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 2026-04-02.

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% 5% 9% 5% 4 4 3 5% 6% 27%	131530
	Night	19% 14% 5% 11% 5% 4 4 3 4 5% 25%	130322
Transparency	Day	55% 7% 11% 18% 8% 1	135008
	Night	59% 7% 10% 16% 8% 1	134931
Seeing	Day	27% 17% 27% 18% 8% 3	16408
	Night	26% 16% 25% 24% 8% 1	25655

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% 19% 11% 24% 9% 4 3 2 2 3 11%	4661
	11th to 20th	Day	12% 18% 12% 22% 9% 6% 3 3 3 3 11%	4650
	21st to end	Day	13% 18% 6% 14% 9% 7% 4 3 4 5% 17%	5459
February	1st to 10th	Day	16% 18% 5% 16% 9% 4 4 4 4 4 17%	1890
	11th to 20th	Day	16% 18% 6% 17% 8% 3 4 3 3 4 18%	1942
	21st to end	Day	20% 17% 6% 15% 7% 3 3 3 4 3 19%	1730
March	1st to 10th	Day	21% 17% 8% 12% 4 4 3 2 3 4 22%	4576
	11th to 20th	Day	14% 18% 7% 9% 4 3 3 3 4 6% 28%	4993
	21st to end	Day	20% 14% 5% 10% 5% 4 3 2 4 5% 28%	5975
April	1st to 10th	Day	21% 17% 3 7% 4 3 3 2 4 5% 30%	2412
	11th to 20th	Day	19% 14% 3 7% 5% 4 4 3 4 5% 32%	2374
	21st to end	Day	22% 13% 4 6% 4 4 4 4 4 4 30%	2440
May	1st to 10th	Day	33% 11% 4 7% 4 3 4 3 4 4 23%	6244
	11th to 20th	Day	19% 10% 3 8% 6% 5% 5% 4 5% 7% 28%	6318
	21st to end	Day	24% 9% 3 7% 5% 4 4 4 5% 7% 28%	6949
June	1st to 10th	Day	21% 8% 3 7% 5% 5% 4 4 6% 7% 30%	2800
	11th to 20th	Day	21% 7% 2 6% 5% 5% 4 5% 6% 9% 30%	2800
	21st to end	Day	21% 7% 2 7% 5% 5% 5% 4 5% 8% 31%	2800
July	1st to 10th	Day	14% 7% 3 6% 5% 5% 5% 4 7% 10% 35%	6360
	11th to 20th	Day	15% 7% 2 7% 5% 5% 5% 4 7% 9% 34%	6360
	21st to end	Day	16% 5% 3 6% 4 4 4 4 5% 7% 40%	6926
August	1st to 10th	Day	19% 7% 2 6% 4 4 5% 4 6% 8% 34%	4441
	11th to 20th	Day	18% 8% 3 7% 4 5% 5% 4 6% 9% 31%	3959
	21st to end	Day	18% 7% 3 5% 4 4 5% 4 6% 8% 36%	4312
September	1st to 10th	Day	17% 5% 3 5% 4 4 5% 4 6% 8% 40%	2400
	11th to 20th	Day	17% 5% 2 6% 3 3 4 3 6% 7% 44%	2360
	21st to end	Day	21% 7% 2 6% 4 4 4 3 7% 7% 36%	2200
October	1st to 10th	Day	25% 8% 2 7% 5% 4 4 3 5% 5% 33%	2002
	11th to 20th	Day	23% 11% 3 6% 4 4 5% 3 5% 6% 32%	2000
	21st to end	Day	27% 13% 3 7% 4 4 4 3 5% 6% 24%	2040
November	1st to 10th	Day	17% 16% 3 8% 4 4 4 3 5% 5% 30%	1851
	11th to 20th	Day	18% 19% 5% 8% 5% 4 3 3 4 4 27%	1832
	21st to end	Day	21% 21% 6% 10% 4 3 3 3 5% 5% 21%	1680
December	1st to 10th	Day	20% 25% 8% 17% 4 3 3 2 3 3 10%	2680
	11th to 20th	Day	21% 20% 11% 13% 6% 3 3 2 2 3 14%	3066
	21st to end	Day	22% 21% 7% 11% 5% 3 3 3 3 4 18%	4048
January	1st to 10th	Night	15% 19% 11% 22% 7% 4 3 2 2 3 12%	8234
	11th to 20th	Night	12% 21% 10% 23% 11% 5% 3 2 3 3 8%	8235
	21st to end	Night	13% 16% 6% 15% 9% 8% 5% 3 4 4 17%	8717
February	1st to 10th	Night	17% 17% 6% 16% 8% 5% 3 3 5% 5% 16%	2730
	11th to 20th	Night	15% 15% 7% 17% 8% 5% 5% 3 3 5% 18%	2564
	21st to end	Night	17% 17% 6% 14% 7% 5% 4 3 4 6% 19%	2076
