CNYAC Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the CNYAC Observatory clear sky chart from 2005-11-06 to 2024-12-09.

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	19% 12% 4 8% 5% 4 4 3 5% 6% 28%	101776
	Night	20% 13% 5% 9% 5% 4 4 3 4 5% 27%	98025
Transparency	Day	53% 7% 12% 19% 8% 1	105272
	Night	56% 7% 11% 17% 8% 1	102618
Seeing	Day	27% 17% 27% 18% 9% 3	14793
	Night	26% 15% 25% 24% 9% 1	23945

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	16% 21% 11% 16% 5% 4 3 2 3 3 15%	2517
	11th to 20th	Day	17% 19% 10% 18% 8% 5% 2 3 3 3 14%	2520
	21st to end	Day	18% 24% 7% 12% 7% 6% 4 3 4 4 11%	2941
February	1st to 10th	Day	17% 19% 5% 14% 9% 4 5% 3 4 4 16%	1710
	11th to 20th	Day	16% 17% 7% 17% 7% 3 4 3 3 4 19%	1752
	21st to end	Day	21% 16% 5% 15% 6% 4 3 3 4 3 20%	1570
March	1st to 10th	Day	24% 17% 4 10% 4 3 3 3 3 5% 23%	3116
	11th to 20th	Day	13% 22% 8% 10% 4 4 4 3 4 5% 23%	3413
	21st to end	Day	19% 13% 5% 10% 4 4 3 2 4 6% 31%	4084
April	1st to 10th	Day	20% 17% 3 7% 4 3 3 2 5% 5% 31%	2280
	11th to 20th	Day	19% 13% 3 7% 5% 4 4 3 4 5% 32%	2254
	21st to end	Day	23% 13% 3 6% 4 4 4 4 4 4 29%	2318
May	1st to 10th	Day	26% 10% 3 6% 4 4 4 3 4 5% 29%	4352
	11th to 20th	Day	20% 9% 3 7% 6% 4 5% 4 5% 8% 30%	4398
	21st to end	Day	16% 6% 2 6% 5% 5% 5% 4 6% 8% 37%	4837
June	1st to 10th	Day	21% 7% 3 7% 5% 5% 4 4 6% 7% 29%	2660
	11th to 20th	Day	21% 7% 2 5% 5% 5% 4 5% 6% 9% 31%	2660
	21st to end	Day	21% 7% 2 7% 5% 5% 5% 4 5% 8% 30%	2660
July	1st to 10th	Day	15% 6% 3 6% 5% 5% 5% 4 7% 10% 34%	4440
	11th to 20th	Day	15% 7% 2 6% 5% 5% 5% 5% 8% 10% 32%	4440
	21st to end	Day	16% 6% 3 6% 5% 5% 5% 5% 6% 8% 35%	4814
August	1st to 10th	Day	20% 7% 2 7% 4 4 5% 4 6% 9% 33%	4305
	11th to 20th	Day	18% 8% 3 7% 4 4 5% 4 7% 9% 30%	3835
	21st to end	Day	19% 7% 3 6% 4 4 4 4 6% 8% 36%	4180
September	1st to 10th	Day	17% 6% 3 5% 4 4 5% 4 6% 8% 39%	2280
	11th to 20th	Day	18% 6% 2 6% 3 3 4 3 5% 7% 43%	2242
	21st to end	Day	21% 7% 2 6% 4 4 4 3 6% 7% 36%	2090
October	1st to 10th	Day	26% 9% 3 7% 5% 4 4 3 5% 5% 30%	1902
	11th to 20th	Day	23% 11% 3 6% 4 4 4 3 5% 6% 32%	1900
	21st to end	Day	27% 13% 3 7% 4 4 4 3 5% 6% 24%	1938
November	1st to 10th	Day	17% 15% 3 8% 4 4 4 3 5% 6% 31%	1761
	11th to 20th	Day	18% 17% 4 8% 5% 4 3 3 4 5% 28%	1745
	21st to end	Day	21% 20% 6% 10% 4 3 3 3 5% 5% 21%	1600
December	1st to 10th	Day	22% 26% 6% 10% 5% 3 4 2 4 3 14%	1584
	11th to 20th	Day	24% 17% 11% 11% 5% 3 4 3 3 4 15%	1906
	21st to end	Day	26% 23% 8% 10% 5% 3 3 2 3 4 15%	2772
January	1st to 10th	Night	18% 21% 9% 16% 5% 3 3 2 3 3 16%	4429
	11th to 20th	Night	14% 20% 9% 19% 11% 4 3 3 3 3 12%	4440
	21st to end	Night	22% 19% 7% 13% 6% 5% 4 3 3 3 14%	4710
February	1st to 10th	Night	17% 17% 5% 16% 7% 5% 4 3 5% 5% 16%	2470
	11th to 20th	Night	15% 15% 7% 15% 7% 5% 5% 3 3 5% 19%	2314
