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 2024-11-12.

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% 8% 3 7% 5% 4 4 4 6% 7% 37%	101219
	Night	14% 9% 3 7% 5% 4 4 3 5% 7% 38%	97851
Transparency	Day	44% 6% 11% 23% 15% 2	104937
	Night	43% 5% 10% 22% 16% 3	102271
Seeing	Day	19% 12% 25% 22% 15% 7%	14882
	Night	19% 9% 25% 29% 16% 2	24845

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	16% 15% 7% 10% 6% 4 4 4 6% 6% 23%	2836
	11th to 20th	Day	17% 14% 5% 16% 6% 4 3 3 3 4 26%	2840
	21st to end	Day	28% 15% 7% 7% 3 4 3 2 4 5% 21%	3124
February	1st to 10th	Day	19% 12% 4 9% 5% 4 4 3 5% 5% 30%	1790
	11th to 20th	Day	18% 12% 5% 9% 5% 5% 4 3 5% 5% 31%	1848
	21st to end	Day	19% 12% 4 7% 4 4 4 3 4 6% 32%	1570
March	1st to 10th	Day	23% 13% 3 8% 4 4 3 3 4 5% 29%	3205
	11th to 20th	Day	13% 7% 3 5% 5% 4 4 4 6% 6% 43%	3480
	21st to end	Day	16% 8% 3 7% 5% 4 4 3 5% 8% 37%	3815
April	1st to 10th	Day	14% 10% 3 8% 4 4 4 4 5% 7% 37%	2184
	11th to 20th	Day	16% 7% 3 7% 4 4 4 4 5% 6% 39%	2325
	21st to end	Day	17% 8% 3 7% 5% 5% 5% 4 6% 6% 36%	2470
May	1st to 10th	Day	19% 10% 3 8% 5% 4 5% 4 6% 7% 29%	4082
	11th to 20th	Day	19% 8% 3 7% 5% 4 4 3 6% 7% 35%	4081
	21st to end	Day	10% 6% 2 8% 5% 5% 6% 5% 8% 10% 35%	4476
June	1st to 10th	Day	11% 5% 2 6% 5% 6% 5% 4 7% 7% 41%	2470
	11th to 20th	Day	8% 5% 3 7% 6% 5% 5% 5% 6% 9% 41%	2470
	21st to end	Day	8% 5% 3 7% 6% 6% 6% 4 7% 8% 40%	2548
July	1st to 10th	Day	11% 6% 3 6% 5% 4 5% 4 6% 10% 39%	4129
	11th to 20th	Day	7% 5% 2 6% 4 5% 6% 5% 7% 10% 43%	4120
	21st to end	Day	14% 10% 3 10% 6% 6% 5% 5% 6% 7% 27%	4466
August	1st to 10th	Day	8% 5% 2 6% 5% 5% 5% 5% 7% 11% 42%	4104
	11th to 20th	Day	8% 6% 3 7% 5% 5% 6% 5% 7% 11% 39%	4044
	21st to end	Day	6% 4 2 5% 3 4 4 4 6% 9% 54%	4255
September	1st to 10th	Day	13% 6% 2 5% 4 5% 4 3 5% 8% 46%	2126
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 3 5% 8% 49%	2090
	21st to end	Day	16% 6% 3 6% 4 3 3 3 5% 7% 45%	2090
October	1st to 10th	Day	12% 4 2 5% 4 3 4 3 6% 7% 51%	2090
	11th to 20th	Day	13% 7% 2 5% 4 3 4 4 5% 5% 49%	1907
	21st to end	Day	19% 9% 2 6% 3 3 4 2 4 7% 42%	2006
November	1st to 10th	Day	12% 6% 2 5% 4 3 3 3 4 6% 53%	1762
	11th to 20th	Day	14% 11% 4 6% 4 3 4 3 5% 5% 41%	1719
	21st to end	Day	18% 9% 4 7% 4 3 3 3 4 6% 39%	1710
December	1st to 10th	Day	22% 13% 4 6% 5% 5% 5% 3 4 5% 28%	1710
	11th to 20th	Day	16% 12% 4 8% 5% 5% 5% 3 4 5% 32%	2153
	21st to end	Day	18% 18% 5% 10% 5% 5% 5% 4 5% 6% 20%	3124
January	1st to 10th	Night	15% 15% 5% 12% 5% 5% 4 3 5% 6% 23%	4109
	11th to 20th	Night	18% 14% 6% 13% 6% 4 3 3 4 4 26%	4120
	21st to end	Night	24% 15% 5% 8% 5% 3 3 2 4 5% 26%	4532
February	1st to 10th	Night	22% 12% 5% 7% 5% 3 4 3 4 5% 29%	2385
	11th to 20th	Night	19% 11% 4 8% 6% 4 4 3 4 5% 31%	2218
	21st to end	Night	23% 14% 4 7% 5% 4 3 3 5% 4 29%	1884
