actual departures depend on the specific exchange scenario (number of weapons, targeting, soot generation), stratospheric dynamics, and ocean circulation responses. The range given spans the plausible envelope from Robock et al. (2007), Toon et al. (2007), and Coupe et al. (2021).^{11 12 13}

6.2 Precipitation changes

Nuclear winter models indicate complex precipitation responses for NZ:

• Total rainfall likely decreases 10–30% globally in Years 1–3, but NZ’s maritime position may buffer this effect.¹⁴
• Seasonal distribution may shift. The weakened Hadley circulation alters westerly wind patterns, potentially reducing West Coast orographic rainfall while increasing east coast precipitation variability.
• Snow line drops significantly. Under -5°C cooling, the effective snow line drops approximately 800–1,000 metres (based on the standard environmental lapse rate of approximately 6.5°C per 1,000 m elevation¹⁵), increasing snow cover duration in upland Canterbury, Otago, and Southland, and introducing occasional lowland snow events in areas that rarely experience them.

6.3 UV radiation and ozone depletion

Nuclear detonations inject nitrogen oxides into the stratosphere, catalysing ozone destruction. Mills et al. (2008) modelled 25–45% global ozone column reduction for a regional conflict (5 Tg soot); a full-scale 150 Tg exchange would likely produce greater depletion, though no published study has modelled ozone loss for NZ specifically under the full-exchange scenario.¹⁶ The resulting increase in surface UV-B radiation would be substantial – a factor of 1.5–4 depending on latitude, season, and the actual magnitude of ozone loss. This has direct consequences:

• Crop damage: Many crop species show reduced photosynthesis and cellular damage under elevated UV-B
• Livestock: Increased eye disease (pink-eye, photokeratitis) and skin damage in light-skinned breeds
• Human health: Increased sunburn risk, skin cancer acceleration, eye damage
• Pasture species: Variable UV-B sensitivity — white clover more susceptible than ryegrass

The baseline sunshine hours in this document provide the reference for calculating total UV dose under changed ozone conditions.

6.4 Using the departure framework

For any region and month, the expected nuclear winter condition is calculated as:

Adjusted value = Baseline value + Departure

Worked example — Invercargill, July, Year 2 of nuclear winter:

• Baseline mean temperature: 4.4°C
• Expected departure: -6°C (mid-range for peak nuclear winter)
• Adjusted mean: -1.6°C — below freezing for the monthly mean
• Baseline frost days: 16.5
• Expected adjusted frost days: 25–30 (nearly every day)
• Practical consequence: continuous ground frost, no pasture growth, livestock require stored feed

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7. REGIONAL SUMMARY CARDS — FIELD FORMAT

For field use, each region’s data should be printed on a single laminated A4 card with:

Front: - Region name and station - Monthly table (as in Section 3) - Annual summary row

Back: - Nuclear winter departure table (expected ranges for Years 1, 2, 3, 5, 10) - Frost risk calendar: normal vs. nuclear winter comparison - Key agricultural thresholds: ryegrass base (5°C), wheat vernalisation range (0–7°C), potato frost kill (-2°C), pasture growth rate equation¹⁷

These cards should be printed and laminated during the Phase 1 printing programme (Doc #5) and distributed to every farm, every school running meteorological observations, and every regional civil defence office.

Estimated print volume: 14 regions x 200–500 copies per region (depending on regional population and number of farms) = 2,800–7,000 cards. At one A4 sheet (double-sided) per card, this is a minor printing expenditure relative to the Phase 1 programme.

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8. ESTIMATED PAGE COUNT

Component	Pages
This document (introduction, framework, four sample tables)	12–15
Complete regional tables (10 additional regions)	10–12
Nuclear winter departure tables (all regions)	4–6
Regional summary cards (print separately)	14
Total (bound document)	26–33
Total (with separate cards)	40–47

