Duluth
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Duluth clear sky chart from 2005-11-06 to 2025-10-04.

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	12% 9% 3 8% 5% 5% 4 4 5% 7% 37%	127619
	Night	12% 10% 4 9% 5% 5% 4 4 5% 6% 36%	122019
Transparency	Day	45% 7% 14% 23% 10% 1	131107
	Night	48% 6% 13% 22% 11% 1	126617
Seeing	Day	16% 7% 26% 32% 16% 4	17424
	Night	17% 7% 27% 34% 13% 1	27227

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	6% 12% 6% 12% 6% 5% 5% 3 4 5% 34%	3617
	11th to 20th	Day	5% 13% 6% 12% 9% 7% 6% 6% 9% 7% 21%	3589
	21st to end	Day	5% 14% 7% 16% 8% 6% 4 4 5% 6% 25%	4191
February	1st to 10th	Day	7% 14% 4 11% 6% 4 5% 4 6% 6% 33%	1800
	11th to 20th	Day	7% 12% 4 9% 5% 5% 5% 4 5% 5% 39%	1754
	21st to end	Day	8% 12% 3 10% 6% 5% 6% 3 5% 7% 36%	1571
March	1st to 10th	Day	11% 9% 3 7% 5% 5% 5% 4 5% 8% 40%	4630
	11th to 20th	Day	7% 11% 4 10% 5% 5% 4 4 5% 7% 36%	4980
	21st to end	Day	20% 11% 3 8% 5% 4 3 3 4 6% 33%	5662
April	1st to 10th	Day	16% 11% 4 9% 5% 6% 4 4 4 6% 32%	2400
	11th to 20th	Day	17% 10% 3 8% 6% 5% 5% 4 5% 6% 31%	2374
	21st to end	Day	21% 11% 3 7% 5% 4 4 3 5% 6% 33%	2620
May	1st to 10th	Day	16% 7% 2 5% 3 3 3 3 4 6% 48%	6348
	11th to 20th	Day	15% 8% 3 6% 4 4 4 3 5% 6% 41%	6319
	21st to end	Day	14% 9% 3 7% 4 4 5% 3 5% 7% 39%	6949
June	1st to 10th	Day	10% 6% 2 6% 5% 4 5% 4 7% 9% 42%	2800
	11th to 20th	Day	13% 7% 3 7% 5% 5% 5% 3 7% 7% 37%	2891
	21st to end	Day	13% 6% 2 6% 5% 5% 5% 5% 8% 9% 36%	3000
July	1st to 10th	Day	9% 5% 2 5% 4 5% 5% 5% 7% 11% 43%	6650
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 4 6% 10% 46%	6360
	21st to end	Day	9% 5% 2 5% 4 4 4 4 6% 10% 48%	6926
August	1st to 10th	Day	10% 6% 2 7% 4 5% 6% 5% 7% 8% 41%	4444
	11th to 20th	Day	13% 5% 2 5% 3 4 4 4 6% 9% 46%	4171
	21st to end	Day	14% 7% 2 7% 4 4 4 4 6% 8% 39%	4430
September	1st to 10th	Day	10% 6% 2 6% 3 5% 4 4 6% 9% 45%	2400
	11th to 20th	Day	12% 7% 3 6% 5% 4 5% 4 5% 7% 42%	2400
	21st to end	Day	16% 7% 3 6% 5% 4 4 3 5% 7% 41%	2103
October	1st to 10th	Day	17% 8% 2 7% 5% 3 5% 4 6% 6% 37%	1930
	11th to 20th	Day	14% 12% 3 8% 6% 5% 4 3 4 7% 33%	1900
	21st to end	Day	19% 11% 3 8% 6% 5% 4 3 5% 7% 30%	1967
November	1st to 10th	Day	13% 12% 3 8% 6% 5% 4 5% 6% 6% 32%	1647
	11th to 20th	Day	13% 15% 6% 11% 6% 5% 5% 4 5% 5% 25%	1600
	21st to end	Day	14% 12% 4 11% 6% 6% 4 5% 5% 6% 28%	1600
December	1st to 10th	Day	8% 14% 5% 18% 10% 6% 5% 4 4 6% 20%	2610
	11th to 20th	Day	10% 17% 6% 11% 7% 5% 5% 4 5% 4 27%	3004
	21st to end	Day	22% 19% 6% 15% 6% 5% 4 2 3 3 15%	3982
January	1st to 10th	Night	8% 10% 5% 13% 7% 6% 4 3 5% 5% 33%	6348
	11th to 20th	Night	6% 10% 5% 14% 9% 7% 7% 6% 7% 7% 24%	6316
	21st to end	Night	6% 17% 7% 13% 6% 5% 4 3 5% 5% 29%	6784
February	1st to 10th	Night	7% 12% 5% 11% 6% 5% 5% 4 4 6% 35%	2600
	11th to 20th	Night	6% 10% 4 10% 6% 5% 4 4 5% 5% 40%	2533