March	1st to 10th	Night	19% 13% 3 7% 4 4 4 3 4 5% 33%	3647
	11th to 20th	Night	13% 8% 3 5% 4 4 4 4 5% 6% 45%	3480
	21st to end	Night	13% 8% 3 7% 6% 4 4 4 6% 7% 38%	3817
April	1st to 10th	Night	13% 7% 2 6% 5% 5% 5% 4 6% 8% 38%	2011
	11th to 20th	Night	14% 7% 2 6% 4 4 4 3 6% 7% 42%	1802
	21st to end	Night	16% 7% 2 7% 4 4 4 4 5% 8% 37%	1710
May	1st to 10th	Night	19% 11% 3 8% 5% 5% 5% 4 5% 7% 27%	2813
	11th to 20th	Night	18% 8% 2 6% 4 4 4 4 6% 9% 37%	2812
	21st to end	Night	9% 6% 2 6% 5% 5% 4 4 7% 10% 44%	3079
June	1st to 10th	Night	7% 5% 1 5% 4 5% 4 4 8% 9% 47%	1710
	11th to 20th	Night	5% 5% 2 6% 5% 4 5% 5% 6% 10% 47%	1710
	21st to end	Night	7% 5% 2 4 4 4 5% 5% 8% 10% 46%	1602
July	1st to 10th	Night	7% 6% 2 6% 4 4 5% 4 7% 10% 44%	2778
	11th to 20th	Night	6% 5% 2 7% 4 5% 5% 4 7% 9% 46%	2840
	21st to end	Night	13% 8% 3 9% 5% 4 5% 4 6% 8% 35%	3087
August	1st to 10th	Night	8% 6% 2 7% 4 4 5% 4 6% 11% 44%	2825
	11th to 20th	Night	8% 7% 2 7% 5% 4 5% 5% 6% 9% 43%	2785
	21st to end	Night	4 4 1 4 3 4 4 3 6% 9% 59%	3397
September	1st to 10th	Night	11% 5% 2 4 3 4 4 3 6% 8% 50%	2034
	11th to 20th	Night	9% 6% 2 5% 3 4 4 3 5% 8% 52%	2090
	21st to end	Night	13% 6% 2 6% 4 3 4 3 4 5% 48%	2090
October	1st to 10th	Night	9% 5% 2 5% 3 3 3 3 6% 7% 54%	2090
	11th to 20th	Night	13% 6% 2 6% 3 4 4 3 5% 8% 47%	2271
	21st to end	Night	17% 8% 2 5% 4 4 4 3 4 5% 43%	2589
November	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 5% 6% 49%	2536
	11th to 20th	Night	16% 8% 3 7% 4 4 4 3 4 6% 42%	2484
	21st to end	Night	18% 10% 3 6% 5% 4 4 3 4 6% 37%	2470
December	1st to 10th	Night	22% 12% 3 7% 4 4 4 4 4 6% 30%	2470
	11th to 20th	Night	17% 10% 4 9% 5% 4 4 3 4 6% 32%	3042
	21st to end	Night	18% 17% 5% 11% 6% 5% 5% 3 4 6% 19%	4532

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	60% 4 11% 16% 8% 1	2837
	11th to 20th	Day	65% 3 7% 19% 6% 1	2840
	21st to end	Day	67% 2 8% 18% 4 1	3124
February	1st to 10th	Day	57% 4 9% 18% 11% 1	1790
	11th to 20th	Day	56% 4 8% 20% 10% 2	1858
	21st to end	Day	54% 5% 8% 21% 11% 2	1570
March	1st to 10th	Day	57% 4 7% 18% 10% 3	3205
	11th to 20th	Day	40% 5% 12% 26% 16% 1	3480
	21st to end	Day	46% 5% 11% 25% 12% 1	3815
April	1st to 10th	Day	45% 6% 10% 24% 12% 3	2184
	11th to 20th	Day	45% 5% 8% 21% 16% 4	2325
	21st to end	Day	46% 7% 11% 23% 11% 2	2470
May	1st to 10th	Day	52% 7% 11% 20% 8% 1	4120
	11th to 20th	Day	48% 6% 11% 21% 12% 1	4085
	21st to end	Day	39% 9% 15% 20% 16% 2	4489
June	1st to 10th	Day	38% 8% 11% 25% 17% 1	2470
	11th to 20th	Day	37% 9% 13% 23% 17% 2	2470
	21st to end	Day	39% 8% 13% 22% 15% 3	2548
July	1st to 10th	Day	39% 8% 13% 22% 16% 1	4064
	11th to 20th	Day	33% 8% 12% 32% 14% 1	4051
	21st to end	Day	54% 7% 11% 15% 11% 2	4324
August	1st to 10th	Day	34% 8% 13% 34% 11% 1	4091
	11th to 20th	Day	36% 8% 11% 30% 14% 1	4083
	21st to end	Day	26% 7% 14% 31% 20% 3	4254
