Cosmos Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cosmos Observatory clear sky chart from 2005-11-06 to 2026-10-14.

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	3 3 1 4 3 4 4 4 6% 8% 60%	157071
	Night	3 4 1 4 3 3 4 3 6% 8% 60%	152257
Transparency	Day	21% 5% 9% 20% 27% 19%	152105
	Night	21% 4 9% 20% 27% 18%	148232
Seeing	Day	10% 19% 29% 26% 13% 3	17780
	Night	7% 15% 30% 32% 15% 2	27792

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	1 6% 2 6% 3 3 3 3 5% 7% 61%	5078
	11th to 20th	Day	1 2 1 3 2 1 2 2 3 4 79%	5340
	21st to end	Day	2 5% 2 6% 3 3 4 3 5% 6% 60%	5844
February	1st to 10th	Day	1 5% 2 4 3 3 3 3 6% 7% 63%	1967
	11th to 20th	Day	2 4 1 4 3 3 3 3 5% 8% 63%	2049
	21st to end	Day	1 5% 2 4 2 3 3 2 4 6% 67%	1730
March	1st to 10th	Day	1 5% 1 4 2 2 3 3 5% 7% 68%	4734
	11th to 20th	Day	4 7% 3 5% 3 2 3 3 5% 8% 57%	5110
	21st to end	Day	1 4 1 3 3 3 4 3 6% 9% 63%	5865
April	1st to 10th	Day	2 4 1 3 2 3 3 4 6% 8% 64%	2520
	11th to 20th	Day	3 4 1 4 3 3 3 3 5% 7% 63%	2491
	21st to end	Day	3 2 1 3 3 2 3 3 5% 8% 66%	2520
May	1st to 10th	Day	4 5% 2 4 3 4 5% 4 6% 7% 57%	7057
	11th to 20th	Day	4 2 1 3 2 2 3 3 4 6% 71%	7337
	21st to end	Day	2 3 1 3 3 3 4 4 6% 7% 64%	9026
June	1st to 10th	Day	2 2 1 4 3 3 5% 5% 6% 9% 58%	2940
	11th to 20th	Day	2 2 1 3 3 3 4 3 6% 6% 68%	2940
	21st to end	Day	5% 3 1 5% 4 4 5% 5% 6% 7% 54%	2940
July	1st to 10th	Day	3 3 1 5% 4 5% 6% 5% 7% 8% 54%	8249
	11th to 20th	Day	1 2 1 4 5% 6% 8% 7% 10% 11% 43%	7734
	21st to end	Day	3 3 2 6% 6% 7% 8% 6% 9% 10% 40%	8304
August	1st to 10th	Day	2 2 1 4 4 6% 7% 6% 8% 9% 52%	7219
	11th to 20th	Day	2 2 2 5% 5% 5% 7% 6% 8% 10% 48%	7003
	21st to end	Day	2 2 1 6% 4 6% 7% 6% 9% 12% 44%	7751
September	1st to 10th	Day	5% 5% 2 5% 4 5% 6% 4 7% 8% 50%	2520
	11th to 20th	Day	6% 3 2 4 4 4 5% 4 6% 8% 54%	2515
	21st to end	Day	6% 3 2 4 3 2 3 3 4 7% 64%	2310
October	1st to 10th	Day	5% 2 1 3 2 2 2 2 3 6% 71%	3376
	11th to 20th	Day	6% 4 2 3 4 3 3 3 4 6% 63%	3280
	21st to end	Day	2 2 1 2 1 2 1 2 3 4 79%	3548
November	1st to 10th	Day	2 3 1 2 2 1 3 2 5% 7% 71%	1949
	11th to 20th	Day	2 3 2 3 2 2 3 2 6% 6% 70%	1890
	21st to end	Day	4 4 1 2 3 2 3 3 5% 6% 67%	1890
December	1st to 10th	Day	1 2 1 2 2 2 2 2 3 6% 77%	2847
	11th to 20th	Day	2 3 2 4 2 3 3 3 3 6% 70%	3106
	21st to end	Day	2 3 1 2 2 3 2 3 4 7% 70%	4092
January	1st to 10th	Night	1 5% 2 6% 4 4 4 3 6% 7% 58%	7886
	11th to 20th	Night	1 2 1 3 3 2 2 2 2 4 76%	7650
	21st to end	Night	2 5% 2 5% 3 3 4 3 5% 8% 62%	8370
February	1st to 10th	Night	1 5% 2 4 3 2 3 2 4 7% 66%	2651
	11th to 20th	Night	3 5% 1 4 3 3 4 3 5% 8% 63%	2458
	21st to end	Night	2 5% 1 3 2 2 2 2 4 7% 69%	2076
March	1st to 10th	Night	1 4 1 3 3 2 3 3 4 7% 70%	5363
	11th to 20th	Night	5% 6% 3 4 3 3 3 2 4 6% 62%	5110
