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 2025-03-27.

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% 10% 3 8% 5% 5% 5% 4 6% 7% 33%	108533
	Night	14% 11% 4 8% 5% 4 4 4 5% 6% 35%	110862
Transparency	Day	47% 6% 12% 21% 12% 1	112148
	Night	47% 6% 12% 22% 12% 1	115374
Seeing	Day	17% 10% 29% 24% 14% 6%	15767
	Night	19% 9% 30% 30% 12% 1	25536

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	10% 16% 6% 13% 6% 4 4 3 5% 5% 27%	3617
	11th to 20th	Day	13% 17% 5% 15% 9% 8% 4 4 5% 5% 15%	3620
	21st to end	Day	21% 16% 6% 11% 6% 4 5% 3 4 4 19%	4440
February	1st to 10th	Day	14% 15% 4 13% 8% 6% 5% 3 4 6% 22%	1800
	11th to 20th	Day	14% 13% 5% 12% 7% 5% 4 4 6% 6% 25%	1857
	21st to end	Day	13% 15% 5% 11% 6% 5% 4 4 6% 6% 25%	1650
March	1st to 10th	Day	18% 14% 5% 10% 5% 4 3 2 4 5% 29%	3334
	11th to 20th	Day	13% 12% 4 9% 6% 4 5% 4 5% 7% 30%	3590
	21st to end	Day	18% 13% 4 7% 6% 4 4 3 5% 6% 29%	3881
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%	1900
	21st to end	Day	19% 11% 3 8% 5% 4 5% 4 5% 7% 30%	1952
November	1st to 10th	Day	12% 10% 3 8% 5% 5% 4 4 6% 8% 35%	1762
	11th to 20th	Day	14% 12% 3 9% 5% 5% 5% 4 6% 6% 30%	1800
	21st to end	Day	18% 13% 5% 8% 5% 5% 5% 4 5% 6% 26%	1605
December	1st to 10th	Day	14% 14% 6% 11% 5% 5% 4 4 6% 8% 23%	2600
	11th to 20th	Day	17% 21% 5% 8% 4 4 4 4 5% 5% 23%	2768
	21st to end	Day	26% 21% 6% 10% 4 3 4 3 4 5% 14%	3688
January	1st to 10th	Night	13% 13% 5% 11% 5% 5% 4 3 5% 6% 31%	6348
	11th to 20th	Night	12% 15% 6% 13% 8% 7% 5% 4 6% 6% 20%	6360
	21st to end	Night	17% 16% 8% 10% 4 4 6% 3 4 5% 24%	6514
February	1st to 10th	Night	14% 16% 5% 12% 6% 5% 5% 4 6% 5% 23%	2600
	11th to 20th	Night	13% 13% 5% 10% 6% 5% 5% 4 5% 6% 28%	2426
	21st to end	Night	17% 12% 3 10% 6% 5% 4 4 5% 6% 28%	1980
March	1st to 10th	Night	15% 13% 3 8% 5% 4 4 3 4 5% 35%	3738
	11th to 20th	Night	9% 8% 4 7% 4 5% 5% 4 6% 8% 40%	3590
	21st to end	Night	16% 10% 2 8% 5% 3 4 3 4 5% 39%	3890
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%	2280
	21st to end	Night	22% 8% 3 7% 4 4 5% 4 6% 6% 31%	2641
November	1st to 10th	Night	14% 9% 2 5% 4 3 4 3 6% 6% 43%	2536
	11th to 20th	Night	16% 11% 3 7% 5% 5% 6% 4 6% 6% 31%	2600
	21st to end	Night	18% 14% 4 7% 5% 5% 5% 3 6% 6% 28%	2791
December	1st to 10th	Night	14% 14% 5% 11% 5% 5% 5% 4 6% 6% 24%	4580
	11th to 20th	Night	15% 19% 5% 10% 6% 5% 5% 3 5% 6% 22%	5417
	21st to end	Night	23% 21% 6% 10% 4 4 4 3 4 4 17%	7272

