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 2025-03-12.

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 4 3 3 3 3 5% 6% 58%	113296
	Night	7% 4 2 4 3 3 3 3 5% 7% 59%	114484
Transparency	Day	28% 3 9% 25% 26% 9%	113063
	Night	26% 3 9% 25% 27% 10%	114307
Seeing	Day	10% 8% 29% 32% 18% 4	15850
	Night	9% 9% 36% 36% 10%	25940

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	9% 7% 3 8% 4 4 4 4 4 5% 49%	4077
	11th to 20th	Day	4 6% 3 5% 3 3 3 3 4 5% 61%	4080
	21st to end	Day	9% 7% 3 6% 3 3 3 2 3 5% 56%	4488
February	1st to 10th	Day	8% 10% 3 5% 3 2 3 2 4 6% 54%	1977
	11th to 20th	Day	6% 6% 3 5% 4 3 4 2 4 5% 58%	1949
	21st to end	Day	7% 5% 2 5% 3 3 3 2 5% 5% 59%	1650
March	1st to 10th	Day	8% 6% 2 4 3 2 2 2 3 6% 63%	3294
	11th to 20th	Day	8% 5% 2 5% 3 3 3 2 4 6% 59%	3491
	21st to end	Day	10% 7% 3 7% 4 4 5% 4 5% 7% 45%	3814
April	1st to 10th	Day	8% 4 2 4 3 3 3 3 5% 6% 58%	2171
	11th to 20th	Day	11% 5% 1 6% 4 3 4 3 5% 6% 50%	2270
	21st to end	Day	13% 5% 2 4 3 4 4 3 4 6% 52%	2470
May	1st to 10th	Day	14% 6% 2 5% 3 3 4 2 5% 6% 50%	4120
	11th to 20th	Day	9% 4 1 5% 4 4 5% 3 6% 6% 52%	4082
	21st to end	Day	10% 5% 2 4 3 4 4 3 6% 7% 52%	4489
June	1st to 10th	Day	6% 3 1 4 3 3 4 4 7% 9% 54%	2470
	11th to 20th	Day	5% 3 1 4 3 4 5% 4 6% 7% 58%	2470
	21st to end	Day	6% 4 1 3 4 3 4 4 7% 7% 58%	2470
July	1st to 10th	Day	8% 4 2 5% 4 4 4 4 6% 8% 51%	4120
	11th to 20th	Day	4 2 1 4 3 4 4 4 5% 8% 60%	4120
	21st to end	Day	3 2 1 4 3 3 5% 5% 6% 9% 58%	4466
August	1st to 10th	Day	9% 3 2 4 4 4 4 4 5% 8% 54%	4103
	11th to 20th	Day	5% 3 2 4 3 4 3 3 6% 8% 59%	3956
	21st to end	Day	4 3 1 4 2 3 4 3 5% 8% 63%	4180
September	1st to 10th	Day	10% 4 1 5% 3 3 3 3 4 6% 58%	2145
	11th to 20th	Day	8% 4 2 4 3 3 3 4 5% 7% 57%	2090
	21st to end	Day	7% 3 1 5% 3 3 3 3 4 6% 62%	2090
October	1st to 10th	Day	6% 2 1 2 2 2 2 2 3 4 74%	3480
	11th to 20th	Day	6% 2 1 2 2 2 2 2 4 6% 71%	3214
	21st to end	Day	6% 3 1 3 2 2 2 3 5% 5% 68%	3318
November	1st to 10th	Day	7% 3 1 3 2 4 3 2 4 5% 66%	1763
	11th to 20th	Day	6% 4 1 4 2 2 3 3 4 6% 66%	1800
	21st to end	Day	9% 5% 1 3 2 2 3 3 3 4 64%	1800
December	1st to 10th	Day	4 3 1 3 3 2 2 2 4 5% 71%	2940
	11th to 20th	Day	5% 3 2 3 2 2 3 3 4 7% 65%	3391
	21st to end	Day	11% 9% 3 7% 5% 4 3 2 4 5% 45%	4488
January	1st to 10th	Night	10% 7% 3 6% 4 3 4 3 4 5% 51%	5888
	11th to 20th	Night	3 3 1 5% 4 4 4 3 4 6% 62%	5900
	21st to end	Night	11% 8% 2 6% 3 2 3 2 4 4 56%	6490
February	1st to 10th	Night	9% 8% 3 6% 3 3 3 3 4 6% 53%	2440
	11th to 20th	Night	9% 6% 2 6% 3 3 3 3 5% 6% 52%	2338
	21st to end	Night	7% 5% 2 4 3 2 3 4 5% 6% 60%	1980
March	1st to 10th	Night	6% 5% 1 4 3 3 3 3 4 6% 64%	3774
	11th to 20th	Night	8% 5% 1 4 3 3 3 2 5% 6% 59%	3497
	21st to end	Night	8% 6% 2 5% 4 3 4 3 5% 6% 54%	3817
