Blackhawk Observatory
Clear Sky Chart
History

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

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	13% 9% 3 8% 5% 4 5% 4 6% 7% 37%	131097
	Night	13% 10% 4 8% 5% 4 4 3 5% 6% 38%	130364
Transparency	Day	45% 5% 12% 24% 13% 1	134592
	Night	46% 5% 11% 23% 14% 2	134944
Seeing	Day	16% 10% 29% 25% 14% 5%	17596
	Night	18% 9% 31% 29% 11% 1	27598

By Month

Percentage Cloud Cover SE (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	11% 12% 7% 14% 6% 5% 6% 4 5% 6% 24%	4691
	11th to 20th	Day	7% 10% 6% 13% 5% 4 5% 6% 6% 6% 31%	4710
	21st to end	Day	10% 12% 6% 10% 5% 5% 5% 3 5% 6% 33%	5395
February	1st to 10th	Day	12% 13% 5% 10% 6% 6% 6% 4 5% 6% 28%	1890
	11th to 20th	Day	13% 10% 4 8% 6% 5% 4 3 4 6% 37%	2029
	21st to end	Day	13% 12% 4 11% 5% 4 4 3 5% 6% 32%	1730
March	1st to 10th	Day	15% 9% 3 6% 4 3 3 3 4 5% 44%	4500
	11th to 20th	Day	11% 6% 3 7% 5% 4 4 3 5% 9% 43%	4736
	21st to end	Day	21% 12% 6% 11% 6% 5% 4 3 4 5% 22%	5957
April	1st to 10th	Day	18% 12% 4 8% 5% 4 5% 4 4 6% 31%	2400
	11th to 20th	Day	16% 9% 3 6% 6% 4 5% 4 5% 6% 34%	2374
	21st to end	Day	19% 9% 3 8% 5% 5% 4 4 6% 6% 32%	2480
May	1st to 10th	Day	15% 8% 3 7% 5% 4 5% 4 5% 7% 37%	5901
	11th to 20th	Day	14% 6% 2 6% 4 5% 4 4 6% 8% 41%	6319
	21st to end	Day	15% 6% 3 6% 5% 4 5% 4 7% 9% 37%	6935
June	1st to 10th	Day	10% 6% 2 6% 5% 4 5% 4 6% 8% 45%	2800
	11th to 20th	Day	10% 6% 2 5% 4 4 5% 4 7% 9% 44%	2800
	21st to end	Day	11% 6% 3 7% 4 5% 4 4 8% 10% 39%	2800
July	1st to 10th	Day	10% 7% 3 8% 5% 5% 5% 4 7% 9% 37%	6360
	11th to 20th	Day	8% 5% 2 6% 4 4 5% 4 6% 10% 44%	6360
	21st to end	Day	10% 6% 3 7% 5% 5% 5% 4 6% 9% 40%	6924
August	1st to 10th	Day	9% 6% 2 5% 4 5% 5% 5% 6% 9% 43%	4513
	11th to 20th	Day	11% 5% 2 5% 4 4 5% 4 7% 9% 44%	3855
	21st to end	Day	9% 5% 2 6% 4 4 5% 4 5% 8% 48%	4312
September	1st to 10th	Day	12% 5% 1 5% 4 4 4 3 5% 8% 48%	2400
	11th to 20th	Day	12% 4 2 4 3 3 4 3 5% 8% 52%	2308
	21st to end	Day	12% 6% 2 6% 5% 4 4 3 6% 7% 45%	2200
October	1st to 10th	Day	13% 6% 3 5% 4 4 3 3 5% 6% 48%	2089
	11th to 20th	Day	19% 8% 3 7% 3 4 4 3 6% 6% 36%	2000
	21st to end	Day	19% 10% 3 7% 4 4 4 2 4 5% 38%	1997
November	1st to 10th	Day	13% 8% 3 6% 4 3 3 4 5% 6% 44%	1852
	11th to 20th	Day	14% 13% 4 9% 5% 4 4 4 4 6% 31%	1890
