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 2025-07-06.

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	17% 9% 3 8% 5% 5% 5% 4 6% 7% 33%	119834
	Night	18% 10% 4 8% 5% 4 4 3 5% 6% 32%	118924
Transparency	Day	48% 6% 13% 20% 11% 1	123381
	Night	50% 6% 13% 19% 11% 1	123538
Seeing	Day	24% 16% 29% 20% 9% 3	15670
	Night	23% 15% 29% 24% 7% 1	24742

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% 11% 6% 17% 6% 5% 4 3 4 5% 26%	3597
	11th to 20th	Day	14% 12% 4 11% 5% 5% 5% 4 5% 6% 28%	3570
	21st to end	Day	15% 11% 4 8% 5% 4 5% 3 5% 6% 33%	4098
February	1st to 10th	Day	16% 12% 4 8% 6% 6% 6% 4 6% 6% 26%	1800
	11th to 20th	Day	12% 13% 4 10% 7% 5% 4 4 6% 7% 28%	1936
	21st to end	Day	18% 11% 3 10% 6% 4 4 3 4 5% 31%	1650
March	1st to 10th	Day	21% 13% 6% 9% 5% 3 3 3 4 5% 29%	4476
	11th to 20th	Day	15% 12% 3 7% 4 4 4 3 5% 6% 36%	4695
	21st to end	Day	21% 10% 3 8% 5% 5% 4 4 4 5% 32%	5970
April	1st to 10th	Day	19% 11% 3 7% 6% 4 4 3 4 5% 33%	2400
	11th to 20th	Day	17% 9% 3 6% 5% 5% 5% 4 5% 6% 37%	2374
	21st to end	Day	21% 7% 3 7% 5% 5% 4 3 5% 6% 35%	2492
May	1st to 10th	Day	29% 9% 3 7% 4 4 4 4 5% 7% 24%	5950
	11th to 20th	Day	16% 10% 3 8% 5% 5% 4 3 6% 7% 32%	6318
	21st to end	Day	19% 7% 2 7% 5% 5% 5% 4 7% 8% 32%	6949
June	1st to 10th	Day	16% 7% 3 6% 6% 5% 5% 4 7% 8% 33%	2800
	11th to 20th	Day	16% 6% 2 6% 5% 5% 5% 5% 7% 10% 34%	2800
	21st to end	Day	15% 6% 3 7% 5% 5% 5% 5% 6% 9% 35%	2800
July	1st to 10th	Day	9% 6% 3 7% 5% 5% 6% 5% 7% 10% 37%	4510
	11th to 20th	Day	10% 5% 2 7% 6% 5% 6% 5% 7% 10% 38%	4440
	21st to end	Day	13% 6% 2 8% 6% 6% 5% 5% 7% 9% 33%	4814
August	1st to 10th	Day	16% 7% 3 7% 6% 5% 5% 5% 6% 8% 31%	4376
	11th to 20th	Day	13% 8% 2 7% 5% 5% 6% 4 7% 9% 34%	3730
	21st to end	Day	11% 6% 2 6% 5% 5% 5% 4 8% 11% 38%	4180
September	1st to 10th	Day	14% 6% 2 6% 4 4 4 4 6% 6% 45%	2280
	11th to 20th	Day	13% 5% 2 5% 4 3 4 4 6% 7% 48%	2185
	21st to end	Day	16% 6% 2 6% 3 4 5% 4 7% 8% 41%	2090
October	1st to 10th	Day	19% 8% 2 8% 5% 4 4 3 5% 5% 36%	1996
	11th to 20th	Day	16% 8% 2 6% 3 4 5% 3 5% 7% 40%	1900
	21st to end	Day	22% 9% 4 8% 4 4 4 3 5% 6% 31%	1886
November	1st to 10th	Day	15% 8% 3 6% 4 4 4 3 4 7% 43%	1761
	11th to 20th	Day	16% 9% 3 7% 5% 4 5% 4 5% 6% 37%	1800
	21st to end	Day	19% 9% 3 7% 5% 4 5% 4 5% 7% 32%	1665
December	1st to 10th	Day	16% 14% 4 9% 5% 5% 5% 4 5% 7% 26%	2600
	11th to 20th	Day	21% 12% 3 8% 5% 4 3 3 4 4 33%	2986
	21st to end	Day	21% 15% 4 9% 5% 3 4 3 4 6% 26%	3960
January	1st to 10th	Night	14% 12% 7% 15% 5% 4 4 3 4 5% 27%	6349
	11th to 20th	Night	11% 13% 5% 15% 6% 5% 4 4 5% 6% 25%	6315
	21st to end	Night	17% 10% 4 8% 6% 5% 5% 4 5% 6% 30%	6840
February	1st to 10th	Night	17% 12% 4 10% 6% 4 5% 4 6% 5% 28%	2600
	11th to 20th	Night	14% 11% 3 11% 6% 5% 6% 4 7% 6% 27%	2348
	21st to end	Night	17% 11% 4 10% 5% 5% 4 3 6% 6% 29%	1980
