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 2026-09-29.

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% 5% 5% 4 6% 7% 33%	148472
	Night	16% 10% 4 9% 5% 4 4 3 5% 6% 34%	142910
Transparency	Day	48% 5% 10% 22% 13% 2	152117
	Night	50% 5% 10% 21% 13% 2	147337
Seeing	Day	22% 14% 26% 21% 12% 4	17368
	Night	21% 11% 24% 28% 14% 2	27620

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	18% 20% 7% 14% 6% 3 3 3 3 6% 18%	5258
	11th to 20th	Day	14% 18% 8% 16% 8% 4 4 2 3 4 20%	5280
	21st to end	Day	18% 15% 5% 12% 8% 6% 5% 3 4 5% 19%	5780
February	1st to 10th	Day	22% 13% 4 9% 6% 3 3 3 4 5% 28%	1890
	11th to 20th	Day	20% 15% 4 11% 4 4 4 3 5% 6% 25%	1938
	21st to end	Day	20% 13% 4 9% 5% 4 3 3 5% 5% 30%	1730
March	1st to 10th	Day	17% 16% 4 7% 4 4 4 3 5% 5% 32%	4996
	11th to 20th	Day	13% 8% 3 7% 4 4 4 3 5% 7% 42%	5090
	21st to end	Day	18% 10% 3 8% 5% 4 5% 4 6% 7% 31%	5586
April	1st to 10th	Day	15% 10% 3 8% 5% 5% 5% 4 5% 6% 33%	2336
	11th to 20th	Day	15% 9% 2 7% 5% 4 5% 2 5% 7% 38%	2604
	21st to end	Day	19% 8% 2 6% 5% 5% 4 3 6% 8% 34%	2730
May	1st to 10th	Day	18% 9% 3 8% 6% 5% 6% 5% 7% 7% 28%	7616
	11th to 20th	Day	16% 8% 3 7% 5% 4 5% 4 6% 8% 33%	7626
	21st to end	Day	20% 7% 3 7% 4 4 4 4 6% 8% 32%	8391
June	1st to 10th	Day	13% 7% 3 6% 4 6% 5% 4 7% 9% 38%	2730
	11th to 20th	Day	10% 6% 3 7% 5% 5% 6% 5% 7% 9% 36%	2730
	21st to end	Day	12% 6% 2 6% 5% 5% 6% 5% 6% 10% 37%	2730
July	1st to 10th	Day	10% 6% 2 7% 5% 5% 5% 4 6% 9% 40%	7680
	11th to 20th	Day	13% 7% 3 7% 5% 5% 5% 5% 7% 10% 36%	7680
	21st to end	Day	13% 7% 3 7% 5% 5% 5% 5% 7% 8% 34%	8340
August	1st to 10th	Day	12% 8% 3 9% 5% 6% 6% 5% 7% 9% 29%	6014
	11th to 20th	Day	14% 8% 3 7% 5% 5% 5% 4 7% 9% 31%	5951
	21st to end	Day	5% 4 2 5% 4 5% 5% 4 7% 10% 49%	6388
September	1st to 10th	Day	14% 5% 2 5% 3 4 4 3 6% 6% 48%	2390
	11th to 20th	Day	10% 5% 2 6% 4 5% 5% 4 6% 7% 46%	2310
	21st to end	Day	20% 8% 3 6% 4 3 4 3 5% 7% 38%	2288
October	1st to 10th	Day	13% 6% 2 5% 4 4 3 4 6% 8% 45%	2070
	11th to 20th	Day	15% 8% 2 6% 4 3 4 3 5% 6% 42%	2000
	21st to end	Day	21% 7% 3 7% 4 4 3 2 4 5% 39%	2200
November	1st to 10th	Day	12% 6% 3 6% 4 4 4 3 4 6% 48%	1922
	11th to 20th	Day	16% 13% 4 6% 4 4 3 4 5% 6% 35%	1890
	21st to end	Day	17% 13% 5% 9% 3 4 3 4 4 6% 32%	1824
December	1st to 10th	Day	20% 14% 4 15% 4 3 3 3 4 5% 25%	2680
	11th to 20th	Day	23% 15% 5% 10% 5% 3 4 3 4 5% 23%	3239
	21st to end	Day	21% 17% 5% 11% 5% 5% 5% 4 5% 5% 18%	4565
January	1st to 10th	Night	16% 17% 6% 16% 6% 5% 4 3 4 4 19%	7635
	11th to 20th	Night	16% 17% 7% 14% 7% 5% 4 3 3 4 22%	7680
	21st to end	Night	19% 12% 4 11% 7% 5% 4 3 4 5% 28%	8401
February	1st to 10th	Night	20% 14% 4 9% 5% 4 4 3 4 5% 28%	2730
	11th to 20th	Night	21% 13% 4 9% 6% 4 4 3 4 5% 26%	2565
	21st to end	Night	21% 14% 4 9% 4 4 4 3 4 5% 29%	2076