	21st to end	Day	16% 14% 4 9% 5% 3 2 2 4 6% 34%	1742
December	1st to 10th	Day	9% 9% 4 8% 5% 5% 5% 4 5% 6% 41%	2690
	11th to 20th	Day	12% 14% 5% 12% 6% 5% 5% 4 5% 7% 24%	3088
	21st to end	Day	21% 21% 8% 13% 5% 4 4 2 3 5% 12%	4070
January	1st to 10th	Night	10% 13% 7% 15% 6% 4 3 3 4 4 31%	8232
	11th to 20th	Night	10% 13% 6% 15% 7% 7% 6% 5% 6% 5% 22%	8280
	21st to end	Night	12% 13% 6% 9% 5% 4 4 4 4 7% 32%	8804
February	1st to 10th	Night	13% 14% 4 9% 5% 5% 4 3 4 5% 33%	2730
	11th to 20th	Night	13% 10% 4 7% 5% 5% 4 4 5% 6% 38%	2475
	21st to end	Night	16% 12% 4 8% 5% 3 3 3 5% 6% 36%	2076
March	1st to 10th	Night	16% 9% 3 7% 4 4 3 3 4 5% 43%	5398
	11th to 20th	Night	8% 7% 2 6% 4 5% 4 3 5% 7% 48%	5266
	21st to end	Night	22% 11% 5% 9% 5% 4 3 3 4 5% 28%	4970
April	1st to 10th	Night	17% 10% 3 6% 4 4 5% 4 4 6% 38%	2000
	11th to 20th	Night	18% 10% 3 6% 5% 4 3 3 4 6% 39%	1975
	21st to end	Night	18% 8% 3 6% 4 4 4 3 6% 7% 37%	1920
May	1st to 10th	Night	18% 7% 2 6% 5% 5% 3 4 5% 7% 38%	3993
	11th to 20th	Night	11% 5% 2 6% 4 4 5% 4 6% 8% 44%	3596
	21st to end	Night	15% 8% 2 7% 5% 4 4 3 6% 7% 39%	3942
June	1st to 10th	Night	9% 5% 2 5% 5% 5% 5% 4 7% 7% 48%	1600
	11th to 20th	Night	10% 5% 2 5% 4 5% 5% 4 5% 10% 45%	1600
	21st to end	Night	9% 7% 3 6% 6% 4 5% 4 5% 9% 44%	1600
July	1st to 10th	Night	12% 5% 3 6% 4 4 5% 4 6% 8% 42%	3618
	11th to 20th	Night	7% 5% 3 5% 4 4 4 4 6% 8% 51%	3617
	21st to end	Night	9% 6% 2 6% 4 4 4 5% 6% 8% 46%	3947
August	1st to 10th	Night	7% 4 2 5% 4 4 4 4 6% 9% 51%	2644
	11th to 20th	Night	9% 5% 2 5% 4 4 4 4 6% 8% 49%	3192
	21st to end	Night	8% 4 1 5% 4 4 4 4 5% 9% 52%	3586
September	1st to 10th	Night	9% 5% 1 6% 4 3 4 3 5% 6% 54%	2000
	11th to 20th	Night	11% 4 1 5% 3 4 3 2 5% 7% 55%	2071
	21st to end	Night	11% 5% 2 6% 4 4 4 3 5% 6% 49%	2200
October	1st to 10th	Night	11% 6% 2 6% 4 4 3 3 4 6% 51%	2309
	11th to 20th	Night	17% 8% 3 6% 4 3 3 3 4 6% 44%	2400
	21st to end	Night	19% 8% 2 6% 4 4 5% 3 5% 6% 38%	2841
November	1st to 10th	Night	13% 8% 2 6% 4 3 3 3 5% 6% 48%	2666
	11th to 20th	Night	14% 11% 3 8% 5% 4 4 3 5% 6% 37%	2730
	21st to end	Night	16% 12% 4 8% 5% 4 3 2 4 5% 36%	2877
December	1st to 10th	Night	10% 9% 3 8% 5% 4 5% 4 5% 7% 41%	4720
	11th to 20th	Night	16% 16% 5% 9% 5% 4 4 3 4 5% 29%	5339
	21st to end	Night	22% 19% 6% 10% 5% 4 5% 3 4 4 17%	7150

