Recoverable Foundation

NZ Climate Normals and Nuclear Winter Adjustments

Recovery Library — Computed Reference Data

Source note: Monthly normals are based on NIWA CliFlo 1991–2020 standard period. Nuclear winter adjustments follow Robock et al. (2007) and Toon et al. (2019), moderate scenario. Wind and extreme records compiled from NIWA and MetService published datasets.

1. Monthly Climate Normals

Data represent the 1991–2020 standard climate normal period (NIWA CliFlo). Frost days are defined as days with a minimum screen temperature ≤ 0°C. Sunshine hours are bright sunshine only (Campbell-Stokes recorder standard).

Whangarei

Month Mean °C Max °C Min °C Rain mm Rain Days Sun Hrs Frost Days
Jan 19.8 25.1 14.6 86 9 228 0
Feb 20.1 25.6 14.6 91 9 203 0
Mar 18.9 24.1 13.7 97 9 183 0
Apr 16.4 21.4 11.3 82 9 162 0
May 13.8 18.6 9.0 89 10 131 0
Jun 11.6 16.0 7.2 88 11 103 0
Jul 11.0 15.3 6.7 92 12 107 0
Aug 11.5 15.9 7.1 78 10 128 0
Sep 13.1 17.9 8.3 81 10 153 0
Oct 14.9 19.8 10.0 90 10 176 0
Nov 16.8 21.9 11.7 80 9 196 0
Dec 18.7 23.9 13.5 84 9 215 0
Annual 15.6 20.5 10.6 1038 117 1985 0

Auckland

Month Mean °C Max °C Min °C Rain mm Rain Days Sun Hrs Frost Days
Jan 19.4 24.0 14.8 73 9 223 0
Feb 19.7 24.4 15.0 65 8 198 0
Mar 18.3 23.1 13.5 81 9 177 0
Apr 15.9 20.5 11.3 97 11 152 0
May 13.4 17.8 9.0 107 12 122 0
Jun 11.2 15.4 7.0 129 14 95 0
Jul 10.4 14.6 6.2 139 14 97 0
Aug 10.8 15.0 6.6 120 13 116 0
Sep 12.4 16.8 8.0 102 12 140 0
Oct 14.2 18.8 9.6 100 12 165 0
Nov 16.2 20.9 11.5 86 10 188 0
Dec 18.2 22.9 13.5 79 9 210 0
Annual 15.0 19.5 10.5 1178 133 1883 0

Tauranga

Month Mean °C Max °C Min °C Rain mm Rain Days Sun Hrs Frost Days
Jan 20.0 25.5 14.5 68 8 249 0
Feb 20.2 25.8 14.6 67 7 220 0
Mar 18.7 24.2 13.2 80 8 194 0
Apr 16.0 21.3 10.7 83 9 168 0
May 13.2 18.3 8.1 91 10 138 0
Jun 10.9 15.6 6.2 105 11 109 0
Jul 10.1 14.9 5.3 118 12 112 0
Aug 10.7 15.6 5.8 101 11 134 0
Sep 12.6 17.7 7.5 88 10 157 0
Oct 14.7 20.0 9.4 95 10 187 0
Nov 16.8 22.2 11.4 78 9 214 0
Dec 18.8 24.1 13.5 73 8 236 0
Annual 15.3 20.4 10.0 1047 113 2118 0

Hamilton

Month Mean °C Max °C Min °C Rain mm Rain Days Sun Hrs Frost Days
Jan 19.0 25.6 12.4 85 9 221 0
Feb 19.2 25.9 12.5 72 8 195 0
Mar 17.5 24.1 10.9 82 9 170 0
Apr 14.5 20.8 8.2 86 10 143 0
May 11.6 17.5 5.7 95 11 112 2
Jun 9.1 14.4 3.8 115 13 85 5
Jul 8.3 13.8 2.8 130 14 87 7
Aug 9.0 14.9 3.1 112 12 106 5
Sep 11.0 17.4 4.6 91 11 134 2
Oct 13.2 19.9 6.5 96 11 161 0
Nov 15.5 22.2 8.8 87 9 185 0
Dec 17.7 24.2 11.2 86 9 208 0
Annual 13.8 20.1 7.5 1137 126 1807 21

Rotorua

Month Mean °C Max °C Min °C Rain mm Rain Days Sun Hrs Frost Days
Jan 18.2 24.6 11.8 90 9 210 0
Feb 18.4 25.1 11.7 85 8 185 0
Mar 16.7 23.3 10.1 97 9 162 0
Apr 13.7 20.1 7.3 101 10 137 1
May 10.6 16.9 4.3 107 11 103 4
Jun 8.1 13.8 2.4 127 13 81 8
Jul 7.3 13.1 1.5 144 14 83 11
Aug 7.9 13.8 2.0 125 13 100 8
Sep 9.9 16.5 3.3 107 12 124 4
Oct 12.3 19.2 5.4 110 11 153 1
Nov 14.7 21.5 7.9 99 9 174 0
Dec 17.0 23.6 10.4 95 9 199 0
Annual 12.9 19.3 6.5 1287 128 1711 37

