Grand Cosmic Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grand Cosmic Observatory clear sky chart from 2005-11-06 to 2025-04-26.

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	3 10% 4 9% 5% 5% 4 4 5% 7% 44%	118372
	Night	3 10% 4 10% 5% 4 4 3 5% 7% 45%	118965
Transparency	Day	37% 8% 12% 20% 16% 7%	118118
	Night	37% 8% 13% 22% 15% 5%	118808
Seeing	Day	11% 18% 32% 26% 11% 2	16193
	Night	10% 10% 27% 37% 15% 1	25934

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 13% 7% 18% 7% 5% 4 4 6% 7% 28%	3542
	11th to 20th	Day	1 10% 5% 17% 6% 5% 4 3 5% 6% 39%	4100
	21st to end	Day	1 14% 6% 12% 5% 4 4 3 5% 6% 40%	4510
February	1st to 10th	Day	3 19% 8% 15% 8% 5% 5% 3 6% 6% 21%	1800
	11th to 20th	Day	2 17% 8% 14% 8% 6% 5% 4 5% 6% 25%	1755
	21st to end	Day	2 18% 7% 18% 8% 7% 5% 4 4 5% 22%	1587
March	1st to 10th	Day	2 15% 7% 14% 5% 5% 4 3 5% 5% 35%	4881
	11th to 20th	Day	2 13% 5% 12% 5% 5% 4 4 5% 6% 38%	5000
	21st to end	Day	2 20% 7% 14% 7% 7% 6% 4 6% 6% 21%	5486
April	1st to 10th	Day	6% 16% 6% 10% 7% 5% 5% 4 5% 6% 29%	2305
	11th to 20th	Day	5% 14% 5% 12% 7% 7% 4 4 5% 7% 30%	2510
	21st to end	Day	6% 15% 5% 11% 6% 5% 5% 5% 6% 7% 29%	2537
May	1st to 10th	Day	8% 15% 4 10% 5% 6% 5% 5% 6% 7% 29%	4110
	11th to 20th	Day	5% 5% 2 7% 4 5% 5% 5% 7% 8% 46%	4197
	21st to end	Day	7% 13% 5% 10% 5% 6% 6% 4 5% 6% 34%	4825
June	1st to 10th	Day	9% 7% 3 7% 5% 4 5% 5% 7% 8% 40%	2660
	11th to 20th	Day	8% 7% 2 7% 4 5% 5% 4 7% 7% 44%	2831
	21st to end	Day	3 2 1 3 3 4 4 3 6% 7% 64%	2850
July	1st to 10th	Day	1 2 1 2 2 3 3 4 5% 7% 69%	4740
	11th to 20th	Day	1 2 1 3 4 3 5% 4 6% 8% 63%	4339
	21st to end	Day	3 3 2 4 5% 4 5% 4 7% 7% 58%	4454
August	1st to 10th	Day	3 3 1 4 4 4 5% 4 7% 9% 56%	4092
	11th to 20th	Day	2 2 1 3 3 4 4 3 6% 7% 64%	4028
	21st to end	Day	3 2 1 3 3 3 3 3 5% 6% 67%	4469
September	1st to 10th	Day	4 4 1 5% 4 4 3 3 5% 5% 61%	2267
	11th to 20th	Day	5% 6% 2 6% 4 5% 5% 4 5% 7% 54%	2090
	21st to end	Day	6% 5% 1 6% 3 4 3 3 3 6% 59%	2090
October	1st to 10th	Day	5% 6% 2 5% 3 4 3 2 3 5% 63%	3216
	11th to 20th	Day	4 8% 3 5% 3 3 3 3 4 4 59%	3150
	21st to end	Day	3 9% 3 8% 4 5% 5% 4 7% 9% 44%	3392
November	1st to 10th	Day	8% 13% 5% 9% 5% 4 4 3 5% 6% 38%	1739
	11th to 20th	Day	3 15% 5% 11% 6% 5% 4 3 5% 7% 36%	1600
	21st to end	Day	1 12% 6% 14% 5% 5% 5% 3 5% 5% 39%	1600
December	1st to 10th	Day	2 12% 4 9% 5% 5% 4 3 3 4 50%	2620
	11th to 20th	Day	3 14% 8% 16% 7% 5% 5% 4 5% 6% 27%	3026
	21st to end	Day	4 16% 6% 13% 6% 5% 5% 5% 6% 7% 28%	3974
January	1st to 10th	Night	1 8% 6% 19% 7% 6% 6% 4 5% 7% 33%	6279
	11th to 20th	Night	1 10% 6% 18% 8% 5% 5% 3 5% 6% 33%	5880
	21st to end	Night	1 16% 6% 11% 4 4 4 2 4 5% 42%	6468
February	1st to 10th	Night	3 16% 7% 16% 7% 6% 5% 4 5% 6% 24%	2600
	11th to 20th	Night	2 15% 9% 13% 8% 6% 5% 3 5% 6% 29%	2532
