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 2024-11-25.

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% 7% 2 6% 4 4 4 4 5% 7% 38%	101443
	Night	17% 8% 2 6% 4 4 4 4 5% 6% 39%	98039
Transparency	Day	44% 5% 11% 22% 15% 2	104981
	Night	45% 4 11% 22% 15% 2	102590
Seeing	Day	23% 16% 32% 20% 7% 2	15067
	Night	21% 14% 32% 24% 8% 1	23948

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	15% 8% 2 6% 4 4 4 3 5% 6% 44%	2517
	11th to 20th	Day	17% 6% 3 7% 4 3 4 3 5% 5% 41%	2520
	21st to end	Day	20% 10% 2 6% 5% 5% 4 4 4 5% 35%	2772
February	1st to 10th	Day	18% 8% 3 6% 4 5% 5% 4 6% 6% 36%	1739
	11th to 20th	Day	13% 8% 2 7% 6% 3 5% 4 6% 7% 40%	1848
	21st to end	Day	17% 11% 3 7% 6% 4 4 3 5% 5% 36%	1570
March	1st to 10th	Day	25% 14% 2 6% 4 4 4 3 4 5% 29%	3116
	11th to 20th	Day	14% 8% 2 6% 4 5% 5% 4 6% 7% 39%	3346
	21st to end	Day	22% 9% 3 7% 4 4 4 4 5% 5% 34%	4006
April	1st to 10th	Day	19% 10% 3 7% 4 3 4 3 4 6% 38%	2280
	11th to 20th	Day	17% 7% 2 6% 5% 5% 5% 3 5% 8% 38%	2286
	21st to end	Day	21% 6% 3 7% 5% 4 4 3 5% 6% 36%	2470
May	1st to 10th	Day	22% 12% 3 8% 5% 3 4 3 5% 7% 28%	4106
	11th to 20th	Day	22% 8% 2 8% 5% 4 3 3 5% 7% 33%	4171
	21st to end	Day	15% 5% 2 5% 4 4 4 4 6% 9% 44%	4837
June	1st to 10th	Day	20% 7% 2 6% 5% 4 5% 4 6% 7% 33%	2660
	11th to 20th	Day	16% 6% 2 6% 4 4 5% 4 6% 8% 39%	2660
	21st to end	Day	18% 7% 2 7% 6% 6% 5% 4 5% 8% 32%	2660
July	1st to 10th	Day	13% 6% 3 7% 5% 5% 5% 5% 7% 9% 36%	4440
	11th to 20th	Day	11% 5% 2 6% 4 5% 5% 4 7% 11% 40%	4440
	21st to end	Day	17% 7% 3 7% 5% 4 5% 4 6% 7% 36%	4814
August	1st to 10th	Day	15% 6% 2 7% 5% 5% 6% 5% 6% 8% 34%	4169
	11th to 20th	Day	16% 8% 2 7% 4 5% 5% 4 6% 7% 35%	4045
	21st to end	Day	9% 4 2 5% 4 4 6% 5% 7% 10% 45%	4180
September	1st to 10th	Day	13% 5% 2 5% 3 4 5% 4 5% 5% 48%	2271
	11th to 20th	Day	14% 5% 2 6% 3 3 3 2 4 6% 51%	2090
	21st to end	Day	16% 6% 2 6% 4 3 4 3 5% 6% 45%	2090
October	1st to 10th	Day	22% 6% 1 5% 5% 4 4 3 5% 5% 40%	2090
	11th to 20th	Day	15% 5% 1 6% 4 5% 4 3 6% 6% 46%	1845
	21st to end	Day	22% 7% 2 6% 4 4 3 3 5% 5% 39%	1881
November	1st to 10th	Day	15% 6% 2 5% 5% 3 2 3 4 6% 49%	1761
	11th to 20th	Day	19% 5% 2 5% 3 3 3 3 4 5% 48%	1800
	21st to end	Day	20% 7% 2 5% 3 3 4 3 5% 5% 43%	1746
December	1st to 10th	Day	19% 8% 3 7% 4 3 5% 3 5% 6% 37%	1539
	11th to 20th	Day	22% 7% 2 5% 3 4 3 3 4 5% 42%	1906
	21st to end	Day	23% 10% 2 7% 4 3 4 3 5% 5% 33%	2772
January	1st to 10th	Night	18% 8% 2 5% 3 3 3 3 5% 6% 45%	4429
	11th to 20th	Night	14% 8% 3 8% 5% 5% 4 3 5% 6% 39%	4440
	21st to end	Night	21% 12% 3 7% 5% 4 5% 3 4 5% 31%	4884
February	1st to 10th	Night	15% 9% 3 7% 5% 4 5% 4 5% 6% 38%	2434
	11th to 20th	Night	13% 10% 3 6% 5% 4 4 3 5% 7% 40%	2218
	21st to end	Night	17% 9% 3 5% 4 4 4 3 6% 6% 41%	1884
March	1st to 10th	Night	27% 10% 2 5% 3 3 3 3 4 5% 32%	3724
