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 2024-11-02.

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 4 1 4 3 4 4 4 6% 8% 59%	100904
	Night	3 4 1 4 3 4 4 3 5% 7% 60%	98316
Transparency	Day	21% 5% 9% 20% 27% 17%	100661
	Night	22% 5% 10% 21% 27% 16%	98149
Seeing	Day	9% 18% 29% 26% 14% 3	15139
	Night	6% 14% 30% 32% 16% 2	25210

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 5% 2 5% 3 3 4 3 5% 7% 62%	2728
	11th to 20th	Day	2 3 1 2 3 2 2 3 4 5% 73%	2860
	21st to end	Day	3 6% 2 5% 3 4 4 4 6% 8% 55%	3146
February	1st to 10th	Day	2 5% 3 4 3 3 3 3 5% 7% 63%	1779
	11th to 20th	Day	2 4 1 4 3 3 3 3 5% 8% 63%	1849
	21st to end	Day	1 5% 2 4 3 3 3 3 5% 6% 65%	1570
March	1st to 10th	Day	1 5% 1 3 2 2 3 3 5% 7% 69%	3219
	11th to 20th	Day	5% 9% 3 6% 4 3 3 3 5% 7% 53%	3490
	21st to end	Day	2 5% 1 4 3 3 4 3 6% 8% 62%	4000
April	1st to 10th	Day	3 3 1 3 2 3 3 4 6% 8% 65%	2280
	11th to 20th	Day	3 4 1 4 3 3 3 3 5% 7% 63%	2251
	21st to end	Day	3 3 1 3 3 2 3 3 5% 8% 65%	2280
May	1st to 10th	Day	2 3 1 3 3 3 3 3 4 6% 70%	3800
	11th to 20th	Day	7% 4 1 4 3 3 3 4 5% 7% 59%	3931
	21st to end	Day	2 2 1 2 2 2 2 2 4 6% 74%	4825
June	1st to 10th	Day	2 2 1 4 3 3 5% 5% 7% 10% 59%	2660
	11th to 20th	Day	2 2 1 3 3 3 4 3 6% 6% 68%	2660
	21st to end	Day	6% 3 1 5% 4 5% 5% 5% 6% 7% 53%	2660
July	1st to 10th	Day	3 3 2 5% 5% 5% 6% 6% 8% 8% 48%	4430
	11th to 20th	Day	2 2 2 5% 6% 7% 9% 8% 10% 10% 40%	4152
	21st to end	Day	5% 4 2 6% 7% 7% 8% 7% 9% 9% 36%	4454
August	1st to 10th	Day	3 3 2 5% 5% 6% 8% 6% 9% 10% 43%	3912
	11th to 20th	Day	3 3 1 4 4 4 5% 5% 7% 10% 54%	3726
	21st to end	Day	2 3 2 7% 5% 7% 7% 6% 9% 11% 42%	4180
September	1st to 10th	Day	6% 5% 2 5% 4 5% 5% 4 6% 8% 51%	2280
	11th to 20th	Day	6% 3 2 4 3 4 5% 4 5% 8% 55%	2275
	21st to end	Day	4 2 2 3 2 2 3 3 4 7% 67%	2090
October	1st to 10th	Day	8% 4 2 5% 3 3 3 3 4 7% 59%	1906
	11th to 20th	Day	4 3 1 2 2 2 2 2 3 4 75%	1900
	21st to end	Day	4 3 1 3 2 3 2 2 4 5% 73%	2090
November	1st to 10th	Day	2 2 1 2 2 2 3 2 5% 8% 72%	1770
	11th to 20th	Day	1 3 2 3 1 2 3 2 6% 7% 70%	1710
	21st to end	Day	4 5% 1 3 3 3 3 3 5% 6% 65%	1710
December	1st to 10th	Day	2 4 1 3 2 3 4 3 5% 8% 65%	1611
	11th to 20th	Day	3 3 1 5% 3 4 4 4 5% 7% 61%	1926
	21st to end	Day	2 4 2 3 2 3 3 3 4 8% 66%	2794
January	1st to 10th	Night	1 5% 2 5% 4 4 4 3 5% 6% 61%	4256
	11th to 20th	Night	2 3 1 3 3 3 3 3 3 5% 71%	4110
	21st to end	Night	2 4 1 4 3 4 5% 4 6% 9% 57%	4521
February	1st to 10th	Night	2 6% 2 4 3 2 3 2 5% 8% 64%	2399
	11th to 20th	Night	3 5% 1 4 3 3 3 3 4 8% 63%	2218
	21st to end	Night	2 5% 1 3 2 2 2 2 5% 7% 67%	1884
March	1st to 10th	Night	1 4 1 3 3 2 2 3 4 7% 70%	3645
	11th to 20th	Night	7% 9% 3 5% 3 3 3 3 5% 6% 53%	3490
	21st to end	Night	2 4 1 4 3 4 3 3 5% 6% 65%	3647
