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 2026-08-15.

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	13% 9% 3 8% 5% 5% 5% 4 6% 7% 35%	142870
	Night	14% 10% 4 8% 5% 4 4 4 5% 6% 36%	139121
Transparency	Day	46% 6% 12% 21% 13% 1	146441
	Night	47% 6% 12% 21% 13% 1	143610
Seeing	Day	17% 10% 29% 24% 14% 6%	17723
	Night	18% 9% 30% 30% 12% 1	27253

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	12% 16% 7% 13% 7% 5% 4 3 4 5% 25%	4691
	11th to 20th	Day	13% 17% 6% 15% 10% 8% 4 3 4 5% 14%	4710
	21st to end	Day	17% 14% 5% 11% 8% 5% 6% 4 4 5% 20%	5754
February	1st to 10th	Day	13% 14% 4 13% 8% 6% 5% 3 4 6% 23%	1890
	11th to 20th	Day	13% 14% 5% 12% 7% 5% 4 4 6% 6% 25%	1952
	21st to end	Day	12% 15% 5% 11% 6% 5% 5% 4 6% 6% 25%	1730
March	1st to 10th	Day	16% 12% 5% 10% 5% 4 4 3 4 5% 32%	4736
	11th to 20th	Day	13% 12% 4 8% 5% 4 4 4 5% 8% 33%	5090
	21st to end	Day	19% 14% 4 7% 5% 4 4 3 5% 6% 28%	5586
April	1st to 10th	Day	17% 13% 4 8% 5% 4 5% 3 6% 6% 31%	2415
	11th to 20th	Day	17% 10% 3 6% 5% 5% 4 4 5% 7% 35%	2692
	21st to end	Day	16% 9% 3 8% 5% 4 4 3 5% 7% 34%	2730
May	1st to 10th	Day	17% 8% 3 8% 5% 4 5% 4 5% 7% 35%	7616
	11th to 20th	Day	15% 8% 3 7% 5% 4 5% 4 6% 8% 36%	7626
	21st to end	Day	14% 6% 2 6% 4 4 4 3 5% 7% 45%	8426
June	1st to 10th	Day	10% 6% 2 7% 5% 4 5% 5% 6% 9% 42%	3125
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 5% 6% 9% 41%	3150
	21st to end	Day	10% 6% 2 6% 5% 5% 6% 5% 7% 9% 40%	3131
July	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 4 7% 10% 41%	8280
	11th to 20th	Day	6% 3 2 5% 4 4 5% 4 6% 10% 51%	8280
	21st to end	Day	7% 4 2 6% 4 5% 5% 4 6% 9% 46%	8440
August	1st to 10th	Day	12% 5% 2 6% 5% 5% 5% 4 6% 8% 42%	4364
	11th to 20th	Day	10% 5% 2 6% 4 6% 6% 5% 8% 8% 40%	4226
	21st to end	Day	10% 5% 2 7% 5% 5% 6% 4 7% 9% 40%	4675
September	1st to 10th	Day	9% 6% 3 6% 4 5% 6% 5% 5% 8% 42%	2260
	11th to 20th	Day	10% 5% 1 5% 4 4 4 4 7% 9% 47%	2200
	21st to end	Day	15% 8% 2 7% 5% 5% 5% 4 7% 8% 36%	2200
October	1st to 10th	Day	13% 6% 2 7% 4 4 5% 4 5% 7% 43%	2120
	11th to 20th	Day	18% 9% 3 7% 4 6% 5% 3 5% 6% 35%	2000
	21st to end	Day	18% 11% 3 8% 5% 4 5% 4 5% 7% 31%	2055
November	1st to 10th	Day	12% 10% 3 8% 5% 5% 4 4 6% 8% 35%	1852
	11th to 20th	Day	13% 12% 3 10% 5% 5% 5% 4 6% 6% 30%	1890
	21st to end	Day	17% 14% 5% 8% 6% 5% 5% 4 5% 6% 26%	1685
December	1st to 10th	Day	14% 14% 6% 11% 5% 5% 4 4 6% 8% 23%	2680
	11th to 20th	Day	17% 21% 5% 8% 4 4 4 4 5% 6% 23%	2843
	21st to end	Day	26% 20% 6% 10% 4 3 4 3 4 5% 14%	3770
January	1st to 10th	Night	14% 14% 5% 11% 5% 5% 4 3 5% 5% 29%	8232
	11th to 20th	Night	10% 15% 7% 15% 8% 7% 6% 4 5% 5% 19%	8280
	21st to end	Night	14% 14% 7% 10% 5% 4 6% 4 4 5% 25%	8425
February	1st to 10th	Night	13% 16% 5% 11% 6% 5% 5% 4 5% 5% 25%	2730
	11th to 20th	Night	13% 13% 5% 10% 6% 5% 5% 3 5% 6% 29%	2550