	21st to end	Night	17% 16% 5% 14% 7% 5% 4 3 4 6% 19%	1884
March	1st to 10th	Night	22% 14% 4 12% 5% 4 3 3 4 4 24%	3724
	11th to 20th	Night	18% 21% 7% 8% 5% 4 4 3 3 4 25%	3543
	21st to end	Night	18% 14% 4 12% 5% 4 3 3 3 5% 30%	3547
April	1st to 10th	Night	22% 15% 4 6% 4 3 4 3 6% 6% 27%	1900
	11th to 20th	Night	22% 12% 3 7% 3 4 4 3 4 6% 31%	1875
	21st to end	Night	22% 11% 3 6% 3 4 4 3 5% 6% 34%	1862
May	1st to 10th	Night	28% 9% 4 6% 5% 4 4 3 5% 5% 28%	2616
	11th to 20th	Night	18% 12% 4 8% 4 4 4 3 5% 5% 33%	2495
	21st to end	Night	18% 7% 2 6% 4 3 4 4 6% 7% 38%	2738
June	1st to 10th	Night	20% 7% 2 6% 4 4 4 4 6% 8% 35%	1520
	11th to 20th	Night	21% 7% 2 4 3 3 4 4 4 9% 38%	1520
	21st to end	Night	19% 6% 2 7% 4 4 3 5% 5% 7% 37%	1520
July	1st to 10th	Night	15% 7% 3 8% 6% 5% 5% 5% 6% 6% 35%	2520
	11th to 20th	Night	18% 6% 2 6% 5% 4 4 3 5% 8% 39%	2520
	21st to end	Night	15% 9% 3 5% 4 4 4 3 6% 7% 41%	2739
August	1st to 10th	Night	24% 10% 3 6% 3 3 4 3 4 5% 35%	2621
	11th to 20th	Night	23% 8% 2 6% 3 3 3 3 4 6% 37%	2962
	21st to end	Night	16% 8% 2 5% 3 4 4 4 5% 7% 41%	3476
September	1st to 10th	Night	15% 8% 3 5% 4 3 4 4 5% 7% 41%	1900
	11th to 20th	Night	18% 6% 3 5% 2 4 3 3 4 5% 49%	1937
	21st to end	Night	21% 6% 2 5% 5% 4 4 4 4 7% 39%	2090
October	1st to 10th	Night	25% 9% 3 7% 5% 4 3 3 5% 5% 30%	2275
	11th to 20th	Night	24% 10% 3 6% 4 4 4 3 5% 5% 32%	2280
	21st to end	Night	27% 12% 3 8% 5% 4 5% 3 4 5% 24%	2660
November	1st to 10th	Night	17% 13% 3 8% 5% 4 4 3 5% 5% 33%	2537
	11th to 20th	Night	21% 16% 3 7% 4 4 4 3 5% 6% 28%	2650
	21st to end	Night	19% 18% 5% 9% 5% 4 5% 3 5% 5% 22%	2800
December	1st to 10th	Night	22% 24% 7% 10% 5% 4 4 3 4 3 15%	2779
	11th to 20th	Night	22% 19% 6% 14% 5% 4 4 3 4 4 16%	3288
	21st to end	Night	29% 18% 8% 8% 4 3 3 3 4 4 15%	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	72% 6% 9% 9% 3	2518
	11th to 20th	Day	75% 6% 9% 6% 3 1	2520
	21st to end	Day	74% 8% 8% 5% 3 1	2941
February	1st to 10th	Day	70% 7% 9% 11% 3 1	1710
	11th to 20th	Day	69% 5% 7% 12% 6% 1	1761
	21st to end	Day	68% 5% 10% 11% 6%	1570
March	1st to 10th	Day	63% 6% 12% 12% 7% 1	3119
	11th to 20th	Day	60% 7% 15% 10% 7% 1	3413
	21st to end	Day	53% 7% 9% 16% 14% 2	4084
April	1st to 10th	Day	56% 5% 8% 21% 9% 2	2280
	11th to 20th	Day	52% 6% 11% 20% 10% 1	2254
	21st to end	Day	56% 5% 9% 16% 11% 2	2318
May	1st to 10th	Day	55% 7% 10% 17% 10% 1	4366
	11th to 20th	Day	50% 8% 13% 20% 7% 1	4402
	21st to end	Day	40% 10% 15% 24% 11% 1	4837
June	1st to 10th	Day	50% 8% 14% 19% 9%	2660
	11th to 20th	Day	47% 8% 15% 20% 9% 1	2660
	21st to end	Day	50% 8% 14% 19% 8% 1	2660
July	1st to 10th	Day	42% 9% 14% 24% 11%	4370
	11th to 20th	Day	40% 10% 16% 26% 8%	4365
	21st to end	Day	44% 9% 14% 24% 9%	4661
August	1st to 10th	Day	46% 9% 14% 20% 11%	4291
