Corvus Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Corvus Observatory clear sky chart from 2005-11-06 to 2026-10-29.

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	16% 9% 3 8% 5% 5% 5% 4 6% 7% 34%	149629
	Night	17% 10% 4 8% 5% 4 4 4 5% 7% 33%	142425
Transparency	Day	47% 7% 13% 21% 11% 1	153133
	Night	50% 6% 13% 19% 11% 1	147010
Seeing	Day	24% 16% 29% 20% 9% 3	17403
	Night	23% 15% 29% 25% 7% 1	26392

By Month

Percentage Cloud Cover NE (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	12% 12% 5% 16% 6% 6% 5% 4 5% 6% 25%	4661
	11th to 20th	Day	13% 13% 4 11% 5% 5% 5% 4 5% 6% 27%	4650
	21st to end	Day	13% 11% 4 9% 5% 4 5% 3 5% 6% 35%	5304
February	1st to 10th	Day	15% 11% 4 8% 6% 6% 6% 4 6% 7% 27%	1890
	11th to 20th	Day	13% 13% 4 11% 6% 5% 4 4 6% 6% 28%	2036
	21st to end	Day	18% 11% 4 10% 6% 4 5% 3 4 5% 31%	1730
March	1st to 10th	Day	21% 13% 5% 9% 5% 3 3 3 4 5% 29%	4576
	11th to 20th	Day	15% 12% 3 7% 5% 4 4 3 5% 6% 36%	4799
	21st to end	Day	20% 10% 3 8% 5% 5% 4 4 4 5% 32%	6102
April	1st to 10th	Day	20% 11% 3 7% 5% 4 4 3 4 5% 33%	2520
	11th to 20th	Day	16% 8% 3 6% 5% 5% 5% 4 5% 6% 36%	2494
	21st to end	Day	21% 7% 3 7% 5% 5% 4 3 5% 6% 35%	2617
May	1st to 10th	Day	25% 9% 3 7% 5% 4 4 4 5% 7% 26%	7716
	11th to 20th	Day	17% 9% 3 7% 5% 4 4 3 6% 8% 34%	8221
	21st to end	Day	21% 7% 2 7% 5% 5% 5% 4 7% 8% 30%	9060
June	1st to 10th	Day	15% 6% 2 6% 6% 5% 6% 4 7% 8% 34%	2940
	11th to 20th	Day	15% 6% 2 6% 5% 5% 5% 5% 7% 10% 34%	2940
	21st to end	Day	15% 6% 3 7% 5% 5% 5% 4 5% 9% 35%	2940
July	1st to 10th	Day	13% 6% 3 7% 5% 5% 6% 5% 7% 10% 34%	8280
	11th to 20th	Day	7% 5% 2 6% 5% 5% 6% 5% 7% 11% 42%	8280
	21st to end	Day	14% 6% 2 7% 5% 4 4 4 6% 8% 40%	8993
August	1st to 10th	Day	14% 8% 3 9% 6% 5% 6% 5% 7% 9% 29%	6421
	11th to 20th	Day	12% 7% 2 7% 4 5% 5% 4 6% 10% 38%	5488
	21st to end	Day	10% 5% 2 7% 5% 5% 6% 5% 8% 11% 36%	6080
September	1st to 10th	Day	14% 5% 2 6% 4 4 4 4 5% 6% 45%	2520
	11th to 20th	Day	13% 5% 2 5% 4 3 4 4 6% 7% 49%	2415
	21st to end	Day	16% 6% 2 5% 4 4 5% 4 7% 8% 39%	2310
October	1st to 10th	Day	18% 8% 2 8% 4 4 4 3 5% 5% 39%	2206
	11th to 20th	Day	16% 8% 2 6% 4 4 5% 3 5% 7% 40%	2100
	21st to end	Day	21% 9% 3 8% 4 4 4 3 5% 6% 33%	2057
November	1st to 10th	Day	15% 8% 3 6% 4 4 4 3 4 7% 42%	1851
	11th to 20th	Day	16% 9% 3 7% 5% 5% 5% 4 5% 5% 36%	1890