	21st to end	Night	3 13% 7% 19% 9% 5% 4 4 5% 5% 26%	2040
March	1st to 10th	Night	1 16% 6% 16% 5% 4 3 3 3 4 38%	5011
	11th to 20th	Night	3 12% 5% 9% 5% 4 5% 4 5% 7% 43%	5000
	21st to end	Night	3 13% 7% 12% 6% 6% 5% 5% 6% 6% 30%	5489
April	1st to 10th	Night	4 13% 5% 9% 5% 5% 5% 4 6% 8% 35%	2095
	11th to 20th	Night	5% 11% 4 9% 6% 5% 5% 5% 5% 7% 38%	1835
	21st to end	Night	5% 11% 4 7% 5% 4 4 4 6% 8% 42%	1759
May	1st to 10th	Night	8% 15% 4 8% 4 4 4 4 6% 7% 36%	2860
	11th to 20th	Night	4 4 2 5% 4 4 4 4 7% 10% 53%	2708
	21st to end	Night	6% 11% 2 6% 5% 5% 4 4 6% 8% 42%	2762
June	1st to 10th	Night	8% 6% 2 6% 4 4 5% 3 8% 8% 46%	1520
	11th to 20th	Night	9% 5% 2 5% 4 3 3 3 6% 8% 53%	1343
	21st to end	Night	2 1 3 2 3 4 4 4 7% 70%	1330
July	1st to 10th	Night	1 1 1 2 3 3 4 3 6% 7% 71%	2220
	11th to 20th	Night	1 1 1 2 2 3 4 6% 8% 71%	2619
	21st to end	Night	1 2 1 2 2 3 4 4 6% 9% 66%	3109
August	1st to 10th	Night	2 2 1 4 3 3 5% 4 6% 8% 62%	2847
	11th to 20th	Night	2 2 1 4 3 3 5% 3 6% 7% 64%	2811
	21st to end	Night	3 2 1 3 2 2 3 2 4 5% 73%	3198
September	1st to 10th	Night	4 3 1 3 2 3 3 3 5% 6% 66%	1909
	11th to 20th	Night	4 4 2 4 3 5% 5% 3 5% 7% 57%	2090
	21st to end	Night	8% 5% 2 3 2 3 3 2 3 5% 65%	2090
October	1st to 10th	Night	5% 5% 1 4 3 3 2 2 3 4 67%	3755
	11th to 20th	Night	6% 7% 2 5% 4 4 4 3 4 6% 56%	3778
	21st to end	Night	4 10% 3 7% 4 4 4 3 6% 8% 49%	4163
November	1st to 10th	Night	7% 13% 4 9% 5% 4 4 4 5% 7% 40%	2540
	11th to 20th	Night	4 13% 4 11% 6% 4 4 5% 6% 7% 37%	2800
	21st to end	Night	3 11% 5% 10% 6% 4 4 4 5% 7% 41%	2800
December	1st to 10th	Night	2 10% 4 10% 5% 5% 4 3 4 5% 48%	4570
	11th to 20th	Night	2 14% 6% 15% 7% 6% 5% 4 5% 7% 30%	5181
	21st to end	Night	3 16% 6% 14% 6% 5% 5% 4 6% 7% 28%	6974

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	53% 10% 12% 16% 9%	3542
	11th to 20th	Day	42% 8% 18% 18% 10% 4	4100
	21st to end	Day	44% 8% 10% 21% 15% 2	4510
February	1st to 10th	Day	63% 9% 12% 12% 4 1	1800
	11th to 20th	Day	58% 10% 13% 12% 5% 2	1764
	21st to end	Day	62% 9% 8% 14% 6% 2	1587
March	1st to 10th	Day	48% 8% 12% 17% 11% 4	4881
	11th to 20th	Day	44% 8% 15% 24% 8% 1	5000
	21st to end	Day	59% 11% 14% 13% 2	5486
April	1st to 10th	Day	54% 9% 13% 15% 8% 1	2305
	11th to 20th	Day	52% 9% 11% 18% 9% 1	2513
	21st to end	Day	50% 10% 13% 21% 5%	2537
May	1st to 10th	Day	48% 11% 12% 20% 7% 1	4110
	11th to 20th	Day	30% 11% 13% 30% 13% 4	4201
	21st to end	Day	45% 11% 10% 25% 8%	4825
June	1st to 10th	Day	38% 9% 17% 24% 11% 1	2660
	11th to 20th	Day	36% 8% 10% 20% 17% 8%	2831
	21st to end	Day	17% 7% 9% 24% 32% 12%	2850
July	1st to 10th	Day	12% 4 9% 14% 37% 24%	4666
	11th to 20th	Day	16% 4 7% 14% 25% 34%	4261
	21st to end	Day	21% 6% 7% 18% 33% 15%	4314
August	1st to 10th	Day	21% 6% 8% 17% 31% 17%	4078
	11th to 20th	Day	18% 5% 8% 23% 33% 12%	4067
	21st to end	Day	16% 5% 7% 21% 32% 19%	4468
