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 2024-02-15.

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% 4 7% 6% 3 5% 5% 6% 7% 33%	78155
	Night	16% 10% 4 7% 5% 3 5% 5% 5% 7% 34%	80921
Transparency	Day	49% 6% 12% 20% 11% 1	77865
	Night	50% 6% 11% 21% 11% 1	80732
Seeing	Day	18% 9% 29% 24% 14% 6%	14285
	Night	19% 8% 29% 30% 12% 1	24057

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% 11% 7% 3 5% 4 4 5% 23%	2527
	11th to 20th	Day	15% 18% 7% 10% 10% 6% 4 4 5% 6% 15%	2530
	21st to end	Day	26% 18% 6% 9% 7% 3 3 3 4 4 17%	3098
February	1st to 10th	Day	14% 14% 5% 12% 9% 5% 5% 4 4 6% 22%	1710
	11th to 20th	Day	13% 12% 5% 10% 8% 4 5% 5% 6% 6% 25%	1702
	21st to end	Day	14% 15% 5% 10% 7% 4 5% 4 6% 6% 25%	1480
March	1st to 10th	Day	15% 11% 5% 8% 7% 4 4 5% 5% 7% 30%	1855
	11th to 20th	Day	13% 12% 5% 8% 7% 2 4 4 5% 7% 32%	1980
	21st to end	Day	18% 13% 4 8% 7% 4 5% 3 5% 6% 28%	2178
April	1st to 10th	Day	16% 11% 5% 7% 6% 3 5% 4 6% 7% 31%	2070
	11th to 20th	Day	17% 10% 3 6% 5% 3 5% 5% 5% 8% 34%	2302
	21st to end	Day	18% 9% 3 7% 6% 4 4 4 5% 7% 33%	2340
May	1st to 10th	Day	17% 8% 3 7% 5% 3 6% 5% 6% 7% 34%	2340
	11th to 20th	Day	19% 7% 3 5% 5% 3 5% 4 5% 7% 37%	2301
	21st to end	Day	14% 5% 2 6% 6% 3 4 5% 6% 9% 40%	2539
June	1st to 10th	Day	10% 6% 3 6% 5% 3 5% 5% 6% 9% 41%	2678
	11th to 20th	Day	10% 5% 2 5% 6% 4 5% 5% 6% 9% 43%	2700
	21st to end	Day	10% 5% 3 5% 5% 4 5% 6% 7% 9% 40%	2684
July	1st to 10th	Day	7% 4 2 5% 5% 3 5% 6% 7% 10% 46%	2520
	11th to 20th	Day	9% 5% 3 5% 5% 4 5% 5% 7% 9% 43%	2520
	21st to end	Day	8% 5% 2 5% 6% 4 6% 6% 7% 10% 41%	2578
August	1st to 10th	Day	9% 4 2 5% 6% 4 5% 7% 7% 9% 40%	2327
	11th to 20th	Day	10% 5% 2 5% 6% 4 6% 7% 7% 8% 40%	2303
	21st to end	Day	12% 6% 3 6% 5% 3 5% 5% 6% 8% 41%	2574
September	1st to 10th	Day	10% 6% 3 6% 6% 4 6% 7% 5% 9% 40%	2035
	11th to 20th	Day	11% 5% 2 4 5% 3 4 5% 7% 9% 45%	1980
	21st to end	Day	15% 7% 2 5% 5% 3 6% 6% 7% 8% 36%	1980
October	1st to 10th	Day	14% 7% 2 6% 6% 3 5% 5% 5% 7% 41%	1908
	11th to 20th	Day	19% 9% 3 6% 5% 4 5% 5% 5% 7% 33%	1800
	21st to end	Day	20% 11% 3 7% 6% 3 4 5% 4 7% 30%	1850
November	1st to 10th	Day	13% 8% 3 7% 6% 4 5% 5% 6% 8% 35%	1672
	11th to 20th	Day	13% 11% 4 8% 6% 4 6% 5% 6% 6% 32%	1710
	21st to end	Day	18% 13% 5% 6% 6% 3 5% 5% 5% 6% 28%	1525
December	1st to 10th	Day	14% 13% 7% 11% 8% 4 5% 5% 4 6% 22%	1520