April	1st to 10th	Night	3 3 1 2 2 3 3 3 4 7% 69%	1900
	11th to 20th	Night	3 4 2 4 3 2 3 2 4 6% 66%	1877
	21st to end	Night	3 3 1 3 2 3 3 3 5% 6% 68%	1900
May	1st to 10th	Night	4 4 1 3 3 3 3 3 5% 6% 67%	3170
	11th to 20th	Night	7% 5% 2 5% 4 4 4 3 6% 8% 52%	2968
	21st to end	Night	3 2 1 3 2 2 3 3 3 5% 73%	2762
June	1st to 10th	Night	3 3 1 4 4 4 5% 5% 8% 9% 54%	1520
	11th to 20th	Night	3 2 1 3 2 3 3 3 5% 7% 68%	1520
	21st to end	Night	6% 3 2 4 5% 5% 5% 5% 6% 10% 49%	1520
July	1st to 10th	Night	7% 6% 3 6% 5% 5% 6% 4 6% 7% 46%	2540
	11th to 20th	Night	3 3 1 4 5% 6% 6% 6% 10% 14% 42%	2813
	21st to end	Night	6% 6% 2 5% 3 5% 5% 5% 8% 10% 43%	3109
August	1st to 10th	Night	4 4 2 7% 7% 8% 8% 7% 8% 12% 34%	3022
	11th to 20th	Night	3 3 2 5% 5% 5% 6% 5% 9% 11% 46%	3113
	21st to end	Night	3 5% 2 7% 6% 7% 7% 6% 10% 13% 34%	3487
September	1st to 10th	Night	5% 4 2 4 5% 4 5% 4 8% 7% 53%	1900
	11th to 20th	Night	6% 4 1 5% 3 4 4 4 5% 8% 56%	1905
	21st to end	Night	5% 3 1 3 3 3 3 4 5% 6% 64%	2090
October	1st to 10th	Night	8% 4 2 3 2 3 3 3 4 5% 62%	2270
	11th to 20th	Night	4 2 1 3 2 2 2 1 3 5% 75%	2280
	21st to end	Night	4 3 1 2 2 2 2 2 4 5% 72%	2508
November	1st to 10th	Night	2 3 1 2 2 1 2 2 4 5% 76%	2321
	11th to 20th	Night	1 2 1 2 2 2 3 3 4 5% 74%	2470
	21st to end	Night	5% 4 1 2 2 3 3 3 4 6% 65%	2470
December	1st to 10th	Night	3 3 1 4 2 3 3 2 5% 8% 66%	2567
	11th to 20th	Night	2 3 2 3 3 4 4 4 5% 7% 62%	3271
	21st to end	Night	3 4 1 4 3 3 3 2 5% 6% 65%	4873

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	21% 3 7% 25% 34% 9%	2729
	11th to 20th	Day	12% 4 8% 31% 37% 7%	2860
	21st to end	Day	25% 5% 9% 20% 32% 10%	3146
February	1st to 10th	Day	21% 3 7% 21% 32% 15%	1779
	11th to 20th	Day	20% 3 7% 23% 28% 19%	1859
	21st to end	Day	18% 3 5% 16% 35% 23%	1570
March	1st to 10th	Day	15% 2 5% 13% 39% 26%	3219
	11th to 20th	Day	30% 3 7% 20% 31% 9%	3490
	21st to end	Day	17% 3 8% 20% 38% 13%	4000
April	1st to 10th	Day	16% 3 7% 20% 33% 21%	2280
	11th to 20th	Day	20% 4 6% 19% 28% 24%	2254
	21st to end	Day	16% 3 8% 22% 36% 15%	2280
May	1st to 10th	Day	15% 5% 7% 15% 29% 29%	3800
	11th to 20th	Day	24% 5% 9% 15% 28% 19%	3935
	21st to end	Day	12% 4 6% 14% 20% 45%	4825
June	1st to 10th	Day	18% 6% 12% 21% 23% 21%	2660
	11th to 20th	Day	16% 4 8% 13% 22% 38%	2660
	21st to end	Day	26% 6% 11% 17% 20% 19%	2660
July	1st to 10th	Day	28% 10% 14% 15% 13% 21%	4361
	11th to 20th	Day	28% 11% 17% 27% 12% 5%	4081
	21st to end	Day	34% 11% 14% 28% 10% 3	4314
August	1st to 10th	Day	30% 9% 19% 21% 13% 8%	3898
	11th to 20th	Day	22% 8% 9% 25% 24% 12%	3762
	21st to end	Day	30% 10% 16% 23% 18% 3	4179
September	1st to 10th	Day	29% 7% 11% 20% 24% 10%	2280
	11th to 20th	Day	25% 6% 9% 19% 22% 19%	2275