March	1st to 10th	Night	21% 11% 4 8% 6% 5% 4 3 5% 6% 27%	5364
	11th to 20th	Night	18% 12% 3 7% 4 4 4 3 5% 6% 34%	5260
	21st to end	Night	22% 12% 3 9% 5% 4 4 3 4 5% 30%	4962
April	1st to 10th	Night	19% 11% 3 8% 5% 5% 4 4 5% 6% 30%	2000
	11th to 20th	Night	21% 9% 3 6% 4 4 4 3 5% 6% 35%	1975
	21st to end	Night	20% 9% 3 6% 4 4 3 4 4 6% 37%	1906
May	1st to 10th	Night	30% 10% 3 7% 4 4 4 3 4 6% 25%	4020
	11th to 20th	Night	19% 11% 4 8% 5% 5% 5% 3 6% 7% 28%	3575
	21st to end	Night	21% 8% 3 8% 5% 4 4 3 6% 7% 31%	3926
June	1st to 10th	Night	17% 8% 2 6% 4 4 4 4 6% 7% 38%	1600
	11th to 20th	Night	15% 6% 2 6% 5% 4 5% 5% 6% 9% 37%	1600
	21st to end	Night	12% 5% 2 6% 5% 4 4 4 6% 8% 42%	1600
July	1st to 10th	Night	11% 6% 3 7% 6% 5% 5% 4 7% 8% 38%	2560
	11th to 20th	Night	13% 7% 3 8% 5% 4 5% 4 7% 10% 36%	2520
	21st to end	Night	14% 7% 3 7% 5% 3 6% 5% 5% 8% 37%	2739
August	1st to 10th	Night	20% 7% 3 8% 4 5% 4 4 4 8% 32%	2506
	11th to 20th	Night	14% 6% 2 8% 5% 4 5% 4 6% 8% 37%	3096
	21st to end	Night	11% 6% 2 6% 4 3 4 4 7% 8% 44%	3476
September	1st to 10th	Night	16% 7% 2 7% 3 4 4 4 5% 7% 42%	1900
	11th to 20th	Night	13% 6% 2 4 3 3 3 3 4 7% 52%	1995
	21st to end	Night	14% 7% 3 7% 5% 5% 4 3 5% 6% 42%	2090
October	1st to 10th	Night	18% 9% 3 7% 5% 4 4 3 5% 6% 36%	2180
	11th to 20th	Night	16% 6% 2 6% 5% 4 4 4 5% 6% 41%	2280
	21st to end	Night	24% 9% 3 6% 4 4 4 3 5% 7% 32%	2711
November	1st to 10th	Night	15% 8% 3 6% 4 4 4 3 5% 5% 43%	2537
	11th to 20th	Night	18% 9% 3 8% 4 4 4 4 5% 5% 37%	2600
	21st to end	Night	16% 10% 3 8% 4 5% 5% 4 5% 6% 33%	2730
December	1st to 10th	Night	18% 13% 5% 9% 5% 5% 5% 4 5% 7% 24%	4580
	11th to 20th	Night	26% 11% 3 6% 4 4 3 2 4 5% 31%	5208
	21st to end	Night	23% 15% 4 8% 4 3 4 3 4 5% 26%	6996

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% 4 12% 21% 4	3598
	11th to 20th	Day	55% 6% 12% 16% 10% 2	3600
	21st to end	Day	50% 6% 11% 20% 12% 1	4098
February	1st to 10th	Day	57% 5% 15% 15% 8% 1	1800
	11th to 20th	Day	55% 5% 11% 19% 9% 1	1945
	21st to end	Day	55% 5% 11% 18% 11% 1	1650
March	1st to 10th	Day	58% 5% 10% 15% 8% 4	4479
	11th to 20th	Day	47% 6% 17% 20% 7% 3	4695
	21st to end	Day	53% 4 7% 15% 18% 3	5970
April	1st to 10th	Day	53% 5% 8% 21% 11% 2	2400
	11th to 20th	Day	47% 5% 11% 20% 15% 2	2374
	21st to end	Day	50% 5% 10% 19% 13% 3	2492
May	1st to 10th	Day	60% 7% 11% 14% 8% 1	5961
	11th to 20th	Day	50% 8% 13% 19% 9% 1	6322
	21st to end	Day	48% 7% 11% 22% 10% 1	6949
June	1st to 10th	Day	46% 8% 14% 21% 11% 1	2800
	11th to 20th	Day	44% 7% 16% 20% 12% 1	2800
	21st to end	Day	46% 7% 14% 21% 12% 1	2800
July	1st to 10th	Day	40% 8% 14% 23% 13% 1	4440
	11th to 20th	Day	40% 8% 16% 24% 11%	4365
	21st to end	Day	45% 8% 16% 17% 13% 1	4661
August	1st to 10th	Day	48% 7% 15% 20% 8%	4362
	11th to 20th	Day	43% 7% 19% 25% 6% 1	3766
