Sirius Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sirius Observatory clear sky chart from 2005-11-06 to 2025-12-03.

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	2 4 2 5% 4 4 4 3 5% 6% 61%	136050
	Night	2 4 2 5% 3 3 3 3 5% 6% 64%	132179
Transparency	Day	21% 5% 8% 16% 28% 22%	130882
	Night	19% 5% 8% 17% 29% 23%	128361
Seeing	Day	10% 19% 29% 26% 12% 3	16311
	Night	6% 12% 25% 33% 20% 4	26470

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	1 4 2 7% 3 3 4 3 5% 5% 61%	4098
	11th to 20th	Day	1 4 2 4 4 3 2 2 3 4 70%	4100
	21st to end	Day	3 10% 4 5% 3 2 3 3 4 6% 56%	4510
February	1st to 10th	Day	2 7% 3 6% 4 3 3 2 5% 6% 58%	1812
	11th to 20th	Day	2 8% 3 5% 2 3 3 3 5% 7% 57%	1950
	21st to end	Day	2 8% 3 8% 4 4 4 3 3 6% 55%	1666
March	1st to 10th	Day	2 7% 2 7% 3 3 3 3 5% 5% 59%	4954
	11th to 20th	Day	2 12% 4 9% 5% 5% 5% 4 5% 5% 45%	5000
	21st to end	Day	2 4 3 4 4 3 4 4 6% 7% 59%	5486
April	1st to 10th	Day	1 5% 2 6% 4 4 4 3 4 8% 59%	2200
	11th to 20th	Day	1 4 2 5% 3 3 3 3 4 6% 68%	2326
	21st to end	Day	3 4 1 4 3 3 3 2 4 6% 66%	2587
May	1st to 10th	Day	4 5% 2 5% 3 3 4 3 4 5% 63%	5880
	11th to 20th	Day	1 3 2 5% 4 4 4 4 5% 5% 64%	5841
	21st to end	Day	1 2 1 2 2 3 3 2 4 6% 74%	6423
June	1st to 10th	Day	1 2 1 3 2 3 3 3 6% 6% 70%	2600
	11th to 20th	Day	2 1 1 1 1 1 2 1 3 4 83%	2600
	21st to end	Day	2 2 2 4 3 4 4 3 5% 5% 68%	2600
July	1st to 10th	Day	1 2 1 4 3 4 4 3 5% 5% 68%	5879
	11th to 20th	Day	4 3 2 6% 6% 6% 7% 6% 8% 10% 41%	5880
	21st to end	Day	3 3 2 5% 4 5% 6% 5% 6% 7% 54%	6401
August	1st to 10th	Day	3 3 2 6% 5% 6% 6% 5% 6% 8% 50%	5779
	11th to 20th	Day	2 2 2 5% 4 5% 6% 5% 8% 9% 53%	5798
	21st to end	Day	3 4 2 6% 5% 5% 6% 5% 7% 9% 47%	6066
September	1st to 10th	Day	4 5% 2 7% 6% 5% 7% 5% 6% 6% 48%	2200
	11th to 20th	Day	3 2 1 4 3 3 4 3 4 5% 67%	2200
	21st to end	Day	3 3 1 4 3 3 4 3 4 5% 68%	2200
October	1st to 10th	Day	3 3 2 4 3 3 3 2 3 4 69%	3481
	11th to 20th	Day	5% 3 1 3 2 2 2 2 3 6% 70%	3250
	21st to end	Day	1 2 1 3 2 2 3 2 4 5% 75%	3548
November	1st to 10th	Day	2 3 1 3 3 3 2 3 4 7% 70%	2059
	11th to 20th	Day	1 3 1 3 3 3 4 3 4 5% 69%	1899
	21st to end	Day	4 6% 2 4 3 2 3 2 4 6% 65%	1890
December	1st to 10th	Day	2 4 1 4 2 3 3 2 4 5% 70%	2968
	11th to 20th	Day	2 6% 3 6% 3 3 3 4 8% 9% 53%	3409
	21st to end	Day	3 7% 3 6% 5% 4 3 4 5% 7% 55%	4510
January	1st to 10th	Night	1 4 3 6% 3 3 4 3 5% 7% 62%	5867
	11th to 20th	Night	1 3 2 4 2 2 2 2 4 5% 71%	5880
	21st to end	Night	3 9% 3 7% 4 3 4 3 5% 5% 53%	6468
February	1st to 10th	Night	1 7% 3 5% 3 3 3 2 5% 6% 62%	2585
	11th to 20th	Night	2 7% 2 4 3 3 3 3 6% 6% 60%	2338
	21st to end	Night	2 8% 2 6% 3 2 4 4 6% 5% 57%	1962
March	1st to 10th	Night	1 9% 3 7% 3 3 3 3 5% 6% 57%	4940
	11th to 20th	Night	2 9% 4 10% 5% 5% 5% 3 5% 6% 46%	5000
