Cors Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cors Observatory clear sky chart from 2005-11-06 to 2026-01-17.

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	18% 7% 2 6% 4 4 4 4 5% 7% 38%	127116
	Night	17% 8% 3 6% 4 4 4 3 5% 6% 39%	124569
Transparency	Day	45% 4 11% 22% 15% 3	130636
	Night	45% 4 11% 22% 16% 3	129136
Seeing	Day	23% 16% 32% 19% 7% 2	16460
	Night	22% 14% 32% 24% 7% 1	25417

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	13% 7% 3 7% 5% 5% 5% 4 7% 7% 38%	3677
	11th to 20th	Day	14% 7% 3 6% 3 3 4 3 6% 6% 46%	3618
	21st to end	Day	16% 8% 2 6% 4 4 3 3 4 5% 45%	3960
February	1st to 10th	Day	17% 8% 3 6% 4 5% 5% 5% 6% 6% 36%	1831
	11th to 20th	Day	14% 7% 2 7% 5% 3 4 4 5% 7% 40%	1948
	21st to end	Day	17% 11% 3 7% 6% 4 4 3 5% 5% 37%	1650
March	1st to 10th	Day	22% 11% 3 6% 4 4 4 3 4 5% 33%	4476
	11th to 20th	Day	17% 9% 2 6% 4 4 5% 4 5% 6% 37%	4790
	21st to end	Day	26% 9% 2 6% 4 4 4 4 5% 6% 31%	5740
April	1st to 10th	Day	20% 10% 3 7% 4 3 4 3 4 6% 37%	2400
	11th to 20th	Day	18% 7% 2 6% 4 5% 5% 3 5% 7% 38%	2408
	21st to end	Day	20% 6% 3 7% 5% 4 4 3 5% 6% 37%	2600
May	1st to 10th	Day	31% 10% 2 7% 4 3 4 3 5% 6% 24%	5886
	11th to 20th	Day	20% 8% 3 8% 5% 4 4 4 5% 7% 31%	5993
	21st to end	Day	20% 7% 2 6% 4 4 5% 4 5% 8% 35%	6949
June	1st to 10th	Day	20% 7% 2 6% 5% 4 5% 4 6% 8% 33%	2800
	11th to 20th	Day	17% 6% 2 6% 4 4 5% 4 6% 8% 39%	2800
	21st to end	Day	18% 7% 2 6% 6% 5% 5% 4 5% 8% 34%	2800
July	1st to 10th	Day	16% 7% 3 7% 5% 5% 5% 4 7% 9% 33%	6360
	11th to 20th	Day	9% 6% 3 6% 5% 5% 6% 4 7% 11% 39%	6360
	21st to end	Day	14% 6% 2 6% 4 4 4 3 6% 7% 43%	6926
August	1st to 10th	Day	15% 6% 3 7% 5% 5% 6% 5% 6% 8% 35%	4301
	11th to 20th	Day	16% 8% 2 7% 4 5% 5% 4 6% 8% 36%	4175
	21st to end	Day	8% 4 2 5% 4 4 6% 5% 7% 10% 45%	4312
September	1st to 10th	Day	13% 5% 2 5% 3 4 5% 4 5% 6% 48%	2390
	11th to 20th	Day	14% 5% 2 6% 3 3 4 2 4 6% 51%	2200
	21st to end	Day	17% 6% 2 6% 4 4 4 3 5% 6% 45%	2200
October	1st to 10th	Day	21% 6% 1 5% 5% 4 4 3 4 5% 42%	2200
	11th to 20th	Day	16% 5% 1 6% 3 5% 4 3 5% 6% 46%	1941
	21st to end	Day	21% 7% 2 6% 4 4 4 3 5% 6% 40%	1980
November	1st to 10th	Day	15% 6% 2 5% 5% 3 2 3 4 5% 49%	1851
	11th to 20th	Day	18% 5% 2 5% 3 3 3 3 4 5% 48%	1890
	21st to end	Day	20% 7% 2 6% 3 3 4 3 5% 5% 42%	1890
December	1st to 10th	Day	14% 9% 2 6% 3 3 4 3 4 7% 45%	2700
	11th to 20th	Day	19% 6% 2 5% 3 4 4 2 5% 6% 45%	3066
	21st to end	Day	21% 9% 3 6% 4 3 3 3 5% 5% 37%	4048
January	1st to 10th	Night	14% 7% 3 6% 4 4 4 3 5% 6% 45%	6489
	11th to 20th	Night	14% 8% 3 8% 5% 5% 4 3 5% 6% 38%	6400
	21st to end	Night	15% 9% 3 6% 5% 4 5% 3 5% 5% 39%	6996
February	1st to 10th	Night	16% 9% 3 7% 5% 4 5% 4 5% 6% 38%	2562
	11th to 20th	Night	13% 9% 3 6% 5% 4 4 4 6% 7% 40%	2338
	21st to end	Night	16% 9% 3 5% 4 4 4 3 6% 6% 40%	1980
