Blackhole Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blackhole Observatory clear sky chart from 2005-11-06 to 2026-01-02.

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	14% 9% 3 8% 5% 4 5% 4 6% 7% 36%	127193
	Night	14% 9% 3 8% 5% 4 4 4 5% 7% 37%	124096
Transparency	Day	45% 6% 11% 23% 14% 2	130867
	Night	45% 5% 10% 22% 15% 2	128490
Seeing	Day	19% 12% 26% 22% 15% 7%	16369
	Night	19% 9% 25% 29% 16% 2	26359

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	15% 16% 8% 11% 5% 4 4 3 5% 5% 25%	4065
	11th to 20th	Day	14% 14% 5% 15% 5% 3 3 3 3 4 31%	4060
	21st to end	Day	22% 11% 6% 8% 5% 5% 4 3 4 5% 26%	4466
February	1st to 10th	Day	19% 12% 4 9% 5% 4 4 3 5% 5% 30%	1885
	11th to 20th	Day	18% 11% 5% 9% 5% 5% 4 3 5% 5% 30%	1948
	21st to end	Day	18% 12% 4 7% 4 4 4 3 4 6% 33%	1650
March	1st to 10th	Day	17% 13% 3 8% 4 4 3 3 4 5% 34%	4607
	11th to 20th	Day	12% 7% 3 6% 5% 4 5% 4 6% 7% 41%	4980
	21st to end	Day	16% 8% 3 7% 5% 5% 4 4 5% 8% 36%	5465
April	1st to 10th	Day	15% 10% 3 8% 4 4 4 4 5% 7% 36%	2299
	11th to 20th	Day	16% 7% 3 8% 4 4 4 4 5% 6% 39%	2449
	21st to end	Day	17% 8% 3 7% 5% 5% 5% 4 6% 6% 35%	2600
May	1st to 10th	Day	17% 10% 3 8% 5% 5% 6% 5% 7% 8% 26%	5862
	11th to 20th	Day	18% 9% 3 8% 5% 4 4 4 6% 7% 31%	5861
	21st to end	Day	14% 6% 2 7% 5% 5% 6% 5% 7% 10% 32%	6433
June	1st to 10th	Day	10% 5% 2 6% 5% 6% 5% 5% 7% 7% 42%	2600
	11th to 20th	Day	8% 5% 3 7% 6% 5% 5% 5% 6% 9% 40%	2600
	21st to end	Day	8% 5% 2 7% 6% 6% 6% 4 7% 9% 40%	2683
July	1st to 10th	Day	10% 6% 3 7% 5% 4 5% 4 6% 9% 42%	5923
	11th to 20th	Day	8% 7% 3 8% 5% 6% 7% 5% 8% 10% 34%	5900
	21st to end	Day	11% 8% 3 9% 6% 6% 6% 5% 7% 8% 28%	6424
August	1st to 10th	Day	7% 5% 2 6% 5% 5% 5% 5% 7% 11% 42%	4234
	11th to 20th	Day	7% 6% 3 7% 5% 5% 6% 5% 7% 11% 39%	4174
	21st to end	Day	6% 4 2 5% 3 4 4 4 6% 9% 55%	4389
September	1st to 10th	Day	13% 6% 2 5% 4 5% 4 3 5% 7% 47%	2238
	11th to 20th	Day	9% 5% 2 6% 4 4 4 3 5% 8% 50%	2200
	21st to end	Day	16% 6% 2 6% 4 3 3 3 5% 7% 45%	2200
October	1st to 10th	Day	11% 4 1 5% 4 3 4 3 6% 7% 52%	2200
	11th to 20th	Day	13% 6% 2 5% 4 3 3 3 5% 6% 50%	2007
	21st to end	Day	19% 9% 2 6% 4 3 4 2 4 6% 41%	2111
November	1st to 10th	Day	12% 6% 2 5% 4 3 4 3 4 6% 52%	1852
	11th to 20th	Day	15% 11% 4 7% 4 3 4 3 5% 5% 40%	1890
	21st to end	Day	18% 10% 4 7% 4 3 3 3 4 6% 37%	1890
December	1st to 10th	Day	19% 11% 4 7% 5% 4 4 2 4 4 37%	3020
	11th to 20th	Day	21% 12% 4 8% 5% 5% 5% 3 4 4 28%	3463
	21st to end	Day	19% 17% 5% 11% 5% 5% 4 4 4 5% 21%	4565
January	1st to 10th	Night	15% 14% 6% 16% 6% 5% 4 3 5% 5% 19%	5903
	11th to 20th	Night	17% 13% 6% 13% 6% 4 3 3 4 4 26%	5900
	21st to end	Night	20% 11% 4 8% 5% 4 4 3 4 5% 34%	6490
February	1st to 10th	Night	22% 12% 5% 7% 5% 3 4 3 4 5% 29%	2510
	11th to 20th	Night	20% 12% 4 9% 6% 4 4 3 4 5% 30%	2338
