COSMIC OBSERVATORY
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the COSMIC OBSERVATORY clear sky chart from 2005-11-06 to 2026-06-01.

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	12% 6% 2 6% 5% 5% 5% 4 6% 8% 41%	131880
	Night	12% 6% 2 6% 4 4 4 4 6% 7% 44%	131242
Transparency	Day	39% 7% 11% 22% 16% 5%	135486
	Night	37% 7% 12% 23% 18% 4	135706
Seeing	Day	17% 13% 24% 25% 16% 6%	16988
	Night	16% 9% 25% 34% 15% 2	26851

By Month

Percentage Cloud Cover SE (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	13% 7% 3 8% 4 3 3 3 4 6% 46%	5258
	11th to 20th	Day	10% 7% 3 8% 4 4 3 3 4 4 50%	5280
	21st to end	Day	12% 6% 3 7% 4 4 4 3 5% 5% 47%	5780
February	1st to 10th	Day	11% 8% 3 8% 5% 5% 4 4 5% 6% 42%	1890
	11th to 20th	Day	15% 7% 2 7% 5% 4 4 4 5% 6% 41%	1843
	21st to end	Day	15% 8% 3 6% 4 4 5% 4 5% 7% 41%	1797
March	1st to 10th	Day	15% 7% 2 6% 4 5% 5% 4 5% 6% 42%	5045
	11th to 20th	Day	9% 5% 2 6% 4 3 4 3 5% 7% 52%	5090
	21st to end	Day	12% 8% 3 8% 5% 5% 5% 3 5% 6% 41%	5586
April	1st to 10th	Day	11% 6% 2 5% 5% 5% 5% 3 6% 6% 46%	2457
	11th to 20th	Day	12% 6% 2 6% 3 4 4 3 6% 7% 47%	2494
	21st to end	Day	14% 7% 3 6% 5% 4 5% 3 5% 6% 43%	2605
May	1st to 10th	Day	13% 6% 2 7% 6% 6% 6% 5% 7% 11% 31%	5992
	11th to 20th	Day	20% 7% 3 6% 5% 6% 5% 4 5% 8% 32%	5991
	21st to end	Day	15% 6% 2 6% 5% 5% 5% 5% 7% 8% 36%	6842
June	1st to 10th	Day	10% 5% 2 6% 4 5% 5% 5% 6% 9% 43%	2800
	11th to 20th	Day	9% 5% 2 6% 6% 6% 7% 5% 8% 10% 37%	2800
	21st to end	Day	7% 4 2 6% 5% 5% 5% 4 8% 12% 42%	2800
July	1st to 10th	Day	6% 4 2 8% 5% 5% 6% 5% 8% 10% 40%	5976
	11th to 20th	Day	7% 5% 3 8% 6% 7% 8% 6% 10% 13% 28%	5900
	21st to end	Day	8% 6% 2 8% 6% 6% 6% 6% 9% 11% 33%	6424
August	1st to 10th	Day	12% 5% 2 7% 5% 6% 7% 6% 9% 10% 31%	4234
	11th to 20th	Day	7% 5% 2 7% 5% 6% 7% 6% 9% 10% 36%	4148
	21st to end	Day	7% 4 2 5% 4 5% 5% 5% 7% 10% 45%	4312
September	1st to 10th	Day	11% 6% 2 6% 4 5% 5% 4 6% 7% 46%	2400
	11th to 20th	Day	12% 6% 2 6% 4 4 4 4 6% 7% 44%	2277
	21st to end	Day	20% 7% 3 7% 4 4 4 3 5% 7% 36%	2153
October	1st to 10th	Day	14% 6% 2 5% 3 4 4 3 5% 8% 47%	2000
	11th to 20th	Day	14% 5% 2 5% 3 3 3 3 5% 6% 50%	2000
	21st to end	Day	13% 5% 2 4 4 3 4 3 5% 6% 51%	2200
November	1st to 10th	Day	10% 6% 2 4 2 4 3 3 6% 6% 54%	2057
	11th to 20th	Day	11% 6% 2 6% 4 3 3 3 5% 6% 50%	1848
