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 2026-08-30.

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% 5% 4 5% 7% 33%	144652
	Night	16% 11% 4 9% 5% 5% 4 3 5% 6% 31%	137555
Transparency	Day	51% 3 12% 22% 11% 1	148145
	Night	54% 3 11% 20% 11% 1	142151
Seeing	Day	24% 14% 31% 19% 9% 2	16779
	Night	24% 13% 30% 24% 9% 1	25846

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% 16% 6% 13% 7% 6% 5% 3 5% 5% 22%	4661
	11th to 20th	Day	12% 19% 8% 16% 6% 5% 5% 3 3 4 19%	4650
	21st to end	Day	11% 16% 6% 15% 8% 8% 5% 3 4 5% 19%	5516
February	1st to 10th	Day	15% 15% 3 13% 7% 5% 5% 4 5% 6% 23%	1890
	11th to 20th	Day	12% 15% 5% 12% 6% 5% 5% 3 5% 5% 27%	1843
	21st to end	Day	19% 14% 6% 11% 7% 5% 4 4 4 4 23%	1722
March	1st to 10th	Day	19% 13% 4 10% 6% 5% 3 3 4 5% 27%	4662
	11th to 20th	Day	11% 13% 4 8% 6% 5% 6% 4 5% 7% 31%	5090
	21st to end	Day	17% 12% 4 8% 5% 3 3 3 4 4 37%	5852
April	1st to 10th	Day	20% 13% 3 8% 5% 4 4 2 4 6% 32%	2520
	11th to 20th	Day	22% 11% 2 7% 3 3 3 3 4 4 36%	2494
	21st to end	Day	21% 11% 3 8% 4 4 4 3 5% 5% 32%	2633
May	1st to 10th	Day	22% 9% 3 8% 5% 5% 5% 4 5% 6% 28%	8237
	11th to 20th	Day	16% 8% 3 8% 5% 4 4 3 6% 8% 35%	8221
	21st to end	Day	19% 9% 2 6% 4 4 4 4 5% 7% 35%	9060
June	1st to 10th	Day	20% 9% 3 6% 5% 4 4 4 6% 6% 32%	2940
	11th to 20th	Day	14% 6% 2 7% 5% 4 5% 5% 6% 9% 36%	2940
	21st to end	Day	16% 7% 3 7% 6% 5% 5% 4 5% 7% 34%	2940
July	1st to 10th	Day	11% 6% 2 7% 5% 5% 5% 5% 6% 10% 37%	8280
	11th to 20th	Day	10% 4 2 5% 4 4 4 4 6% 10% 47%	8280
	21st to end	Day	12% 5% 2 6% 4 4 5% 5% 7% 10% 40%	8993
August	1st to 10th	Day	16% 8% 3 7% 4 4 4 3 6% 7% 37%	4534
	11th to 20th	Day	16% 7% 2 6% 4 4 4 4 7% 9% 37%	4271
	21st to end	Day	14% 6% 2 6% 4 4 5% 4 6% 10% 39%	4420
September	1st to 10th	Day	14% 5% 2 6% 4 5% 5% 3 6% 8% 42%	2400
	11th to 20th	Day	12% 5% 2 5% 4 4 4 3 5% 7% 48%	2400
	21st to end	Day	18% 6% 3 6% 5% 4 5% 3 5% 7% 40%	2123
October	1st to 10th	Day	20% 7% 2 7% 4 3 4 3 5% 6% 38%	2000
	11th to 20th	Day	19% 10% 3 7% 4 5% 5% 3 6% 6% 30%	2000
	21st to end	Day	25% 9% 3 8% 4 4 5% 4 4 6% 27%	2075
November	1st to 10th	Day	15% 10% 3 7% 5% 6% 5% 4 5% 7% 34%	1851
	11th to 20th	Day	17% 13% 4 10% 5% 5% 4 3 4 5% 30%	1680
	21st to end	Day	20% 16% 4 9% 6% 6% 5% 3 5% 5% 22%	1680
December	1st to 10th	Day	21% 18% 5% 13% 5% 5% 4 3 5% 5% 17%	2680
	11th to 20th	Day	18% 18% 5% 11% 6% 4 4 2 4 4 23%	3066
	21st to end	Day	22% 17% 4 13% 6% 4 3 2 3 4 22%	4048
January	1st to 10th	Night	14% 16% 7% 14% 7% 5% 4 3 4 4 21%	8234
	11th to 20th	Night	11% 18% 6% 16% 8% 5% 5% 3 4 5% 18%	8235
	21st to end	Night	13% 15% 6% 13% 6% 6% 4 3 5% 5% 23%	8663
February	1st to 10th	Night	13% 16% 6% 10% 6% 6% 4 3 6% 6% 23%	2730
	11th to 20th	Night	12% 17% 5% 10% 5% 6% 4 3 4 6% 29%	2663
	21st to end	Night	14% 15% 5% 9% 6% 5% 4 4 5% 6% 27%	2083