	11th to 20th	Day	16% 14% 5% 8% 6% 3 4 5% 5% 7% 26%	1766
	21st to end	Day	27% 19% 7% 8% 6% 3 4 4 4 5% 12%	2573
January	1st to 10th	Night	17% 14% 6% 9% 6% 4 4 4 4 6% 27%	4428
	11th to 20th	Night	14% 16% 7% 12% 8% 5% 6% 4 5% 5% 18%	4440
	21st to end	Night	23% 16% 9% 9% 6% 3 4 4 3 4 20%	4556
February	1st to 10th	Night	14% 15% 6% 10% 7% 4 6% 5% 5% 5% 23%	2470
	11th to 20th	Night	14% 12% 5% 9% 7% 4 5% 5% 5% 6% 28%	2236
	21st to end	Night	19% 11% 4 8% 6% 4 4 4 5% 5% 29%	1776
March	1st to 10th	Night	16% 10% 5% 7% 6% 3 4 5% 4 5% 34%	2104
	11th to 20th	Night	13% 7% 4 6% 5% 4 5% 5% 6% 8% 38%	1980
	21st to end	Night	19% 9% 3 6% 6% 2 5% 4 5% 5% 34%	2178
April	1st to 10th	Night	18% 9% 3 6% 5% 3 4 4 6% 6% 35%	1904
	11th to 20th	Night	18% 7% 3 5% 5% 3 4 4 5% 8% 38%	1606
	21st to end	Night	17% 7% 3 5% 4 3 5% 5% 5% 9% 37%	1620
May	1st to 10th	Night	16% 9% 3 6% 6% 3 5% 6% 5% 7% 34%	1620
	11th to 20th	Night	17% 5% 2 5% 5% 3 5% 4 5% 7% 42%	1592
	21st to end	Night	10% 4 2 4 5% 3 5% 5% 5% 9% 48%	1731
June	1st to 10th	Night	10% 4 3 6% 3 2 4 5% 5% 10% 47%	1278
	11th to 20th	Night	10% 5% 3 5% 4 2 4 5% 7% 9% 46%	1260
	21st to end	Night	10% 5% 2 4 5% 3 4 3 6% 8% 49%	1269
July	1st to 10th	Night	7% 5% 1 3 3 3 5% 4 5% 8% 55%	1440
	11th to 20th	Night	8% 3 2 4 4 2 4 5% 6% 10% 51%	1440
	21st to end	Night	6% 3 2 4 4 3 5% 6% 8% 9% 50%	1736
August	1st to 10th	Night	8% 4 1 4 4 3 4 6% 7% 10% 49%	1611
	11th to 20th	Night	7% 4 2 4 5% 3 4 6% 7% 8% 50%	1598
	21st to end	Night	9% 4 2 4 4 3 4 5% 5% 6% 53%	1782
September	1st to 10th	Night	11% 5% 3 5% 6% 4 4 5% 5% 7% 47%	1903
	11th to 20th	Night	10% 4 2 4 4 2 5% 5% 4 7% 53%	1980
	21st to end	Night	15% 6% 2 5% 5% 2 4 5% 5% 7% 45%	1980
October	1st to 10th	Night	15% 6% 2 5% 4 3 4 4 5% 7% 44%	2051
	11th to 20th	Night	20% 8% 3 5% 6% 3 5% 5% 5% 7% 35%	2160
	21st to end	Night	23% 8% 4 6% 5% 3 5% 5% 6% 6% 30%	2502
November	1st to 10th	Night	14% 8% 3 4 4 2 4 4 6% 7% 44%	2406
	11th to 20th	Night	16% 10% 4 6% 6% 3 6% 5% 6% 7% 32%	2470
	21st to end	Night	19% 13% 4 6% 5% 3 5% 4 5% 7% 29%	2651
December	1st to 10th	Night	18% 13% 5% 9% 6% 4 5% 5% 6% 6% 23%	2660
	11th to 20th	Night	15% 13% 5% 10% 6% 4 5% 5% 5% 7% 26%	3427
	21st to end	Night	27% 21% 7% 8% 5% 3 3 3 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% 2	1711
	21st to end	Day	59% 6% 9% 16% 8% 2	1480