	21st to end	Day	13% 5% 2 6% 4 5% 4 3 5% 6% 49%	1680
December	1st to 10th	Day	10% 5% 2 5% 4 4 5% 4 4 7% 50%	2680
	11th to 20th	Day	17% 5% 2 5% 4 4 3 2 5% 7% 45%	3070
	21st to end	Day	16% 8% 3 6% 4 5% 5% 4 6% 7% 35%	4171
January	1st to 10th	Night	10% 7% 2 6% 4 5% 5% 4 6% 6% 46%	7635
	11th to 20th	Night	10% 8% 2 5% 3 3 3 3 4 5% 54%	7680
	21st to end	Night	13% 7% 2 5% 3 3 3 3 4 5% 52%	8401
February	1st to 10th	Night	11% 8% 2 6% 4 4 5% 4 5% 6% 44%	2730
	11th to 20th	Night	17% 8% 2 6% 5% 3 4 3 5% 6% 41%	2663
	21st to end	Night	13% 7% 3 6% 5% 4 5% 3 6% 7% 42%	2004
March	1st to 10th	Night	16% 6% 2 6% 4 3 3 3 4 6% 46%	5028
	11th to 20th	Night	8% 4 2 6% 3 4 4 3 5% 9% 52%	5090
	21st to end	Night	11% 7% 2 7% 5% 5% 4 3 5% 6% 45%	5588
April	1st to 10th	Night	13% 7% 3 5% 4 4 4 4 6% 6% 44%	2162
	11th to 20th	Night	10% 6% 2 5% 4 4 4 4 7% 8% 45%	2075
	21st to end	Night	15% 5% 1 5% 4 4 4 4 5% 8% 45%	2032
May	1st to 10th	Night	15% 8% 3 7% 4 5% 5% 4 6% 8% 34%	4123
	11th to 20th	Night	21% 7% 3 7% 5% 4 5% 4 6% 8% 32%	4122
	21st to end	Night	15% 6% 2 6% 4 5% 5% 4 6% 9% 37%	4250
June	1st to 10th	Night	10% 5% 1 6% 4 4 5% 3 7% 10% 44%	1600
	11th to 20th	Night	10% 5% 2 7% 5% 4 5% 4 7% 9% 43%	1600
	21st to end	Night	8% 4 1 5% 4 4 6% 5% 8% 10% 45%	1600
July	1st to 10th	Night	6% 5% 2 7% 6% 5% 6% 5% 8% 10% 40%	3962
	11th to 20th	Night	8% 7% 4 11% 6% 7% 7% 6% 8% 9% 27%	4057
	21st to end	Night	6% 6% 3 9% 6% 7% 7% 5% 9% 11% 31%	4427
August	1st to 10th	Night	14% 6% 3 6% 5% 5% 5% 5% 7% 10% 32%	2915
	11th to 20th	Night	9% 6% 2 8% 6% 7% 7% 5% 7% 8% 36%	2875
	21st to end	Night	9% 5% 2 6% 5% 4 4 4 6% 10% 45%	3577
September	1st to 10th	Night	12% 5% 2 5% 5% 4 3 4 5% 7% 49%	2000
	11th to 20th	Night	13% 5% 2 6% 4 5% 4 3 5% 7% 45%	2119
	21st to end	Night	22% 8% 2 6% 4 4 4 3 4 5% 39%	2245
October	1st to 10th	Night	15% 6% 2 5% 3 4 4 4 5% 7% 45%	2400
	11th to 20th	Night	12% 5% 1 6% 3 4 4 3 4 6% 51%	2400
	21st to end	Night	13% 5% 1 5% 4 3 4 3 4 6% 52%	2640
November	1st to 10th	Night	11% 7% 2 5% 4 3 4 3 4 5% 53%	2461
	11th to 20th	Night	12% 6% 2 5% 4 4 4 3 5% 6% 49%	2767
	21st to end	Night	13% 5% 2 5% 4 4 4 4 5% 6% 48%	2940
December	1st to 10th	Night	9% 5% 2 5% 4 4 4 4 6% 6% 51%	4720
	11th to 20th	Night	15% 8% 3 7% 4 4 4 4 4 6% 41%	5345
	21st to end	Night	15% 7% 2 6% 5% 5% 5% 5% 6% 8% 37%	7009