	21st to end	Night	3 5% 2 4 3 3 4 3 6% 8% 60%	5489
April	1st to 10th	Night	1 5% 2 5% 3 3 3 2 4 6% 66%	2200
	11th to 20th	Night	1 4 2 4 3 3 3 3 5% 5% 69%	2022
	21st to end	Night	1 2 2 3 2 3 2 4 7% 74%	1810
May	1st to 10th	Night	2 3 2 5% 3 3 3 3 4 4 68%	4100
	11th to 20th	Night	1 2 1 3 2 2 2 2 3 5% 77%	4074
	21st to end	Night	1 1 1 1 2 3 2 4 5% 79%	4474
June	1st to 10th	Night	1 3 1 2 2 2 2 3 4 5% 73%	1800
	11th to 20th	Night	1 1 1 2 1 1 1 1 2 4 85%	1800
	21st to end	Night	2 2 1 3 2 2 3 3 4 5% 73%	1800
July	1st to 10th	Night	1 1 1 4 3 3 3 3 4 5% 71%	4100
	11th to 20th	Night	2 2 1 5% 5% 6% 7% 6% 9% 11% 46%	4100
	21st to end	Night	3 3 1 5% 4 5% 5% 4 7% 9% 55%	4472
August	1st to 10th	Night	2 3 2 6% 5% 5% 6% 5% 7% 10% 50%	4033
	11th to 20th	Night	1 1 1 2 2 4 4 4 7% 10% 63%	4051
	21st to end	Night	3 4 2 6% 5% 5% 6% 5% 7% 10% 49%	4898
September	1st to 10th	Night	3 4 2 6% 4 5% 4 3 7% 7% 57%	2200
	11th to 20th	Night	2 2 1 2 2 3 3 2 4 4 75%	2200
	21st to end	Night	2 2 1 3 3 3 3 3 4 4 73%	2200
October	1st to 10th	Night	2 2 1 2 2 2 3 3 4 4 75%	3713
	11th to 20th	Night	3 3 2 3 2 2 3 2 3 6% 71%	3898
	21st to end	Night	2 3 1 4 2 2 3 2 4 5% 73%	4252
November	1st to 10th	Night	2 3 1 4 2 2 3 3 4 5% 70%	2460
	11th to 20th	Night	1 3 1 3 2 2 3 3 5% 5% 71%	2718
	21st to end	Night	3 6% 2 5% 3 3 3 3 4 5% 64%	2730
December	1st to 10th	Night	2 4 1 3 2 2 3 2 5% 7% 67%	4266
	11th to 20th	Night	2 5% 3 5% 3 3 4 3 5% 6% 61%	4811
	21st to end	Night	2 6% 3 6% 4 3 3 3 4 5% 62%	6468

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	22% 3 6% 26% 28% 14%	4099
	11th to 20th	Day	16% 6% 7% 20% 20% 31%	4100
	21st to end	Day	25% 6% 5% 12% 30% 22%	4510
February	1st to 10th	Day	27% 3 9% 17% 25% 20%	1812
	11th to 20th	Day	25% 4 8% 20% 24% 18%	1960
	21st to end	Day	29% 4 6% 15% 25% 20%	1666
March	1st to 10th	Day	22% 6% 8% 18% 29% 18%	4954
	11th to 20th	Day	33% 7% 11% 17% 23% 10%	5000
	21st to end	Day	19% 6% 13% 30% 25% 8%	5486
April	1st to 10th	Day	22% 3 7% 18% 35% 16%	2200
	11th to 20th	Day	18% 3 7% 14% 35% 23%	2329
	21st to end	Day	18% 3 6% 14% 40% 18%	2587
May	1st to 10th	Day	20% 6% 6% 13% 36% 19%	5880
	11th to 20th	Day	16% 3 9% 11% 36% 25%	5844
	21st to end	Day	11% 3 3 8% 31% 43%	6424
June	1st to 10th	Day	13% 3 6% 11% 33% 35%	2600
	11th to 20th	Day	8% 2 2 6% 28% 55%	2600
	21st to end	Day	17% 3 6% 9% 25% 40%	2600
July	1st to 10th	Day	16% 4 7% 11% 18% 44%	5816
	11th to 20th	Day	28% 10% 12% 21% 18% 10%	5811
	21st to end	Day	24% 8% 15% 15% 23% 15%	6217
August	1st to 10th	Day	37% 10% 21% 12% 11% 10%	4208
	11th to 20th	Day	24% 8% 15% 21% 20% 12%	4197
	21st to end	Day	28% 7% 12% 21% 22% 10%	4369
September	1st to 10th	Day	31% 6% 11% 14% 19% 20%	2200
	11th to 20th	Day	18% 4 7% 13% 34% 24%	2200