March	1st to 10th	Night	21% 12% 6% 14% 7% 4 4 3 4 4 21%	5484
	11th to 20th	Night	18% 17% 6% 7% 4 3 3 3 4 6% 29%	5183
	21st to end	Night	23% 13% 5% 13% 5% 3 3 2 3 4 25%	5198
April	1st to 10th	Night	23% 15% 4 7% 4 3 4 3 5% 6% 26%	2011
	11th to 20th	Night	22% 12% 3 7% 3 4 4 3 4 6% 31%	1975
	21st to end	Night	21% 11% 3 6% 3 4 4 3 4 6% 35%	1960
May	1st to 10th	Night	35% 11% 4 6% 4 4 3 3 4 5% 21%	3738
	11th to 20th	Night	16% 13% 4 8% 5% 4 4 3 5% 6% 32%	3575
	21st to end	Night	24% 11% 3 8% 5% 3 4 3 5% 5% 29%	3926
June	1st to 10th	Night	20% 7% 2 6% 4 4 4 4 5% 8% 35%	1600
	11th to 20th	Night	22% 7% 2 5% 3 3 4 4 4 9% 38%	1600
	21st to end	Night	20% 6% 2 7% 4 4 3 5% 5% 7% 38%	1600
July	1st to 10th	Night	14% 8% 3 7% 6% 5% 5% 5% 6% 8% 34%	3600
	11th to 20th	Night	20% 7% 2 6% 4 4 4 3 5% 7% 38%	3600
	21st to end	Night	12% 8% 2 5% 3 3 4 3 6% 8% 46%	3926
August	1st to 10th	Night	23% 9% 3 6% 3 3 4 3 4 5% 35%	2705
	11th to 20th	Night	23% 8% 2 6% 3 3 3 3 4 6% 38%	3057
	21st to end	Night	16% 8% 2 5% 3 4 4 4 5% 7% 41%	3586
September	1st to 10th	Night	15% 8% 3 5% 4 3 4 3 5% 7% 42%	2000
	11th to 20th	Night	17% 6% 3 5% 3 4 3 3 4 5% 49%	2039
	21st to end	Night	21% 6% 2 5% 5% 4 5% 4 4 6% 39%	2200
October	1st to 10th	Night	24% 9% 3 7% 5% 3 3 3 5% 5% 33%	2395
	11th to 20th	Night	24% 10% 3 6% 4 4 4 3 5% 6% 32%	2400
	21st to end	Night	27% 12% 3 7% 5% 4 5% 3 4 5% 24%	2800
November	1st to 10th	Night	18% 13% 3 7% 5% 4 4 3 5% 5% 32%	2667
	11th to 20th	Night	21% 16% 4 7% 4 4 3 3 5% 5% 28%	2783
	21st to end	Night	19% 19% 6% 9% 5% 4 5% 3 5% 5% 21%	2940
December	1st to 10th	Night	19% 24% 12% 14% 4 3 3 2 4 3 12%	4720
	11th to 20th	Night	24% 21% 6% 15% 4 4 3 2 3 3 15%	5348
	21st to end	Night	25% 19% 7% 9% 5% 3 3 3 3 4 20%	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	80% 5% 6% 7% 2	4662
	11th to 20th	Day	77% 6% 7% 6% 4 1	4680
	21st to end	Day	67% 6% 9% 12% 5% 1	5459
February	1st to 10th	Day	69% 7% 9% 11% 3	1890
	11th to 20th	Day	70% 5% 8% 12% 5% 1	1951
	21st to end	Day	69% 4 9% 11% 6%	1730
March	1st to 10th	Day	65% 6% 10% 11% 6% 2	4579
	11th to 20th	Day	55% 6% 19% 11% 7% 2	4993
	21st to end	Day	57% 6% 8% 13% 15% 1	5975
April	1st to 10th	Day	57% 5% 8% 20% 9% 2	2412
	11th to 20th	Day	53% 6% 10% 20% 11% 1	2374
	21st to end	Day	56% 5% 9% 17% 11% 2	2440
May	1st to 10th	Day	64% 6% 8% 14% 8% 1	6255
	11th to 20th	Day	52% 8% 11% 20% 7% 1	6322
	21st to end	Day	52% 8% 12% 19% 8%	6949
June	1st to 10th	Day	50% 7% 13% 19% 10%	2800
	11th to 20th	Day	48% 8% 14% 20% 9% 1	2800
	21st to end	Day	50% 8% 14% 20% 9% 1	2800
July	1st to 10th	Day	41% 9% 15% 24% 11%	6290
	11th to 20th	Day	41% 9% 15% 24% 11%	6285
	21st to end	Day	40% 8% 12% 31% 10%	6728
August	1st to 10th	Day	45% 9% 14% 20% 11%	4427
	11th to 20th	Day	47% 9% 16% 23% 5%	3998
