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 2024-10-25.

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%	101496
	Night	17% 11% 3 8% 5% 4 4 3 5% 6% 32%	97311
Transparency	Day	51% 3 11% 22% 11% 1	101254
	Night	54% 3 11% 21% 10% 1	97173
Seeing	Day	24% 13% 31% 20% 9% 3	14473
	Night	24% 12% 29% 25% 9% 1	23581

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	15% 16% 6% 10% 6% 6% 5% 3 4 4 25%	2517
	11th to 20th	Day	14% 16% 6% 14% 7% 5% 5% 3 4 5% 22%	2520
	21st to end	Day	17% 19% 6% 14% 7% 6% 4 3 3 4 17%	2971
February	1st to 10th	Day	16% 15% 3 12% 7% 5% 5% 4 5% 6% 23%	1710
	11th to 20th	Day	12% 16% 5% 11% 6% 5% 5% 4 4 5% 28%	1663
	21st to end	Day	20% 14% 5% 10% 7% 5% 4 4 4 4 24%	1562
March	1st to 10th	Day	21% 13% 4 10% 5% 5% 3 2 4 4 28%	3172
	11th to 20th	Day	11% 16% 5% 8% 6% 5% 5% 3 4 7% 30%	3480
	21st to end	Day	12% 10% 3 7% 4 3 4 3 5% 5% 42%	3992
April	1st to 10th	Day	20% 13% 3 7% 5% 4 4 2 4 6% 32%	2280
	11th to 20th	Day	22% 10% 2 7% 3 3 3 3 4 5% 37%	2254
	21st to end	Day	22% 10% 3 8% 4 4 4 4 5% 5% 32%	2382
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	26% 10% 3 8% 4 4 5% 4 4 6% 26%	1915
November	1st to 10th	Day	15% 9% 3 7% 5% 6% 5% 4 5% 7% 34%	1671
	11th to 20th	Day	18% 13% 3 10% 5% 4 4 3 5% 5% 30%	1520
	21st to end	Day	20% 15% 4 9% 6% 6% 6% 3 5% 5% 21%	1520
December	1st to 10th	Day	22% 18% 5% 11% 5% 5% 5% 3 5% 5% 18%	1520
	11th to 20th	Day	21% 14% 5% 10% 5% 4 5% 2 5% 4 25%	1906
	21st to end	Day	27% 17% 5% 12% 5% 4 3 2 2 3 20%	2772
January	1st to 10th	Night	17% 15% 6% 12% 7% 5% 4 3 5% 5% 21%	4429
	11th to 20th	Night	13% 15% 5% 14% 8% 5% 5% 4 5% 5% 20%	4440
	21st to end	Night	19% 17% 6% 12% 6% 6% 4 3 4 4 20%	4683
February	1st to 10th	Night	14% 16% 5% 10% 6% 6% 4 3 6% 5% 23%	2470
	11th to 20th	Night	12% 16% 5% 10% 5% 5% 4 3 4 5% 29%	2403
	21st to end	Night	15% 14% 4 8% 6% 5% 4 4 5% 6% 28%	1891
March	1st to 10th	Night	20% 12% 3 9% 5% 4 4 3 4 5% 32%	3665
	11th to 20th	Night	18% 17% 4 8% 5% 4 3 3 5% 7% 26%	3479
	21st to end	Night	15% 10% 3 7% 4 4 3 3 4 6% 41%	3638
April	1st to 10th	Night	21% 10% 4 8% 5% 4 3 4 4 5% 35%	1900
	11th to 20th	Night	20% 10% 2 7% 5% 4 4 3 4 5% 37%	1875
	21st to end	Night	20% 10% 4 7% 5% 3 4 4 5% 6% 32%	1814
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%	2545
November	1st to 10th	Night	18% 11% 2 6% 4 4 4 4 5% 6% 35%	2407
	11th to 20th	Night	17% 11% 4 8% 5% 4 4 4 5% 6% 32%	2655
	21st to end	Night	21% 13% 4 9% 5% 5% 5% 3 5% 6% 25%	2660
December	1st to 10th	Night	19% 18% 6% 10% 6% 4 4 3 4 5% 20%	2660
	11th to 20th	Night	21% 15% 4 9% 6% 4 4 3 4 4 27%	3288
	21st to end	Night	30% 16% 3 11% 5% 4 3 3 4 4 16%	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	63% 2 15% 13% 6% 2	2518
	11th to 20th	Day	68% 1 10% 12% 8%	2520
	21st to end	Day	73% 3 9% 9% 5% 1	2971
February	1st to 10th	Day	63% 3 8% 20% 5% 1	1710
	11th to 20th	Day	59% 2 10% 18% 10% 1	1672
	21st to end	Day	65% 2 9% 12% 10% 1	1562
March	1st to 10th	Day	61% 2 8% 19% 9% 1	3175
	11th to 20th	Day	56% 3 12% 15% 12% 2	3480
	21st to end	Day	45% 3 6% 25% 18% 4	3992
April	1st to 10th	Day	55% 3 6% 20% 13% 3	2280
	11th to 20th	Day	51% 3 5% 22% 17% 2	2254
	21st to end	Day	54% 3 8% 17% 14% 4	2382
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	50% 2 11% 24% 12% 1	1900
