Black Lake Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Black Lake Observatory clear sky chart from 2005-11-06 to 2025-04-26.

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% 3 8% 5% 5% 4 4 5% 7% 32%	113748
	Night	17% 12% 4 9% 5% 5% 4 3 5% 6% 31%	114853
Transparency	Day	52% 3 11% 22% 11% 1	117287
	Night	54% 3 11% 20% 11% 1	119478
Seeing	Day	25% 14% 30% 20% 9% 3	15008
	Night	24% 12% 30% 24% 9% 1	24312

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	13% 15% 6% 12% 6% 6% 5% 3 4 5% 24%	3597
	11th to 20th	Day	12% 17% 7% 15% 6% 5% 5% 3 4 5% 21%	3570
	21st to end	Day	14% 18% 6% 14% 7% 6% 4 3 4 5% 20%	4257
February	1st to 10th	Day	16% 15% 3 13% 7% 5% 5% 4 5% 6% 23%	1800
	11th to 20th	Day	12% 16% 5% 12% 6% 5% 5% 3 4 5% 28%	1753
	21st to end	Day	20% 14% 5% 10% 7% 5% 4 4 4 4 23%	1642
March	1st to 10th	Day	19% 14% 4 10% 6% 5% 3 3 4 5% 27%	4560
	11th to 20th	Day	11% 13% 4 8% 6% 5% 6% 4 5% 7% 31%	4980
	21st to end	Day	18% 12% 4 8% 4 3 3 3 4 4 38%	5726
April	1st to 10th	Day	20% 13% 3 7% 5% 4 4 2 4 6% 32%	2400
	11th to 20th	Day	21% 10% 2 7% 3 3 3 3 4 5% 37%	2374
	21st to end	Day	22% 10% 3 8% 4 4 4 3 5% 5% 32%	2442
May	1st to 10th	Day	20% 7% 3 8% 5% 5% 4 3 5% 6% 32%	4426
	11th to 20th	Day	16% 9% 3 8% 5% 4 5% 3 5% 8% 33%	4398
	21st to end	Day	13% 7% 2 6% 4 5% 5% 4 6% 9% 40%	4837
June	1st to 10th	Day	20% 9% 3 6% 5% 5% 4 4 6% 6% 30%	2660
	11th to 20th	Day	15% 6% 2 7% 5% 4 5% 5% 6% 9% 37%	2660
	21st to end	Day	17% 8% 3 7% 6% 5% 5% 4 5% 7% 33%	2660
July	1st to 10th	Day	11% 6% 2 7% 5% 5% 5% 5% 6% 10% 36%	4440
	11th to 20th	Day	13% 6% 2 5% 4 5% 5% 5% 6% 10% 40%	4440
	21st to end	Day	10% 6% 2 6% 5% 4 5% 5% 7% 9% 41%	4814
August	1st to 10th	Day	17% 8% 3 7% 4 5% 5% 3 6% 7% 36%	4264
	11th to 20th	Day	17% 7% 2 6% 4 4 4 4 7% 9% 37%	4013
	21st to end	Day	15% 6% 2 6% 4 4 5% 4 6% 10% 39%	4180
September	1st to 10th	Day	14% 5% 2 6% 4 5% 6% 3 6% 8% 42%	2280
	11th to 20th	Day	12% 5% 2 5% 4 4 4 3 5% 7% 48%	2280
	21st to end	Day	18% 6% 3 6% 5% 4 5% 3 5% 7% 40%	2017
October	1st to 10th	Day	21% 7% 3 8% 4 3 4 3 5% 6% 36%	1900
	11th to 20th	Day	20% 10% 3 8% 5% 5% 5% 3 6% 6% 30%	1900
	21st to end	Day	25% 10% 3 8% 4 4 5% 4 4 6% 26%	1971
November	1st to 10th	Day	15% 9% 3 7% 5% 6% 5% 4 6% 7% 34%	1761
	11th to 20th	Day	17% 13% 3 10% 5% 5% 4 3 5% 5% 30%	1600
	21st to end	Day	20% 15% 4 9% 6% 6% 6% 3 5% 5% 21%	1600
December	1st to 10th	Day	21% 17% 5% 13% 5% 5% 5% 3 5% 5% 17%	2600
	11th to 20th	Day	18% 18% 5% 11% 6% 4 4 2 4 4 23%	2986
	21st to end	Day	22% 17% 4 12% 6% 4 3 2 3 4 23%	3960
January	1st to 10th	Night	14% 16% 7% 14% 7% 5% 4 3 4 4 22%	6349
	11th to 20th	Night	11% 17% 7% 15% 8% 5% 5% 3 4 5% 19%	6315
	21st to end	Night	16% 16% 7% 14% 6% 6% 4 3 4 4 20%	6697
February	1st to 10th	Night	14% 16% 6% 10% 6% 6% 4 3 6% 6% 23%	2600
	11th to 20th	Night	12% 16% 5% 10% 5% 6% 4 3 4 5% 29%	2533
