Copper Harbor
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Copper Harbor clear sky chart from 2005-11-06 to 2026-03-18.

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	11% 11% 5% 11% 7% 6% 4 3 4 6% 32%	130919
	Night	11% 12% 5% 14% 9% 6% 5% 4 5% 5% 24%	130841
Transparency	Day	35% 37% 8% 14% 6% 1	134336
	Night	40% 36% 8% 11% 5%	135510
Seeing	Day	19% 12% 31% 25% 11% 3	17658
	Night	20% 11% 28% 28% 12% 1	27608

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% 16% 10% 27% 17% 8% 4 2 2 2 6%	4691
	11th to 20th	Day	7% 17% 9% 20% 20% 12% 8% 2 2 1 3	4679
	21st to end	Day	5% 13% 10% 18% 12% 15% 8% 5% 4 3 7%	5156
February	1st to 10th	Day	8% 14% 6% 16% 15% 12% 7% 4 4 4 11%	1733
	11th to 20th	Day	7% 13% 6% 16% 11% 10% 6% 4 4 4 18%	1950
	21st to end	Day	9% 15% 5% 16% 12% 8% 4 4 6% 5% 16%	1730
March	1st to 10th	Day	10% 10% 5% 14% 7% 7% 5% 3 5% 6% 29%	4578
	11th to 20th	Day	9% 12% 7% 16% 9% 6% 4 3 5% 5% 25%	4734
	21st to end	Day	20% 13% 6% 15% 6% 4 4 2 3 4 23%	5969
April	1st to 10th	Day	13% 14% 4 10% 5% 6% 4 4 6% 6% 30%	2400
	11th to 20th	Day	17% 13% 4 9% 6% 5% 4 3 4 6% 29%	2413
	21st to end	Day	16% 11% 4 6% 5% 5% 4 4 5% 6% 34%	2600
May	1st to 10th	Day	13% 8% 2 5% 3 3 3 3 4 6% 49%	6273
	11th to 20th	Day	10% 9% 3 7% 4 4 4 3 5% 7% 42%	6319
	21st to end	Day	9% 8% 3 8% 5% 5% 5% 4 5% 7% 41%	6949
June	1st to 10th	Day	11% 6% 3 6% 4 4 5% 4 5% 7% 45%	2885
	11th to 20th	Day	11% 6% 3 7% 5% 5% 5% 4 6% 7% 41%	3000
	21st to end	Day	13% 6% 3 6% 5% 4 4 3 6% 7% 42%	2894
July	1st to 10th	Day	11% 5% 2 5% 3 4 4 3 5% 7% 51%	6359
	11th to 20th	Day	10% 5% 2 5% 3 3 4 3 5% 8% 51%	6360
	21st to end	Day	8% 5% 2 5% 3 3 3 3 4 9% 54%	6926
August	1st to 10th	Day	8% 6% 2 7% 5% 5% 5% 4 6% 8% 43%	4563
	11th to 20th	Day	12% 6% 2 4 3 3 3 3 5% 7% 51%	4337
	21st to end	Day	11% 7% 3 6% 3 4 4 4 5% 8% 46%	4331
September	1st to 10th	Day	12% 8% 3 6% 4 4 4 3 5% 7% 43%	2400
	11th to 20th	Day	13% 7% 3 6% 5% 4 5% 4 6% 8% 38%	2290
	21st to end	Day	16% 10% 4 7% 5% 4 4 3 5% 7% 35%	2200
October	1st to 10th	Day	15% 12% 3 9% 5% 5% 4 3 5% 6% 33%	2095
	11th to 20th	Day	19% 18% 4 11% 5% 5% 5% 3 4 5% 20%	1889
	21st to end	Day	20% 19% 6% 12% 5% 5% 3 3 4 4 18%	1980
November	1st to 10th	Day	16% 22% 5% 12% 5% 5% 4 3 4 5% 19%	1852
	11th to 20th	Day	15% 20% 8% 17% 8% 5% 5% 3 3 3 13%	1769
	21st to end	Day	14% 21% 9% 22% 9% 6% 3 2 2 3 8%	1627
December	1st to 10th	Day	9% 20% 13% 29% 11% 5% 4 3 3 2 3	2350
	11th to 20th	Day	10% 17% 10% 25% 15% 8% 5% 2 2 2 4	2691
	21st to end	Day	17% 17% 8% 21% 9% 5% 5% 3 3 3 8%	3947
January	1st to 10th	Night	7% 16% 9% 26% 14% 9% 4 2 2 2 8%	8233
	11th to 20th	Night	6% 17% 10% 23% 18% 9% 8% 3 2 2 3	8236
	21st to end	Night	5% 10% 7% 17% 12% 14% 12% 5% 5% 4 8%	9058
February	1st to 10th	Night	8% 15% 5% 14% 14% 12% 6% 4 3 4 14%	2882
	11th to 20th	Night	9% 12% 5% 15% 12% 11% 7% 5% 5% 5% 16%	2555
