Sirius Astronomical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sirius Astronomical 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	17% 7% 3 6% 4 4 4 4 5% 7% 38%	126814
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 39%	123893
Transparency	Day	44% 5% 11% 22% 15% 2	130346
	Night	44% 4 11% 22% 16% 2	128448
Seeing	Day	24% 16% 30% 19% 8% 2	16390
	Night	23% 14% 32% 25% 7% 1	25390

By Month

Percentage Cloud Cover NE (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	12% 9% 3 8% 6% 4 4 3 4 6% 41%	3597
	11th to 20th	Day	14% 8% 3 7% 4 3 4 4 5% 6% 42%	3570
	21st to end	Day	19% 9% 2 7% 4 3 4 3 5% 5% 40%	4022
February	1st to 10th	Day	18% 8% 2 5% 5% 5% 5% 5% 6% 6% 36%	1880
	11th to 20th	Day	15% 7% 3 7% 4 5% 5% 3 5% 7% 39%	1948
	21st to end	Day	20% 9% 3 5% 4 4 4 4 5% 6% 38%	1650
March	1st to 10th	Day	22% 10% 3 6% 4 3 3 2 4 5% 38%	4476
	11th to 20th	Day	15% 7% 2 7% 5% 5% 4 4 5% 6% 39%	4648
	21st to end	Day	22% 8% 3 6% 4 4 4 4 5% 7% 34%	5967
April	1st to 10th	Day	20% 8% 2 7% 4 4 4 3 5% 8% 35%	2400
	11th to 20th	Day	20% 7% 2 6% 4 4 4 4 5% 5% 41%	2374
	21st to end	Day	20% 5% 2 6% 4 5% 4 3 4 6% 39%	2533
May	1st to 10th	Day	30% 8% 3 6% 4 4 4 3 5% 8% 25%	5886
	11th to 20th	Day	20% 8% 3 7% 5% 4 5% 3 5% 7% 33%	6106
	21st to end	Day	20% 8% 3 6% 4 4 4 3 6% 8% 35%	6949
June	1st to 10th	Day	17% 6% 2 6% 4 5% 5% 4 6% 9% 34%	2800
	11th to 20th	Day	15% 7% 3 7% 5% 4 5% 4 6% 9% 36%	2800
	21st to end	Day	13% 8% 3 7% 4 5% 5% 4 7% 9% 37%	2800
July	1st to 10th	Day	14% 8% 3 7% 5% 5% 6% 5% 7% 9% 33%	6360
	11th to 20th	Day	11% 6% 2 7% 5% 5% 6% 5% 8% 11% 34%	6360
	21st to end	Day	15% 6% 2 6% 4 4 4 3 5% 7% 41%	6926
August	1st to 10th	Day	17% 8% 3 8% 5% 5% 5% 5% 6% 9% 29%	4376
	11th to 20th	Day	10% 6% 2 8% 5% 5% 5% 4 6% 8% 41%	3922
	21st to end	Day	8% 5% 2 7% 5% 5% 5% 4 6% 8% 44%	4312
September	1st to 10th	Day	14% 6% 3 5% 5% 4 4 3 5% 6% 45%	2400
	11th to 20th	Day	13% 6% 2 5% 3 3 3 4 4 6% 50%	2249
	21st to end	Day	18% 6% 2 6% 3 4 4 4 5% 7% 40%	2200
October	1st to 10th	Day	21% 6% 2 5% 3 4 4 3 4 5% 42%	2188
	11th to 20th	Day	15% 6% 2 5% 3 3 4 3 6% 7% 46%	1963
	21st to end	Day	19% 7% 3 5% 5% 3 3 3 5% 7% 41%	1980
November	1st to 10th	Day	13% 7% 2 6% 4 4 5% 3 4 5% 47%	1851
	11th to 20th	Day	19% 6% 2 4 3 3 4 2 5% 6% 46%	1890
	21st to end	Day	18% 8% 3 6% 5% 4 3 3 4 5% 42%	1869
December	1st to 10th	Day	16% 8% 2 6% 4 2 3 3 4 6% 45%	2616
	11th to 20th	Day	21% 7% 2 4 3 3 3 3 4 5% 45%	2986
	21st to end	Day	21% 9% 3 7% 4 3 3 2 5% 6% 37%	3960
January	1st to 10th	Night	14% 7% 2 8% 4 4 4 3 5% 6% 42%	6349
	11th to 20th	Night	14% 8% 3 8% 5% 4 3 3 4 6% 42%	6315
	21st to end	Night	18% 9% 3 7% 5% 4 4 3 4 5% 37%	6932
February	1st to 10th	Night	17% 10% 3 6% 5% 3 3 4 5% 6% 38%	2518
	11th to 20th	Night	16% 8% 3 6% 4 4 4 4 5% 7% 39%	2338
	21st to end	Night	19% 9% 2 7% 4 4 4 3 5% 5% 39%	1980