March	1st to 10th	Night	24% 9% 2 6% 3 4 4 4 5% 5% 35%	5364
	11th to 20th	Night	16% 10% 3 7% 5% 4 5% 3 5% 6% 36%	5168
	21st to end	Night	26% 8% 2 5% 4 3 4 4 6% 6% 32%	5175
April	1st to 10th	Night	19% 8% 3 6% 4 4 4 3 4 6% 36%	2000
	11th to 20th	Night	21% 8% 2 5% 4 4 4 3 4 6% 39%	1941
	21st to end	Night	20% 7% 3 7% 4 4 4 4 5% 6% 38%	1800
May	1st to 10th	Night	33% 10% 3 6% 4 4 4 3 4 6% 23%	4054
	11th to 20th	Night	15% 8% 3 8% 6% 6% 7% 5% 6% 8% 29%	3942
	21st to end	Night	20% 8% 3 8% 5% 4 4 4 5% 5% 33%	3926
June	1st to 10th	Night	18% 9% 3 6% 5% 4 4 3 4 6% 37%	1600
	11th to 20th	Night	19% 6% 2 5% 4 4 3 5% 6% 8% 38%	1600
	21st to end	Night	17% 8% 3 5% 5% 4 4 4 6% 8% 37%	1600
July	1st to 10th	Night	12% 7% 2 7% 6% 5% 5% 4 6% 9% 37%	3600
	11th to 20th	Night	10% 6% 3 7% 5% 5% 4 5% 8% 9% 38%	3600
	21st to end	Night	11% 6% 2 6% 4 4 4 4 5% 6% 47%	3926
August	1st to 10th	Night	19% 7% 3 7% 4 4 5% 4 5% 7% 35%	2816
	11th to 20th	Night	12% 8% 3 7% 5% 5% 5% 5% 7% 9% 35%	2872
	21st to end	Night	9% 4 2 5% 3 4 4 3 6% 8% 52%	3583
September	1st to 10th	Night	12% 6% 2 6% 3 4 4 4 5% 7% 49%	2010
	11th to 20th	Night	13% 5% 2 5% 3 3 4 3 4 7% 52%	2200
	21st to end	Night	16% 6% 2 5% 4 4 4 3 5% 7% 45%	2200
October	1st to 10th	Night	19% 7% 2 6% 4 4 3 2 5% 6% 42%	2200
	11th to 20th	Night	16% 5% 1 5% 4 4 4 4 5% 5% 47%	2457
	21st to end	Night	22% 7% 2 6% 3 3 4 3 5% 6% 39%	2860
November	1st to 10th	Night	15% 5% 2 5% 3 4 4 4 5% 6% 47%	2667
	11th to 20th	Night	17% 7% 2 5% 3 4 4 3 4 6% 47%	2730
	21st to end	Night	16% 6% 2 6% 4 4 4 4 4 7% 44%	2730
December	1st to 10th	Night	16% 9% 3 6% 4 4 4 3 5% 6% 40%	4685
	11th to 20th	Night	22% 6% 2 6% 3 4 3 3 5% 4 42%	5348
	21st to end	Night	18% 10% 3 7% 4 3 4 3 4 5% 39%	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	44% 4 8% 27% 16% 1	3678
	11th to 20th	Day	40% 4 11% 23% 19% 3	3648
	21st to end	Day	42% 4 8% 22% 20% 4	3960
February	1st to 10th	Day	47% 4 12% 21% 13% 3	1831
	11th to 20th	Day	43% 5% 11% 22% 15% 4	1957
	21st to end	Day	49% 4 9% 21% 15% 3	1650
March	1st to 10th	Day	53% 3 8% 14% 12% 10%	4479
	11th to 20th	Day	45% 4 12% 23% 12% 3	4790
	21st to end	Day	54% 3 5% 16% 16% 7%	5740
April	1st to 10th	Day	50% 3 7% 18% 17% 4	2400
	11th to 20th	Day	45% 5% 10% 20% 17% 3	2408
	21st to end	Day	49% 5% 7% 18% 18% 4	2600
May	1st to 10th	Day	62% 4 12% 13% 8% 1	5897
	11th to 20th	Day	52% 4 14% 18% 10% 2	5997
	21st to end	Day	47% 5% 12% 20% 14% 1	6949
June	1st to 10th	Day	48% 6% 11% 22% 12% 1	2800
	11th to 20th	Day	43% 5% 13% 20% 16% 3	2800
	21st to end	Day	48% 6% 10% 21% 13% 1	2800
July	1st to 10th	Day	48% 5% 12% 21% 13% 1	6290
	11th to 20th	Day	39% 5% 15% 26% 14% 1	6285
	21st to end	Day	39% 4 12% 27% 16% 1	6728
August	1st to 10th	Day	46% 6% 15% 22% 10% 1	4287
	11th to 20th	Day	45% 5% 13% 26% 10% 1	4214
