Black Skies Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Black Skies Observatory clear sky chart from 2005-11-06 to 2024-11-05.

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	15% 11% 4 8% 5% 5% 4 3 5% 7% 33%	102071
	Night	15% 13% 5% 10% 5% 4 4 3 5% 6% 29%	96903
Transparency	Day	50% 7% 12% 20% 10% 1	105666
	Night	54% 7% 12% 18% 9% 1	101397
Seeing	Day	22% 17% 31% 19% 8% 3	14943
	Night	23% 15% 29% 23% 9% 1	23765

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	17% 17% 11% 12% 7% 4 4 2 3 5% 20%	2369
	11th to 20th	Day	10% 15% 8% 20% 9% 6% 6% 4 4 5% 14%	2693
	21st to end	Day	15% 22% 9% 14% 6% 4 4 3 3 4 16%	3124
February	1st to 10th	Day	12% 18% 6% 16% 9% 6% 4 3 3 4 18%	1710
	11th to 20th	Day	11% 16% 6% 12% 7% 5% 4 4 4 5% 25%	1663
	21st to end	Day	16% 16% 6% 11% 6% 5% 5% 4 4 5% 21%	1448
March	1st to 10th	Day	17% 13% 5% 8% 6% 5% 5% 3 4 5% 30%	3427
	11th to 20th	Day	11% 15% 7% 11% 6% 5% 5% 3 5% 6% 27%	3480
	21st to end	Day	14% 11% 4 8% 5% 3 4 3 6% 7% 36%	3815
April	1st to 10th	Day	19% 13% 3 8% 5% 4 5% 3 4 5% 31%	2242
	11th to 20th	Day	19% 11% 2 6% 5% 4 4 3 4 5% 36%	2365
	21st to end	Day	21% 10% 3 6% 4 4 3 3 4 6% 35%	2470
May	1st to 10th	Day	17% 9% 3 7% 4 4 4 4 6% 6% 36%	4130
	11th to 20th	Day	13% 8% 2 7% 4 4 5% 4 6% 8% 39%	4398
	21st to end	Day	14% 6% 2 5% 4 4 4 3 6% 7% 43%	4837
June	1st to 10th	Day	18% 7% 3 6% 4 4 4 4 5% 8% 37%	2660
	11th to 20th	Day	12% 6% 2 5% 5% 4 5% 4 6% 9% 41%	2836
	21st to end	Day	17% 6% 2 7% 4 4 4 4 6% 8% 39%	2850
July	1st to 10th	Day	11% 5% 2 7% 5% 5% 5% 4 7% 9% 39%	4760
	11th to 20th	Day	12% 6% 2 6% 4 4 4 4 6% 8% 46%	4509
	21st to end	Day	9% 5% 2 7% 4 4 4 3 6% 8% 47%	4570
August	1st to 10th	Day	13% 7% 2 7% 4 4 5% 4 5% 8% 39%	4104
	11th to 20th	Day	14% 8% 3 6% 4 4 4 4 6% 9% 39%	4044
	21st to end	Day	9% 4 2 6% 5% 5% 4 4 6% 9% 45%	4404
September	1st to 10th	Day	15% 7% 2 6% 5% 4 4 4 5% 7% 41%	2280
	11th to 20th	Day	13% 6% 3 6% 4 4 4 3 6% 9% 44%	2159
	21st to end	Day	15% 7% 3 8% 5% 5% 5% 4 4 6% 36%	2057
October	1st to 10th	Day	19% 9% 3 7% 5% 4 4 3 6% 7% 33%	1900
	11th to 20th	Day	23% 10% 4 10% 5% 5% 4 4 5% 6% 23%	1900
	21st to end	Day	27% 14% 4 9% 5% 5% 4 3 4 5% 19%	2090
November	1st to 10th	Day	17% 17% 4 9% 6% 4 4 3 5% 7% 25%	1583
	11th to 20th	Day	17% 16% 4 11% 6% 5% 7% 4 4 7% 18%	1520
	21st to end	Day	19% 22% 6% 10% 6% 6% 6% 3 5% 5% 12%	1520
December	1st to 10th	Day	16% 24% 6% 13% 7% 6% 4 4 4 4 10%	1520
	11th to 20th	Day	17% 19% 6% 12% 7% 5% 4 3 5% 5% 17%	1906
	21st to end	Day	24% 18% 6% 14% 8% 5% 4 3 3 3 12%	2728
January	1st to 10th	Night	15% 23% 10% 16% 6% 5% 4 3 4 4 10%	4533
	11th to 20th	Night	12% 15% 7% 17% 9% 7% 5% 4 5% 5% 13%	4266
	21st to end	Night	17% 18% 7% 13% 7% 5% 4 3 5% 5% 16%	4532
February	1st to 10th	Night	12% 20% 6% 15% 9% 5% 5% 3 4 4 16%	2470
	11th to 20th	Night	12% 17% 6% 13% 7% 5% 5% 3 4 5% 23%	2403