	21st to end	Day	43% 7% 14% 29% 6% 1	4312
September	1st to 10th	Day	41% 8% 12% 28% 11% 1	2400
	11th to 20th	Day	39% 6% 13% 27% 15% 1	2360
	21st to end	Day	46% 7% 12% 23% 11% 1	2200
October	1st to 10th	Day	48% 6% 11% 24% 9% 1	3262
	11th to 20th	Day	45% 6% 14% 22% 13% 1	3240
	21st to end	Day	44% 8% 15% 24% 9% 1	3278
November	1st to 10th	Day	53% 7% 13% 18% 9%	1851
	11th to 20th	Day	58% 7% 10% 15% 9% 2	1832
	21st to end	Day	65% 6% 14% 12% 4 1	1680
December	1st to 10th	Day	78% 6% 7% 7% 2	2680
	11th to 20th	Day	73% 6% 8% 11% 3	3066
	21st to end	Day	69% 6% 7% 11% 6%	4048
January	1st to 10th	Night	78% 6% 8% 6% 3	8234
	11th to 20th	Night	77% 4 8% 6% 3 1	8280
	21st to end	Night	67% 6% 8% 11% 7% 1	8717
February	1st to 10th	Night	69% 7% 8% 11% 4	2730
	11th to 20th	Night	68% 7% 8% 12% 5%	2566
	21st to end	Night	65% 7% 11% 11% 6%	2076
March	1st to 10th	Night	65% 7% 10% 11% 6% 1	5494
	11th to 20th	Night	54% 7% 17% 13% 6% 2	5184
	21st to end	Night	61% 5% 6% 19% 9% 1	5198
April	1st to 10th	Night	58% 6% 11% 17% 7% 1	2011
	11th to 20th	Night	53% 7% 10% 18% 12% 1	1975
	21st to end	Night	49% 7% 10% 18% 14% 2	1960
May	1st to 10th	Night	65% 5% 8% 11% 9% 1	3744
	11th to 20th	Night	52% 9% 11% 20% 8% 1	3576
	21st to end	Night	54% 8% 11% 16% 11% 1	3927
June	1st to 10th	Night	45% 7% 12% 22% 13% 1	1600
	11th to 20th	Night	44% 7% 15% 23% 11% 1	1600
	21st to end	Night	43% 8% 13% 24% 11% 1	1600
July	1st to 10th	Night	44% 8% 14% 20% 12% 1	3560
	11th to 20th	Night	44% 8% 12% 21% 14%	3552
	21st to end	Night	35% 7% 12% 33% 13% 1	3809
August	1st to 10th	Night	49% 6% 14% 22% 8%	2695
	11th to 20th	Night	48% 6% 13% 26% 8%	3084
	21st to end	Night	39% 8% 14% 30% 9%	3586
September	1st to 10th	Night	41% 7% 15% 25% 12% 1	2000
	11th to 20th	Night	37% 6% 11% 28% 17% 1	2039
	21st to end	Night	43% 8% 12% 24% 11% 2	2200
October	1st to 10th	Night	53% 8% 11% 19% 9% 1	3915
	11th to 20th	Night	44% 6% 14% 20% 15% 1	3899
	21st to end	Night	52% 6% 11% 24% 6%	4512
November	1st to 10th	Night	51% 8% 11% 18% 11% 2	2667
	11th to 20th	Night	56% 7% 10% 17% 9% 2	2783
	21st to end	Night	62% 8% 12% 14% 4	2940
December	1st to 10th	Night	75% 6% 10% 7% 2	4720
	11th to 20th	Night	71% 6% 7% 12% 4	5348
	21st to end	Night	69% 7% 6% 10% 7% 1	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	35% 22% 29% 12% 2	220
	11th to 20th	Day	38% 27% 28% 6% 1	220
	21st to end	Day	28% 27% 35% 8% 2	303
February	1st to 10th	Day	39% 25% 28% 6% 1	330
	11th to 20th	Day	39% 29% 25% 6% 1	429
	21st to end	Day	37% 23% 28% 11% 1	446
March	1st to 10th	Day	33% 25% 26% 14% 2	540
	11th to 20th	Day	33% 27% 25% 13% 2	540
	21st to end	Day	36% 25% 22% 13% 4 1	594
April	1st to 10th	Day	38% 25% 19% 12% 5% 1	504
	11th to 20th	Day	35% 23% 20% 12% 8% 2	492
	21st to end	Day	36% 16% 29% 12% 5% 2	500