	21st to end	Night	11% 15% 5% 11% 9% 7% 5% 3 6% 6% 22%	2076
March	1st to 10th	Night	8% 9% 3 11% 7% 7% 6% 5% 7% 8% 29%	5482
	11th to 20th	Night	8% 12% 5% 14% 7% 7% 6% 4 5% 7% 25%	5421
	21st to end	Night	20% 11% 5% 13% 8% 5% 5% 3 5% 5% 21%	4962
April	1st to 10th	Night	14% 11% 4 10% 7% 7% 6% 4 5% 5% 27%	2000
	11th to 20th	Night	19% 10% 4 7% 5% 4 5% 3 4 6% 33%	1934
	21st to end	Night	18% 10% 4 8% 4 4 4 4 5% 6% 35%	1800
May	1st to 10th	Night	13% 7% 3 6% 4 4 4 3 5% 7% 46%	3671
	11th to 20th	Night	11% 7% 3 8% 5% 4 4 5% 6% 7% 39%	3596
	21st to end	Night	11% 9% 3 7% 5% 5% 5% 4 6% 7% 37%	3949
June	1st to 10th	Night	13% 6% 2 8% 6% 5% 4 3 7% 8% 38%	1506
	11th to 20th	Night	11% 7% 3 7% 5% 5% 6% 5% 8% 9% 34%	1400
	21st to end	Night	14% 7% 2 6% 5% 6% 4 4 6% 7% 39%	1497
July	1st to 10th	Night	8% 6% 3 6% 4 5% 5% 4 6% 9% 44%	3620
	11th to 20th	Night	12% 5% 2 5% 3 3 4 4 6% 8% 48%	3620
	21st to end	Night	7% 4 2 6% 4 4 5% 5% 6% 9% 47%	3948
August	1st to 10th	Night	10% 5% 3 6% 5% 5% 4 4 6% 10% 43%	2597
	11th to 20th	Night	11% 5% 2 5% 3 4 4 3 5% 6% 51%	2717
	21st to end	Night	13% 6% 2 7% 5% 4 4 3 5% 6% 45%	3552
September	1st to 10th	Night	11% 7% 2 6% 5% 4 4 3 6% 6% 46%	2000
	11th to 20th	Night	10% 7% 3 7% 6% 5% 5% 4 6% 8% 40%	2090
	21st to end	Night	16% 9% 3 8% 5% 3 4 3 5% 7% 38%	2200
October	1st to 10th	Night	16% 9% 3 8% 4 5% 4 3 5% 6% 36%	2302
	11th to 20th	Night	15% 15% 4 10% 5% 5% 5% 4 6% 6% 26%	2510
	21st to end	Night	18% 16% 5% 11% 7% 5% 4 3 4 4 23%	2860
November	1st to 10th	Night	15% 20% 6% 10% 5% 5% 5% 3 5% 6% 21%	2666
	11th to 20th	Night	15% 19% 7% 16% 7% 4 4 3 4 4 16%	2849
	21st to end	Night	14% 19% 9% 19% 8% 5% 4 3 4 4 12%	2989
December	1st to 10th	Night	11% 17% 9% 28% 12% 6% 4 2 3 2 6%	5060
	11th to 20th	Night	9% 19% 8% 20% 15% 8% 4 3 3 3 9%	5736
	21st to end	Night	15% 19% 8% 19% 10% 5% 4 2 3 3 12%	7267

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	70% 21% 7% 2	4692
	11th to 20th	Day	75% 21% 2 2	4710
	21st to end	Day	61% 23% 8% 6% 2	5156
February	1st to 10th	Day	31% 59% 5% 3 1	1733
	11th to 20th	Day	28% 58% 4 8% 2	1959
	21st to end	Day	26% 58% 3 7% 5%	1730
March	1st to 10th	Day	38% 28% 10% 14% 8% 1	4580
	11th to 20th	Day	47% 28% 10% 9% 6%	4734
	21st to end	Day	51% 28% 4 11% 7%	5969
April	1st to 10th	Day	23% 61% 4 7% 4	2400
	11th to 20th	Day	22% 62% 3 9% 3 1	2413
	21st to end	Day	20% 61% 3 11% 4 1	2600
May	1st to 10th	Day	25% 28% 7% 22% 15% 2	6282
	11th to 20th	Day	30% 29% 8% 20% 9% 4	6323
	21st to end	Day	31% 29% 9% 17% 12% 1	6949
June	1st to 10th	Day	15% 62% 6% 9% 7% 1	2885
	11th to 20th	Day	18% 63% 5% 9% 4 1	3000
	21st to end	Day	17% 61% 5% 12% 4	2894
July	1st to 10th	Day	26% 27% 13% 25% 8%	6291
	11th to 20th	Day	24% 27% 15% 27% 7%	6285
	21st to end	Day	21% 26% 12% 31% 10%	6728
August	1st to 10th	Day	27% 36% 12% 18% 6%	4548