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	62% 3 14% 15% 6%	4692
	11th to 20th	Day	52% 3 11% 16% 15% 2	4710
	21st to end	Day	52% 4 13% 22% 9%	5395
February	1st to 10th	Day	57% 5% 12% 15% 10% 1	1890
	11th to 20th	Day	49% 5% 10% 25% 9% 2	2039
	21st to end	Day	54% 5% 9% 21% 10% 1	1730
March	1st to 10th	Day	42% 5% 11% 27% 11% 4	4500
	11th to 20th	Day	39% 5% 12% 32% 12% 1	4736
	21st to end	Day	63% 6% 11% 13% 7% 1	5959
April	1st to 10th	Day	55% 4 10% 21% 9%	2400
	11th to 20th	Day	49% 6% 9% 23% 11% 2	2374
	21st to end	Day	52% 5% 11% 18% 13% 1	2480
May	1st to 10th	Day	45% 6% 9% 22% 17% 1	5936
	11th to 20th	Day	41% 6% 10% 26% 16% 1	6323
	21st to end	Day	42% 5% 12% 26% 13% 3	6949
June	1st to 10th	Day	36% 6% 14% 28% 16% 1	2800
	11th to 20th	Day	34% 6% 14% 25% 18% 2	2800
	21st to end	Day	39% 8% 13% 24% 15% 2	2800
July	1st to 10th	Day	41% 6% 13% 28% 12% 1	6291
	11th to 20th	Day	35% 6% 14% 28% 16% 1	6285
	21st to end	Day	40% 7% 15% 21% 16% 2	6728
August	1st to 10th	Day	36% 6% 16% 29% 12% 1	4498
	11th to 20th	Day	34% 6% 13% 33% 13% 1	3891
	21st to end	Day	34% 5% 13% 29% 18% 2	4311
September	1st to 10th	Day	35% 6% 11% 28% 18% 2	2400
	11th to 20th	Day	30% 5% 11% 33% 19% 2	2308
	21st to end	Day	38% 5% 9% 29% 19% 1	2200
October	1st to 10th	Day	25% 3 7% 37% 27% 1	3401
	11th to 20th	Day	34% 4 11% 27% 23% 1	3240
	21st to end	Day	44% 4 7% 29% 13% 3	3197
November	1st to 10th	Day	40% 5% 14% 25% 15% 2	1852
	11th to 20th	Day	54% 3 11% 21% 9% 1	1890
	21st to end	Day	53% 4 13% 23% 8%	1742
December	1st to 10th	Day	44% 3 19% 26% 7% 1	2690
	11th to 20th	Day	60% 4 12% 17% 7% 1	3085
	21st to end	Day	76% 4 6% 9% 5%	4070
January	1st to 10th	Night	57% 4 10% 19% 9%	8233
	11th to 20th	Night	62% 4 9% 11% 13% 1	8280
	21st to end	Night	54% 3 12% 21% 10%	8804
February	1st to 10th	Night	55% 3 10% 18% 12% 1	2730
	11th to 20th	Night	48% 4 10% 24% 13% 1	2476
	21st to end	Night	50% 5% 11% 20% 11% 4	2076
March	1st to 10th	Night	46% 3 12% 23% 11% 6%	5398
	11th to 20th	Night	35% 5% 11% 33% 13% 3	5266
	21st to end	Night	60% 4 9% 17% 10% 1	4970
April	1st to 10th	Night	46% 5% 10% 23% 15% 1	2000
	11th to 20th	Night	48% 4 6% 21% 17% 3	1975
	21st to end	Night	45% 5% 12% 24% 12% 2	1920
May	1st to 10th	Night	45% 6% 12% 18% 18% 2	4020
	11th to 20th	Night	36% 8% 14% 24% 15% 4	3597
	21st to end	Night	43% 6% 11% 22% 16% 3	3950
June	1st to 10th	Night	32% 8% 14% 26% 19% 2	1600
	11th to 20th	Night	34% 7% 10% 24% 23% 2	1600
	21st to end	Night	37% 6% 14% 22% 19% 2	1600
July	1st to 10th	Night	41% 5% 13% 26% 13% 2	3580
	11th to 20th	Night	31% 6% 14% 29% 20% 1	3572
	21st to end	Night	35% 7% 14% 24% 18% 2	3832
August	1st to 10th	Night	29% 6% 14% 30% 19% 2	2635
	11th to 20th	Night	32% 6% 14% 31% 17%	3222
	21st to end	Night	29% 5% 12% 31% 18% 3	3586
September	1st to 10th	Night	31% 5% 10% 28% 24% 3	2000
	11th to 20th	Night	30% 4 10% 33% 21% 3	2071
	21st to end	Night	35% 5% 10% 30% 19% 2	2200
October	1st to 10th	Night	25% 3 6% 36% 28% 2	3777
	11th to 20th	Night	36% 3 9% 26% 25% 3	3899
	21st to end	Night	42% 4 9% 26% 17% 3	4591
November	1st to 10th	Night	39% 4 10% 26% 18% 3	2666
	11th to 20th	Night	49% 3 12% 23% 12% 1	2730
	21st to end	Night	52% 4 10% 23% 10% 1	2877
December	1st to 10th	Night	44% 4 14% 27% 10% 2	4720
	11th to 20th	Night	59% 3 12% 16% 10% 1	5341
	21st to end	Night	70% 5% 8% 11% 6%	7150

