Leaking Light Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Leaking Light Observatory clear sky chart from 2005-11-06 to 2025-05-26.

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	15% 10% 3 8% 5% 4 4 4 5% 7% 33%	114011
	Night	16% 11% 4 8% 5% 4 4 3 5% 6% 34%	115242
Transparency	Day	48% 5% 10% 22% 13% 2	117700
	Night	49% 5% 9% 21% 13% 2	119692
Seeing	Day	22% 13% 26% 21% 12% 4	15541
	Night	21% 10% 25% 28% 14% 2	25886

By Month

Percentage Cloud Cover SE (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	17% 22% 8% 15% 6% 3 2 2 3 5% 17%	4056
	11th to 20th	Day	15% 20% 8% 12% 8% 5% 4 2 3 3 19%	4060
	21st to end	Day	20% 17% 5% 10% 7% 5% 4 3 4 4 21%	4466
February	1st to 10th	Day	23% 14% 4 9% 6% 4 3 3 4 5% 27%	1800
	11th to 20th	Day	20% 15% 4 11% 4 3 4 3 5% 6% 25%	1843
	21st to end	Day	21% 13% 4 9% 5% 4 3 3 5% 5% 29%	1650
March	1st to 10th	Day	17% 16% 4 7% 4 4 4 3 4 5% 32%	4887
	11th to 20th	Day	13% 8% 3 7% 4 4 4 3 5% 7% 43%	4980
	21st to end	Day	18% 10% 3 8% 5% 4 5% 4 6% 7% 31%	5465
April	1st to 10th	Day	16% 11% 3 8% 5% 5% 5% 4 6% 6% 32%	2225
	11th to 20th	Day	15% 10% 3 7% 5% 4 5% 2 5% 7% 38%	2479
	21st to end	Day	19% 8% 2 6% 5% 5% 4 3 6% 8% 34%	2600
May	1st to 10th	Day	21% 10% 4 8% 5% 5% 5% 4 6% 6% 27%	4212
	11th to 20th	Day	18% 9% 3 7% 6% 4 5% 4 5% 7% 32%	4211
	21st to end	Day	11% 6% 2 7% 5% 5% 6% 5% 7% 10% 37%	4541
June	1st to 10th	Day	13% 7% 2 5% 4 6% 5% 5% 7% 9% 38%	2470
	11th to 20th	Day	10% 6% 3 7% 5% 5% 6% 5% 7% 9% 37%	2470
	21st to end	Day	12% 6% 2 6% 5% 4 6% 5% 6% 11% 37%	2470
July	1st to 10th	Day	9% 6% 2 6% 5% 6% 5% 4 7% 9% 40%	4120
	11th to 20th	Day	10% 5% 2 6% 4 4 4 4 6% 10% 43%	4120
	21st to end	Day	15% 8% 3 7% 5% 5% 5% 4 6% 8% 33%	4466
August	1st to 10th	Day	9% 7% 3 8% 5% 6% 6% 4 7% 10% 35%	4104
	11th to 20th	Day	9% 7% 2 7% 5% 5% 6% 4 8% 10% 38%	4044
	21st to end	Day	6% 4 2 5% 4 5% 5% 4 7% 9% 48%	4386
September	1st to 10th	Day	14% 5% 2 5% 4 4 3 3 6% 6% 48%	2162
	11th to 20th	Day	11% 5% 2 6% 4 5% 4 4 6% 7% 46%	2090
	21st to end	Day	19% 9% 3 6% 4 3 4 3 5% 7% 39%	2090
October	1st to 10th	Day	14% 6% 2 5% 4 4 3 4 6% 8% 43%	1967
	11th to 20th	Day	16% 8% 2 6% 5% 3 4 4 5% 6% 41%	1900
	21st to end	Day	22% 7% 3 7% 4 3 3 3 4 5% 39%	2090
November	1st to 10th	Day	12% 6% 2 6% 3 4 4 3 4 6% 49%	1829
	11th to 20th	Day	17% 13% 3 6% 4 4 3 4 5% 6% 35%	1800
