Sept-Īles
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sept-Īles clear sky chart from 2005-11-06 to 2025-10-19.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	19% 10% 3 7% 5% 4 4 3 5% 6% 33%	127218
	Night	18% 12% 4 8% 5% 4 4 3 5% 6% 33%	122723
Transparency	Day	52% 5% 11% 21% 10% 1	130606
	Night	53% 4 11% 20% 10% 1	127424
Seeing	Day	27% 15% 29% 20% 8% 2	17549
	Night	25% 12% 26% 26% 10% 1	25059

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	16% 18% 4 10% 6% 4 4 3 4 5% 28%	3157
	11th to 20th	Day	9% 13% 5% 13% 4 4 4 2 4 6% 35%	3134
	21st to end	Day	4 7% 3 7% 5% 4 6% 4 5% 6% 48%	3842
February	1st to 10th	Day	7% 13% 3 9% 7% 5% 4 3 5% 6% 36%	1739
	11th to 20th	Day	8% 16% 6% 10% 6% 4 5% 4 6% 6% 30%	1753
	21st to end	Day	8% 14% 5% 9% 5% 5% 5% 4 6% 7% 34%	1520
March	1st to 10th	Day	13% 15% 5% 9% 5% 5% 4 3 4 6% 31%	4649
	11th to 20th	Day	20% 17% 4 8% 5% 4 4 3 5% 5% 24%	4980
	21st to end	Day	21% 17% 4 8% 4 3 3 2 4 5% 29%	5466
April	1st to 10th	Day	19% 13% 4 9% 5% 6% 5% 3 5% 5% 27%	2452
	11th to 20th	Day	15% 12% 3 8% 5% 4 4 3 5% 6% 35%	2572
	21st to end	Day	24% 10% 3 6% 4 4 4 3 4 5% 33%	2600
May	1st to 10th	Day	23% 11% 3 7% 5% 4 4 3 5% 5% 30%	6197
	11th to 20th	Day	24% 7% 3 7% 4 4 4 3 5% 6% 33%	6628
	21st to end	Day	25% 9% 2 7% 5% 4 4 3 5% 6% 31%	7451
June	1st to 10th	Day	24% 8% 3 7% 4 4 5% 4 5% 5% 32%	3000
	11th to 20th	Day	21% 6% 2 6% 4 5% 5% 5% 7% 7% 33%	3000
	21st to end	Day	26% 7% 2 7% 4 4 4 4 6% 7% 29%	3000
July	1st to 10th	Day	17% 6% 2 6% 5% 5% 5% 4 6% 7% 36%	6820
	11th to 20th	Day	18% 8% 3 7% 5% 5% 6% 4 7% 8% 30%	6820
	21st to end	Day	14% 5% 2 5% 5% 4 5% 4 6% 8% 42%	7428
August	1st to 10th	Day	16% 5% 2 7% 4 5% 6% 5% 7% 8% 34%	4384
	11th to 20th	Day	22% 8% 3 8% 5% 5% 5% 4 6% 7% 27%	4175
	21st to end	Day	16% 5% 2 6% 4 4 4 4 5% 7% 42%	4675
September	1st to 10th	Day	20% 6% 2 7% 4 5% 4 3 5% 6% 37%	2465
	11th to 20th	Day	24% 6% 2 5% 3 3 4 3 4 6% 39%	2223
	21st to end	Day	23% 6% 2 6% 4 3 3 4 4 7% 38%	2200
October	1st to 10th	Day	21% 7% 2 6% 4 4 4 3 5% 6% 37%	2010
	11th to 20th	Day	26% 8% 2 6% 5% 4 5% 4 4 7% 29%	1974
	21st to end	Day	29% 10% 3 7% 4 3 4 3 4 4 29%	1865
November	1st to 10th	Day	23% 9% 2 7% 5% 4 5% 3 5% 5% 31%	1565
	11th to 20th	Day	22% 14% 3 6% 5% 4 5% 3 4 5% 28%	1600
	21st to end	Day	20% 16% 5% 7% 5% 4 4 3 3 5% 28%	1500
December	1st to 10th	Day	12% 16% 5% 10% 5% 5% 4 3 6% 5% 28%	2280
	11th to 20th	Day	18% 12% 4 8% 4 4 4 3 5% 5% 32%	2618
	21st to end	Day	14% 12% 4 7% 4 4 3 3 4 7% 39%	3476
January	1st to 10th	Night	17% 21% 5% 9% 5% 5% 4 4 4 4 23%	6808
	11th to 20th	Night	11% 16% 5% 11% 5% 6% 5% 4 6% 6% 25%	6771
	21st to end	Night	4 7% 4 8% 4 4 4 4 6% 6% 49%	7116
February	1st to 10th	Night	7% 12% 4 11% 6% 4 4 3 5% 7% 36%	2660
	11th to 20th	Night	10% 17% 5% 9% 6% 4 4 3 5% 5% 32%	2533
	21st to end	Night	10% 14% 5% 9% 5% 5% 5% 4 5% 4 33%	2108