March	1st to 10th	Day	55% 7% 12% 16% 9% 1	1855
	11th to 20th	Day	51% 5% 9% 20% 12% 3	1980
	21st to end	Day	58% 4 10% 18% 8% 2	2178
April	1st to 10th	Day	52% 6% 11% 20% 11% 1	2070
	11th to 20th	Day	49% 6% 10% 19% 14% 2	2302
	21st to end	Day	51% 6% 13% 16% 13% 2	2340
May	1st to 10th	Day	49% 6% 11% 21% 11% 2	2340
	11th to 20th	Day	46% 6% 11% 21% 13% 2	2305
	21st to end	Day	41% 6% 13% 23% 15% 2	2539
June	1st to 10th	Day	39% 6% 12% 26% 15% 2	2678
	11th to 20th	Day	37% 7% 14% 25% 16% 2	2700
	21st to end	Day	38% 7% 13% 25% 16% 2	2684
July	1st to 10th	Day	32% 8% 12% 27% 19% 3	2451
	11th to 20th	Day	34% 7% 14% 27% 16% 2	2445
	21st to end	Day	36% 8% 14% 23% 17% 3	2435
August	1st to 10th	Day	37% 8% 15% 25% 13% 2	2314
	11th to 20th	Day	39% 8% 13% 26% 12% 2	2303
	21st to end	Day	39% 6% 11% 27% 15% 1	2574
September	1st to 10th	Day	41% 7% 14% 24% 12% 2	2035
	11th to 20th	Day	35% 6% 13% 33% 13%	1980
	21st to end	Day	43% 6% 14% 25% 11% 1	1980
October	1st to 10th	Day	42% 7% 11% 25% 14% 2	1908
	11th to 20th	Day	51% 5% 13% 21% 10% 1	1800
	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	2238
	21st to end	Night	59% 4 9% 18% 9% 1	1776
March	1st to 10th	Night	52% 5% 9% 22% 10% 1	2104
	11th to 20th	Night	45% 5% 10% 23% 14% 2	1980
	21st to end	Night	52% 5% 8% 22% 11% 1	2178
April	1st to 10th	Night	48% 5% 11% 21% 14% 2	1904
	11th to 20th	Night	44% 6% 10% 19% 19% 2	1606
	21st to end	Night	43% 6% 10% 20% 16% 4	1620
May	1st to 10th	Night	48% 6% 11% 21% 11% 3	1620
	11th to 20th	Night	42% 5% 10% 20% 18% 4	1593
	21st to end	Night	34% 7% 13% 25% 17% 4	1732
June	1st to 10th	Night	34% 6% 10% 30% 17% 2	1278
	11th to 20th	Night	32% 8% 14% 23% 20% 2	1260
	21st to end	Night	33% 6% 12% 27% 20% 2	1269
July	1st to 10th	Night	28% 5% 11% 29% 23% 4	1400
	11th to 20th	Night	28% 8% 12% 34% 16% 2	1392
	21st to end	Night	27% 10% 18% 23% 19% 3	1639
August	1st to 10th	Night	28% 8% 16% 32% 14% 2	1600
	11th to 20th	Night	30% 6% 16% 33% 12% 3	1598
	21st to end	Night	30% 7% 13% 29% 19% 1	1782
September	1st to 10th	Night	37% 6% 12% 28% 14% 2	1903
	11th to 20th	Night	30% 7% 11% 32% 19% 1	1980
	21st to end	Night	39% 5% 12% 31% 12% 1	1980
October	1st to 10th	Night	39% 5% 11% 25% 18% 1	2051
	11th to 20th	Night	48% 6% 12% 21% 12% 1	2160
	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	25% 16% 39% 19% 2	252
