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 2025-05-11.

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	7% 3 1 3 2 2 3 3 4 5% 67%	117729
	Night	8% 4 1 3 2 2 3 2 4 5% 65%	120094
Transparency	Day	20% 4 9% 18% 24% 26%	117479
	Night	22% 4 10% 19% 24% 21%	119934
Seeing	Day	7% 5% 20% 34% 25% 8%	15894
	Night	9% 2 13% 39% 31% 7%	25495

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	10% 7% 2 5% 4 3 4 4 6% 8% 48%	3639
	11th to 20th	Day	10% 4 2 5% 4 5% 4 3 4 4 55%	3944
	21st to end	Day	14% 6% 2 6% 4 4 4 4 6% 7% 44%	4510
February	1st to 10th	Day	15% 6% 1 5% 3 3 4 4 4 6% 48%	1877
	11th to 20th	Day	11% 6% 2 4 3 4 4 4 6% 7% 49%	1950
	21st to end	Day	15% 6% 2 5% 4 4 3 3 4 6% 48%	1650
March	1st to 10th	Day	14% 7% 1 6% 4 4 4 5% 8% 9% 36%	4518
	11th to 20th	Day	14% 5% 2 4 3 3 3 3 4 7% 53%	4944
	21st to end	Day	14% 5% 2 4 3 3 4 3 5% 8% 48%	5835
April	1st to 10th	Day	10% 5% 2 4 3 3 3 3 4 7% 58%	2400
	11th to 20th	Day	8% 4 1 4 3 3 3 3 5% 6% 59%	2369
	21st to end	Day	7% 3 1 3 3 3 3 3 5% 6% 63%	2400
May	1st to 10th	Day	5% 2 1 2 1 2 1 1 2 4 78%	4005
	11th to 20th	Day	2 1 1 2 1 1 2 2 3 5% 81%	4124
	21st to end	Day	3 1 2 1 2 2 2 3 4 80%	4813
June	1st to 10th	Day	3 2 1 2 2 2 2 3 4 7% 72%	2660
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 87%	2660
	21st to end	Day	1 1 1 1 1 1 1 2 3 86%	2660
July	1st to 10th	Day	1 1 1 1 2 2 92%	4420
	11th to 20th	Day	1 1 1 1 1 1 2 2 3 3 84%	4420
	21st to end	Day	1 1 1 1 1 1 1 2 3 87%	4519
August	1st to 10th	Day	2 1 1 1 1 1 1 2 2 3 85%	3916
	11th to 20th	Day	1 1 1 1 1 3 91%	3724
	21st to end	Day	1 1 1 1 1 1 1 1 2 90%	4180
September	1st to 10th	Day	1 1 1 1 2 1 2 2 88%	2280
	11th to 20th	Day	2 2 2 1 1 1 2 3 4 82%	2237
	21st to end	Day	2 1 1 2 1 1 1 2 3 4 83%	2090
October	1st to 10th	Day	1 2 1 2 1 2 2 2 4 4 78%	3278
	11th to 20th	Day	2 2 1 2 1 2 2 2 3 3 80%	3161
	21st to end	Day	4 2 1 3 2 2 4 2 4 6% 68%	3431
November	1st to 10th	Day	7% 4 1 2 3 3 3 3 5% 7% 61%	1764
	11th to 20th	Day	8% 3 1 4 3 3 3 4 5% 7% 60%	1800
	21st to end	Day	11% 4 1 4 3 3 3 3 4 7% 57%	1800
December	1st to 10th	Day	8% 4 1 3 3 4 4 4 5% 6% 57%	2719
	11th to 20th	Day	19% 6% 2 5% 4 4 4 3 4 8% 41%	3028
	21st to end	Day	16% 6% 3 8% 5% 5% 5% 5% 6% 10% 32%	4004
January	1st to 10th	Night	11% 6% 2 4 3 3 3 3 5% 7% 51%	6306
	11th to 20th	Night	13% 5% 2 4 3 3 3 3 4 5% 56%	6012
	21st to end	Night	16% 6% 2 5% 3 3 3 2 4 6% 50%	6468
February	1st to 10th	Night	17% 5% 1 4 2 3 3 3 4 6% 51%	2520
	11th to 20th	Night	13% 4 1 4 2 3 3 3 5% 6% 56%	2338
	21st to end	Night	14% 5% 2 5% 3 3 2 3 4 6% 53%	1980
March	1st to 10th	Night	16% 6% 2 6% 4 4 4 3 6% 7% 43%	5376
	11th to 20th	Night	13% 6% 2 6% 4 3 3 2 4 5% 52%	5053
