Black Star Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Black Star Observatory clear sky chart from 2005-11-06 to 2024-12-24.

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	17% 10% 4 8% 5% 4 5% 4 5% 7% 32%	101746
	Night	17% 11% 4 8% 5% 4 4 4 5% 6% 32%	98366
Transparency	Day	51% 4 11% 21% 12% 1	105268
	Night	50% 4 12% 21% 12% 1	102933
Seeing	Day	26% 19% 28% 18% 8% 2	15006
	Night	24% 18% 27% 22% 8% 1	24097

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% 14% 6% 12% 6% 5% 5% 3 5% 6% 26%	2517
	11th to 20th	Day	16% 13% 6% 13% 9% 6% 4 3 4 4 22%	2520
	21st to end	Day	19% 18% 9% 11% 5% 4 3 2 4 4 23%	2833
February	1st to 10th	Day	16% 13% 4 10% 7% 6% 5% 4 6% 5% 25%	1720
	11th to 20th	Day	12% 12% 6% 12% 7% 5% 4 4 4 6% 28%	1848
	21st to end	Day	18% 13% 4 12% 6% 4 4 3 4 4 28%	1570
March	1st to 10th	Day	22% 16% 5% 9% 4 3 3 2 4 4 27%	3116
	11th to 20th	Day	11% 13% 6% 11% 6% 5% 5% 4 5% 6% 28%	3270
	21st to end	Day	20% 8% 4 11% 6% 4 4 3 3 5% 32%	4166
April	1st to 10th	Day	18% 14% 3 9% 5% 4 4 3 5% 5% 31%	2280
	11th to 20th	Day	17% 10% 3 7% 4 5% 6% 4 5% 6% 34%	2254
	21st to end	Day	19% 9% 4 8% 6% 5% 4 4 5% 5% 31%	2387
May	1st to 10th	Day	26% 11% 3 7% 4 4 4 3 5% 6% 28%	4106
	11th to 20th	Day	19% 9% 3 8% 5% 5% 4 4 6% 7% 31%	4394
	21st to end	Day	13% 6% 2 6% 4 4 5% 5% 7% 8% 39%	4837
June	1st to 10th	Day	17% 7% 2 7% 6% 6% 5% 5% 7% 9% 30%	2660
	11th to 20th	Day	19% 6% 2 6% 5% 5% 4 5% 6% 10% 32%	2660
	21st to end	Day	17% 7% 3 8% 5% 6% 5% 4 5% 8% 30%	2660
July	1st to 10th	Day	12% 7% 3 7% 6% 5% 5% 5% 7% 9% 35%	4440
	11th to 20th	Day	11% 6% 2 7% 5% 5% 6% 5% 8% 10% 35%	4440
	21st to end	Day	15% 9% 3 8% 5% 5% 5% 5% 6% 7% 32%	4814
August	1st to 10th	Day	18% 8% 3 8% 4 5% 5% 4 6% 8% 30%	4346
	11th to 20th	Day	14% 8% 3 7% 5% 5% 6% 4 8% 8% 33%	3765
	21st to end	Day	12% 6% 2 7% 5% 5% 5% 5% 7% 8% 37%	4180
September	1st to 10th	Day	15% 6% 2 6% 4 4 4 4 5% 7% 41%	2280
	11th to 20th	Day	16% 6% 2 6% 4 3 4 3 5% 7% 45%	2170
	21st to end	Day	17% 6% 3 7% 4 4 4 4 6% 7% 39%	2090
October	1st to 10th	Day	22% 9% 3 8% 4 4 4 2 5% 5% 35%	2023
	11th to 20th	Day	18% 8% 3 6% 5% 3 4 4 5% 6% 39%	1886
	21st to end	Day	22% 13% 4 8% 4 4 3 3 5% 5% 30%	1881
November	1st to 10th	Day	12% 9% 4 7% 5% 3 3 3 4 6% 44%	1761
	11th to 20th	Day	17% 11% 3 7% 6% 3 5% 3 6% 6% 35%	1800
	21st to end	Day	17% 12% 4 8% 5% 4 4 3 4 6% 31%	1690
December	1st to 10th	Day	18% 15% 5% 10% 6% 5% 5% 3 5% 5% 23%	1600
	11th to 20th	Day	20% 14% 6% 8% 5% 3 3 3 3 4 32%	1986
	21st to end	Day	18% 15% 5% 12% 5% 5% 3 3 5% 5% 22%	2796
January	1st to 10th	Night	14% 12% 5% 11% 6% 4 4 3 4 6% 31%	4429
	11th to 20th	Night	14% 11% 4 14% 9% 7% 6% 4 5% 5% 22%	4440
	21st to end	Night	19% 18% 5% 9% 5% 5% 4 3 4 4 22%	4820
February	1st to 10th	Night	15% 11% 4 9% 7% 7% 6% 5% 6% 5% 25%	2458
	11th to 20th	Night	12% 10% 6% 11% 6% 6% 6% 5% 6% 5% 27%	2218