April	1st to 10th	Night	8% 4 1 3 3 3 3 3 4 7% 60%	2005
	11th to 20th	Night	11% 4 2 5% 3 3 4 3 5% 7% 53%	1858
	21st to end	Night	12% 4 2 4 4 4 4 4 5% 6% 53%	1710
May	1st to 10th	Night	15% 6% 1 4 3 3 4 3 5% 5% 50%	2850
	11th to 20th	Night	9% 4 1 5% 3 4 4 4 6% 10% 51%	2823
	21st to end	Night	8% 3 1 4 3 3 3 3 6% 9% 56%	3098
June	1st to 10th	Night	5% 4 1 4 3 4 5% 3 7% 11% 55%	1710
	11th to 20th	Night	5% 3 1 5% 3 3 4 4 7% 9% 56%	1710
	21st to end	Night	5% 3 1 4 2 3 4 3 6% 9% 59%	1710
July	1st to 10th	Night	5% 3 2 4 4 4 4 4 7% 9% 55%	2850
	11th to 20th	Night	3 2 1 3 3 4 4 4 7% 11% 58%	2850
	21st to end	Night	3 3 1 4 3 3 4 4 7% 11% 58%	3097
August	1st to 10th	Night	5% 3 1 5% 4 4 5% 5% 7% 10% 52%	2836
	11th to 20th	Night	5% 4 2 4 4 4 3 4 7% 10% 52%	2879
	21st to end	Night	4 2 1 4 3 3 4 4 6% 10% 59%	3487
September	1st to 10th	Night	9% 4 1 4 3 2 3 3 4 6% 62%	2033
	11th to 20th	Night	10% 4 2 5% 3 3 4 4 5% 7% 56%	2090
	21st to end	Night	6% 2 1 4 3 3 2 3 5% 7% 64%	2090
October	1st to 10th	Night	6% 2 1 2 2 2 2 2 3 4 75%	3480
	11th to 20th	Night	5% 2 1 2 2 2 2 2 4 6% 72%	3672
	21st to end	Night	5% 2 1 2 2 2 2 2 4 7% 72%	4231
November	1st to 10th	Night	8% 3 1 2 2 2 3 2 4 6% 67%	2535
	11th to 20th	Night	5% 4 1 3 2 3 3 3 3 6% 67%	2600
	21st to end	Night	9% 4 1 3 3 3 3 2 3 5% 63%	2600
December	1st to 10th	Night	4 3 1 3 2 2 2 2 3 6% 72%	4250
	11th to 20th	Night	5% 4 1 4 2 3 3 2 4 6% 67%	4816
	21st to end	Night	10% 8% 2 5% 3 4 4 2 4 5% 51%	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	38% 2 9% 24% 24% 4	4078
	11th to 20th	Day	28% 1 10% 19% 30% 11%	4080
	21st to end	Day	34% 1 6% 20% 34% 5%	4488
February	1st to 10th	Day	35% 2 10% 25% 22% 7%	1977
	11th to 20th	Day	31% 3 9% 26% 22% 8%	1959
	21st to end	Day	28% 2 5% 24% 26% 14%	1650
March	1st to 10th	Day	26% 2 6% 19% 25% 22%	3294
	11th to 20th	Day	28% 3 9% 22% 29% 9%	3491
	21st to end	Day	40% 3 10% 26% 17% 4	3814
April	1st to 10th	Day	27% 3 9% 25% 25% 12%	2171
	11th to 20th	Day	35% 4 8% 25% 19% 9%	2273
	21st to end	Day	33% 3 10% 27% 22% 4	2470
May	1st to 10th	Day	37% 4 10% 20% 19% 11%	4120
	11th to 20th	Day	32% 3 13% 28% 20% 4	4086
	21st to end	Day	32% 4 10% 20% 19% 15%	4489
June	1st to 10th	Day	25% 5% 19% 28% 16% 7%	2470
	11th to 20th	Day	25% 4 12% 26% 25% 8%	2470
	21st to end	Day	24% 6% 11% 24% 24% 10%	2470
July	1st to 10th	Day	31% 4 10% 24% 24% 7%	4056
	11th to 20th	Day	21% 5% 10% 30% 28% 6%	4051
	21st to end	Day	20% 6% 12% 28% 30% 4	4325
August	1st to 10th	Day	29% 5% 10% 31% 21% 4	4090
	11th to 20th	Day	23% 4 13% 26% 29% 4	3995
	21st to end	Day	20% 4 11% 30% 28% 6%	4179
September	1st to 10th	Day	28% 4 9% 22% 24% 13%	2145
	11th to 20th	Day	27% 5% 9% 23% 30% 6%	2090
	21st to end	Day	24% 3 8% 24% 29% 11%	2090