	21st to end	Night	22% 13% 4 7% 4 4 3 3 5% 4 30%	1980
March	1st to 10th	Night	17% 13% 3 7% 4 4 3 3 4 5% 37%	5245
	11th to 20th	Night	12% 7% 2 6% 4 4 4 4 6% 6% 45%	4980
	21st to end	Night	14% 9% 4 9% 6% 5% 5% 4 5% 7% 32%	5467
April	1st to 10th	Night	14% 7% 2 7% 5% 5% 5% 4 6% 8% 37%	2117
	11th to 20th	Night	13% 6% 2 6% 5% 4 4 4 6% 7% 41%	1898
	21st to end	Night	16% 7% 2 7% 4 4 5% 4 6% 8% 36%	1800
May	1st to 10th	Night	18% 11% 3 9% 5% 4 5% 4 5% 8% 28%	4033
	11th to 20th	Night	16% 8% 2 7% 4 5% 6% 5% 7% 10% 31%	4032
	21st to end	Night	12% 7% 2 7% 5% 5% 4 4 7% 9% 37%	4421
June	1st to 10th	Night	8% 5% 1 5% 4 5% 4 4 8% 9% 47%	1800
	11th to 20th	Night	6% 5% 2 6% 5% 4 4 5% 6% 10% 46%	1800
	21st to end	Night	7% 5% 2 4 4 4 5% 5% 8% 10% 46%	1686
July	1st to 10th	Night	7% 6% 3 7% 4 5% 5% 4 6% 9% 45%	3970
	11th to 20th	Night	7% 6% 3 8% 5% 6% 5% 5% 7% 11% 38%	4057
	21st to end	Night	11% 7% 3 8% 6% 5% 5% 4 6% 8% 36%	4427
August	1st to 10th	Night	7% 6% 2 7% 5% 4 5% 4 6% 11% 44%	2915
	11th to 20th	Night	8% 7% 2 7% 5% 4 4 5% 6% 9% 43%	2875
	21st to end	Night	4 4 1 4 3 4 4 3 6% 9% 60%	3504
September	1st to 10th	Night	11% 5% 2 4 4 4 4 3 5% 8% 51%	2141
	11th to 20th	Night	9% 5% 2 5% 3 4 4 3 5% 8% 52%	2200
	21st to end	Night	13% 6% 2 6% 4 3 4 3 4 5% 48%	2200
October	1st to 10th	Night	9% 5% 2 5% 3 3 3 3 6% 7% 55%	2200
	11th to 20th	Night	13% 5% 2 6% 3 4 4 3 5% 8% 48%	2391
	21st to end	Night	18% 8% 2 5% 4 4 4 3 4 5% 43%	2725
November	1st to 10th	Night	13% 7% 2 5% 4 3 3 3 5% 6% 49%	2666
	11th to 20th	Night	15% 9% 3 7% 4 4 4 3 4 6% 42%	2730
	21st to end	Night	18% 11% 3 6% 5% 4 4 3 4 6% 36%	2730
December	1st to 10th	Night	18% 12% 4 8% 4 3 4 3 4 5% 37%	4380
	11th to 20th	Night	19% 12% 5% 9% 5% 4 4 3 4 6% 31%	4952
	21st to end	Night	19% 17% 6% 11% 6% 5% 5% 3 4 6% 19%	6633

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	61% 4 9% 19% 7%	4066
	11th to 20th	Day	59% 3 6% 18% 12% 2	4060
	21st to end	Day	60% 3 8% 21% 6% 2	4466
February	1st to 10th	Day	57% 4 9% 18% 11% 1	1885
	11th to 20th	Day	57% 4 8% 19% 10% 2	1958
	21st to end	Day	53% 4 8% 21% 11% 2	1650
March	1st to 10th	Day	52% 4 6% 19% 14% 5%	4607
	11th to 20th	Day	40% 5% 10% 27% 16% 2	4980
	21st to end	Day	47% 5% 11% 24% 12% 1	5465
April	1st to 10th	Day	46% 6% 10% 23% 12% 3	2299
	11th to 20th	Day	45% 5% 8% 22% 16% 4	2449
	21st to end	Day	47% 7% 11% 22% 11% 2	2600
May	1st to 10th	Day	52% 8% 12% 18% 10% 1	5897
	11th to 20th	Day	51% 6% 11% 22% 10% 1	5865
	21st to end	Day	42% 8% 15% 21% 12% 1	6447
June	1st to 10th	Day	38% 8% 11% 24% 18% 1	2600
	11th to 20th	Day	38% 9% 13% 22% 17% 1	2600
	21st to end	Day	38% 8% 13% 22% 15% 3	2683
July	1st to 10th	Day	37% 8% 13% 24% 18% 1	5858
	11th to 20th	Day	40% 10% 13% 25% 10% 1	5831
	21st to end	Day	49% 9% 12% 17% 11% 1	6240
August	1st to 10th	Day	34% 8% 13% 34% 10% 1	4221
	11th to 20th	Day	36% 8% 11% 30% 14% 1	4213