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	58% 6% 11% 16% 7% 2	3618
	11th to 20th	Day	71% 6% 9% 7% 6% 1	3620
	21st to end	Day	68% 5% 12% 9% 6%	4440
February	1st to 10th	Day	64% 5% 8% 16% 7%	1800
	11th to 20th	Day	59% 4 9% 17% 8% 1	1866
	21st to end	Day	58% 7% 10% 16% 7% 2	1650
March	1st to 10th	Day	58% 5% 11% 19% 6% 1	3334
	11th to 20th	Day	51% 6% 12% 21% 8% 2	3590
	21st to end	Day	55% 5% 13% 15% 10% 2	3881
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	32% 6% 10% 32% 18% 2	3352
	11th to 20th	Day	40% 5% 11% 24% 18%	3140
	21st to end	Day	46% 5% 14% 23% 12% 1	3189
November	1st to 10th	Day	46% 6% 10% 25% 11% 1	1762
	11th to 20th	Day	53% 5% 13% 18% 10% 1	1800
	21st to end	Day	58% 6% 12% 18% 6% 1	1605
December	1st to 10th	Day	58% 7% 17% 14% 5%	2600
	11th to 20th	Day	62% 5% 12% 13% 8%	2764
	21st to end	Day	74% 5% 7% 9% 5%	3688
January	1st to 10th	Night	53% 6% 10% 21% 8% 2	6348
	11th to 20th	Night	63% 7% 12% 12% 6% 1	6360
	21st to end	Night	62% 5% 10% 16% 7%	6514
February	1st to 10th	Night	62% 6% 9% 15% 8% 1	2600
	11th to 20th	Night	56% 5% 8% 20% 10% 1	2428
	21st to end	Night	57% 4 10% 18% 9% 1	1980
March	1st to 10th	Night	52% 4 13% 24% 7% 1	3738
	11th to 20th	Night	40% 7% 14% 22% 16% 1	3590
	21st to end	Night	47% 5% 9% 24% 13% 1	3890
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	26% 4 11% 32% 26% 1	3607
	11th to 20th	Night	43% 5% 10% 25% 17% 1	3779
	21st to end	Night	42% 5% 12% 28% 12% 1	4341
November	1st to 10th	Night	40% 6% 11% 27% 14% 1	2536
	11th to 20th	Night	52% 6% 13% 19% 9% 1	2600
	21st to end	Night	56% 5% 15% 18% 6% 1	2791
December	1st to 10th	Night	58% 7% 17% 15% 3	4580
	11th to 20th	Night	61% 6% 11% 14% 7% 1	5420
	21st to end	Night	70% 5% 10% 9% 6%	7272

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	19% 18% 38% 18% 6% 1	300
	11th to 20th	Day	23% 14% 43% 16% 2 1	300
	21st to end	Day	27% 13% 38% 17% 5% 1	330
February	1st to 10th	Day	23% 18% 40% 15% 3 1	300
	11th to 20th	Day	24% 18% 37% 19% 2	300
	21st to end	Day	24% 24% 36% 14% 2	249
March	1st to 10th	Day	23% 18% 38% 18% 3 1	362
	11th to 20th	Day	22% 13% 34% 23% 7% 1	500
	21st to end	Day	27% 10% 33% 24% 5% 1	530
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	19% 13% 34% 23% 9% 2	490
	21st to end	Day	26% 12% 36% 20% 5% 1	401
November	1st to 10th	Day	14% 17% 39% 19% 9% 1	300
	11th to 20th	Day	21% 20% 34% 16% 7% 2	300
	21st to end	Day	27% 14% 39% 16% 4	203
December	1st to 10th	Day	22% 17% 37% 17% 7% 2	200
	11th to 20th	Day	26% 22% 35% 15% 3 1	200
	21st to end	Day	29% 15% 35% 16% 4	267
January	1st to 10th	Night	21% 14% 38% 20% 5% 1	1000
	11th to 20th	Night	24% 14% 34% 24% 4 1	1000
	21st to end	Night	21% 11% 35% 27% 6%	891
February	1st to 10th	Night	21% 14% 40% 23% 2	800
	11th to 20th	Night	22% 14% 34% 25% 5%	783
	21st to end	Night	23% 15% 38% 21% 3	660
March	1st to 10th	Night	22% 11% 36% 26% 5%	800
	11th to 20th	Night	17% 13% 38% 26% 6% 1	800
	21st to end	Night	23% 10% 33% 27% 7% 1	863
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	760
	21st to end	Night	25% 10% 32% 28% 5%	836
November	1st to 10th	Night	18% 11% 30% 27% 13% 1	780
	11th to 20th	Night	20% 14% 34% 23% 8% 1	800
	21st to end	Night	23% 12% 37% 23% 6%	800
December	1st to 10th	Night	24% 11% 39% 20% 6% 1	800
	11th to 20th	Night	23% 14% 36% 24% 3	900
	21st to end	Night	30% 11% 33% 23% 2	1100

Page generated at 2025-03-24 09:04:26. Version ac07

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