	21st to end	Night	17% 12% 4 10% 5% 5% 4 4 5% 6% 28%	2076
March	1st to 10th	Night	15% 11% 3 8% 4 4 4 3 5% 6% 35%	5336
	11th to 20th	Night	10% 8% 3 8% 5% 5% 5% 4 6% 8% 39%	5090
	21st to end	Night	15% 11% 2 8% 5% 4 5% 3 5% 5% 39%	5588
April	1st to 10th	Night	18% 10% 3 7% 4 5% 4 4 6% 6% 34%	2221
	11th to 20th	Night	18% 8% 3 6% 5% 4 4 3 6% 7% 37%	1876
	21st to end	Night	16% 7% 2 6% 4 4 4 4 5% 8% 38%	1890
May	1st to 10th	Night	18% 9% 3 6% 5% 4 5% 4 5% 7% 35%	5237
	11th to 20th	Night	15% 7% 3 7% 4 4 4 3 5% 6% 41%	5234
	21st to end	Night	15% 6% 2 5% 4 4 4 3 5% 7% 46%	5707
June	1st to 10th	Night	10% 5% 2 6% 3 4 4 4 5% 9% 48%	1490
	11th to 20th	Night	10% 6% 3 6% 4 4 3 5% 7% 9% 44%	1470
	21st to end	Night	10% 5% 2 5% 4 4 4 3 6% 8% 50%	1482
July	1st to 10th	Night	9% 5% 2 6% 3 4 4 4 6% 10% 48%	4708
	11th to 20th	Night	7% 3 2 5% 4 3 4 4 6% 9% 54%	4707
	21st to end	Night	8% 4 2 5% 4 4 5% 4 6% 7% 50%	5637
August	1st to 10th	Night	11% 5% 2 5% 4 3 3 4 6% 8% 47%	3005
	11th to 20th	Night	9% 6% 2 5% 4 4 4 3 7% 7% 48%	2911
	21st to end	Night	8% 5% 2 6% 4 4 5% 4 6% 7% 50%	3223
September	1st to 10th	Night	10% 5% 2 5% 5% 4 4 4 5% 7% 49%	2116
	11th to 20th	Night	9% 4 1 4 3 3 4 4 4 7% 56%	2200
	21st to end	Night	15% 7% 2 6% 4 3 4 4 5% 7% 43%	2200
October	1st to 10th	Night	14% 5% 2 5% 4 3 4 3 5% 7% 47%	2279
	11th to 20th	Night	19% 8% 2 6% 5% 4 4 4 5% 6% 37%	2400
	21st to end	Night	21% 8% 3 7% 4 4 5% 4 5% 6% 33%	2780
November	1st to 10th	Night	14% 9% 2 5% 4 3 4 3 6% 6% 43%	2666
	11th to 20th	Night	15% 10% 3 7% 5% 5% 5% 4 6% 7% 32%	2730
	21st to end	Night	18% 14% 4 8% 5% 4 5% 3 6% 6% 27%	2931
December	1st to 10th	Night	14% 14% 5% 11% 5% 5% 5% 4 6% 6% 25%	4720
	11th to 20th	Night	15% 18% 5% 10% 5% 5% 5% 3 5% 6% 22%	5562
	21st to end	Night	23% 21% 6% 10% 5% 4 4 3 4 4 17%	7432

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	60% 6% 11% 16% 6% 1	4692
	11th to 20th	Day	72% 6% 8% 7% 6%	4710
	21st to end	Day	66% 6% 10% 11% 7%	5754
February	1st to 10th	Day	63% 5% 8% 17% 7%	1890
	11th to 20th	Day	59% 4 9% 17% 8% 1	1961
	21st to end	Day	58% 7% 10% 16% 7% 2	1730
March	1st to 10th	Day	54% 5% 13% 17% 9% 2	4736
	11th to 20th	Day	49% 5% 14% 22% 8% 2	5090
	21st to end	Day	57% 5% 12% 15% 10% 2	5586
April	1st to 10th	Day	52% 6% 10% 20% 10% 1	2415
	11th to 20th	Day	48% 6% 10% 20% 14% 2	2692
	21st to end	Day	49% 6% 12% 19% 12% 2	2730
May	1st to 10th	Day	47% 7% 11% 20% 11% 4	7651
	11th to 20th	Day	45% 7% 12% 21% 13% 2	7630
	21st to end	Day	39% 5% 9% 21% 22% 4	8441
June	1st to 10th	Day	37% 6% 12% 26% 16% 2	3125
	11th to 20th	Day	37% 7% 13% 24% 16% 2	3150
	21st to end	Day	38% 7% 14% 25% 15% 1	3131
July	1st to 10th	Day	37% 8% 14% 24% 16% 1	8211
	11th to 20th	Day	26% 7% 13% 33% 19% 1	8205
	21st to end	Day	31% 8% 14% 24% 23% 1	8255
August	1st to 10th	Day	38% 8% 14% 22% 16% 2	4351
	11th to 20th	Day	37% 9% 13% 30% 10% 1	4265