	21st to end	Night	14% 15% 4 8% 6% 5% 4 4 5% 6% 28%	1987
March	1st to 10th	Night	21% 13% 4 10% 5% 5% 4 2 3 4 30%	5277
	11th to 20th	Night	14% 13% 3 7% 4 5% 4 5% 7% 9% 30%	4979
	21st to end	Night	18% 11% 3 8% 4 4 4 3 4 6% 35%	5204
April	1st to 10th	Night	21% 10% 4 8% 5% 4 3 4 4 5% 34%	2000
	11th to 20th	Night	20% 10% 2 7% 5% 4 4 3 4 5% 37%	1975
	21st to end	Night	20% 10% 4 7% 4 4 4 4 4 6% 32%	1871
May	1st to 10th	Night	22% 10% 3 7% 5% 4 5% 4 5% 7% 30%	2514
	11th to 20th	Night	19% 8% 3 8% 4 4 4 3 5% 7% 37%	2495
	21st to end	Night	14% 6% 2 6% 4 4 4 4 6% 8% 42%	2738
June	1st to 10th	Night	16% 7% 2 7% 5% 5% 6% 4 6% 8% 33%	1520
	11th to 20th	Night	13% 7% 2 6% 3 5% 4 4 6% 8% 41%	1520
	21st to end	Night	18% 6% 2 7% 5% 4 4 5% 7% 8% 34%	1520
July	1st to 10th	Night	10% 5% 3 8% 5% 6% 6% 5% 7% 8% 38%	2520
	11th to 20th	Night	13% 6% 2 6% 4 4 4 3 5% 7% 45%	2520
	21st to end	Night	10% 6% 2 6% 4 5% 5% 4 7% 7% 45%	2739
August	1st to 10th	Night	11% 6% 3 7% 4 4 4 4 6% 7% 43%	2662
	11th to 20th	Night	12% 7% 2 7% 4 4 4 3 6% 8% 42%	2785
	21st to end	Night	10% 5% 2 6% 4 5% 6% 4 7% 9% 42%	3473
September	1st to 10th	Night	11% 6% 2 6% 5% 3 4 3 6% 7% 46%	1900
	11th to 20th	Night	13% 4 1 5% 3 3 4 4 5% 6% 51%	1900
	21st to end	Night	17% 6% 2 7% 3 4 4 3 5% 7% 42%	2159
October	1st to 10th	Night	17% 7% 3 7% 4 5% 4 3 5% 7% 37%	2280
	11th to 20th	Night	21% 9% 3 8% 5% 4 5% 3 6% 6% 32%	2280
	21st to end	Night	23% 10% 3 7% 5% 4 3 4 5% 6% 29%	2625
November	1st to 10th	Night	17% 11% 3 6% 4 4 4 4 5% 6% 36%	2537
	11th to 20th	Night	17% 11% 4 8% 5% 4 4 4 5% 6% 33%	2795
	21st to end	Night	21% 13% 4 9% 5% 5% 5% 3 5% 6% 24%	2800
December	1st to 10th	Night	19% 17% 5% 12% 7% 5% 5% 3 4 6% 18%	4580
	11th to 20th	Night	22% 15% 4 9% 5% 4 4 3 4 5% 26%	5208
	21st to end	Night	26% 16% 3 11% 5% 4 3 3 4 4 20%	6996

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	63% 2 12% 17% 5% 1	3598
	11th to 20th	Day	68% 2 10% 12% 8%	3600
	21st to end	Day	67% 3 9% 10% 9% 1	4257
February	1st to 10th	Day	62% 3 8% 19% 6% 1	1800
	11th to 20th	Day	59% 2 9% 18% 10% 1	1762
	21st to end	Day	66% 2 9% 12% 10% 1	1642
March	1st to 10th	Day	61% 1 9% 14% 12% 3	4563
	11th to 20th	Day	52% 3 16% 15% 12% 2	4980
	21st to end	Day	51% 2 5% 20% 19% 3	5726
April	1st to 10th	Day	55% 3 6% 19% 14% 3	2400
	11th to 20th	Day	51% 3 5% 21% 17% 2	2374
	21st to end	Day	55% 3 8% 17% 14% 4	2442
May	1st to 10th	Day	53% 3 9% 22% 11% 2	4440
	11th to 20th	Day	51% 2 13% 23% 9% 1	4402
	21st to end	Day	42% 3 13% 26% 15% 2	4837
June	1st to 10th	Day	53% 4 9% 20% 12% 1	2660
	11th to 20th	Day	43% 4 10% 27% 13% 2	2660
	21st to end	Day	50% 4 13% 21% 11% 1	2660
July	1st to 10th	Day	41% 5% 13% 28% 12% 1	4370
	11th to 20th	Day	38% 5% 15% 33% 9%	4365
	21st to end	Day	39% 5% 13% 32% 11%	4661
August	1st to 10th	Day	47% 3 14% 24% 11% 1	4250