	21st to end	Day	24% 25% 36% 13% 2	198
March	1st to 10th	Day	22% 19% 38% 16% 4 1	295
	11th to 20th	Day	21% 12% 33% 23% 9% 1	420
	21st to end	Day	28% 9% 32% 24% 5% 2	462
April	1st to 10th	Day	19% 15% 34% 22% 9% 1	420
	11th to 20th	Day	26% 12% 30% 16% 9% 6%	412
	21st to end	Day	24% 11% 27% 25% 10% 3	420
May	1st to 10th	Day	22% 9% 30% 24% 13% 4	423
	11th to 20th	Day	21% 6% 30% 23% 13% 7%	447
	21st to end	Day	15% 6% 23% 27% 20% 9%	485
June	1st to 10th	Day	13% 8% 29% 31% 14% 5%	608
	11th to 20th	Day	12% 6% 29% 30% 18% 5%	630
	21st to end	Day	13% 5% 27% 33% 16% 6%	630
July	1st to 10th	Day	9% 4 19% 32% 23% 14%	621
	11th to 20th	Day	11% 5% 25% 32% 19% 8%	618
	21st to end	Day	8% 4 23% 30% 25% 11%	519
August	1st to 10th	Day	8% 4 17% 27% 26% 18%	446
	11th to 20th	Day	11% 4 14% 31% 25% 16%	439
	21st to end	Day	13% 6% 19% 25% 24% 14%	495
September	1st to 10th	Day	14% 5% 23% 24% 21% 14%	450
	11th to 20th	Day	11% 3 18% 23% 28% 16%	450
	21st to end	Day	17% 10% 22% 22% 21% 8%	450
October	1st to 10th	Day	18% 6% 27% 23% 18% 8%	450
	11th to 20th	Day	20% 14% 34% 22% 8% 1	450
	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%	722
	21st to end	Night	24% 15% 36% 21% 3	592
March	1st to 10th	Night	22% 11% 35% 27% 5%	720
	11th to 20th	Night	17% 11% 38% 27% 7% 1	720
	21st to end	Night	24% 9% 32% 27% 7% 1	792
April	1st to 10th	Night	22% 13% 34% 27% 4	644
	11th to 20th	Night	22% 8% 31% 28% 9% 2	533
	21st to end	Night	21% 5% 31% 34% 9%	540
May	1st to 10th	Night	18% 5% 30% 34% 11% 1	540
	11th to 20th	Night	20% 5% 27% 34% 13% 1	531
	21st to end	Night	12% 3 22% 40% 22% 2	571
June	1st to 10th	Night	12% 4 25% 45% 14%	360
	11th to 20th	Night	12% 5% 24% 43% 15% 1	360
	21st to end	Night	12% 2 22% 47% 16% 1	369
July	1st to 10th	Night	9% 1 16% 43% 27% 4	525
	11th to 20th	Night	11% 2 17% 42% 25% 4	522
	21st to end	Night	6% 2 20% 47% 23% 3	591
August	1st to 10th	Night	9% 2 13% 41% 32% 2	537
	11th to 20th	Night	7% 2 16% 40% 32% 3	534
	21st to end	Night	9% 3 18% 34% 31% 4	594
September	1st to 10th	Night	12% 2 18% 36% 27% 4	661
	11th to 20th	Night	11% 4 18% 36% 26% 5%	720
	21st to end	Night	16% 4 22% 34% 19% 5%	720
October	1st to 10th	Night	17% 6% 25% 32% 17% 3	720
	11th to 20th	Night	23% 11% 30% 27% 8% 1	720
	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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