	11th to 20th	Night	13% 8% 3 8% 6% 5% 6% 4 6% 6% 37%	3612
	21st to end	Night	24% 8% 2 5% 4 3 5% 4 6% 6% 33%	3609
April	1st to 10th	Night	18% 8% 3 6% 4 5% 4 3 4 6% 36%	1900
	11th to 20th	Night	21% 8% 2 5% 4 4 4 4 4 6% 40%	1843
	21st to end	Night	20% 7% 2 7% 4 4 4 4 5% 6% 38%	1710
May	1st to 10th	Night	23% 12% 3 8% 4 4 4 3 5% 7% 25%	2834
	11th to 20th	Night	18% 8% 3 8% 6% 6% 6% 5% 6% 7% 27%	2750
	21st to end	Night	16% 7% 3 6% 4 4 4 4 6% 6% 41%	2738
June	1st to 10th	Night	18% 9% 3 6% 5% 4 4 3 4 7% 37%	1520
	11th to 20th	Night	20% 6% 2 5% 4 4 3 5% 6% 7% 38%	1520
	21st to end	Night	17% 8% 3 5% 4 4 4 4 6% 8% 36%	1520
July	1st to 10th	Night	11% 7% 2 7% 6% 6% 7% 4 6% 10% 34%	2520
	11th to 20th	Night	11% 5% 3 7% 5% 5% 5% 5% 8% 8% 39%	2520
	21st to end	Night	16% 7% 3 7% 4 4 4 4 5% 7% 38%	2739
August	1st to 10th	Night	19% 7% 3 7% 4 5% 5% 4 5% 7% 34%	2729
	11th to 20th	Night	12% 8% 3 7% 5% 5% 5% 5% 7% 9% 34%	2782
	21st to end	Night	9% 4 2 5% 3 4 4 3 7% 8% 52%	3473
September	1st to 10th	Night	12% 6% 2 6% 3 4 4 4 5% 7% 49%	1909
	11th to 20th	Night	14% 5% 2 4 3 3 4 3 4 6% 52%	2090
	21st to end	Night	16% 6% 2 5% 3 4 3 3 4 7% 45%	2090
October	1st to 10th	Night	20% 7% 3 6% 4 4 4 3 5% 6% 39%	2090
	11th to 20th	Night	16% 5% 1 5% 4 4 4 4 5% 5% 47%	2333
	21st to end	Night	22% 7% 2 6% 3 3 4 3 5% 5% 39%	2717
November	1st to 10th	Night	15% 5% 2 5% 3 4 4 4 5% 5% 47%	2537
	11th to 20th	Night	17% 7% 2 5% 3 4 3 3 4 5% 47%	2600
	21st to end	Night	17% 6% 2 6% 3 4 4 4 4 7% 43%	2529
December	1st to 10th	Night	19% 8% 3 7% 4 4 4 3 4 6% 38%	2640
	11th to 20th	Night	20% 5% 2 6% 4 4 4 4 6% 5% 41%	3288
	21st to end	Night	22% 9% 3 7% 4 4 4 3 4 6% 36%	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	41% 4 10% 33% 12% 1	2518
	11th to 20th	Day	45% 4 11% 25% 12% 2	2520
	21st to end	Day	51% 4 10% 23% 10% 2	2772
February	1st to 10th	Day	47% 5% 12% 21% 12% 3	1739
	11th to 20th	Day	43% 5% 11% 22% 15% 4	1857
	21st to end	Day	49% 4 9% 20% 14% 3	1570
March	1st to 10th	Day	58% 4 9% 14% 12% 3	3119
	11th to 20th	Day	42% 5% 13% 21% 15% 4	3346
	21st to end	Day	51% 4 5% 16% 17% 8%	4006
April	1st to 10th	Day	49% 3 7% 19% 17% 4	2280
	11th to 20th	Day	45% 5% 10% 20% 17% 3	2286
	21st to end	Day	49% 5% 7% 18% 18% 3	2470
May	1st to 10th	Day	56% 5% 11% 15% 11% 1	4120
	11th to 20th	Day	51% 5% 15% 17% 10% 2	4175
	21st to end	Day	38% 5% 13% 23% 19% 2	4837
June	1st to 10th	Day	48% 7% 12% 22% 12% 1	2660
	11th to 20th	Day	42% 5% 13% 21% 16% 3	2660
	21st to end	Day	49% 6% 10% 21% 13% 1	2660
July	1st to 10th	Day	44% 6% 12% 23% 14% 1	4370
	11th to 20th	Day	38% 5% 15% 27% 13% 1	4365
	21st to end	Day	46% 6% 12% 21% 13% 2	4661
August	1st to 10th	Day	46% 6% 15% 22% 10% 1	4155
	11th to 20th	Day	46% 5% 13% 26% 9% 1	4084
	21st to end	Day	32% 5% 13% 29% 19% 2	4180