	21st to end	Day	18% 4 6% 14% 32% 27%	2200
October	1st to 10th	Day	18% 4 4 11% 23% 40%	3481
	11th to 20th	Day	17% 2 4 13% 42% 22%	3250
	21st to end	Day	12% 3 4 13% 36% 31%	3548
November	1st to 10th	Day	15% 3 6% 22% 33% 21%	2059
	11th to 20th	Day	17% 3 5% 16% 33% 25%	1899
	21st to end	Day	22% 2 6% 17% 33% 20%	1890
December	1st to 10th	Day	17% 3 5% 21% 34% 21%	2968
	11th to 20th	Day	26% 4 6% 19% 35% 10%	3408
	21st to end	Day	27% 5% 8% 23% 28% 9%	4510
January	1st to 10th	Night	19% 5% 7% 20% 35% 14%	5867
	11th to 20th	Night	13% 6% 6% 21% 27% 27%	5880
	21st to end	Night	27% 8% 5% 10% 26% 24%	6468
February	1st to 10th	Night	22% 3 8% 18% 27% 22%	2585
	11th to 20th	Night	25% 3 6% 21% 26% 19%	2339
	21st to end	Night	25% 3 8% 14% 28% 22%	1962
March	1st to 10th	Night	19% 10% 8% 16% 31% 17%	4940
	11th to 20th	Night	29% 8% 9% 18% 25% 11%	5000
	21st to end	Night	19% 7% 8% 30% 27% 9%	5489
April	1st to 10th	Night	19% 3 8% 16% 36% 17%	2200
	11th to 20th	Night	17% 3 5% 12% 36% 27%	2022
	21st to end	Night	11% 2 6% 19% 41% 20%	1810
May	1st to 10th	Night	19% 3 7% 15% 35% 20%	4097
	11th to 20th	Night	10% 3 5% 13% 40% 29%	4075
	21st to end	Night	8% 2 3 8% 36% 43%	4474
June	1st to 10th	Night	12% 2 5% 8% 33% 39%	1800
	11th to 20th	Night	7% 1 2 5% 27% 57%	1800
	21st to end	Night	14% 2 6% 7% 24% 47%	1800
July	1st to 10th	Night	15% 4 6% 11% 18% 46%	4055
	11th to 20th	Night	24% 9% 14% 24% 18% 11%	4046
	21st to end	Night	22% 6% 18% 16% 21% 17%	4344
August	1st to 10th	Night	34% 9% 21% 12% 12% 11%	2928
	11th to 20th	Night	17% 6% 18% 25% 22% 12%	2928
	21st to end	Night	22% 5% 12% 22% 29% 10%	3536
September	1st to 10th	Night	25% 5% 11% 16% 23% 20%	2200
	11th to 20th	Night	13% 4 5% 16% 32% 30%	2200
	21st to end	Night	15% 3 6% 12% 32% 31%	2200
October	1st to 10th	Night	12% 4 4 13% 27% 40%	3713
	11th to 20th	Night	15% 4 4 16% 39% 23%	3898
	21st to end	Night	13% 3 3 11% 34% 34%	4252
November	1st to 10th	Night	17% 3 5% 19% 35% 22%	2460
	11th to 20th	Night	15% 4 6% 15% 35% 25%	2718
	21st to end	Night	23% 3 5% 16% 29% 24%	2730
December	1st to 10th	Night	16% 3 5% 19% 31% 25%	4266
	11th to 20th	Night	23% 3 5% 16% 37% 16%	4811
	21st to end	Night	24% 4 8% 24% 27% 14%	6468

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	17% 21% 33% 19% 10%	300
	11th to 20th	Day	14% 21% 32% 22% 9% 2	300
	21st to end	Day	9% 26% 32% 20% 13% 1	330
February	1st to 10th	Day	17% 22% 32% 19% 9% 1	312
	11th to 20th	Day	16% 31% 31% 19% 4	495
	21st to end	Day	19% 31% 31% 14% 4	414
March	1st to 10th	Day	22% 29% 28% 17% 4	500
	11th to 20th	Day	18% 27% 27% 18% 8% 1	500
	21st to end	Day	18% 30% 33% 14% 4 1	550
April	1st to 10th	Day	11% 28% 34% 18% 7% 3	500
	11th to 20th	Day	15% 33% 29% 15% 7% 1	492
	21st to end	Day	11% 33% 30% 17% 7% 2	500
May	1st to 10th	Day	9% 28% 39% 17% 7% 1	503