	21st to end	Day	37% 8% 17% 28% 10% 1	4180
September	1st to 10th	Day	39% 7% 12% 26% 14% 1	2280
	11th to 20th	Day	36% 6% 12% 25% 21% 1	2185
	21st to end	Day	40% 7% 13% 24% 14% 2	2090
October	1st to 10th	Day	43% 7% 9% 21% 18% 1	3321
	11th to 20th	Day	38% 6% 12% 26% 17% 1	3140
	21st to end	Day	39% 6% 14% 27% 12% 2	3086
November	1st to 10th	Day	40% 6% 15% 25% 13% 1	1761
	11th to 20th	Day	47% 6% 11% 20% 15% 1	1800
	21st to end	Day	51% 5% 15% 21% 7% 1	1665
December	1st to 10th	Day	56% 6% 14% 19% 4	2600
	11th to 20th	Day	53% 5% 13% 18% 10% 1	2986
	21st to end	Day	58% 5% 8% 17% 10% 2	3960
January	1st to 10th	Night	60% 6% 12% 15% 7% 1	6349
	11th to 20th	Night	58% 6% 13% 13% 7% 1	6360
	21st to end	Night	53% 5% 13% 15% 12% 3	6840
February	1st to 10th	Night	56% 6% 13% 16% 7% 1	2600
	11th to 20th	Night	56% 6% 11% 17% 9% 1	2350
	21st to end	Night	55% 5% 13% 18% 8% 2	1980
March	1st to 10th	Night	56% 6% 12% 14% 10% 1	5374
	11th to 20th	Night	51% 5% 16% 17% 7% 2	5261
	21st to end	Night	55% 5% 6% 16% 16% 2	4962
April	1st to 10th	Night	55% 5% 11% 18% 10% 1	2000
	11th to 20th	Night	50% 5% 10% 18% 16% 1	1975
	21st to end	Night	48% 5% 10% 19% 14% 3	1906
May	1st to 10th	Night	61% 6% 11% 13% 8% 1	4026
	11th to 20th	Night	55% 7% 11% 17% 8% 1	3576
	21st to end	Night	53% 7% 11% 17% 11% 1	3927
June	1st to 10th	Night	44% 6% 13% 24% 12% 2	1600
	11th to 20th	Night	43% 7% 15% 23% 11%	1600
	21st to end	Night	38% 8% 16% 24% 13% 1	1600
July	1st to 10th	Night	42% 8% 15% 19% 16% 1	2520
	11th to 20th	Night	40% 8% 16% 24% 12%	2472
	21st to end	Night	43% 8% 14% 23% 12% 1	2651
August	1st to 10th	Night	50% 7% 14% 20% 8% 1	2496
	11th to 20th	Night	42% 8% 19% 23% 8%	3126
	21st to end	Night	34% 7% 15% 33% 11%	3476
September	1st to 10th	Night	41% 7% 10% 26% 15% 1	1900
	11th to 20th	Night	32% 5% 12% 28% 22% 1	1995
	21st to end	Night	41% 6% 11% 24% 17% 1	2090
October	1st to 10th	Night	45% 7% 13% 20% 13% 1	3635
	11th to 20th	Night	35% 6% 12% 24% 21% 1	3779
	21st to end	Night	39% 6% 14% 29% 12% 1	4461
November	1st to 10th	Night	42% 6% 12% 22% 16% 2	2537
	11th to 20th	Night	49% 5% 12% 19% 13% 2	2600
	21st to end	Night	50% 6% 17% 20% 7% 1	2730
December	1st to 10th	Night	57% 7% 13% 18% 4	4580
	11th to 20th	Night	56% 4 13% 14% 12%	5208
	21st to end	Night	59% 5% 9% 14% 9% 3	6996

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	32% 27% 25% 12% 5%	200
	11th to 20th	Day	33% 26% 27% 13% 2 1	200
	21st to end	Day	28% 24% 30% 14% 3 1	250
February	1st to 10th	Day	38% 28% 27% 6% 1	300
	11th to 20th	Day	35% 27% 26% 11% 2	484
	21st to end	Day	28% 28% 30% 13% 2	414
March	1st to 10th	Day	32% 21% 31% 14% 2	500
	11th to 20th	Day	30% 24% 28% 15% 2	500
	21st to end	Day	35% 21% 26% 11% 5% 2	550
April	1st to 10th	Day	36% 21% 23% 15% 4 1	500
	11th to 20th	Day	30% 24% 26% 14% 5% 1	492
	21st to end	Day	28% 18% 31% 17% 5% 1	500