September	1st to 10th	Day	38% 6% 11% 20% 19% 6%	2126
	11th to 20th	Day	34% 6% 11% 24% 23% 3	2090
	21st to end	Day	39% 4 10% 25% 20% 2	2090
October	1st to 10th	Day	22% 4 8% 27% 37% 2	3480
	11th to 20th	Day	26% 4 10% 30% 29% 2	3147
	21st to end	Day	32% 3 10% 28% 23% 4	3282
November	1st to 10th	Day	35% 4 8% 28% 22% 4	1762
	11th to 20th	Day	44% 4 7% 23% 18% 4	1719
	21st to end	Day	47% 3 10% 21% 17% 2	1710
December	1st to 10th	Day	58% 5% 11% 17% 9% 1	1710
	11th to 20th	Day	54% 5% 8% 18% 14% 1	2150
	21st to end	Day	64% 5% 10% 14% 6% 1	3124
January	1st to 10th	Night	60% 5% 9% 15% 9% 1	4109
	11th to 20th	Night	62% 4 7% 18% 8% 1	4120
	21st to end	Night	62% 4 8% 18% 8% 1	4532
February	1st to 10th	Night	57% 5% 10% 19% 8% 1	2385
	11th to 20th	Night	55% 4 8% 20% 12% 1	2219
	21st to end	Night	58% 4 10% 15% 10% 3	1884
March	1st to 10th	Night	52% 5% 6% 23% 11% 1	3647
	11th to 20th	Night	38% 4 12% 28% 17% 1	3480
	21st to end	Night	44% 6% 9% 25% 15% 1	3817
April	1st to 10th	Night	41% 8% 10% 23% 16% 2	2011
	11th to 20th	Night	40% 4 11% 20% 18% 7%	1802
	21st to end	Night	45% 6% 12% 22% 13% 3	1710
May	1st to 10th	Night	55% 8% 9% 19% 8% 2	2840
	11th to 20th	Night	44% 7% 14% 24% 11% 1	2813
	21st to end	Night	33% 9% 13% 24% 17% 4	3087
June	1st to 10th	Night	31% 7% 16% 26% 16% 4	1710
	11th to 20th	Night	29% 9% 15% 24% 21% 3	1710
	21st to end	Night	30% 10% 16% 24% 17% 4	1602
July	1st to 10th	Night	34% 9% 13% 22% 17% 4	2733
	11th to 20th	Night	32% 7% 13% 28% 17% 2	2786
	21st to end	Night	46% 7% 12% 19% 13% 2	2988
August	1st to 10th	Night	34% 7% 14% 32% 13% 1	2814
	11th to 20th	Night	36% 8% 11% 26% 18% 1	2812
	21st to end	Night	20% 7% 14% 33% 22% 4	3397
September	1st to 10th	Night	33% 6% 14% 23% 18% 7%	2034
	11th to 20th	Night	32% 6% 10% 23% 26% 4	2090
	21st to end	Night	38% 5% 8% 21% 25% 3	2090
October	1st to 10th	Night	24% 3 8% 25% 37% 3	3480
	11th to 20th	Night	26% 4 8% 26% 33% 4	3770
	21st to end	Night	31% 3 8% 22% 28% 8%	4263
November	1st to 10th	Night	37% 4 9% 23% 23% 4	2536
	11th to 20th	Night	44% 4 8% 19% 20% 5%	2484
	21st to end	Night	48% 4 9% 23% 14% 2	2470
December	1st to 10th	Night	55% 5% 11% 18% 10% 1	2470
	11th to 20th	Night	53% 3 9% 22% 12% 1	3044
	21st to end	Night	66% 4 9% 14% 7%	4532

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	28% 20% 34% 13% 4	270
	11th to 20th	Day	34% 26% 29% 10% 1	270
	21st to end	Day	30% 24% 32% 11% 3	297
February	1st to 10th	Day	30% 22% 33% 13% 3	270
	11th to 20th	Day	32% 28% 29% 9% 1	270
	21st to end	Day	36% 19% 35% 9% 2	225
March	1st to 10th	Day	28% 25% 33% 12% 3	316
	11th to 20th	Day	24% 24% 31% 16% 4	460
	21st to end	Day	30% 18% 30% 15% 7%	506
April	1st to 10th	Day	25% 18% 27% 20% 8% 1	460
	11th to 20th	Day	22% 20% 25% 21% 8% 4	452
	21st to end	Day	23% 15% 34% 18% 8% 2	460