	21st to end	Day	37% 8% 14% 28% 12% 1	4674
September	1st to 10th	Day	38% 7% 14% 27% 13% 2	2260
	11th to 20th	Day	33% 6% 13% 33% 16% 1	2200
	21st to end	Day	44% 6% 13% 24% 12% 1	2200
October	1st to 10th	Day	31% 6% 10% 32% 19% 2	3458
	11th to 20th	Day	40% 5% 11% 25% 18%	3240
	21st to end	Day	45% 5% 14% 22% 12% 1	3292
November	1st to 10th	Day	46% 6% 10% 26% 11% 1	1852
	11th to 20th	Day	53% 5% 13% 18% 10% 1	1890
	21st to end	Day	58% 6% 11% 18% 6% 1	1685
December	1st to 10th	Day	58% 7% 16% 14% 5%	2680
	11th to 20th	Day	62% 5% 12% 13% 8%	2839
	21st to end	Day	73% 5% 7% 9% 6%	3770
January	1st to 10th	Night	56% 6% 10% 18% 8% 2	8233
	11th to 20th	Night	63% 7% 11% 12% 7% 1	8280
	21st to end	Night	59% 6% 9% 18% 7%	8425
February	1st to 10th	Night	60% 6% 9% 16% 8% 1	2730
	11th to 20th	Night	55% 5% 9% 20% 10% 1	2552
	21st to end	Night	57% 5% 10% 18% 9% 1	2076
March	1st to 10th	Night	49% 4 14% 21% 10% 1	5336
	11th to 20th	Night	41% 6% 15% 23% 13% 1	5090
	21st to end	Night	47% 5% 11% 21% 14% 2	5588
April	1st to 10th	Night	50% 5% 11% 20% 12% 2	2221
	11th to 20th	Night	45% 6% 10% 19% 18% 2	1876
	21st to end	Night	43% 6% 11% 21% 16% 3	1890
May	1st to 10th	Night	47% 7% 11% 17% 14% 4	5264
	11th to 20th	Night	43% 7% 11% 24% 14% 2	5235
	21st to end	Night	37% 6% 10% 23% 18% 5%	5716
June	1st to 10th	Night	34% 6% 10% 28% 20% 2	1490
	11th to 20th	Night	34% 8% 14% 23% 19% 2	1470
	21st to end	Night	33% 6% 12% 27% 20% 2	1482
July	1st to 10th	Night	31% 7% 15% 27% 16% 4	4670
	11th to 20th	Night	26% 7% 12% 34% 20% 2	4662
	21st to end	Night	30% 8% 13% 21% 26% 2	5509
August	1st to 10th	Night	33% 7% 15% 27% 17% 2	2994
	11th to 20th	Night	33% 6% 16% 32% 11% 2	2938
	21st to end	Night	32% 8% 13% 31% 15% 1	3223
September	1st to 10th	Night	36% 5% 12% 30% 15% 2	2116
	11th to 20th	Night	27% 7% 11% 33% 20% 1	2200
	21st to end	Night	40% 5% 12% 29% 12% 1	2200
October	1st to 10th	Night	25% 4 11% 32% 27% 1	3721
	11th to 20th	Night	43% 5% 9% 24% 18% 1	3899
	21st to end	Night	42% 5% 12% 28% 13% 1	4480
November	1st to 10th	Night	41% 6% 11% 27% 14% 1	2666
	11th to 20th	Night	51% 5% 14% 20% 10% 1	2730
	21st to end	Night	57% 5% 14% 18% 6% 1	2931
December	1st to 10th	Night	58% 7% 17% 15% 3	4720
	11th to 20th	Night	60% 6% 11% 14% 7% 1	5565
	21st to end	Night	70% 5% 10% 9% 6%	7432

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	20% 18% 38% 18% 5% 1	330
	11th to 20th	Day	24% 16% 42% 15% 2 1	330
	21st to end	Day	26% 15% 36% 17% 5% 1	363
February	1st to 10th	Day	22% 19% 39% 16% 3 1	330
	11th to 20th	Day	23% 19% 36% 19% 2	330
	21st to end	Day	22% 25% 38% 14% 2	273
March	1st to 10th	Day	24% 18% 38% 17% 3 1	395
	11th to 20th	Day	24% 13% 33% 22% 7% 1	540
	21st to end	Day	27% 12% 33% 22% 5% 1	594
April	1st to 10th	Day	21% 17% 34% 20% 8% 1	540
	11th to 20th	Day	26% 14% 32% 15% 9% 5%	532
	21st to end	Day	21% 11% 28% 26% 10% 3	540