	21st to end	Day	26% 7% 13% 31% 20% 3	4388
September	1st to 10th	Day	37% 6% 12% 20% 19% 6%	2238
	11th to 20th	Day	33% 6% 10% 24% 24% 3	2200
	21st to end	Day	39% 5% 10% 24% 20% 3	2200
October	1st to 10th	Day	22% 4 8% 28% 37% 2	3590
	11th to 20th	Day	26% 4 10% 30% 29% 2	3247
	21st to end	Day	33% 3 9% 28% 23% 4	3387
November	1st to 10th	Day	36% 4 8% 28% 21% 3	1852
	11th to 20th	Day	45% 4 7% 23% 18% 3	1890
	21st to end	Day	49% 3 10% 21% 16% 1	1890
December	1st to 10th	Day	51% 4 8% 21% 15%	3020
	11th to 20th	Day	59% 5% 7% 18% 10% 1	3460
	21st to end	Day	65% 5% 11% 14% 6% 1	4565
January	1st to 10th	Night	64% 5% 8% 14% 8% 1	5903
	11th to 20th	Night	60% 4 7% 16% 9% 4	5900
	21st to end	Night	54% 3 7% 22% 11% 2	6490
February	1st to 10th	Night	57% 5% 10% 18% 8% 1	2510
	11th to 20th	Night	57% 4 8% 19% 11% 1	2339
	21st to end	Night	57% 4 10% 16% 11% 3	1980
March	1st to 10th	Night	49% 4 5% 25% 13% 3	5245
	11th to 20th	Night	37% 4 10% 28% 20% 1	4980
	21st to end	Night	50% 5% 8% 22% 14% 1	5467
April	1st to 10th	Night	43% 8% 10% 22% 16% 2	2117
	11th to 20th	Night	40% 5% 11% 21% 17% 6%	1898
	21st to end	Night	45% 6% 12% 22% 13% 3	1800
May	1st to 10th	Night	53% 8% 10% 19% 8% 3	4060
	11th to 20th	Night	47% 8% 14% 21% 9% 1	4033
	21st to end	Night	40% 9% 15% 20% 13% 3	4429
June	1st to 10th	Night	31% 7% 15% 26% 17% 4	1800
	11th to 20th	Night	31% 9% 14% 23% 20% 3	1800
	21st to end	Night	29% 10% 16% 24% 17% 4	1686
July	1st to 10th	Night	35% 9% 12% 22% 20% 3	3927
	11th to 20th	Night	38% 9% 16% 23% 12% 1	4006
	21st to end	Night	43% 8% 13% 21% 14% 2	4297
August	1st to 10th	Night	34% 7% 14% 32% 13% 1	2904
	11th to 20th	Night	35% 8% 11% 27% 18% 1	2902
	21st to end	Night	20% 7% 13% 33% 22% 4	3504
September	1st to 10th	Night	32% 6% 13% 23% 19% 7%	2141
	11th to 20th	Night	31% 5% 10% 24% 26% 4	2200
	21st to end	Night	39% 5% 8% 21% 25% 3	2200
October	1st to 10th	Night	24% 3 8% 25% 37% 3	3590
	11th to 20th	Night	26% 4 8% 26% 33% 4	3890
	21st to end	Night	32% 3 8% 22% 28% 8%	4399
November	1st to 10th	Night	36% 4 9% 24% 23% 4	2666
	11th to 20th	Night	44% 4 8% 20% 20% 4	2730
	21st to end	Night	50% 4 8% 22% 14% 2	2730
December	1st to 10th	Night	51% 4 11% 21% 12% 1	4380
	11th to 20th	Night	55% 4 11% 20% 10% 1	4954
	21st to end	Night	66% 4 9% 14% 6%	6633

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	27% 21% 35% 14% 4	303
	11th to 20th	Day	32% 27% 30% 9% 1	300
	21st to end	Day	28% 23% 35% 10% 3	330
February	1st to 10th	Day	29% 22% 35% 12% 2	300
	11th to 20th	Day	33% 28% 30% 8% 1	300
	21st to end	Day	33% 19% 36% 9% 2	249
March	1st to 10th	Day	27% 24% 33% 13% 3	349
	11th to 20th	Day	23% 24% 31% 17% 4 1	500
	21st to end	Day	30% 19% 30% 15% 6%	550
April	1st to 10th	Day	25% 17% 28% 20% 8% 1	500
	11th to 20th	Day	21% 20% 27% 20% 8% 4	492
	21st to end	Day	23% 15% 34% 18% 8% 2	500