May	1st to 10th	Day	22% 18% 29% 19% 8% 4	463
	11th to 20th	Day	19% 10% 28% 24% 13% 6%	487
	21st to end	Day	14% 8% 25% 25% 19% 9%	529
June	1st to 10th	Day	12% 8% 21% 29% 22% 8%	490
	11th to 20th	Day	10% 6% 22% 29% 23% 10%	490
	21st to end	Day	11% 6% 25% 30% 18% 10%	568
July	1st to 10th	Day	16% 5% 15% 19% 23% 22%	494
	11th to 20th	Day	11% 5% 18% 27% 26% 14%	484
	21st to end	Day	14% 3 16% 28% 25% 14%	532
August	1st to 10th	Day	13% 4 16% 25% 27% 15%	486
	11th to 20th	Day	11% 5% 18% 34% 22% 11%	479
	21st to end	Day	9% 3 14% 29% 27% 18%	539
September	1st to 10th	Day	12% 4 14% 28% 28% 14%	490
	11th to 20th	Day	11% 3 14% 29% 28% 14%	490
	21st to end	Day	21% 8% 19% 23% 23% 6%	490
October	1st to 10th	Day	13% 6% 22% 29% 22% 7%	490
	11th to 20th	Day	15% 13% 31% 23% 14% 4	490
	21st to end	Day	22% 12% 33% 23% 9% 1	455
November	1st to 10th	Day	16% 14% 33% 26% 7% 3	300
	11th to 20th	Day	24% 19% 28% 21% 7% 1	273
	21st to end	Day	25% 14% 38% 17% 4 1	270
December	1st to 10th	Day	31% 18% 30% 13% 6%	270
	11th to 20th	Day	24% 17% 39% 15% 5%	270
	21st to end	Day	31% 20% 37% 11% 1	297
January	1st to 10th	Night	23% 19% 32% 22% 5%	760
	11th to 20th	Night	31% 16% 31% 19% 3	760
	21st to end	Night	25% 15% 33% 24% 2	836
February	1st to 10th	Night	30% 12% 33% 23% 3	760
	11th to 20th	Night	27% 16% 35% 21% 1	743
	21st to end	Night	32% 15% 34% 18% 1	628
March	1st to 10th	Night	24% 15% 31% 24% 6%	760
	11th to 20th	Night	24% 13% 32% 23% 7%	760
	21st to end	Night	21% 12% 33% 27% 7%	836
April	1st to 10th	Night	18% 15% 30% 30% 7%	681
	11th to 20th	Night	16% 13% 29% 28% 13% 1	563
	21st to end	Night	20% 7% 35% 29% 8% 1	570
May	1st to 10th	Night	20% 8% 29% 29% 12% 2	570
	11th to 20th	Night	18% 4 23% 35% 17% 2	561
	21st to end	Night	10% 3 22% 32% 27% 5%	617
June	1st to 10th	Night	9% 2 17% 43% 25% 4	570
	11th to 20th	Night	7% 2 16% 41% 30% 4	570
	21st to end	Night	8% 4 19% 39% 27% 3	570
July	1st to 10th	Night	11% 2 13% 31% 33% 9%	555
	11th to 20th	Night	5% 2 13% 39% 38% 4	552
	21st to end	Night	11% 3 11% 36% 35% 5%	624
August	1st to 10th	Night	10% 2 8% 37% 37% 7%	567
	11th to 20th	Night	9% 3 9% 38% 34% 7%	564
	21st to end	Night	8% 1 12% 33% 39% 8%	627
September	1st to 10th	Night	14% 2 12% 32% 35% 6%	704
	11th to 20th	Night	11% 3 13% 33% 34% 6%	760
	21st to end	Night	16% 4 19% 33% 26% 2	760
October	1st to 10th	Night	12% 5% 19% 34% 25% 4	760
	11th to 20th	Night	16% 7% 26% 33% 17% 2	760
	21st to end	Night	21% 10% 27% 28% 12% 1	836
November	1st to 10th	Night	16% 8% 26% 31% 17% 2	780
	11th to 20th	Night	21% 12% 29% 28% 10% 1	765
	21st to end	Night	24% 9% 28% 28% 9% 1	760
December	1st to 10th	Night	29% 13% 27% 26% 5%	760
	11th to 20th	Night	24% 11% 33% 26% 6% 1	760
	21st to end	Night	28% 14% 31% 23% 4	836

Page generated at 2024-11-08 22:45:39. Version ac07

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