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 2024-10-20.

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	14% 9% 3 8% 5% 5% 5% 4 6% 7% 34%	100897
	Night	15% 10% 3 7% 5% 4 4 4 5% 7% 36%	97764
Transparency	Day	47% 6% 12% 21% 12% 1	100697
	Night	48% 6% 12% 22% 12% 2	97635
Seeing	Day	17% 9% 29% 24% 14% 6%	15306
	Night	19% 9% 29% 30% 12% 1	24868

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	14% 18% 7% 12% 6% 4 4 3 4 5% 23%	2527
	11th to 20th	Day	15% 19% 6% 12% 9% 7% 4 4 5% 6% 15%	2530
	21st to end	Day	26% 19% 6% 10% 6% 4 3 3 4 4 16%	3098
February	1st to 10th	Day	14% 14% 4 13% 8% 6% 5% 3 4 6% 22%	1710
	11th to 20th	Day	13% 13% 5% 11% 7% 5% 4 4 6% 6% 26%	1761
	21st to end	Day	13% 15% 5% 10% 6% 5% 4 4 6% 6% 26%	1570
March	1st to 10th	Day	18% 15% 5% 9% 5% 4 3 2 4 5% 29%	3231
	11th to 20th	Day	13% 12% 4 9% 6% 5% 5% 4 5% 7% 30%	3480
	21st to end	Day	18% 13% 4 7% 6% 4 4 3 5% 6% 29%	3815
April	1st to 10th	Day	16% 13% 4 8% 5% 4 5% 3 6% 6% 31%	2185
	11th to 20th	Day	17% 10% 3 6% 5% 4 4 4 5% 7% 35%	2432
	21st to end	Day	17% 10% 3 8% 6% 5% 4 3 5% 7% 34%	2470
May	1st to 10th	Day	17% 8% 3 8% 5% 4 5% 3 5% 7% 36%	4082
	11th to 20th	Day	17% 7% 3 6% 5% 4 5% 4 6% 8% 35%	4081
	21st to end	Day	10% 5% 2 7% 5% 4 4 3 5% 8% 46%	4497
June	1st to 10th	Day	10% 6% 2 7% 5% 4 5% 5% 7% 8% 40%	2827
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 5% 7% 9% 42%	2850
	21st to end	Day	10% 6% 2 6% 5% 5% 6% 5% 7% 9% 39%	2833
July	1st to 10th	Day	11% 4 2 7% 5% 5% 5% 5% 8% 9% 38%	4440
	11th to 20th	Day	8% 4 2 5% 4 4 5% 4 7% 10% 47%	4440
	21st to end	Day	8% 5% 2 7% 5% 6% 6% 5% 7% 9% 41%	4511
August	1st to 10th	Day	13% 5% 2 6% 5% 5% 5% 4 6% 8% 41%	4104
	11th to 20th	Day	10% 5% 2 6% 4 6% 6% 5% 7% 8% 40%	4044
	21st to end	Day	10% 5% 2 7% 5% 5% 6% 4 7% 9% 39%	4532
September	1st to 10th	Day	10% 6% 3 7% 4 5% 6% 5% 5% 8% 41%	2147
	11th to 20th	Day	10% 5% 1 5% 5% 5% 4 4 6% 8% 47%	2090
	21st to end	Day	15% 8% 2 6% 4 5% 5% 4 7% 8% 35%	2090
October	1st to 10th	Day	13% 7% 2 7% 4 4 5% 4 5% 7% 42%	2014
	11th to 20th	Day	18% 9% 3 7% 4 6% 5% 3 5% 6% 34%	1890
	21st to end	Day	20% 12% 2 8% 5% 4 5% 4 5% 7% 30%	1850
November	1st to 10th	Day	12% 10% 3 8% 5% 5% 4 4 6% 8% 35%	1672
	11th to 20th	Day	13% 12% 3 9% 5% 5% 5% 4 6% 6% 32%	1710
	21st to end	Day	18% 13% 5% 7% 5% 5% 4 4 5% 6% 28%	1525
December	1st to 10th	Day	14% 14% 6% 13% 6% 6% 5% 4 5% 6% 22%	1520
	11th to 20th	Day	16% 16% 4 9% 5% 4 4 5% 5% 7% 26%	1766
	21st to end	Day	27% 20% 6% 9% 4 3 4 3 5% 5% 12%	2573
January	1st to 10th	Night	17% 15% 6% 10% 5% 5% 4 3 4 6% 26%	4428
	11th to 20th	Night	14% 17% 7% 14% 6% 7% 5% 3 5% 5% 18%	4440
	21st to end	Night	23% 17% 8% 10% 5% 3 4 3 3 4 20%	4556
February	1st to 10th	Night	14% 16% 5% 11% 6% 5% 5% 4 5% 5% 23%	2470
	11th to 20th	Night	13% 13% 5% 10% 6% 5% 5% 4 5% 6% 28%	2302