March	1st to 10th	Night	21% 13% 4 9% 5% 5% 4 2 3 4 30%	5395
	11th to 20th	Night	14% 13% 3 7% 4 5% 4 4 7% 9% 29%	5089
	21st to end	Night	18% 11% 3 8% 4 4 4 3 4 6% 34%	5320
April	1st to 10th	Night	21% 10% 4 8% 5% 4 3 4 4 5% 34%	2100
	11th to 20th	Night	21% 10% 3 6% 4 4 4 3 4 5% 36%	2075
	21st to end	Night	20% 10% 4 7% 4 3 4 3 4 6% 33%	2004
May	1st to 10th	Night	22% 10% 3 8% 5% 5% 5% 4 5% 7% 25%	4660
	11th to 20th	Night	14% 8% 3 7% 4 4 4 3 5% 7% 40%	4639
	21st to end	Night	19% 7% 2 6% 4 4 4 4 6% 7% 37%	5114
June	1st to 10th	Night	16% 7% 2 7% 5% 5% 6% 4 6% 8% 35%	1680
	11th to 20th	Night	13% 7% 2 6% 3 5% 4 4 6% 7% 41%	1680
	21st to end	Night	17% 6% 2 8% 5% 4 5% 5% 6% 8% 34%	1680
July	1st to 10th	Night	10% 5% 2 7% 5% 5% 5% 4 7% 10% 39%	4680
	11th to 20th	Night	10% 5% 2 6% 3 4 4 3 5% 7% 50%	4680
	21st to end	Night	12% 6% 2 6% 5% 4 5% 3 6% 6% 45%	5084
August	1st to 10th	Night	11% 6% 3 7% 4 4 4 4 6% 7% 43%	2832
	11th to 20th	Night	12% 7% 2 7% 4 4 4 4 6% 8% 43%	2965
	21st to end	Night	10% 5% 2 6% 4 5% 6% 4 7% 9% 42%	3674
September	1st to 10th	Night	11% 6% 2 6% 5% 3 4 3 6% 7% 47%	2000
	11th to 20th	Night	12% 4 2 5% 4 3 4 3 5% 6% 52%	2000
	21st to end	Night	17% 6% 2 7% 3 4 4 3 5% 7% 42%	2273
October	1st to 10th	Night	17% 7% 3 7% 4 5% 4 3 5% 7% 39%	2400
	11th to 20th	Night	20% 9% 2 8% 5% 4 5% 3 6% 6% 32%	2400
	21st to end	Night	22% 10% 3 7% 5% 4 3 4 5% 6% 31%	2763
November	1st to 10th	Night	17% 11% 3 7% 4 4 4 4 5% 6% 35%	2667
	11th to 20th	Night	17% 11% 4 8% 5% 4 4 4 5% 6% 33%	2935
	21st to end	Night	20% 13% 4 9% 5% 5% 5% 3 5% 6% 24%	2940
December	1st to 10th	Night	19% 17% 5% 12% 7% 5% 5% 3 4 5% 18%	4720
	11th to 20th	Night	22% 15% 4 9% 5% 4 4 3 4 5% 26%	5348
	21st to end	Night	26% 16% 3 11% 5% 3 3 3 4 4 20%	7150

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	64% 2 13% 15% 4 1	4662
	11th to 20th	Day	71% 2 8% 10% 9% 1	4680
	21st to end	Day	68% 3 9% 10% 10% 1	5516
February	1st to 10th	Day	62% 3 8% 20% 6% 1	1890
	11th to 20th	Day	59% 2 9% 19% 9% 1	1852
	21st to end	Day	67% 2 8% 12% 10% 1	1722
March	1st to 10th	Day	60% 2 9% 14% 12% 3	4665
	11th to 20th	Day	53% 3 16% 15% 12% 2	5090
	21st to end	Day	52% 2 5% 20% 19% 3	5852
April	1st to 10th	Day	56% 3 6% 19% 13% 3	2520
	11th to 20th	Day	52% 3 5% 21% 17% 2	2494
	21st to end	Day	54% 3 8% 17% 15% 3	2633
May	1st to 10th	Day	56% 3 9% 19% 11% 2	8249
	11th to 20th	Day	48% 3 13% 23% 12% 1	8225
	21st to end	Day	49% 3 11% 25% 11% 1	9061
June	1st to 10th	Day	52% 4 9% 21% 13% 1	2940
	11th to 20th	Day	44% 4 10% 27% 13% 2	2940
	21st to end	Day	49% 4 13% 22% 12% 1	2940
July	1st to 10th	Day	40% 5% 15% 28% 11%	8210
	11th to 20th	Day	32% 4 13% 34% 15% 1	8205
	21st to end	Day	38% 5% 15% 33% 9%	8795
August	1st to 10th	Day	46% 3 15% 24% 11% 1	4520
	11th to 20th	Day	43% 4 14% 28% 10% 1	4310