	11th to 20th	Day	47% 9% 17% 22% 5%	3874
	21st to end	Day	43% 7% 14% 29% 6%	4180
September	1st to 10th	Day	41% 8% 12% 27% 10% 1	2280
	11th to 20th	Day	40% 6% 13% 26% 15% 1	2242
	21st to end	Day	46% 7% 12% 22% 11% 1	2090
October	1st to 10th	Day	50% 6% 11% 24% 9% 1	3162
	11th to 20th	Day	45% 6% 14% 23% 13% 1	3140
	21st to end	Day	44% 9% 15% 24% 9%	3176
November	1st to 10th	Day	52% 8% 13% 18% 9%	1761
	11th to 20th	Day	57% 7% 10% 15% 9% 2	1745
	21st to end	Day	64% 6% 14% 12% 4 1	1600
December	1st to 10th	Day	74% 5% 10% 8% 2	1584
	11th to 20th	Day	71% 7% 10% 8% 4	1906
	21st to end	Day	75% 6% 5% 11% 3	2772
January	1st to 10th	Night	71% 8% 9% 7% 4	4429
	11th to 20th	Night	75% 5% 10% 7% 3 1	4440
	21st to end	Night	75% 7% 6% 8% 3 1	4710
February	1st to 10th	Night	70% 7% 9% 11% 4	2470
	11th to 20th	Night	66% 7% 8% 13% 6%	2316
	21st to end	Night	64% 8% 11% 11% 6%	1884
March	1st to 10th	Night	62% 7% 9% 14% 7% 1	3734
	11th to 20th	Night	60% 7% 12% 15% 5%	3544
	21st to end	Night	56% 5% 6% 20% 10% 2	3547
April	1st to 10th	Night	56% 6% 11% 17% 8% 1	1900
	11th to 20th	Night	52% 7% 10% 18% 12% 1	1875
	21st to end	Night	49% 7% 10% 18% 13% 3	1862
May	1st to 10th	Night	57% 6% 10% 14% 11% 2	2622
	11th to 20th	Night	51% 9% 12% 20% 8% 1	2496
	21st to end	Night	42% 9% 13% 20% 15% 1	2739
June	1st to 10th	Night	44% 7% 12% 23% 12% 1	1520
	11th to 20th	Night	43% 7% 15% 23% 11% 1	1520
	21st to end	Night	43% 8% 13% 24% 11% 1	1520
July	1st to 10th	Night	45% 8% 13% 19% 14% 1	2480
	11th to 20th	Night	42% 8% 14% 25% 12%	2472
	21st to end	Night	41% 7% 10% 28% 12% 1	2651
August	1st to 10th	Night	50% 6% 14% 22% 8%	2611
	11th to 20th	Night	49% 6% 13% 25% 7%	2989
	21st to end	Night	38% 8% 14% 31% 8%	3476
September	1st to 10th	Night	41% 7% 16% 24% 12% 1	1900
	11th to 20th	Night	38% 5% 11% 28% 17% 1	1937
	21st to end	Night	43% 8% 13% 25% 11% 2	2090
October	1st to 10th	Night	54% 8% 11% 19% 8%	3795
	11th to 20th	Night	44% 6% 14% 20% 15% 1	3779
	21st to end	Night	52% 6% 11% 25% 6%	4372
November	1st to 10th	Night	50% 8% 11% 18% 11% 2	2537
	11th to 20th	Night	55% 7% 10% 17% 9% 2	2650
	21st to end	Night	61% 8% 12% 14% 4	2800
December	1st to 10th	Night	71% 6% 11% 10% 2	2779
	11th to 20th	Night	70% 7% 9% 9% 4	3288
	21st to end	Night	73% 8% 8% 10% 2	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	36% 18% 29% 14% 3	180
	11th to 20th	Day	41% 26% 25% 7% 2 1	180
	21st to end	Day	24% 27% 37% 9% 3	247
February	1st to 10th	Day	39% 26% 29% 6%	270
	11th to 20th	Day	35% 30% 27% 6% 2	359
	21st to end	Day	38% 21% 27% 13% 1	382
March	1st to 10th	Day	30% 26% 26% 15% 3	460
	11th to 20th	Day	31% 28% 25% 13% 2	460
	21st to end	Day	35% 23% 23% 14% 4 1	506
April	1st to 10th	Day	36% 24% 20% 13% 5% 1	460
	11th to 20th	Day	35% 22% 20% 13% 8% 2	452
	21st to end	Day	36% 15% 29% 13% 6% 2	460