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CRITICAL UNCERTAINTIES

Uncertainty	Impact	Mitigation
Normal period (1991–2020) may not reflect conditions at time of event due to ongoing climate change	Baseline temperatures may be 0.5–1.0°C low relative to actual pre-event conditions	Note the baseline year range; adjust if more recent data available
Nuclear winter magnitude for NZ is modelled, not observed	Actual cooling could be outside the 2–8°C range; models disagree on Southern Hemisphere response	Use the range, not point estimates; establish observation network early to measure actual departure
Regional variation within each region is large	A single station cannot represent altitude, aspect, and local effects	Use the data as a starting point; local observers compare against their own experience
UV/ozone depletion estimates vary widely between models; published figures are for regional (5 Tg) scenarios, not the full 150 Tg exchange	Surface UV-B increase could be lower or higher than the 1.5–4× estimate used here	Monitor with UV meters where available; use conservative exposure guidance

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CROSS-REFERENCES

• Doc #5 — National Printing Capability (for printing this document and regional cards)
• Doc #74 — Pastoral Farming Under Nuclear Winter (uses baseline for growth modelling)
• Doc #75 — Cropping and Dairy Adaptation (uses baseline for growing season calculations)
• Doc #77 — Seed Preservation and Distribution (planting dates depend on frost data)
• Doc #79 — Geothermal Greenhouses (uses baseline for heating requirement estimates)
• Doc #125 — Public Health (UV exposure guidance depends on ozone depletion estimates)
• Doc #8 — National Asset and Skills Census (identifies meteorological observation capability)

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1. NIWA (National Institute of Water and Atmospheric Research). Climate normals are calculated for the standard WMO 30-year period (1991–2020). Published via NIWA’s Climate Summaries and CliFlo database. https://cliflo.niwa.co.nz/ — Note: the previous normal period (1981–2010) is also widely referenced in older NZ publications.↩︎

2. NIWA CliFlo — The National Climate Database. Contains records from approximately 6,500 stations dating back to the 1850s for some locations. Approximately 600 stations active as of 2024. https://cliflo.niwa.co.nz/ — Access requires registration but data is publicly available.↩︎

3. NIWA CliFlo — The National Climate Database. Contains records from approximately 6,500 stations dating back to the 1850s for some locations. Approximately 600 stations active as of 2024. https://cliflo.niwa.co.nz/ — Access requires registration but data is publicly available.↩︎

4. World Meteorological Organisation (WMO). WMO Guidelines on the Calculation of Climate Normals (WMO-No. 1203), 2017. Specifies 30-year averaging periods for climate normal computation.↩︎

5. Sturman, A.P. and Tapper, N.J. The Weather and Climate of Australia and New Zealand, 2nd edition, Oxford University Press, 2006. Standard reference for NZ climate geography. Also: Salinger, M.J. et al., “Climate trends in New Zealand,” International Journal of Climatology, various papers. NIWA’s Climate Overview pages provide accessible summaries: https://niwa.co.nz/education-and-training/schools/resourc...↩︎

6. Individual station normals derived from NIWA CliFlo database records and NIWA published climate summaries. Auckland Airport (Agent No. 1962), Wellington Kelburn (Agent No. 3385), Christchurch Airport (Agent No. 4843), Invercargill Airport (Agent No. 5814). Figures are approximate 1991–2020 normals based on available published data. Some values are interpolated from the 1981–2010 period where 1991–2020 figures have not yet been published for all variables. These figures should be verified against the CliFlo database directly before final printing.↩︎

7. Robock, A., Oman, L., and Stenchikov, G.L. “Nuclear winter revisited with a modern climate model and current nuclear arsenals: Still catastrophic consequences,” Journal of Geophysical Research: Atmospheres, 112, D13107, 2007. This study models a 150 Tg soot scenario (full-scale nuclear war) and estimates global average cooling of 7–8°C at peak, with Southern Hemisphere cooling of 3–6°C at peak.↩︎

8. Mitchell, K.J. “Growth of pasture species under controlled environment. I. Growth at various levels of constant temperature,” New Zealand Journal of Science and Technology, Section A, 38(2): 203–216, 1956. Also: DairyNZ Pasture Growth Data, https://www.dairynz.co.nz/feed/pasture-management/ — The 5°C base temperature is a rounded approximation; actual base varies by cultivar and is closer to 4–6°C for most NZ ryegrass varieties.↩︎