	11th to 20th	Day	55% 3 14% 20% 8% 1	1900
	21st to end	Day	60% 3 10% 19% 7% 1	1915
November	1st to 10th	Day	50% 3 14% 23% 9%	1671
	11th to 20th	Day	56% 3 12% 17% 10% 2	1520
	21st to end	Day	64% 4 12% 12% 8%	1520
December	1st to 10th	Day	69% 3 12% 11% 4 1	1520
	11th to 20th	Day	65% 4 14% 13% 5%	1906
	21st to end	Day	73% 2 7% 14% 4	2772
January	1st to 10th	Night	66% 2 15% 12% 4	4429
	11th to 20th	Night	69% 2 11% 12% 5%	4440
	21st to end	Night	71% 2 10% 11% 6% 1	4683
February	1st to 10th	Night	63% 3 10% 17% 7% 1	2470
	11th to 20th	Night	58% 3 9% 20% 9%	2405
	21st to end	Night	58% 4 9% 18% 11% 1	1891
March	1st to 10th	Night	57% 2 8% 23% 10% 1	3675
	11th to 20th	Night	60% 2 13% 15% 9% 1	3480
	21st to end	Night	47% 3 5% 25% 17% 3	3638
April	1st to 10th	Night	53% 2 7% 21% 15% 2	1900
	11th to 20th	Night	51% 3 7% 20% 17% 2	1875
	21st to end	Night	53% 3 8% 17% 16% 3	1814
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	48% 4 10% 24% 14% 1	2280
	11th to 20th	Night	53% 3 13% 21% 10% 1	2280
	21st to end	Night	56% 3 13% 17% 10%	2545
November	1st to 10th	Night	50% 3 14% 22% 11% 1	2407
	11th to 20th	Night	53% 3 11% 20% 12% 2	2655
	21st to end	Night	62% 3 12% 17% 6%	2660
December	1st to 10th	Night	69% 3 9% 15% 4	2660
	11th to 20th	Night	62% 3 10% 16% 8% 1	3288
	21st to end	Night	73% 2 9% 13% 3	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	29% 17% 36% 13% 4 1	180
	11th to 20th	Day	36% 13% 32% 16% 2 1	180
	21st to end	Day	24% 18% 34% 18% 5%	256
February	1st to 10th	Day	35% 20% 33% 11% 1	270
	11th to 20th	Day	30% 24% 39% 7% 1	270
	21st to end	Day	36% 16% 35% 11% 2	374
March	1st to 10th	Day	28% 19% 32% 17% 5%	460
	11th to 20th	Day	31% 15% 33% 15% 5%	460
	21st to end	Day	32% 19% 29% 12% 7% 1	506
April	1st to 10th	Day	32% 19% 28% 14% 7%	460
	11th to 20th	Day	35% 17% 23% 14% 8% 3	452
	21st to end	Day	30% 16% 34% 13% 5% 2	460
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	31% 17% 28% 19% 5%	458
November	1st to 10th	Day	25% 17% 28% 21% 8%	365
	11th to 20th	Day	31% 14% 34% 13% 6% 2	180
	21st to end	Day	29% 21% 34% 13% 3	180
December	1st to 10th	Day	32% 17% 34% 13% 4	180
	11th to 20th	Day	36% 21% 31% 9% 4 1	180
	21st to end	Day	38% 14% 32% 12% 3 1	198
January	1st to 10th	Night	30% 17% 30% 20% 3	760
	11th to 20th	Night	31% 15% 30% 20% 4 1	760
	21st to end	Night	22% 14% 33% 25% 6%	836
February	1st to 10th	Night	24% 19% 34% 20% 2	760
	11th to 20th	Night	23% 18% 38% 18% 3	743
	21st to end	Night	25% 17% 35% 19% 4	628
March	1st to 10th	Night	27% 15% 31% 24% 4	760
	11th to 20th	Night	28% 19% 28% 21% 4	760
	21st to end	Night	25% 14% 31% 21% 8%	836
April	1st to 10th	Night	29% 13% 33% 22% 3	760
	11th to 20th	Night	27% 18% 28% 19% 7% 1	751
	21st to end	Night	27% 12% 28% 24% 8% 1	674
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	29% 13% 27% 25% 6%	812
November	1st to 10th	Night	28% 12% 29% 21% 8% 1	740
	11th to 20th	Night	27% 17% 30% 17% 9% 1	760
	21st to end	Night	32% 15% 27% 20% 6%	760
December	1st to 10th	Night	30% 16% 32% 18% 4	760
	11th to 20th	Night	28% 18% 32% 18% 3 1	760
	21st to end	Night	35% 15% 29% 19% 2	836

Page generated at 2024-10-22 20:21:02. Version ac07

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