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 2024-01-12.

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	16% 10% 4 7% 6% 3 5% 4 5% 7% 33%	75095
	Night	16% 12% 5% 8% 7% 3 4 4 5% 6% 29%	74558
Transparency	Day	52% 7% 12% 19% 10% 1	74801
	Night	56% 6% 11% 17% 9% 1	74387
Seeing	Day	22% 17% 31% 19% 8% 3	13779
	Night	23% 15% 29% 23% 9% 1	22784

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	16% 14% 7% 12% 10% 5% 5% 4 4 6% 17%	1429
	11th to 20th	Day	15% 17% 7% 12% 9% 5% 5% 4 4 4 19%	1557
	21st to end	Day	12% 15% 8% 12% 9% 4 6% 5% 4 5% 19%	1782
February	1st to 10th	Day	13% 17% 7% 13% 11% 5% 5% 4 4 4 17%	1620
	11th to 20th	Day	12% 14% 7% 11% 8% 3 4 5% 5% 5% 26%	1573
	21st to end	Day	16% 15% 7% 10% 7% 4 5% 5% 5% 5% 21%	1365
March	1st to 10th	Day	14% 12% 5% 8% 7% 5% 5% 4 4 5% 31%	1972
	11th to 20th	Day	12% 12% 6% 8% 8% 3 5% 4 4 7% 32%	1980
	21st to end	Day	18% 10% 4 6% 6% 3 4 4 4 6% 36%	2178
April	1st to 10th	Day	19% 12% 4 7% 6% 3 5% 4 4 5% 31%	2124
	11th to 20th	Day	18% 10% 2 5% 6% 3 5% 4 4 5% 37%	2239
	21st to end	Day	21% 9% 3 5% 4 3 4 4 4 7% 35%	2340
May	1st to 10th	Day	18% 8% 3 5% 5% 3 5% 5% 5% 7% 36%	2348
	11th to 20th	Day	14% 7% 3 5% 5% 3 5% 4 4 6% 43%	2478
	21st to end	Day	12% 6% 3 4 5% 3 5% 5% 7% 8% 43%	2725
June	1st to 10th	Day	18% 6% 3 5% 4 3 5% 5% 5% 8% 39%	2520
	11th to 20th	Day	13% 6% 2 5% 5% 3 5% 5% 6% 9% 42%	2686
	21st to end	Day	16% 6% 3 6% 5% 3 4 5% 5% 8% 40%	2700
July	1st to 10th	Day	10% 4 2 5% 4 3 4 5% 6% 8% 48%	2700
	11th to 20th	Day	12% 6% 3 5% 5% 3 5% 5% 6% 8% 43%	2561
	21st to end	Day	10% 5% 2 7% 6% 4 5% 5% 6% 9% 41%	2609
August	1st to 10th	Day	12% 6% 2 5% 4 3 5% 5% 5% 9% 44%	2327
	11th to 20th	Day	12% 5% 3 5% 4 3 5% 5% 6% 9% 44%	2303
	21st to end	Day	11% 5% 3 6% 6% 3 4 5% 6% 9% 41%	2502
September	1st to 10th	Day	14% 6% 3 6% 6% 3 4 5% 5% 7% 41%	2160
	11th to 20th	Day	14% 6% 3 5% 5% 3 4 4 5% 9% 43%	2046
	21st to end	Day	16% 7% 3 6% 6% 4 5% 5% 5% 6% 37%	1949
October	1st to 10th	Day	20% 9% 3 6% 6% 3 4 3 6% 7% 33%	1800
	11th to 20th	Day	24% 9% 5% 9% 7% 4 4 5% 5% 6% 22%	1800
	21st to end	Day	28% 14% 4 8% 7% 3 4 4 4 5% 19%	1980
November	1st to 10th	Day	17% 16% 5% 8% 7% 3 4 4 5% 7% 25%	1548
	11th to 20th	Day	17% 14% 6% 10% 8% 4 7% 5% 5% 7% 18%	1520
	21st to end	Day	19% 20% 6% 9% 7% 5% 6% 5% 5% 5% 12%	1520
December	1st to 10th	Day	17% 22% 8% 11% 9% 4 6% 4 4 5% 10%	1520
	11th to 20th	Day	18% 18% 7% 11% 8% 4 5% 4 5% 6% 17%	1906
	21st to end	Day	24% 17% 7% 12% 9% 4 4 4 3 4 12%	2728
January	1st to 10th	Night	17% 19% 8% 12% 9% 4 5% 5% 4 5% 13%	2714
	11th to 20th	Night	17% 18% 7% 10% 9% 4 4 4 5% 5% 17%	2442