	21st to end	Night	18% 5% 2 4 3 2 3 3 4 7% 49%	5138
April	1st to 10th	Night	10% 4 2 4 2 2 2 3 5% 7% 60%	2000
	11th to 20th	Night	6% 4 1 3 3 3 3 3 5% 6% 63%	1979
	21st to end	Night	5% 4 1 2 3 3 3 2 3 5% 69%	2000
May	1st to 10th	Night	4 2 1 3 1 1 2 2 3 4 79%	3191
	11th to 20th	Night	2 1 1 1 1 1 1 2 3 4 82%	2779
	21st to end	Night	2 1 1 2 1 1 1 1 2 4 84%	2762
June	1st to 10th	Night	3 2 1 2 2 2 2 2 3 6% 75%	1520
	11th to 20th	Night	1 1 1 1 1 2 2 3 88%	1520
	21st to end	Night	1 1 1 1 1 2 1 2 3 87%	1520
July	1st to 10th	Night	1 1 1 1 1 1 2 94%	2540
	11th to 20th	Night	2 2 1 2 2 2 2 2 3 4 80%	2540
	21st to end	Night	1 1 1 1 1 2 2 2 4 85%	3037
August	1st to 10th	Night	1 2 1 1 1 1 2 1 3 4 83%	3027
	11th to 20th	Night	1 1 1 1 1 2 2 91%	3126
	21st to end	Night	2 1 1 1 1 1 1 1 2 89%	3498
September	1st to 10th	Night	1 1 1 1 1 1 1 2 90%	1900
	11th to 20th	Night	1 2 2 1 1 1 1 3 3 83%	1942
	21st to end	Night	2 1 1 1 1 2 1 1 2 4 84%	2090
October	1st to 10th	Night	2 2 1 1 1 2 1 2 2 3 83%	3698
	11th to 20th	Night	2 1 2 1 1 2 2 3 4 82%	3777
	21st to end	Night	4 2 1 2 1 2 2 1 3 5% 76%	4135
November	1st to 10th	Night	8% 3 1 3 3 2 3 2 4 6% 66%	2535
	11th to 20th	Night	6% 3 1 4 3 2 2 2 4 7% 65%	2600
	21st to end	Night	10% 3 2 4 3 3 3 3 5% 6% 59%	2600
December	1st to 10th	Night	8% 4 1 4 3 3 3 3 4 6% 62%	4459
	11th to 20th	Night	18% 7% 2 5% 3 3 4 4 6% 9% 40%	5176
	21st to end	Night	14% 7% 3 8% 5% 6% 6% 5% 6% 8% 33%	6952

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	33% 7% 25% 19% 14% 3	3639
	11th to 20th	Day	31% 8% 9% 14% 22% 16%	3944
	21st to end	Day	37% 7% 11% 15% 25% 4	4510
February	1st to 10th	Day	36% 5% 11% 24% 18% 6%	1877
	11th to 20th	Day	33% 5% 12% 25% 20% 6%	1960
	21st to end	Day	36% 6% 14% 25% 17% 2	1650
March	1st to 10th	Day	38% 7% 15% 19% 18% 2	4518
	11th to 20th	Day	30% 6% 17% 28% 12% 8%	4944
	21st to end	Day	32% 6% 17% 28% 14% 3	5835
April	1st to 10th	Day	27% 3 13% 28% 21% 8%	2400
	11th to 20th	Day	25% 5% 10% 27% 23% 9%	2372
	21st to end	Day	20% 5% 12% 29% 28% 6%	2400
May	1st to 10th	Day	13% 2 7% 25% 34% 18%	4005
	11th to 20th	Day	9% 2 6% 25% 41% 17%	4127
	21st to end	Day	10% 2 6% 27% 38% 17%	4814
June	1st to 10th	Day	13% 3 10% 20% 31% 22%	2660
	11th to 20th	Day	5% 1 5% 15% 38% 35%	2660
	21st to end	Day	6% 2 2 8% 29% 53%	2660
July	1st to 10th	Day	3 1 1 3 12% 79%	4351
	11th to 20th	Day	7% 2 3 5% 15% 69%	4345
	21st to end	Day	6% 1 2 5% 26% 60%	4376
August	1st to 10th	Day	8% 1 2 5% 25% 59%	3901
	11th to 20th	Day	3 1 5% 24% 66%	3760
	21st to end	Day	5% 1 2 7% 27% 59%	4179
September	1st to 10th	Day	6% 1 3 6% 27% 57%	2280
	11th to 20th	Day	8% 2 3 19% 38% 30%	2237
	21st to end	Day	8% 2 3 17% 37% 34%	2090
October	1st to 10th	Day	10% 2 4 11% 19% 53%	3278
