Is Thunder Bay or Duluth better for the northern lights?

Thunder Bay sits closer to the oval, at 57.1 degrees geomagnetic against 55.4. Darkness, cloud and your dates decide the rest; the tables on this page show both towns month by month.

Which has longer nights in December, Thunder Bay or Duluth?

Thunder Bay, with 14.3 hours of civil night on December 15 against 14.2 in Duluth.

What Kp do Thunder Bay and Duluth need?

By SWPC's rule of 66 degrees at Kp 0 and 2 degrees per step, Thunder Bay needs about Kp 5 and Duluth Kp 6 for the average oval edge to sit overhead.

When is it too light for aurora in Thunder Bay and Duluth?

Thunder Bay: never, every night has civil darkness. Duluth: never, every night has civil darkness.

Which is clearer, Thunder Bay or Duluth?

Duluth, with 61% mean cloud in the months with 4 hours or more of civil night against 70% in Thunder Bay (NASA POWER, 2001 to 2020).

Comparison

Thunder Bay vs Duluth for the northern lights

Answer

Thunder Bay sits 1.8 degrees higher on the IGRF-14 dipole, 57.1 against 55.4. By SWPC's oval-edge rule Thunder Bay needs about Kp 5 for the average edge to sit overhead and Duluth Kp 6. On the map Thunder Bay is also closer to the pole, by 1.6 degrees.

57.1° / 55.4°

dipole latitude

Side by side

Computed figures for both towns
Figure	Thunder Bay	Duluth
Dipole latitude	57.1°	55.4°
Kp for the oval edge overhead	Kp 5	Kp 6
Civil night, December 15	14.3 h	14.2 h
Dark-season days	365	365
Nights with no civil darkness	0	0
Dark-month cloud, NASA POWER	70%	61%
Solar midnight, December 15	00:52	00:04

Thunder Bay sits at 57.1 degrees geomagnetic. By SWPC's rule the average oval edge is overhead at Kp 5, and aurora is usually visible on the northern horizon at G1 (Kp 5) or less. Duluth sits at 55.4 degrees geomagnetic. By SWPC's rule the average oval edge is overhead at Kp 6, and aurora is usually visible on the northern horizon at G1 (Kp 5) or less.

Civil night month by month

On December 15 Thunder Bay has 14.3 hours of civil night and Duluth 14.2. Across the 15th of each month, Thunder Bay is darker in 4 months and Duluth in 6. The widest gap comes in June, when Duluth has 0.3 more hours.

Hours of civil night on the 15th of each month, 2026
Month	Thunder Bay	Duluth	Difference
January	13.9 h	13.8 h	+0.1 h
February	12.6 h	12.5 h	+0.1 h
March	11.0 h	11.0 h	+0.0 h
April	9.1 h	9.2 h	-0.1 h
May	7.4 h	7.6 h	-0.2 h
June	6.4 h	6.7 h	-0.3 h
July	6.9 h	7.2 h	-0.3 h
August	8.4 h	8.6 h	-0.2 h
September	10.3 h	10.4 h	-0.1 h
October	12.0 h	12.0 h	+0.0 h
November	13.6 h	13.4 h	+0.2 h
December	14.3 h	14.2 h	+0.1 h

Difference: Thunder Bay minus Duluth. Each figure links to that town's month page.

Season and midnight sun

Thunder Bay has some civil darkness every night of 2026, and clears Glow's 4-hour dark-season bar every evening. Duluth has some civil darkness every night of 2026, and clears Glow's 4-hour dark-season bar every evening.

Distance, cloud and time zones

The two towns are 278 km apart by great circle. In the dark months Duluth averages 61% cloud and Thunder Bay 70%, from NASA POWER's 2001 to 2020 record. Thunder Bay's best month by civil night times clear sky is January. Duluth's best month by civil night times clear sky is February. Time zones: America/Toronto and America/Chicago.

For tonight, open the live pages for Thunder Bay and Duluth, which carry the live OVATION cell.

Questions

Is Thunder Bay or Duluth better for the northern lights?: Thunder Bay sits closer to the oval, at 57.1 degrees geomagnetic against 55.4. Darkness, cloud and your dates decide the rest; the tables on this page show both towns month by month.
Which has longer nights in December, Thunder Bay or Duluth?: Thunder Bay, with 14.3 hours of civil night on December 15 against 14.2 in Duluth.
What Kp do Thunder Bay and Duluth need?: By SWPC's rule of 66 degrees at Kp 0 and 2 degrees per step, Thunder Bay needs about Kp 5 and Duluth Kp 6 for the average oval edge to sit overhead.
When is it too light for aurora in Thunder Bay and Duluth?: Thunder Bay: never, every night has civil darkness. Duluth: never, every night has civil darkness.
Which is clearer, Thunder Bay or Duluth?: Duluth, with 61% mean cloud in the months with 4 hours or more of civil night against 70% in Thunder Bay (NASA POWER, 2001 to 2020).

Sources and verification

IGRF-14 coefficients, NOAA NCEIhttps://www.ngdc.noaa.gov/IAGA/vmod/coeffs/igrf14coeffs.txt
SunCalchttps://github.com/mourner/suncalc
GeoNameshttps://www.geonames.org/export/
NASA POWER climatologyhttps://power.larc.nasa.gov/docs/services/api/temporal/climatology/
NOAA SWPC, Tips on viewing the aurorahttps://www.spaceweather.gov/content/tips-viewing-aurora

Quoted figures last checked against their sources on 2026-09-30; next review 2026-12-31. Computed figures come from SunCalc and the IGRF-14 dipole as described in the methodology. Every quote is listed in the sources ledger.