September	1st to 10th	Day	35% 6% 12% 25% 21% 2	2271
	11th to 20th	Day	36% 4 7% 28% 23% 3	2090
	21st to end	Day	40% 4 8% 24% 21% 2	2090
October	1st to 10th	Day	45% 4 8% 23% 18% 2	3480
	11th to 20th	Day	32% 3 7% 30% 24% 3	3035
	21st to end	Day	35% 3 7% 30% 22% 3	3081
November	1st to 10th	Day	38% 3 13% 27% 17% 2	1761
	11th to 20th	Day	38% 4 8% 23% 24% 3	1800
	21st to end	Day	43% 4 16% 22% 15% 1	1746
December	1st to 10th	Day	47% 4 14% 17% 16% 2	1539
	11th to 20th	Day	46% 3 11% 18% 17% 5%	1906
	21st to end	Day	54% 3 12% 19% 10% 2	2772
January	1st to 10th	Night	41% 4 13% 28% 12% 3	4429
	11th to 20th	Night	45% 5% 11% 23% 12% 4	4440
	21st to end	Night	56% 3 10% 18% 11% 2	4884
February	1st to 10th	Night	47% 5% 12% 19% 14% 3	2434
	11th to 20th	Night	44% 4 10% 22% 17% 3	2220
	21st to end	Night	43% 4 10% 24% 14% 5%	1884
March	1st to 10th	Night	54% 4 9% 18% 12% 4	3734
	11th to 20th	Night	47% 5% 12% 17% 17% 3	3613
	21st to end	Night	51% 3 7% 14% 19% 7%	3609
April	1st to 10th	Night	48% 4 10% 19% 17% 3	1900
	11th to 20th	Night	46% 4 8% 21% 19% 3	1843
	21st to end	Night	47% 4 9% 19% 16% 3	1710
May	1st to 10th	Night	58% 5% 11% 17% 8% 2	2840
	11th to 20th	Night	54% 6% 16% 15% 8% 2	2751
	21st to end	Night	43% 5% 16% 20% 14% 2	2739
June	1st to 10th	Night	49% 4 11% 23% 13% 1	1520
	11th to 20th	Night	43% 7% 12% 20% 14% 3	1520
	21st to end	Night	45% 6% 13% 22% 12% 1	1520
July	1st to 10th	Night	45% 6% 13% 22% 13% 1	2480
	11th to 20th	Night	39% 6% 16% 24% 15%	2472
	21st to end	Night	44% 5% 12% 24% 15% 1	2651
August	1st to 10th	Night	50% 6% 13% 19% 11% 1	2719
	11th to 20th	Night	45% 7% 16% 20% 11%	2809
	21st to end	Night	30% 4 16% 28% 22% 1	3473
September	1st to 10th	Night	36% 4 12% 27% 20% 1	1909
	11th to 20th	Night	34% 4 10% 26% 25% 2	2090
	21st to end	Night	39% 4 9% 27% 19% 2	2090
October	1st to 10th	Night	43% 4 14% 22% 14% 2	3480
	11th to 20th	Night	28% 3 10% 28% 27% 3	3882
	21st to end	Night	36% 3 10% 32% 17% 2	4467
November	1st to 10th	Night	39% 4 10% 25% 19% 3	2537
	11th to 20th	Night	40% 3 9% 23% 22% 3	2600
	21st to end	Night	42% 3 13% 26% 14% 1	2529
December	1st to 10th	Night	49% 4 11% 23% 12% 1	2640
	11th to 20th	Night	44% 4 13% 19% 19% 1	3288
	21st to end	Night	52% 4 9% 24% 11% 1	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	27% 23% 29% 19% 2	180
	11th to 20th	Day	27% 25% 34% 11% 2 1	180
	21st to end	Day	20% 26% 37% 13% 3 1	198
February	1st to 10th	Day	27% 25% 33% 13% 2	351
	11th to 20th	Day	29% 28% 31% 10% 2	456
	21st to end	Day	27% 21% 33% 16% 2	382
March	1st to 10th	Day	28% 19% 37% 13% 3	460
	11th to 20th	Day	26% 27% 33% 11% 2 1	460
	21st to end	Day	30% 21% 33% 10% 5% 2	506
April	1st to 10th	Day	30% 22% 30% 12% 5% 1	460
	11th to 20th	Day	28% 23% 29% 13% 5% 1	452
	21st to end	Day	28% 17% 34% 15% 5% 1	460