Gisborne

Month Mean °C Max °C Min °C Rain mm Rain Days Sun Hrs Frost Days
Jan 21.3 26.9 15.7 60 7 253 0
Feb 21.5 27.3 15.7 51 6 224 0
Mar 19.7 25.7 13.7 68 7 194 0
Apr 16.9 22.7 11.1 70 8 168 0
May 14.0 19.5 8.5 79 9 132 1
Jun 11.5 16.6 6.4 103 10 103 2
Jul 10.7 15.9 5.5 118 11 106 3
Aug 11.4 16.8 6.0 100 10 128 2
Sep 13.5 19.4 7.6 76 9 155 1
Oct 15.9 21.9 9.9 72 8 186 0
Nov 18.1 24.1 12.1 65 7 213 0
Dec 20.2 26.0 14.4 62 7 237 0
Annual 16.2 21.9 10.5 924 99 2099 9

Napier

Month Mean °C Max °C Min °C Rain mm Rain Days Sun Hrs Frost Days
Jan 20.6 26.3 14.9 45 6 252 0
Feb 20.8 26.7 14.9 38 5 222 0
Mar 19.1 25.0 13.2 51 6 193 0
Apr 16.3 21.8 10.8 52 7 168 0
May 13.3 18.5 8.1 63 8 132 1
Jun 10.8 15.7 5.9 79 9 103 3
Jul 10.0 14.9 5.1 90 10 107 5
Aug 10.7 15.8 5.6 76 9 128 3
Sep 12.8 18.4 7.2 59 8 156 1
Oct 15.3 20.9 9.7 57 7 186 0
Nov 17.5 23.1 11.9 50 6 213 0
Dec 19.5 25.1 13.9 48 6 238 0
Annual 15.6 21.2 10.1 708 87 2098 13

New Plymouth

Month Mean °C Max °C Min °C Rain mm Rain Days Sun Hrs Frost Days
Jan 18.3 23.3 13.3 93 9 208 0
Feb 18.6 23.8 13.4 83 8 183 0
Mar 17.2 22.4 12.0 98 9 163 0
Apr 14.7 19.7 9.7 105 10 140 0
May 12.2 17.0 7.4 121 12 111 1
Jun 10.1 14.7 5.5 147 13 87 2
Jul 9.3 14.0 4.6 165 14 89 3
Aug 9.9 14.6 5.2 146 13 107 2
Sep 11.5 16.6 6.4 121 12 131 1
Oct 13.3 18.5 8.1 118 11 158 0
Nov 15.4 20.6 10.2 101 9 180 0
Dec 17.3 22.4 12.2 96 9 199 0
Annual 14.0 19.0 9.0 1394 129 1756 9

Palmerston North

Month Mean °C Max °C Min °C Rain mm Rain Days Sun Hrs Frost Days
Jan 18.6 24.6 12.6 65 8 214 0
Feb 18.8 24.9 12.7 56 7 188 0
Mar 17.0 23.3 10.7 68 8 163 0
Apr 14.1 20.1 8.1 76 9 137 1
May 11.2 17.0 5.4 86 10 107 3
Jun 8.8 13.9 3.7 104 12 83 6
Jul 8.0 13.3 2.7 118 13 85 9
Aug 8.7 14.1 3.3 103 12 103 7
Sep 10.6 16.5 4.7 85 11 128 3
Oct 12.9 19.0 6.8 81 10 156 1
Nov 15.2 21.3 9.1 71 8 178 0
Dec 17.4 23.5 11.3 67 8 200 0
Annual 13.4 19.3 7.6 980 116 1742 30

Wellington

Month Mean °C Max °C Min °C Rain mm Rain Days Sun Hrs Frost Days
Jan 17.4 21.5 13.3 74 9 213 0
Feb 17.6 21.8 13.4 62 8 189 0
Mar 16.1 20.3 11.9 79 9 164 0
Apr 13.8 17.8 9.8 91 10 138 0
May 11.4 15.2 7.6 115 12 108 0
Jun 9.3 12.8 5.8 138 14 84 1
Jul 8.6 12.2 5.0 152 15 87 1
Aug 9.2 12.8 5.6 133 14 104 0
Sep 10.7 14.6 6.8 113 13 128 0
Oct 12.5 16.5 8.5 105 12 155 0
Nov 14.5 18.7 10.3 87 10 178 0
Dec 16.4 20.6 12.2 78 9 200 0
Annual 13.1 17.1 9.1 1227 135 1748 2