	21st to end	Night	1 3 1 3 2 3 3 2 5% 7% 70%	5340
April	1st to 10th	Night	3 3 1 3 2 3 3 3 4 7% 69%	2100
	11th to 20th	Night	3 4 2 3 3 3 3 2 5% 6% 66%	2077
	21st to end	Night	3 3 1 3 2 3 3 3 5% 7% 68%	2100
May	1st to 10th	Night	8% 5% 2 4 3 3 3 3 5% 7% 57%	5890
	11th to 20th	Night	4 3 1 3 2 3 3 3 5% 7% 66%	5522
	21st to end	Night	5% 5% 2 4 3 3 4 3 4 6% 62%	5181
June	1st to 10th	Night	3 4 1 5% 4 4 5% 4 7% 9% 54%	1680
	11th to 20th	Night	2 2 1 3 2 4 3 3 5% 7% 68%	1680
	21st to end	Night	6% 3 1 4 5% 4 5% 4 6% 10% 50%	1680
July	1st to 10th	Night	6% 5% 2 5% 4 5% 5% 4 6% 7% 49%	4740
	11th to 20th	Night	3 3 1 5% 4 5% 6% 6% 10% 14% 43%	5237
	21st to end	Night	4 4 2 6% 5% 6% 7% 6% 10% 13% 37%	5805
August	1st to 10th	Night	2 3 1 5% 6% 7% 7% 6% 8% 12% 43%	5598
	11th to 20th	Night	2 3 2 6% 6% 6% 7% 6% 10% 13% 40%	5853
	21st to end	Night	3 4 2 8% 6% 7% 8% 6% 10% 14% 32%	6465
September	1st to 10th	Night	5% 4 2 4 5% 4 5% 4 8% 7% 52%	2100
	11th to 20th	Night	6% 4 1 4 3 4 4 4 6% 8% 56%	2105
	21st to end	Night	6% 4 1 3 3 3 3 4 5% 6% 61%	2310
October	1st to 10th	Night	5% 3 1 2 2 2 3 2 3 5% 72%	4029
	11th to 20th	Night	7% 3 1 4 2 2 2 2 4 7% 65%	3934
	21st to end	Night	3 2 2 2 2 2 2 3 5% 77%	4252
November	1st to 10th	Night	2 3 1 2 2 1 2 2 4 5% 75%	2546
	11th to 20th	Night	1 2 1 2 2 2 3 3 4 5% 74%	2730
	21st to end	Night	5% 4 1 2 2 3 3 4 4 6% 65%	2730
December	1st to 10th	Night	2 2 1 3 2 2 2 2 4 7% 73%	4560
	11th to 20th	Night	1 2 1 2 2 3 3 3 5% 7% 70%	5321
	21st to end	Night	2 3 1 3 3 3 2 2 5% 7% 68%	7128

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	23% 2 5% 26% 35% 9%	5081
	11th to 20th	Day	10% 3 6% 22% 36% 23%	5340
	21st to end	Day	23% 3 6% 14% 35% 19%	5844
February	1st to 10th	Day	20% 3 7% 20% 32% 17%	1967
	11th to 20th	Day	20% 3 7% 23% 29% 18%	2059
	21st to end	Day	17% 3 4 15% 35% 25%	1730
March	1st to 10th	Day	16% 2 4 9% 35% 34%	4734
	11th to 20th	Day	24% 2 6% 15% 34% 19%	5110
	21st to end	Day	16% 3 7% 23% 42% 10%	5865
April	1st to 10th	Day	17% 3 7% 20% 34% 20%	2520
	11th to 20th	Day	20% 4 6% 18% 28% 24%	2494
	21st to end	Day	15% 3 8% 21% 36% 17%	2520
May	1st to 10th	Day	24% 5% 8% 17% 27% 18%	7057
	11th to 20th	Day	15% 3 6% 13% 25% 37%	7341
	21st to end	Day	18% 5% 11% 14% 16% 36%	9027
June	1st to 10th	Day	19% 6% 11% 20% 22% 21%	2940
	11th to 20th	Day	15% 4 8% 13% 22% 37%	2940
	21st to end	Day	26% 5% 11% 17% 20% 21%	2940
July	1st to 10th	Day	25% 7% 11% 16% 17% 25%	8181
	11th to 20th	Day	25% 11% 19% 28% 12% 4	7663
	21st to end	Day	32% 10% 18% 27% 11% 3	8120
August	1st to 10th	Day	27% 8% 16% 25% 17% 6%	5714
	11th to 20th	Day	25% 9% 14% 25% 19% 8%	5519
	21st to end	Day	26% 10% 18% 29% 15% 2	6078
September	1st to 10th	Day	29% 7% 11% 20% 23% 10%	2520
	11th to 20th	Day	26% 6% 9% 19% 22% 18%	2515