	11th to 20th	Day	11% 2 5% 21% 40% 21%	3161
	21st to end	Day	18% 2 6% 16% 39% 19%	3431
November	1st to 10th	Day	23% 4 13% 24% 25% 12%	1764
	11th to 20th	Day	23% 5% 15% 26% 24% 7%	1800
	21st to end	Day	28% 5% 11% 19% 27% 9%	1800
December	1st to 10th	Day	27% 5% 10% 28% 22% 8%	2719
	11th to 20th	Day	42% 5% 13% 19% 17% 3	3028
	21st to end	Day	45% 10% 21% 17% 6% 2	4004
January	1st to 10th	Night	30% 7% 24% 22% 15% 2	6306
	11th to 20th	Night	31% 5% 13% 14% 27% 9%	6012
	21st to end	Night	35% 5% 10% 21% 22% 7%	6468
February	1st to 10th	Night	34% 4 11% 23% 20% 8%	2520
	11th to 20th	Night	29% 4 12% 27% 22% 6%	2339
	21st to end	Night	33% 4 13% 27% 18% 5%	1980
March	1st to 10th	Night	38% 6% 13% 20% 21% 3	5376
	11th to 20th	Night	33% 4 16% 27% 15% 6%	5053
	21st to end	Night	35% 5% 19% 22% 15% 4	5138
April	1st to 10th	Night	25% 4 15% 29% 21% 8%	2000
	11th to 20th	Night	22% 4 11% 30% 24% 9%	1979
	21st to end	Night	18% 3 13% 31% 30% 5%	2000
May	1st to 10th	Night	11% 2 9% 29% 35% 14%	3191
	11th to 20th	Night	8% 2 5% 24% 43% 18%	2780
	21st to end	Night	8% 2 6% 28% 45% 11%	2762
June	1st to 10th	Night	12% 3 9% 24% 31% 21%	1520
	11th to 20th	Night	5% 2 4 17% 39% 34%	1520
	21st to end	Night	5% 2 3 8% 26% 57%	1520
July	1st to 10th	Night	2 1 2 13% 81%	2500
	11th to 20th	Night	9% 2 2 5% 14% 68%	2492
	21st to end	Night	6% 1 2 5% 24% 62%	2942
August	1st to 10th	Night	8% 1 1 6% 22% 61%	3018
	11th to 20th	Night	4 1 2 4 18% 72%	3156
	21st to end	Night	6% 1 1 6% 25% 61%	3498
September	1st to 10th	Night	4 1 2 6% 25% 62%	1900
	11th to 20th	Night	8% 2 4 14% 40% 32%	1942
	21st to end	Night	8% 2 2 14% 37% 37%	2090
October	1st to 10th	Night	8% 2 3 10% 21% 56%	3698
	11th to 20th	Night	9% 2 4 22% 41% 22%	3777
	21st to end	Night	14% 2 4 19% 38% 24%	4135
November	1st to 10th	Night	21% 3 10% 25% 25% 17%	2535
	11th to 20th	Night	19% 4 13% 27% 25% 12%	2600
	21st to end	Night	25% 4 11% 22% 29% 10%	2600
December	1st to 10th	Night	24% 4 13% 28% 23% 9%	4459
	11th to 20th	Night	38% 5% 17% 18% 18% 3	5176
	21st to end	Night	45% 8% 17% 21% 6% 2	6952

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	24% 5% 24% 28% 15% 5%	200
	11th to 20th	Day	14% 4 18% 34% 21% 7%	335
	21st to end	Day	16% 4 19% 35% 18% 9%	440
February	1st to 10th	Day	18% 7% 23% 26% 19% 8%	439
	11th to 20th	Day	15% 7% 27% 31% 16% 4	495
	21st to end	Day	12% 9% 26% 35% 16% 2	414
March	1st to 10th	Day	16% 9% 25% 34% 14% 2	500
	11th to 20th	Day	15% 11% 28% 25% 17% 4	500
	21st to end	Day	16% 11% 27% 27% 15% 3	550
April	1st to 10th	Day	12% 12% 25% 28% 18% 4	500
	11th to 20th	Day	11% 11% 25% 33% 15% 3	492
	21st to end	Day	5% 11% 26% 33% 19% 7%	500
May	1st to 10th	Day	4 10% 33% 32% 15% 6%	503