	21st to end	Night	18% 12% 4 10% 5% 5% 4 4 5% 6% 28%	1884
March	1st to 10th	Night	15% 13% 3 8% 5% 4 4 3 4 5% 35%	3621
	11th to 20th	Night	9% 8% 4 7% 4 5% 5% 4 6% 8% 40%	3480
	21st to end	Night	16% 10% 2 8% 5% 4 4 3 4 5% 39%	3817
April	1st to 10th	Night	18% 10% 3 7% 4 4 4 4 6% 6% 35%	2009
	11th to 20th	Night	18% 8% 3 5% 4 4 4 3 6% 7% 37%	1696
	21st to end	Night	17% 7% 3 6% 4 4 4 4 5% 9% 37%	1710
May	1st to 10th	Night	17% 9% 2 6% 4 4 4 4 5% 8% 37%	2813
	11th to 20th	Night	14% 7% 3 7% 4 5% 4 3 5% 7% 41%	2812
	21st to end	Night	11% 5% 2 4 4 4 4 3 5% 8% 50%	3053
June	1st to 10th	Night	10% 5% 2 7% 3 4 4 4 5% 9% 47%	1348
	11th to 20th	Night	9% 6% 3 6% 4 4 3 5% 7% 9% 45%	1330
	21st to end	Night	10% 5% 2 5% 4 4 4 3 6% 8% 49%	1340
July	1st to 10th	Night	8% 4 1 5% 3 4 4 4 6% 11% 50%	2530
	11th to 20th	Night	9% 3 2 5% 4 3 4 4 5% 8% 54%	2530
	21st to end	Night	5% 3 2 5% 5% 5% 6% 5% 8% 9% 47%	3039
August	1st to 10th	Night	12% 5% 2 5% 4 3 3 4 5% 8% 47%	2825
	11th to 20th	Night	9% 6% 2 5% 4 4 4 3 7% 7% 48%	2785
	21st to end	Night	9% 5% 2 6% 4 4 5% 4 5% 7% 49%	3124
September	1st to 10th	Night	10% 6% 2 5% 5% 5% 4 4 5% 7% 48%	2010
	11th to 20th	Night	10% 4 1 4 3 3 4 4 4 7% 54%	2090
	21st to end	Night	15% 6% 2 6% 4 3 4 4 5% 7% 43%	2090
October	1st to 10th	Night	14% 5% 2 6% 4 3 4 3 5% 7% 46%	2165
	11th to 20th	Night	20% 8% 2 6% 5% 4 4 4 5% 6% 36%	2275
	21st to end	Night	23% 8% 3 7% 4 4 5% 4 6% 6% 30%	2502
November	1st to 10th	Night	14% 9% 2 5% 4 3 4 3 6% 7% 43%	2406
	11th to 20th	Night	16% 10% 3 7% 5% 5% 5% 4 7% 6% 31%	2470
	21st to end	Night	19% 14% 4 7% 5% 4 5% 3 6% 6% 29%	2651
December	1st to 10th	Night	18% 14% 5% 10% 6% 5% 5% 4 6% 6% 23%	2660
	11th to 20th	Night	15% 14% 5% 11% 5% 5% 5% 4 5% 7% 26%	3427
	21st to end	Night	26% 22% 6% 9% 4 4 3 2 4 4 14%	5076

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	63% 7% 10% 15% 5% 1	2528
	11th to 20th	Day	71% 6% 11% 8% 4	2530
	21st to end	Day	73% 5% 11% 8% 4	3098
February	1st to 10th	Day	64% 5% 8% 16% 7%	1710
	11th to 20th	Day	59% 4 9% 17% 9% 1	1770
	21st to end	Day	58% 7% 10% 16% 8% 2	1570
March	1st to 10th	Day	58% 5% 11% 19% 6% 1	3231
	11th to 20th	Day	52% 6% 12% 21% 8% 2	3480
	21st to end	Day	55% 5% 13% 15% 10% 2	3815
April	1st to 10th	Day	52% 6% 11% 20% 11% 1	2185
	11th to 20th	Day	48% 6% 10% 20% 14% 2	2432
	21st to end	Day	50% 6% 12% 17% 13% 2	2470
May	1st to 10th	Day	46% 7% 11% 21% 12% 3	4120
	11th to 20th	Day	45% 8% 14% 23% 9% 2	4085
	21st to end	Day	36% 6% 11% 19% 23% 5%	4511
June	1st to 10th	Day	39% 7% 12% 26% 15% 2	2827
	11th to 20th	Day	36% 7% 14% 24% 16% 2	2850
	21st to end	Day	39% 7% 14% 24% 15% 2	2833
July	1st to 10th	Day	39% 8% 15% 20% 15% 2	4371
	11th to 20th	Day	29% 8% 14% 34% 14% 1	4365
	21st to end	Day	36% 9% 15% 18% 21% 2	4368
August	1st to 10th	Day	39% 8% 13% 22% 16% 2	4091
	11th to 20th	Day	37% 9% 13% 30% 10% 1	4083
