SLL Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the SLL Observatory clear sky chart from 2005-11-06 to 2026-07-16.

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	8% 5% 2 5% 3 3 4 3 5% 6% 57%	146763
	Night	7% 4 2 4 3 3 3 3 5% 7% 58%	143322
Transparency	Day	28% 3 9% 25% 26% 9%	141640
	Night	27% 3 9% 25% 26% 10%	139470
Seeing	Day	10% 8% 29% 32% 17% 4	17781
	Night	9% 9% 36% 36% 10%	27654

By Month

Percentage Cloud Cover SW (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	8% 6% 2 7% 3 4 4 4 5% 5% 52%	5287
	11th to 20th	Day	4 5% 3 4 3 3 3 3 3 5% 64%	5310
	21st to end	Day	11% 7% 3 6% 3 3 3 2 3 5% 55%	5812
February	1st to 10th	Day	8% 10% 3 5% 3 2 3 2 4 5% 54%	2076
	11th to 20th	Day	6% 6% 3 5% 4 3 4 3 5% 5% 58%	2049
	21st to end	Day	6% 5% 2 4 3 3 3 2 5% 5% 61%	1730
March	1st to 10th	Day	8% 7% 2 4 3 2 3 2 3 5% 61%	4690
	11th to 20th	Day	6% 5% 1 4 3 3 3 3 5% 7% 60%	5090
	21st to end	Day	7% 5% 2 5% 4 4 4 4 5% 8% 51%	5585
April	1st to 10th	Day	9% 4 2 4 3 3 3 3 5% 6% 58%	2399
	11th to 20th	Day	11% 5% 1 5% 4 3 4 3 5% 7% 51%	2512
	21st to end	Day	13% 5% 2 4 3 4 3 3 4 6% 52%	2730
May	1st to 10th	Day	15% 6% 2 5% 3 3 4 3 4 6% 48%	7655
	11th to 20th	Day	6% 4 1 4 3 3 4 3 5% 6% 61%	7627
	21st to end	Day	16% 6% 2 6% 4 4 4 3 5% 6% 45%	8404
June	1st to 10th	Day	5% 3 1 4 4 4 4 4 7% 9% 54%	2730
	11th to 20th	Day	5% 3 1 4 4 4 5% 4 6% 7% 57%	2730
	21st to end	Day	6% 4 2 4 4 3 4 4 7% 7% 56%	2730
July	1st to 10th	Day	6% 3 2 6% 5% 4 4 5% 7% 8% 49%	6029
	11th to 20th	Day	6% 3 2 5% 3 3 4 3 5% 8% 58%	5966
	21st to end	Day	4 2 1 3 3 4 4 4 6% 9% 60%	6424
August	1st to 10th	Day	10% 4 2 4 4 4 4 3 5% 7% 53%	5801
	11th to 20th	Day	4 3 2 4 3 4 4 4 7% 10% 55%	5694
	21st to end	Day	12% 4 2 4 3 3 4 3 6% 7% 52%	5984
September	1st to 10th	Day	10% 4 2 5% 4 3 3 3 4 6% 58%	2258
	11th to 20th	Day	8% 4 2 4 3 3 3 4 6% 7% 57%	2200
	21st to end	Day	7% 3 2 5% 3 3 3 3 4 6% 61%	2200
October	1st to 10th	Day	6% 2 1 3 2 2 2 2 3 4 73%	3590
	11th to 20th	Day	6% 2 1 3 2 2 2 2 4 6% 71%	3316
	21st to end	Day	6% 3 1 3 2 2 2 3 5% 5% 67%	3424
November	1st to 10th	Day	6% 3 1 3 2 3 3 2 4 5% 67%	1853
	11th to 20th	Day	6% 4 1 4 2 2 3 2 4 7% 66%	1890
	21st to end	Day	9% 6% 1 3 2 2 3 3 3 4 63%	1890
December	1st to 10th	Day	4 3 1 3 3 2 2 2 4 5% 70%	3030
	11th to 20th	Day	5% 3 2 3 2 2 3 3 4 7% 65%	3481
	21st to end	Day	11% 9% 3 7% 5% 4 3 2 4 5% 46%	4587
January	1st to 10th	Night	10% 7% 2 6% 3 3 4 3 5% 6% 51%	7634
	11th to 20th	Night	3 3 1 4 4 3 3 3 4 6% 65%	7680
	21st to end	Night	12% 7% 2 6% 3 2 3 2 4 4 53%	8402
February	1st to 10th	Night	9% 7% 3 6% 3 3 3 2 4 6% 54%	2562
	11th to 20th	Night	9% 7% 2 6% 4 3 3 3 5% 6% 52%	2458
	21st to end	Night	6% 5% 2 4 3 2 3 4 4 6% 61%	2076
March	1st to 10th	Night	5% 5% 1 4 3 3 3 3 4 6% 64%	5386
