Observaterry
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observaterry clear sky chart from 2005-11-06 to 2025-05-26.

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	2 2 1 3 3 3 4 3 5% 7% 66%	117880
	Night	3 3 1 4 3 3 3 3 5% 7% 65%	120333
Transparency	Day	16% 4 7% 15% 29% 29%	117644
	Night	18% 4 6% 15% 29% 28%	120158
Seeing	Day	5% 8% 19% 29% 26% 13%	15047
	Night	4 3 21% 43% 26% 2	25492

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	3 6% 2 3 3 3 4 3 5% 5% 63%	4098
	11th to 20th	Day	1 2 1 2 2 3 3 2 4 7% 73%	4100
	21st to end	Day	4 3 1 3 2 2 3 4 5% 6% 66%	4510
February	1st to 10th	Day	2 2 1 3 2 2 3 2 4 7% 69%	1805
	11th to 20th	Day	3 3 1 3 3 4 3 3 4 6% 66%	1949
	21st to end	Day	1 3 1 3 2 3 2 2 5% 6% 72%	1697
March	1st to 10th	Day	1 3 1 3 3 2 3 3 4 6% 71%	4954
	11th to 20th	Day	5% 4 2 5% 3 3 4 3 4 6% 63%	5000
	21st to end	Day	1 3 2 4 3 4 4 4 5% 7% 64%	5486
April	1st to 10th	Day	1 2 1 2 2 3 2 3 4 6% 74%	2399
	11th to 20th	Day	2 2 1 3 2 2 2 2 3 5% 75%	2371
	21st to end	Day	1 1 1 2 2 2 2 2 3 5% 78%	2400
May	1st to 10th	Day	1 1 1 1 2 2 2 3 4 82%	3920
	11th to 20th	Day	2 2 1 3 2 3 4 3 5% 6% 68%	3971
	21st to end	Day	1 1 1 1 1 2 1 2 3 86%	4544
June	1st to 10th	Day	2 2 1 3 2 3 4 4 6% 10% 64%	2470
	11th to 20th	Day	2 2 1 1 2 2 3 3 5% 6% 74%	2470
	21st to end	Day	5% 3 1 4 3 5% 4 4 8% 9% 53%	2470
July	1st to 10th	Day	3 2 1 4 4 5% 5% 5% 7% 10% 54%	4110
	11th to 20th	Day	2 2 1 4 4 5% 6% 5% 8% 11% 54%	4110
	21st to end	Day	3 3 2 6% 5% 6% 7% 7% 10% 12% 39%	4454
August	1st to 10th	Day	2 2 1 3 4 5% 6% 5% 9% 13% 51%	4000
	11th to 20th	Day	3 2 1 3 3 3 4 4 8% 11% 58%	3726
	21st to end	Day	3 3 1 5% 4 5% 6% 6% 8% 11% 48%	4180
September	1st to 10th	Day	6% 4 2 5% 4 4 5% 4 6% 9% 51%	2280
	11th to 20th	Day	5% 4 1 4 4 3 4 3 5% 6% 59%	2280
	21st to end	Day	3 2 1 2 2 2 3 2 5% 7% 71%	2013
October	1st to 10th	Day	2 1 2 2 2 2 2 3 5% 79%	3170
	11th to 20th	Day	2 2 1 3 2 2 3 2 4 5% 75%	3150
	21st to end	Day	1 1 1 2 1 1 1 2 3 86%	3438
November	1st to 10th	Day	2 2 1 2 2 2 2 3 4 6% 74%	1959
	11th to 20th	Day	2 3 2 2 2 1 2 3 5% 6% 73%	2000
	21st to end	Day	5% 4 1 4 3 2 3 3 5% 7% 64%	1863
December	1st to 10th	Day	3 4 1 4 2 2 2 3 4 7% 68%	2726
	11th to 20th	Day	2 4 2 5% 3 3 3 2 3 7% 67%	3297
	21st to end	Day	1 2 1 2 2 3 3 3 4 7% 71%	4510
January	1st to 10th	Night	3 5% 2 6% 3 3 3 3 4 6% 61%	5867
	11th to 20th	Night	2 2 1 2 1 2 3 3 5% 9% 72%	5880
	21st to end	Night	3 4 1 5% 3 2 3 3 5% 7% 63%	6468
February	1st to 10th	Night	1 3 1 2 2 2 3 3 5% 7% 70%	2593
	11th to 20th	Night	4 4 1 3 3 3 3 3 4 5% 66%	2338
	21st to end	Night	3 3 1 3 3 3 3 2 5% 7% 68%	1930
March	1st to 10th	Night	2 3 1 3 1 2 3 3 4 6% 71%	4940
	11th to 20th	Night	6% 3 1 3 3 3 3 2 4 6% 66%	5000