	21st to end	Night	16% 13% 4 10% 6% 4 5% 4 6% 5% 29%	1884
March	1st to 10th	Night	22% 12% 4 10% 5% 4 5% 3 5% 4 25%	3724
	11th to 20th	Night	11% 12% 5% 9% 7% 6% 5% 4 5% 5% 30%	3683
	21st to end	Night	17% 15% 5% 10% 5% 4 3 2 4 4 31%	3466
April	1st to 10th	Night	18% 12% 3 8% 5% 4 5% 4 5% 6% 30%	1900
	11th to 20th	Night	18% 11% 3 7% 5% 4 4 3 4 6% 35%	1875
	21st to end	Night	20% 10% 3 6% 4 3 4 4 5% 5% 36%	1791
May	1st to 10th	Night	26% 11% 3 6% 4 4 4 3 5% 6% 29%	2834
	11th to 20th	Night	17% 10% 3 7% 6% 6% 5% 4 5% 7% 30%	2527
	21st to end	Night	15% 8% 2 7% 4 4 4 3 6% 8% 38%	2738
June	1st to 10th	Night	19% 7% 2 7% 5% 4 4 3 6% 7% 37%	1520
	11th to 20th	Night	17% 7% 2 6% 4 4 5% 4 6% 8% 37%	1520
	21st to end	Night	15% 6% 2 5% 4 5% 5% 5% 6% 7% 40%	1520
July	1st to 10th	Night	13% 6% 3 7% 5% 4 5% 4 7% 9% 37%	2520
	11th to 20th	Night	16% 7% 2 6% 4 4 4 5% 6% 8% 38%	2520
	21st to end	Night	17% 8% 3 6% 4 4 5% 4 5% 6% 39%	2739
August	1st to 10th	Night	25% 8% 3 7% 3 4 4 3 5% 6% 34%	2538
	11th to 20th	Night	15% 7% 3 7% 5% 4 5% 4 5% 7% 39%	3059
	21st to end	Night	9% 8% 2 6% 3 4 4 3 5% 8% 47%	3476
September	1st to 10th	Night	14% 7% 2 7% 4 4 4 4 5% 6% 44%	1900
	11th to 20th	Night	14% 5% 2 5% 3 3 4 3 4 7% 51%	2009
	21st to end	Night	17% 7% 2 6% 4 3 3 3 4 6% 43%	2090
October	1st to 10th	Night	21% 8% 2 6% 4 4 3 3 5% 5% 37%	2156
	11th to 20th	Night	15% 8% 3 6% 5% 4 4 4 5% 5% 41%	2292
	21st to end	Night	20% 12% 3 7% 5% 3 4 3 5% 6% 32%	2717
November	1st to 10th	Night	14% 10% 2 6% 4 3 4 4 5% 6% 42%	2537
	11th to 20th	Night	17% 9% 3 9% 5% 5% 5% 3 4 5% 35%	2600
	21st to end	Night	13% 13% 3 9% 5% 5% 5% 4 6% 6% 32%	2705
December	1st to 10th	Night	19% 14% 4 11% 6% 5% 4 4 5% 6% 23%	2800
	11th to 20th	Night	20% 13% 5% 11% 5% 4 4 4 4 5% 26%	3428
	21st to end	Night	20% 14% 6% 13% 5% 4 4 3 4 4 22%	4933

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	54% 4 10% 25% 6% 1	2518
	11th to 20th	Day	63% 4 10% 17% 6% 1	2520
	21st to end	Day	66% 2 8% 17% 6% 1	2833
February	1st to 10th	Day	58% 5% 11% 16% 9% 1	1720
	11th to 20th	Day	54% 5% 8% 19% 12% 2	1857
	21st to end	Day	58% 3 9% 18% 11% 1	1570
March	1st to 10th	Day	62% 2 10% 14% 10% 1	3119
	11th to 20th	Day	54% 3 14% 19% 9% 1	3270
	21st to end	Day	55% 3 5% 16% 18% 3	4166
April	1st to 10th	Day	55% 3 8% 21% 11% 2	2280
	11th to 20th	Day	50% 4 10% 21% 15% 1	2254
	21st to end	Day	54% 3 8% 19% 13% 3	2387
May	1st to 10th	Day	59% 3 10% 17% 10% 1	4120
	11th to 20th	Day	52% 3 14% 21% 8% 1	4398
	21st to end	Day	41% 5% 13% 26% 14% 1	4837
June	1st to 10th	Day	49% 5% 13% 23% 10%	2660
	11th to 20th	Day	47% 4 15% 22% 11% 1	2660
	21st to end	Day	50% 5% 13% 21% 10% 1	2660
July	1st to 10th	Day	46% 5% 13% 23% 14% 1	4370
	11th to 20th	Day	43% 4 16% 25% 12% 1	4365
	21st to end	Day	50% 3 14% 18% 14% 1	4661
August	1st to 10th	Day	51% 4 14% 21% 9%	4332