March	1st to 10th	Night	20% 13% 4 6% 4 4 3 2 4 6% 35%	5076
	11th to 20th	Night	12% 9% 3 7% 4 4 4 4 5% 7% 41%	5090
	21st to end	Night	17% 10% 3 8% 5% 5% 4 3 5% 6% 34%	5588
April	1st to 10th	Night	19% 10% 2 7% 5% 4 4 3 5% 6% 35%	2282
	11th to 20th	Night	15% 8% 3 6% 4 4 4 3 6% 8% 38%	1983
	21st to end	Night	14% 8% 3 7% 4 4 3 4 5% 8% 40%	1890
May	1st to 10th	Night	21% 10% 3 8% 5% 5% 4 4 5% 7% 28%	5237
	11th to 20th	Night	13% 8% 3 7% 5% 4 4 4 7% 10% 36%	5234
	21st to end	Night	20% 10% 3 8% 4 4 4 3 5% 7% 32%	5761
June	1st to 10th	Night	11% 6% 2 5% 4 4 4 4 5% 8% 45%	1890
	11th to 20th	Night	9% 6% 2 8% 5% 5% 5% 4 7% 8% 42%	1890
	21st to end	Night	13% 5% 2 5% 3 4 4 3 8% 10% 42%	1890
July	1st to 10th	Night	10% 7% 3 8% 5% 5% 4 4 5% 8% 42%	5278
	11th to 20th	Night	13% 7% 3 7% 5% 5% 5% 4 6% 10% 36%	5277
	21st to end	Night	11% 7% 2 8% 4 4 5% 4 7% 8% 38%	5736
August	1st to 10th	Night	14% 9% 4 8% 6% 5% 5% 5% 7% 8% 30%	4135
	11th to 20th	Night	16% 10% 3 8% 5% 4 4 3 5% 7% 35%	4095
	21st to end	Night	6% 4 2 6% 4 4 5% 4 6% 10% 48%	4689
September	1st to 10th	Night	12% 4 1 5% 3 4 4 3 5% 7% 51%	2226
	11th to 20th	Night	10% 5% 1 5% 3 3 4 3 5% 7% 53%	2310
	21st to end	Night	17% 7% 3 7% 3 3 4 4 5% 6% 41%	2289
October	1st to 10th	Night	13% 5% 2 5% 3 4 4 4 5% 7% 48%	2330
	11th to 20th	Night	15% 7% 1 5% 4 4 4 3 5% 7% 46%	2400
	21st to end	Night	21% 7% 2 6% 4 4 3 3 3 6% 41%	2640
November	1st to 10th	Night	13% 8% 2 5% 4 4 3 3 5% 5% 50%	2592
	11th to 20th	Night	18% 10% 3 7% 4 4 5% 3 5% 7% 35%	2730
	21st to end	Night	19% 12% 4 7% 5% 4 4 3 4 5% 33%	2788
December	1st to 10th	Night	19% 13% 5% 13% 5% 4 4 3 5% 5% 25%	4720
	11th to 20th	Night	19% 16% 7% 9% 5% 4 4 3 4 5% 25%	5144
	21st to end	Night	19% 17% 5% 10% 5% 5% 5% 3 5% 6% 21%	6633

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% 3 12% 13% 3	5259
	11th to 20th	Day	70% 3 6% 11% 9% 1	5280
	21st to end	Day	68% 3 8% 15% 5%	5780
February	1st to 10th	Day	60% 3 9% 19% 8% 1	1890
	11th to 20th	Day	60% 4 9% 17% 9% 1	1947
	21st to end	Day	56% 4 11% 18% 10% 1	1730
March	1st to 10th	Day	55% 3 6% 23% 10% 3	4996
	11th to 20th	Day	41% 4 8% 30% 15% 3	5090
	21st to end	Day	52% 4 7% 22% 12% 2	5586
April	1st to 10th	Day	49% 5% 10% 20% 13% 3	2336
	11th to 20th	Day	45% 4 11% 19% 18% 3	2604
	21st to end	Day	47% 6% 11% 22% 12% 2	2730
May	1st to 10th	Day	52% 6% 10% 22% 9% 2	7651
	11th to 20th	Day	48% 6% 10% 24% 11% 1	7630
	21st to end	Day	48% 6% 10% 17% 14% 5%	8405
June	1st to 10th	Day	41% 6% 12% 23% 16% 2	2730
	11th to 20th	Day	41% 8% 14% 21% 15% 1	2730
	21st to end	Day	38% 9% 14% 23% 14% 1	2730
July	1st to 10th	Day	41% 7% 11% 24% 16% 2	7615
	11th to 20th	Day	42% 7% 9% 30% 11%	7611
	21st to end	Day	45% 7% 9% 20% 17% 2	8156
August	1st to 10th	Day	48% 8% 12% 22% 9%	6001
	11th to 20th	Day	46% 8% 11% 24% 11%	5990