	21st to end	Night	2 2 1 4 3 3 4 3 5% 8% 66%	5489
April	1st to 10th	Night	1 1 1 3 2 2 3 3 5% 6% 72%	2000
	11th to 20th	Night	3 2 1 3 3 2 3 2 4 5% 72%	1977
	21st to end	Night	2 1 1 2 2 3 3 3 4 5% 74%	2000
May	1st to 10th	Night	1 1 1 2 2 2 2 3 4 81%	3270
	11th to 20th	Night	4 2 1 3 3 3 3 3 5% 6% 66%	3148
	21st to end	Night	1 1 1 1 1 2 1 2 4 85%	3159
June	1st to 10th	Night	3 3 1 3 3 3 4 4 6% 7% 63%	1710
	11th to 20th	Night	2 2 3 2 2 3 3 5% 6% 71%	1710
	21st to end	Night	3 4 2 5% 5% 4 4 4 8% 9% 51%	1710
July	1st to 10th	Night	3 3 1 4 4 5% 6% 5% 9% 11% 48%	2860
	11th to 20th	Night	2 5% 2 6% 5% 5% 6% 5% 8% 11% 45%	2860
	21st to end	Night	5% 5% 3 8% 6% 7% 7% 7% 9% 12% 32%	3109
August	1st to 10th	Night	2 3 1 5% 4 6% 7% 7% 11% 16% 38%	2938
	11th to 20th	Night	4 3 1 6% 6% 5% 6% 6% 10% 11% 43%	3113
	21st to end	Night	4 5% 2 8% 6% 7% 7% 6% 9% 11% 35%	3487
September	1st to 10th	Night	6% 3 2 6% 5% 5% 6% 3 7% 9% 48%	1900
	11th to 20th	Night	5% 4 1 3 3 3 4 4 6% 7% 59%	1900
	21st to end	Night	4 3 1 4 3 3 2 3 5% 5% 68%	2161
October	1st to 10th	Night	2 2 1 2 2 2 3 3 5% 5% 73%	3800
	11th to 20th	Night	3 2 1 3 2 2 2 2 4 6% 73%	3778
	21st to end	Night	1 1 1 2 1 1 2 2 3 5% 81%	4120
November	1st to 10th	Night	2 1 1 2 1 2 3 2 4 6% 76%	2340
	11th to 20th	Night	2 3 1 3 2 2 2 2 3 5% 75%	2400
	21st to end	Night	5% 4 1 3 3 2 3 2 4 7% 66%	2508
December	1st to 10th	Night	3 3 1 3 2 2 3 2 5% 7% 67%	4459
	11th to 20th	Night	2 2 1 4 3 3 3 3 6% 7% 66%	4943
	21st to end	Night	3 3 1 3 2 2 3 3 4 7% 70%	6468

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	19% 4 4 13% 42% 17%	4099
	11th to 20th	Day	11% 3 5% 16% 44% 21%	4100
	21st to end	Day	17% 4 5% 18% 29% 27%	4510
February	1st to 10th	Day	16% 2 5% 16% 29% 31%	1805
	11th to 20th	Day	19% 3 5% 16% 31% 26%	1959
	21st to end	Day	14% 3 4 9% 31% 40%	1697
March	1st to 10th	Day	14% 3 3 6% 33% 42%	4954
	11th to 20th	Day	21% 6% 4 12% 26% 31%	5000
	21st to end	Day	18% 5% 5% 12% 31% 30%	5486
April	1st to 10th	Day	12% 3 4 11% 31% 40%	2399
	11th to 20th	Day	14% 2 2 8% 27% 47%	2374
	21st to end	Day	10% 2 4 11% 30% 44%	2400
May	1st to 10th	Day	8% 2 2 7% 24% 58%	3920
	11th to 20th	Day	12% 6% 7% 11% 26% 39%	3974
	21st to end	Day	6% 1 3 6% 14% 70%	4544
June	1st to 10th	Day	15% 4 6% 14% 23% 38%	2470
	11th to 20th	Day	10% 3 4 8% 21% 54%	2470
	21st to end	Day	23% 6% 10% 15% 16% 29%	2470
July	1st to 10th	Day	22% 7% 13% 16% 18% 24%	4046
	11th to 20th	Day	21% 7% 14% 24% 28% 6%	4041
	21st to end	Day	30% 9% 16% 29% 14% 2	4314
August	1st to 10th	Day	19% 8% 15% 30% 25% 4	3986
	11th to 20th	Day	18% 6% 11% 21% 33% 11%	3762
	21st to end	Day	25% 9% 15% 23% 22% 6%	4179
September	1st to 10th	Day	27% 6% 10% 18% 23% 16%	2280
	11th to 20th	Day	23% 6% 7% 18% 24% 22%	2280