	21st to end	Day	32% 5% 13% 29% 19% 2	4312
September	1st to 10th	Day	35% 6% 12% 25% 20% 2	2390
	11th to 20th	Day	35% 4 7% 28% 24% 3	2200
	21st to end	Day	41% 4 8% 24% 21% 2	2200
October	1st to 10th	Day	44% 4 8% 23% 18% 3	3590
	11th to 20th	Day	33% 3 7% 31% 24% 3	3131
	21st to end	Day	35% 3 7% 30% 21% 3	3180
November	1st to 10th	Day	39% 3 13% 26% 17% 2	1851
	11th to 20th	Day	38% 4 7% 24% 23% 3	1890
	21st to end	Day	43% 3 15% 23% 14% 1	1890
December	1st to 10th	Day	41% 3 13% 30% 12% 1	2700
	11th to 20th	Day	42% 3 12% 18% 20% 6%	3066
	21st to end	Day	50% 3 9% 20% 14% 5%	4048
January	1st to 10th	Night	39% 3 10% 31% 14% 2	6489
	11th to 20th	Night	47% 5% 11% 21% 13% 3	6445
	21st to end	Night	47% 3 9% 18% 17% 6%	6996
February	1st to 10th	Night	47% 5% 12% 20% 14% 3	2562
	11th to 20th	Night	44% 4 10% 22% 17% 4	2340
	21st to end	Night	44% 3 10% 24% 14% 5%	1980
March	1st to 10th	Night	51% 3 8% 15% 16% 7%	5374
	11th to 20th	Night	49% 4 15% 17% 13% 2	5169
	21st to end	Night	53% 2 5% 13% 21% 6%	5175
April	1st to 10th	Night	49% 4 9% 18% 17% 3	2000
	11th to 20th	Night	46% 4 8% 21% 19% 3	1941
	21st to end	Night	47% 4 9% 19% 17% 3	1800
May	1st to 10th	Night	62% 4 12% 13% 8% 2	4060
	11th to 20th	Night	52% 5% 15% 17% 9% 1	3943
	21st to end	Night	53% 4 13% 18% 10% 1	3927
June	1st to 10th	Night	49% 4 10% 23% 13% 1	1600
	11th to 20th	Night	43% 7% 13% 21% 14% 2	1600
	21st to end	Night	45% 6% 13% 22% 13% 1	1600
July	1st to 10th	Night	44% 5% 12% 24% 14% 1	3560
	11th to 20th	Night	41% 5% 18% 20% 15%	3552
	21st to end	Night	36% 5% 11% 32% 15% 1	3809
August	1st to 10th	Night	50% 5% 13% 20% 11% 1	2806
	11th to 20th	Night	45% 7% 16% 21% 11% 1	2899
	21st to end	Night	30% 4 15% 28% 22% 1	3583
September	1st to 10th	Night	35% 4 12% 28% 20% 1	2010
	11th to 20th	Night	34% 4 10% 26% 25% 2	2200
	21st to end	Night	40% 4 9% 27% 19% 2	2200
October	1st to 10th	Night	42% 4 14% 22% 16% 2	3590
	11th to 20th	Night	29% 3 10% 28% 27% 3	4006
	21st to end	Night	36% 3 10% 32% 17% 2	4610
November	1st to 10th	Night	39% 4 10% 25% 19% 3	2667
	11th to 20th	Night	40% 3 9% 23% 21% 3	2730
	21st to end	Night	42% 3 13% 26% 14% 1	2730
December	1st to 10th	Night	46% 3 9% 30% 11% 1	4685
	11th to 20th	Night	47% 3 12% 16% 21% 2	5348
	21st to end	Night	48% 3 8% 22% 16% 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	28% 23% 31% 16% 2	220
	11th to 20th	Day	29% 25% 33% 10% 2 1	212
	21st to end	Day	22% 26% 37% 12% 2	220
February	1st to 10th	Day	28% 25% 33% 13% 2	383
	11th to 20th	Day	31% 27% 30% 10% 2	496
	21st to end	Day	27% 23% 33% 15% 2	414
March	1st to 10th	Day	30% 20% 36% 12% 3	500
	11th to 20th	Day	27% 27% 32% 11% 3 1	500
	21st to end	Day	31% 21% 33% 9% 4 2	550
April	1st to 10th	Day	33% 21% 29% 11% 5% 1	500
	11th to 20th	Day	29% 23% 29% 13% 5% 1	492
	21st to end	Day	27% 17% 35% 15% 5% 1	500