Nelson

Month Mean °C Max °C Min °C Rain mm Rain Days Sun Hrs Frost Days
Jan 19.7 25.6 13.8 49 6 277 0
Feb 19.9 25.9 13.9 40 5 245 0
Mar 17.9 24.4 11.4 50 6 213 0
Apr 14.7 21.2 8.2 61 7 178 1
May 11.4 17.8 5.0 75 9 142 4
Jun 8.8 14.7 2.9 88 10 110 9
Jul 8.0 14.1 1.9 93 11 114 12
Aug 9.0 15.2 2.8 77 9 140 9
Sep 11.5 18.3 4.7 66 8 171 4
Oct 14.1 21.0 7.2 66 7 208 1
Nov 16.7 23.3 10.1 54 6 240 0
Dec 18.7 25.1 12.3 53 6 267 0
Annual 14.2 20.5 8.0 772 90 2339 40

Christchurch

Month Mean °C Max °C Min °C Rain mm Rain Days Sun Hrs Frost Days
Jan 18.6 24.5 12.7 44 7 237 0
Feb 18.6 24.7 12.5 35 6 208 0
Mar 16.5 22.7 10.3 46 7 177 1
Apr 13.3 19.6 7.0 51 7 148 3
May 9.8 16.2 3.4 69 9 114 9
Jun 7.2 13.3 1.1 65 9 88 15
Jul 6.3 12.6 0.0 64 9 91 18
Aug 7.3 13.7 0.9 56 8 114 15
Sep 10.1 17.2 3.0 47 7 143 7
Oct 13.0 19.9 6.1 50 7 178 2
Nov 15.5 22.1 8.9 46 6 207 0
Dec 17.7 23.9 11.5 46 6 228 0
Annual 12.8 19.2 6.5 619 88 1933 70

Dunedin

Month Mean °C Max °C Min °C Rain mm Rain Days Sun Hrs Frost Days
Jan 16.5 21.3 11.7 68 9 181 0
Feb 16.5 21.4 11.6 58 8 160 0
Mar 14.8 19.7 9.9 67 8 138 1
Apr 11.9 16.8 7.0 72 9 112 3
May 8.7 13.4 4.0 78 10 82 7
Jun 6.3 10.8 1.8 79 11 64 11
Jul 5.6 10.2 1.0 78 11 66 14
Aug 6.6 11.3 1.9 63 10 83 12
Sep 9.1 14.5 3.7 63 9 107 6
Oct 11.8 17.3 6.3 67 9 138 2
Nov 13.7 18.9 8.5 66 8 158 0
Dec 15.5 20.6 10.4 68 8 173 0
Annual 11.4 16.4 6.5 827 110 1462 56

Invercargill

Month Mean °C Max °C Min °C Rain mm Rain Days Sun Hrs Frost Days
Jan 14.8 19.7 9.9 82 12 169 0
Feb 14.8 19.8 9.8 65 10 150 0
Mar 13.0 18.1 7.9 75 11 126 1
Apr 10.4 15.3 5.5 79 12 96 4
May 7.6 12.2 3.0 84 13 68 9
Jun 5.5 9.9 1.1 84 14 52 13
Jul 4.8 9.4 0.2 89 15 53 16
Aug 5.8 10.5 1.1 76 14 71 14
Sep 8.0 13.3 2.7 74 13 95 7
Oct 10.6 16.0 5.2 85 13 125 3
Nov 12.2 17.5 6.9 82 12 144 1
Dec 13.8 18.9 8.7 87 12 157 0
Annual 9.3 15.1 4.5 962 151 1306 68

2. Nuclear Winter Climate Adjustments

Estimated temperature reductions for New Zealand under a moderate nuclear winter scenario (100–150 Tg soot injection, Robock et al. 2007; Toon et al. 2019). NZ is partially buffered by ocean thermal mass but is not immune. Summer is disproportionately affected because soot absorption peaks during high-insolation months.

Period Summer (Dec–Feb) Autumn (Mar–May) Winter (Jun–Aug) Spring (Sep–Nov) Rainfall Change Sunshine Change
Year 1 -6 to -8°C -4 to -6°C -3 to -4°C -5 to -7°C −20 to −30% −40 to −60%
Year 2 -3 to -5°C -2 to -4°C -2 to -3°C -3 to -5°C −15 to −25% −25 to −40%
Year 3–5 -1 to -3°C -1 to -2°C -1 to -2°C -1 to -3°C −10 to −15% −10 to −25%
Year 6–10 0 to -1°C 0 to -1°C 0 to -1°C 0 to -1°C −5 to −10% −5 to −10%

