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 2024-11-03.

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 7% 5% 5% 5% 4 6% 7% 34%	101350
	Night	18% 10% 3 8% 5% 4 4 3 5% 6% 33%	97636
Transparency	Day	48% 6% 13% 20% 11% 1	101105
	Night	50% 6% 13% 19% 10% 1	97501
Seeing	Day	23% 16% 29% 21% 9% 3	14856
	Night	23% 15% 29% 25% 8% 1	23891

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	14% 11% 4 8% 6% 6% 5% 4 5% 6% 31%	2517
	11th to 20th	Day	16% 13% 5% 11% 6% 5% 5% 4 5% 5% 24%	2520
	21st to end	Day	19% 13% 4 10% 6% 5% 5% 3 5% 6% 24%	2868
February	1st to 10th	Day	16% 12% 4 7% 5% 5% 6% 4 6% 6% 27%	1710
	11th to 20th	Day	12% 13% 4 10% 6% 5% 4 3 6% 7% 30%	1836
	21st to end	Day	19% 11% 3 9% 6% 4 4 3 4 4 31%	1570
March	1st to 10th	Day	24% 14% 5% 9% 5% 3 2 3 3 4 29%	3116
	11th to 20th	Day	13% 15% 4 8% 5% 5% 4 4 5% 7% 31%	3279
	21st to end	Day	20% 9% 3 8% 5% 4 4 4 4 5% 34%	4166
April	1st to 10th	Day	19% 11% 3 6% 6% 4 4 3 4 5% 34%	2280
	11th to 20th	Day	17% 8% 3 6% 5% 5% 5% 3 5% 6% 37%	2254
	21st to end	Day	21% 7% 3 7% 5% 5% 4 3 5% 6% 34%	2367
May	1st to 10th	Day	24% 9% 3 7% 5% 4 4 3 5% 7% 30%	4156
	11th to 20th	Day	17% 9% 3 7% 5% 4 4 3 6% 8% 32%	4398
	21st to end	Day	12% 5% 2 6% 4 5% 5% 4 7% 9% 42%	4837
June	1st to 10th	Day	15% 7% 2 6% 6% 5% 5% 4 7% 8% 33%	2660
	11th to 20th	Day	15% 6% 2 6% 5% 5% 5% 5% 7% 10% 34%	2660
	21st to end	Day	16% 6% 3 7% 5% 5% 5% 5% 5% 9% 34%	2660
July	1st to 10th	Day	9% 6% 3 7% 5% 5% 6% 5% 7% 10% 37%	4440
	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 3 4 3 4 7% 43%	1689
	11th to 20th	Day	17% 9% 3 7% 5% 5% 5% 4 5% 5% 35%	1710
	21st to end	Day	18% 9% 3 7% 5% 4 5% 4 5% 7% 32%	1582
December	1st to 10th	Day	18% 13% 4 8% 6% 5% 6% 4 4 7% 24%	1520
	11th to 20th	Day	23% 11% 3 8% 5% 4 2 3 4 4 32%	1906
	21st to end	Day	25% 16% 5% 9% 6% 4 3 3 4 5% 21%	2772
January	1st to 10th	Night	16% 12% 4 8% 4 4 4 3 5% 5% 34%	4429
	11th to 20th	Night	14% 14% 5% 12% 6% 5% 4 3 4 6% 27%	4440
	21st to end	Night	24% 13% 4 9% 6% 4 4 3 4 5% 25%	4784
February	1st to 10th	Night	17% 11% 4 10% 6% 4 5% 4 6% 5% 29%	2470
	11th to 20th	Night	14% 11% 3 10% 6% 5% 6% 4 7% 6% 28%	2228
	21st to end	Night	18% 11% 4 9% 5% 5% 4 3 6% 6% 30%	1884
March	1st to 10th	Night	25% 11% 3 8% 5% 5% 4 3 4 5% 27%	3724
	11th to 20th	Night	16% 14% 3 8% 5% 5% 4 3 4 5% 31%	3676
	21st to end	Night	17% 13% 3 9% 4 4 4 3 4 5% 32%	3466
April	1st to 10th	Night	18% 11% 3 8% 5% 5% 4 4 5% 6% 31%	1900
	11th to 20th	Night	21% 8% 3 6% 4 4 4 3 5% 6% 36%	1875
	21st to end	Night	21% 9% 3 6% 4 4 3 4 4 6% 37%	1811
May	1st to 10th	Night	25% 10% 2 8% 4 4 4 3 5% 6% 30%	2814
	11th to 20th	Night	18% 9% 4 8% 6% 5% 5% 4 6% 7% 30%	2495
	21st to end	Night	12% 6% 3 8% 5% 4 5% 3 7% 9% 38%	2738
June	1st to 10th	Night	17% 8% 2 6% 4 4 4 4 6% 7% 38%	1520
	11th to 20th	Night	15% 6% 2 5% 5% 4 5% 5% 6% 9% 38%	1520
	21st to end	Night	12% 5% 2 7% 5% 4 4 4 6% 8% 42%	1520
July	1st to 10th	Night	11% 6% 3 7% 6% 5% 5% 4 7% 8% 38%	2520
	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% 31%	2711
November	1st to 10th	Night	16% 8% 3 6% 5% 4 4 3 5% 5% 42%	2434
	11th to 20th	Night	19% 9% 3 8% 4 4 4 4 5% 5% 36%	2470
	21st to end	Night	16% 10% 3 8% 4 5% 5% 4 5% 7% 34%	2593
December	1st to 10th	Night	19% 13% 4 8% 5% 5% 5% 4 5% 7% 26%	2660
	11th to 20th	Night	22% 11% 4 8% 5% 5% 4 3 5% 6% 28%	3288
	21st to end	Night	26% 15% 4 8% 4 4 4 3 5% 5% 23%	4884