	21st to end	Day	14% 4 6% 16% 32% 27%	2013
October	1st to 10th	Day	10% 2 5% 10% 29% 43%	3170
	11th to 20th	Day	13% 3 4 12% 36% 32%	3150
	21st to end	Day	7% 1 2 14% 34% 42%	3438
November	1st to 10th	Day	12% 3 4 18% 35% 29%	1959
	11th to 20th	Day	13% 3 5% 18% 31% 30%	2000
	21st to end	Day	20% 3 6% 16% 33% 21%	1863
December	1st to 10th	Day	17% 3 4 22% 35% 19%	2726
	11th to 20th	Day	21% 2 4 15% 42% 16%	3296
	21st to end	Day	12% 3 6% 20% 39% 20%	4510
January	1st to 10th	Night	22% 4 4 13% 43% 14%	5867
	11th to 20th	Night	11% 5% 5% 13% 47% 20%	5880
	21st to end	Night	20% 4 5% 16% 26% 28%	6468
February	1st to 10th	Night	14% 4 6% 17% 27% 32%	2593
	11th to 20th	Night	20% 3 5% 15% 29% 28%	2338
	21st to end	Night	18% 3 5% 9% 28% 37%	1930
March	1st to 10th	Night	13% 4 3 8% 32% 40%	4940
	11th to 20th	Night	18% 4 4 13% 33% 28%	5000
	21st to end	Night	15% 4 4 14% 34% 29%	5489
April	1st to 10th	Night	13% 2 5% 11% 34% 35%	2000
	11th to 20th	Night	15% 3 3 10% 27% 42%	1977
	21st to end	Night	12% 3 4 12% 28% 41%	2000
May	1st to 10th	Night	9% 2 3 8% 21% 58%	3270
	11th to 20th	Night	15% 7% 8% 11% 23% 38%	3149
	21st to end	Night	8% 2 3 5% 11% 71%	3159
June	1st to 10th	Night	17% 5% 6% 15% 22% 35%	1710
	11th to 20th	Night	13% 3 5% 9% 18% 52%	1710
	21st to end	Night	27% 6% 12% 13% 15% 26%	1710
July	1st to 10th	Night	25% 7% 15% 15% 17% 20%	2815
	11th to 20th	Night	30% 8% 12% 20% 25% 6%	2806
	21st to end	Night	40% 9% 18% 24% 8% 2	3011
August	1st to 10th	Night	25% 9% 16% 34% 13% 4	2929
	11th to 20th	Night	27% 8% 12% 17% 29% 8%	3143
	21st to end	Night	39% 8% 14% 19% 16% 3	3487
September	1st to 10th	Night	30% 7% 12% 18% 18% 16%	1900
	11th to 20th	Night	21% 6% 10% 19% 21% 23%	1900
	21st to end	Night	18% 4 7% 15% 30% 26%	2161
October	1st to 10th	Night	13% 4 5% 9% 29% 40%	3800
	11th to 20th	Night	14% 3 4 14% 28% 38%	3778
	21st to end	Night	9% 2 2 9% 32% 46%	4120
November	1st to 10th	Night	11% 3 4 16% 34% 32%	2340
	11th to 20th	Night	13% 2 4 15% 32% 34%	2400
	21st to end	Night	19% 3 5% 16% 33% 23%	2508
December	1st to 10th	Night	17% 3 4 21% 37% 18%	4459
	11th to 20th	Night	17% 4 6% 13% 41% 19%	4943
	21st to end	Night	15% 3 3 18% 42% 19%	6468

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	9% 6% 21% 43% 19% 2	300
	11th to 20th	Day	6% 4 21% 45% 20% 4	300
	21st to end	Day	5% 8% 25% 35% 22% 5%	330
February	1st to 10th	Day	4 10% 20% 38% 21% 6%	300
	11th to 20th	Day	9% 9% 26% 40% 15% 1	300
	21st to end	Day	9% 18% 30% 32% 8% 3	296
March	1st to 10th	Day	9% 19% 31% 30% 9% 2	500
	11th to 20th	Day	9% 19% 26% 29% 14% 3	500
	21st to end	Day	6% 22% 33% 25% 12% 1	550
April	1st to 10th	Day	6% 14% 36% 26% 14% 4	500
	11th to 20th	Day	6% 21% 30% 27% 12% 2	492
	21st to end	Day	4 19% 32% 26% 15% 3	500