	21st to end	Day	28% 6% 15% 30% 19% 2	6388
September	1st to 10th	Day	35% 6% 10% 23% 23% 5%	2390
	11th to 20th	Day	35% 6% 10% 26% 20% 3	2310
	21st to end	Day	46% 5% 8% 21% 18% 1	2288
October	1st to 10th	Day	27% 4 8% 27% 31% 2	3369
	11th to 20th	Day	34% 4 11% 27% 18% 5%	3240
	21st to end	Day	34% 3 10% 29% 21% 2	3538
November	1st to 10th	Day	38% 3 10% 28% 17% 3	1922
	11th to 20th	Day	49% 3 9% 20% 15% 3	1890
	21st to end	Day	54% 3 9% 20% 12% 1	1824
December	1st to 10th	Day	63% 4 10% 19% 4	2680
	11th to 20th	Day	64% 3 9% 18% 5%	3236
	21st to end	Day	68% 5% 10% 13% 4	4565
January	1st to 10th	Night	69% 3 9% 15% 5%	7636
	11th to 20th	Night	68% 3 6% 13% 9% 2	7680
	21st to end	Night	60% 3 11% 18% 7% 1	8401
February	1st to 10th	Night	59% 4 10% 18% 9% 1	2730
	11th to 20th	Night	61% 4 9% 17% 10%	2567
	21st to end	Night	59% 4 9% 17% 9% 2	2076
March	1st to 10th	Night	52% 3 5% 25% 13% 2	5076
	11th to 20th	Night	41% 5% 7% 29% 16% 2	5090
	21st to end	Night	51% 4 7% 22% 13% 2	5588
April	1st to 10th	Night	50% 4 9% 20% 15% 2	2282
	11th to 20th	Night	43% 5% 11% 21% 15% 5%	1983
	21st to end	Night	43% 5% 10% 23% 17% 2	1890
May	1st to 10th	Night	55% 6% 11% 16% 9% 3	5264
	11th to 20th	Night	43% 8% 12% 22% 14% 1	5235
	21st to end	Night	50% 6% 9% 18% 12% 4	5771
June	1st to 10th	Night	36% 7% 10% 26% 19% 3	1890
	11th to 20th	Night	38% 8% 13% 21% 17% 3	1890
	21st to end	Night	35% 8% 14% 22% 19% 2	1890
July	1st to 10th	Night	41% 6% 8% 25% 18% 2	5235
	11th to 20th	Night	43% 7% 13% 26% 10% 1	5226
	21st to end	Night	41% 8% 10% 19% 19% 2	5606
August	1st to 10th	Night	51% 7% 12% 20% 9% 1	4124
	11th to 20th	Night	49% 6% 10% 20% 15% 1	4122
	21st to end	Night	30% 7% 13% 32% 17% 2	4689
September	1st to 10th	Night	32% 5% 12% 21% 23% 7%	2226
	11th to 20th	Night	30% 6% 11% 26% 25% 3	2310
	21st to end	Night	43% 6% 10% 19% 21% 2	2289
October	1st to 10th	Night	29% 4 7% 28% 27% 5%	3811
	11th to 20th	Night	32% 4 10% 24% 23% 8%	3899
	21st to end	Night	34% 3 6% 27% 26% 4	4252
November	1st to 10th	Night	37% 4 10% 23% 21% 4	2592
	11th to 20th	Night	49% 3 8% 21% 15% 3	2730
	21st to end	Night	54% 3 9% 22% 11% 1	2788
December	1st to 10th	Night	62% 4 11% 19% 4	4720
	11th to 20th	Night	63% 4 11% 17% 5%	5146
	21st to end	Night	64% 4 10% 15% 7%	6633

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% 3 1	330
	11th to 20th	Day	39% 26% 26% 7% 2	330
	21st to end	Day	35% 28% 28% 7% 3	363
February	1st to 10th	Day	35% 21% 30% 11% 3	330
	11th to 20th	Day	44% 24% 26% 4 2	330
	21st to end	Day	33% 23% 38% 5% 1	273
March	1st to 10th	Day	31% 21% 26% 17% 4	515
	11th to 20th	Day	30% 22% 30% 13% 5% 1	540
	21st to end	Day	32% 17% 30% 17% 3	594
April	1st to 10th	Day	27% 22% 28% 16% 7% 1	540
	11th to 20th	Day	28% 18% 27% 18% 7% 2	532
	21st to end	Day	26% 18% 29% 18% 7% 2	540
May	1st to 10th	Day	28% 17% 27% 17% 8% 3	543