	11th to 20th	Day	47% 4 16% 27% 6%	3802
	21st to end	Day	42% 3 12% 31% 11% 1	4180
September	1st to 10th	Day	42% 4 12% 26% 15% 1	2280
	11th to 20th	Day	40% 4 8% 25% 21% 2	2170
	21st to end	Day	45% 4 10% 24% 14% 2	2090
October	1st to 10th	Day	51% 3 6% 19% 19% 2	3361
	11th to 20th	Day	42% 2 10% 23% 18% 5%	3114
	21st to end	Day	41% 3 13% 26% 14% 3	3081
November	1st to 10th	Day	41% 4 15% 24% 14% 2	1761
	11th to 20th	Day	48% 4 9% 20% 17% 2	1800
	21st to end	Day	53% 3 13% 21% 8% 1	1690
December	1st to 10th	Day	59% 4 12% 15% 10% 1	1600
	11th to 20th	Day	55% 3 12% 18% 11% 1	1986
	21st to end	Day	64% 3 10% 13% 11%	2796
January	1st to 10th	Night	53% 2 12% 22% 9% 2	4429
	11th to 20th	Night	56% 5% 13% 16% 8% 2	4440
	21st to end	Night	64% 2 9% 17% 7% 1	4820
February	1st to 10th	Night	55% 3 12% 18% 9% 2	2458
	11th to 20th	Night	53% 5% 10% 17% 13% 1	2220
	21st to end	Night	54% 3 11% 20% 10% 3	1884
March	1st to 10th	Night	60% 3 9% 18% 9% 1	3734
	11th to 20th	Night	52% 3 13% 23% 8% 1	3684
	21st to end	Night	54% 4 5% 14% 19% 4	3466
April	1st to 10th	Night	53% 3 10% 21% 11% 2	1900
	11th to 20th	Night	50% 4 9% 22% 15% 1	1875
	21st to end	Night	49% 4 9% 20% 15% 3	1791
May	1st to 10th	Night	58% 3 11% 16% 10% 1	2840
	11th to 20th	Night	54% 4 14% 18% 8% 1	2528
	21st to end	Night	44% 4 16% 21% 13% 1	2739
June	1st to 10th	Night	46% 5% 12% 26% 12%	1520
	11th to 20th	Night	45% 4 16% 23% 11%	1520
	21st to end	Night	41% 6% 15% 25% 12% 1	1520
July	1st to 10th	Night	43% 4 16% 22% 15% 1	2480
	11th to 20th	Night	43% 4 14% 27% 12%	2472
	21st to end	Night	46% 4 14% 21% 14% 1	2651
August	1st to 10th	Night	52% 3 13% 23% 7%	2528
	11th to 20th	Night	44% 4 18% 28% 7%	3088
	21st to end	Night	35% 3 14% 35% 13% 1	3476
September	1st to 10th	Night	40% 4 12% 28% 15% 1	1900
	11th to 20th	Night	35% 4 10% 28% 22% 1	2009
	21st to end	Night	42% 3 10% 25% 17% 2	2090
October	1st to 10th	Night	47% 2 12% 22% 15% 1	3598
	11th to 20th	Night	39% 2 11% 24% 19% 5%	3803
	21st to end	Night	40% 3 14% 31% 12% 1	4467
November	1st to 10th	Night	42% 4 11% 23% 18% 2	2537
	11th to 20th	Night	49% 4 9% 20% 15% 3	2600
	21st to end	Night	50% 3 14% 23% 8% 1	2705
December	1st to 10th	Night	58% 4 12% 18% 8%	2800
	11th to 20th	Night	58% 4 12% 14% 11% 1	3428
	21st to end	Night	63% 4 8% 16% 9%	4933

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	32% 27% 31% 6% 3 1	180
	11th to 20th	Day	36% 27% 29% 7% 1 1	180
	21st to end	Day	25% 30% 30% 11% 3 1	215
February	1st to 10th	Day	38% 31% 25% 4 1	280
	11th to 20th	Day	36% 29% 24% 10% 2	456
	21st to end	Day	32% 26% 31% 10% 1	382
March	1st to 10th	Day	31% 26% 29% 12% 2	460
	11th to 20th	Day	33% 27% 25% 11% 3	460
	21st to end	Day	34% 26% 22% 10% 4 3	506
April	1st to 10th	Day	37% 22% 23% 12% 5% 1	460
	11th to 20th	Day	33% 25% 24% 12% 5% 1	452
	21st to end	Day	29% 22% 31% 13% 4 2	460
