Observatoire POLARIS
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire POLARIS clear sky chart from 2005-11-06 to 2025-09-04.

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% 10% 4 8% 5% 5% 5% 4 5% 6% 32%	126418
	Night	16% 11% 4 9% 5% 4 4 4 5% 6% 33%	122313
Transparency	Day	51% 7% 12% 19% 10% 1	129850
	Night	51% 6% 12% 19% 10% 1	126955
Seeing	Day	22% 17% 32% 19% 8% 2	15554
	Night	22% 12% 31% 26% 9% 1	24414

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% 17% 7% 12% 6% 4 4 3 4 5% 25%	3597
	11th to 20th	Day	11% 14% 7% 15% 7% 5% 4 3 4 4 25%	3570
	21st to end	Day	8% 12% 7% 13% 6% 5% 4 3 5% 6% 31%	3960
February	1st to 10th	Day	11% 19% 5% 11% 7% 4 4 3 4 5% 26%	1710
	11th to 20th	Day	12% 17% 5% 11% 5% 5% 5% 3 5% 4 28%	1856
	21st to end	Day	14% 17% 6% 9% 6% 4 4 3 5% 5% 27%	1650
March	1st to 10th	Day	20% 15% 5% 11% 7% 4 4 3 4 4 24%	4476
	11th to 20th	Day	11% 14% 4 10% 5% 5% 4 3 5% 6% 33%	4685
	21st to end	Day	17% 14% 3 8% 4 4 4 3 4 5% 33%	5970
April	1st to 10th	Day	19% 14% 4 8% 5% 4 4 3 4 6% 31%	2400
	11th to 20th	Day	21% 8% 3 7% 5% 4 4 3 4 5% 35%	2374
	21st to end	Day	20% 12% 3 8% 4 4 5% 3 5% 5% 31%	2595
May	1st to 10th	Day	26% 10% 3 7% 5% 5% 4 4 4 6% 27%	6290
	11th to 20th	Day	20% 9% 3 7% 4 4 4 3 5% 8% 33%	6318
	21st to end	Day	21% 10% 3 7% 5% 5% 5% 4 5% 6% 29%	6949
June	1st to 10th	Day	21% 8% 3 7% 5% 5% 5% 4 6% 7% 28%	2800
	11th to 20th	Day	16% 6% 2 7% 6% 5% 5% 4 7% 8% 33%	2800
	21st to end	Day	22% 8% 3 8% 6% 5% 5% 4 6% 7% 26%	2800
July	1st to 10th	Day	16% 7% 3 9% 5% 5% 6% 5% 7% 8% 31%	6360
	11th to 20th	Day	13% 6% 2 6% 5% 5% 6% 6% 7% 9% 35%	6360
	21st to end	Day	10% 5% 2 6% 4 5% 6% 4 7% 9% 42%	6926
August	1st to 10th	Day	13% 6% 2 7% 5% 6% 8% 5% 7% 9% 32%	4563
	11th to 20th	Day	16% 6% 3 7% 5% 4 5% 4 7% 9% 34%	4203
	21st to end	Day	17% 6% 2 6% 5% 4 5% 4 5% 7% 38%	4312
September	1st to 10th	Day	14% 5% 2 5% 5% 4 4 4 6% 8% 42%	2316
	11th to 20th	Day	14% 6% 2 4 4 4 5% 3 5% 6% 47%	2190
	21st to end	Day	21% 7% 2 6% 4 3 3 3 5% 6% 40%	2090
October	1st to 10th	Day	22% 6% 3 7% 4 3 4 4 5% 7% 35%	1973
	11th to 20th	Day	21% 10% 3 9% 4 6% 5% 4 5% 6% 28%	1832
	21st to end	Day	25% 12% 3 6% 5% 4 3 3 4 5% 30%	1881
November	1st to 10th	Day	15% 12% 3 7% 4 5% 5% 3 5% 6% 34%	1761
	11th to 20th	Day	19% 11% 4 8% 4 4 5% 4 4 7% 28%	1705
	21st to end	Day	20% 15% 4 7% 5% 4 6% 3 5% 6% 26%	1600
December	1st to 10th	Day	19% 18% 6% 12% 5% 4 4 3 4 5% 21%	2600
	11th to 20th	Day	18% 16% 5% 9% 5% 3 3 3 4 5% 29%	2986
	21st to end	Day	20% 15% 5% 10% 5% 4 4 3 4 4 28%	3960
January	1st to 10th	Night	12% 19% 7% 14% 5% 4 4 3 4 4 23%	6349
	11th to 20th	Night	9% 15% 7% 16% 8% 5% 4 3 4 4 25%	6315
	21st to end	Night	8% 12% 5% 13% 8% 7% 6% 4 5% 6% 27%	6996
February	1st to 10th	Night	11% 14% 6% 10% 7% 5% 5% 4 5% 5% 29%	2686
	11th to 20th	Night	11% 13% 5% 11% 7% 5% 4 4 4 5% 31%	2429