March	1st to 10th	Night	23% 8% 3 6% 4 3 3 3 5% 5% 37%	5364
	11th to 20th	Night	17% 9% 3 6% 4 4 4 3 5% 6% 38%	5311
	21st to end	Night	23% 9% 3 6% 4 4 4 3 5% 5% 35%	4962
April	1st to 10th	Night	20% 7% 3 6% 5% 5% 5% 4 6% 5% 35%	2000
	11th to 20th	Night	20% 7% 3 6% 5% 4 3 4 5% 5% 39%	1975
	21st to end	Night	21% 7% 2 7% 3 3 3 3 5% 5% 40%	1865
May	1st to 10th	Night	28% 11% 3 7% 4 4 3 3 5% 6% 26%	4054
	11th to 20th	Night	20% 11% 4 8% 4 4 4 3 6% 7% 29%	3816
	21st to end	Night	22% 7% 3 7% 4 4 4 3 5% 7% 32%	3926
June	1st to 10th	Night	18% 7% 2 7% 3 4 4 4 6% 7% 38%	1600
	11th to 20th	Night	17% 7% 3 6% 5% 4 4 3 6% 8% 38%	1600
	21st to end	Night	12% 6% 2 7% 5% 5% 4 4 7% 9% 42%	1600
July	1st to 10th	Night	14% 7% 3 7% 5% 4 4 4 6% 9% 36%	3600
	11th to 20th	Night	15% 7% 2 8% 4 4 4 4 7% 9% 35%	3600
	21st to end	Night	13% 7% 3 6% 4 4 4 4 5% 6% 43%	3926
August	1st to 10th	Night	22% 6% 2 7% 5% 4 5% 4 7% 7% 30%	2741
	11th to 20th	Night	12% 8% 3 7% 5% 4 5% 4 6% 7% 39%	3123
	21st to end	Night	9% 4 1 5% 3 4 4 3 6% 8% 52%	3586
September	1st to 10th	Night	14% 6% 3 6% 4 3 5% 4 6% 6% 44%	2000
	11th to 20th	Night	12% 5% 2 4 4 3 3 3 5% 6% 52%	2150
	21st to end	Night	17% 7% 3 6% 4 3 4 4 4 6% 42%	2200
October	1st to 10th	Night	19% 7% 2 5% 3 4 3 3 5% 6% 42%	2210
	11th to 20th	Night	17% 5% 2 5% 3 4 4 3 5% 6% 47%	2435
	21st to end	Night	22% 7% 2 5% 3 3 4 3 5% 5% 40%	2860
November	1st to 10th	Night	14% 5% 2 5% 4 3 3 4 5% 6% 48%	2667
	11th to 20th	Night	19% 7% 2 5% 3 3 3 3 5% 5% 44%	2730
	21st to end	Night	16% 8% 2 6% 4 3 4 3 5% 6% 44%	2747
December	1st to 10th	Night	19% 7% 2 5% 4 4 4 3 4 5% 44%	4609
	11th to 20th	Night	25% 8% 2 5% 3 3 3 3 3 4 41%	5208
	21st to end	Night	19% 10% 3 6% 4 4 3 3 5% 5% 39%	6996

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	44% 4 7% 29% 14% 2	3598
	11th to 20th	Day	43% 3 10% 23% 14% 7%	3600
	21st to end	Day	47% 3 10% 23% 16% 1	4022
February	1st to 10th	Day	47% 4 13% 21% 13% 2	1880
	11th to 20th	Day	45% 4 9% 23% 15% 3	1957
	21st to end	Day	48% 3 10% 22% 14% 3	1650
March	1st to 10th	Day	50% 3 10% 16% 12% 9%	4479
	11th to 20th	Day	43% 6% 12% 28% 11% 2	4648
	21st to end	Day	49% 5% 6% 16% 19% 5%	5967
April	1st to 10th	Day	48% 4 9% 21% 15% 3	2400
	11th to 20th	Day	45% 4 10% 21% 19% 3	2374
	21st to end	Day	47% 4 9% 21% 16% 4	2533
May	1st to 10th	Day	58% 4 14% 14% 8% 1	5897
	11th to 20th	Day	51% 4 12% 19% 12% 1	6110
	21st to end	Day	47% 6% 10% 23% 13% 1	6949
June	1st to 10th	Day	45% 5% 12% 24% 12% 1	2800
	11th to 20th	Day	45% 5% 14% 22% 13% 2	2800
	21st to end	Day	43% 6% 13% 22% 15% 1	2800
July	1st to 10th	Day	47% 7% 11% 20% 14% 1	6290
	11th to 20th	Day	41% 8% 17% 23% 11% 1	6285
	21st to end	Day	41% 5% 11% 23% 18% 1	6728
August	1st to 10th	Day	50% 6% 14% 20% 8% 1	4362
	11th to 20th	Day	39% 6% 14% 30% 10% 1	3960
