Artis Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Artis Observatory clear sky chart from 2005-11-06 to 2026-09-14.

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	6% 3 1 3 2 2 2 2 4 5% 71%	157110
	Night	8% 3 1 3 2 2 2 2 4 5% 67%	151543
Transparency	Day	18% 3 8% 17% 27% 27%	152136
	Night	21% 3 9% 19% 26% 22%	147497
Seeing	Day	7% 5% 20% 35% 25% 8%	17719
	Night	8% 2 13% 39% 31% 6%	27152

By Month

Percentage Cloud Cover SW (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% 6% 2 5% 4 3 4 3 6% 8% 45%	4722
	11th to 20th	Day	7% 3 2 4 3 4 4 2 4 4 62%	5142
	21st to end	Day	11% 5% 2 6% 4 4 4 4 6% 8% 45%	5843
February	1st to 10th	Day	14% 6% 2 5% 3 3 4 4 4 6% 48%	1971
	11th to 20th	Day	12% 6% 2 4 4 4 4 4 5% 7% 48%	2050
	21st to end	Day	15% 5% 2 5% 4 4 3 3 5% 7% 47%	1730
March	1st to 10th	Day	14% 7% 1 6% 4 4 4 5% 8% 9% 37%	4618
	11th to 20th	Day	14% 5% 2 4 3 3 3 3 4 7% 54%	5052
	21st to end	Day	14% 5% 2 4 3 3 4 3 5% 8% 48%	5964
April	1st to 10th	Day	9% 5% 2 4 3 3 3 3 4 7% 58%	2520
	11th to 20th	Day	8% 4 1 4 3 3 3 3 5% 6% 58%	2489
	21st to end	Day	7% 3 1 3 3 3 3 3 5% 6% 63%	2520
May	1st to 10th	Day	4 3 1 3 2 2 1 1 3 4 76%	7203
	11th to 20th	Day	1 1 1 1 1 2 2 3 4 84%	7699
	21st to end	Day	2 2 1 2 2 2 2 2 3 4 80%	8992
June	1st to 10th	Day	2 2 1 2 2 2 2 3 4 7% 74%	2940
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 87%	2940
	21st to end	Day	1 1 1 1 1 1 1 2 3 86%	2940
July	1st to 10th	Day	1 1 1 1 1 2 93%	8219
	11th to 20th	Day	1 1 1 1 1 1 2 1 3 3 85%	8220
	21st to end	Day	1 1 1 1 1 1 2 3 88%	8419
August	1st to 10th	Day	1 1 1 1 1 1 1 2 91%	7235
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 88%	7001
	21st to end	Day	3 2 1 2 1 2 2 2 3 5% 80%	7750
September	1st to 10th	Day	1 1 1 1 2 1 2 2 88%	2520
	11th to 20th	Day	2 2 2 1 1 1 2 3 4 82%	2392
	21st to end	Day	2 1 1 2 1 1 1 2 3 4 83%	2200
October	1st to 10th	Day	2 2 1 2 1 2 2 2 4 4 78%	3381
	11th to 20th	Day	3 2 1 2 1 2 2 2 3 3 80%	3261
	21st to end	Day	4 2 1 3 2 3 4 2 4 6% 69%	3541
November	1st to 10th	Day	7% 3 1 2 3 3 3 3 5% 7% 62%	1854
	11th to 20th	Day	10% 3 1 4 3 3 3 4 5% 7% 58%	1890