October	1st to 10th	Day	17% 2 6% 16% 42% 17%	3480
	11th to 20th	Day	16% 2 9% 26% 31% 16%	3214
	21st to end	Day	19% 3 6% 25% 34% 13%	3318
November	1st to 10th	Day	22% 3 7% 23% 31% 14%	1763
	11th to 20th	Day	21% 3 7% 28% 30% 12%	1800
	21st to end	Day	26% 2 6% 25% 29% 11%	1800
December	1st to 10th	Day	18% 2 7% 38% 31% 5%	2940
	11th to 20th	Day	21% 1 6% 28% 37% 7%	3389
	21st to end	Day	44% 2 8% 22% 19% 6%	4488
January	1st to 10th	Night	37% 2 10% 29% 18% 4	5888
	11th to 20th	Night	25% 2 10% 20% 34% 9%	5900
	21st to end	Night	34% 1 6% 18% 34% 7%	6490
February	1st to 10th	Night	35% 3 7% 26% 21% 9%	2440
	11th to 20th	Night	34% 4 9% 23% 23% 8%	2338
	21st to end	Night	26% 3 5% 22% 27% 16%	1980
March	1st to 10th	Night	24% 2 6% 22% 24% 21%	3774
	11th to 20th	Night	27% 2 8% 26% 26% 10%	3497
	21st to end	Night	32% 3 10% 29% 21% 6%	3817
April	1st to 10th	Night	26% 3 9% 22% 25% 15%	2005
	11th to 20th	Night	32% 4 10% 25% 19% 10%	1858
	21st to end	Night	31% 4 10% 28% 22% 4	1710
May	1st to 10th	Night	37% 3 10% 15% 20% 14%	2850
	11th to 20th	Night	30% 4 16% 28% 16% 5%	2824
	21st to end	Night	26% 4 13% 21% 22% 14%	3099
June	1st to 10th	Night	25% 3 17% 33% 16% 6%	1710
	11th to 20th	Night	23% 6% 12% 27% 23% 8%	1710
	21st to end	Night	23% 3 12% 29% 24% 11%	1710
July	1st to 10th	Night	26% 4 15% 26% 25% 5%	2805
	11th to 20th	Night	18% 5% 12% 29% 29% 8%	2796
	21st to end	Night	20% 4 14% 27% 30% 5%	2999
August	1st to 10th	Night	26% 6% 11% 25% 26% 6%	2826
	11th to 20th	Night	26% 3 12% 29% 24% 6%	2906
	21st to end	Night	20% 5% 12% 34% 23% 5%	3487
September	1st to 10th	Night	24% 5% 8% 24% 27% 11%	2033
	11th to 20th	Night	29% 4 9% 21% 28% 9%	2090
	21st to end	Night	22% 4 9% 23% 30% 13%	2090
October	1st to 10th	Night	16% 2 5% 17% 39% 21%	3480
	11th to 20th	Night	16% 2 6% 24% 34% 18%	3672
	21st to end	Night	15% 2 5% 26% 37% 15%	4231
November	1st to 10th	Night	21% 2 9% 23% 27% 17%	2535
	11th to 20th	Night	21% 3 6% 27% 31% 13%	2600
	21st to end	Night	26% 2 6% 25% 29% 13%	2600
December	1st to 10th	Night	17% 2 6% 40% 31% 4	4250
	11th to 20th	Night	22% 1 4 28% 34% 11%	4817
	21st to end	Night	36% 2 9% 26% 20% 6%	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	12% 9% 37% 31% 11% 1	300
	11th to 20th	Day	13% 14% 39% 25% 9%	300
	21st to end	Day	7% 14% 42% 27% 9% 1	330
February	1st to 10th	Day	12% 13% 42% 28% 5% 1	300
	11th to 20th	Day	14% 10% 48% 21% 6% 1	300
	21st to end	Day	11% 18% 43% 20% 7% 1	249
March	1st to 10th	Day	7% 16% 44% 26% 6%	300
	11th to 20th	Day	17% 14% 40% 21% 5% 2	273
	21st to end	Day	17% 19% 36% 24% 4	297
April	1st to 10th	Day	13% 18% 40% 22% 6%	351
	11th to 20th	Day	13% 19% 45% 20% 2	471
	21st to end	Day	17% 15% 38% 21% 9% 1	650
May	1st to 10th	Day	12% 13% 40% 28% 6% 1	653