	21st to end	Day	35% 7% 11% 29% 16% 2	4312
September	1st to 10th	Day	40% 4 11% 23% 20% 2	2400
	11th to 20th	Day	34% 4 10% 24% 24% 3	2249
	21st to end	Day	44% 4 8% 22% 19% 3	2200
October	1st to 10th	Day	38% 5% 9% 21% 24% 4	3565
	11th to 20th	Day	30% 3 8% 26% 29% 4	3179
	21st to end	Day	33% 4 13% 27% 21% 3	3180
November	1st to 10th	Day	40% 4 12% 22% 20% 2	1851
	11th to 20th	Day	40% 3 9% 24% 20% 4	1890
	21st to end	Day	45% 4 11% 27% 11% 3	1869
December	1st to 10th	Day	41% 3 15% 29% 12% 1	2616
	11th to 20th	Day	42% 3 14% 19% 19% 3	2986
	21st to end	Day	50% 3 8% 20% 15% 5%	3960
January	1st to 10th	Night	42% 3 9% 29% 15% 2	6349
	11th to 20th	Night	44% 4 13% 21% 14% 5%	6360
	21st to end	Night	49% 3 10% 18% 16% 3	6932
February	1st to 10th	Night	47% 4 13% 23% 12% 2	2518
	11th to 20th	Night	45% 4 9% 21% 17% 4	2340
	21st to end	Night	47% 4 7% 23% 15% 4	1980
March	1st to 10th	Night	49% 4 10% 18% 15% 5%	5374
	11th to 20th	Night	46% 4 14% 22% 11% 3	5312
	21st to end	Night	51% 3 5% 13% 23% 5%	4962
April	1st to 10th	Night	50% 4 8% 20% 15% 3	2000
	11th to 20th	Night	47% 4 9% 20% 19% 1	1975
	21st to end	Night	46% 4 9% 22% 14% 5%	1865
May	1st to 10th	Night	60% 4 14% 12% 9% 1	4060
	11th to 20th	Night	55% 5% 12% 15% 10% 3	3817
	21st to end	Night	50% 6% 13% 21% 10% 1	3927
June	1st to 10th	Night	45% 6% 11% 22% 15% 1	1600
	11th to 20th	Night	45% 4 14% 25% 11% 1	1600
	21st to end	Night	39% 7% 14% 25% 14% 1	1600
July	1st to 10th	Night	44% 6% 15% 21% 12% 1	3560
	11th to 20th	Night	44% 7% 18% 21% 9% 1	3552
	21st to end	Night	40% 5% 12% 24% 18% 1	3809
August	1st to 10th	Night	52% 7% 13% 18% 10% 1	2731
	11th to 20th	Night	43% 6% 14% 29% 8% 1	3151
	21st to end	Night	27% 6% 16% 33% 16% 1	3586
September	1st to 10th	Night	39% 5% 10% 27% 18% 1	2000
	11th to 20th	Night	34% 3 10% 24% 26% 2	2150
	21st to end	Night	43% 6% 8% 22% 20% 2	2200
October	1st to 10th	Night	39% 5% 12% 23% 19% 2	3613
	11th to 20th	Night	29% 3 9% 27% 30% 2	3958
	21st to end	Night	33% 3 13% 29% 20% 2	4610
November	1st to 10th	Night	37% 4 12% 24% 20% 4	2667
	11th to 20th	Night	42% 4 9% 22% 20% 4	2730
	21st to end	Night	41% 3 13% 28% 12% 1	2747
December	1st to 10th	Night	43% 4 12% 30% 11%	4609
	11th to 20th	Night	48% 3 12% 16% 19% 1	5208
	21st to end	Night	48% 3 8% 20% 17% 3	6996

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	32% 25% 27% 15% 3	200
	11th to 20th	Day	31% 27% 28% 11% 4	200
	21st to end	Day	28% 26% 31% 9% 5% 1	233
February	1st to 10th	Day	33% 24% 27% 14% 1	380
	11th to 20th	Day	33% 26% 31% 9% 1	496
	21st to end	Day	30% 21% 33% 11% 4	414
March	1st to 10th	Day	29% 23% 31% 14% 3	500
	11th to 20th	Day	31% 23% 32% 12% 3	500
	21st to end	Day	33% 21% 30% 10% 4 2	550
April	1st to 10th	Day	34% 24% 26% 11% 4 1	500
	11th to 20th	Day	34% 19% 28% 14% 4	492
	21st to end	Day	31% 18% 33% 12% 4 2	500