	11th to 20th	Day	22% 13% 29% 21% 11% 4	567
	21st to end	Day	17% 11% 26% 26% 15% 5%	617
June	1st to 10th	Day	15% 8% 26% 28% 17% 5%	570
	11th to 20th	Day	15% 12% 24% 28% 15% 6%	570
	21st to end	Day	14% 9% 28% 25% 19% 5%	570
July	1st to 10th	Day	14% 5% 18% 27% 22% 15%	565
	11th to 20th	Day	9% 6% 23% 32% 21% 9%	564
	21st to end	Day	17% 6% 20% 29% 19% 10%	620
August	1st to 10th	Day	11% 5% 20% 33% 21% 9%	566
	11th to 20th	Day	14% 6% 19% 31% 20% 10%	559
	21st to end	Day	11% 5% 21% 29% 25% 10%	627
September	1st to 10th	Day	15% 5% 20% 29% 22% 9%	570
	11th to 20th	Day	11% 3 16% 28% 30% 12%	570
	21st to end	Day	27% 9% 23% 22% 16% 3	562
October	1st to 10th	Day	14% 8% 25% 29% 19% 5%	530
	11th to 20th	Day	20% 13% 28% 23% 13% 3	530
	21st to end	Day	25% 15% 30% 20% 8% 1	583
November	1st to 10th	Day	20% 14% 36% 20% 8% 3	400
	11th to 20th	Day	29% 18% 28% 18% 5% 1	330
	21st to end	Day	27% 22% 32% 16% 2 1	294
December	1st to 10th	Day	31% 19% 36% 11% 3	220
	11th to 20th	Day	33% 20% 31% 14% 3	261
	21st to end	Day	36% 20% 26% 13% 4	363
January	1st to 10th	Night	28% 20% 29% 20% 3	840
	11th to 20th	Night	33% 17% 28% 19% 3	840
	21st to end	Night	28% 18% 32% 20% 2	924
February	1st to 10th	Night	27% 15% 30% 27% 1	840
	11th to 20th	Night	33% 19% 29% 18% 1	823
	21st to end	Night	31% 18% 31% 19% 2	692
March	1st to 10th	Night	27% 14% 31% 23% 4	840
	11th to 20th	Night	27% 17% 28% 20% 7% 1	840
	21st to end	Night	27% 16% 30% 21% 5%	924
April	1st to 10th	Night	26% 17% 31% 20% 6%	840
	11th to 20th	Night	20% 14% 32% 23% 10% 1	739
	21st to end	Night	18% 9% 34% 31% 7% 1	630
May	1st to 10th	Night	21% 9% 29% 29% 12% 1	630
	11th to 20th	Night	17% 7% 28% 32% 14% 1	621
	21st to end	Night	15% 5% 22% 33% 22% 4	683
June	1st to 10th	Night	13% 3 24% 36% 23% 2	630
	11th to 20th	Night	11% 5% 19% 38% 23% 3	630
	21st to end	Night	14% 5% 18% 37% 25% 2	630
July	1st to 10th	Night	13% 2 13% 35% 31% 6%	615
	11th to 20th	Night	10% 4 15% 34% 32% 5%	612
	21st to end	Night	13% 3 15% 35% 30% 4	690
August	1st to 10th	Night	10% 4 10% 42% 32% 2	627
	11th to 20th	Night	11% 3 14% 37% 30% 5%	624
	21st to end	Night	11% 3 13% 35% 33% 5%	759
September	1st to 10th	Night	13% 3 15% 36% 28% 5%	840
	11th to 20th	Night	11% 3 14% 34% 33% 4	840
	21st to end	Night	20% 6% 18% 33% 19% 3	833
October	1st to 10th	Night	16% 5% 20% 33% 24% 3	800
	11th to 20th	Night	19% 7% 29% 29% 15% 1	800
	21st to end	Night	25% 10% 23% 30% 11% 1	880
November	1st to 10th	Night	17% 9% 27% 32% 13%	820
	11th to 20th	Night	25% 14% 26% 25% 8% 1	840
	21st to end	Night	26% 13% 25% 27% 7% 1	840
December	1st to 10th	Night	33% 14% 27% 22% 5%	840
	11th to 20th	Night	26% 15% 29% 24% 5%	840
	21st to end	Night	29% 14% 29% 26% 2	924

Page generated at 2026-09-26 04:18:42. Version ac07

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