	21st to end	Night	16% 15% 5% 11% 7% 5% 4 3 5% 6% 24%	2003
March	1st to 10th	Night	15% 12% 4 9% 5% 4 5% 3 4 6% 31%	3413
	11th to 20th	Night	12% 16% 7% 9% 5% 4 4 3 4 5% 30%	3479
	21st to end	Night	13% 11% 6% 10% 5% 4 5% 4 6% 6% 30%	3817
April	1st to 10th	Night	21% 13% 4 8% 5% 3 4 3 4 5% 29%	1938
	11th to 20th	Night	19% 10% 2 7% 4 4 5% 3 5% 5% 35%	1781
	21st to end	Night	17% 10% 4 8% 5% 5% 4 3 6% 6% 32%	1710
May	1st to 10th	Night	15% 8% 3 6% 5% 5% 5% 4 5% 7% 38%	2811
	11th to 20th	Night	15% 8% 3 7% 5% 4 3 4 6% 7% 39%	2495
	21st to end	Night	11% 5% 2 5% 5% 4 4 4 5% 8% 46%	2738
June	1st to 10th	Night	15% 8% 2 7% 5% 4 5% 5% 5% 8% 36%	1520
	11th to 20th	Night	11% 5% 2 6% 4 4 5% 4 7% 10% 43%	1339
	21st to end	Night	15% 6% 2 7% 4 4 4 4 7% 8% 38%	1330
July	1st to 10th	Night	8% 5% 3 8% 5% 4 5% 3 6% 7% 46%	2210
	11th to 20th	Night	14% 5% 2 5% 3 3 4 3 5% 7% 49%	2450
	21st to end	Night	8% 6% 1 6% 4 4 4 3 6% 8% 47%	2980
August	1st to 10th	Night	10% 6% 3 6% 4 4 5% 3 6% 8% 46%	2825
	11th to 20th	Night	15% 7% 2 6% 4 4 4 3 4 7% 44%	2785
	21st to end	Night	15% 5% 2 6% 4 3 4 4 5% 8% 45%	3218
September	1st to 10th	Night	10% 6% 3 7% 5% 4 5% 3 6% 8% 45%	1900
	11th to 20th	Night	14% 5% 2 6% 4 4 4 4 5% 6% 46%	2019
	21st to end	Night	16% 8% 2 7% 5% 4 4 3 5% 7% 40%	2121
October	1st to 10th	Night	18% 9% 3 7% 5% 4 5% 4 4 5% 36%	2280
	11th to 20th	Night	23% 13% 3 8% 4 5% 4 4 5% 6% 25%	2280
	21st to end	Night	24% 13% 4 10% 5% 5% 4 3 5% 6% 21%	2508
November	1st to 10th	Night	18% 15% 3 9% 5% 5% 4 3 4 5% 28%	2592
	11th to 20th	Night	16% 19% 5% 11% 5% 5% 4 4 5% 5% 22%	2660
	21st to end	Night	21% 20% 6% 11% 6% 5% 4 4 4 6% 14%	2660
December	1st to 10th	Night	18% 24% 8% 13% 7% 4 4 3 4 4 10%	2660
	11th to 20th	Night	16% 20% 7% 14% 7% 5% 4 3 4 6% 15%	3288
	21st to end	Night	23% 22% 7% 16% 6% 5% 3 2 3 3 9%	4889

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	69% 6% 13% 8% 4	2370
	11th to 20th	Day	72% 6% 12% 6% 4	2693
	21st to end	Day	74% 5% 8% 8% 4 1	3124
February	1st to 10th	Day	71% 6% 8% 12% 3 1	1710
	11th to 20th	Day	61% 7% 10% 14% 7% 1	1672
	21st to end	Day	65% 6% 9% 14% 6%	1448
March	1st to 10th	Day	56% 7% 14% 13% 9%	3430
	11th to 20th	Day	58% 7% 11% 16% 6% 1	3480
	21st to end	Day	45% 9% 11% 21% 12% 2	3815
April	1st to 10th	Day	57% 5% 10% 18% 9% 1	2242
	11th to 20th	Day	51% 5% 9% 22% 13% 1	2365
	21st to end	Day	51% 6% 10% 19% 12% 3	2470
May	1st to 10th	Day	46% 8% 11% 21% 13% 1	4142
	11th to 20th	Day	42% 9% 14% 25% 10% 1	4402
	21st to end	Day	38% 7% 13% 21% 17% 3	4837
June	1st to 10th	Day	45% 7% 11% 23% 13%	2660
	11th to 20th	Day	38% 9% 13% 24% 16% 1	2836
	21st to end	Day	43% 7% 14% 23% 12% 1	2850
July	1st to 10th	Day	39% 8% 16% 23% 13% 1	4686
	11th to 20th	Day	35% 7% 14% 33% 10% 1	4431
	21st to end	Day	35% 7% 11% 28% 18% 1	4424
August	1st to 10th	Day	41% 8% 15% 24% 12% 1	4091