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% 33% 28% 7%	330
	11th to 20th	Day	17% 17% 43% 18% 3 1	330
	21st to end	Day	20% 14% 38% 23% 4 1	363
February	1st to 10th	Day	17% 17% 39% 20% 6%	330
	11th to 20th	Day	18% 21% 35% 21% 5%	330
	21st to end	Day	22% 18% 42% 16% 2	273
March	1st to 10th	Day	16% 20% 40% 19% 4	306
	11th to 20th	Day	23% 17% 37% 14% 7% 2	300
	21st to end	Day	27% 14% 32% 19% 6% 2	540
April	1st to 10th	Day	26% 14% 33% 23% 4	500
	11th to 20th	Day	23% 20% 26% 20% 9% 2	492
	21st to end	Day	24% 13% 30% 21% 9% 2	500
May	1st to 10th	Day	18% 15% 32% 22% 10% 3	516
	11th to 20th	Day	21% 12% 27% 24% 12% 3	725
	21st to end	Day	17% 10% 27% 24% 15% 6%	789
June	1st to 10th	Day	10% 7% 32% 28% 17% 5%	730
	11th to 20th	Day	13% 5% 32% 30% 15% 5%	730
	21st to end	Day	12% 7% 29% 32% 15% 5%	730
July	1st to 10th	Day	10% 3 19% 30% 26% 13%	721
	11th to 20th	Day	9% 4 24% 32% 20% 10%	718
	21st to end	Day	8% 2 24% 37% 21% 8%	794
August	1st to 10th	Day	9% 3 17% 33% 26% 12%	726
	11th to 20th	Day	11% 5% 18% 27% 22% 17%	525
	21st to end	Day	10% 5% 15% 30% 28% 11%	583
September	1st to 10th	Day	14% 7% 20% 28% 23% 8%	530
	11th to 20th	Day	10% 3 19% 29% 25% 14%	530
	21st to end	Day	14% 9% 27% 23% 18% 8%	530
October	1st to 10th	Day	16% 7% 29% 26% 19% 5%	419
	11th to 20th	Day	22% 18% 31% 21% 8% 1	330
	21st to end	Day	29% 12% 31% 22% 5% 1	363
November	1st to 10th	Day	17% 15% 38% 19% 9% 2	330
	11th to 20th	Day	22% 17% 38% 16% 7% 1	330
	21st to end	Day	25% 11% 41% 17% 5%	330
December	1st to 10th	Day	17% 17% 42% 16% 7% 1	330
	11th to 20th	Day	17% 14% 39% 21% 8%	330
	21st to end	Day	27% 18% 36% 17% 2	363
January	1st to 10th	Night	18% 13% 37% 25% 6%	1050
	11th to 20th	Night	20% 13% 39% 22% 5% 1	1050
	21st to end	Night	19% 12% 37% 27% 5%	1155
February	1st to 10th	Night	21% 10% 40% 24% 5%	1050
	11th to 20th	Night	18% 14% 37% 27% 5%	840
	21st to end	Night	23% 12% 40% 22% 3	692
March	1st to 10th	Night	21% 11% 38% 23% 6%	809
	11th to 20th	Night	16% 12% 37% 28% 8%	730
	21st to end	Night	23% 12% 32% 26% 7% 1	660
April	1st to 10th	Night	21% 14% 35% 25% 5%	600
	11th to 20th	Night	23% 13% 31% 27% 6%	593
	21st to end	Night	22% 8% 35% 27% 8% 1	600
May	1st to 10th	Night	19% 8% 35% 30% 8% 1	600
	11th to 20th	Night	17% 7% 31% 33% 12%	591
	21st to end	Night	16% 4 29% 34% 17%	650
June	1st to 10th	Night	10% 4 27% 44% 15% 1	600
	11th to 20th	Night	13% 3 32% 40% 11% 1	600
	21st to end	Night	9% 5% 29% 38% 19% 1	600
July	1st to 10th	Night	9% 1 17% 40% 30% 3	585
	11th to 20th	Night	8% 4 20% 41% 24% 2	582
	21st to end	Night	7% 3 19% 45% 24% 2	657
August	1st to 10th	Night	6% 2 14% 47% 27% 4	597
	11th to 20th	Night	10% 2 12% 41% 30% 5%	594
	21st to end	Night	9% 2 16% 42% 28% 3	660
September	1st to 10th	Night	9% 4 20% 36% 26% 5%	600
	11th to 20th	Night	12% 3 19% 34% 27% 4	671
	21st to end	Night	12% 8% 24% 33% 21% 3	800
October	1st to 10th	Night	15% 5% 29% 35% 16% 2	800
	11th to 20th	Night	22% 10% 31% 26% 10%	800
	21st to end	Night	23% 11% 33% 25% 7%	880
November	1st to 10th	Night	17% 11% 37% 25% 9% 1	820
	11th to 20th	Night	20% 16% 35% 23% 6%	840
	21st to end	Night	21% 13% 35% 24% 6%	987
December	1st to 10th	Night	21% 12% 39% 22% 5%	1050
	11th to 20th	Night	22% 13% 36% 23% 5% 1	1050
	21st to end	Night	30% 11% 35% 21% 3	1155

Page generated at 2026-02-28 00:40:23. Version ac07

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