	21st to end	Night	14% 14% 4 8% 6% 4 5% 3 5% 6% 32%	1980
March	1st to 10th	Night	21% 15% 4 11% 5% 4 4 3 3 5% 25%	5364
	11th to 20th	Night	12% 13% 4 9% 5% 5% 5% 4 6% 6% 31%	5271
	21st to end	Night	15% 11% 4 10% 5% 5% 4 4 5% 5% 32%	4962
April	1st to 10th	Night	20% 12% 3 7% 5% 4 3 3 4 6% 31%	2000
	11th to 20th	Night	18% 7% 3 8% 5% 4 4 3 5% 6% 37%	1975
	21st to end	Night	21% 10% 3 6% 4 4 3 3 5% 6% 35%	1805
May	1st to 10th	Night	25% 10% 3 7% 5% 4 4 4 5% 6% 26%	3693
	11th to 20th	Night	23% 7% 3 5% 4 4 4 3 5% 6% 35%	3575
	21st to end	Night	22% 8% 3 8% 5% 5% 5% 4 5% 6% 29%	3926
June	1st to 10th	Night	20% 6% 2 6% 5% 4 4 4 6% 7% 35%	1600
	11th to 20th	Night	18% 7% 2 6% 5% 4 5% 4 5% 7% 37%	1600
	21st to end	Night	19% 7% 3 6% 5% 5% 4 4 4 8% 34%	1600
July	1st to 10th	Night	13% 5% 2 6% 5% 5% 6% 5% 8% 10% 35%	3600
	11th to 20th	Night	14% 5% 2 6% 5% 5% 4 4 6% 7% 44%	3600
	21st to end	Night	10% 4 2 5% 4 4 5% 4 6% 8% 48%	3926
August	1st to 10th	Night	12% 5% 2 7% 6% 5% 5% 4 6% 7% 41%	2586
	11th to 20th	Night	17% 6% 2 6% 4 4 4 4 5% 6% 43%	2796
	21st to end	Night	19% 6% 2 5% 4 4 4 3 5% 6% 41%	3586
September	1st to 10th	Night	12% 5% 2 5% 3 5% 5% 4 5% 6% 48%	1931
	11th to 20th	Night	15% 5% 2 4 3 3 4 3 4 6% 51%	1990
	21st to end	Night	19% 5% 2 6% 4 3 3 3 4 5% 46%	2090
October	1st to 10th	Night	18% 8% 3 7% 4 4 4 3 4 6% 40%	2204
	11th to 20th	Night	22% 8% 3 6% 4 4 4 3 5% 7% 36%	2347
	21st to end	Night	23% 9% 3 7% 4 4 4 3 5% 6% 31%	2717
November	1st to 10th	Night	16% 10% 3 8% 4 4 4 3 5% 6% 35%	2537
	11th to 20th	Night	18% 11% 3 6% 6% 4 5% 4 5% 6% 32%	2693
	21st to end	Night	21% 13% 3 9% 6% 4 3 3 5% 5% 28%	2800
December	1st to 10th	Night	15% 15% 5% 10% 6% 5% 5% 4 6% 6% 23%	4580
	11th to 20th	Night	23% 12% 4 9% 5% 4 4 3 4 4 28%	5208
	21st to end	Night	20% 16% 5% 10% 4 3 3 2 3 3 29%	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	60% 8% 9% 18% 5% 1	3598
	11th to 20th	Day	60% 7% 9% 15% 7% 2	3600
	21st to end	Day	54% 6% 8% 17% 14% 1	3960
February	1st to 10th	Day	60% 4 8% 16% 9% 3	1710
	11th to 20th	Day	58% 6% 11% 14% 11% 1	1865
	21st to end	Day	59% 5% 7% 16% 9% 3	1650
March	1st to 10th	Day	62% 7% 5% 11% 11% 3	4479
	11th to 20th	Day	50% 7% 16% 12% 12% 3	4685
	21st to end	Day	52% 5% 7% 15% 17% 3	5970
April	1st to 10th	Day	55% 6% 8% 16% 13% 2	2400
	11th to 20th	Day	50% 6% 9% 21% 13% 3	2374
	21st to end	Day	53% 7% 9% 16% 13% 3	2595
May	1st to 10th	Day	58% 5% 9% 16% 10% 1	6301
	11th to 20th	Day	49% 8% 16% 17% 7% 2	6322
	21st to end	Day	53% 8% 11% 15% 12% 2	6949
June	1st to 10th	Day	52% 9% 10% 17% 10% 2	2800
	11th to 20th	Day	44% 9% 13% 22% 11% 2	2800
	21st to end	Day	56% 8% 11% 16% 9% 1	2800
July	1st to 10th	Day	49% 7% 14% 23% 7%	6290
	11th to 20th	Day	41% 8% 17% 27% 7%	6285
	21st to end	Day	36% 8% 17% 30% 9%	6728
August	1st to 10th	Day	45% 9% 18% 21% 7%	4549