	21st to end	Day	20% 4 7% 22% 31% 17%	2310
October	1st to 10th	Day	16% 3 5% 13% 32% 31%	3376
	11th to 20th	Day	22% 3 6% 16% 28% 25%	3280
	21st to end	Day	11% 2 3 14% 42% 28%	3548
November	1st to 10th	Day	13% 3 5% 18% 41% 20%	1949
	11th to 20th	Day	15% 2 6% 22% 39% 16%	1890
	21st to end	Day	18% 3 5% 19% 32% 23%	1890
December	1st to 10th	Day	11% 2 6% 23% 40% 19%	2847
	11th to 20th	Day	17% 2 6% 21% 40% 15%	3104
	21st to end	Day	15% 2 5% 23% 37% 18%	4092
January	1st to 10th	Night	24% 3 5% 23% 34% 11%	7887
	11th to 20th	Night	14% 2 4 22% 36% 22%	7650
	21st to end	Night	21% 3 6% 15% 37% 18%	8370
February	1st to 10th	Night	18% 3 8% 22% 30% 19%	2651
	11th to 20th	Night	19% 4 8% 23% 30% 16%	2458
	21st to end	Night	16% 2 3 16% 36% 26%	2076
March	1st to 10th	Night	15% 2 4 10% 36% 33%	5363
	11th to 20th	Night	24% 3 5% 17% 34% 18%	5110
	21st to end	Night	14% 3 6% 23% 41% 13%	5340
April	1st to 10th	Night	17% 3 6% 20% 33% 22%	2100
	11th to 20th	Night	19% 4 6% 18% 28% 26%	2077
	21st to end	Night	16% 4 8% 22% 34% 17%	2100
May	1st to 10th	Night	26% 4 9% 21% 22% 18%	5888
	11th to 20th	Night	18% 4 8% 15% 22% 33%	5523
	21st to end	Night	23% 4 12% 11% 12% 38%	5182
June	1st to 10th	Night	24% 7% 15% 16% 18% 20%	1680
	11th to 20th	Night	15% 3 9% 16% 19% 37%	1680
	21st to end	Night	28% 6% 14% 17% 16% 21%	1680
July	1st to 10th	Night	32% 6% 10% 19% 15% 18%	4700
	11th to 20th	Night	25% 10% 26% 25% 9% 5%	5185
	21st to end	Night	33% 9% 23% 23% 11% 1	5677
August	1st to 10th	Night	32% 8% 18% 24% 12% 6%	4427
	11th to 20th	Night	31% 8% 19% 22% 15% 5%	4613
	21st to end	Night	35% 10% 23% 23% 7% 3	5069
September	1st to 10th	Night	27% 7% 16% 22% 19% 8%	2100
	11th to 20th	Night	26% 5% 10% 19% 23% 17%	2105
	21st to end	Night	22% 4 8% 19% 30% 17%	2310
October	1st to 10th	Night	17% 3 5% 15% 31% 30%	4029
	11th to 20th	Night	21% 3 8% 14% 27% 28%	3934
	21st to end	Night	12% 2 4 16% 38% 29%	4252
November	1st to 10th	Night	12% 2 5% 21% 39% 21%	2546
	11th to 20th	Night	13% 3 5% 21% 39% 19%	2730
	21st to end	Night	20% 3 6% 19% 30% 22%	2730
December	1st to 10th	Night	14% 2 5% 28% 34% 17%	4560
	11th to 20th	Night	14% 3 5% 19% 38% 21%	5322
	21st to end	Night	16% 3 6% 23% 37% 15%	7128

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% 25% 31% 17% 5%	330
	11th to 20th	Day	17% 30% 29% 17% 6% 1	330
	21st to end	Day	7% 26% 38% 19% 8% 2	363
February	1st to 10th	Day	14% 35% 30% 17% 4	330
	11th to 20th	Day	15% 38% 29% 15% 3	330
	21st to end	Day	21% 37% 29% 11% 2	273
March	1st to 10th	Day	15% 35% 30% 16% 4	386
	11th to 20th	Day	22% 30% 27% 15% 4 1	540
	21st to end	Day	17% 35% 33% 12% 3 1	594
April	1st to 10th	Day	14% 33% 34% 14% 5% 1	540
	11th to 20th	Day	17% 37% 32% 12% 2	532
	21st to end	Day	14% 33% 36% 14% 3	540
May	1st to 10th	Day	12% 29% 39% 17% 3 1	543