May	1st to 10th	Day	32% 18% 27% 13% 6% 3	463
	11th to 20th	Day	24% 14% 31% 18% 9% 2	487
	21st to end	Day	21% 13% 30% 21% 12% 4	529
June	1st to 10th	Day	21% 18% 29% 22% 9% 2	490
	11th to 20th	Day	20% 15% 28% 25% 10% 1	490
	21st to end	Day	24% 16% 31% 20% 8% 2	490
July	1st to 10th	Day	16% 8% 30% 27% 14% 4	485
	11th to 20th	Day	15% 8% 29% 32% 13% 4	484
	21st to end	Day	15% 9% 36% 25% 12% 3	532
August	1st to 10th	Day	16% 10% 25% 27% 16% 5%	486
	11th to 20th	Day	19% 9% 27% 25% 13% 8%	479
	21st to end	Day	17% 14% 26% 24% 13% 6%	539
September	1st to 10th	Day	18% 11% 25% 21% 17% 8%	490
	11th to 20th	Day	16% 10% 25% 20% 18% 12%	490
	21st to end	Day	20% 13% 26% 23% 13% 4	490
October	1st to 10th	Day	30% 12% 23% 19% 13% 3	490
	11th to 20th	Day	26% 18% 28% 18% 9% 1	490
	21st to end	Day	34% 17% 25% 17% 6%	459
November	1st to 10th	Day	31% 19% 19% 19% 11%	395
	11th to 20th	Day	32% 22% 23% 18% 3 1	345
	21st to end	Day	44% 17% 28% 11% 1	200
December	1st to 10th	Day	36% 25% 27% 8% 4	196
	11th to 20th	Day	37% 24% 27% 11% 1	180
	21st to end	Day	36% 24% 30% 9% 1	198
January	1st to 10th	Night	31% 24% 28% 16% 1	760
	11th to 20th	Night	33% 22% 26% 16% 3	760
	21st to end	Night	23% 25% 32% 17% 3	836
February	1st to 10th	Night	27% 23% 33% 16% 1	760
	11th to 20th	Night	26% 26% 32% 13% 3	743
	21st to end	Night	30% 20% 32% 17% 2	628
March	1st to 10th	Night	26% 21% 30% 21% 2	760
	11th to 20th	Night	29% 20% 29% 19% 2 1	760
	21st to end	Night	27% 20% 26% 20% 6% 1	836
April	1st to 10th	Night	28% 18% 29% 21% 3	760
	11th to 20th	Night	27% 17% 29% 19% 7% 1	751
	21st to end	Night	26% 17% 26% 24% 7%	722
May	1st to 10th	Night	30% 11% 25% 25% 9%	443
	11th to 20th	Night	21% 10% 29% 33% 7%	374
	21st to end	Night	17% 6% 25% 37% 14% 1	411
June	1st to 10th	Night	18% 8% 22% 38% 12% 1	380
	11th to 20th	Night	22% 7% 20% 36% 16%	380
	21st to end	Night	18% 5% 25% 37% 14%	380
July	1st to 10th	Night	16% 3 18% 43% 19% 1	370
	11th to 20th	Night	13% 4 19% 43% 19% 2	368
	21st to end	Night	13% 5% 20% 44% 16% 1	416
August	1st to 10th	Night	17% 4 18% 37% 23% 1	441
	11th to 20th	Night	19% 6% 16% 33% 23% 3	663
	21st to end	Night	17% 6% 19% 36% 21% 1	836
September	1st to 10th	Night	18% 6% 20% 34% 19% 2	760
	11th to 20th	Night	20% 5% 18% 30% 22% 5%	760
	21st to end	Night	21% 8% 25% 27% 17% 1	760
October	1st to 10th	Night	28% 10% 21% 23% 15% 2	760
	11th to 20th	Night	28% 12% 25% 28% 6%	760
	21st to end	Night	31% 17% 26% 23% 3	836
November	1st to 10th	Night	24% 16% 25% 28% 7%	780
	11th to 20th	Night	28% 18% 26% 21% 6% 1	800
	21st to end	Night	31% 19% 27% 19% 5%	800
December	1st to 10th	Night	33% 22% 26% 16% 3	795
	11th to 20th	Night	33% 21% 26% 16% 4 1	760
	21st to end	Night	33% 24% 26% 15% 1	836

Page generated at 2024-12-06 03:54:51. Version ac07

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