Blackstar II Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blackstar II Observatory clear sky chart from 2005-11-06 to 2024-11-28.

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	16% 9% 3 8% 5% 5% 5% 4 6% 7% 32%	101526
	Night	16% 10% 3 8% 5% 4 4 4 5% 6% 34%	98012
Transparency	Day	47% 7% 12% 20% 12% 1	105049
	Night	46% 8% 12% 21% 12% 1	102577
Seeing	Day	24% 20% 29% 17% 7% 2	14970
	Night	22% 20% 27% 22% 8% 1	23996

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	12% 11% 5% 9% 6% 6% 5% 4 6% 7% 30%	2517
	11th to 20th	Day	15% 12% 5% 12% 9% 6% 5% 4 3 5% 23%	2520
	21st to end	Day	19% 16% 9% 10% 5% 4 4 3 4 4 24%	2809
February	1st to 10th	Day	14% 11% 5% 9% 7% 5% 6% 4 6% 7% 26%	1732
	11th to 20th	Day	11% 9% 5% 12% 6% 5% 5% 4 7% 6% 30%	1848
	21st to end	Day	18% 12% 4 10% 7% 4 5% 3 4 4 30%	1570
March	1st to 10th	Day	21% 16% 4 9% 5% 3 3 2 4 4 28%	3116
	11th to 20th	Day	11% 10% 7% 10% 6% 5% 6% 4 6% 6% 30%	3268
	21st to end	Day	19% 8% 3 11% 5% 5% 4 3 4 5% 33%	4166
April	1st to 10th	Day	16% 13% 3 9% 5% 4 4 3 5% 5% 32%	2280
	11th to 20th	Day	16% 9% 3 6% 6% 5% 5% 4 5% 6% 35%	2254
	21st to end	Day	18% 9% 3 8% 5% 6% 5% 4 5% 5% 32%	2396
May	1st to 10th	Day	26% 10% 3 8% 4 4 4 3 4 6% 27%	4106
	11th to 20th	Day	20% 9% 3 7% 5% 4 4 4 5% 7% 31%	4380
	21st to end	Day	13% 6% 2 7% 5% 5% 5% 4 7% 8% 39%	4837
June	1st to 10th	Day	16% 7% 3 6% 7% 6% 5% 4 7% 8% 30%	2660
	11th to 20th	Day	18% 8% 2 5% 4 5% 5% 4 7% 9% 32%	2660
	21st to end	Day	17% 7% 3 8% 6% 6% 5% 4 6% 8% 31%	2660
July	1st to 10th	Day	12% 6% 3 7% 6% 5% 6% 5% 6% 9% 34%	4440
	11th to 20th	Day	11% 6% 2 7% 5% 5% 6% 5% 8% 11% 35%	4440
	21st to end	Day	15% 8% 3 7% 5% 5% 5% 4 7% 7% 32%	4814
August	1st to 10th	Day	18% 7% 3 8% 5% 4 5% 4 6% 8% 31%	4335
	11th to 20th	Day	13% 8% 3 7% 5% 5% 6% 5% 8% 8% 33%	3776
	21st to end	Day	11% 6% 2 7% 5% 5% 6% 5% 8% 8% 37%	4180
September	1st to 10th	Day	12% 7% 2 8% 4 5% 5% 3 5% 6% 42%	2280
	11th to 20th	Day	14% 6% 2 5% 4 4 5% 2 4 6% 46%	2161
	21st to end	Day	16% 6% 3 6% 4 4 4 4 5% 7% 38%	2090
October	1st to 10th	Day	21% 10% 3 7% 5% 3 4 3 4 4 34%	2033
	11th to 20th	Day	15% 9% 3 6% 4 5% 4 3 5% 6% 39%	1877
	21st to end	Day	21% 13% 3 7% 5% 3 4 4 5% 5% 30%	1881
November	1st to 10th	Day	11% 9% 3 7% 4 4 3 3 5% 6% 45%	1761
	11th to 20th	Day	16% 8% 3 7% 4 4 4 5% 6% 6% 37%	1800
	21st to end	Day	16% 12% 3 7% 5% 5% 4 4 5% 6% 33%	1681
December	1st to 10th	Day	17% 13% 5% 10% 6% 5% 3 4 5% 6% 25%	1520
	11th to 20th	Day	20% 11% 4 8% 5% 4 3 3 4 6% 33%	1906
	21st to end	Day	17% 14% 5% 12% 7% 5% 4 3 5% 6% 23%	2772
January	1st to 10th	Night	15% 9% 4 8% 6% 5% 5% 4 5% 6% 34%	4429
	11th to 20th	Night	13% 10% 4 12% 9% 6% 5% 4 6% 6% 26%	4440
	21st to end	Night	18% 17% 5% 9% 5% 5% 4 4 4 5% 24%	4843
February	1st to 10th	Night	14% 9% 3 9% 6% 6% 6% 5% 6% 7% 28%	2447
	11th to 20th	Night	11% 10% 3 11% 6% 6% 6% 5% 7% 6% 30%	2218
