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 2024-02-25.

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	5% 10% 5% 9% 7% 4 5% 5% 5% 7% 40%	78912
	Night	4 10% 5% 9% 7% 3 4 5% 5% 7% 41%	80298
Transparency	Day	43% 8% 12% 18% 14% 5%	78618
	Night	43% 8% 13% 19% 12% 4	80113
Seeing	Day	11% 17% 32% 26% 12% 2	14579
	Night	10% 10% 26% 37% 16% 2	24380

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 14% 9% 18% 12% 4 5% 4 4 5% 24%	2456
	11th to 20th	Day	2 14% 7% 16% 9% 4 5% 4 4 5% 29%	2860
	21st to end	Day	1 19% 8% 14% 8% 4 4 4 5% 5% 27%	3146
February	1st to 10th	Day	3 18% 9% 14% 9% 4 5% 4 6% 6% 23%	1710
	11th to 20th	Day	3 16% 9% 13% 9% 5% 5% 5% 4 5% 26%	1665
	21st to end	Day	2 16% 8% 18% 9% 5% 5% 6% 5% 5% 21%	1461
March	1st to 10th	Day	2 13% 8% 15% 9% 4 6% 5% 5% 6% 28%	1944
	11th to 20th	Day	4 12% 7% 11% 6% 4 5% 6% 6% 8% 31%	1980
	21st to end	Day	4 19% 8% 12% 8% 5% 6% 5% 6% 6% 21%	2178
April	1st to 10th	Day	7% 15% 7% 9% 8% 4 6% 5% 4 7% 27%	2074
	11th to 20th	Day	6% 14% 6% 11% 8% 5% 5% 5% 5% 7% 27%	2255
	21st to end	Day	6% 14% 6% 10% 7% 4 6% 6% 5% 7% 30%	2340
May	1st to 10th	Day	8% 9% 5% 9% 7% 4 6% 6% 7% 7% 32%	2340
	11th to 20th	Day	8% 7% 2 7% 7% 5% 6% 7% 8% 10% 33%	2371
	21st to end	Day	11% 12% 5% 10% 8% 5% 6% 6% 6% 7% 24%	2724
June	1st to 10th	Day	9% 6% 4 6% 5% 3 5% 6% 7% 8% 40%	2520
	11th to 20th	Day	9% 7% 2 6% 5% 3 5% 4 6% 8% 44%	2682
	21st to end	Day	2 2 1 3 3 3 4 4 5% 7% 64%	2700
July	1st to 10th	Day	2 2 1 3 3 3 5% 5% 6% 9% 61%	2700
	11th to 20th	Day	2 3 2 3 4 2 5% 6% 5% 7% 63%	2473
	21st to end	Day	4 3 2 4 5% 3 5% 5% 7% 8% 56%	2545
August	1st to 10th	Day	5% 4 2 4 5% 4 5% 5% 6% 7% 53%	2328
	11th to 20th	Day	2 3 1 3 4 2 4 5% 6% 7% 63%	2299
	21st to end	Day	4 3 1 3 4 3 4 5% 6% 8% 59%	2548
September	1st to 10th	Day	5% 4 1 4 5% 3 3 5% 5% 5% 60%	2148
	11th to 20th	Day	6% 5% 2 5% 4 3 4 5% 5% 7% 54%	1980
	21st to end	Day	7% 5% 1 5% 4 3 3 4 3 7% 58%	1980
October	1st to 10th	Day	9% 10% 4 6% 5% 5% 5% 3 4 7% 43%	1832
	11th to 20th	Day	7% 9% 4 6% 5% 4 4 4 5% 4 49%	1800
	21st to end	Day	5% 10% 4 8% 6% 3 4 4 5% 7% 44%	1958
November	1st to 10th	Day	9% 12% 6% 9% 5% 3 4 4 5% 7% 37%	1651
	11th to 20th	Day	4 14% 6% 11% 7% 3 5% 3 4 7% 36%	1520
	21st to end	Day	2 11% 6% 13% 7% 4 5% 4 4 5% 39%	1520
December	1st to 10th	Day	4 14% 7% 12% 10% 6% 6% 5% 3 4 29%	1520
	11th to 20th	Day	4 16% 7% 14% 8% 4 5% 5% 5% 7% 23%	1926
	21st to end	Day	3 13% 6% 11% 8% 4 5% 6% 6% 7% 33%	2778
January	1st to 10th	Night	2 10% 7% 20% 11% 4 5% 5% 5% 6% 27%	4397
	11th to 20th	Night	2 12% 9% 19% 11% 4 4 3 4 4 26%	4110
	21st to end	Night	2 22% 9% 14% 8% 4 5% 3 4 5% 26%	4521
February	1st to 10th	Night	3 15% 8% 14% 9% 5% 5% 5% 5% 6% 25%	2470
	11th to 20th	Night	3 14% 10% 12% 8% 4 5% 4 5% 5% 30%	2402
	21st to end	Night	3 12% 8% 17% 11% 4 5% 5% 5% 5% 24%	1886