	11th to 20th	Day	43% 4 14% 28% 11% 1	4052
	21st to end	Day	41% 3 15% 29% 11%	4180
September	1st to 10th	Day	41% 4 14% 29% 13% 1	2280
	11th to 20th	Day	36% 4 13% 31% 16%	2280
	21st to end	Day	45% 4 13% 26% 12%	2017
October	1st to 10th	Day	45% 3 12% 32% 8%	3160
	11th to 20th	Day	46% 3 15% 26% 10%	3140
	21st to end	Day	49% 4 15% 25% 7%	3222
November	1st to 10th	Day	50% 3 14% 23% 9%	1761
	11th to 20th	Day	55% 3 11% 18% 11% 2	1600
	21st to end	Day	65% 4 13% 11% 8%	1600
December	1st to 10th	Day	70% 4 15% 9% 3	2600
	11th to 20th	Day	66% 3 11% 14% 6%	2986
	21st to end	Day	68% 2 8% 16% 6%	3960
January	1st to 10th	Night	67% 1 13% 14% 5%	6349
	11th to 20th	Night	70% 2 10% 11% 6%	6360
	21st to end	Night	68% 2 9% 9% 11% 1	6697
February	1st to 10th	Night	62% 3 10% 17% 7% 1	2600
	11th to 20th	Night	59% 3 9% 20% 9% 1	2535
	21st to end	Night	58% 3 9% 18% 10% 1	1987
March	1st to 10th	Night	61% 2 6% 17% 11% 4	5287
	11th to 20th	Night	51% 3 18% 17% 9% 3	4980
	21st to end	Night	53% 3 6% 23% 14% 2	5204
April	1st to 10th	Night	54% 2 7% 20% 14% 2	2000
	11th to 20th	Night	51% 3 7% 21% 17% 1	1975
	21st to end	Night	53% 3 8% 17% 16% 3	1871
May	1st to 10th	Night	56% 2 9% 20% 10% 3	2520
	11th to 20th	Night	49% 3 11% 25% 10% 2	2496
	21st to end	Night	41% 3 13% 23% 18% 2	2739
June	1st to 10th	Night	48% 5% 11% 23% 12% 1	1520
	11th to 20th	Night	41% 3 12% 23% 18% 2	1520
	21st to end	Night	46% 6% 13% 21% 13% 1	1520
July	1st to 10th	Night	43% 5% 13% 22% 15% 2	2480
	11th to 20th	Night	38% 4 12% 35% 10%	2472
	21st to end	Night	38% 6% 14% 26% 14% 2	2651
August	1st to 10th	Night	40% 4 14% 27% 13% 1	2652
	11th to 20th	Night	41% 4 14% 32% 9% 1	2812
	21st to end	Night	38% 5% 15% 29% 11% 1	3473
September	1st to 10th	Night	38% 4 12% 30% 15% 1	1900
	11th to 20th	Night	33% 4 14% 34% 14% 1	1900
	21st to end	Night	43% 3 11% 31% 12% 1	2159
October	1st to 10th	Night	42% 3 12% 29% 14%	3800
	11th to 20th	Night	45% 3 15% 27% 10%	3779
	21st to end	Night	49% 4 14% 25% 8%	4324
November	1st to 10th	Night	50% 3 13% 22% 11% 1	2537
	11th to 20th	Night	53% 3 11% 20% 12% 2	2795
	21st to end	Night	62% 3 12% 17% 6%	2800
December	1st to 10th	Night	70% 4 12% 12% 3	4580
	11th to 20th	Night	62% 4 10% 14% 10% 1	5208
	21st to end	Night	70% 2 7% 12% 10%	6996

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	29% 16% 39% 12% 4 1	200
	11th to 20th	Day	34% 14% 35% 15% 3 1	200
	21st to end	Day	23% 22% 33% 17% 5%	285
February	1st to 10th	Day	34% 21% 32% 11% 1	300
	11th to 20th	Day	31% 25% 36% 7% 1	300
	21st to end	Day	35% 16% 35% 11% 2	406
March	1st to 10th	Day	28% 19% 32% 16% 4	500
	11th to 20th	Day	31% 17% 32% 15% 5%	500
	21st to end	Day	33% 19% 28% 12% 7% 1	550
April	1st to 10th	Day	32% 20% 28% 13% 6%	500
	11th to 20th	Day	35% 17% 23% 14% 8% 2	492
	21st to end	Day	30% 17% 34% 13% 5% 2	480