March	1st to 10th	Night	16% 13% 4 8% 5% 5% 4 3 5% 5% 33%	5190
	11th to 20th	Night	21% 18% 5% 9% 5% 4 4 3 4 4 23%	4979
	21st to end	Night	20% 16% 5% 10% 4 4 4 3 4 4 26%	5466
April	1st to 10th	Night	21% 13% 3 8% 5% 3 4 3 4 5% 31%	1945
	11th to 20th	Night	20% 9% 3 5% 4 4 4 3 4 5% 38%	1778
	21st to end	Night	25% 10% 3 5% 4 4 2 3 3 4 37%	1800
May	1st to 10th	Night	27% 11% 3 7% 4 3 4 3 5% 5% 29%	3756
	11th to 20th	Night	24% 8% 2 7% 5% 5% 4 3 5% 5% 31%	3327
	21st to end	Night	25% 11% 3 7% 4 4 4 4 5% 7% 26%	3446
June	1st to 10th	Night	24% 6% 3 5% 4 3 4 4 6% 6% 35%	1400
	11th to 20th	Night	21% 7% 2 6% 4 5% 5% 5% 7% 6% 33%	1400
	21st to end	Night	26% 7% 2 7% 4 4 4 4 6% 5% 31%	1400
July	1st to 10th	Night	17% 7% 3 6% 4 4 4 4 7% 8% 38%	3160
	11th to 20th	Night	19% 7% 3 6% 5% 4 5% 4 7% 7% 33%	3160
	21st to end	Night	11% 5% 2 5% 4 4 5% 4 7% 8% 45%	3446
August	1st to 10th	Night	16% 6% 3 6% 5% 5% 5% 4 6% 8% 35%	2732
	11th to 20th	Night	19% 8% 3 8% 6% 4 5% 5% 6% 6% 29%	2872
	21st to end	Night	16% 6% 2 6% 4 4 5% 4 6% 8% 39%	3223
September	1st to 10th	Night	22% 6% 2 5% 4 3 4 4 4 6% 39%	1935
	11th to 20th	Night	22% 6% 2 4 4 4 4 3 5% 6% 41%	2175
	21st to end	Night	21% 6% 2 5% 3 3 4 3 5% 7% 41%	2200
October	1st to 10th	Night	23% 7% 2 7% 4 3 3 3 5% 6% 37%	2390
	11th to 20th	Night	27% 8% 3 8% 4 4 4 3 5% 5% 31%	2382
	21st to end	Night	27% 10% 3 6% 4 4 4 3 4 6% 30%	2732
November	1st to 10th	Night	24% 11% 2 6% 4 5% 5% 4 5% 6% 30%	2733
	11th to 20th	Night	23% 14% 4 6% 4 4 4 3 5% 5% 28%	2800
	21st to end	Night	22% 17% 5% 8% 4 4 4 3 4 5% 25%	2897
December	1st to 10th	Night	14% 16% 5% 9% 5% 4 3 3 4 5% 32%	4910
	11th to 20th	Night	21% 10% 4 7% 4 4 4 3 4 5% 33%	5591
	21st to end	Night	14% 13% 4 9% 5% 5% 5% 4 5% 5% 32%	7502

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	60% 3 6% 20% 10% 1	3158
	11th to 20th	Day	50% 4 15% 24% 6% 1	3160
	21st to end	Day	37% 3 12% 22% 23% 3	3842
February	1st to 10th	Day	49% 3 9% 22% 13% 3	1739
	11th to 20th	Day	54% 4 10% 20% 11% 1	1761
	21st to end	Day	50% 5% 7% 24% 12% 2	1520
March	1st to 10th	Day	56% 3 7% 18% 15% 2	4653
	11th to 20th	Day	63% 4 7% 15% 8% 3	4980
	21st to end	Day	60% 3 5% 17% 12% 3	5466
April	1st to 10th	Day	59% 5% 7% 14% 13% 2	2452
	11th to 20th	Day	50% 5% 8% 22% 13% 2	2572
	21st to end	Day	54% 4 11% 17% 13% 2	2600
May	1st to 10th	Day	56% 5% 8% 21% 9% 1	6209
	11th to 20th	Day	53% 5% 11% 22% 8% 1	6632
	21st to end	Day	55% 5% 8% 20% 11% 1	7451
June	1st to 10th	Day	54% 5% 9% 21% 10% 2	3000
	11th to 20th	Day	49% 7% 13% 18% 11% 2	3000
	21st to end	Day	54% 7% 10% 19% 9% 1	3000
July	1st to 10th	Day	47% 6% 11% 28% 8%	6745
	11th to 20th	Day	51% 7% 16% 19% 7%	6739
	21st to end	Day	39% 6% 14% 30% 10%	7216
August	1st to 10th	Day	46% 8% 20% 22% 5%	4369
	11th to 20th	Day	55% 8% 16% 16% 4	4214