	21st to end	Day	17% 13% 5% 9% 3 4 3 4 4 6% 33%	1737
December	1st to 10th	Day	20% 13% 4 15% 4 3 3 3 4 5% 26%	2600
	11th to 20th	Day	23% 15% 5% 10% 4 3 4 3 4 5% 23%	3155
	21st to end	Day	21% 17% 5% 11% 5% 5% 5% 4 5% 5% 18%	4466
January	1st to 10th	Night	15% 18% 7% 18% 5% 4 3 3 4 4 20%	5889
	11th to 20th	Night	17% 17% 6% 13% 7% 5% 4 3 3 4 21%	5900
	21st to end	Night	21% 12% 5% 11% 5% 4 4 2 3 4 28%	6490
February	1st to 10th	Night	21% 14% 4 9% 5% 4 4 3 5% 5% 28%	2600
	11th to 20th	Night	22% 13% 4 8% 6% 4 4 3 4 5% 26%	2440
	21st to end	Night	22% 14% 4 9% 4 4 4 3 4 5% 29%	1980
March	1st to 10th	Night	20% 13% 3 6% 4 4 3 2 4 6% 36%	4965
	11th to 20th	Night	11% 9% 3 7% 4 4 4 4 5% 7% 42%	4980
	21st to end	Night	17% 10% 3 8% 5% 5% 4 3 5% 6% 34%	5467
April	1st to 10th	Night	19% 10% 2 7% 5% 3 4 3 5% 6% 35%	2173
	11th to 20th	Night	15% 8% 3 6% 4 4 4 3 6% 8% 38%	1887
	21st to end	Night	15% 8% 3 7% 4 4 3 4 5% 8% 40%	1800
May	1st to 10th	Night	25% 11% 3 8% 5% 5% 4 3 4 6% 25%	2903
	11th to 20th	Night	17% 9% 3 6% 5% 3 3 3 5% 8% 37%	2902
	21st to end	Night	11% 8% 2 7% 4 4 4 4 6% 9% 41%	3130
June	1st to 10th	Night	10% 5% 2 5% 5% 4 5% 4 5% 9% 45%	1710
	11th to 20th	Night	9% 5% 2 7% 5% 5% 5% 4 7% 9% 43%	1710
	21st to end	Night	12% 5% 2 5% 3 4 4 4 8% 10% 43%	1710
July	1st to 10th	Night	10% 7% 2 7% 5% 5% 5% 4 5% 9% 41%	2840
	11th to 20th	Night	9% 6% 3 6% 5% 4 5% 4 6% 9% 42%	2840
	21st to end	Night	10% 8% 2 8% 5% 5% 6% 5% 6% 8% 38%	3087
August	1st to 10th	Night	10% 7% 3 7% 6% 5% 5% 4 7% 9% 36%	2825
	11th to 20th	Night	11% 7% 3 7% 5% 4 4 4 5% 7% 43%	2785
	21st to end	Night	6% 4 2 6% 4 5% 5% 4 6% 10% 48%	3236
September	1st to 10th	Night	12% 4 1 6% 3 4 4 3 5% 7% 50%	2014
	11th to 20th	Night	9% 5% 1 5% 3 3 5% 3 5% 7% 52%	2090
	21st to end	Night	16% 7% 3 7% 3 4 4 4 5% 6% 42%	2090
October	1st to 10th	Night	13% 5% 2 6% 3 4 4 4 5% 7% 47%	2213
	11th to 20th	Night	15% 7% 1 6% 4 4 4 3 5% 7% 45%	2280
	21st to end	Night	21% 7% 2 6% 4 4 3 3 3 6% 41%	2508
November	1st to 10th	Night	13% 8% 2 5% 4 4 3 3 5% 4 50%	2465
	11th to 20th	Night	18% 10% 3 7% 4 4 5% 3 5% 7% 35%	2600
	21st to end	Night	19% 12% 3 6% 5% 5% 4 3 4 6% 33%	2655
December	1st to 10th	Night	19% 13% 5% 13% 5% 4 4 3 5% 5% 25%	4580
	11th to 20th	Night	19% 16% 7% 9% 5% 4 4 3 3 5% 25%	5008
	21st to end	Night	19% 17% 5% 10% 5% 5% 5% 3 5% 6% 21%	6490