	21st to end	Night	16% 11% 4 9% 5% 5% 4 4 6% 5% 32%	1884
March	1st to 10th	Night	21% 13% 4 8% 5% 4 5% 4 5% 5% 27%	3724
	11th to 20th	Night	11% 10% 4 8% 6% 6% 6% 4 7% 7% 31%	3689
	21st to end	Night	17% 13% 4 9% 5% 4 3 3 4 5% 32%	3466
April	1st to 10th	Night	16% 12% 3 7% 5% 5% 4 5% 6% 7% 31%	1900
	11th to 20th	Night	18% 11% 3 7% 4 3 4 4 5% 6% 36%	1875
	21st to end	Night	20% 10% 3 7% 4 3 3 4 5% 5% 36%	1782
May	1st to 10th	Night	25% 11% 2 7% 4 5% 4 3 5% 5% 29%	2834
	11th to 20th	Night	17% 10% 4 8% 5% 5% 5% 4 6% 6% 30%	2541
	21st to end	Night	14% 9% 2 5% 5% 3 4 3 6% 9% 39%	2738
June	1st to 10th	Night	17% 8% 3 7% 5% 3 3 4 5% 7% 38%	1520
	11th to 20th	Night	17% 7% 2 6% 5% 4 4 5% 5% 8% 37%	1520
	21st to end	Night	15% 6% 2 6% 4 4 5% 4 5% 8% 41%	1520
July	1st to 10th	Night	11% 6% 3 7% 6% 4 5% 4 7% 9% 38%	2520
	11th to 20th	Night	15% 7% 2 7% 4 3 4 4 6% 9% 39%	2520
	21st to end	Night	16% 8% 3 7% 5% 4 3 4 5% 7% 39%	2739
August	1st to 10th	Night	23% 7% 3 7% 4 3 4 3 5% 6% 35%	2562
	11th to 20th	Night	15% 7% 2 7% 5% 4 5% 3 6% 7% 39%	3036
	21st to end	Night	9% 5% 2 6% 4 4 4 4 5% 8% 49%	3476
September	1st to 10th	Night	13% 7% 2 6% 4 4 4 4 5% 7% 44%	1900
	11th to 20th	Night	13% 5% 2 5% 3 3 5% 3 4 7% 50%	2019
	21st to end	Night	17% 8% 2 5% 4 4 4 4 4 6% 42%	2090
October	1st to 10th	Night	20% 8% 3 6% 4 4 4 3 4 5% 38%	2142
	11th to 20th	Night	14% 8% 3 6% 5% 4 4 3 5% 5% 42%	2301
	21st to end	Night	20% 10% 3 7% 5% 4 4 3 4 7% 34%	2717
November	1st to 10th	Night	13% 8% 3 6% 4 3 4 4 5% 6% 44%	2537
	11th to 20th	Night	16% 8% 4 7% 6% 5% 4 4 5% 6% 36%	2600
	21st to end	Night	13% 11% 4 7% 5% 4 5% 4 6% 6% 35%	2651
December	1st to 10th	Night	18% 12% 4 8% 5% 4 5% 4 7% 7% 26%	2660
	11th to 20th	Night	19% 11% 4 9% 5% 4 5% 4 5% 6% 27%	3288
	21st to end	Night	19% 13% 6% 12% 5% 4 4 4 5% 5% 23%	4884

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	45% 12% 10% 26% 6% 1	2518
	11th to 20th	Day	55% 12% 9% 17% 6% 1	2520
	21st to end	Day	63% 5% 7% 17% 7% 1	2809
February	1st to 10th	Day	55% 8% 12% 15% 10% 1	1732
	11th to 20th	Day	49% 9% 9% 19% 12% 2	1857
	21st to end	Day	54% 7% 8% 18% 11% 1	1570
March	1st to 10th	Day	58% 6% 10% 14% 10% 1	3119
	11th to 20th	Day	45% 13% 13% 19% 9% 1	3268
	21st to end	Day	50% 8% 6% 15% 18% 3	4166
April	1st to 10th	Day	52% 6% 8% 21% 11% 2	2280
	11th to 20th	Day	47% 7% 9% 20% 14% 2	2254
	21st to end	Day	50% 8% 8% 18% 12% 3	2396
May	1st to 10th	Day	58% 6% 10% 16% 10% 1	4120
	11th to 20th	Day	51% 6% 15% 19% 8% 1	4384
	21st to end	Day	37% 10% 14% 25% 13% 1	4837
June	1st to 10th	Day	47% 9% 13% 21% 9%	2660
	11th to 20th	Day	45% 8% 15% 21% 11% 1	2660
	21st to end	Day	49% 7% 14% 19% 11%	2660
July	1st to 10th	Day	43% 7% 13% 22% 13% 1	4370
	11th to 20th	Day	40% 8% 16% 23% 12% 1	4365
	21st to end	Day	48% 7% 14% 17% 13% 1	4661
August	1st to 10th	Day	49% 7% 14% 21% 9%	4321