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	51% 4 16% 24% 5%	2518
	11th to 20th	Day	61% 6% 13% 15% 5% 1	2520
	21st to end	Day	60% 7% 12% 15% 5% 1	2868
February	1st to 10th	Day	56% 5% 15% 15% 8% 1	1710
	11th to 20th	Day	54% 4 11% 19% 9% 1	1845
	21st to end	Day	56% 4 11% 18% 11% 1	1570
March	1st to 10th	Day	60% 4 11% 16% 7% 1	3119
	11th to 20th	Day	50% 7% 16% 18% 8% 1	3279
	21st to end	Day	52% 4 7% 17% 18% 3	4166
April	1st to 10th	Day	53% 5% 8% 21% 10% 2	2280
	11th to 20th	Day	47% 5% 11% 21% 15% 2	2254
	21st to end	Day	51% 5% 10% 19% 13% 3	2367
May	1st to 10th	Day	54% 6% 12% 16% 11% 1	4170
	11th to 20th	Day	49% 7% 14% 20% 9% 1	4402
	21st to end	Day	37% 7% 14% 26% 14% 2	4837
June	1st to 10th	Day	46% 8% 14% 21% 11% 1	2660
	11th to 20th	Day	43% 8% 16% 20% 12% 1	2660
	21st to end	Day	46% 7% 14% 20% 11% 1	2660
July	1st to 10th	Day	41% 8% 14% 23% 13% 1	4370
	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	49% 5% 10% 21% 13% 2	1996
	11th to 20th	Day	43% 6% 14% 25% 11% 1	1900
	21st to end	Day	55% 5% 12% 16% 10% 1	1886
November	1st to 10th	Day	41% 6% 15% 25% 13% 1	1689
	11th to 20th	Day	49% 6% 12% 19% 13% 1	1710
	21st to end	Day	50% 5% 15% 22% 7% 1	1582
December	1st to 10th	Day	60% 6% 13% 14% 7% 1	1520
	11th to 20th	Day	56% 4 14% 18% 7% 1	1906
	21st to end	Day	65% 6% 9% 15% 5%	2772
January	1st to 10th	Night	52% 5% 16% 18% 8% 1	4429
	11th to 20th	Night	59% 5% 12% 16% 7% 1	4440
	21st to end	Night	62% 5% 12% 15% 6% 1	4784
February	1st to 10th	Night	56% 6% 13% 16% 8% 2	2470
	11th to 20th	Night	55% 6% 11% 17% 10% 1	2230
	21st to end	Night	54% 4 13% 18% 8% 2	1884
March	1st to 10th	Night	58% 6% 10% 16% 8% 1	3734
	11th to 20th	Night	54% 6% 13% 19% 7% 1	3677
	21st to end	Night	54% 4 6% 17% 17% 3	3466
April	1st to 10th	Night	54% 5% 11% 19% 10% 1	1900
	11th to 20th	Night	49% 5% 10% 19% 16% 1	1875
	21st to end	Night	48% 4 11% 19% 14% 4	1811
May	1st to 10th	Night	55% 7% 11% 15% 10% 2	2820
	11th to 20th	Night	53% 8% 13% 17% 8% 2	2496
	21st to end	Night	41% 8% 14% 20% 15% 2	2739
June	1st to 10th	Night	43% 6% 13% 25% 11% 2	1520
	11th to 20th	Night	42% 7% 15% 23% 12%	1520
	21st to end	Night	39% 9% 16% 24% 13% 1	1520
July	1st to 10th	Night	42% 8% 15% 18% 15% 1	2480
	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	48% 7% 12% 20% 12% 1	2180
	11th to 20th	Night	43% 6% 14% 24% 12% 1	2280
	21st to end	Night	52% 5% 14% 18% 10% 1	2711
November	1st to 10th	Night	43% 6% 12% 21% 16% 2	2434
	11th to 20th	Night	50% 5% 12% 19% 11% 2	2470
	21st to end	Night	50% 6% 17% 20% 7% 1	2593
December	1st to 10th	Night	57% 7% 14% 16% 6%	2660
	11th to 20th	Night	57% 5% 14% 15% 9%	3288
	21st to end	Night	63% 6% 10% 16% 5%	4884