	21st to end	Night	11% 12% 4 8% 5% 5% 4 3 5% 6% 36%	2058
March	1st to 10th	Night	9% 8% 2 6% 4 4 4 3 5% 6% 47%	5252
	11th to 20th	Night	11% 10% 5% 12% 6% 5% 5% 4 5% 6% 31%	4979
	21st to end	Night	21% 14% 5% 8% 4 3 3 3 4 4 31%	5268
April	1st to 10th	Night	17% 12% 3 7% 5% 4 4 3 6% 6% 34%	2000
	11th to 20th	Night	18% 10% 3 8% 4 4 4 4 5% 6% 34%	1975
	21st to end	Night	21% 11% 3 6% 4 3 4 3 5% 5% 34%	1778
May	1st to 10th	Night	15% 6% 2 5% 4 3 3 3 5% 7% 47%	3613
	11th to 20th	Night	15% 6% 2 6% 4 4 4 3 4 6% 46%	3596
	21st to end	Night	14% 8% 3 7% 5% 5% 5% 4 6% 7% 36%	3949
June	1st to 10th	Night	13% 7% 2 7% 5% 5% 5% 4 6% 8% 38%	1600
	11th to 20th	Night	13% 7% 3 6% 4 5% 4 5% 7% 7% 39%	1501
	21st to end	Night	13% 8% 1 5% 5% 5% 5% 4 7% 8% 39%	1400
July	1st to 10th	Night	7% 7% 3 5% 3 3 4 5% 6% 10% 46%	3312
	11th to 20th	Night	9% 5% 2 5% 3 3 4 4 6% 9% 52%	3620
	21st to end	Night	9% 7% 2 6% 4 4 4 4 6% 9% 45%	3948
August	1st to 10th	Night	10% 5% 3 6% 5% 4 4 3 6% 8% 47%	2713
	11th to 20th	Night	13% 7% 2 5% 3 4 3 3 5% 8% 46%	2888
	21st to end	Night	17% 8% 3 6% 3 4 4 4 6% 7% 39%	3458
September	1st to 10th	Night	10% 6% 2 5% 3 4 5% 3 5% 7% 50%	2000
	11th to 20th	Night	11% 6% 2 6% 3 4 4 4 4 8% 48%	2000
	21st to end	Night	15% 7% 3 6% 4 3 4 4 6% 7% 42%	2275
October	1st to 10th	Night	15% 7% 2 7% 5% 4 4 3 5% 7% 41%	2317
	11th to 20th	Night	14% 9% 3 7% 6% 4 5% 4 5% 7% 36%	2280
	21st to end	Night	19% 10% 3 8% 5% 5% 4 3 5% 6% 32%	2631
November	1st to 10th	Night	14% 9% 4 8% 5% 5% 5% 3 4 8% 35%	2648
	11th to 20th	Night	15% 11% 5% 9% 5% 5% 4 4 5% 7% 31%	2800
	21st to end	Night	12% 10% 3 8% 6% 5% 5% 4 6% 6% 35%	2800
December	1st to 10th	Night	9% 13% 6% 13% 8% 7% 6% 4 6% 6% 21%	4580
	11th to 20th	Night	11% 16% 5% 13% 6% 6% 5% 4 5% 5% 24%	5203
	21st to end	Night	24% 19% 5% 10% 5% 3 4 3 4 4 20%	6996

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	52% 5% 10% 19% 14%	3618
	11th to 20th	Day	59% 6% 10% 18% 7%	3620
	21st to end	Day	59% 7% 10% 18% 6%	4191
February	1st to 10th	Day	51% 5% 13% 19% 11% 1	1800
	11th to 20th	Day	45% 5% 14% 24% 10% 1	1763
	21st to end	Day	47% 7% 14% 19% 12% 1	1571
March	1st to 10th	Day	42% 5% 15% 23% 12% 3	4632
	11th to 20th	Day	47% 6% 15% 20% 12%	4980
	21st to end	Day	52% 6% 11% 21% 10% 1	5662
April	1st to 10th	Day	53% 5% 10% 18% 12% 2	2400
	11th to 20th	Day	53% 5% 13% 18% 10% 1	2374
	21st to end	Day	52% 6% 11% 21% 10% 1	2620
May	1st to 10th	Day	38% 5% 15% 25% 17% 1	6357
	11th to 20th	Day	43% 8% 13% 22% 9% 6%	6323
	21st to end	Day	44% 7% 9% 26% 14%	6949
June	1st to 10th	Day	36% 9% 16% 25% 13% 1	2800
	11th to 20th	Day	43% 8% 16% 20% 13%	2891
	21st to end	Day	40% 9% 16% 25% 9% 1	3000
July	1st to 10th	Day	33% 8% 21% 27% 10%	6580
	11th to 20th	Day	33% 6% 20% 31% 10% 1	6285
	21st to end	Day	30% 7% 19% 31% 13% 1	6728
August	1st to 10th	Day	38% 8% 17% 25% 11% 1	4429