March	1st to 10th	Night	2 13% 7% 14% 7% 4 5% 5% 6% 6% 31%	2014
	11th to 20th	Night	3 9% 5% 9% 6% 3 6% 6% 6% 8% 40%	1980
	21st to end	Night	4 13% 7% 10% 8% 4 5% 6% 7% 6% 29%	2178
April	1st to 10th	Night	4 12% 5% 9% 7% 3 5% 5% 6% 8% 34%	1886
	11th to 20th	Night	5% 10% 5% 9% 7% 4 5% 6% 5% 7% 36%	1650
	21st to end	Night	5% 11% 5% 5% 6% 3 4 5% 5% 8% 43%	1620
May	1st to 10th	Night	7% 10% 2 6% 6% 2 5% 5% 6% 9% 43%	1620
	11th to 20th	Night	7% 6% 2 4 5% 4 4 6% 7% 10% 45%	1524
	21st to end	Night	9% 10% 3 6% 7% 4 6% 6% 7% 9% 33%	1552
June	1st to 10th	Night	9% 6% 2 5% 4 3 5% 4 7% 9% 46%	1440
	11th to 20th	Night	9% 5% 2 4 5% 2 3 4 6% 8% 53%	1273
	21st to end	Night	3 1 2 2 2 4 5% 4 7% 70%	1260
July	1st to 10th	Night	1 1 1 2 3 2 4 5% 8% 10% 65%	1260
	11th to 20th	Night	2 1 1 2 2 1 4 3 5% 8% 72%	1477
	21st to end	Night	2 2 2 2 3 2 3 4 6% 7% 65%	1773
August	1st to 10th	Night	3 3 1 4 4 3 3 5% 6% 9% 58%	1611
	11th to 20th	Night	2 2 1 2 3 2 3 3 5% 8% 69%	1602
	21st to end	Night	4 3 1 3 2 3 4 4 5% 6% 66%	1808
September	1st to 10th	Night	4 3 2 3 3 3 3 4 5% 6% 65%	1808
	11th to 20th	Night	4 4 2 3 4 4 5% 4 6% 7% 58%	1980
	21st to end	Night	8% 5% 2 3 3 2 3 2 4 5% 63%	1980
October	1st to 10th	Night	9% 8% 3 6% 5% 3 4 4 5% 6% 47%	2128
	11th to 20th	Night	8% 9% 3 5% 3 3 3 3 5% 5% 53%	2160
	21st to end	Night	5% 9% 3 6% 5% 3 4 4 5% 7% 50%	2395
November	1st to 10th	Night	8% 12% 5% 8% 5% 3 4 5% 5% 7% 38%	2409
	11th to 20th	Night	5% 12% 5% 9% 7% 3 4 6% 6% 7% 37%	2660
	21st to end	Night	3 11% 6% 9% 6% 4 4 4 5% 7% 41%	2660
December	1st to 10th	Night	3 15% 7% 12% 9% 5% 6% 5% 4 5% 29%	2660
	11th to 20th	Night	4 14% 6% 14% 10% 4 5% 4 5% 5% 28%	3271
	21st to end	Night	3 15% 6% 12% 8% 4 5% 5% 5% 6% 32%	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	62% 8% 11% 15% 3 1	2456
	11th to 20th	Day	56% 9% 14% 12% 7% 3	2860
	21st to end	Day	59% 8% 11% 13% 8% 1	3146
February	1st to 10th	Day	62% 9% 12% 12% 4 1	1710
	11th to 20th	Day	59% 9% 13% 12% 5% 2	1674
	21st to end	Day	63% 9% 8% 14% 5% 2	1461
March	1st to 10th	Day	56% 10% 12% 15% 6% 1	1944
	11th to 20th	Day	49% 10% 16% 15% 8% 1	1980
	21st to end	Day	63% 9% 12% 13% 3	2178
April	1st to 10th	Day	57% 9% 12% 13% 7% 1	2074
	11th to 20th	Day	55% 9% 11% 16% 7%	2258
	21st to end	Day	51% 10% 13% 21% 5%	2340
May	1st to 10th	Day	47% 10% 13% 22% 8% 1	2340
	11th to 20th	Day	42% 11% 14% 25% 7%	2375
	21st to end	Day	55% 11% 13% 16% 5%	2724
June	1st to 10th	Day	39% 9% 17% 23% 11% 1	2520
	11th to 20th	Day	37% 8% 10% 21% 16% 8%	2682
	21st to end	Day	17% 7% 9% 24% 31% 11%	2700
July	1st to 10th	Day	17% 7% 9% 22% 31% 13%	2626
	11th to 20th	Day	20% 6% 7% 16% 26% 25%	2395
	21st to end	Day	23% 6% 9% 15% 26% 21%	2404
August	1st to 10th	Day	27% 8% 11% 17% 25% 12%	2313
	11th to 20th	Day	19% 6% 9% 22% 30% 14%	2299
	21st to end	Day	22% 7% 9% 20% 31% 12%	2548
