Can you see the northern lights in Great Falls in September?

On dark, clear nights with enough activity. 30 of 30 nights at Great Falls have civil darkness, and 5% of past September days reached Kp 6.

How dark is Great Falls in September?

Great Falls averages 10.3 hours of civil night a night in September 2027, from 9 h 30 min on the 1st to 11 h 10 min on the last night.

How cloudy is Great Falls in September?

NASA POWER's 2001 to 2020 climatology gives Great Falls's 1 degree cell 52% mean cloud in September, against 62% for the whole year. It is an all-hours average, not a forecast.

What is the best night in September 2027 for aurora in Great Falls?

By darkness and moon alone, Sep 1, with 4 h of moon-free darkness inside 10 PM to 2 AM at Great Falls. Activity on that date will only be known hours ahead.

Is September or October better in Great Falls?

By civil night times the clear share, September scores 4.9 hours at Great Falls and October 4.4. September comes out ahead.

Great Falls · September 2027

Northern lights in Great Falls in September 2027

Answer

September gives Great Falls 10.3 hours of civil night on the 15th, the 7th darkest of its 12 months, under 52% mean cloud. 5% of September days since 1996 reached Kp 6, enough for the oval edge to reach Great Falls.

10.3 h

civil night, average night

SunCalc for 47.50, -111.30; NASA POWER; GFZ Kp

September against the rest of the year

4.1

3.8

3.2

2.8

2.6

2.8

4.3

4.8

4.9

4.4

4.2

4.4

September cloud at Great Falls averages 52% in NASA POWER's 2001 to 2020 record, 10 points clearer than its yearly mean. July is Great Falls's clearest month at 38%. That makes September the best month of Great Falls's year by civil night times clear sky, 4.9 hours. Lethbridge, 269 km away, averages 56%, cloudier by 4 points.

Civil night at Great Falls starts September at 9 h 30 min and reaches 11 h 10 min by September 30 as the nights lengthen by 1.7 hours. SWPC's 10 PM to 2 AM window is fully dark on every night. True astronomical darkness, the sun 18 degrees down, lasts 7.8 hours on the 15th.

September night by night

Full moon comes first, on Sep 15, then the new moon on Sep 29, in America/Denver dates. Great Falls gets 157 hours of civil night with the moon below the horizon across the month. The best of them for moon-free darkness inside 10 PM to 2 AM: Sep 1 (4 h), Sep 2 (4 h), Sep 3 (4 h).

1
3% lit
9.5 h
2
9% lit
9.6 h
3
16% lit
9.3 h
4
25% lit
8.9 h
5
35% lit
8.4 h
6
45% lit
7.8 h
7
55% lit
7.1 h
8
64% lit
6.2 h
9
73% lit
5.2 h
10
81% lit
4.1 h
11
88% lit
3.0 h
12
93% lit
1.9 h
13
97% lit
0.8 h
14
99% lit
0.0 h
15
100% lit
0.0 h
Full
16
98% lit
0.0 h
17
95% lit
0.0 h
18
89% lit
0.1 h
19
82% lit
0.6 h
20
73% lit
1.2 h
21
63% lit
1.9 h
22
52% lit
3.0 h
23
41% lit
4.2 h
24
30% lit
5.5 h
25
20% lit
7.0 h
26
11% lit
8.4 h
27
5% lit
9.8 h
28
1% lit
11.1 h
29
0% lit
11.1 h
New
30
2% lit
11.2 h

Each cell is the night that starts on that date. Hours are civil-dark time with the moon below the horizon; the green rule scales with them. Green outline: new moon. Bone outline: full moon.

Activity in past Septembers

Great Falls needs Kp 6 for the average oval edge to sit overhead. 47 of the 900 September days from 1996 to 2025 (5%) reached it in the GFZ Potsdam record, one of the three most active months for that level; storm days with Kp 5 or more made up 14%. Great Falls sits at 54.3 degrees geomagnetic. The lowest NOAA storm level whose typical viewing latitude reaches it is G3 (Kp = 7). By SWPC's rule the average oval edge is overhead only at Kp 6.

September across United States

Great Falls ranks 6th in United States for civil night on September 15, behind Buffalo at 10.5 hours. Coming from August (8.5 hours at Great Falls), September adds darkness, and the nights keep growing into October (12.0 hours on the 15th). The September equinox falls on September 22, 23:56 Great Falls time, inside the weeks SWPC links with larger storms.