	11th to 20th	Day	35% 7% 19% 31% 9%	4210
	21st to end	Day	40% 9% 16% 25% 10%	4429
September	1st to 10th	Day	36% 7% 19% 30% 9%	2400
	11th to 20th	Day	41% 8% 18% 27% 7%	2400
	21st to end	Day	43% 6% 14% 28% 8%	2103
October	1st to 10th	Day	36% 5% 16% 31% 11% 1	3190
	11th to 20th	Day	45% 5% 11% 26% 13%	3140
	21st to end	Day	51% 5% 17% 21% 6%	3218
November	1st to 10th	Day	52% 7% 12% 19% 9% 1	1647
	11th to 20th	Day	62% 5% 12% 14% 6%	1600
	21st to end	Day	56% 8% 13% 17% 5% 1	1600
December	1st to 10th	Day	65% 9% 10% 14% 2	2610
	11th to 20th	Day	59% 6% 9% 13% 13%	3005
	21st to end	Day	75% 5% 6% 9% 5%	3982
January	1st to 10th	Night	52% 6% 12% 16% 13%	6348
	11th to 20th	Night	57% 6% 15% 14% 8%	6360
	21st to end	Night	57% 6% 11% 18% 7%	6784
February	1st to 10th	Night	50% 5% 12% 21% 11% 1	2600
	11th to 20th	Night	46% 5% 12% 23% 12% 1	2534
	21st to end	Night	49% 5% 12% 23% 10% 1	2058
March	1st to 10th	Night	37% 5% 16% 20% 18% 5%	5263
	11th to 20th	Night	53% 5% 12% 20% 9%	4980
	21st to end	Night	58% 4 9% 16% 13% 1	5268
April	1st to 10th	Night	52% 5% 9% 18% 16% 1	2000
	11th to 20th	Night	50% 5% 11% 19% 13% 1	1975
	21st to end	Night	51% 4 9% 23% 12% 1	1778
May	1st to 10th	Night	37% 6% 13% 29% 14% 1	3620
	11th to 20th	Night	38% 6% 10% 24% 15% 6%	3597
	21st to end	Night	45% 8% 10% 26% 10% 1	3950
June	1st to 10th	Night	41% 8% 14% 24% 12% 2	1600
	11th to 20th	Night	40% 9% 12% 22% 16% 2	1501
	21st to end	Night	41% 8% 13% 25% 12% 1	1400
July	1st to 10th	Night	32% 8% 20% 26% 13% 1	3274
	11th to 20th	Night	28% 7% 17% 37% 10% 1	3572
	21st to end	Night	35% 7% 15% 28% 13% 1	3832
August	1st to 10th	Night	35% 7% 13% 27% 15% 2	2704
	11th to 20th	Night	36% 6% 15% 33% 8% 1	2915
	21st to end	Night	44% 6% 15% 25% 10% 1	3458
September	1st to 10th	Night	32% 8% 15% 33% 12% 1	2000
	11th to 20th	Night	34% 8% 19% 31% 9%	2000
	21st to end	Night	41% 7% 13% 29% 10%	2275
October	1st to 10th	Night	30% 4 10% 35% 19% 2	3837
	11th to 20th	Night	44% 5% 12% 24% 15%	3779
	21st to end	Night	53% 6% 12% 21% 8%	4330
November	1st to 10th	Night	49% 6% 14% 21% 8% 1	2648
	11th to 20th	Night	54% 6% 15% 17% 8%	2800
	21st to end	Night	50% 8% 17% 18% 6% 1	2800
December	1st to 10th	Night	61% 9% 14% 13% 2	4580
	11th to 20th	Night	62% 6% 10% 14% 8%	5201
	21st to end	Night	68% 6% 8% 13% 4	6996

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	11% 6% 26% 37% 20% 1	300
	11th to 20th	Day	17% 8% 22% 37% 16% 1	300
	21st to end	Day	13% 4 28% 35% 16% 3	330
February	1st to 10th	Day	13% 9% 34% 34% 8% 1	300
	11th to 20th	Day	11% 8% 39% 29% 10% 3	300
	21st to end	Day	16% 7% 34% 37% 7%	249
March	1st to 10th	Day	12% 10% 29% 32% 14% 2	300
	11th to 20th	Day	12% 7% 30% 38% 8% 4	300
	21st to end	Day	21% 7% 27% 29% 12% 3	412
April	1st to 10th	Day	21% 12% 26% 30% 10% 1	500
	11th to 20th	Day	24% 8% 22% 32% 12% 2	492