May	1st to 10th	Day	35% 17% 28% 12% 7% 1	503
	11th to 20th	Day	26% 17% 35% 15% 6% 1	527
	21st to end	Day	22% 15% 31% 21% 8% 2	573
June	1st to 10th	Day	20% 13% 31% 23% 10% 3	530
	11th to 20th	Day	17% 15% 38% 21% 8% 1	530
	21st to end	Day	17% 18% 31% 24% 9% 2	530
July	1st to 10th	Day	19% 8% 29% 29% 12% 3	525
	11th to 20th	Day	15% 6% 31% 32% 12% 3	524
	21st to end	Day	16% 9% 35% 30% 9% 2	576
August	1st to 10th	Day	14% 7% 32% 33% 10% 4	526
	11th to 20th	Day	14% 8% 31% 27% 15% 5%	519
	21st to end	Day	13% 11% 31% 26% 13% 6%	583
September	1st to 10th	Day	19% 11% 25% 25% 15% 5%	530
	11th to 20th	Day	15% 11% 27% 25% 12% 10%	530
	21st to end	Day	21% 11% 34% 19% 9% 5%	530
October	1st to 10th	Day	29% 9% 24% 21% 12% 5%	530
	11th to 20th	Day	20% 18% 28% 25% 7% 2	509
	21st to end	Day	28% 14% 31% 18% 7% 2	462
November	1st to 10th	Day	22% 16% 31% 20% 9% 2	425
	11th to 20th	Day	30% 18% 29% 16% 7% 1	430
	21st to end	Day	29% 18% 29% 17% 5% 1	409
December	1st to 10th	Day	24% 27% 30% 15% 4	204
	11th to 20th	Day	38% 22% 30% 9% 2	200
	21st to end	Day	27% 22% 30% 15% 4 2	220
January	1st to 10th	Night	23% 20% 38% 17% 3	800
	11th to 20th	Night	29% 19% 31% 19% 2	800
	21st to end	Night	22% 21% 34% 19% 3	880
February	1st to 10th	Night	24% 20% 34% 21% 2	800
	11th to 20th	Night	26% 21% 38% 14% 1	783
	21st to end	Night	27% 20% 35% 16% 2	660
March	1st to 10th	Night	27% 20% 34% 17% 3	800
	11th to 20th	Night	24% 20% 36% 17% 3	800
	21st to end	Night	27% 19% 35% 16% 3	880
April	1st to 10th	Night	26% 18% 34% 19% 3	800
	11th to 20th	Night	25% 19% 33% 19% 4	791
	21st to end	Night	25% 18% 34% 19% 4	665
May	1st to 10th	Night	30% 15% 30% 21% 4	600
	11th to 20th	Night	23% 14% 37% 21% 5%	505
	21st to end	Night	22% 7% 33% 30% 6% 1	433
June	1st to 10th	Night	21% 6% 32% 33% 8%	400
	11th to 20th	Night	21% 7% 30% 33% 9%	400
	21st to end	Night	16% 4 35% 37% 8% 1	400
July	1st to 10th	Night	14% 4 29% 39% 14%	390
	11th to 20th	Night	15% 3 26% 40% 15% 1	388
	21st to end	Night	12% 7% 31% 39% 11%	438
August	1st to 10th	Night	12% 6% 27% 42% 12% 1	488
	11th to 20th	Night	14% 4 25% 38% 17% 1	740
	21st to end	Night	15% 5% 26% 36% 17% 2	880
September	1st to 10th	Night	17% 5% 30% 34% 12% 2	800
	11th to 20th	Night	16% 7% 24% 34% 17% 2	800
	21st to end	Night	20% 7% 27% 30% 14% 2	800
October	1st to 10th	Night	24% 9% 27% 25% 13% 2	800
	11th to 20th	Night	22% 12% 29% 27% 9% 1	800
	21st to end	Night	27% 13% 30% 24% 6%	880
November	1st to 10th	Night	19% 12% 33% 26% 8% 1	820
	11th to 20th	Night	24% 15% 33% 21% 6% 1	840
	21st to end	Night	24% 17% 31% 23% 5%	840
December	1st to 10th	Night	28% 14% 33% 19% 5% 1	809
	11th to 20th	Night	26% 19% 31% 22% 2	800
	21st to end	Night	29% 16% 34% 20% 2	880

Page generated at 2025-11-30 01:00:51. Version ac07

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