	11th to 20th	Day	15% 7% 29% 32% 14% 2	675
	21st to end	Day	13% 8% 33% 32% 13% 2	734
June	1st to 10th	Day	6% 4 25% 39% 19% 7%	680
	11th to 20th	Day	5% 5% 27% 40% 20% 2	680
	21st to end	Day	6% 3 28% 39% 20% 3	680
July	1st to 10th	Day	7% 2 13% 39% 30% 9%	671
	11th to 20th	Day	3 1 13% 42% 33% 9%	668
	21st to end	Day	6% 1 11% 42% 32% 10%	739
August	1st to 10th	Day	7% 1 14% 40% 33% 5%	676
	11th to 20th	Day	8% 1 13% 40% 32% 5%	619
	21st to end	Day	5% 2 12% 40% 32% 8%	539
September	1st to 10th	Day	8% 2 13% 35% 35% 8%	355
	11th to 20th	Day	5% 2 21% 41% 25% 6%	300
	21st to end	Day	9% 2 25% 32% 25% 7%	300
October	1st to 10th	Day	9% 4 29% 33% 20% 4	300
	11th to 20th	Day	10% 6% 34% 33% 15% 2	300
	21st to end	Day	16% 11% 34% 23% 11% 5%	330
November	1st to 10th	Day	9% 7% 40% 28% 13% 3	300
	11th to 20th	Day	9% 11% 41% 24% 12% 3	300
	21st to end	Day	15% 12% 33% 24% 13% 3	300
December	1st to 10th	Day	7% 8% 41% 32% 11% 1	300
	11th to 20th	Day	9% 11% 37% 33% 10% 1	300
	21st to end	Day	14% 17% 36% 27% 6%	330
January	1st to 10th	Night	8% 10% 42% 32% 8%	1000
	11th to 20th	Night	11% 12% 36% 34% 7%	1000
	21st to end	Night	9% 9% 41% 34% 6%	1100
February	1st to 10th	Night	13% 10% 42% 31% 5%	840
	11th to 20th	Night	11% 13% 42% 29% 4	784
	21st to end	Night	10% 14% 44% 28% 4	660
March	1st to 10th	Night	9% 13% 44% 29% 5%	759
	11th to 20th	Night	14% 11% 41% 30% 4	575
	21st to end	Night	13% 15% 44% 24% 4	627
April	1st to 10th	Night	11% 13% 45% 25% 6%	570
	11th to 20th	Night	13% 17% 45% 21% 4	563
	21st to end	Night	14% 12% 44% 23% 6%	570
May	1st to 10th	Night	12% 13% 44% 27% 4	570
	11th to 20th	Night	10% 8% 37% 34% 10% 1	561
	21st to end	Night	9% 10% 37% 33% 11% 1	617
June	1st to 10th	Night	7% 6% 35% 38% 15% 1	570
	11th to 20th	Night	6% 6% 40% 40% 8%	570
	21st to end	Night	6% 3 34% 47% 11%	570
July	1st to 10th	Night	6% 2 21% 47% 24% 2	555
	11th to 20th	Night	4 1 19% 53% 22% 1	552
	21st to end	Night	5% 1 17% 53% 21% 2	624
August	1st to 10th	Night	6% 1 22% 48% 22% 1	567
	11th to 20th	Night	7% 2 19% 54% 19%	564
	21st to end	Night	4 2 19% 46% 27% 2	627
September	1st to 10th	Night	9% 2 21% 47% 20% 1	570
	11th to 20th	Night	10% 3 30% 44% 12% 1	570
	21st to end	Night	6% 4 30% 42% 17% 1	570
October	1st to 10th	Night	11% 6% 30% 41% 12%	570
	11th to 20th	Night	8% 8% 41% 36% 6%	692
	21st to end	Night	9% 11% 37% 28% 14%	898
November	1st to 10th	Night	9% 7% 35% 37% 10% 1	975
	11th to 20th	Night	7% 11% 43% 31% 8%	1000
	21st to end	Night	11% 10% 33% 37% 9%	1000
December	1st to 10th	Night	8% 9% 39% 36% 8% 1	1000
	11th to 20th	Night	12% 10% 41% 33% 5%	1000
	21st to end	Night	12% 11% 40% 31% 5%	1100

Page generated at 2025-03-09 07:08:48. Version ac07

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