	11th to 20th	Day	44% 9% 18% 23% 6%	4245
	21st to end	Day	43% 7% 15% 20% 13% 1	4312
September	1st to 10th	Day	38% 7% 17% 25% 12% 1	2316
	11th to 20th	Day	37% 8% 14% 28% 12% 1	2190
	21st to end	Day	44% 6% 13% 26% 11%	2090
October	1st to 10th	Day	47% 6% 13% 26% 7% 1	3272
	11th to 20th	Day	47% 6% 16% 25% 6% 1	3022
	21st to end	Day	44% 5% 21% 19% 9% 1	3081
November	1st to 10th	Day	50% 7% 13% 20% 9% 1	1761
	11th to 20th	Day	54% 7% 11% 17% 11% 1	1705
	21st to end	Day	60% 8% 11% 15% 6% 1	1600
December	1st to 10th	Day	66% 4 10% 15% 5%	2600
	11th to 20th	Day	57% 4 8% 18% 13%	2986
	21st to end	Day	61% 5% 8% 15% 11%	3960
January	1st to 10th	Night	64% 7% 10% 14% 5%	6349
	11th to 20th	Night	62% 6% 11% 16% 5% 1	6360
	21st to end	Night	55% 8% 9% 14% 14% 1	6996
February	1st to 10th	Night	56% 6% 9% 18% 10% 1	2686
	11th to 20th	Night	54% 6% 10% 18% 11% 1	2431
	21st to end	Night	52% 6% 10% 18% 13% 2	1980
March	1st to 10th	Night	62% 6% 8% 11% 11% 3	5374
	11th to 20th	Night	51% 7% 12% 14% 12% 3	5272
	21st to end	Night	52% 4 7% 22% 11% 3	4962
April	1st to 10th	Night	54% 6% 9% 15% 15% 2	2000
	11th to 20th	Night	46% 6% 10% 21% 14% 2	1975
	21st to end	Night	49% 6% 8% 20% 15% 2	1805
May	1st to 10th	Night	56% 7% 10% 16% 9% 1	3699
	11th to 20th	Night	48% 8% 13% 20% 9% 2	3576
	21st to end	Night	51% 8% 12% 16% 12% 1	3927
June	1st to 10th	Night	45% 8% 14% 20% 13% 1	1600
	11th to 20th	Night	45% 7% 13% 19% 13% 3	1600
	21st to end	Night	48% 8% 12% 18% 13% 1	1600
July	1st to 10th	Night	43% 6% 23% 21% 7%	3560
	11th to 20th	Night	39% 6% 19% 29% 6%	3552
	21st to end	Night	33% 6% 18% 32% 11%	3809
August	1st to 10th	Night	42% 7% 17% 26% 8%	2576
	11th to 20th	Night	42% 5% 21% 24% 8%	2820
	21st to end	Night	44% 6% 15% 23% 12%	3586
September	1st to 10th	Night	35% 7% 17% 26% 14% 1	1931
	11th to 20th	Night	34% 6% 16% 32% 11% 1	1990
	21st to end	Night	40% 6% 12% 29% 13%	2090
October	1st to 10th	Night	46% 4 14% 25% 10%	3672
	11th to 20th	Night	41% 6% 13% 31% 9%	3896
	21st to end	Night	45% 5% 17% 19% 12% 2	4467
November	1st to 10th	Night	49% 6% 14% 22% 8% 1	2537
	11th to 20th	Night	51% 6% 13% 18% 11% 1	2693
	21st to end	Night	58% 5% 12% 16% 8% 1	2800
December	1st to 10th	Night	61% 5% 14% 15% 5%	4580
	11th to 20th	Night	60% 5% 6% 17% 11% 1	5208
	21st to end	Night	61% 5% 6% 15% 14% 1	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	22% 22% 35% 18% 4	200
	11th to 20th	Day	24% 18% 36% 17% 5% 1	200
	21st to end	Day	16% 20% 43% 18% 3 1	220
February	1st to 10th	Day	27% 26% 37% 8% 1	255
	11th to 20th	Day	24% 23% 39% 13% 1	403
	21st to end	Day	25% 23% 33% 15% 4	414
March	1st to 10th	Day	25% 21% 34% 16% 5%	500
	11th to 20th	Day	22% 21% 37% 15% 4	500
	21st to end	Day	27% 23% 27% 14% 6% 1	550
April	1st to 10th	Day	29% 19% 30% 16% 6%	500
	11th to 20th	Day	30% 22% 25% 15% 7% 1	492