	21st to end	Day	18% 10% 3 7% 5% 4 5% 4 5% 7% 31%	1748
December	1st to 10th	Day	16% 13% 4 9% 5% 5% 5% 4 5% 7% 26%	2680
	11th to 20th	Day	21% 12% 3 8% 5% 4 3 3 4 4 33%	3066
	21st to end	Day	21% 15% 4 9% 5% 4 4 3 4 6% 26%	4048
January	1st to 10th	Night	15% 12% 7% 15% 5% 5% 4 3 5% 5% 24%	8234
	11th to 20th	Night	13% 15% 6% 14% 6% 5% 4 4 5% 5% 23%	8235
	21st to end	Night	14% 9% 3 8% 7% 6% 5% 4 6% 7% 32%	8859
February	1st to 10th	Night	16% 11% 4 9% 6% 5% 5% 4 6% 6% 29%	2730
	11th to 20th	Night	14% 11% 3 11% 6% 5% 6% 4 7% 6% 26%	2468
	21st to end	Night	17% 12% 4 10% 5% 5% 4 3 6% 6% 29%	2076
March	1st to 10th	Night	21% 11% 4 8% 6% 5% 4 3 5% 6% 28%	5484
	11th to 20th	Night	18% 12% 3 7% 4 4 4 3 4 6% 33%	5376
	21st to end	Night	22% 12% 3 9% 5% 4 4 3 4 5% 29%	5072
April	1st to 10th	Night	20% 11% 3 8% 5% 5% 4 4 5% 6% 30%	2100
	11th to 20th	Night	20% 9% 3 6% 4 4 4 3 5% 6% 35%	2075
	21st to end	Night	20% 8% 3 6% 4 4 3 4 4 6% 37%	2001
May	1st to 10th	Night	26% 9% 3 7% 4 4 4 3 5% 6% 28%	5225
	11th to 20th	Night	18% 10% 3 8% 5% 5% 5% 4 6% 8% 30%	4639
	21st to end	Night	22% 8% 3 7% 5% 4 5% 3 6% 7% 31%	5114
June	1st to 10th	Night	17% 7% 2 6% 4 4 4 4 5% 7% 39%	1680
	11th to 20th	Night	15% 6% 3 6% 5% 4 5% 5% 6% 9% 36%	1680
	21st to end	Night	13% 5% 2 6% 5% 4 4 4 6% 9% 42%	1680
July	1st to 10th	Night	14% 7% 3 7% 5% 4 5% 4 6% 9% 37%	4680
	11th to 20th	Night	9% 6% 3 7% 5% 5% 5% 4 7% 9% 40%	4680
	21st to end	Night	12% 6% 3 6% 4 3 4 4 5% 8% 44%	5084
August	1st to 10th	Night	18% 8% 4 9% 5% 5% 4 4 5% 9% 30%	3666
	11th to 20th	Night	15% 5% 2 7% 4 4 5% 4 6% 8% 38%	4556
	21st to end	Night	11% 7% 3 7% 5% 4 5% 4 7% 9% 39%	5045
September	1st to 10th	Night	16% 7% 2 6% 3 4 4 4 5% 7% 42%	2100
	11th to 20th	Night	13% 6% 2 4 3 4 3 3 4 7% 51%	2205
	21st to end	Night	16% 7% 3 7% 5% 4 4 3 5% 6% 41%	2310
October	1st to 10th	Night	17% 8% 3 7% 5% 4 4 3 5% 6% 38%	2410
	11th to 20th	Night	16% 7% 3 6% 5% 4 4 4 5% 6% 41%	2520
	21st to end	Night	23% 8% 3 6% 4 4 4 3 5% 7% 33%	2959
November	1st to 10th	Night	16% 8% 3 6% 4 4 4 3 5% 6% 42%	2667
	11th to 20th	Night	18% 9% 3 8% 4 4 4 4 5% 5% 36%	2730
	21st to end	Night	16% 10% 3 8% 4 4 5% 4 5% 6% 33%	2867
December	1st to 10th	Night	18% 13% 5% 9% 5% 5% 5% 4 5% 7% 24%	4720
	11th to 20th	Night	26% 11% 4 7% 4 4 3 3 4 5% 31%	5348
	21st to end	Night	23% 15% 4 8% 4 3 4 3 4 5% 26%	7150