	11th to 20th	Day	23% 36% 16% 21% 4	4379
	21st to end	Day	24% 37% 12% 18% 8%	4330
September	1st to 10th	Day	18% 58% 7% 12% 4	2400
	11th to 20th	Day	18% 58% 10% 11% 3	2290
	21st to end	Day	18% 57% 9% 11% 4	2200
October	1st to 10th	Day	22% 37% 8% 26% 6%	3407
	11th to 20th	Day	37% 36% 7% 11% 10%	3053
	21st to end	Day	39% 37% 9% 11% 3	3180
November	1st to 10th	Day	34% 54% 5% 5% 2	1852
	11th to 20th	Day	34% 54% 5% 6% 1	1769
	21st to end	Day	41% 54% 3 2 1	1627
December	1st to 10th	Day	56% 36% 5% 3	2350
	11th to 20th	Day	64% 31% 3 1 1	2691
	21st to end	Day	58% 26% 5% 11% 1	3947
January	1st to 10th	Night	68% 21% 6% 4 1	8233
	11th to 20th	Night	73% 21% 3 2	8280
	21st to end	Night	58% 24% 11% 6% 1	9058
February	1st to 10th	Night	30% 59% 5% 5% 1	2882
	11th to 20th	Night	28% 57% 5% 8% 2	2556
	21st to end	Night	23% 59% 5% 9% 3	2076
March	1st to 10th	Night	34% 29% 13% 13% 9% 1	5493
	11th to 20th	Night	45% 28% 10% 12% 5%	5422
	21st to end	Night	52% 29% 3 12% 4 1	4962
April	1st to 10th	Night	23% 61% 4 7% 5%	2000
	11th to 20th	Night	20% 62% 3 10% 4 1	1934
	21st to end	Night	20% 61% 3 9% 7%	1800
May	1st to 10th	Night	24% 29% 9% 23% 12% 3	3678
	11th to 20th	Night	31% 29% 7% 16% 12% 5%	3597
	21st to end	Night	34% 30% 7% 15% 13% 1	3950
June	1st to 10th	Night	17% 61% 5% 9% 8%	1506
	11th to 20th	Night	20% 63% 6% 7% 4	1400
	21st to end	Night	18% 62% 5% 10% 4 1	1497
July	1st to 10th	Night	28% 27% 16% 19% 9% 1	3580
	11th to 20th	Night	24% 27% 15% 26% 7%	3572
	21st to end	Night	25% 27% 11% 23% 14% 1	3832
August	1st to 10th	Night	26% 36% 9% 21% 8% 1	2588
	11th to 20th	Night	24% 37% 12% 20% 6%	2741
	21st to end	Night	27% 38% 10% 18% 7%	3552
September	1st to 10th	Night	19% 58% 6% 13% 3	2000
	11th to 20th	Night	19% 58% 8% 13% 3	2090
	21st to end	Night	20% 57% 8% 12% 3 1	2200
October	1st to 10th	Night	21% 36% 7% 26% 10%	3770
	11th to 20th	Night	28% 36% 8% 19% 9%	4085
	21st to end	Night	37% 36% 12% 11% 3	4610
November	1st to 10th	Night	33% 53% 6% 7% 1	2666
	11th to 20th	Night	32% 54% 5% 5% 3	2849
	21st to end	Night	39% 55% 4 3	2989
December	1st to 10th	Night	55% 37% 5% 2	5060
	11th to 20th	Night	58% 31% 5% 5%	5735
	21st to end	Night	55% 26% 6% 9% 3	7267

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	18% 14% 41% 21% 5% 1	330
	11th to 20th	Day	18% 17% 38% 20% 6% 1	330
	21st to end	Day	20% 15% 45% 17% 2	363
February	1st to 10th	Day	19% 24% 38% 15% 3 1	330
	11th to 20th	Day	16% 26% 34% 17% 7%	330
	21st to end	Day	21% 28% 37% 13% 1	273
March	1st to 10th	Day	16% 22% 41% 17% 3 1	330
	11th to 20th	Day	17% 20% 37% 20% 6% 1	353
	21st to end	Day	24% 16% 30% 21% 7% 2	550
April	1st to 10th	Day	23% 12% 33% 24% 7% 1	500
	11th to 20th	Day	29% 13% 26% 23% 7% 2	492
	21st to end	Day	27% 11% 30% 21% 9% 3	500
May	1st to 10th	Day	17% 13% 34% 25% 9% 3	667