September	1st to 10th	Day	24% 5% 8% 17% 33% 12%	2267
	11th to 20th	Day	28% 7% 9% 23% 24% 8%	2090
	21st to end	Day	27% 5% 13% 27% 24% 5%	2090
October	1st to 10th	Day	26% 5% 7% 21% 33% 8%	3216
	11th to 20th	Day	29% 5% 10% 36% 16% 4	3150
	21st to end	Day	34% 9% 18% 28% 11% 1	3392
November	1st to 10th	Day	45% 8% 15% 21% 8% 2	1739
	11th to 20th	Day	47% 8% 19% 17% 8% 1	1600
	21st to end	Day	47% 9% 17% 19% 9% 1	1600
December	1st to 10th	Day	38% 6% 27% 21% 7% 2	2620
	11th to 20th	Day	56% 9% 20% 13% 3	3024
	21st to end	Day	50% 10% 14% 14% 7% 6%	3974
January	1st to 10th	Night	47% 11% 12% 21% 8%	6279
	11th to 20th	Night	49% 9% 13% 18% 8% 3	5880
	21st to end	Night	44% 5% 14% 21% 14% 2	6468
February	1st to 10th	Night	61% 11% 11% 11% 6%	2600
	11th to 20th	Night	56% 9% 11% 15% 8% 1	2533
	21st to end	Night	58% 9% 11% 14% 6% 2	2040
March	1st to 10th	Night	48% 8% 10% 18% 11% 4	5011
	11th to 20th	Night	38% 9% 15% 27% 10% 1	5000
	21st to end	Night	47% 15% 16% 20% 2	5489
April	1st to 10th	Night	43% 13% 16% 17% 10% 1	2095
	11th to 20th	Night	43% 11% 16% 21% 8% 1	1835
	21st to end	Night	37% 10% 17% 28% 8%	1759
May	1st to 10th	Night	43% 10% 14% 22% 11% 1	2860
	11th to 20th	Night	22% 9% 20% 34% 12% 3	2709
	21st to end	Night	34% 13% 13% 29% 10% 1	2762
June	1st to 10th	Night	33% 10% 19% 25% 12% 1	1520
	11th to 20th	Night	28% 7% 13% 25% 20% 8%	1343
	21st to end	Night	13% 5% 11% 24% 33% 14%	1330
July	1st to 10th	Night	11% 4 9% 16% 42% 18%	2185
	11th to 20th	Night	9% 3 6% 18% 35% 29%	2574
	21st to end	Night	12% 7% 8% 23% 33% 17%	3011
August	1st to 10th	Night	17% 5% 9% 18% 36% 15%	2838
	11th to 20th	Night	18% 6% 9% 27% 26% 14%	2838
	21st to end	Night	14% 4 7% 23% 36% 16%	3198
September	1st to 10th	Night	18% 6% 8% 19% 32% 17%	1909
	11th to 20th	Night	24% 7% 12% 23% 25% 9%	2090
	21st to end	Night	24% 4 10% 29% 28% 6%	2090
October	1st to 10th	Night	21% 3 9% 27% 34% 5%	3755
	11th to 20th	Night	29% 6% 9% 37% 17% 3	3778
	21st to end	Night	31% 9% 15% 30% 14% 1	4163
November	1st to 10th	Night	43% 9% 14% 22% 9% 2	2540
	11th to 20th	Night	44% 9% 17% 21% 7% 1	2800
	21st to end	Night	43% 9% 17% 21% 10% 2	2800
December	1st to 10th	Night	37% 7% 27% 22% 6% 1	4570
	11th to 20th	Night	53% 10% 20% 14% 3	5182
	21st to end	Night	51% 9% 15% 15% 7% 3	6974

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	12% 23% 27% 27% 10% 1	254
	11th to 20th	Day	5% 16% 39% 25% 11% 3	300
	21st to end	Day	7% 16% 35% 32% 10% 1	330
February	1st to 10th	Day	19% 31% 33% 15% 2	300
	11th to 20th	Day	14% 27% 30% 23% 6%	300
	21st to end	Day	14% 27% 35% 18% 6%	351
March	1st to 10th	Day	14% 29% 33% 18% 4 1	500
	11th to 20th	Day	12% 25% 33% 23% 7%	500
	21st to end	Day	16% 27% 34% 19% 4	550
April	1st to 10th	Day	18% 25% 30% 20% 6% 1	500
	11th to 20th	Day	17% 26% 36% 16% 4 1	492