May	1st to 10th	Day	3 14% 36% 27% 17% 4	503
	11th to 20th	Day	4 11% 26% 33% 20% 7%	528
	21st to end	Day	3 10% 34% 30% 19% 4	550
June	1st to 10th	Day	3 3 23% 31% 31% 9%	490
	11th to 20th	Day	2 6% 21% 32% 28% 11%	490
	21st to end	Day	7% 2 12% 29% 32% 18%	490
July	1st to 10th	Day	4 2 7% 23% 38% 27%	486
	11th to 20th	Day	2 3 20% 37% 37%	484
	21st to end	Day	3 1 3 17% 39% 37%	531
August	1st to 10th	Day	2 1 3 20% 39% 34%	487
	11th to 20th	Day	4 1 6% 19% 43% 27%	477
	21st to end	Day	4 1 4 21% 40% 30%	539
September	1st to 10th	Day	6% 1 8% 23% 37% 26%	490
	11th to 20th	Day	4 1 10% 31% 34% 18%	490
	21st to end	Day	3 6% 11% 28% 30% 21%	384
October	1st to 10th	Day	4 2 14% 25% 37% 17%	300
	11th to 20th	Day	6% 4 15% 30% 30% 15%	300
	21st to end	Day	6% 6% 13% 24% 36% 14%	330
November	1st to 10th	Day	7% 6% 23% 34% 24% 7%	300
	11th to 20th	Day	3 6% 14% 35% 35% 8%	300
	21st to end	Day	8% 9% 20% 38% 21% 5%	300
December	1st to 10th	Day	8% 4 17% 40% 28% 2	300
	11th to 20th	Day	4 6% 23% 41% 23% 3	300
	21st to end	Day	7% 8% 20% 45% 17% 3	330
January	1st to 10th	Night	5% 3 22% 49% 19% 2	800
	11th to 20th	Night	5% 2 26% 46% 18% 2	800
	21st to end	Night	5% 5% 28% 45% 16% 1	880
February	1st to 10th	Night	3 5% 28% 43% 20% 1	800
	11th to 20th	Night	5% 5% 30% 49% 10% 1	784
	21st to end	Night	4 8% 39% 41% 8%	660
March	1st to 10th	Night	4 6% 37% 43% 10% 1	800
	11th to 20th	Night	4 6% 30% 43% 17% 1	800
	21st to end	Night	3 7% 36% 44% 11%	880
April	1st to 10th	Night	3 6% 37% 46% 8%	600
	11th to 20th	Night	4 6% 39% 42% 9%	593
	21st to end	Night	2 8% 35% 45% 11%	600
May	1st to 10th	Night	3 5% 33% 49% 11%	600
	11th to 20th	Night	4 4 23% 51% 18% 1	591
	21st to end	Night	2 2 27% 56% 12% 1	634
June	1st to 10th	Night	3 1 19% 52% 25% 1	570
	11th to 20th	Night	2 1 18% 55% 24%	570
	21st to end	Night	3 2 11% 47% 34% 3	570
July	1st to 10th	Night	5% 1 6% 34% 49% 5%	555
	11th to 20th	Night	3 3 27% 59% 7%	552
	21st to end	Night	7% 1 5% 19% 59% 9%	624
August	1st to 10th	Night	3 1 3 21% 59% 13%	567
	11th to 20th	Night	5% 1 5% 25% 55% 8%	564
	21st to end	Night	4 1 3 25% 59% 7%	627
September	1st to 10th	Night	7% 1 5% 36% 47% 4	570
	11th to 20th	Night	5% 1 5% 44% 42% 2	570
	21st to end	Night	4 1 12% 42% 38% 2	727
October	1st to 10th	Night	6% 1 15% 46% 29% 3	760
	11th to 20th	Night	5% 2 17% 47% 27% 3	760
	21st to end	Night	3 3 16% 41% 32% 4	836
November	1st to 10th	Night	3 2 16% 47% 30% 3	780
	11th to 20th	Night	3 2 19% 46% 27% 4	800
	21st to end	Night	7% 4 23% 40% 23% 3	908
December	1st to 10th	Night	6% 3 20% 48% 21% 1	1000
	11th to 20th	Night	5% 3 26% 45% 20% 1	880
	21st to end	Night	5% 3 27% 49% 15% 2	880

Page generated at 2025-05-23 07:21:02. Version ac07

Copyright © 2025 Attilla Danko.