9. Robock, A., Oman, L., and Stenchikov, G.L. “Nuclear winter revisited with a modern climate model and current nuclear arsenals: Still catastrophic consequences,” Journal of Geophysical Research: Atmospheres, 112, D13107, 2007. This study models a 150 Tg soot scenario (full-scale nuclear war) and estimates global average cooling of 7–8°C at peak, with Southern Hemisphere cooling of 3–6°C at peak.↩︎

10. Toon, O.B., Robock, A., Turco, R.P. et al. “Consequences of regional-scale nuclear conflicts,” Science, 315(5818): 1224–1225, 2007. This and the companion paper in Atmospheric Chemistry and Physics provide the scenario framework used in the Recovery Library.↩︎

11. Robock, A., Oman, L., and Stenchikov, G.L. “Nuclear winter revisited with a modern climate model and current nuclear arsenals: Still catastrophic consequences,” Journal of Geophysical Research: Atmospheres, 112, D13107, 2007. This study models a 150 Tg soot scenario (full-scale nuclear war) and estimates global average cooling of 7–8°C at peak, with Southern Hemisphere cooling of 3–6°C at peak.↩︎

12. Toon, O.B., Robock, A., Turco, R.P. et al. “Consequences of regional-scale nuclear conflicts,” Science, 315(5818): 1224–1225, 2007. This and the companion paper in Atmospheric Chemistry and Physics provide the scenario framework used in the Recovery Library.↩︎

13. Coupe, J., Bardeen, C.G., Robock, A., and Toon, O.B. “Nuclear Niño response observed in simulations of nuclear war scenarios,” Communications Earth & Environment, 2, 18, 2021. Updates nuclear winter modelling with WACCM6 (Whole Atmosphere Community Climate Model). Confirms persistent multi-year cooling and identifies “Nuclear Niño” — warming of the eastern tropical Pacific that alters global precipitation patterns.↩︎

14. Toon, O.B., Robock, A., Turco, R.P. et al. “Consequences of regional-scale nuclear conflicts,” Science, 315(5818): 1224–1225, 2007. This and the companion paper in Atmospheric Chemistry and Physics provide the scenario framework used in the Recovery Library.↩︎

15. The environmental lapse rate averages approximately 6.5°C per 1,000 m of elevation in the troposphere. This is a standard meteorological value (see any introductory atmospheric science text, e.g., Wallace, J.M. and Hobbs, P.V., Atmospheric Science: An Introductory Survey, 2nd edition, Academic Press, 2006). Under -5°C uniform cooling, the altitude at which temperatures reach a given isotherm drops by approximately 770–1,000 m, depending on local conditions and moisture content.↩︎

16. Mills, M.J., Toon, O.B., Turco, R.P. et al. “Massive global ozone loss predicted following regional nuclear conflict,” Proceedings of the National Academy of Sciences, 105(14): 5307–5312, 2008. This study modelled a regional conflict (5 Tg soot injection — India-Pakistan scale), not the full 150 Tg exchange. It found 25–45% global ozone column loss peaking 1–5 years after the conflict. The full-exchange scenario used in the Recovery Library would likely produce greater ozone depletion, but no published model has quantified NZ-specific ozone loss under 150 Tg conditions. Also: Bardeen, C.G. et al., “On transient climate change at the Cretaceous-Paleogene boundary due to atmospheric soot injections,” PNAS, 2017, which provides updated ozone depletion modelling. NZ, at mid-latitudes in the Southern Hemisphere, is expected to experience significant but not the most extreme ozone losses — the worst depletion concentrates at high northern latitudes.↩︎

17. Wheat vernalisation range (0–7°C): Brooking, I.R., “Vernalization and photoperiod response of selected Australasian wheat cultivars,” NZ Journal of Agricultural Research, 39(3): 395–403, 1996. Also: Chouard, P., “Vernalization and its relations to dormancy,” Annual Review of Plant Physiology, 11: 191–238, 1960. Potato frost kill (-2°C): van der Zaag, D.E. and Burton, W.G., “Potato,” in The Physiology of Tropical Field Crops, Wiley, 1984. The -2°C threshold is approximate; actual damage onset varies by cultivar and exposure duration.↩︎