	11th to 20th	Day	44% 8% 15% 26% 7%	3813
	21st to end	Day	39% 6% 13% 29% 11% 1	4180
September	1st to 10th	Day	42% 6% 12% 26% 14% 1	2280
	11th to 20th	Day	38% 6% 9% 24% 20% 2	2161
	21st to end	Day	43% 7% 11% 22% 15% 2	2090
October	1st to 10th	Day	48% 5% 7% 19% 19% 2	3384
	11th to 20th	Day	39% 5% 10% 23% 18% 5%	3093
	21st to end	Day	38% 6% 12% 26% 14% 4	3081
November	1st to 10th	Day	39% 6% 14% 25% 14% 2	1761
	11th to 20th	Day	43% 9% 9% 19% 17% 3	1800
	21st to end	Day	48% 7% 14% 21% 8% 2	1681
December	1st to 10th	Day	56% 7% 13% 13% 11% 1	1520
	11th to 20th	Day	49% 8% 12% 18% 11% 1	1906
	21st to end	Day	58% 9% 10% 12% 11%	2772
January	1st to 10th	Night	45% 8% 12% 22% 10% 2	4429
	11th to 20th	Night	49% 13% 10% 17% 9% 2	4440
	21st to end	Night	59% 7% 10% 15% 8% 1	4843
February	1st to 10th	Night	50% 9% 12% 18% 9% 2	2447
	11th to 20th	Night	47% 10% 11% 18% 13% 2	2220
	21st to end	Night	49% 8% 10% 21% 9% 3	1884
March	1st to 10th	Night	55% 7% 10% 18% 9% 1	3734
	11th to 20th	Night	44% 11% 12% 22% 8% 2	3690
	21st to end	Night	49% 9% 7% 12% 19% 4	3466
April	1st to 10th	Night	50% 7% 10% 19% 12% 2	1900
	11th to 20th	Night	48% 6% 9% 20% 16% 1	1875
	21st to end	Night	47% 7% 10% 18% 15% 3	1782
May	1st to 10th	Night	55% 7% 11% 15% 10% 1	2840
	11th to 20th	Night	52% 7% 15% 16% 8% 1	2542
	21st to end	Night	40% 10% 14% 22% 13% 1	2739
June	1st to 10th	Night	43% 8% 13% 25% 11%	1520
	11th to 20th	Night	43% 7% 16% 22% 11%	1520
	21st to end	Night	40% 9% 15% 23% 13% 1	1520
July	1st to 10th	Night	40% 7% 16% 22% 15% 1	2480
	11th to 20th	Night	41% 6% 14% 27% 12%	2472
	21st to end	Night	43% 6% 14% 22% 14% 1	2651
August	1st to 10th	Night	50% 6% 14% 22% 8% 1	2552
	11th to 20th	Night	42% 6% 17% 27% 8%	3064
	21st to end	Night	30% 6% 14% 35% 14% 1	3476
September	1st to 10th	Night	39% 6% 13% 25% 16% 1	1900
	11th to 20th	Night	35% 6% 11% 27% 21% 1	2019
	21st to end	Night	42% 6% 9% 24% 17% 2	2090
October	1st to 10th	Night	44% 4 12% 22% 16% 1	3571
	11th to 20th	Night	35% 4 12% 23% 20% 6%	3824
	21st to end	Night	37% 5% 14% 30% 12% 1	4467
November	1st to 10th	Night	39% 7% 11% 22% 19% 3	2537
	11th to 20th	Night	44% 8% 10% 19% 16% 2	2600
	21st to end	Night	45% 9% 14% 22% 9%	2651
December	1st to 10th	Night	51% 11% 12% 17% 9%	2660
	11th to 20th	Night	48% 14% 13% 14% 11% 1	3288
	21st to end	Night	56% 11% 8% 16% 9%	4884

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% 30% 30% 8% 3 1	180
	11th to 20th	Day	31% 34% 26% 7% 2	180
	21st to end	Day	23% 30% 31% 13% 2 1	210
February	1st to 10th	Day	34% 36% 23% 5% 2	292
	11th to 20th	Day	31% 32% 26% 9% 1	456
	21st to end	Day	31% 27% 29% 13% 1	382
March	1st to 10th	Day	29% 27% 29% 12% 2	460
	11th to 20th	Day	31% 29% 27% 9% 3 1	460
	21st to end	Day	33% 27% 24% 9% 5% 3	506
April	1st to 10th	Day	34% 23% 25% 11% 6% 1	460
	11th to 20th	Day	31% 25% 27% 11% 4 2	452
	21st to end	Day	28% 22% 31% 13% 4 2	460