September	1st to 10th	Day	25% 5% 8% 16% 34% 13%	2148
	11th to 20th	Day	28% 7% 9% 22% 25% 9%	1980
	21st to end	Day	28% 5% 13% 27% 23% 4	1980
October	1st to 10th	Day	42% 7% 12% 23% 13% 3	1832
	11th to 20th	Day	39% 6% 15% 28% 10% 2	1800
	21st to end	Day	39% 8% 17% 23% 11% 1	1958
November	1st to 10th	Day	47% 8% 15% 21% 8% 2	1651
	11th to 20th	Day	48% 8% 18% 17% 8% 1	1520
	21st to end	Day	47% 9% 16% 18% 9% 1	1520
December	1st to 10th	Day	59% 9% 10% 11% 9% 3	1520
	11th to 20th	Day	61% 9% 12% 15% 4	1924
	21st to end	Day	49% 8% 12% 14% 9% 8%	2778
January	1st to 10th	Night	56% 11% 13% 15% 5%	4397
	11th to 20th	Night	62% 8% 11% 12% 6% 2	4110
	21st to end	Night	62% 7% 13% 12% 5% 1	4521
February	1st to 10th	Night	60% 11% 11% 12% 5%	2470
	11th to 20th	Night	57% 8% 11% 15% 7% 1	2403
	21st to end	Night	59% 9% 10% 13% 6% 2	1886
March	1st to 10th	Night	52% 11% 14% 17% 6% 1	2014
	11th to 20th	Night	41% 12% 16% 21% 8% 2	1980
	21st to end	Night	52% 12% 15% 17% 3 1	2178
April	1st to 10th	Night	45% 12% 16% 15% 10% 2	1886
	11th to 20th	Night	45% 11% 16% 20% 7% 1	1650
	21st to end	Night	37% 10% 17% 28% 8%	1620
May	1st to 10th	Night	35% 10% 16% 29% 11%	1620
	11th to 20th	Night	31% 12% 18% 27% 12%	1525
	21st to end	Night	43% 12% 18% 20% 6% 1	1552
June	1st to 10th	Night	34% 10% 19% 25% 12% 1	1440
	11th to 20th	Night	28% 8% 13% 25% 18% 8%	1273
	21st to end	Night	13% 5% 11% 25% 33% 12%	1260
July	1st to 10th	Night	13% 7% 12% 24% 32% 12%	1225
	11th to 20th	Night	12% 4 6% 21% 32% 25%	1432
	21st to end	Night	15% 6% 9% 18% 30% 22%	1674
August	1st to 10th	Night	23% 7% 12% 18% 24% 17%	1602
	11th to 20th	Night	14% 5% 8% 25% 32% 15%	1602
	21st to end	Night	19% 5% 9% 20% 32% 15%	1808
September	1st to 10th	Night	19% 6% 8% 18% 32% 18%	1808
	11th to 20th	Night	23% 6% 12% 23% 26% 10%	1980
	21st to end	Night	26% 4 10% 29% 27% 5%	1980
October	1st to 10th	Night	37% 6% 14% 25% 16% 3	2128
	11th to 20th	Night	33% 6% 14% 33% 12% 2	2160
	21st to end	Night	33% 9% 18% 27% 12% 1	2395
November	1st to 10th	Night	45% 9% 14% 22% 9% 2	2409
	11th to 20th	Night	45% 9% 17% 21% 7% 2	2660
	21st to end	Night	43% 8% 16% 20% 10% 2	2660
December	1st to 10th	Night	57% 9% 14% 11% 7% 2	2660
	11th to 20th	Night	58% 8% 14% 14% 5%	3272
	21st to end	Night	52% 7% 12% 15% 9% 5%	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	13% 21% 25% 28% 11% 1	228
	11th to 20th	Day	6% 18% 38% 25% 10% 3	270
	21st to end	Day	7% 16% 36% 30% 9% 1	297
February	1st to 10th	Day	16% 30% 36% 16% 2	270
	11th to 20th	Day	14% 27% 30% 23% 6%	270
	21st to end	Day	14% 28% 33% 19% 6%	303
March	1st to 10th	Day	14% 32% 33% 18% 3	420
	11th to 20th	Day	12% 25% 35% 22% 7%	420
	21st to end	Day	15% 28% 36% 18% 3	462
April	1st to 10th	Day	20% 27% 30% 19% 5% 1	420
	11th to 20th	Day	18% 29% 35% 13% 4 1	412
	21st to end	Day	17% 22% 29% 20% 10% 2	420