May	1st to 10th	Day	22% 17% 30% 19% 8% 4	503
	11th to 20th	Day	19% 11% 28% 24% 13% 6%	527
	21st to end	Day	15% 9% 25% 25% 18% 9%	573
June	1st to 10th	Day	12% 7% 22% 30% 21% 8%	530
	11th to 20th	Day	10% 6% 22% 30% 23% 9%	530
	21st to end	Day	10% 6% 24% 29% 20% 12%	613
July	1st to 10th	Day	15% 5% 15% 19% 24% 21%	535
	11th to 20th	Day	10% 6% 17% 28% 25% 14%	524
	21st to end	Day	14% 3 15% 27% 27% 15%	576
August	1st to 10th	Day	12% 4 16% 26% 27% 16%	526
	11th to 20th	Day	11% 4 17% 32% 24% 13%	519
	21st to end	Day	8% 3 15% 29% 27% 18%	583
September	1st to 10th	Day	12% 4 15% 27% 29% 14%	530
	11th to 20th	Day	10% 2 14% 28% 29% 17%	530
	21st to end	Day	20% 8% 18% 24% 24% 5%	530
October	1st to 10th	Day	13% 6% 22% 28% 24% 8%	530
	11th to 20th	Day	15% 12% 30% 25% 15% 4	530
	21st to end	Day	22% 13% 33% 22% 8% 1	494
November	1st to 10th	Day	17% 15% 34% 25% 7% 2	330
	11th to 20th	Day	25% 19% 29% 20% 6% 1	330
	21st to end	Day	25% 15% 37% 17% 4 1	330
December	1st to 10th	Day	29% 19% 34% 12% 6%	330
	11th to 20th	Day	25% 17% 39% 15% 4	330
	21st to end	Day	31% 18% 36% 13% 1	363
January	1st to 10th	Night	23% 18% 32% 22% 5%	805
	11th to 20th	Night	31% 16% 31% 19% 3	800
	21st to end	Night	25% 14% 35% 24% 2	880
February	1st to 10th	Night	30% 12% 34% 22% 2	800
	11th to 20th	Night	28% 16% 35% 20% 1	783
	21st to end	Night	32% 14% 34% 18% 1	660
March	1st to 10th	Night	24% 15% 31% 25% 5%	800
	11th to 20th	Night	24% 13% 32% 24% 7%	800
	21st to end	Night	21% 12% 33% 26% 7%	880
April	1st to 10th	Night	19% 15% 30% 29% 7%	717
	11th to 20th	Night	15% 13% 31% 28% 12% 1	593
	21st to end	Night	20% 7% 34% 30% 9% 1	600
May	1st to 10th	Night	20% 8% 29% 30% 11% 2	600
	11th to 20th	Night	19% 5% 23% 35% 17% 2	591
	21st to end	Night	11% 3 23% 32% 26% 5%	650
June	1st to 10th	Night	10% 2 17% 43% 25% 3	600
	11th to 20th	Night	8% 3 16% 41% 29% 4	600
	21st to end	Night	8% 3 18% 38% 29% 4	600
July	1st to 10th	Night	11% 2 13% 31% 34% 9%	585
	11th to 20th	Night	5% 2 13% 37% 38% 4	582
	21st to end	Night	11% 3 10% 35% 36% 5%	657
August	1st to 10th	Night	9% 2 8% 38% 37% 6%	597
	11th to 20th	Night	9% 3 9% 37% 36% 7%	594
	21st to end	Night	7% 1 12% 34% 38% 8%	660
September	1st to 10th	Night	13% 2 12% 33% 34% 5%	741
	11th to 20th	Night	11% 3 13% 33% 34% 7%	800
	21st to end	Night	16% 4 18% 34% 26% 2	800
October	1st to 10th	Night	12% 5% 18% 34% 27% 4	800
	11th to 20th	Night	16% 7% 25% 34% 17% 2	800
	21st to end	Night	22% 10% 28% 27% 12% 1	880
November	1st to 10th	Night	16% 9% 26% 31% 16% 2	820
	11th to 20th	Night	21% 12% 30% 27% 10% 1	840
	21st to end	Night	25% 9% 28% 27% 9% 1	840
December	1st to 10th	Night	28% 13% 28% 26% 5%	840
	11th to 20th	Night	25% 11% 33% 25% 5% 1	840
	21st to end	Night	28% 15% 31% 23% 4	924

Page generated at 2025-12-30 01:14:08. Version ac07

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