	11th to 20th	Day	17% 10% 36% 23% 10% 3	725
	21st to end	Day	18% 8% 31% 29% 13% 2	789
June	1st to 10th	Day	15% 9% 32% 31% 12% 2	730
	11th to 20th	Day	17% 9% 32% 28% 12% 3	730
	21st to end	Day	17% 9% 27% 29% 14% 3	730
July	1st to 10th	Day	9% 5% 30% 35% 17% 4	721
	11th to 20th	Day	11% 8% 30% 29% 17% 5%	718
	21st to end	Day	9% 7% 29% 35% 15% 6%	794
August	1st to 10th	Day	13% 8% 27% 31% 18% 4	726
	11th to 20th	Day	14% 7% 20% 30% 21% 6%	621
	21st to end	Day	18% 11% 21% 28% 19% 4	583
September	1st to 10th	Day	18% 7% 26% 26% 17% 5%	530
	11th to 20th	Day	15% 8% 27% 31% 17% 2	530
	21st to end	Day	22% 11% 25% 27% 13% 3	530
October	1st to 10th	Day	23% 13% 27% 24% 10% 3	425
	11th to 20th	Day	30% 19% 32% 16% 3	330
	21st to end	Day	34% 17% 32% 13% 3 1	363
November	1st to 10th	Day	29% 18% 27% 19% 5% 1	330
	11th to 20th	Day	26% 22% 36% 11% 4 1	330
	21st to end	Day	27% 20% 36% 13% 4 1	302
December	1st to 10th	Day	22% 20% 36% 15% 6% 1	220
	11th to 20th	Day	28% 21% 34% 14% 3 1	220
	21st to end	Day	28% 15% 36% 17% 4	333
January	1st to 10th	Night	17% 16% 38% 23% 5% 1	1050
	11th to 20th	Night	18% 13% 39% 22% 6% 1	1050
	21st to end	Night	18% 14% 36% 25% 7%	1155
February	1st to 10th	Night	18% 17% 32% 24% 7% 1	1050
	11th to 20th	Night	18% 16% 33% 23% 9% 1	921
	21st to end	Night	20% 11% 36% 27% 6%	692
March	1st to 10th	Night	15% 12% 32% 31% 9% 1	840
	11th to 20th	Night	19% 14% 30% 28% 8% 1	824
	21st to end	Night	23% 9% 24% 28% 14% 2	660
April	1st to 10th	Night	24% 6% 30% 32% 8% 2	600
	11th to 20th	Night	23% 11% 25% 30% 10% 1	593
	21st to end	Night	23% 7% 24% 32% 13% 2	600
May	1st to 10th	Night	18% 8% 27% 32% 14% 1	600
	11th to 20th	Night	19% 5% 27% 34% 12% 3	591
	21st to end	Night	15% 8% 21% 36% 19% 2	650
June	1st to 10th	Night	17% 5% 20% 43% 14% 2	506
	11th to 20th	Night	14% 6% 25% 43% 13% 1	400
	21st to end	Night	17% 4 21% 37% 18% 2	497
July	1st to 10th	Night	9% 4 19% 42% 24% 2	585
	11th to 20th	Night	12% 6% 25% 32% 24% 2	582
	21st to end	Night	10% 3 21% 41% 23% 3	657
August	1st to 10th	Night	11% 3 20% 38% 26% 2	597
	11th to 20th	Night	12% 3 19% 32% 29% 5%	594
	21st to end	Night	15% 5% 22% 32% 23% 2	660
September	1st to 10th	Night	18% 8% 20% 30% 22% 4	600
	11th to 20th	Night	16% 5% 23% 34% 18% 2	690
	21st to end	Night	22% 7% 22% 31% 16% 1	800
October	1st to 10th	Night	24% 11% 25% 26% 13% 1	800
	11th to 20th	Night	28% 12% 27% 25% 8% 1	800
	21st to end	Night	33% 10% 23% 26% 7% 1	880
November	1st to 10th	Night	28% 13% 27% 26% 6%	820
	11th to 20th	Night	27% 16% 30% 19% 8% 1	959
	21st to end	Night	27% 17% 30% 22% 4	1050
December	1st to 10th	Night	23% 19% 34% 18% 6% 1	1050
	11th to 20th	Night	21% 18% 33% 19% 8% 1	1050
	21st to end	Night	28% 14% 33% 19% 5% 1	1155

Page generated at 2026-03-15 02:23:52. Version ac07

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