	21st to end	Day	42% 6% 19% 25% 8%	4675
September	1st to 10th	Day	47% 6% 16% 22% 8%	2465
	11th to 20th	Day	46% 6% 15% 26% 7%	2223
	21st to end	Day	47% 7% 10% 25% 10%	2200
October	1st to 10th	Day	53% 5% 12% 23% 7% 1	3270
	11th to 20th	Day	50% 6% 13% 24% 6%	3202
	21st to end	Day	51% 4 13% 18% 13% 1	3054
November	1st to 10th	Day	54% 6% 13% 17% 9% 1	1565
	11th to 20th	Day	59% 6% 11% 17% 7%	1600
	21st to end	Day	61% 4 9% 17% 9%	1500
December	1st to 10th	Day	58% 4 11% 16% 10% 1	2280
	11th to 20th	Day	55% 4 9% 22% 9%	2618
	21st to end	Day	48% 3 12% 24% 10% 4	3476
January	1st to 10th	Night	65% 4 7% 15% 9% 1	6808
	11th to 20th	Night	59% 4 12% 20% 5% 1	6820
	21st to end	Night	35% 3 13% 24% 23% 1	7116
February	1st to 10th	Night	49% 3 8% 21% 16% 4	2660
	11th to 20th	Night	55% 3 9% 18% 13% 1	2536
	21st to end	Night	53% 3 7% 20% 14% 2	2108
March	1st to 10th	Night	54% 3 7% 13% 21% 2	5199
	11th to 20th	Night	66% 2 5% 14% 11% 1	4980
	21st to end	Night	63% 3 5% 13% 14% 3	5466
April	1st to 10th	Night	56% 3 8% 14% 15% 3	1945
	11th to 20th	Night	49% 4 8% 19% 17% 3	1778
	21st to end	Night	54% 2 9% 16% 17% 2	1800
May	1st to 10th	Night	57% 4 8% 18% 10% 3	3761
	11th to 20th	Night	55% 4 8% 20% 12% 1	3328
	21st to end	Night	57% 5% 9% 20% 9% 1	3447
June	1st to 10th	Night	48% 4 10% 23% 13% 1	1400
	11th to 20th	Night	49% 5% 12% 19% 13% 2	1400
	21st to end	Night	54% 5% 12% 19% 10% 1	1400
July	1st to 10th	Night	43% 6% 14% 26% 9% 2	3125
	11th to 20th	Night	48% 6% 16% 22% 8% 1	3118
	21st to end	Night	36% 5% 20% 27% 11% 1	3343
August	1st to 10th	Night	45% 6% 20% 24% 6%	2723
	11th to 20th	Night	53% 6% 18% 18% 4	2899
	21st to end	Night	41% 6% 14% 30% 9% 1	3223
September	1st to 10th	Night	45% 5% 13% 29% 8%	1935
	11th to 20th	Night	44% 4 17% 26% 9% 1	2175
	21st to end	Night	44% 5% 12% 28% 11%	2200
October	1st to 10th	Night	54% 4 9% 26% 6% 1	3910
	11th to 20th	Night	54% 4 14% 20% 7%	3893
	21st to end	Night	56% 5% 8% 24% 6% 1	4494
November	1st to 10th	Night	56% 6% 11% 18% 9% 1	2733
	11th to 20th	Night	59% 3 12% 18% 8%	2800
	21st to end	Night	63% 4 11% 16% 7%	2897
December	1st to 10th	Night	55% 3 12% 20% 9%	4910
	11th to 20th	Night	55% 4 10% 23% 8%	5592
	21st to end	Night	54% 3 12% 21% 7% 3	7502

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	25% 14% 34% 18% 9% 1	200
	11th to 20th	Day	22% 15% 32% 20% 10% 3	200
	21st to end	Day	20% 20% 29% 24% 6% 1	220
February	1st to 10th	Day	16% 15% 38% 20% 9% 1	339
	11th to 20th	Day	15% 22% 37% 20% 5% 1	396
	21st to end	Day	19% 17% 34% 21% 6% 2	349
March	1st to 10th	Day	21% 20% 34% 18% 6% 1	500
	11th to 20th	Day	26% 14% 32% 19% 8% 2	500
	21st to end	Day	29% 23% 26% 16% 5% 1	550
April	1st to 10th	Day	29% 25% 27% 14% 4 1	500
	11th to 20th	Day	25% 16% 36% 17% 4 1	492
	21st to end	Day	31% 16% 30% 15% 6% 1	500
May	1st to 10th	Day	29% 14% 30% 18% 8% 2	633