May	1st to 10th	Day	35% 21% 24% 12% 4 3	463
	11th to 20th	Day	21% 23% 34% 17% 5% 1	487
	21st to end	Day	20% 15% 32% 22% 10% 2	529
June	1st to 10th	Day	18% 18% 32% 20% 11% 1	490
	11th to 20th	Day	20% 16% 31% 22% 10% 1	490
	21st to end	Day	19% 18% 33% 19% 8% 2	490
July	1st to 10th	Day	16% 10% 34% 26% 11% 2	485
	11th to 20th	Day	14% 7% 36% 30% 10% 2	484
	21st to end	Day	14% 12% 39% 24% 9% 2	532
August	1st to 10th	Day	15% 11% 28% 30% 11% 5%	486
	11th to 20th	Day	16% 11% 29% 26% 11% 7%	479
	21st to end	Day	14% 14% 27% 26% 14% 6%	539
September	1st to 10th	Day	18% 12% 27% 22% 16% 6%	490
	11th to 20th	Day	17% 13% 24% 22% 14% 11%	490
	21st to end	Day	19% 15% 29% 20% 11% 6%	490
October	1st to 10th	Day	27% 15% 26% 17% 11% 4	490
	11th to 20th	Day	22% 19% 29% 21% 8% 2	477
	21st to end	Day	32% 19% 27% 17% 5% 1	429
November	1st to 10th	Day	24% 23% 25% 20% 8% 1	395
	11th to 20th	Day	30% 28% 24% 13% 5% 1	400
	21st to end	Day	33% 24% 24% 16% 2 1	299
December	1st to 10th	Day	32% 31% 24% 10% 3	180
	11th to 20th	Day	38% 24% 30% 6% 2	180
	21st to end	Day	29% 31% 27% 10% 2 1	198
January	1st to 10th	Night	23% 29% 31% 14% 2	760
	11th to 20th	Night	26% 26% 30% 15% 2	760
	21st to end	Night	20% 30% 30% 17% 2	836
February	1st to 10th	Night	24% 27% 33% 15% 1	760
	11th to 20th	Night	24% 31% 31% 13% 1	743
	21st to end	Night	28% 29% 26% 14% 2	628
March	1st to 10th	Night	27% 26% 27% 17% 3	760
	11th to 20th	Night	23% 28% 29% 17% 3 1	760
	21st to end	Night	25% 25% 26% 18% 5% 1	836
April	1st to 10th	Night	26% 22% 32% 17% 4	760
	11th to 20th	Night	25% 23% 28% 18% 6%	751
	21st to end	Night	28% 22% 26% 20% 5%	642
May	1st to 10th	Night	28% 16% 27% 21% 7%	570
	11th to 20th	Night	20% 16% 32% 27% 5%	407
	21st to end	Night	20% 8% 28% 30% 13%	411
June	1st to 10th	Night	20% 10% 30% 29% 11% 1	380
	11th to 20th	Night	18% 9% 27% 36% 10% 1	380
	21st to end	Night	18% 9% 26% 34% 12% 1	380
July	1st to 10th	Night	14% 6% 24% 39% 15% 1	370
	11th to 20th	Night	15% 3 24% 41% 15% 1	368
	21st to end	Night	11% 7% 25% 40% 17%	416
August	1st to 10th	Night	14% 4 24% 40% 18%	408
	11th to 20th	Night	16% 7% 23% 32% 19% 3	713
	21st to end	Night	16% 7% 22% 33% 20% 2	836
September	1st to 10th	Night	15% 10% 27% 30% 15% 2	760
	11th to 20th	Night	14% 10% 22% 27% 20% 5%	760
	21st to end	Night	20% 12% 24% 28% 14% 3	760
October	1st to 10th	Night	26% 12% 25% 23% 11% 2	760
	11th to 20th	Night	21% 19% 26% 24% 9% 1	760
	21st to end	Night	27% 19% 28% 21% 5%	836
November	1st to 10th	Night	20% 22% 29% 22% 8%	780
	11th to 20th	Night	24% 26% 25% 17% 7% 1	800
	21st to end	Night	23% 25% 28% 19% 4 1	789
December	1st to 10th	Night	25% 29% 27% 14% 4	760
	11th to 20th	Night	25% 28% 28% 16% 3	760
	21st to end	Night	27% 32% 26% 14% 2	836

Page generated at 2024-11-25 00:22:20. Version ac07

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