	21st to end	Day	41% 4 15% 29% 11%	4420
September	1st to 10th	Day	40% 4 14% 29% 12% 1	2400
	11th to 20th	Day	36% 4 12% 32% 16%	2400
	21st to end	Day	46% 4 13% 26% 12%	2123
October	1st to 10th	Day	44% 3 11% 32% 9%	3260
	11th to 20th	Day	46% 3 15% 26% 10% 1	3240
	21st to end	Day	49% 4 15% 25% 7%	3326
November	1st to 10th	Day	51% 3 14% 23% 9%	1851
	11th to 20th	Day	56% 3 12% 17% 10% 1	1680
	21st to end	Day	65% 3 12% 12% 8%	1680
December	1st to 10th	Day	70% 3 14% 9% 3	2680
	11th to 20th	Day	66% 3 11% 14% 6%	3066
	21st to end	Day	68% 2 8% 15% 6%	4048
January	1st to 10th	Night	67% 2 13% 13% 5%	8234
	11th to 20th	Night	71% 2 9% 10% 7%	8280
	21st to end	Night	64% 2 8% 13% 12% 1	8663
February	1st to 10th	Night	62% 3 9% 18% 7% 1	2730
	11th to 20th	Night	59% 3 9% 20% 9% 1	2665
	21st to end	Night	59% 3 9% 17% 10% 1	2083
March	1st to 10th	Night	60% 2 6% 17% 11% 4	5405
	11th to 20th	Night	51% 3 17% 17% 9% 3	5090
	21st to end	Night	53% 3 6% 22% 14% 2	5320
April	1st to 10th	Night	54% 2 7% 20% 14% 2	2100
	11th to 20th	Night	52% 3 7% 21% 17% 1	2075
	21st to end	Night	53% 3 7% 17% 16% 3	2004
May	1st to 10th	Night	59% 3 8% 18% 9% 2	4666
	11th to 20th	Night	46% 3 13% 23% 14% 1	4640
	21st to end	Night	46% 3 12% 21% 16% 1	5115
June	1st to 10th	Night	47% 5% 10% 23% 14% 1	1680
	11th to 20th	Night	41% 4 13% 23% 18% 2	1680
	21st to end	Night	47% 6% 12% 22% 13% 1	1680
July	1st to 10th	Night	39% 5% 18% 23% 14% 1	4640
	11th to 20th	Night	33% 4 13% 34% 15% 1	4632
	21st to end	Night	40% 5% 12% 29% 14% 1	4967
August	1st to 10th	Night	40% 5% 14% 27% 13% 1	2822
	11th to 20th	Night	40% 4 14% 32% 10% 1	2992
	21st to end	Night	38% 5% 16% 29% 11% 1	3674
September	1st to 10th	Night	37% 4 12% 31% 15% 1	2000
	11th to 20th	Night	33% 4 13% 35% 14% 1	2000
	21st to end	Night	43% 3 11% 30% 13% 1	2273
October	1st to 10th	Night	41% 3 11% 30% 15%	3920
	11th to 20th	Night	45% 3 15% 27% 10%	3899
	21st to end	Night	48% 4 14% 25% 8%	4462
November	1st to 10th	Night	50% 3 13% 22% 11% 1	2667
	11th to 20th	Night	53% 3 11% 20% 11% 2	2935
	21st to end	Night	62% 3 12% 17% 5%	2940
December	1st to 10th	Night	69% 4 11% 12% 3	4720
	11th to 20th	Night	62% 4 10% 14% 10% 1	5348
	21st to end	Night	70% 2 7% 12% 9%	7150

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% 15% 40% 13% 4	220
	11th to 20th	Day	36% 14% 33% 13% 3	220
	21st to end	Day	23% 21% 33% 18% 5%	314
February	1st to 10th	Day	32% 20% 34% 12% 2	330
	11th to 20th	Day	29% 24% 38% 8% 1	330
	21st to end	Day	35% 17% 36% 11% 2	438
March	1st to 10th	Day	28% 20% 32% 16% 4	540
	11th to 20th	Day	33% 17% 31% 14% 4	540
	21st to end	Day	33% 20% 28% 12% 7% 1	594
April	1st to 10th	Day	32% 21% 29% 12% 6%	540
	11th to 20th	Day	36% 17% 24% 14% 8% 2	532
	21st to end	Day	30% 16% 34% 13% 5% 2	540
May	1st to 10th	Day	26% 16% 31% 19% 6% 2	543