May	1st to 10th	Day	26% 14% 31% 21% 7% 2	463
	11th to 20th	Day	20% 13% 35% 21% 9% 2	487
	21st to end	Day	19% 13% 32% 23% 11% 2	529
June	1st to 10th	Day	24% 10% 32% 23% 8% 2	490
	11th to 20th	Day	19% 13% 30% 23% 12% 3	490
	21st to end	Day	22% 11% 30% 23% 11% 3	490
July	1st to 10th	Day	15% 6% 29% 32% 14% 4	485
	11th to 20th	Day	17% 7% 32% 26% 14% 4	484
	21st to end	Day	14% 8% 32% 30% 13% 4	532
August	1st to 10th	Day	17% 6% 29% 31% 14% 4	486
	11th to 20th	Day	14% 6% 28% 30% 16% 7%	479
	21st to end	Day	17% 9% 31% 26% 12% 6%	539
September	1st to 10th	Day	16% 11% 30% 24% 12% 7%	490
	11th to 20th	Day	13% 8% 24% 25% 19% 11%	490
	21st to end	Day	21% 9% 27% 24% 14% 4	490
October	1st to 10th	Day	27% 14% 27% 18% 11% 4	490
	11th to 20th	Day	30% 16% 29% 17% 6% 2	490
	21st to end	Day	30% 18% 28% 19% 4	476
November	1st to 10th	Day	26% 17% 29% 21% 8%	395
	11th to 20th	Day	29% 15% 36% 13% 6% 2	200
	21st to end	Day	31% 21% 33% 13% 3	200
December	1st to 10th	Day	33% 17% 33% 13% 5%	200
	11th to 20th	Day	37% 21% 30% 8% 5% 1	200
	21st to end	Day	35% 15% 31% 14% 5%	220
January	1st to 10th	Night	29% 17% 32% 19% 3	800
	11th to 20th	Night	30% 16% 30% 20% 4 1	800
	21st to end	Night	22% 15% 34% 24% 6%	880
February	1st to 10th	Night	25% 19% 34% 20% 2	800
	11th to 20th	Night	24% 18% 37% 18% 3	783
	21st to end	Night	25% 17% 35% 19% 4	660
March	1st to 10th	Night	28% 15% 31% 23% 4	800
	11th to 20th	Night	27% 19% 28% 22% 4	800
	21st to end	Night	26% 15% 31% 21% 7%	880
April	1st to 10th	Night	29% 14% 32% 22% 3	800
	11th to 20th	Night	28% 18% 27% 19% 7% 1	791
	21st to end	Night	27% 12% 29% 25% 7% 1	697
May	1st to 10th	Night	25% 8% 26% 29% 11% 2	380
	11th to 20th	Night	16% 10% 33% 32% 9% 1	374
	21st to end	Night	12% 11% 38% 27% 12%	411
June	1st to 10th	Night	16% 7% 30% 37% 10% 1	380
	11th to 20th	Night	16% 8% 29% 31% 14% 1	380
	21st to end	Night	19% 5% 25% 37% 14% 1	380
July	1st to 10th	Night	12% 5% 31% 35% 16% 1	370
	11th to 20th	Night	15% 3 27% 38% 16% 1	368
	21st to end	Night	10% 4 29% 41% 14% 1	416
August	1st to 10th	Night	14% 4 25% 36% 21% 1	469
	11th to 20th	Night	14% 3 22% 35% 24% 2	564
	21st to end	Night	12% 6% 29% 33% 18% 2	833
September	1st to 10th	Night	15% 6% 30% 31% 17% 1	760
	11th to 20th	Night	16% 6% 20% 29% 23% 6%	760
	21st to end	Night	20% 8% 25% 25% 19% 2	760
October	1st to 10th	Night	23% 11% 27% 23% 15% 3	760
	11th to 20th	Night	27% 11% 30% 24% 8% 1	760
	21st to end	Night	30% 13% 27% 25% 6%	836
November	1st to 10th	Night	28% 13% 29% 21% 8% 1	780
	11th to 20th	Night	27% 16% 31% 17% 9% 1	800
	21st to end	Night	31% 16% 27% 19% 6%	800
December	1st to 10th	Night	30% 16% 32% 17% 4	800
	11th to 20th	Night	28% 18% 32% 18% 3 1	800
	21st to end	Night	34% 14% 29% 19% 3	880

Page generated at 2025-04-23 09:25:06. Version ac07

Copyright © 2025 Attilla Danko.