	21st to end	Night	12% 16% 6% 11% 10% 4 5% 4 6% 6% 19%	2574
February	1st to 10th	Night	12% 18% 8% 14% 10% 4 6% 5% 4 5% 15%	2340
	11th to 20th	Night	13% 15% 7% 11% 9% 4 4 4 4 5% 23%	2273
	21st to end	Night	17% 14% 6% 10% 8% 3 5% 4 5% 6% 24%	1888
March	1st to 10th	Night	14% 11% 5% 8% 7% 3 4 4 4 6% 34%	1975
	11th to 20th	Night	14% 12% 6% 7% 6% 4 4 3 4 5% 34%	1979
	21st to end	Night	15% 11% 4 8% 7% 2 4 4 5% 5% 35%	2178
April	1st to 10th	Night	21% 11% 5% 7% 6% 2 4 4 4 6% 30%	1836
	11th to 20th	Night	19% 10% 2 7% 4 3 5% 4 4 6% 36%	1687
	21st to end	Night	17% 10% 5% 7% 6% 4 4 4 5% 6% 32%	1620
May	1st to 10th	Night	17% 8% 3 4 5% 3 5% 4 5% 8% 38%	1612
	11th to 20th	Night	14% 7% 3 5% 5% 3 4 4 6% 6% 43%	1415
	21st to end	Night	11% 4 2 4 5% 3 4 5% 6% 9% 47%	1550
June	1st to 10th	Night	15% 7% 3 6% 6% 3 5% 6% 5% 8% 36%	1440
	11th to 20th	Night	12% 5% 2 5% 4 2 5% 5% 7% 10% 43%	1269
	21st to end	Night	15% 6% 3 5% 5% 3 4 5% 6% 8% 39%	1260
July	1st to 10th	Night	8% 3 3 4 4 3 4 4 5% 7% 54%	1260
	11th to 20th	Night	11% 6% 2 5% 5% 3 4 5% 6% 8% 45%	1396
	21st to end	Night	10% 5% 2 6% 5% 3 5% 5% 6% 8% 46%	1706
August	1st to 10th	Night	9% 4 3 4 4 4 5% 5% 6% 8% 49%	1611
	11th to 20th	Night	9% 5% 2 6% 6% 3 5% 5% 5% 8% 47%	1598
	21st to end	Night	15% 6% 3 6% 6% 3 4 4 5% 7% 41%	1834
September	1st to 10th	Night	10% 5% 3 6% 6% 3 5% 4 5% 8% 44%	1800
	11th to 20th	Night	15% 5% 2 5% 5% 3 4 4 5% 6% 45%	1912
	21st to end	Night	16% 7% 3 6% 6% 3 4 4 5% 7% 40%	2009
October	1st to 10th	Night	19% 9% 4 6% 6% 3 4 5% 4 6% 35%	2160
	11th to 20th	Night	24% 13% 3 7% 5% 4 4 4 5% 5% 24%	2160
	21st to end	Night	25% 14% 4 8% 7% 4 4 4 4 6% 20%	2376
November	1st to 10th	Night	19% 14% 3 8% 6% 4 5% 4 4 5% 29%	2527
	11th to 20th	Night	16% 17% 6% 9% 7% 4 4 4 4 5% 22%	2660
	21st to end	Night	21% 19% 6% 10% 7% 4 5% 4 4 6% 14%	2660
December	1st to 10th	Night	19% 23% 9% 12% 8% 4 4 4 3 4 10%	2660
	11th to 20th	Night	16% 18% 8% 13% 8% 4 4 4 4 6% 15%	3288
	21st to end	Night	23% 22% 7% 14% 8% 3 4 3 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% 7% 10% 10% 3	1430
	11th to 20th	Day	70% 6% 8% 9% 7% 1	1557
	21st to end	Day	68% 5% 8% 10% 8% 1	1782
February	1st to 10th	Day	72% 6% 7% 11% 3 1	1620
	11th to 20th	Day	62% 6% 10% 14% 7% 2	1582
	21st to end	Day	66% 6% 9% 14% 5%	1365
March	1st to 10th	Day	55% 7% 14% 15% 9% 1	1975
	11th to 20th	Day	54% 7% 10% 17% 10% 2	1980
	21st to end	Day	50% 5% 9% 17% 15% 3	2178
April	1st to 10th	Day	56% 5% 10% 18% 9% 1	2124
	11th to 20th	Day	50% 5% 9% 23% 13% 1	2239
	21st to end	Day	51% 6% 10% 19% 12% 3	2340
May	1st to 10th	Day	47% 7% 10% 21% 13% 2	2348