Civil night on September 15 and September cloud, month-page towns in United States
Town	Civil night	Cloud
Buffalo	10.5 h	57%
Portland, Maine	10.5 h	50%
Minneapolis	10.4 h	49%
Duluth	10.4 h	56%
Fargo	10.4 h	50%
Great Falls	10.3 h	52%
Seattle	10.3 h	53%
Juneau	9.8 h	81%
Anchorage	9.5 h	78%
Eagle River	9.5 h	78%
Fairbanks	9.2 h	72%

Darkness and moon, every night

Great Falls, September 2027: each night from local noon to the next noon
Night	Civil	Astronomical	Moon-free dark	Moon-free, 10 PM to 2 AM	Moon lit
Wed Sep 1	9 h 30 min	6 h 50 min	9 h 30 min	4 h	3%
Thu Sep 2	9 h 33 min	6 h 54 min	9 h 33 min	4 h	9%
Fri Sep 3	9 h 37 min	6 h 58 min	9 h 19 min	4 h	16%
Sat Sep 4	9 h 40 min	7 h 3 min	8 h 55 min	4 h	25%
Sun Sep 5	9 h 44 min	7 h 7 min	8 h 26 min	4 h	35%
Mon Sep 6	9 h 47 min	7 h 11 min	7 h 50 min	3 h 32 min	45%
Tue Sep 7	9 h 51 min	7 h 16 min	7 h 4 min	2 h 45 min	55%
Wed Sep 8	9 h 54 min	7 h 20 min	6 h 10 min	1 h 50 min	64%
Thu Sep 9	9 h 58 min	7 h 24 min	5 h 9 min	47 min	73%
Fri Sep 10	10 h 1 min	7 h 28 min	4 h 4 min	0 min	81%
Sat Sep 11	10 h 5 min	7 h 32 min	2 h 58 min	0 min	88%
Sun Sep 12	10 h 8 min	7 h 36 min	1 h 53 min	0 min	93%
Mon Sep 13	10 h 12 min	7 h 41 min	47 min	0 min	97%
Tue Sep 14	10 h 15 min	7 h 45 min	0 min	0 min	99%
Wed Sep 15	10 h 19 min	7 h 49 min	0 min	0 min	100%
Thu Sep 16	10 h 22 min	7 h 53 min	0 min	0 min	98%
Fri Sep 17	10 h 26 min	7 h 57 min	0 min	0 min	95%
Sat Sep 18	10 h 29 min	8 h 1 min	7 min	0 min	89%
Sun Sep 19	10 h 33 min	8 h 5 min	35 min	0 min	82%
Mon Sep 20	10 h 36 min	8 h 8 min	1 h 11 min	0 min	73%
Tue Sep 21	10 h 40 min	8 h 12 min	1 h 57 min	0 min	63%
Wed Sep 22	10 h 43 min	8 h 16 min	2 h 57 min	54 min	52%
Thu Sep 23	10 h 47 min	8 h 20 min	4 h 10 min	2 h 5 min	41%
Fri Sep 24	10 h 50 min	8 h 24 min	5 h 32 min	3 h 24 min	30%
Sat Sep 25	10 h 53 min	8 h 28 min	6 h 57 min	4 h	20%
Sun Sep 26	10 h 57 min	8 h 31 min	8 h 24 min	4 h	11%
Mon Sep 27	11 h	8 h 35 min	9 h 49 min	4 h	5%
Tue Sep 28	11 h 4 min	8 h 39 min	11 h 4 min	4 h	1%
Wed Sep 29	11 h 7 min	8 h 42 min	11 h 7 min	4 h	0%
Thu Sep 30	11 h 10 min	8 h 46 min	11 h 10 min	4 h	2%

Moon lit: SunCalc illuminated fraction at the middle of the 10 PM to 2 AM window.

Questions

Can you see the northern lights in Great Falls in September?: On dark, clear nights with enough activity. 30 of 30 nights at Great Falls have civil darkness, and 5% of past September days reached Kp 6.
How dark is Great Falls in September?: Great Falls averages 10.3 hours of civil night a night in September 2027, from 9 h 30 min on the 1st to 11 h 10 min on the last night.
How cloudy is Great Falls in September?: NASA POWER's 2001 to 2020 climatology gives Great Falls's 1 degree cell 52% mean cloud in September, against 62% for the whole year. It is an all-hours average, not a forecast.
What is the best night in September 2027 for aurora in Great Falls?: By darkness and moon alone, Sep 1, with 4 h of moon-free darkness inside 10 PM to 2 AM at Great Falls. Activity on that date will only be known hours ahead.
Is September or October better in Great Falls?: By civil night times the clear share, September scores 4.9 hours at Great Falls and October 4.4. September comes out ahead.

Sources and verification

SunCalchttps://github.com/mourner/suncalc
NASA POWER climatologyhttps://power.larc.nasa.gov/docs/services/api/temporal/climatology/
GFZ Potsdam Kp indexhttps://kp.gfz.de/en/data
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.