	21st to end	Day	10% 4 1 4 3 3 3 3 4 7% 57%	1890
December	1st to 10th	Day	8% 4 1 3 3 4 4 4 6% 6% 57%	2802
	11th to 20th	Day	18% 6% 2 5% 4 4 4 3 4 8% 42%	3108
	21st to end	Day	16% 6% 3 8% 5% 5% 5% 5% 7% 10% 31%	4092
January	1st to 10th	Night	16% 7% 2 5% 3 3 3 3 5% 7% 47%	8172
	11th to 20th	Night	10% 4 1 3 2 2 3 3 4 6% 61%	7821
	21st to end	Night	13% 6% 2 6% 3 3 3 3 4 7% 49%	8371
February	1st to 10th	Night	16% 5% 1 4 2 2 3 3 4 6% 52%	2646
	11th to 20th	Night	14% 5% 2 4 2 3 3 3 5% 6% 54%	2458
	21st to end	Night	14% 5% 2 5% 3 3 3 3 4 6% 53%	2076
March	1st to 10th	Night	16% 6% 2 6% 4 4 4 3 6% 6% 44%	5496
	11th to 20th	Night	13% 6% 2 5% 3 3 3 2 4 5% 53%	5164
	21st to end	Night	18% 5% 2 4 3 2 3 3 4 7% 50%	5251
April	1st to 10th	Night	10% 4 2 4 2 2 2 3 5% 7% 59%	2100
	11th to 20th	Night	6% 4 1 3 3 3 3 3 5% 6% 63%	2079
	21st to end	Night	6% 4 1 2 3 3 3 2 3 5% 68%	2100
May	1st to 10th	Night	3 2 1 3 2 2 2 2 3 5% 76%	5765
	11th to 20th	Night	2 1 1 1 1 1 2 2 3 4 83%	5181
	21st to end	Night	2 1 1 2 1 1 1 1 2 3 85%	5181
June	1st to 10th	Night	3 2 1 2 2 2 2 2 3 6% 75%	1680
	11th to 20th	Night	1 1 1 1 1 2 2 3 89%	1680
	21st to end	Night	1 1 1 1 1 2 1 2 3 88%	1680
July	1st to 10th	Night	1 1 1 1 1 1 94%	4740
	11th to 20th	Night	3 2 1 2 1 1 1 1 2 3 82%	4740
	21st to end	Night	1 1 1 1 1 1 1 2 3 88%	5679
August	1st to 10th	Night	1 1 1 1 1 1 1 2 3 89%	5602
	11th to 20th	Night	1 1 1 2 1 3 4 87%	5886
	21st to end	Night	3 1 2 1 2 2 2 4 5% 78%	6499
September	1st to 10th	Night	1 1 1 1 1 1 1 3 89%	2100
	11th to 20th	Night	2 2 2 1 1 1 1 3 3 83%	2074
	21st to end	Night	2 1 1 2 1 2 1 1 2 4 83%	2200
October	1st to 10th	Night	2 1 1 1 2 1 2 2 3 83%	3815
	11th to 20th	Night	3 1 2 1 1 2 2 2 4 82%	3897
	21st to end	Night	4 2 1 2 1 2 2 1 3 5% 76%	4267
November	1st to 10th	Night	8% 3 1 2 3 2 3 2 4 6% 67%	2665
	11th to 20th	Night	7% 3 1 4 3 2 2 2 4 7% 63%	2730
	21st to end	Night	9% 3 2 3 2 3 3 3 5% 7% 59%	2730
December	1st to 10th	Night	8% 3 1 3 2 3 3 3 4 6% 62%	4596
	11th to 20th	Night	17% 7% 2 5% 3 3 3 4 6% 9% 41%	5316
	21st to end	Night	14% 7% 3 8% 5% 6% 6% 5% 6% 8% 33%	7106