	11th to 20th	Day	43% 6% 9% 26% 14% 1	2482
	21st to end	Day	38% 9% 14% 22% 14% 2	2725
June	1st to 10th	Day	43% 7% 12% 24% 13%	2520
	11th to 20th	Day	37% 9% 13% 24% 16% 1	2686
	21st to end	Day	42% 7% 14% 24% 12% 1	2700
July	1st to 10th	Day	32% 9% 14% 26% 17% 1	2626
	11th to 20th	Day	38% 8% 14% 30% 10% 1	2483
	21st to end	Day	40% 9% 13% 25% 12%	2463
August	1st to 10th	Day	36% 9% 17% 24% 13% 1	2314
	11th to 20th	Day	37% 9% 18% 26% 11%	2303
	21st to end	Day	40% 9% 16% 24% 10% 1	2502
September	1st to 10th	Day	42% 7% 13% 27% 10% 1	2160
	11th to 20th	Day	38% 7% 15% 29% 10%	2046
	21st to end	Day	47% 8% 13% 25% 7%	1949
October	1st to 10th	Day	51% 7% 12% 19% 10% 1	1800
	11th to 20th	Day	62% 7% 14% 12% 5% 1	1800
	21st to end	Day	68% 7% 11% 9% 5%	1980
November	1st to 10th	Day	60% 7% 12% 16% 6%	1548
	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	74% 6% 8% 8% 3	2714
	11th to 20th	Night	69% 7% 9% 9% 5% 1	2442
	21st to end	Night	65% 7% 9% 11% 7% 1	2574
February	1st to 10th	Night	72% 6% 8% 11% 3 1	2340
	11th to 20th	Night	63% 6% 10% 15% 6%	2275
	21st to end	Night	63% 6% 10% 15% 6%	1888
March	1st to 10th	Night	52% 6% 15% 16% 10% 1	1985
	11th to 20th	Night	54% 5% 10% 18% 11% 2	1980
	21st to end	Night	50% 4 10% 19% 14% 2	2178
April	1st to 10th	Night	56% 6% 8% 18% 10% 1	1836
	11th to 20th	Night	49% 5% 8% 22% 15% 2	1687
	21st to end	Night	53% 6% 9% 15% 15% 1	1620
May	1st to 10th	Night	44% 5% 11% 20% 17% 3	1612
	11th to 20th	Night	41% 6% 10% 22% 20% 2	1416
	21st to end	Night	34% 7% 15% 25% 16% 3	1551
June	1st to 10th	Night	44% 6% 12% 22% 15% 1	1440
	11th to 20th	Night	35% 6% 13% 25% 19% 1	1269
	21st to end	Night	42% 8% 14% 22% 13% 1	1260
July	1st to 10th	Night	28% 7% 13% 27% 22% 3	1225
	11th to 20th	Night	35% 6% 15% 29% 14% 1	1351
	21st to end	Night	36% 7% 12% 29% 15% 1	1611
August	1st to 10th	Night	33% 7% 17% 29% 14% 1	1600
	11th to 20th	Night	34% 8% 16% 24% 17% 1	1598
	21st to end	Night	41% 8% 13% 26% 11% 1	1834
September	1st to 10th	Night	36% 8% 16% 26% 13% 1	1800
	11th to 20th	Night	38% 6% 15% 28% 13%	1912
	21st to end	Night	44% 6% 14% 26% 9%	2009
October	1st to 10th	Night	51% 8% 12% 18% 11% 1	2160
	11th to 20th	Night	60% 7% 12% 16% 4	2160
	21st to end	Night	64% 7% 9% 14% 6%	2376
November	1st to 10th	Night	56% 6% 12% 18% 7%	2527
	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% 30% 32% 12% 5%	180
	11th to 20th	Day	20% 28% 37% 11% 4	210
	21st to end	Day	28% 22% 33% 15% 2	264
February	1st to 10th	Day	29% 35% 27% 6% 3	240
	11th to 20th	Day	25% 28% 40% 5% 1	240
	21st to end	Day	39% 27% 31% 3	198
March	1st to 10th	Day	22% 23% 38% 13% 3	415