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	40% 4 8% 20% 21% 7%	5259
	11th to 20th	Day	37% 4 5% 17% 22% 15%	5280
	21st to end	Day	41% 3 6% 26% 20% 5%	5780
February	1st to 10th	Day	42% 5% 10% 21% 18% 4	1890
	11th to 20th	Day	44% 5% 8% 21% 19% 4	1852
	21st to end	Day	42% 6% 11% 18% 16% 6%	1797
March	1st to 10th	Day	42% 5% 9% 18% 16% 10%	5045
	11th to 20th	Day	32% 5% 9% 25% 22% 8%	5090
	21st to end	Day	44% 6% 9% 17% 17% 7%	5586
April	1st to 10th	Day	37% 7% 11% 20% 18% 7%	2457
	11th to 20th	Day	36% 6% 11% 21% 20% 5%	2494
	21st to end	Day	42% 6% 11% 23% 14% 4	2605
May	1st to 10th	Day	45% 10% 12% 20% 11% 2	6027
	11th to 20th	Day	50% 8% 10% 20% 11% 1	5995
	21st to end	Day	42% 10% 13% 18% 14% 3	6857
June	1st to 10th	Day	37% 9% 14% 24% 16% 1	2800
	11th to 20th	Day	40% 11% 14% 20% 13% 2	2800
	21st to end	Day	32% 12% 15% 26% 13% 1	2800
July	1st to 10th	Day	36% 11% 16% 20% 12% 4	5911
	11th to 20th	Day	42% 15% 16% 21% 5% 1	5831
	21st to end	Day	41% 10% 12% 23% 13% 1	6240
August	1st to 10th	Day	44% 12% 16% 21% 7%	4221
	11th to 20th	Day	38% 10% 12% 27% 12% 1	4187
	21st to end	Day	32% 10% 17% 27% 13% 1	4312
September	1st to 10th	Day	37% 9% 12% 18% 19% 5%	2400
	11th to 20th	Day	37% 7% 8% 26% 19% 3	2277
	21st to end	Day	48% 6% 10% 18% 16% 3	2153
October	1st to 10th	Day	31% 6% 8% 20% 28% 6%	3260
	11th to 20th	Day	25% 4 9% 28% 22% 12%	3240
	21st to end	Day	22% 4 9% 30% 23% 11%	3538
November	1st to 10th	Day	30% 5% 9% 25% 25% 5%	2057
	11th to 20th	Day	34% 5% 9% 21% 23% 8%	1848
	21st to end	Day	36% 4 9% 26% 21% 3	1680
December	1st to 10th	Day	34% 5% 10% 25% 20% 6%	2680
	11th to 20th	Day	40% 4 11% 22% 17% 5%	3066
	21st to end	Day	45% 6% 13% 20% 15% 1	4171
January	1st to 10th	Night	36% 6% 9% 22% 22% 6%	7636
	11th to 20th	Night	33% 4 7% 21% 24% 11%	7680
	21st to end	Night	35% 3 8% 27% 22% 5%	8401
February	1st to 10th	Night	39% 6% 12% 24% 16% 3	2730
	11th to 20th	Night	43% 5% 9% 21% 16% 6%	2665
	21st to end	Night	40% 6% 13% 20% 14% 6%	2004
March	1st to 10th	Night	38% 5% 8% 22% 18% 9%	5028
	11th to 20th	Night	28% 5% 10% 27% 25% 6%	5090
	21st to end	Night	39% 4 13% 19% 19% 5%	5588
April	1st to 10th	Night	39% 6% 11% 20% 19% 4	2162
	11th to 20th	Night	34% 6% 15% 22% 19% 4	2075
	21st to end	Night	37% 6% 13% 27% 15% 3	2032
May	1st to 10th	Night	46% 10% 14% 19% 9% 2	4150
	11th to 20th	Night	48% 8% 11% 23% 10%	4123
	21st to end	Night	42% 9% 16% 23% 7% 2	4257
June	1st to 10th	Night	33% 10% 16% 27% 14% 1	1600
	11th to 20th	Night	35% 11% 16% 24% 12% 2	1600
	21st to end	Night	30% 12% 17% 21% 18% 2	1600
July	1st to 10th	Night	37% 12% 18% 19% 13% 2	3919
	11th to 20th	Night	48% 14% 15% 16% 6% 1	4006
	21st to end	Night	41% 11% 17% 20% 10%	4297
August	1st to 10th	Night	43% 11% 17% 22% 6% 1	2904
	11th to 20th	Night	41% 10% 14% 24% 11%	2902
	21st to end	Night	34% 8% 17% 29% 10% 2	3577
September	1st to 10th	Night	34% 8% 14% 22% 18% 4	2000
	11th to 20th	Night	37% 7% 9% 27% 18% 2	2119
	21st to end	Night	47% 7% 9% 19% 16% 2	2245
October	1st to 10th	Night	40% 5% 10% 20% 21% 4	3920
	11th to 20th	Night	25% 5% 11% 22% 30% 8%	3899
	21st to end	Night	22% 4 10% 30% 28% 7%	4252
November	1st to 10th	Night	34% 5% 9% 23% 24% 6%	2461
	11th to 20th	Night	35% 5% 8% 22% 23% 6%	2767
	21st to end	Night	36% 5% 10% 24% 21% 4	2940
December	1st to 10th	Night	33% 6% 10% 26% 22% 3	4720
	11th to 20th	Night	44% 4 11% 18% 16% 6%	5348
	21st to end	Night	41% 7% 12% 22% 16% 1	7009