	11th to 20th	Day	41% 7% 17% 25% 9%	4083
	21st to end	Day	34% 7% 14% 34% 10% 1	4404
September	1st to 10th	Day	41% 7% 13% 27% 11% 1	2280
	11th to 20th	Day	37% 7% 15% 29% 12%	2159
	21st to end	Day	48% 7% 12% 24% 8%	2057
October	1st to 10th	Day	45% 6% 13% 27% 9% 1	3160
	11th to 20th	Day	50% 6% 15% 21% 8%	3140
	21st to end	Day	59% 8% 14% 16% 4	3428
November	1st to 10th	Day	59% 7% 12% 16% 6%	1583
	11th to 20th	Day	64% 8% 12% 11% 4	1520
	21st to end	Day	73% 7% 10% 9% 1	1520
December	1st to 10th	Day	75% 8% 9% 7% 2	1520
	11th to 20th	Day	68% 9% 9% 10% 4	1906
	21st to end	Day	79% 4 8% 7% 2	2728
January	1st to 10th	Night	78% 6% 7% 6% 2	4533
	11th to 20th	Night	70% 10% 9% 7% 3	4266
	21st to end	Night	70% 7% 12% 7% 4 1	4532
February	1st to 10th	Night	70% 7% 8% 12% 3	2470
	11th to 20th	Night	63% 6% 10% 15% 5%	2405
	21st to end	Night	62% 6% 10% 15% 6%	2003
March	1st to 10th	Night	52% 7% 13% 18% 9% 1	3423
	11th to 20th	Night	54% 6% 14% 17% 8% 1	3480
	21st to end	Night	50% 9% 10% 20% 10% 1	3817
April	1st to 10th	Night	56% 6% 8% 18% 10% 1	1938
	11th to 20th	Night	50% 5% 7% 21% 15% 2	1781
	21st to end	Night	52% 6% 9% 15% 16% 1	1710
May	1st to 10th	Night	43% 6% 14% 19% 16% 2	2818
	11th to 20th	Night	41% 8% 12% 23% 14% 2	2496
	21st to end	Night	35% 7% 13% 25% 18% 2	2739
June	1st to 10th	Night	45% 5% 12% 22% 15% 1	1520
	11th to 20th	Night	35% 6% 13% 26% 19% 1	1339
	21st to end	Night	43% 8% 13% 23% 13% 1	1330
July	1st to 10th	Night	36% 7% 13% 24% 17% 2	2175
	11th to 20th	Night	34% 5% 14% 33% 13% 1	2405
	21st to end	Night	33% 7% 12% 31% 17% 1	2885
August	1st to 10th	Night	36% 6% 15% 27% 16% 1	2814
	11th to 20th	Night	39% 7% 17% 23% 13% 1	2812
	21st to end	Night	36% 6% 11% 33% 12% 1	3218
September	1st to 10th	Night	36% 8% 16% 26% 13% 1	1900
	11th to 20th	Night	37% 6% 14% 27% 15%	2019
	21st to end	Night	44% 5% 14% 26% 10%	2121
October	1st to 10th	Night	38% 7% 12% 25% 17%	3800
	11th to 20th	Night	49% 6% 16% 20% 8%	3779
	21st to end	Night	52% 8% 15% 21% 5%	4120
November	1st to 10th	Night	56% 6% 12% 19% 7%	2592
	11th to 20th	Night	62% 8% 13% 11% 4 1	2660
	21st to end	Night	70% 8% 11% 10% 1	2660
December	1st to 10th	Night	76% 7% 9% 6% 2	2660
	11th to 20th	Night	71% 7% 11% 9% 2	3288
	21st to end	Night	81% 5% 6% 7% 1	4889

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	21% 31% 32% 12% 5%	180
	11th to 20th	Day	20% 30% 35% 10% 4	231
	21st to end	Day	29% 21% 32% 16% 2	297
February	1st to 10th	Day	27% 34% 28% 8% 3	270
	11th to 20th	Day	25% 28% 39% 7% 1	270
	21st to end	Day	38% 27% 32% 4	225
March	1st to 10th	Day	23% 23% 38% 13% 3	455
	11th to 20th	Day	22% 25% 34% 15% 4 1	460
	21st to end	Day	32% 21% 27% 15% 5%	506
April	1st to 10th	Day	29% 21% 32% 12% 5% 1	460
	11th to 20th	Day	36% 19% 22% 15% 6% 3	452
	21st to end	Day	30% 18% 33% 11% 6% 2	460