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	58% 5% 14% 18% 5%	4662
	11th to 20th	Day	53% 7% 11% 15% 12% 2	4680
	21st to end	Day	48% 6% 11% 22% 12% 1	5304
February	1st to 10th	Day	56% 5% 14% 16% 8% 1	1890
	11th to 20th	Day	56% 5% 11% 18% 9% 1	2045
	21st to end	Day	54% 5% 12% 18% 11% 1	1730
March	1st to 10th	Day	57% 5% 10% 15% 8% 4	4579
	11th to 20th	Day	47% 6% 17% 20% 7% 3	4799
	21st to end	Day	53% 4 7% 15% 18% 3	6102
April	1st to 10th	Day	53% 5% 9% 21% 11% 2	2520
	11th to 20th	Day	47% 5% 11% 20% 15% 2	2494
	21st to end	Day	50% 4 10% 19% 13% 3	2617
May	1st to 10th	Day	56% 7% 12% 16% 9% 1	7728
	11th to 20th	Day	48% 7% 13% 21% 10% 1	8225
	21st to end	Day	50% 7% 11% 22% 9% 1	9061
June	1st to 10th	Day	46% 8% 13% 22% 11% 1	2940
	11th to 20th	Day	44% 7% 16% 20% 12% 1	2940
	21st to end	Day	46% 7% 14% 21% 11% 1	2940
July	1st to 10th	Day	44% 8% 15% 22% 11% 1	8210
	11th to 20th	Day	35% 8% 17% 28% 12%	8205
	21st to end	Day	41% 7% 14% 24% 14%	8795
August	1st to 10th	Day	49% 8% 15% 17% 9%	6407
	11th to 20th	Day	39% 7% 21% 27% 5%	5524
	21st to end	Day	38% 9% 17% 26% 10% 1	6080
September	1st to 10th	Day	39% 6% 12% 27% 15% 1	2520
	11th to 20th	Day	34% 6% 12% 25% 21% 2	2415
	21st to end	Day	41% 7% 13% 23% 14% 2	2310
October	1st to 10th	Day	42% 6% 9% 22% 19% 1	3531
	11th to 20th	Day	39% 6% 12% 26% 17% 1	3340
	21st to end	Day	38% 6% 14% 27% 13% 2	3257
November	1st to 10th	Day	42% 6% 14% 25% 13% 1	1851
	11th to 20th	Day	48% 6% 11% 20% 14% 1	1890
	21st to end	Day	51% 5% 15% 21% 7% 1	1748
December	1st to 10th	Day	56% 6% 14% 19% 5%	2680
	11th to 20th	Day	53% 6% 13% 18% 10% 1	3066
	21st to end	Day	58% 5% 8% 17% 10% 2	4048
January	1st to 10th	Night	61% 6% 13% 13% 6% 1	8234
	11th to 20th	Night	61% 6% 11% 12% 8% 2	8280
	21st to end	Night	50% 6% 13% 17% 13% 2	8859
February	1st to 10th	Night	54% 6% 13% 17% 8% 1	2730
	11th to 20th	Night	56% 6% 11% 17% 9% 1	2470
	21st to end	Night	55% 5% 12% 18% 8% 2	2076
March	1st to 10th	Night	55% 6% 12% 14% 10% 1	5494
	11th to 20th	Night	52% 6% 16% 17% 7% 2	5377
	21st to end	Night	56% 5% 5% 16% 16% 2	5072
April	1st to 10th	Night	55% 5% 11% 18% 10% 1	2100
	11th to 20th	Night	50% 5% 10% 19% 16% 1	2075
	21st to end	Night	48% 4 10% 19% 15% 4	2001
May	1st to 10th	Night	56% 6% 13% 15% 9% 1	5231
	11th to 20th	Night	52% 7% 12% 20% 8% 1	4640
	21st to end	Night	53% 6% 11% 20% 9% 1	5115
June	1st to 10th	Night	42% 6% 13% 25% 13% 1	1680
	11th to 20th	Night	43% 8% 14% 23% 11%	1680
	21st to end	Night	38% 8% 16% 24% 13% 1	1680
July	1st to 10th	Night	44% 7% 14% 20% 14% 1	4640
	11th to 20th	Night	38% 8% 14% 25% 14% 1	4632
	21st to end	Night	37% 7% 14% 29% 13%	4967
August	1st to 10th	Night	51% 7% 14% 17% 10% 1	3656
	11th to 20th	Night	41% 7% 19% 26% 6%	4586
	21st to end	Night	38% 9% 16% 27% 10%	5045
September	1st to 10th	Night	41% 7% 10% 26% 15% 1	2100
	11th to 20th	Night	33% 5% 11% 27% 22% 1	2205
	21st to end	Night	43% 6% 11% 23% 17% 1	2310
October	1st to 10th	Night	44% 7% 13% 21% 13% 1	3865
	11th to 20th	Night	36% 6% 12% 24% 21% 1	4019
	21st to end	Night	38% 6% 14% 28% 13% 1	4709
November	1st to 10th	Night	43% 6% 11% 22% 16% 2	2667
	11th to 20th	Night	50% 5% 12% 19% 12% 2	2730
	21st to end	Night	50% 6% 17% 20% 7%	2867
December	1st to 10th	Night	57% 7% 13% 18% 4	4720
	11th to 20th	Night	56% 4 13% 14% 12%	5348
	21st to end	Night	59% 5% 9% 14% 9% 3	7150