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	72% 3 10% 11% 4	4057
	11th to 20th	Day	71% 3 6% 12% 7% 1	4060
	21st to end	Day	67% 3 8% 14% 7% 1	4466
February	1st to 10th	Day	61% 3 10% 18% 8% 1	1800
	11th to 20th	Day	60% 4 9% 17% 9% 1	1852
	21st to end	Day	57% 4 10% 18% 10% 1	1650
March	1st to 10th	Day	55% 3 6% 23% 10% 3	4887
	11th to 20th	Day	40% 4 7% 31% 15% 3	4980
	21st to end	Day	51% 4 7% 22% 12% 2	5465
April	1st to 10th	Day	50% 5% 10% 19% 13% 3	2225
	11th to 20th	Day	46% 4 11% 19% 17% 3	2479
	21st to end	Day	48% 6% 11% 21% 13% 2	2600
May	1st to 10th	Day	56% 6% 9% 20% 8% 1	4250
	11th to 20th	Day	50% 6% 10% 20% 13% 1	4215
	21st to end	Day	39% 8% 14% 20% 14% 4	4554
June	1st to 10th	Day	41% 6% 13% 24% 15% 1	2470
	11th to 20th	Day	41% 7% 14% 21% 15% 1	2470
	21st to end	Day	39% 9% 14% 23% 14% 1	2470
July	1st to 10th	Day	40% 7% 12% 23% 15% 3	4055
	11th to 20th	Day	35% 6% 10% 32% 16% 1	4051
	21st to end	Day	48% 6% 10% 18% 15% 3	4324
August	1st to 10th	Day	41% 8% 13% 28% 10%	4091
	11th to 20th	Day	38% 8% 9% 30% 15% 1	4083
	21st to end	Day	30% 6% 13% 29% 19% 2	4386
September	1st to 10th	Day	36% 6% 10% 21% 22% 5%	2162
	11th to 20th	Day	35% 6% 10% 26% 20% 3	2090
	21st to end	Day	46% 5% 8% 22% 19% 1	2090
October	1st to 10th	Day	28% 4 8% 27% 30% 2	3266
	11th to 20th	Day	35% 4 12% 27% 18% 5%	3140
	21st to end	Day	34% 3 11% 29% 21% 2	3428
November	1st to 10th	Day	38% 3 9% 28% 17% 3	1829
	11th to 20th	Day	50% 3 9% 20% 16% 3	1800
	21st to end	Day	53% 3 9% 21% 13% 1	1737
December	1st to 10th	Day	62% 4 10% 20% 4	2600
	11th to 20th	Day	64% 3 9% 18% 5%	3152
	21st to end	Day	67% 5% 10% 14% 4	4466
January	1st to 10th	Night	69% 3 8% 15% 5%	5889
	11th to 20th	Night	69% 3 7% 12% 6% 2	5900
	21st to end	Night	61% 3 9% 18% 8% 1	6490
February	1st to 10th	Night	59% 4 10% 18% 9% 1	2600
	11th to 20th	Night	61% 4 9% 16% 10%	2442
	21st to end	Night	59% 4 9% 16% 9% 3	1980
March	1st to 10th	Night	52% 3 5% 25% 13% 2	4965
	11th to 20th	Night	41% 5% 7% 29% 16% 2	4980
	21st to end	Night	51% 3 7% 22% 14% 3	5467
April	1st to 10th	Night	50% 4 9% 20% 15% 2	2173
	11th to 20th	Night	43% 5% 11% 21% 15% 5%	1887
	21st to end	Night	43% 5% 10% 23% 17% 2	1800
May	1st to 10th	Night	59% 6% 9% 15% 9% 2	2930
	11th to 20th	Night	47% 7% 10% 22% 13% 1	2903
	21st to end	Night	36% 8% 13% 25% 12% 5%	3138
June	1st to 10th	Night	35% 8% 10% 27% 17% 3	1710
	11th to 20th	Night	36% 8% 14% 22% 17% 3	1710
	21st to end	Night	34% 8% 15% 23% 18% 2	1710
July	1st to 10th	Night	40% 6% 9% 23% 17% 4	2795
	11th to 20th	Night	36% 7% 12% 28% 15% 2	2786
	21st to end	Night	43% 8% 9% 20% 16% 4	2988
August	1st to 10th	Night	43% 7% 14% 25% 10% 1	2814
	11th to 20th	Night	39% 6% 9% 23% 21% 1	2812
	21st to end	Night	31% 7% 12% 29% 19% 3	3236
September	1st to 10th	Night	33% 6% 11% 21% 22% 7%	2014
	11th to 20th	Night	30% 6% 11% 25% 25% 3	2090
	21st to end	Night	41% 6% 11% 20% 21% 2	2090
October	1st to 10th	Night	30% 4 7% 28% 26% 5%	3694
	11th to 20th	Night	32% 4 10% 24% 22% 9%	3779
	21st to end	Night	33% 3 6% 27% 26% 4	4120
November	1st to 10th	Night	38% 4 10% 23% 21% 5%	2465
	11th to 20th	Night	49% 3 8% 20% 16% 3	2600
	21st to end	Night	53% 3 9% 22% 11% 1	2655
December	1st to 10th	Night	61% 4 11% 20% 4	4580
	11th to 20th	Night	63% 4 11% 17% 5%	5010
	21st to end	Night	64% 4 10% 15% 7%	6490