May	1st to 10th	Day	27% 13% 32% 18% 7% 3	463
	11th to 20th	Day	18% 14% 32% 22% 11% 3	487
	21st to end	Day	16% 17% 31% 23% 11% 3	529
June	1st to 10th	Day	21% 14% 31% 26% 6% 1	490
	11th to 20th	Day	16% 12% 33% 26% 11% 3	666
	21st to end	Day	21% 11% 36% 22% 8% 2	680
July	1st to 10th	Day	11% 10% 34% 28% 14% 3	671
	11th to 20th	Day	15% 12% 27% 29% 12% 5%	524
	21st to end	Day	12% 9% 34% 27% 13% 5%	532
August	1st to 10th	Day	14% 8% 28% 28% 15% 8%	486
	11th to 20th	Day	16% 6% 25% 29% 14% 9%	479
	21st to end	Day	13% 13% 30% 26% 13% 6%	539
September	1st to 10th	Day	18% 10% 27% 26% 14% 6%	490
	11th to 20th	Day	12% 9% 28% 21% 20% 10%	490
	21st to end	Day	18% 17% 25% 26% 11% 4	490
October	1st to 10th	Day	24% 18% 26% 17% 12% 3	490
	11th to 20th	Day	24% 23% 31% 15% 6%	490
	21st to end	Day	35% 19% 30% 12% 3	539
November	1st to 10th	Day	31% 18% 33% 16% 3	224
	11th to 20th	Day	29% 24% 33% 12% 1 1	180
	21st to end	Day	28% 22% 40% 8% 2	180
December	1st to 10th	Day	29% 29% 29% 10% 3	180
	11th to 20th	Day	29% 35% 23% 10% 2 1	180
	21st to end	Day	31% 33% 29% 7% 1	198
January	1st to 10th	Night	27% 25% 29% 15% 4	825
	11th to 20th	Night	32% 17% 30% 16% 4	760
	21st to end	Night	21% 15% 34% 23% 6% 1	836
February	1st to 10th	Night	24% 24% 32% 18% 2	760
	11th to 20th	Night	22% 20% 35% 20% 4	743
	21st to end	Night	27% 22% 28% 19% 4	628
March	1st to 10th	Night	22% 21% 34% 20% 3	760
	11th to 20th	Night	23% 20% 33% 19% 4 1	760
	21st to end	Night	22% 16% 32% 23% 7%	836
April	1st to 10th	Night	28% 16% 32% 20% 4	760
	11th to 20th	Night	26% 15% 29% 21% 9%	657
	21st to end	Night	22% 14% 31% 22% 10%	570
May	1st to 10th	Night	21% 9% 25% 32% 11% 1	561
	11th to 20th	Night	18% 10% 29% 31% 10%	374
	21st to end	Night	11% 7% 33% 33% 15%	411
June	1st to 10th	Night	16% 9% 31% 33% 10% 1	380
	11th to 20th	Night	12% 9% 30% 37% 12%	380
	21st to end	Night	16% 6% 29% 34% 15% 1	380
July	1st to 10th	Night	8% 6% 19% 48% 17% 1	370
	11th to 20th	Night	11% 8% 26% 33% 20% 1	368
	21st to end	Night	10% 5% 28% 37% 20% 1	556
August	1st to 10th	Night	10% 6% 21% 35% 25% 2	567
	11th to 20th	Night	11% 5% 21% 36% 25% 2	564
	21st to end	Night	18% 7% 24% 28% 23% 1	682
September	1st to 10th	Night	13% 8% 29% 28% 18% 4	760
	11th to 20th	Night	15% 7% 23% 31% 20% 4	760
	21st to end	Night	20% 10% 22% 30% 14% 3	760
October	1st to 10th	Night	22% 14% 29% 22% 12% 1	760
	11th to 20th	Night	31% 16% 26% 19% 8% 1	760
	21st to end	Night	30% 20% 29% 17% 3	836
November	1st to 10th	Night	29% 15% 29% 19% 6% 1	760
	11th to 20th	Night	27% 22% 29% 16% 5%	760
	21st to end	Night	33% 18% 30% 15% 3	760
December	1st to 10th	Night	34% 24% 28% 11% 3	760
	11th to 20th	Night	28% 23% 35% 11% 3 1	760
	21st to end	Night	33% 20% 30% 15% 2	841

Page generated at 2024-11-03 02:11:08. Version ac07

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