	11th to 20th	Night	6% 4 1 4 3 2 3 2 5% 7% 64%	5090
	21st to end	Night	6% 5% 2 5% 4 3 4 3 5% 6% 57%	5588
April	1st to 10th	Night	8% 4 1 3 3 3 3 3 4 7% 59%	2216
	11th to 20th	Night	11% 4 1 5% 3 3 4 3 5% 7% 54%	2056
	21st to end	Night	12% 3 2 4 3 3 4 4 5% 7% 53%	1890
May	1st to 10th	Night	14% 6% 2 5% 4 3 5% 3 5% 6% 47%	5292
	11th to 20th	Night	5% 3 1 3 2 3 4 3 5% 8% 63%	5265
	21st to end	Night	14% 6% 2 5% 3 3 3 3 5% 7% 48%	5803
June	1st to 10th	Night	5% 4 1 5% 3 4 4 3 7% 11% 53%	1890
	11th to 20th	Night	5% 3 1 5% 3 3 4 5% 7% 9% 56%	1890
	21st to end	Night	5% 3 1 4 3 3 4 3 6% 9% 58%	1890
July	1st to 10th	Night	5% 3 2 5% 4 4 4 5% 7% 10% 51%	4170
	11th to 20th	Night	4 3 1 4 3 4 5% 4 7% 10% 55%	4125
	21st to end	Night	3 3 1 4 3 3 4 4 7% 9% 59%	4449
August	1st to 10th	Night	6% 3 1 4 3 4 5% 5% 7% 11% 52%	4011
	11th to 20th	Night	4 4 2 5% 4 5% 4 4 7% 10% 51%	4137
	21st to end	Night	11% 5% 1 5% 3 3 4 4 5% 9% 49%	4993
September	1st to 10th	Night	9% 4 1 4 3 2 3 3 4 6% 61%	2140
	11th to 20th	Night	9% 4 2 5% 3 3 4 4 5% 7% 56%	2200
	21st to end	Night	6% 2 1 4 3 3 3 3 5% 7% 63%	2200
October	1st to 10th	Night	6% 2 1 2 2 2 2 2 3 4 75%	3590
	11th to 20th	Night	5% 2 1 2 2 2 2 2 4 6% 72%	3789
	21st to end	Night	5% 2 1 2 2 2 2 2 4 7% 72%	4366
November	1st to 10th	Night	8% 3 1 2 2 2 3 2 4 6% 67%	2665
	11th to 20th	Night	5% 4 1 3 2 2 3 3 3 6% 67%	2730
	21st to end	Night	9% 4 2 3 3 3 3 2 3 5% 62%	2730
December	1st to 10th	Night	4 3 1 3 2 2 2 2 3 6% 72%	4380
	11th to 20th	Night	5% 4 1 4 2 3 3 2 4 5% 67%	4946
	21st to end	Night	10% 8% 2 5% 3 3 4 2 4 5% 52%	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	35% 2 7% 25% 25% 6%	5290
	11th to 20th	Day	25% 1 8% 22% 31% 12%	5310
	21st to end	Day	34% 1 6% 20% 34% 4	5812
February	1st to 10th	Day	34% 2 9% 26% 21% 7%	2076
	11th to 20th	Day	31% 3 10% 26% 22% 8%	2059
	21st to end	Day	27% 2 5% 24% 28% 15%	1730
March	1st to 10th	Day	28% 1 5% 15% 23% 27%	4690
	11th to 20th	Day	26% 2 7% 17% 29% 19%	5090
	21st to end	Day	32% 3 9% 32% 20% 3	5585
April	1st to 10th	Day	27% 3 8% 25% 25% 11%	2399
	11th to 20th	Day	34% 4 8% 24% 20% 10%	2515
	21st to end	Day	34% 3 10% 27% 22% 4	2730
May	1st to 10th	Day	39% 4 10% 20% 22% 7%	7655
	11th to 20th	Day	24% 3 9% 24% 27% 13%	7631
	21st to end	Day	41% 4 9% 19% 17% 10%	8405
June	1st to 10th	Day	26% 5% 20% 27% 16% 7%	2730
	11th to 20th	Day	25% 4 12% 25% 25% 8%	2730
	21st to end	Day	26% 6% 11% 24% 23% 10%	2730
July	1st to 10th	Day	30% 5% 13% 21% 24% 7%	5966
	11th to 20th	Day	25% 5% 10% 28% 28% 5%	5897
	21st to end	Day	20% 6% 12% 36% 23% 3	6240
August	1st to 10th	Day	29% 5% 11% 31% 21% 4	4220
	11th to 20th	Day	23% 4 13% 26% 29% 5%	4122
	21st to end	Day	21% 4 11% 29% 28% 6%	4311
September	1st to 10th	Day	29% 4 9% 22% 24% 12%	2258