May	1st to 10th	Day	12% 21% 33% 26% 8% 2	423
	11th to 20th	Day	15% 14% 30% 25% 13% 2	518
	21st to end	Day	17% 12% 26% 26% 16% 3	679
June	1st to 10th	Day	9% 16% 25% 29% 17% 3	630
	11th to 20th	Day	14% 18% 34% 24% 10%	630
	21st to end	Day	5% 10% 31% 32% 18% 3	630
July	1st to 10th	Day	3 7% 31% 39% 18% 3	621
	11th to 20th	Day	4 12% 39% 30% 13% 2	530
	21st to end	Day	3 9% 32% 38% 15% 3	487
August	1st to 10th	Day	6% 10% 35% 33% 15% 1	447
	11th to 20th	Day	4 8% 33% 30% 20% 4	437
	21st to end	Day	6% 13% 31% 31% 16% 4	495
September	1st to 10th	Day	6% 13% 35% 28% 16% 3	450
	11th to 20th	Day	12% 18% 28% 27% 12% 4	450
	21st to end	Day	9% 14% 30% 30% 14% 3	450
October	1st to 10th	Day	11% 14% 28% 25% 19% 3	302
	11th to 20th	Day	11% 20% 23% 27% 15% 5%	270
	21st to end	Day	16% 16% 32% 28% 6% 1	297
November	1st to 10th	Day	17% 24% 29% 19% 8% 3	270
	11th to 20th	Day	11% 19% 36% 22% 10% 2	270
	21st to end	Day	7% 15% 38% 29% 11% 1	270
December	1st to 10th	Day	15% 16% 31% 26% 11% 1	270
	11th to 20th	Day	17% 19% 32% 25% 7%	270
	21st to end	Day	14% 19% 31% 23% 11% 2	291
January	1st to 10th	Night	9% 14% 29% 37% 10% 1	917
	11th to 20th	Night	9% 12% 33% 33% 11% 2	760
	21st to end	Night	9% 8% 36% 35% 10% 1	836
February	1st to 10th	Night	9% 16% 35% 33% 7% 1	760
	11th to 20th	Night	10% 14% 32% 34% 9% 1	744
	21st to end	Night	10% 14% 36% 31% 9% 1	610
March	1st to 10th	Night	7% 17% 31% 35% 9% 1	720
	11th to 20th	Night	8% 12% 28% 39% 13% 1	720
	21st to end	Night	11% 12% 32% 33% 11%	792
April	1st to 10th	Night	11% 15% 29% 33% 11% 1	626
	11th to 20th	Night	13% 11% 32% 34% 10%	533
	21st to end	Night	11% 9% 26% 35% 18% 1	540
May	1st to 10th	Night	12% 10% 22% 38% 18% 1	540
	11th to 20th	Night	12% 6% 22% 35% 22% 2	531
	21st to end	Night	14% 8% 20% 38% 20% 1	584
June	1st to 10th	Night	13% 7% 22% 34% 23% 1	540
	11th to 20th	Night	10% 8% 24% 43% 14%	373
	21st to end	Night	3 1 16% 48% 30% 3	360
July	1st to 10th	Night	1 1 16% 46% 34% 2	350
	11th to 20th	Night	3 2 16% 51% 28%	479
	21st to end	Night	2 1 13% 49% 33% 2	591
August	1st to 10th	Night	5% 2 14% 47% 31% 1	537
	11th to 20th	Night	3 2 11% 49% 33% 2	534
	21st to end	Night	4 4 17% 39% 34% 3	594
September	1st to 10th	Night	7% 3 16% 43% 29% 3	548
	11th to 20th	Night	6% 4 17% 40% 29% 4	720
	21st to end	Night	12% 3 19% 38% 24% 4	720
October	1st to 10th	Night	12% 7% 23% 36% 21% 3	720
	11th to 20th	Night	13% 8% 19% 38% 20% 2	720
	21st to end	Night	10% 9% 23% 44% 12% 3	792
November	1st to 10th	Night	14% 12% 26% 34% 11% 2	744
	11th to 20th	Night	12% 14% 32% 35% 7% 1	950
	21st to end	Night	10% 11% 31% 36% 11% 2	950
December	1st to 10th	Night	14% 12% 32% 34% 7% 1	950
	11th to 20th	Night	14% 14% 29% 36% 7%	950
	21st to end	Night	13% 15% 32% 29% 9% 2	1045

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