	21st to end	Day	18% 4 7% 22% 33% 18%	2090
October	1st to 10th	Day	25% 4 7% 17% 26% 20%	1906
	11th to 20th	Day	15% 2 4 14% 32% 32%	1900
	21st to end	Day	16% 3 5% 19% 36% 22%	2090
November	1st to 10th	Day	12% 3 5% 19% 40% 21%	1770
	11th to 20th	Day	14% 2 6% 23% 39% 15%	1710
	21st to end	Day	20% 3 4 18% 32% 23%	1710
December	1st to 10th	Day	18% 3 9% 29% 28% 13%	1611
	11th to 20th	Day	21% 3 9% 22% 29% 15%	1924
	21st to end	Day	17% 2 6% 19% 33% 22%	2794
January	1st to 10th	Night	22% 3 8% 24% 33% 10%	4256
	11th to 20th	Night	17% 4 7% 30% 32% 12%	4110
	21st to end	Night	20% 5% 10% 24% 32% 9%	4521
February	1st to 10th	Night	19% 3 9% 23% 30% 16%	2399
	11th to 20th	Night	19% 4 7% 23% 29% 17%	2218
	21st to end	Night	17% 3 4 17% 34% 25%	1884
March	1st to 10th	Night	15% 2 6% 14% 39% 24%	3645
	11th to 20th	Night	31% 4 6% 23% 28% 9%	3490
	21st to end	Night	18% 4 7% 22% 37% 12%	3647
April	1st to 10th	Night	16% 3 6% 19% 32% 23%	1900
	11th to 20th	Night	19% 4 6% 18% 28% 25%	1877
	21st to end	Night	16% 4 8% 22% 34% 15%	1900
May	1st to 10th	Night	18% 4 8% 13% 28% 30%	3170
	11th to 20th	Night	27% 7% 11% 22% 19% 15%	2969
	21st to end	Night	14% 3 9% 12% 15% 47%	2762
June	1st to 10th	Night	23% 8% 15% 16% 18% 20%	1520
	11th to 20th	Night	15% 3 9% 16% 19% 38%	1520
	21st to end	Night	29% 6% 14% 16% 16% 19%	1520
July	1st to 10th	Night	36% 7% 12% 16% 13% 16%	2500
	11th to 20th	Night	25% 10% 22% 30% 8% 4	2761
	21st to end	Night	32% 8% 18% 28% 13% 1	3011
August	1st to 10th	Night	38% 9% 17% 17% 13% 6%	3013
	11th to 20th	Night	26% 7% 15% 24% 20% 8%	3143
	21st to end	Night	35% 9% 19% 23% 9% 4	3487
September	1st to 10th	Night	26% 7% 15% 23% 20% 8%	1900
	11th to 20th	Night	26% 5% 10% 18% 23% 18%	1905
	21st to end	Night	20% 4 9% 19% 31% 18%	2090
October	1st to 10th	Night	24% 5% 7% 19% 22% 23%	2270
	11th to 20th	Night	15% 2 4 14% 32% 32%	2280
	21st to end	Night	16% 2 6% 19% 35% 22%	2508
November	1st to 10th	Night	12% 2 5% 21% 38% 21%	2321
	11th to 20th	Night	12% 3 5% 22% 40% 17%	2470
	21st to end	Night	20% 3 6% 18% 30% 23%	2470
December	1st to 10th	Night	18% 3 8% 28% 29% 13%	2567
	11th to 20th	Night	21% 4 7% 22% 32% 15%	3272
	21st to end	Night	19% 3 7% 23% 31% 17%	4873

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	15% 27% 33% 18% 7%	270
	11th to 20th	Day	16% 31% 30% 18% 5% 1	270
	21st to end	Day	3 25% 40% 20% 9% 2	297
February	1st to 10th	Day	9% 36% 33% 19% 3	270
	11th to 20th	Day	9% 40% 31% 17% 4	270
	21st to end	Day	21% 38% 28% 11% 2	225
March	1st to 10th	Day	10% 36% 33% 17% 4	320
	11th to 20th	Day	18% 30% 30% 17% 5% 1	460
	21st to end	Day	17% 36% 32% 12% 3 1	506
April	1st to 10th	Day	12% 33% 36% 14% 5% 1	460
	11th to 20th	Day	13% 38% 34% 13% 2	452
	21st to end	Day	11% 32% 40% 15% 3	460