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	31% 28% 24% 12% 5%	220
	11th to 20th	Day	35% 25% 26% 11% 1	220
	21st to end	Day	28% 26% 30% 13% 3 1	275
February	1st to 10th	Day	36% 26% 29% 7% 2	330
	11th to 20th	Day	34% 25% 28% 11% 2	524
	21st to end	Day	28% 29% 30% 12% 2	446
March	1st to 10th	Day	32% 20% 31% 14% 2	540
	11th to 20th	Day	31% 24% 27% 15% 2	540
	21st to end	Day	36% 22% 25% 10% 5% 2	594
April	1st to 10th	Day	37% 22% 22% 14% 4 1	540
	11th to 20th	Day	30% 23% 27% 14% 5% 1	532
	21st to end	Day	28% 18% 31% 16% 5% 1	540
May	1st to 10th	Day	32% 19% 28% 14% 6% 1	543
	11th to 20th	Day	22% 16% 34% 19% 8% 1	567
	21st to end	Day	21% 12% 31% 24% 8% 3	617
June	1st to 10th	Day	17% 13% 32% 24% 12% 2	570
	11th to 20th	Day	21% 11% 31% 24% 11% 2	570
	21st to end	Day	19% 11% 33% 22% 12% 2	570
July	1st to 10th	Day	15% 8% 30% 28% 16% 4	565
	11th to 20th	Day	13% 7% 28% 32% 17% 3	564
	21st to end	Day	12% 9% 32% 30% 14% 3	620
August	1st to 10th	Day	11% 7% 26% 31% 18% 6%	566
	11th to 20th	Day	14% 7% 30% 27% 15% 7%	559
	21st to end	Day	12% 9% 33% 25% 15% 6%	627
September	1st to 10th	Day	19% 10% 24% 28% 14% 6%	570
	11th to 20th	Day	15% 10% 25% 21% 18% 11%	570
	21st to end	Day	20% 9% 29% 25% 13% 5%	570
October	1st to 10th	Day	23% 13% 25% 21% 13% 5%	570
	11th to 20th	Day	24% 17% 29% 19% 9% 2	570
	21st to end	Day	28% 14% 30% 19% 8% 1	489
November	1st to 10th	Day	28% 17% 26% 18% 9% 2	425
	11th to 20th	Day	26% 20% 30% 18% 5% 1	430
	21st to end	Day	32% 24% 25% 16% 2 1	288
December	1st to 10th	Day	27% 26% 27% 14% 5%	220
	11th to 20th	Day	34% 25% 30% 10% 3	220
	21st to end	Day	29% 22% 32% 14% 2	242
January	1st to 10th	Night	24% 21% 36% 16% 2	840
	11th to 20th	Night	28% 23% 29% 18% 2	840
	21st to end	Night	23% 22% 34% 19% 2	924
February	1st to 10th	Night	25% 22% 33% 19% 2	840
	11th to 20th	Night	26% 22% 35% 15% 1	823
	21st to end	Night	28% 23% 32% 15% 3	692
March	1st to 10th	Night	27% 23% 32% 15% 2	840
	11th to 20th	Night	26% 24% 32% 16% 2	840
	21st to end	Night	29% 21% 29% 17% 4	924
April	1st to 10th	Night	28% 21% 30% 19% 3	840
	11th to 20th	Night	29% 17% 29% 19% 6%	831
	21st to end	Night	27% 15% 31% 23% 4	741
May	1st to 10th	Night	27% 16% 31% 19% 7%	622
	11th to 20th	Night	22% 13% 33% 28% 4	414
	21st to end	Night	16% 10% 29% 35% 9% 1	455
June	1st to 10th	Night	19% 6% 31% 34% 10%	420
	11th to 20th	Night	17% 8% 30% 33% 11%	420
	21st to end	Night	14% 7% 27% 40% 12%	420
July	1st to 10th	Night	14% 4 20% 45% 15% 1	410
	11th to 20th	Night	14% 4 23% 44% 14% 1	408
	21st to end	Night	16% 3 21% 48% 12%	460
August	1st to 10th	Night	12% 4 22% 44% 17% 1	418
	11th to 20th	Night	15% 4 20% 38% 21% 3	829
	21st to end	Night	16% 6% 23% 34% 19% 2	924
September	1st to 10th	Night	18% 6% 26% 34% 14% 2	840
	11th to 20th	Night	16% 7% 24% 30% 20% 3	840
	21st to end	Night	19% 9% 28% 29% 14% 2	840
October	1st to 10th	Night	24% 9% 24% 29% 13% 2	840
	11th to 20th	Night	21% 13% 30% 27% 9%	840
	21st to end	Night	29% 12% 30% 24% 4	913
November	1st to 10th	Night	22% 15% 33% 23% 7%	820
	11th to 20th	Night	25% 16% 33% 18% 7% 1	840
	21st to end	Night	24% 20% 30% 22% 4	840
December	1st to 10th	Night	26% 21% 32% 17% 4	840
	11th to 20th	Night	30% 18% 29% 22% 2	840
	21st to end	Night	32% 20% 31% 16% 2	924

Page generated at 2026-10-26 04:37:45. Version ac07

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