	11th to 20th	Day	6% 7% 26% 34% 20% 6%	489
	21st to end	Day	6% 9% 19% 36% 23% 7%	529
June	1st to 10th	Day	2 5% 24% 33% 27% 9%	490
	11th to 20th	Day	2 5% 24% 38% 24% 8%	490
	21st to end	Day	1 3 18% 40% 28% 10%	490
July	1st to 10th	Day	1 10% 42% 35% 11%	486
	11th to 20th	Day	1 7% 36% 40% 15%	484
	21st to end	Day	1 9% 40% 37% 13%	531
August	1st to 10th	Day	3 1 14% 43% 27% 11%	487
	11th to 20th	Day	1 1 16% 40% 30% 12%	477
	21st to end	Day	2 1 10% 39% 36% 13%	539
September	1st to 10th	Day	1 2 13% 40% 31% 12%	490
	11th to 20th	Day	3 3 22% 37% 28% 7%	490
	21st to end	Day	2 3 18% 36% 34% 8%	490
October	1st to 10th	Day	2 4 21% 34% 31% 9%	490
	11th to 20th	Day	3 4 21% 33% 30% 9%	490
	21st to end	Day	4 5% 16% 33% 34% 8%	534
November	1st to 10th	Day	8% 5% 17% 30% 30% 10%	300
	11th to 20th	Day	5% 3 21% 34% 26% 11%	300
	21st to end	Day	12% 3 18% 32% 28% 7%	300
December	1st to 10th	Day	13% 2 16% 34% 27% 8%	230
	11th to 20th	Day	17% 4 19% 31% 20% 10%	200
	21st to end	Day	21% 3 21% 32% 19% 4	220
January	1st to 10th	Night	17% 2 14% 35% 23% 9%	800
	11th to 20th	Night	11% 3 14% 39% 26% 8%	800
	21st to end	Night	18% 2 11% 39% 25% 6%	880
February	1st to 10th	Night	18% 3 14% 36% 21% 8%	800
	11th to 20th	Night	14% 4 15% 35% 26% 6%	784
	21st to end	Night	16% 2 13% 46% 19% 3	660
March	1st to 10th	Night	18% 4 15% 40% 20% 4	800
	11th to 20th	Night	13% 4 18% 34% 26% 6%	800
	21st to end	Night	15% 3 19% 37% 25% 2	880
April	1st to 10th	Night	11% 4 22% 36% 25% 3	800
	11th to 20th	Night	6% 5% 22% 38% 23% 5%	791
	21st to end	Night	6% 3 22% 38% 26% 6%	800
May	1st to 10th	Night	4 4 21% 44% 23% 4	750
	11th to 20th	Night	5% 2 20% 46% 27% 1	531
	21st to end	Night	4 3 20% 43% 29% 2	411
June	1st to 10th	Night	4 2 17% 48% 26% 3	380
	11th to 20th	Night	1 1 16% 52% 31% 1	380
	21st to end	Night	1 11% 52% 34% 2	380
July	1st to 10th	Night	5% 48% 44% 2	370
	11th to 20th	Night	3 4 44% 46% 3	368
	21st to end	Night	1 6% 28% 56% 10%	578
August	1st to 10th	Night	2 8% 41% 42% 7%	668
	11th to 20th	Night	1 1 5% 42% 45% 7%	752
	21st to end	Night	2 4 32% 54% 8%	836
September	1st to 10th	Night	1 5% 34% 51% 8%	760
	11th to 20th	Night	1 2 9% 41% 37% 10%	760
	21st to end	Night	2 1 8% 35% 44% 10%	760
October	1st to 10th	Night	4 1 8% 41% 39% 8%	760
	11th to 20th	Night	4 1 8% 39% 38% 10%	760
	21st to end	Night	5% 10% 36% 32% 16%	836
November	1st to 10th	Night	8% 3 10% 33% 34% 12%	780
	11th to 20th	Night	6% 1 12% 42% 30% 8%	800
	21st to end	Night	10% 1 13% 38% 29% 9%	800
December	1st to 10th	Night	14% 2 12% 38% 27% 7%	800
	11th to 20th	Night	18% 2 11% 41% 23% 6%	800
	21st to end	Night	19% 2 15% 42% 19% 4	880

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