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	21% 28% 29% 16% 5% 1	330
	11th to 20th	Day	22% 31% 28% 17% 2	330
	21st to end	Day	21% 28% 32% 16% 3 1	363
February	1st to 10th	Day	22% 25% 33% 15% 4	330
	11th to 20th	Day	32% 27% 31% 9% 2	330
	21st to end	Day	24% 20% 33% 18% 4	403
March	1st to 10th	Day	18% 24% 29% 23% 6%	540
	11th to 20th	Day	19% 23% 31% 19% 6% 1	540
	21st to end	Day	20% 19% 31% 22% 7% 1	594
April	1st to 10th	Day	20% 24% 26% 21% 9% 1	540
	11th to 20th	Day	18% 14% 28% 23% 13% 4	532
	21st to end	Day	20% 15% 29% 22% 12% 3	540
May	1st to 10th	Day	18% 17% 27% 24% 10% 4	543
	11th to 20th	Day	20% 11% 27% 23% 15% 4	567
	21st to end	Day	19% 9% 19% 27% 18% 8%	617
June	1st to 10th	Day	12% 8% 23% 30% 21% 7%	530
	11th to 20th	Day	12% 6% 22% 30% 20% 10%	530
	21st to end	Day	8% 5% 21% 37% 22% 9%	530
July	1st to 10th	Day	10% 4 15% 30% 29% 14%	525
	11th to 20th	Day	5% 2 13% 36% 30% 15%	524
	21st to end	Day	11% 2 13% 33% 27% 14%	576
August	1st to 10th	Day	12% 6% 15% 28% 25% 14%	526
	11th to 20th	Day	13% 4 14% 30% 25% 14%	519
	21st to end	Day	10% 2 12% 31% 28% 17%	583
September	1st to 10th	Day	12% 7% 16% 25% 25% 15%	530
	11th to 20th	Day	14% 4 16% 26% 29% 12%	530
	21st to end	Day	28% 8% 17% 24% 18% 5%	530
October	1st to 10th	Day	18% 7% 15% 29% 23% 7%	530
	11th to 20th	Day	12% 14% 25% 27% 18% 3	530
	21st to end	Day	16% 14% 29% 27% 11% 3	583
November	1st to 10th	Day	16% 8% 30% 30% 13% 3	535
	11th to 20th	Day	16% 17% 29% 25% 11% 2	346
	21st to end	Day	18% 20% 31% 21% 9% 1	220
December	1st to 10th	Day	20% 19% 33% 21% 5% 1	220
	11th to 20th	Day	24% 16% 31% 20% 8% 1	220
	21st to end	Day	21% 19% 32% 18% 8% 1	272
January	1st to 10th	Night	17% 16% 36% 27% 4	840
	11th to 20th	Night	19% 14% 32% 31% 4	840
	21st to end	Night	17% 11% 42% 27% 3	924
February	1st to 10th	Night	16% 12% 37% 32% 3	840
	11th to 20th	Night	23% 16% 37% 23% 1	823
	21st to end	Night	20% 14% 38% 26% 3	692
March	1st to 10th	Night	16% 13% 37% 30% 4	840
	11th to 20th	Night	19% 15% 33% 26% 7% 1	840
	21st to end	Night	19% 11% 34% 29% 6% 1	924
April	1st to 10th	Night	18% 16% 32% 27% 7%	840
	11th to 20th	Night	14% 11% 31% 32% 11%	831
	21st to end	Night	17% 7% 28% 38% 9% 1	772
May	1st to 10th	Night	18% 7% 23% 35% 15% 1	630
	11th to 20th	Night	18% 3 24% 36% 17% 1	621
	21st to end	Night	16% 3 15% 39% 25% 2	568
June	1st to 10th	Night	9% 2 17% 48% 23% 2	400
	11th to 20th	Night	10% 3 14% 46% 26% 2	400
	21st to end	Night	9% 2 13% 48% 26% 3	400
July	1st to 10th	Night	12% 2 9% 36% 36% 6%	541
	11th to 20th	Night	5% 1 9% 36% 42% 6%	582
	21st to end	Night	8% 2 7% 41% 36% 6%	657
August	1st to 10th	Night	12% 3 8% 38% 35% 4	597
	11th to 20th	Night	13% 3 9% 37% 36% 3	594
	21st to end	Night	12% 2 11% 36% 34% 5%	871
September	1st to 10th	Night	14% 3 11% 35% 32% 6%	800
	11th to 20th	Night	14% 4 11% 35% 32% 5%	800
	21st to end	Night	24% 5% 17% 31% 21% 2	800
October	1st to 10th	Night	17% 6% 17% 31% 25% 4	800
	11th to 20th	Night	15% 7% 26% 34% 16% 1	800
	21st to end	Night	15% 9% 25% 36% 13% 1	880
November	1st to 10th	Night	15% 7% 22% 41% 12% 2	820
	11th to 20th	Night	15% 13% 25% 36% 10% 1	840
	21st to end	Night	15% 11% 31% 34% 8% 1	840
December	1st to 10th	Night	19% 9% 33% 33% 6%	840
	11th to 20th	Night	16% 10% 32% 36% 6%	840
	21st to end	Night	17% 11% 30% 38% 4	924

Page generated at 2026-05-29 01:53:28. Version ac07

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