May	1st to 10th	Day	34% 21% 26% 11% 6% 1	463
	11th to 20th	Day	23% 18% 33% 19% 5% 1	487
	21st to end	Day	20% 14% 29% 25% 9% 2	529
June	1st to 10th	Day	18% 18% 33% 19% 10% 1	490
	11th to 20th	Day	21% 13% 30% 21% 13% 1	490
	21st to end	Day	21% 17% 33% 19% 9% 2	490
July	1st to 10th	Day	15% 9% 34% 27% 12% 2	485
	11th to 20th	Day	14% 7% 32% 33% 12% 2	484
	21st to end	Day	14% 11% 38% 25% 10% 2	532
August	1st to 10th	Day	14% 12% 27% 30% 11% 5%	486
	11th to 20th	Day	15% 10% 30% 24% 14% 6%	479
	21st to end	Day	14% 13% 27% 27% 14% 6%	539
September	1st to 10th	Day	19% 11% 25% 24% 14% 6%	490
	11th to 20th	Day	19% 12% 23% 20% 16% 11%	490
	21st to end	Day	21% 13% 29% 21% 10% 6%	490
October	1st to 10th	Day	28% 13% 25% 19% 10% 4	490
	11th to 20th	Day	25% 18% 29% 18% 8% 2	483
	21st to end	Day	34% 19% 22% 19% 5% 1	429
November	1st to 10th	Day	25% 22% 26% 19% 8%	395
	11th to 20th	Day	30% 26% 25% 14% 4 1	400
	21st to end	Day	36% 23% 24% 14% 2 1	290
December	1st to 10th	Day	32% 30% 25% 11% 4	200
	11th to 20th	Day	39% 27% 26% 8% 2	200
	21st to end	Day	36% 24% 28% 10% 2	204
January	1st to 10th	Night	25% 28% 32% 13% 2	760
	11th to 20th	Night	31% 24% 28% 15% 3	760
	21st to end	Night	24% 25% 33% 16% 2	836
February	1st to 10th	Night	27% 26% 32% 14% 1	760
	11th to 20th	Night	27% 28% 31% 13% 1	743
	21st to end	Night	31% 27% 26% 14% 3	628
March	1st to 10th	Night	29% 24% 29% 15% 3	760
	11th to 20th	Night	25% 27% 29% 16% 3	760
	21st to end	Night	30% 21% 25% 18% 5%	836
April	1st to 10th	Night	28% 21% 30% 18% 3	760
	11th to 20th	Night	27% 23% 26% 18% 6%	751
	21st to end	Night	29% 20% 27% 20% 5%	651
May	1st to 10th	Night	29% 15% 29% 20% 7%	570
	11th to 20th	Night	22% 13% 34% 27% 4	393
	21st to end	Night	18% 8% 29% 32% 13% 1	411
June	1st to 10th	Night	19% 10% 29% 29% 11%	380
	11th to 20th	Night	18% 8% 29% 34% 11% 1	380
	21st to end	Night	16% 9% 26% 37% 12% 1	380
July	1st to 10th	Night	16% 5% 23% 40% 15% 1	370
	11th to 20th	Night	14% 5% 23% 42% 14% 1	368
	21st to end	Night	14% 6% 23% 41% 16%	416
August	1st to 10th	Night	14% 4 24% 41% 17% 1	397
	11th to 20th	Night	16% 6% 21% 35% 20% 2	724
	21st to end	Night	16% 6% 23% 34% 19% 2	836
September	1st to 10th	Night	17% 9% 28% 28% 16% 2	760
	11th to 20th	Night	16% 10% 24% 24% 21% 4	760
	21st to end	Night	21% 11% 24% 28% 13% 2	760
October	1st to 10th	Night	28% 10% 24% 26% 11% 3	760
	11th to 20th	Night	23% 15% 29% 23% 9% 1	760
	21st to end	Night	27% 20% 28% 20% 5%	836
November	1st to 10th	Night	21% 22% 28% 22% 8%	780
	11th to 20th	Night	25% 24% 27% 16% 6% 1	800
	21st to end	Night	27% 23% 28% 19% 4 1	800
December	1st to 10th	Night	30% 26% 24% 15% 4	800
	11th to 20th	Night	33% 22% 27% 16% 3	800
	21st to end	Night	31% 28% 26% 14% 2	851

Page generated at 2024-12-21 04:13:48. Version ac07

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