May	1st to 10th	Day	20% 11% 30% 25% 11% 3	543
	11th to 20th	Day	18% 9% 31% 23% 13% 6%	567
	21st to end	Day	14% 7% 27% 25% 18% 8%	617
June	1st to 10th	Day	12% 9% 30% 30% 14% 5%	755
	11th to 20th	Day	12% 8% 31% 28% 16% 4	780
	21st to end	Day	12% 6% 29% 33% 15% 5%	780
July	1st to 10th	Day	9% 5% 20% 32% 21% 12%	771
	11th to 20th	Day	10% 5% 26% 32% 20% 8%	768
	21st to end	Day	7% 4 23% 30% 25% 11%	656
August	1st to 10th	Day	9% 5% 16% 28% 26% 16%	566
	11th to 20th	Day	10% 4 15% 33% 25% 14%	535
	21st to end	Day	12% 6% 19% 27% 23% 14%	583
September	1st to 10th	Day	12% 5% 24% 24% 20% 15%	530
	11th to 20th	Day	10% 3 16% 22% 30% 19%	530
	21st to end	Day	17% 10% 22% 23% 21% 8%	530
October	1st to 10th	Day	16% 7% 27% 23% 20% 8%	530
	11th to 20th	Day	19% 12% 35% 24% 8% 2	530
	21st to end	Day	25% 11% 36% 21% 5% 1	438
November	1st to 10th	Day	15% 18% 38% 19% 9% 1	330
	11th to 20th	Day	22% 22% 34% 15% 7% 2	330
	21st to end	Day	26% 16% 38% 17% 4	223
December	1st to 10th	Day	22% 17% 36% 16% 7% 2	220
	11th to 20th	Day	26% 23% 34% 14% 3	220
	21st to end	Day	30% 15% 35% 16% 4	294
January	1st to 10th	Night	21% 14% 38% 20% 5% 1	1050
	11th to 20th	Night	23% 15% 34% 23% 3 1	1050
	21st to end	Night	20% 12% 36% 27% 5%	935
February	1st to 10th	Night	21% 13% 39% 24% 3	840
	11th to 20th	Night	21% 14% 34% 26% 5%	823
	21st to end	Night	22% 16% 37% 21% 3	692
March	1st to 10th	Night	22% 11% 36% 26% 5%	840
	11th to 20th	Night	18% 13% 37% 25% 6% 1	840
	21st to end	Night	23% 11% 33% 26% 6% 1	924
April	1st to 10th	Night	23% 14% 35% 25% 3	751
	11th to 20th	Night	22% 9% 32% 27% 9% 2	623
	21st to end	Night	20% 6% 30% 36% 8%	630
May	1st to 10th	Night	18% 6% 32% 33% 10% 1	630
	11th to 20th	Night	19% 5% 29% 33% 13% 1	621
	21st to end	Night	12% 4 23% 40% 19% 2	667
June	1st to 10th	Night	12% 4 26% 45% 13%	420
	11th to 20th	Night	12% 5% 25% 43% 14%	420
	21st to end	Night	11% 3 22% 48% 16%	432
July	1st to 10th	Night	9% 1 16% 45% 26% 4	615
	11th to 20th	Night	9% 3 17% 42% 26% 4	612
	21st to end	Night	6% 2 19% 47% 23% 2	690
August	1st to 10th	Night	9% 3 14% 41% 31% 2	627
	11th to 20th	Night	7% 2 17% 39% 32% 3	606
	21st to end	Night	9% 3 17% 36% 31% 4	660
September	1st to 10th	Night	11% 2 20% 34% 27% 5%	735
	11th to 20th	Night	10% 4 16% 35% 30% 6%	800
	21st to end	Night	17% 4 22% 35% 18% 5%	800
October	1st to 10th	Night	15% 6% 26% 33% 18% 3	800
	11th to 20th	Night	23% 10% 30% 27% 9% 1	800
	21st to end	Night	24% 9% 32% 29% 5%	880
November	1st to 10th	Night	18% 12% 30% 26% 12% 1	820
	11th to 20th	Night	19% 15% 34% 23% 8% 1	840
	21st to end	Night	23% 12% 38% 22% 5%	840
December	1st to 10th	Night	24% 11% 39% 20% 5%	840
	11th to 20th	Night	22% 14% 37% 23% 3	945
	21st to end	Night	30% 11% 34% 23% 2	1155

Page generated at 2026-08-12 03:37:19. Version ac07

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