	11th to 20th	Day	12% 18% 36% 24% 8% 2	660
	21st to end	Day	7% 17% 37% 28% 11% 1	844
June	1st to 10th	Day	5% 10% 29% 35% 19% 3	780
	11th to 20th	Day	5% 13% 29% 34% 17% 2	780
	21st to end	Day	7% 10% 27% 31% 20% 4	780
July	1st to 10th	Day	6% 4 21% 35% 29% 6%	771
	11th to 20th	Day	4 2 12% 40% 33% 9%	594
	21st to end	Day	5% 1 12% 36% 37% 9%	619
August	1st to 10th	Day	4 3 20% 38% 29% 6%	567
	11th to 20th	Day	4 4 20% 42% 25% 6%	557
	21st to end	Day	3 3 25% 40% 22% 7%	627
September	1st to 10th	Day	6% 4 24% 42% 19% 4	570
	11th to 20th	Day	4 9% 34% 34% 14% 4	565
	21st to end	Day	8% 15% 32% 25% 16% 4	360
October	1st to 10th	Day	9% 16% 30% 25% 15% 4	360
	11th to 20th	Day	13% 21% 32% 22% 10% 2	339
	21st to end	Day	15% 19% 34% 20% 10% 3	363
November	1st to 10th	Day	11% 33% 33% 17% 6% 1	330
	11th to 20th	Day	10% 21% 38% 22% 7% 2	330
	21st to end	Day	16% 28% 30% 17% 8% 1	330
December	1st to 10th	Day	17% 26% 29% 20% 7% 1	330
	11th to 20th	Day	10% 33% 36% 16% 4 1	330
	21st to end	Day	19% 34% 30% 13% 3	363
January	1st to 10th	Night	7% 23% 34% 26% 10% 1	943
	11th to 20th	Night	7% 20% 37% 25% 9% 1	840
	21st to end	Night	6% 21% 38% 26% 9%	924
February	1st to 10th	Night	6% 22% 39% 27% 5%	840
	11th to 20th	Night	8% 25% 38% 24% 5%	824
	21st to end	Night	8% 26% 45% 18% 2	692
March	1st to 10th	Night	6% 24% 38% 26% 6%	840
	11th to 20th	Night	11% 20% 31% 30% 9%	840
	21st to end	Night	6% 21% 41% 27% 6%	809
April	1st to 10th	Night	8% 21% 38% 27% 5%	630
	11th to 20th	Night	8% 20% 37% 29% 5%	623
	21st to end	Night	7% 20% 37% 30% 5%	630
May	1st to 10th	Night	7% 15% 35% 34% 9%	630
	11th to 20th	Night	8% 12% 31% 33% 15% 1	621
	21st to end	Night	6% 11% 35% 36% 11% 1	683
June	1st to 10th	Night	4 6% 25% 45% 19%	630
	11th to 20th	Night	3 5% 30% 44% 17%	630
	21st to end	Night	7% 4 24% 39% 26% 1	630
July	1st to 10th	Night	7% 3 10% 44% 33% 3	615
	11th to 20th	Night	3 1 6% 36% 49% 5%	612
	21st to end	Night	6% 1 7% 33% 47% 6%	690
August	1st to 10th	Night	5% 2 8% 32% 45% 7%	627
	11th to 20th	Night	4 2 10% 41% 38% 4	624
	21st to end	Night	4 2 10% 42% 37% 5%	693
September	1st to 10th	Night	5% 3 14% 41% 33% 4	630
	11th to 20th	Night	8% 3 16% 47% 24% 2	630
	21st to end	Night	7% 5% 24% 42% 20% 2	630
October	1st to 10th	Night	8% 6% 22% 43% 18% 2	829
	11th to 20th	Night	7% 11% 31% 35% 12% 2	812
	21st to end	Night	6% 14% 33% 29% 15% 3	880
November	1st to 10th	Night	6% 13% 35% 33% 12% 1	906
	11th to 20th	Night	4 17% 36% 31% 10% 2	1050
	21st to end	Night	8% 20% 34% 27% 10% 2	1050
December	1st to 10th	Night	8% 23% 35% 27% 7% 1	1050
	11th to 20th	Night	6% 26% 36% 23% 8%	1050
	21st to end	Night	10% 25% 40% 22% 3	1155

Page generated at 2026-10-11 03:06:33. Version ac07

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