	21st to end	Day	30% 21% 33% 11% 5% 1	500
May	1st to 10th	Day	28% 19% 33% 15% 4 1	503
	11th to 20th	Day	20% 19% 34% 20% 6% 1	527
	21st to end	Day	22% 21% 34% 16% 5% 2	573
June	1st to 10th	Day	22% 17% 38% 17% 7%	530
	11th to 20th	Day	18% 18% 33% 20% 10% 1	530
	21st to end	Day	25% 14% 29% 23% 7% 1	530
July	1st to 10th	Day	16% 11% 36% 25% 10% 2	525
	11th to 20th	Day	14% 13% 37% 27% 8% 2	524
	21st to end	Day	13% 10% 33% 28% 12% 2	576
August	1st to 10th	Day	13% 10% 31% 31% 13% 2	526
	11th to 20th	Day	14% 8% 29% 27% 15% 6%	519
	21st to end	Day	17% 15% 26% 23% 15% 4	583
September	1st to 10th	Day	15% 12% 32% 24% 11% 6%	502
	11th to 20th	Day	14% 11% 28% 26% 15% 7%	490
	21st to end	Day	23% 11% 29% 20% 13% 4	490
October	1st to 10th	Day	24% 15% 26% 23% 10% 2	490
	11th to 20th	Day	25% 19% 29% 17% 9% 1	453
	21st to end	Day	31% 18% 27% 19% 4	429
November	1st to 10th	Day	24% 20% 29% 19% 7% 1	395
	11th to 20th	Day	23% 21% 31% 18% 6%	305
	21st to end	Day	27% 28% 28% 16% 3	200
December	1st to 10th	Day	28% 15% 29% 21% 7% 2	200
	11th to 20th	Day	32% 23% 31% 13% 3	200
	21st to end	Day	30% 19% 30% 15% 5% 1	220
January	1st to 10th	Night	24% 13% 38% 22% 4	800
	11th to 20th	Night	24% 14% 34% 22% 7%	800
	21st to end	Night	15% 12% 39% 27% 6%	880
February	1st to 10th	Night	17% 14% 39% 24% 5%	800
	11th to 20th	Night	20% 17% 36% 21% 4	783
	21st to end	Night	21% 15% 36% 23% 4	660
March	1st to 10th	Night	24% 15% 33% 23% 4	800
	11th to 20th	Night	22% 18% 34% 20% 5% 1	800
	21st to end	Night	20% 17% 33% 24% 6%	880
April	1st to 10th	Night	28% 14% 30% 23% 5%	800
	11th to 20th	Night	24% 16% 32% 23% 6%	791
	21st to end	Night	27% 16% 31% 20% 5%	605
May	1st to 10th	Night	27% 14% 30% 22% 6% 1	447
	11th to 20th	Night	21% 13% 31% 27% 7%	394
	21st to end	Night	22% 9% 40% 20% 9%	433
June	1st to 10th	Night	22% 9% 30% 32% 8%	400
	11th to 20th	Night	19% 8% 28% 32% 12% 1	400
	21st to end	Night	21% 6% 30% 30% 13%	400
July	1st to 10th	Night	13% 5% 31% 37% 13% 1	390
	11th to 20th	Night	13% 4 27% 41% 14% 1	388
	21st to end	Night	15% 6% 24% 40% 14% 1	438
August	1st to 10th	Night	11% 3 23% 46% 17% 1	398
	11th to 20th	Night	14% 4 23% 36% 20% 1	538
	21st to end	Night	18% 6% 26% 32% 17% 2	880
September	1st to 10th	Night	14% 6% 30% 35% 14% 1	773
	11th to 20th	Night	18% 7% 27% 27% 20% 2	760
	21st to end	Night	21% 9% 28% 25% 15% 2	760
October	1st to 10th	Night	22% 11% 30% 26% 11% 1	760
	11th to 20th	Night	24% 12% 29% 25% 8% 1	760
	21st to end	Night	28% 12% 31% 23% 6%	836
November	1st to 10th	Night	24% 13% 30% 24% 7% 1	780
	11th to 20th	Night	23% 15% 32% 21% 9% 1	800
	21st to end	Night	27% 17% 27% 23% 6% 1	800
December	1st to 10th	Night	27% 13% 30% 23% 6%	800
	11th to 20th	Night	26% 16% 32% 21% 5%	800
	21st to end	Night	29% 14% 30% 21% 5%	880

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