	11th to 20th	Day	27% 5% 9% 24% 29% 6%	2200
	21st to end	Day	25% 3 8% 24% 28% 11%	2200
October	1st to 10th	Day	18% 2 6% 16% 42% 16%	3590
	11th to 20th	Day	16% 2 9% 26% 31% 15%	3316
	21st to end	Day	20% 3 6% 24% 34% 13%	3424
November	1st to 10th	Day	21% 2 7% 24% 32% 13%	1853
	11th to 20th	Day	21% 2 7% 28% 30% 12%	1890
	21st to end	Day	26% 2 7% 26% 28% 11%	1890
December	1st to 10th	Day	18% 2 7% 38% 31% 5%	3030
	11th to 20th	Day	21% 1 6% 28% 37% 7%	3479
	21st to end	Day	43% 2 7% 22% 19% 6%	4587
January	1st to 10th	Night	36% 2 8% 28% 21% 5%	7635
	11th to 20th	Night	23% 1 10% 21% 35% 10%	7680
	21st to end	Night	37% 1 5% 19% 30% 8%	8402
February	1st to 10th	Night	34% 3 7% 26% 21% 9%	2562
	11th to 20th	Night	34% 4 9% 23% 22% 8%	2458
	21st to end	Night	25% 3 5% 22% 29% 16%	2076
March	1st to 10th	Night	24% 2 5% 19% 24% 26%	5386
	11th to 20th	Night	21% 2 6% 20% 25% 26%	5090
	21st to end	Night	29% 3 9% 32% 23% 5%	5588
April	1st to 10th	Night	27% 3 9% 22% 25% 14%	2216
	11th to 20th	Night	31% 4 9% 25% 20% 11%	2056
	21st to end	Night	31% 4 11% 29% 21% 4	1890
May	1st to 10th	Night	39% 4 11% 17% 21% 8%	5293
	11th to 20th	Night	20% 3 11% 25% 27% 14%	5266
	21st to end	Night	37% 5% 12% 20% 16% 11%	5805
June	1st to 10th	Night	27% 4 17% 31% 16% 5%	1890
	11th to 20th	Night	23% 6% 12% 28% 23% 8%	1890
	21st to end	Night	24% 3 12% 28% 23% 10%	1890
July	1st to 10th	Night	27% 6% 18% 26% 20% 4	4125
	11th to 20th	Night	22% 5% 11% 27% 28% 6%	4071
	21st to end	Night	22% 4 13% 31% 27% 3	4320
August	1st to 10th	Night	26% 6% 11% 26% 25% 6%	2916
	11th to 20th	Night	26% 3 12% 29% 24% 6%	2998
	21st to end	Night	21% 5% 12% 33% 23% 5%	3597
September	1st to 10th	Night	25% 5% 8% 25% 27% 11%	2140
	11th to 20th	Night	29% 4 9% 22% 28% 8%	2200
	21st to end	Night	22% 3 9% 23% 30% 12%	2200
October	1st to 10th	Night	16% 2 5% 17% 39% 20%	3590
	11th to 20th	Night	16% 2 6% 24% 34% 18%	3789
	21st to end	Night	15% 2 5% 26% 37% 15%	4366
November	1st to 10th	Night	20% 2 9% 23% 28% 16%	2665
	11th to 20th	Night	20% 2 6% 27% 31% 13%	2730
	21st to end	Night	26% 2 7% 26% 27% 12%	2730
December	1st to 10th	Night	17% 2 6% 40% 31% 4	4380
	11th to 20th	Night	22% 1 4 28% 34% 11%	4947
	21st to end	Night	36% 2 9% 26% 21% 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	12% 9% 37% 30% 12% 1	330
	11th to 20th	Day	12% 14% 39% 25% 10%	330
	21st to end	Day	8% 13% 41% 28% 10% 1	363
February	1st to 10th	Day	11% 12% 42% 28% 6% 1	330
	11th to 20th	Day	14% 11% 46% 22% 6% 1	330
	21st to end	Day	10% 16% 44% 22% 6% 1	273
March	1st to 10th	Day	7% 15% 45% 27% 5%	330
	11th to 20th	Day	17% 14% 40% 21% 5% 2	330
	21st to end	Day	14% 17% 39% 25% 4	363
April	1st to 10th	Day	14% 17% 39% 23% 7%	419
	11th to 20th	Day	13% 18% 46% 20% 2	553
	21st to end	Day	18% 14% 38% 21% 9% 1	750