May	1st to 10th	Day	31% 18% 27% 15% 6% 1	503
	11th to 20th	Day	22% 16% 33% 20% 8% 1	527
	21st to end	Day	20% 12% 31% 25% 9% 3	573
June	1st to 10th	Day	18% 13% 32% 23% 12% 2	530
	11th to 20th	Day	21% 10% 30% 25% 12% 2	530
	21st to end	Day	18% 12% 33% 22% 12% 3	530
July	1st to 10th	Day	14% 9% 29% 29% 16% 4	505
	11th to 20th	Day	13% 7% 27% 33% 17% 3	484
	21st to end	Day	12% 8% 34% 30% 14% 2	532
August	1st to 10th	Day	12% 7% 27% 32% 16% 5%	486
	11th to 20th	Day	14% 6% 30% 27% 15% 7%	479
	21st to end	Day	13% 9% 31% 26% 15% 6%	539
September	1st to 10th	Day	19% 9% 22% 30% 14% 7%	490
	11th to 20th	Day	16% 10% 24% 20% 19% 11%	490
	21st to end	Day	19% 10% 28% 26% 12% 6%	490
October	1st to 10th	Day	25% 12% 24% 22% 12% 5%	490
	11th to 20th	Day	21% 16% 29% 21% 10% 2	490
	21st to end	Day	31% 14% 29% 19% 7% 1	432
November	1st to 10th	Day	26% 16% 27% 19% 10% 2	395
	11th to 20th	Day	27% 21% 30% 18% 5% 1	400
	21st to end	Day	32% 22% 26% 17% 3 1	265
December	1st to 10th	Day	26% 27% 28% 14% 6%	200
	11th to 20th	Day	32% 25% 31% 10% 3	200
	21st to end	Day	27% 21% 33% 15% 3	220
January	1st to 10th	Night	24% 21% 36% 17% 2	800
	11th to 20th	Night	27% 22% 30% 19% 2	800
	21st to end	Night	22% 22% 35% 20% 2	880
February	1st to 10th	Night	26% 22% 33% 18% 2	800
	11th to 20th	Night	27% 23% 34% 15% 1	783
	21st to end	Night	28% 23% 31% 15% 3	660
March	1st to 10th	Night	27% 23% 32% 16% 2	800
	11th to 20th	Night	25% 24% 33% 16% 2	800
	21st to end	Night	28% 21% 29% 18% 4	880
April	1st to 10th	Night	27% 21% 30% 20% 3	800
	11th to 20th	Night	29% 17% 29% 18% 6%	791
	21st to end	Night	27% 15% 30% 23% 5%	706
May	1st to 10th	Night	28% 15% 30% 20% 7%	592
	11th to 20th	Night	22% 12% 32% 28% 5% 1	394
	21st to end	Night	15% 9% 29% 36% 9% 1	433
June	1st to 10th	Night	19% 6% 32% 32% 11%	400
	11th to 20th	Night	17% 7% 30% 34% 11% 1	400
	21st to end	Night	13% 7% 27% 40% 13% 1	400
July	1st to 10th	Night	14% 5% 19% 45% 16% 1	380
	11th to 20th	Night	15% 4 21% 43% 15% 1	368
	21st to end	Night	17% 3 20% 48% 12%	416
August	1st to 10th	Night	13% 3 23% 43% 17% 1	378
	11th to 20th	Night	15% 3 21% 38% 20% 3	749
	21st to end	Night	16% 6% 22% 34% 20% 2	836
September	1st to 10th	Night	17% 6% 28% 32% 15% 2	760
	11th to 20th	Night	16% 7% 25% 29% 19% 3	760
	21st to end	Night	18% 9% 29% 29% 14% 2	760
October	1st to 10th	Night	25% 9% 23% 29% 12% 2	760
	11th to 20th	Night	20% 12% 31% 27% 9%	760
	21st to end	Night	31% 12% 30% 22% 4	836
November	1st to 10th	Night	21% 15% 33% 24% 7%	780
	11th to 20th	Night	24% 16% 33% 18% 7% 1	800
	21st to end	Night	24% 20% 30% 22% 5%	800
December	1st to 10th	Night	26% 21% 32% 17% 4	800
	11th to 20th	Night	30% 17% 29% 22% 2	800
	21st to end	Night	31% 20% 31% 16% 2	880

Page generated at 2025-07-03 01:30:17. Version ac07

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