	21st to end	Day	24% 9% 26% 27% 13% 2	690
May	1st to 10th	Day	20% 10% 26% 31% 10% 3	703
	11th to 20th	Day	20% 9% 24% 29% 14% 5%	725
	21st to end	Day	18% 7% 23% 31% 19% 2	789
June	1st to 10th	Day	10% 6% 29% 34% 17% 3	730
	11th to 20th	Day	15% 8% 28% 34% 13% 2	730
	21st to end	Day	15% 6% 25% 30% 19% 4	730
July	1st to 10th	Day	9% 4 21% 32% 25% 9%	721
	11th to 20th	Day	11% 5% 24% 31% 21% 8%	718
	21st to end	Day	8% 3 22% 39% 22% 8%	794
August	1st to 10th	Day	12% 4 20% 32% 27% 6%	726
	11th to 20th	Day	15% 4 18% 31% 27% 6%	715
	21st to end	Day	15% 6% 20% 30% 23% 6%	662
September	1st to 10th	Day	12% 5% 22% 35% 23% 4	530
	11th to 20th	Day	16% 5% 21% 34% 20% 5%	530
	21st to end	Day	21% 8% 27% 24% 15% 5%	379
October	1st to 10th	Day	21% 9% 23% 29% 13% 5%	309
	11th to 20th	Day	18% 10% 36% 24% 11% 1	300
	21st to end	Day	24% 8% 35% 25% 6% 1	330
November	1st to 10th	Day	19% 13% 29% 30% 10%	300
	11th to 20th	Day	18% 13% 31% 28% 9% 1	300
	21st to end	Day	18% 10% 28% 33% 10%	300
December	1st to 10th	Day	15% 9% 34% 31% 9% 1	300
	11th to 20th	Day	19% 12% 32% 28% 9%	300
	21st to end	Day	28% 8% 26% 29% 8% 2	330
January	1st to 10th	Night	13% 8% 32% 36% 11% 1	1000
	11th to 20th	Night	15% 6% 34% 34% 10% 1	1000
	21st to end	Night	16% 7% 34% 33% 10% 1	1100
February	1st to 10th	Night	12% 8% 34% 36% 8% 1	1000
	11th to 20th	Night	12% 5% 38% 33% 10% 1	979
	21st to end	Night	17% 5% 31% 36% 10% 1	738
March	1st to 10th	Night	16% 7% 30% 36% 10% 1	755
	11th to 20th	Night	18% 5% 33% 34% 8% 1	600
	21st to end	Night	20% 7% 25% 33% 12% 3	660
April	1st to 10th	Night	24% 5% 29% 33% 9% 1	600
	11th to 20th	Night	22% 6% 23% 38% 10% 1	593
	21st to end	Night	27% 4 19% 34% 13% 2	600
May	1st to 10th	Night	18% 5% 30% 33% 12% 2	600
	11th to 20th	Night	19% 4 25% 35% 15% 2	591
	21st to end	Night	16% 5% 18% 39% 21% 1	650
June	1st to 10th	Night	15% 5% 21% 40% 18% 1	600
	11th to 20th	Night	16% 4 21% 46% 13% 1	600
	21st to end	Night	15% 5% 19% 40% 19% 2	600
July	1st to 10th	Night	7% 2 15% 40% 31% 4	585
	11th to 20th	Night	10% 2 22% 38% 26% 3	582
	21st to end	Night	7% 3 16% 45% 26% 3	657
August	1st to 10th	Night	10% 3 17% 43% 24% 3	597
	11th to 20th	Night	14% 4 16% 34% 29% 3	594
	21st to end	Night	15% 5% 18% 35% 24% 3	660
September	1st to 10th	Night	12% 4 19% 39% 24% 3	600
	11th to 20th	Night	14% 5% 18% 38% 23% 3	600
	21st to end	Night	19% 5% 25% 31% 18% 2	725
October	1st to 10th	Night	19% 7% 25% 35% 13% 1	773
	11th to 20th	Night	20% 10% 33% 28% 9% 1	760
	21st to end	Night	25% 8% 30% 30% 8%	836
November	1st to 10th	Night	19% 9% 31% 33% 8%	892
	11th to 20th	Night	20% 11% 31% 27% 9% 1	1000
	21st to end	Night	18% 10% 35% 31% 7%	1000
December	1st to 10th	Night	16% 11% 32% 33% 7% 1	1000
	11th to 20th	Night	20% 11% 34% 24% 9% 1	1000
	21st to end	Night	28% 8% 30% 26% 7% 1	1100

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