	11th to 20th	Day	27% 15% 29% 20% 8% 2	724
	21st to end	Day	31% 16% 27% 18% 7% 1	790
June	1st to 10th	Day	29% 12% 31% 20% 7% 1	730
	11th to 20th	Day	25% 16% 28% 18% 11% 2	730
	21st to end	Day	29% 12% 27% 22% 8% 2	730
July	1st to 10th	Day	24% 12% 31% 22% 9% 2	720
	11th to 20th	Day	24% 11% 28% 25% 10% 1	718
	21st to end	Day	26% 10% 26% 25% 11% 2	795
August	1st to 10th	Day	24% 10% 27% 27% 8% 3	558
	11th to 20th	Day	24% 11% 31% 24% 8% 2	519
	21st to end	Day	25% 10% 31% 22% 9% 3	583
September	1st to 10th	Day	27% 14% 30% 21% 5% 2	530
	11th to 20th	Day	28% 12% 32% 18% 7% 2	530
	21st to end	Day	26% 17% 29% 18% 8% 2	530
October	1st to 10th	Day	26% 20% 27% 18% 8% 1	425
	11th to 20th	Day	33% 14% 26% 15% 9% 3	414
	21st to end	Day	37% 13% 22% 16% 10% 2	429
November	1st to 10th	Day	33% 18% 26% 18% 5% 1	395
	11th to 20th	Day	31% 16% 27% 19% 7% 1	400
	21st to end	Day	32% 17% 28% 16% 7%	300
December	1st to 10th	Day	25% 11% 25% 28% 10% 2	200
	11th to 20th	Day	24% 25% 27% 18% 7%	200
	21st to end	Day	27% 17% 29% 20% 6%	220
January	1st to 10th	Night	23% 14% 30% 26% 7% 1	900
	11th to 20th	Night	25% 15% 28% 25% 7%	900
	21st to end	Night	15% 15% 34% 28% 7%	907
February	1st to 10th	Night	13% 16% 36% 26% 9%	800
	11th to 20th	Night	21% 17% 31% 20% 9% 1	783
	21st to end	Night	19% 15% 31% 25% 8% 3	660
March	1st to 10th	Night	20% 16% 31% 25% 8% 2	800
	11th to 20th	Night	24% 13% 28% 26% 9% 1	800
	21st to end	Night	23% 18% 26% 23% 9% 1	880
April	1st to 10th	Night	30% 15% 26% 23% 5% 1	660
	11th to 20th	Night	24% 13% 29% 22% 10% 1	594
	21st to end	Night	32% 10% 26% 24% 7% 2	600
May	1st to 10th	Night	30% 8% 23% 31% 7% 2	600
	11th to 20th	Night	27% 8% 23% 29% 11% 2	479
	21st to end	Night	27% 8% 24% 30% 9% 1	433
June	1st to 10th	Night	28% 6% 20% 36% 10% 1	400
	11th to 20th	Night	25% 7% 17% 34% 15% 2	400
	21st to end	Night	29% 7% 19% 32% 12% 2	400
July	1st to 10th	Night	23% 5% 22% 33% 16% 1	390
	11th to 20th	Night	23% 5% 16% 38% 16% 1	388
	21st to end	Night	20% 6% 16% 40% 17% 1	438
August	1st to 10th	Night	23% 4 14% 33% 22% 3	513
	11th to 20th	Night	24% 6% 15% 37% 18% 1	592
	21st to end	Night	22% 7% 20% 31% 19% 2	660
September	1st to 10th	Night	24% 8% 23% 33% 11% 1	600
	11th to 20th	Night	25% 8% 19% 32% 14% 2	775
	21st to end	Night	26% 9% 23% 31% 11% 1	800
October	1st to 10th	Night	29% 12% 24% 26% 10% 1	800
	11th to 20th	Night	32% 8% 22% 22% 14% 1	793
	21st to end	Night	35% 9% 22% 24% 9% 1	844
November	1st to 10th	Night	33% 13% 26% 22% 6% 1	880
	11th to 20th	Night	29% 14% 26% 24% 7% 1	900
	21st to end	Night	32% 13% 25% 23% 7% 1	900
December	1st to 10th	Night	27% 15% 26% 22% 9% 1	900
	11th to 20th	Night	25% 18% 33% 18% 6%	900
	21st to end	Night	23% 16% 31% 22% 8% 1	990

Page generated at 2025-10-16 01:50:27. Version ac07

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