May	1st to 10th	Day	34% 18% 31% 11% 5% 1	503
	11th to 20th	Day	20% 17% 37% 19% 5% 2	527
	21st to end	Day	20% 13% 36% 23% 6% 1	573
June	1st to 10th	Day	22% 10% 38% 20% 9% 1	530
	11th to 20th	Day	18% 17% 33% 24% 6% 2	530
	21st to end	Day	18% 13% 38% 21% 8% 2	530
July	1st to 10th	Day	17% 6% 36% 26% 12% 3	525
	11th to 20th	Day	17% 6% 33% 29% 12% 3	524
	21st to end	Day	14% 9% 35% 32% 8% 2	576
August	1st to 10th	Day	13% 8% 30% 31% 15% 2	526
	11th to 20th	Day	17% 8% 28% 28% 15% 4	519
	21st to end	Day	13% 9% 34% 27% 12% 5%	583
September	1st to 10th	Day	17% 9% 31% 24% 13% 6%	530
	11th to 20th	Day	18% 10% 28% 20% 15% 8%	530
	21st to end	Day	18% 10% 34% 22% 9% 5%	530
October	1st to 10th	Day	24% 13% 26% 22% 11% 4	530
	11th to 20th	Day	21% 14% 30% 23% 9% 2	458
	21st to end	Day	29% 14% 30% 20% 6%	462
November	1st to 10th	Day	17% 23% 31% 20% 7% 2	425
	11th to 20th	Day	27% 22% 30% 17% 4	430
	21st to end	Day	31% 20% 29% 15% 5%	430
December	1st to 10th	Day	23% 26% 30% 16% 5%	240
	11th to 20th	Day	31% 25% 31% 11% 1	220
	21st to end	Day	29% 24% 26% 14% 6% 1	242
January	1st to 10th	Night	21% 24% 35% 17% 2	840
	11th to 20th	Night	27% 19% 33% 18% 3	825
	21st to end	Night	21% 22% 37% 18% 3	880
February	1st to 10th	Night	24% 19% 37% 20% 1	800
	11th to 20th	Night	25% 21% 37% 15% 3	783
	21st to end	Night	23% 19% 35% 20% 3	660
March	1st to 10th	Night	27% 19% 36% 16% 3	800
	11th to 20th	Night	22% 21% 35% 17% 5%	800
	21st to end	Night	26% 17% 34% 19% 4	880
April	1st to 10th	Night	25% 18% 33% 20% 3	800
	11th to 20th	Night	26% 17% 33% 19% 5%	757
	21st to end	Night	23% 17% 33% 23% 5%	600
May	1st to 10th	Night	28% 13% 32% 22% 6%	600
	11th to 20th	Night	21% 12% 35% 25% 6% 1	553
	21st to end	Night	20% 9% 30% 31% 9%	433
June	1st to 10th	Night	22% 8% 30% 30% 9% 2	400
	11th to 20th	Night	21% 7% 30% 34% 8% 1	400
	21st to end	Night	22% 5% 30% 33% 11%	400
July	1st to 10th	Night	17% 4 25% 42% 11% 1	390
	11th to 20th	Night	13% 3 24% 43% 16% 1	388
	21st to end	Night	16% 3 27% 40% 13% 1	438
August	1st to 10th	Night	15% 3 24% 42% 16% 1	537
	11th to 20th	Night	15% 6% 28% 33% 17% 1	592
	21st to end	Night	14% 4 27% 37% 17% 2	877
September	1st to 10th	Night	14% 6% 32% 31% 17% 1	800
	11th to 20th	Night	16% 6% 28% 29% 19% 3	800
	21st to end	Night	19% 6% 29% 28% 15% 3	800
October	1st to 10th	Night	22% 9% 28% 26% 14% 1	800
	11th to 20th	Night	20% 13% 29% 27% 10% 1	800
	21st to end	Night	26% 14% 31% 23% 6%	880
November	1st to 10th	Night	18% 14% 36% 24% 7% 1	820
	11th to 20th	Night	22% 19% 32% 20% 6% 1	840
	21st to end	Night	22% 21% 32% 19% 5% 1	840
December	1st to 10th	Night	24% 17% 37% 19% 3	840
	11th to 20th	Night	24% 20% 32% 20% 3	840
	21st to end	Night	26% 19% 33% 20% 2	924

Page generated at 2026-01-14 01:14:29. Version ac07

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