May	1st to 10th	Day	12% 12% 39% 28% 8% 1	753
	11th to 20th	Day	14% 8% 32% 30% 14% 3	775
	21st to end	Day	14% 8% 32% 31% 12% 2	844
June	1st to 10th	Day	6% 4 26% 39% 18% 6%	780
	11th to 20th	Day	5% 5% 28% 39% 19% 3	780
	21st to end	Day	7% 3 30% 37% 19% 3	780
July	1st to 10th	Day	6% 2 15% 38% 30% 9%	771
	11th to 20th	Day	3 1 12% 41% 34% 9%	744
	21st to end	Day	6% 1 11% 41% 32% 10%	794
August	1st to 10th	Day	7% 1 15% 40% 32% 5%	726
	11th to 20th	Day	8% 1 12% 41% 33% 5%	666
	21st to end	Day	7% 2 12% 40% 31% 8%	583
September	1st to 10th	Day	8% 2 13% 35% 35% 7%	388
	11th to 20th	Day	5% 2 21% 41% 25% 6%	330
	21st to end	Day	9% 4 23% 32% 25% 7%	330
October	1st to 10th	Day	9% 4 28% 34% 21% 4	330
	11th to 20th	Day	10% 7% 33% 34% 15% 2	330
	21st to end	Day	17% 12% 33% 24% 11% 4	363
November	1st to 10th	Day	8% 7% 41% 29% 12% 2	330
	11th to 20th	Day	9% 10% 38% 27% 14% 2	330
	21st to end	Day	14% 13% 33% 25% 13% 3	330
December	1st to 10th	Day	7% 7% 42% 33% 10% 1	330
	11th to 20th	Day	8% 11% 36% 32% 11% 1	330
	21st to end	Day	13% 16% 34% 26% 10%	363
January	1st to 10th	Night	8% 10% 42% 32% 7%	1050
	11th to 20th	Night	10% 12% 37% 34% 7%	1050
	21st to end	Night	9% 9% 41% 34% 6%	1155
February	1st to 10th	Night	12% 9% 42% 32% 5%	882
	11th to 20th	Night	12% 14% 42% 28% 4	824
	21st to end	Night	10% 13% 45% 28% 4	692
March	1st to 10th	Night	9% 13% 44% 29% 5%	797
	11th to 20th	Night	13% 12% 42% 29% 4	630
	21st to end	Night	12% 14% 45% 25% 4	693
April	1st to 10th	Night	12% 13% 44% 26% 5%	630
	11th to 20th	Night	14% 17% 45% 20% 4	623
	21st to end	Night	14% 12% 45% 23% 6%	630
May	1st to 10th	Night	12% 12% 43% 29% 4	630
	11th to 20th	Night	10% 10% 38% 32% 10% 1	621
	21st to end	Night	11% 9% 36% 33% 11% 1	683
June	1st to 10th	Night	7% 6% 34% 39% 14%	630
	11th to 20th	Night	6% 6% 40% 40% 8%	630
	21st to end	Night	6% 4 35% 46% 10%	630
July	1st to 10th	Night	6% 2 22% 46% 23% 2	615
	11th to 20th	Night	4 1 19% 53% 22% 1	597
	21st to end	Night	5% 1 18% 52% 21% 2	657
August	1st to 10th	Night	6% 2 23% 48% 21% 1	597
	11th to 20th	Night	6% 2 19% 54% 19%	594
	21st to end	Night	5% 2 19% 45% 27% 2	660
September	1st to 10th	Night	9% 2 21% 48% 19% 1	600
	11th to 20th	Night	10% 4 29% 44% 12% 1	600
	21st to end	Night	7% 4 29% 43% 16% 1	600
October	1st to 10th	Night	11% 6% 30% 41% 11%	600
	11th to 20th	Night	8% 8% 42% 36% 6%	729
	21st to end	Night	10% 11% 37% 28% 13%	945
November	1st to 10th	Night	9% 7% 35% 37% 10% 1	1025
	11th to 20th	Night	7% 11% 42% 33% 8%	1050
	21st to end	Night	11% 10% 33% 37% 9%	1050
December	1st to 10th	Night	8% 8% 40% 36% 8%	1050
	11th to 20th	Night	12% 10% 41% 34% 4	1050
	21st to end	Night	12% 11% 40% 32% 5%	1155

Page generated at 2026-07-13 02:14:52. Version ac07

Copyright © 2026 Attilla Danko.