	21st to end	Day	38% 8% 14% 28% 12% 1	4531
September	1st to 10th	Day	39% 7% 14% 26% 13% 2	2147
	11th to 20th	Day	34% 6% 13% 32% 16% 1	2090
	21st to end	Day	45% 6% 13% 24% 11% 1	2090
October	1st to 10th	Day	41% 7% 10% 26% 15% 2	2014
	11th to 20th	Day	50% 4 13% 21% 11% 1	1890
	21st to end	Day	55% 4 14% 18% 9%	1850
November	1st to 10th	Day	46% 6% 10% 25% 11% 1	1672
	11th to 20th	Day	52% 5% 13% 19% 10% 1	1710
	21st to end	Day	57% 6% 12% 18% 6% 1	1525
December	1st to 10th	Day	63% 6% 11% 15% 5%	1520
	11th to 20th	Day	57% 6% 14% 16% 6%	1762
	21st to end	Day	75% 5% 9% 8% 3	2573
January	1st to 10th	Night	59% 6% 11% 16% 6% 1	4428
	11th to 20th	Night	66% 7% 11% 12% 4	4440
	21st to end	Night	69% 4 9% 15% 3	4556
February	1st to 10th	Night	62% 6% 9% 15% 7% 1	2470
	11th to 20th	Night	56% 5% 8% 20% 10% 1	2304
	21st to end	Night	58% 5% 10% 18% 9% 1	1884
March	1st to 10th	Night	52% 4 12% 24% 7% 1	3621
	11th to 20th	Night	40% 7% 14% 21% 17% 1	3480
	21st to end	Night	47% 5% 10% 24% 13% 1	3817
April	1st to 10th	Night	49% 5% 11% 21% 13% 2	2009
	11th to 20th	Night	45% 6% 10% 19% 18% 2	1696
	21st to end	Night	43% 6% 10% 20% 16% 3	1710
May	1st to 10th	Night	45% 6% 10% 21% 13% 4	2840
	11th to 20th	Night	42% 7% 11% 26% 12% 2	2813
	21st to end	Night	32% 8% 11% 22% 19% 8%	3060
June	1st to 10th	Night	34% 6% 10% 30% 18% 2	1348
	11th to 20th	Night	33% 8% 14% 23% 20% 2	1330
	21st to end	Night	33% 6% 12% 27% 20% 2	1340
July	1st to 10th	Night	28% 6% 15% 29% 16% 7%	2490
	11th to 20th	Night	28% 7% 12% 35% 17% 2	2482
	21st to end	Night	28% 11% 17% 21% 21% 2	2942
August	1st to 10th	Night	34% 7% 14% 28% 16% 2	2814
	11th to 20th	Night	33% 6% 16% 32% 11% 2	2812
	21st to end	Night	33% 8% 13% 30% 15% 1	3124
September	1st to 10th	Night	36% 6% 12% 29% 15% 2	2010
	11th to 20th	Night	29% 7% 11% 32% 21% 1	2090
	21st to end	Night	40% 5% 11% 30% 12% 1	2090
October	1st to 10th	Night	37% 5% 11% 26% 19% 1	2165
	11th to 20th	Night	48% 6% 12% 21% 12% 1	2275
	21st to end	Night	54% 5% 10% 21% 10% 1	2502
November	1st to 10th	Night	40% 6% 12% 27% 14% 1	2406
	11th to 20th	Night	51% 5% 13% 20% 9% 1	2470
	21st to end	Night	55% 4 15% 18% 6%	2651
December	1st to 10th	Night	61% 5% 12% 16% 5%	2660
	11th to 20th	Night	57% 7% 13% 16% 7%	3430
	21st to end	Night	74% 5% 10% 8% 3	5076

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	21% 18% 36% 17% 6% 1	270
	11th to 20th	Day	24% 13% 41% 18% 3 1	270
	21st to end	Day	29% 11% 37% 18% 5% 1	297
February	1st to 10th	Day	23% 17% 41% 15% 3 1	270
	11th to 20th	Day	24% 16% 39% 19% 2	270
	21st to end	Day	23% 23% 38% 14% 2	225
March	1st to 10th	Day	24% 17% 37% 18% 4 1	329
	11th to 20th	Day	22% 12% 33% 24% 8% 1	460
	21st to end	Day	28% 9% 33% 24% 5% 1	506
April	1st to 10th	Day	19% 15% 34% 22% 9% 1	460
	11th to 20th	Day	27% 13% 30% 15% 8% 6%	452