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	33% 23% 31% 10% 2 1	300
	11th to 20th	Day	38% 25% 28% 8% 2	300
	21st to end	Day	33% 28% 28% 8% 3	330
February	1st to 10th	Day	37% 23% 28% 9% 3	300
	11th to 20th	Day	44% 24% 26% 4 2	300
	21st to end	Day	35% 24% 36% 5% 1	249
March	1st to 10th	Day	30% 21% 27% 18% 5%	476
	11th to 20th	Day	28% 23% 30% 13% 5% 1	500
	21st to end	Day	33% 16% 30% 17% 3	550
April	1st to 10th	Day	29% 21% 28% 15% 7% 1	500
	11th to 20th	Day	28% 18% 26% 18% 8% 2	492
	21st to end	Day	27% 18% 29% 18% 7% 1	500
May	1st to 10th	Day	27% 16% 27% 18% 8% 3	503
	11th to 20th	Day	23% 11% 29% 21% 12% 4	527
	21st to end	Day	13% 11% 26% 27% 17% 6%	549
June	1st to 10th	Day	16% 7% 25% 29% 17% 5%	490
	11th to 20th	Day	14% 10% 22% 30% 17% 7%	490
	21st to end	Day	13% 9% 30% 24% 18% 5%	490
July	1st to 10th	Day	13% 6% 20% 25% 20% 15%	485
	11th to 20th	Day	9% 6% 23% 33% 19% 9%	484
	21st to end	Day	17% 5% 19% 29% 20% 10%	532
August	1st to 10th	Day	11% 6% 18% 33% 22% 10%	486
	11th to 20th	Day	13% 6% 19% 32% 19% 11%	479
	21st to end	Day	12% 4 20% 27% 26% 11%	539
September	1st to 10th	Day	15% 6% 19% 27% 23% 10%	490
	11th to 20th	Day	11% 3 16% 28% 30% 11%	490
	21st to end	Day	26% 9% 22% 22% 17% 3	490
October	1st to 10th	Day	14% 8% 26% 28% 18% 4	490
	11th to 20th	Day	21% 12% 28% 23% 13% 3	490
	21st to end	Day	25% 14% 30% 21% 9% 1	539
November	1st to 10th	Day	18% 14% 35% 21% 8% 3	367
	11th to 20th	Day	28% 17% 29% 19% 6% 1	300
	21st to end	Day	25% 22% 33% 17% 2 1	267
December	1st to 10th	Day	30% 19% 37% 12% 3 1	200
	11th to 20th	Day	32% 20% 30% 15% 3	237
	21st to end	Day	35% 19% 27% 14% 5%	330
January	1st to 10th	Night	28% 20% 29% 20% 3	800
	11th to 20th	Night	33% 16% 28% 20% 3	800
	21st to end	Night	28% 17% 32% 21% 2	880
February	1st to 10th	Night	28% 15% 30% 26% 1	800
	11th to 20th	Night	33% 19% 29% 18% 1	783
	21st to end	Night	31% 17% 31% 19% 2	660
March	1st to 10th	Night	28% 14% 30% 24% 5%	800
	11th to 20th	Night	27% 17% 28% 21% 7% 1	800
	21st to end	Night	28% 16% 29% 21% 6%	880
April	1st to 10th	Night	26% 16% 31% 20% 6%	800
	11th to 20th	Night	20% 14% 32% 23% 10% 1	703
	21st to end	Night	18% 9% 34% 32% 7% 1	600
May	1st to 10th	Night	21% 10% 28% 29% 12% 1	600
	11th to 20th	Night	18% 7% 27% 31% 15% 1	591
	21st to end	Night	13% 5% 22% 33% 23% 4	634
June	1st to 10th	Night	11% 3 25% 37% 23% 1	570
	11th to 20th	Night	11% 4 19% 38% 25% 3	570
	21st to end	Night	13% 4 19% 37% 25% 1	570
July	1st to 10th	Night	13% 2 14% 36% 29% 6%	555
	11th to 20th	Night	9% 4 16% 34% 33% 5%	552
	21st to end	Night	13% 3 15% 35% 30% 4	624
August	1st to 10th	Night	9% 3 10% 41% 34% 2	567
	11th to 20th	Night	11% 3 14% 36% 31% 5%	564
	21st to end	Night	12% 3 13% 34% 33% 6%	687
September	1st to 10th	Night	13% 3 15% 35% 29% 6%	760
	11th to 20th	Night	11% 3 15% 34% 33% 4	760
	21st to end	Night	19% 7% 19% 33% 20% 4	760
October	1st to 10th	Night	16% 5% 21% 33% 23% 3	760
	11th to 20th	Night	19% 8% 28% 28% 16% 1	760
	21st to end	Night	25% 10% 22% 31% 12% 1	836
November	1st to 10th	Night	17% 9% 27% 32% 14% 1	780
	11th to 20th	Night	24% 14% 27% 26% 9% 1	800
	21st to end	Night	26% 13% 26% 28% 7% 1	800
December	1st to 10th	Night	34% 14% 26% 22% 5%	800
	11th to 20th	Night	26% 16% 29% 25% 5%	800
	21st to end	Night	29% 14% 29% 26% 2	880

Page generated at 2025-05-23 09:21:03. Version ac07

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