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	36% 6% 22% 20% 13% 3	4723
	11th to 20th	Day	25% 6% 7% 14% 27% 22%	5142
	21st to end	Day	35% 6% 10% 18% 27% 4	5843
February	1st to 10th	Day	36% 5% 11% 24% 18% 7%	1971
	11th to 20th	Day	34% 5% 11% 24% 19% 6%	2060
	21st to end	Day	35% 7% 15% 26% 16% 2	1730
March	1st to 10th	Day	38% 7% 15% 19% 19% 2	4618
	11th to 20th	Day	29% 6% 17% 28% 12% 8%	5052
	21st to end	Day	32% 6% 16% 27% 15% 3	5964
April	1st to 10th	Day	27% 3 13% 27% 21% 8%	2520
	11th to 20th	Day	25% 6% 11% 27% 23% 9%	2492
	21st to end	Day	21% 5% 12% 29% 28% 6%	2520
May	1st to 10th	Day	14% 3 7% 23% 40% 14%	7203
	11th to 20th	Day	7% 2 4 25% 43% 20%	7702
	21st to end	Day	11% 2 5% 19% 49% 14%	8994
June	1st to 10th	Day	12% 3 10% 20% 32% 23%	2940
	11th to 20th	Day	5% 1 5% 14% 37% 38%	2940
	21st to end	Day	6% 2 2 7% 29% 53%	2940
July	1st to 10th	Day	3 1 3 18% 75%	8151
	11th to 20th	Day	7% 1 2 7% 14% 69%	8145
	21st to end	Day	4 1 2 8% 31% 53%	8231
August	1st to 10th	Day	5% 1 1 5% 27% 60%	5730
	11th to 20th	Day	7% 1 2 14% 31% 47%	5517
	21st to end	Day	9% 1 2 12% 33% 42%	6077
September	1st to 10th	Day	5% 1 3 8% 30% 53%	2520
	11th to 20th	Day	8% 2 3 19% 38% 30%	2392
	21st to end	Day	8% 2 4 17% 37% 33%	2200
October	1st to 10th	Day	10% 2 4 12% 20% 52%	3381
	11th to 20th	Day	11% 2 5% 21% 40% 21%	3261
	21st to end	Day	18% 2 6% 16% 39% 18%	3541
November	1st to 10th	Day	22% 4 13% 23% 26% 12%	1854
	11th to 20th	Day	26% 5% 14% 26% 23% 7%	1890
	21st to end	Day	28% 5% 12% 21% 26% 9%	1890
December	1st to 10th	Day	26% 5% 10% 29% 22% 8%	2802
	11th to 20th	Day	41% 6% 13% 19% 17% 4	3108
	21st to end	Day	45% 10% 20% 17% 6% 2	4092
January	1st to 10th	Night	35% 6% 23% 20% 13% 2	8172
	11th to 20th	Night	25% 4 10% 17% 32% 12%	7821
	21st to end	Night	34% 5% 9% 24% 22% 6%	8371
February	1st to 10th	Night	33% 4 10% 23% 21% 8%	2646
	11th to 20th	Night	30% 4 11% 27% 22% 6%	2459
	21st to end	Night	33% 5% 13% 27% 18% 5%	2076
March	1st to 10th	Night	37% 6% 13% 20% 21% 3	5496
	11th to 20th	Night	33% 4 15% 27% 15% 7%	5164
	21st to end	Night	35% 5% 18% 23% 16% 4	5251
April	1st to 10th	Night	25% 4 14% 28% 22% 8%	2100
	11th to 20th	Night	23% 4 11% 30% 24% 8%	2079
	21st to end	Night	19% 3 12% 30% 30% 5%	2100
May	1st to 10th	Night	13% 2 10% 27% 37% 11%	5763
	11th to 20th	Night	7% 2 4 23% 44% 20%	5182
	21st to end	Night	8% 2 4 22% 52% 12%	5182
June	1st to 10th	Night	12% 3 8% 24% 32% 22%	1680
	11th to 20th	Night	5% 1 4 15% 38% 37%	1680
	21st to end	Night	4 1 2 8% 26% 57%	1680
July	1st to 10th	Night	2 1 2 18% 77%	4700
	11th to 20th	Night	10% 1 2 6% 12% 69%	4692
	21st to end	Night	4 1 2 7% 31% 56%	5555
August	1st to 10th	Night	6% 1 1 5% 24% 63%	4424
	11th to 20th	Night	6% 1 3 13% 27% 50%	4636
	21st to end	Night	11% 1 2 10% 32% 45%	5092
September	1st to 10th	Night	5% 1 2 8% 27% 57%	2100
	11th to 20th	Night	9% 2 4 14% 41% 31%	2074
	21st to end	Night	8% 2 3 15% 36% 36%	2200
October	1st to 10th	Night	8% 2 3 11% 21% 55%	3815
	11th to 20th	Night	10% 2 4 22% 41% 22%	3897
	21st to end	Night	14% 2 4 19% 38% 23%	4267
November	1st to 10th	Night	21% 3 10% 25% 25% 16%	2665
	11th to 20th	Night	21% 4 13% 26% 24% 12%	2730
	21st to end	Night	25% 4 11% 23% 28% 9%	2730
December	1st to 10th	Night	24% 3 13% 29% 23% 8%	4596
	11th to 20th	Night	38% 6% 17% 18% 18% 4	5316
	21st to end	Night	45% 8% 17% 20% 6% 2	7106