	11th to 20th	Day	22% 24% 35% 15% 4 1	420
	21st to end	Day	32% 20% 27% 15% 5%	462
April	1st to 10th	Day	29% 21% 32% 12% 5% 1	420
	11th to 20th	Day	33% 18% 23% 16% 7% 3	412
	21st to end	Day	29% 18% 34% 11% 6% 2	420
May	1st to 10th	Day	27% 13% 32% 18% 7% 3	423
	11th to 20th	Day	19% 14% 32% 21% 10% 3	447
	21st to end	Day	16% 15% 31% 23% 12% 3	485
June	1st to 10th	Day	21% 13% 32% 26% 6% 1	450
	11th to 20th	Day	17% 13% 32% 25% 11% 3	616
	21st to end	Day	19% 10% 37% 23% 9% 2	630
July	1st to 10th	Day	10% 10% 33% 29% 14% 4	621
	11th to 20th	Day	15% 11% 26% 29% 12% 6%	482
	21st to end	Day	11% 9% 35% 28% 12% 4	488
August	1st to 10th	Day	14% 7% 27% 28% 15% 8%	446
	11th to 20th	Day	15% 6% 26% 30% 14% 9%	439
	21st to end	Day	14% 14% 30% 25% 13% 6%	495
September	1st to 10th	Day	16% 10% 29% 26% 13% 6%	450
	11th to 20th	Day	13% 10% 31% 20% 19% 8%	450
	21st to end	Day	18% 16% 26% 26% 10% 4	450
October	1st to 10th	Day	24% 17% 26% 17% 12% 3	450
	11th to 20th	Day	26% 23% 31% 15% 5%	450
	21st to end	Day	36% 19% 30% 12% 3	495
November	1st to 10th	Day	31% 19% 31% 16% 3	213
	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% 31% 16% 4	728
	21st to end	Night	21% 15% 33% 23% 6% 1	792
February	1st to 10th	Night	25% 23% 33% 17% 2	720
	11th to 20th	Night	22% 19% 34% 20% 4	703
	21st to end	Night	28% 22% 28% 19% 4	592
March	1st to 10th	Night	22% 21% 33% 20% 3	720
	11th to 20th	Night	23% 20% 33% 19% 4 1	720
	21st to end	Night	23% 14% 32% 24% 7%	792
April	1st to 10th	Night	28% 16% 31% 21% 4	720
	11th to 20th	Night	26% 14% 29% 22% 9%	623
	21st to end	Night	22% 14% 31% 23% 10%	540
May	1st to 10th	Night	22% 9% 25% 31% 12% 1	532
	11th to 20th	Night	18% 11% 29% 31% 10%	354
	21st to end	Night	11% 6% 35% 32% 15%	389
June	1st to 10th	Night	16% 8% 31% 34% 11% 1	360
	11th to 20th	Night	13% 9% 30% 36% 13%	360
	21st to end	Night	15% 6% 29% 34% 15% 1	360
July	1st to 10th	Night	9% 6% 20% 47% 17% 1	350
	11th to 20th	Night	11% 8% 27% 32% 20% 1	348
	21st to end	Night	10% 4 28% 37% 19% 1	526
August	1st to 10th	Night	10% 6% 21% 36% 25% 2	537
	11th to 20th	Night	10% 5% 21% 36% 25% 3	534
	21st to end	Night	17% 7% 24% 28% 22% 1	646
September	1st to 10th	Night	13% 8% 29% 29% 18% 4	720
	11th to 20th	Night	16% 8% 24% 31% 17% 4	720
	21st to end	Night	19% 11% 23% 30% 14% 3	720
October	1st to 10th	Night	23% 14% 28% 22% 12% 1	720
	11th to 20th	Night	32% 16% 27% 18% 7% 1	720
	21st to end	Night	32% 20% 28% 17% 3	792
November	1st to 10th	Night	29% 15% 30% 19% 6% 1	740
	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 2024-01-09 16:24:25. Version ac07

Copyright 2024 Attilla Danko