	21st to end	Day	22% 11% 28% 26% 10% 3	460
May	1st to 10th	Day	21% 10% 29% 25% 13% 3	463
	11th to 20th	Day	20% 6% 30% 24% 14% 7%	487
	21st to end	Day	14% 6% 24% 27% 19% 9%	529
June	1st to 10th	Day	13% 9% 28% 31% 14% 5%	657
	11th to 20th	Day	11% 6% 30% 30% 17% 5%	680
	21st to end	Day	13% 6% 28% 32% 15% 6%	680
July	1st to 10th	Day	9% 5% 19% 32% 22% 13%	671
	11th to 20th	Day	10% 5% 26% 31% 19% 9%	668
	21st to end	Day	8% 4 21% 30% 26% 11%	564
August	1st to 10th	Day	9% 5% 17% 27% 26% 17%	486
	11th to 20th	Day	10% 4 15% 32% 25% 15%	479
	21st to end	Day	12% 6% 18% 26% 24% 14%	539
September	1st to 10th	Day	13% 4 24% 24% 20% 14%	490
	11th to 20th	Day	10% 3 17% 22% 30% 17%	490
	21st to end	Day	17% 10% 23% 22% 20% 8%	490
October	1st to 10th	Day	17% 7% 27% 24% 18% 7%	490
	11th to 20th	Day	20% 13% 34% 22% 9% 2	486
	21st to end	Day	27% 10% 37% 20% 5% 1	365
November	1st to 10th	Day	14% 17% 39% 19% 10% 1	270
	11th to 20th	Day	19% 20% 36% 17% 7% 2	270
	21st to end	Day	28% 14% 38% 16% 4	183
December	1st to 10th	Day	22% 16% 38% 16% 7% 2	180
	11th to 20th	Day	26% 21% 36% 15% 3 1	180
	21st to end	Day	29% 14% 36% 17% 3	240
January	1st to 10th	Night	22% 14% 37% 20% 5% 1	950
	11th to 20th	Night	25% 14% 33% 23% 4 1	950
	21st to end	Night	21% 11% 34% 28% 6%	847
February	1st to 10th	Night	21% 13% 40% 23% 3	760
	11th to 20th	Night	22% 13% 34% 26% 5%	743
	21st to end	Night	23% 15% 37% 21% 3	628
March	1st to 10th	Night	22% 10% 36% 26% 5%	760
	11th to 20th	Night	17% 12% 38% 26% 6% 1	760
	21st to end	Night	23% 9% 33% 27% 7% 1	836
April	1st to 10th	Night	23% 13% 35% 26% 4	679
	11th to 20th	Night	23% 8% 32% 27% 9% 2	563
	21st to end	Night	21% 5% 31% 35% 9%	570
May	1st to 10th	Night	19% 5% 30% 34% 11% 1	570
	11th to 20th	Night	19% 5% 27% 34% 13% 1	561
	21st to end	Night	12% 3 22% 40% 21% 2	603
June	1st to 10th	Night	12% 4 26% 45% 13%	380
	11th to 20th	Night	12% 5% 24% 43% 15% 1	380
	21st to end	Night	12% 2 23% 47% 16% 1	390
July	1st to 10th	Night	9% 1 17% 43% 26% 4	555
	11th to 20th	Night	10% 3 17% 41% 25% 4	552
	21st to end	Night	6% 2 19% 47% 23% 3	624
August	1st to 10th	Night	9% 3 14% 41% 31% 2	567
	11th to 20th	Night	7% 2 17% 39% 32% 3	564
	21st to end	Night	9% 3 17% 35% 31% 4	627
September	1st to 10th	Night	12% 2 19% 35% 27% 4	698
	11th to 20th	Night	11% 4 17% 36% 28% 5%	760
	21st to end	Night	17% 4 22% 34% 18% 4	760
October	1st to 10th	Night	16% 6% 26% 32% 17% 3	760
	11th to 20th	Night	23% 11% 30% 27% 9% 1	759
	21st to end	Night	27% 10% 31% 27% 5%	792
November	1st to 10th	Night	18% 10% 30% 27% 14% 1	740
	11th to 20th	Night	20% 13% 34% 22% 9% 1	760
	21st to end	Night	24% 11% 36% 23% 6%	760
December	1st to 10th	Night	24% 11% 40% 19% 6% 1	760
	11th to 20th	Night	23% 13% 36% 24% 3	855
	21st to end	Night	30% 11% 33% 23% 2	1045

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