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	25% 4 25% 26% 16% 5%	220
	11th to 20th	Day	13% 4 17% 33% 24% 9%	362
	21st to end	Day	15% 4 19% 35% 20% 8%	473
February	1st to 10th	Day	17% 7% 22% 27% 19% 8%	473
	11th to 20th	Day	16% 8% 27% 30% 15% 4	535
	21st to end	Day	11% 9% 24% 37% 17% 2	446
March	1st to 10th	Day	15% 9% 26% 34% 14% 3	540
	11th to 20th	Day	14% 10% 27% 26% 19% 4	540
	21st to end	Day	15% 11% 27% 28% 15% 3	594
April	1st to 10th	Day	12% 12% 24% 29% 18% 5%	540
	11th to 20th	Day	11% 11% 25% 33% 17% 4	532
	21st to end	Day	5% 11% 26% 32% 19% 7%	540
May	1st to 10th	Day	4 10% 31% 33% 16% 6%	543
	11th to 20th	Day	6% 8% 26% 34% 20% 6%	568
	21st to end	Day	6% 8% 19% 36% 23% 8%	617
June	1st to 10th	Day	2 5% 22% 35% 27% 9%	570
	11th to 20th	Day	2 5% 24% 38% 24% 8%	570
	21st to end	Day	1 4 19% 39% 27% 10%	570
July	1st to 10th	Day	1 1 11% 43% 33% 10%	566
	11th to 20th	Day	2 7% 37% 39% 15%	564
	21st to end	Day	1 9% 40% 36% 14%	619
August	1st to 10th	Day	3 1 14% 40% 30% 13%	567
	11th to 20th	Day	1 1 14% 39% 32% 13%	557
	21st to end	Day	2 1 9% 40% 35% 13%	627
September	1st to 10th	Day	1 2 14% 40% 30% 12%	570
	11th to 20th	Day	3 3 21% 37% 28% 8%	543
	21st to end	Day	2 3 19% 36% 33% 8%	530
October	1st to 10th	Day	2 4 22% 33% 31% 8%	530
	11th to 20th	Day	4 5% 20% 33% 29% 9%	530
	21st to end	Day	5% 5% 17% 32% 34% 8%	578
November	1st to 10th	Day	7% 5% 16% 30% 31% 11%	330
	11th to 20th	Day	9% 4 21% 33% 24% 10%	330
	21st to end	Day	11% 2 17% 34% 29% 7%	330
December	1st to 10th	Day	12% 2 15% 34% 26% 10%	253
	11th to 20th	Day	15% 4 19% 33% 20% 10%	220
	21st to end	Day	22% 4 22% 31% 17% 4	242
January	1st to 10th	Night	18% 3 15% 34% 22% 8%	840
	11th to 20th	Night	11% 3 13% 39% 28% 7%	840
	21st to end	Night	17% 2 11% 39% 24% 5%	924
February	1st to 10th	Night	18% 3 14% 37% 21% 8%	840
	11th to 20th	Night	15% 4 15% 35% 25% 6%	824
	21st to end	Night	16% 2 14% 47% 18% 3	692
March	1st to 10th	Night	17% 4 15% 41% 20% 3	840
	11th to 20th	Night	13% 4 17% 34% 27% 6%	840
	21st to end	Night	14% 2 18% 38% 25% 2	924
April	1st to 10th	Night	11% 4 21% 36% 26% 3	840
	11th to 20th	Night	7% 5% 22% 39% 23% 5%	831
	21st to end	Night	6% 3 21% 38% 25% 5%	840
May	1st to 10th	Night	4 4 21% 43% 24% 4	788
	11th to 20th	Night	5% 2 21% 45% 25% 1	585
	21st to end	Night	4 3 19% 45% 28% 2	455
June	1st to 10th	Night	4 2 15% 49% 27% 3	420
	11th to 20th	Night	15% 53% 30% 1	420
	21st to end	Night	1 11% 51% 35% 1	420
July	1st to 10th	Night	5% 50% 42% 2	410
	11th to 20th	Night	3 3 44% 46% 3	408
	21st to end	Night	6% 29% 56% 9%	640
August	1st to 10th	Night	2 8% 40% 44% 6%	739
	11th to 20th	Night	1 1 5% 41% 45% 7%	832
	21st to end	Night	2 4 33% 53% 8%	924
September	1st to 10th	Night	1 6% 36% 49% 8%	840
	11th to 20th	Night	2 2 9% 41% 37% 9%	812
	21st to end	Night	2 1 8% 37% 42% 10%	800
October	1st to 10th	Night	4 1 8% 42% 38% 7%	800
	11th to 20th	Night	5% 1 8% 38% 37% 10%	800
	21st to end	Night	5% 10% 36% 32% 15%	880
November	1st to 10th	Night	8% 3 10% 33% 35% 12%	820
	11th to 20th	Night	8% 1 12% 42% 29% 8%	840
	21st to end	Night	10% 1 13% 39% 29% 8%	840
December	1st to 10th	Night	13% 2 11% 39% 27% 7%	840
	11th to 20th	Night	17% 2 11% 41% 24% 6%	840
	21st to end	Night	19% 2 16% 41% 18% 4	924

Page generated at 2026-09-11 01:11:56. Version ac07

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