May	1st to 10th	Day	32% 19% 32% 10% 5% 1	463
	11th to 20th	Day	20% 17% 38% 19% 5% 2	487
	21st to end	Day	19% 13% 35% 24% 7% 1	529
June	1st to 10th	Day	22% 10% 37% 20% 9% 1	490
	11th to 20th	Day	17% 16% 33% 26% 6% 2	490
	21st to end	Day	19% 13% 38% 21% 7% 2	490
July	1st to 10th	Day	18% 5% 35% 27% 13% 3	485
	11th to 20th	Day	17% 6% 33% 30% 11% 3	484
	21st to end	Day	14% 9% 34% 32% 9% 2	532
August	1st to 10th	Day	14% 8% 29% 32% 14% 2	486
	11th to 20th	Day	18% 9% 28% 26% 15% 4	479
	21st to end	Day	14% 8% 32% 28% 13% 5%	539
September	1st to 10th	Day	17% 9% 30% 24% 14% 6%	490
	11th to 20th	Day	20% 11% 28% 18% 15% 8%	490
	21st to end	Day	18% 10% 35% 22% 10% 6%	490
October	1st to 10th	Day	26% 12% 28% 21% 10% 3	490
	11th to 20th	Day	20% 14% 31% 24% 9% 2	425
	21st to end	Day	29% 13% 31% 21% 6%	429
November	1st to 10th	Day	16% 22% 32% 21% 8% 2	395
	11th to 20th	Day	28% 21% 29% 17% 4 1	400
	21st to end	Day	30% 19% 28% 16% 6% 1	382
December	1st to 10th	Day	20% 27% 31% 16% 6%	199
	11th to 20th	Day	29% 25% 32% 13% 1	180
	21st to end	Day	26% 27% 26% 14% 6% 1	198
January	1st to 10th	Night	21% 23% 36% 18% 3	760
	11th to 20th	Night	26% 19% 34% 18% 3	760
	21st to end	Night	21% 22% 36% 18% 3	836
February	1st to 10th	Night	23% 19% 37% 20% 1	760
	11th to 20th	Night	23% 21% 38% 16% 3	743
	21st to end	Night	24% 18% 34% 21% 4	628
March	1st to 10th	Night	26% 18% 36% 17% 3	760
	11th to 20th	Night	21% 22% 36% 16% 5%	760
	21st to end	Night	26% 17% 34% 19% 4	836
April	1st to 10th	Night	24% 18% 34% 21% 4	760
	11th to 20th	Night	25% 17% 33% 19% 6%	719
	21st to end	Night	22% 16% 33% 24% 5%	570
May	1st to 10th	Night	26% 13% 32% 23% 6%	570
	11th to 20th	Night	22% 12% 34% 25% 6% 1	525
	21st to end	Night	20% 9% 30% 32% 10%	411
June	1st to 10th	Night	21% 8% 31% 29% 9% 2	380
	11th to 20th	Night	21% 7% 30% 33% 8% 1	380
	21st to end	Night	21% 5% 31% 33% 10%	380
July	1st to 10th	Night	17% 4 25% 42% 11% 1	370
	11th to 20th	Night	13% 3 24% 43% 16% 1	368
	21st to end	Night	17% 3 26% 40% 13% 1	416
August	1st to 10th	Night	15% 3 24% 41% 16% 1	510
	11th to 20th	Night	15% 6% 28% 33% 17% 1	562
	21st to end	Night	15% 4 25% 37% 18% 2	833
September	1st to 10th	Night	14% 6% 32% 30% 17% 1	760
	11th to 20th	Night	16% 6% 28% 29% 18% 3	760
	21st to end	Night	19% 7% 29% 28% 15% 3	760
October	1st to 10th	Night	23% 9% 28% 26% 13% 1	760
	11th to 20th	Night	19% 13% 30% 27% 10% 1	760
	21st to end	Night	26% 14% 30% 23% 6%	836
November	1st to 10th	Night	17% 13% 37% 24% 7% 1	780
	11th to 20th	Night	22% 19% 32% 20% 6% 1	800
	21st to end	Night	22% 22% 31% 19% 5% 1	779
December	1st to 10th	Night	24% 17% 37% 19% 3	760
	11th to 20th	Night	23% 21% 32% 21% 3	760
	21st to end	Night	25% 19% 34% 19% 1	836

Page generated at 2024-11-22 08:02:25. Version ac07

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