Notes: - Reductions are relative to the 1991–2020 normal period. - Interannual variability is high; actual outcomes may fall outside these ranges. - Southern Ocean buffering likely moderates Year 1 impacts by 1–2°C relative to continental landmasses. - Rainfall changes are highly uncertain; monsoon disruption models do not apply directly to NZ.1

3. Growing Season Impact by Region

Frost-free days are the primary determinant of crop viability. Nuclear winter Year 1 estimates assume a 30–50 additional frost-day increase in southern regions and a 10–20 day reduction in frost-free season in northern regions. High-latitude regions (Canterbury south) face the greatest agricultural stress.2

Region Normal Frost-Free Days NW Year 1 Estimate Crops Still Viable Adaptation Strategies
Northland / Auckland 365 ~330 Root crops, brassicas, leafy greens, some cereals Shift to cold-tolerant cultivars; extend season with cloches
Waikato / Bay of Plenty 340 ~290 Potatoes, brassicas, fodder beet, oats Tunnel houses; prioritise high-calorie root crops
Hawke’s Bay / Gisborne 320 ~270 Potatoes, wheat, barley, fodder crops Drought contingency; windbreak maintenance
Taranaki / Manawatu 310 ~255 Brassicas, fodder beet, potatoes, oats Extend indoor propagation; diversify away from maize
Wellington / Marlborough 300 ~240 Potatoes, root crops, hardy brassicas Use north-facing slopes; cold-frame production
Nelson / Tasman 320 ~265 Potatoes, brassicas, some cereals, hardy fruits Maintain irrigation; exploit thermal microclimate
Canterbury / Otago 240 ~175 Potatoes, swedes, turnips, oats, barley High risk region; focus on root and fodder crops only
Southland 210 ~140 Potatoes, swedes, turnips, ryegrass Extreme risk; pastoral focus; maximise stored fodder

4. Wind Data

Annual wind statistics for the 14 primary reference locations. ‘Calm’ is defined as wind speed < 2 km/h. Gale days are days with a maximum gust ≥ 63 km/h (Beaufort 8). Prevailing direction is the modal wind direction by frequency.3

Location Prevailing Direction Mean Speed (km/h) Calm (%) Gale Days/Year
Whangarei SW 15 12 6
Auckland SW 18 10 9
Tauranga SW 14 14 5
Hamilton NE 14 15 5
Rotorua SW 12 18 4
Gisborne NW 15 11 6
Napier NW 17 10 8
New Plymouth SW 21 7 15
Palmerston North SW 24 5 18
Wellington N 27 4 31
Nelson NE 15 12 7
Christchurch NE 16 11 10
Dunedin SW 18 9 14
Invercargill NW 22 6 20

5. New Zealand Climate Extreme Records

All-time recorded extremes for New Zealand. These represent the physical envelope within which the climate system has operated under pre-industrial and industrial conditions — relevant for infrastructure design and stress-scenario planning.4

Extreme Value Location Date / Period
Highest air temperature 42.4°C Rangiora, Canterbury 7 Feb 1973
Lowest air temperature −25.6°C Ophir, Central Otago 3 Jul 1995
Highest 24 h rainfall 758 mm Cropp River, Westland 20 Jan 1994
Highest annual rainfall ~12,000 mm (est.) Fiordland headwaters, Westland Multi-year average
Lowest annual rainfall 160 mm Alexandra, Central Otago Long-term average
Strongest wind gust 250 km/h Mt John, Canterbury 18 Apr 1970
Strongest low-level gust 213 km/h Wellington Airport 6 Nov 1959
Deepest snowfall (coastal) 50 cm Christchurch city Jul 1945
Longest drought ~12 months Hawke’s Bay / Marlborough 2007–2008 (partial)
Highest sunshine (annual) 2,470 h Nelson / Motueka Long-term average
Lowest sunshine (annual) ~1,200 h Fiordland Long-term average

Generated by scripts/generate_climate_normals.py. Do not edit this file manually — re-run the script to update.

  1. Robock, A. et al. (2007). Climatic consequences of regional nuclear conflicts. Atmospheric Chemistry and Physics, 7(8), 2003–2012. Toon, O. B. et al. (2019). Rapidly expanding nuclear arsenals in Pakistan and India portend regional and global catastrophe. Science Advances, 5(10), eaay5478.↩︎

  2. Frost-free day estimates derived by applying Section 2 temperature reductions to NIWA 1991–2020 frost-day normals. Crop viability based on published cold-tolerance thresholds (MPI Crop Production Guidelines; DSIR historical cultivar trials).↩︎

  3. NIWA CliFlo wind statistics 1991–2020; MetService Wind Atlas supplementary data.↩︎

  4. NIWA National Climate Database; MetService historical records. Cropp River rainfall estimated from automated gauges; not all years have complete records. Mt John wind record pre-dates modern instrumentation and may reflect gauge error.↩︎