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% 24% 26% 13% 6%	180
	11th to 20th	Day	33% 25% 27% 13% 2 1	180
	21st to end	Day	27% 24% 30% 15% 3 1	225
February	1st to 10th	Day	37% 27% 27% 7% 1	270
	11th to 20th	Day	33% 27% 27% 11% 2	444
	21st to end	Day	29% 27% 29% 14% 2	382
March	1st to 10th	Day	30% 20% 33% 15% 2	460
	11th to 20th	Day	30% 23% 28% 15% 3	460
	21st to end	Day	35% 20% 26% 11% 6% 2	506
April	1st to 10th	Day	34% 21% 23% 16% 5% 1	460
	11th to 20th	Day	28% 24% 26% 14% 6% 2	452
	21st to end	Day	27% 18% 31% 17% 5% 2	460
May	1st to 10th	Day	31% 17% 28% 16% 6% 1	463
	11th to 20th	Day	21% 14% 34% 21% 8% 1	487
	21st to end	Day	18% 11% 30% 27% 10% 4	529
June	1st to 10th	Day	17% 13% 33% 23% 12% 2	490
	11th to 20th	Day	19% 10% 30% 26% 13% 2	490
	21st to end	Day	19% 12% 32% 22% 12% 3	490
July	1st to 10th	Day	15% 8% 28% 29% 16% 4	485
	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% 26% 20% 10% 2	371
	11th to 20th	Day	27% 21% 29% 17% 5% 1	370
	21st to end	Day	30% 22% 26% 18% 3 1	242
December	1st to 10th	Day	27% 27% 28% 13% 5%	180
	11th to 20th	Day	31% 24% 32% 9% 3	180
	21st to end	Day	28% 22% 32% 16% 2 1	198
January	1st to 10th	Night	24% 20% 37% 17% 2	760
	11th to 20th	Night	28% 22% 30% 19% 2	760
	21st to end	Night	22% 22% 34% 20% 2	836
February	1st to 10th	Night	25% 21% 33% 18% 2	760
	11th to 20th	Night	25% 23% 35% 16% 1	743
	21st to end	Night	29% 23% 30% 15% 3	628
March	1st to 10th	Night	27% 23% 32% 16% 2	760
	11th to 20th	Night	25% 24% 34% 16% 2	760
	21st to end	Night	27% 21% 28% 19% 5%	836
April	1st to 10th	Night	26% 20% 30% 20% 3	760
	11th to 20th	Night	27% 17% 29% 19% 7%	751
	21st to end	Night	27% 14% 30% 23% 5%	671
May	1st to 10th	Night	27% 15% 30% 20% 7%	563
	11th to 20th	Night	22% 12% 33% 28% 5% 1	374
	21st to end	Night	14% 10% 30% 37% 9% 1	411
June	1st to 10th	Night	19% 5% 32% 32% 11%	380
	11th to 20th	Night	17% 7% 29% 34% 12% 1	380
	21st to end	Night	13% 7% 27% 40% 12% 1	380
July	1st to 10th	Night	14% 5% 19% 45% 16% 1	370
	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% 32% 24% 7%	749
	11th to 20th	Night	25% 15% 33% 18% 7% 1	760
	21st to end	Night	23% 20% 30% 22% 5%	760
December	1st to 10th	Night	26% 21% 32% 17% 4	760
	11th to 20th	Night	29% 17% 29% 22% 2	760
	21st to end	Night	31% 19% 32% 16% 2	836

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