May	1st to 10th	Day	9% 31% 40% 17% 3 1	463
	11th to 20th	Day	9% 17% 37% 26% 9% 2	571
	21st to end	Day	7% 16% 37% 28% 10% 1	734
June	1st to 10th	Day	4 9% 28% 36% 20% 3	680
	11th to 20th	Day	5% 12% 28% 35% 18% 2	680
	21st to end	Day	7% 8% 26% 33% 22% 4	680
July	1st to 10th	Day	6% 3 19% 34% 31% 7%	671
	11th to 20th	Day	5% 1 11% 38% 35% 9%	511
	21st to end	Day	6% 1 10% 34% 39% 10%	531
August	1st to 10th	Day	5% 3 18% 38% 31% 6%	487
	11th to 20th	Day	4 4 22% 40% 25% 6%	477
	21st to end	Day	4 3 23% 40% 23% 7%	539
September	1st to 10th	Day	7% 3 22% 42% 20% 4	490
	11th to 20th	Day	5% 7% 33% 36% 15% 4	485
	21st to end	Day	6% 13% 30% 28% 18% 4	300
October	1st to 10th	Day	10% 16% 30% 23% 16% 4	300
	11th to 20th	Day	10% 19% 33% 25% 11% 3	300
	21st to end	Day	15% 19% 33% 20% 11% 3	330
November	1st to 10th	Day	9% 33% 33% 18% 6% 1	273
	11th to 20th	Day	6% 20% 39% 25% 9% 2	270
	21st to end	Day	14% 27% 30% 18% 10% 1	270
December	1st to 10th	Day	11% 29% 31% 21% 7% 1	270
	11th to 20th	Day	8% 33% 38% 17% 4	270
	21st to end	Day	16% 36% 31% 13% 4	297
January	1st to 10th	Night	4 22% 34% 27% 11% 1	854
	11th to 20th	Night	7% 20% 37% 26% 9% 2	760
	21st to end	Night	5% 20% 38% 27% 10%	836
February	1st to 10th	Night	6% 22% 39% 28% 6%	760
	11th to 20th	Night	7% 25% 38% 25% 6%	744
	21st to end	Night	8% 26% 45% 19% 2	628
March	1st to 10th	Night	5% 23% 39% 26% 6% 1	760
	11th to 20th	Night	9% 18% 31% 33% 9%	760
	21st to end	Night	5% 21% 40% 27% 6%	732
April	1st to 10th	Night	7% 23% 39% 27% 5%	570
	11th to 20th	Night	8% 18% 37% 31% 6%	563
	21st to end	Night	6% 20% 37% 31% 6% 1	570
May	1st to 10th	Night	5% 16% 35% 34% 9%	570
	11th to 20th	Night	7% 10% 31% 35% 16% 1	561
	21st to end	Night	5% 12% 35% 37% 10% 1	617
June	1st to 10th	Night	4 6% 24% 45% 21%	570
	11th to 20th	Night	4 5% 29% 45% 16% 1	570
	21st to end	Night	6% 3 22% 39% 28% 1	570
July	1st to 10th	Night	8% 2 10% 42% 35% 3	555
	11th to 20th	Night	3 1 6% 35% 49% 5%	552
	21st to end	Night	6% 1 7% 30% 48% 7%	624
August	1st to 10th	Night	6% 2 7% 31% 47% 8%	567
	11th to 20th	Night	4 2 11% 39% 39% 5%	564
	21st to end	Night	4 1 9% 42% 38% 6%	627
September	1st to 10th	Night	5% 3 14% 41% 33% 4	570
	11th to 20th	Night	8% 3 15% 47% 25% 2	570
	21st to end	Night	6% 5% 23% 43% 22% 2	570
October	1st to 10th	Night	8% 7% 23% 43% 18% 2	750
	11th to 20th	Night	6% 11% 31% 37% 13% 3	760
	21st to end	Night	6% 13% 32% 29% 16% 3	836
November	1st to 10th	Night	4 12% 34% 34% 13% 2	825
	11th to 20th	Night	3 16% 36% 33% 11% 2	950
	21st to end	Night	7% 18% 34% 28% 11% 2	950
December	1st to 10th	Night	7% 21% 36% 28% 7% 1	950
	11th to 20th	Night	5% 25% 36% 24% 8%	950
	21st to end	Night	9% 24% 40% 23% 4	1045

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