	11th to 20th	Day	9% 20% 35% 24% 9% 3	528
	21st to end	Day	9% 25% 33% 23% 8% 2	573
June	1st to 10th	Day	3 17% 38% 28% 12% 2	530
	11th to 20th	Day	5% 22% 37% 22% 10% 4	530
	21st to end	Day	3 14% 34% 36% 11% 2	530
July	1st to 10th	Day	3 8% 32% 36% 16% 5%	526
	11th to 20th	Day	5% 3 21% 39% 24% 8%	524
	21st to end	Day	6% 3 19% 41% 23% 9%	575
August	1st to 10th	Day	4 3 24% 40% 21% 7%	527
	11th to 20th	Day	3 5% 23% 43% 18% 7%	517
	21st to end	Day	4 4 19% 37% 26% 8%	583
September	1st to 10th	Day	6% 7% 25% 34% 21% 8%	530
	11th to 20th	Day	6% 12% 31% 33% 14% 4	530
	21st to end	Day	7% 14% 28% 31% 16% 4	530
October	1st to 10th	Day	8% 19% 29% 26% 16% 3	463
	11th to 20th	Day	10% 17% 26% 31% 13% 3	330
	21st to end	Day	7% 20% 29% 26% 15% 2	363
November	1st to 10th	Day	12% 17% 27% 28% 13% 2	330
	11th to 20th	Day	10% 18% 28% 25% 17% 2	330
	21st to end	Day	14% 21% 28% 23% 12% 2	330
December	1st to 10th	Day	9% 23% 28% 26% 12% 1	306
	11th to 20th	Day	6% 19% 31% 29% 11% 3	300
	21st to end	Day	21% 19% 27% 26% 5% 1	330
January	1st to 10th	Night	9% 14% 32% 26% 14% 5%	800
	11th to 20th	Night	9% 14% 33% 27% 15% 3	800
	21st to end	Night	11% 19% 29% 27% 12% 3	880
February	1st to 10th	Night	8% 18% 29% 29% 13% 3	800
	11th to 20th	Night	8% 20% 34% 28% 9% 2	784
	21st to end	Night	11% 20% 30% 27% 9% 2	660
March	1st to 10th	Night	11% 19% 36% 25% 9% 1	800
	11th to 20th	Night	11% 14% 27% 30% 15% 3	800
	21st to end	Night	7% 17% 32% 31% 11% 2	880
April	1st to 10th	Night	5% 19% 33% 31% 11% 1	800
	11th to 20th	Night	7% 20% 32% 31% 8% 2	638
	21st to end	Night	4 20% 29% 33% 11% 3	600
May	1st to 10th	Night	4 16% 32% 36% 11% 1	600
	11th to 20th	Night	4 13% 28% 34% 18% 2	591
	21st to end	Night	3 13% 32% 35% 14% 2	650
June	1st to 10th	Night	2 8% 29% 45% 16% 1	600
	11th to 20th	Night	1 9% 30% 43% 15% 2	600
	21st to end	Night	3 5% 23% 46% 23% 2	600
July	1st to 10th	Night	3 1 14% 47% 34% 2	585
	11th to 20th	Night	4 6% 38% 46% 6%	582
	21st to end	Night	4 2 8% 32% 47% 7%	657
August	1st to 10th	Night	5% 1 8% 37% 40% 10%	597
	11th to 20th	Night	3 2 7% 37% 44% 7%	594
	21st to end	Night	3 2 7% 34% 45% 9%	704
September	1st to 10th	Night	4 3 11% 33% 42% 8%	800
	11th to 20th	Night	4 4 16% 41% 31% 5%	800
	21st to end	Night	3 9% 15% 36% 31% 5%	800
October	1st to 10th	Night	6% 9% 26% 37% 20% 2	800
	11th to 20th	Night	7% 7% 24% 38% 19% 4	800
	21st to end	Night	3 14% 23% 32% 21% 7%	880
November	1st to 10th	Night	5% 11% 27% 36% 16% 5%	820
	11th to 20th	Night	5% 12% 27% 34% 18% 5%	840
	21st to end	Night	8% 15% 27% 31% 15% 4	840
December	1st to 10th	Night	10% 14% 27% 29% 18% 2	808
	11th to 20th	Night	9% 13% 31% 30% 15% 2	800
	21st to end	Night	10% 15% 34% 28% 10% 3	880

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