May	1st to 10th	Day	34% 19% 27% 12% 6% 3	503
	11th to 20th	Day	24% 16% 31% 18% 9% 2	527
	21st to end	Day	23% 13% 29% 20% 12% 3	573
June	1st to 10th	Day	21% 17% 29% 23% 8% 2	530
	11th to 20th	Day	22% 15% 28% 25% 9% 1	530
	21st to end	Day	24% 15% 31% 20% 7% 2	530
July	1st to 10th	Day	16% 9% 31% 27% 14% 4	525
	11th to 20th	Day	14% 9% 28% 32% 13% 3	524
	21st to end	Day	15% 10% 36% 25% 11% 3	576
August	1st to 10th	Day	15% 10% 24% 28% 17% 6%	526
	11th to 20th	Day	18% 8% 27% 25% 14% 8%	519
	21st to end	Day	17% 14% 28% 24% 12% 6%	583
September	1st to 10th	Day	18% 10% 25% 22% 16% 8%	530
	11th to 20th	Day	15% 9% 25% 22% 18% 11%	530
	21st to end	Day	20% 13% 27% 24% 13% 4	530
October	1st to 10th	Day	29% 12% 22% 19% 15% 3	530
	11th to 20th	Day	27% 18% 28% 18% 8% 1	530
	21st to end	Day	34% 17% 26% 17% 6%	495
November	1st to 10th	Day	33% 20% 19% 18% 11%	425
	11th to 20th	Day	33% 22% 23% 18% 3 1	372
	21st to end	Day	45% 17% 26% 10% 1	220
December	1st to 10th	Day	36% 25% 28% 8% 4	220
	11th to 20th	Day	39% 23% 26% 10% 1	220
	21st to end	Day	36% 23% 29% 10% 2	242
January	1st to 10th	Night	31% 25% 28% 15% 1	840
	11th to 20th	Night	33% 22% 27% 15% 3	840
	21st to end	Night	24% 24% 32% 17% 3	924
February	1st to 10th	Night	27% 23% 32% 17% 1	840
	11th to 20th	Night	28% 26% 32% 13% 2	823
	21st to end	Night	30% 20% 32% 16% 2	692
March	1st to 10th	Night	27% 21% 30% 20% 2	840
	11th to 20th	Night	30% 20% 29% 19% 2	840
	21st to end	Night	29% 20% 27% 19% 5% 1	924
April	1st to 10th	Night	30% 18% 28% 20% 3	805
	11th to 20th	Night	27% 17% 30% 18% 7% 1	791
	21st to end	Night	26% 17% 26% 24% 7%	760
May	1st to 10th	Night	31% 11% 24% 24% 8%	466
	11th to 20th	Night	21% 11% 29% 32% 7%	394
	21st to end	Night	18% 7% 25% 36% 13% 1	433
June	1st to 10th	Night	20% 8% 22% 37% 12% 1	400
	11th to 20th	Night	23% 7% 20% 35% 15%	400
	21st to end	Night	19% 5% 26% 36% 15%	400
July	1st to 10th	Night	16% 3 18% 43% 19% 1	390
	11th to 20th	Night	13% 4 19% 44% 18% 2	388
	21st to end	Night	13% 5% 21% 45% 15% 1	438
August	1st to 10th	Night	16% 4 18% 38% 23% 1	465
	11th to 20th	Night	19% 6% 16% 33% 24% 3	698
	21st to end	Night	17% 7% 19% 35% 20% 1	880
September	1st to 10th	Night	18% 6% 20% 35% 19% 2	800
	11th to 20th	Night	19% 5% 17% 32% 22% 4	800
	21st to end	Night	22% 8% 24% 28% 17% 1	800
October	1st to 10th	Night	28% 10% 21% 24% 16% 2	800
	11th to 20th	Night	28% 12% 25% 28% 6%	800
	21st to end	Night	31% 17% 25% 23% 4	880
November	1st to 10th	Night	25% 16% 25% 27% 7%	820
	11th to 20th	Night	28% 18% 26% 20% 6% 1	840
	21st to end	Night	31% 19% 26% 19% 5%	840
December	1st to 10th	Night	33% 22% 26% 16% 3	840
	11th to 20th	Night	35% 21% 26% 15% 3	840
	21st to end	Night	33% 24% 26% 15% 2	924

Page generated at 2026-03-30 02:18:43. Version ac07

Copyright © 2026 Attilla Danko.