	21st to end	Day	17% 23% 29% 20% 9% 2	481
May	1st to 10th	Day	13% 22% 31% 25% 7% 2	463
	11th to 20th	Day	15% 15% 30% 24% 13% 2	562
	21st to end	Day	17% 13% 26% 26% 16% 3	734
June	1st to 10th	Day	9% 17% 26% 29% 16% 3	680
	11th to 20th	Day	14% 19% 33% 24% 9%	680
	21st to end	Day	6% 12% 32% 31% 17% 3	680
July	1st to 10th	Day	3 8% 31% 38% 17% 2	671
	11th to 20th	Day	4 11% 39% 31% 14% 2	575
	21st to end	Day	3 10% 32% 38% 15% 3	531
August	1st to 10th	Day	5% 10% 37% 32% 15% 2	487
	11th to 20th	Day	4 9% 35% 29% 19% 4	477
	21st to end	Day	6% 14% 31% 29% 15% 5%	539
September	1st to 10th	Day	5% 12% 33% 28% 17% 4	490
	11th to 20th	Day	12% 18% 29% 26% 12% 4	490
	21st to end	Day	9% 13% 30% 29% 15% 3	490
October	1st to 10th	Day	10% 14% 29% 25% 19% 3	333
	11th to 20th	Day	10% 18% 23% 25% 17% 7%	300
	21st to end	Day	15% 18% 32% 28% 6% 1	330
November	1st to 10th	Day	15% 22% 29% 21% 9% 3	300
	11th to 20th	Day	11% 21% 35% 22% 9% 2	300
	21st to end	Day	7% 14% 39% 29% 10% 1	300
December	1st to 10th	Day	14% 15% 30% 29% 12% 1	300
	11th to 20th	Day	16% 20% 33% 24% 6%	300
	21st to end	Day	15% 20% 31% 22% 10% 2	323
January	1st to 10th	Night	9% 14% 30% 36% 10% 1	965
	11th to 20th	Night	9% 12% 33% 33% 11% 2	800
	21st to end	Night	8% 8% 37% 36% 10% 1	880
February	1st to 10th	Night	10% 17% 35% 32% 6% 1	800
	11th to 20th	Night	10% 15% 32% 34% 9% 1	784
	21st to end	Night	10% 14% 37% 30% 8% 1	660
March	1st to 10th	Night	8% 18% 32% 33% 9% 1	800
	11th to 20th	Night	8% 12% 30% 37% 12% 1	800
	21st to end	Night	10% 13% 32% 33% 11%	880
April	1st to 10th	Night	11% 14% 30% 33% 11% 1	695
	11th to 20th	Night	12% 11% 34% 33% 9%	593
	21st to end	Night	11% 10% 26% 36% 17% 1	587
May	1st to 10th	Night	12% 11% 23% 36% 17% 1	570
	11th to 20th	Night	12% 6% 23% 36% 21% 2	561
	21st to end	Night	14% 8% 20% 38% 19% 1	617
June	1st to 10th	Night	12% 8% 22% 34% 22% 1	570
	11th to 20th	Night	10% 8% 25% 42% 14%	393
	21st to end	Night	3 1 18% 48% 29% 2	380
July	1st to 10th	Night	1 1 18% 45% 32% 2	370
	11th to 20th	Night	3 2 16% 51% 28%	507
	21st to end	Night	2 1 13% 50% 31% 2	624
August	1st to 10th	Night	5% 2 14% 47% 29% 2	567
	11th to 20th	Night	3 2 13% 49% 31% 2	564
	21st to end	Night	4 4 19% 38% 32% 3	627
September	1st to 10th	Night	6% 2 16% 44% 29% 3	579
	11th to 20th	Night	7% 4 18% 39% 28% 4	760
	21st to end	Night	11% 3 19% 38% 24% 4	760
October	1st to 10th	Night	12% 6% 23% 37% 20% 2	760
	11th to 20th	Night	13% 8% 19% 38% 20% 2	760
	21st to end	Night	9% 9% 25% 43% 11% 3	836
November	1st to 10th	Night	14% 12% 27% 35% 11% 2	785
	11th to 20th	Night	12% 14% 34% 34% 6% 1	1000
	21st to end	Night	10% 11% 32% 35% 11% 2	1000
December	1st to 10th	Night	13% 11% 33% 35% 7% 1	1000
	11th to 20th	Night	14% 14% 30% 35% 7%	1000
	21st to end	Night	14% 15% 32% 29% 9% 1	1100

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