	11th to 20th	Day	19% 14% 37% 19% 9% 2	567
	21st to end	Day	22% 14% 32% 21% 10% 2	617
June	1st to 10th	Day	23% 11% 33% 23% 8% 2	570
	11th to 20th	Day	19% 15% 33% 21% 11% 2	570
	21st to end	Day	21% 11% 31% 23% 11% 4	570
July	1st to 10th	Day	14% 6% 32% 30% 13% 3	565
	11th to 20th	Day	16% 9% 33% 25% 13% 4	564
	21st to end	Day	15% 8% 31% 30% 12% 4	620
August	1st to 10th	Day	16% 5% 31% 30% 14% 4	566
	11th to 20th	Day	14% 6% 30% 30% 15% 6%	559
	21st to end	Day	16% 9% 31% 26% 12% 6%	619
September	1st to 10th	Day	16% 11% 29% 24% 12% 7%	530
	11th to 20th	Day	12% 7% 24% 27% 19% 11%	530
	21st to end	Day	21% 9% 29% 23% 14% 4	530
October	1st to 10th	Day	25% 13% 27% 19% 12% 4	530
	11th to 20th	Day	29% 16% 31% 16% 6% 2	530
	21st to end	Day	30% 18% 29% 20% 4	514
November	1st to 10th	Day	27% 17% 28% 20% 7%	425
	11th to 20th	Day	28% 15% 37% 12% 6% 2	220
	21st to end	Day	32% 20% 34% 11% 3	220
December	1st to 10th	Day	34% 16% 33% 12% 5%	220
	11th to 20th	Day	36% 20% 32% 7% 4	220
	21st to end	Day	36% 15% 31% 12% 5%	242
January	1st to 10th	Night	28% 16% 33% 19% 3	840
	11th to 20th	Night	31% 16% 29% 20% 4	840
	21st to end	Night	22% 15% 34% 23% 6%	924
February	1st to 10th	Night	24% 18% 34% 20% 3	840
	11th to 20th	Night	24% 18% 37% 18% 3	823
	21st to end	Night	25% 17% 35% 18% 4	692
March	1st to 10th	Night	28% 15% 31% 23% 4	840
	11th to 20th	Night	28% 18% 28% 21% 4	840
	21st to end	Night	26% 15% 31% 20% 7%	924
April	1st to 10th	Night	29% 14% 32% 22% 3	840
	11th to 20th	Night	28% 18% 27% 20% 7% 1	831
	21st to end	Night	26% 12% 29% 25% 7% 1	744
May	1st to 10th	Night	26% 9% 26% 27% 10% 1	420
	11th to 20th	Night	15% 12% 33% 31% 8%	414
	21st to end	Night	14% 11% 38% 27% 11%	455
June	1st to 10th	Night	16% 7% 30% 36% 10% 1	420
	11th to 20th	Night	17% 10% 30% 30% 13% 1	420
	21st to end	Night	18% 5% 26% 37% 13%	420
July	1st to 10th	Night	11% 4 32% 36% 15% 1	410
	11th to 20th	Night	14% 5% 27% 38% 15% 1	408
	21st to end	Night	11% 5% 28% 41% 14% 1	460
August	1st to 10th	Night	14% 3 24% 36% 21% 1	519
	11th to 20th	Night	14% 3 23% 35% 23% 2	624
	21st to end	Night	13% 6% 29% 33% 17% 2	914
September	1st to 10th	Night	15% 6% 31% 30% 18% 1	800
	11th to 20th	Night	16% 5% 20% 30% 24% 5%	800
	21st to end	Night	20% 8% 26% 25% 19% 2	800
October	1st to 10th	Night	22% 10% 27% 23% 16% 2	800
	11th to 20th	Night	27% 11% 31% 24% 7% 1	800
	21st to end	Night	29% 13% 27% 25% 6%	880
November	1st to 10th	Night	29% 13% 29% 21% 8%	820
	11th to 20th	Night	26% 16% 31% 17% 9%	840
	21st to end	Night	31% 16% 27% 19% 6%	840
December	1st to 10th	Night	30% 16% 32% 18% 4	840
	11th to 20th	Night	28% 19% 32% 17% 3 1	840
	21st to end	Night	35% 15% 29% 19% 3	924

Page generated at 2026-08-27 04:07:15. Version ac07

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