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 2024-07-08.

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	20% 12% 4 9% 5% 4 4 3 5% 6% 28%	89688
	Night	20% 14% 5% 10% 5% 4 4 3 4 5% 27%	89653
Transparency	Day	55% 7% 12% 17% 8% 1	89407
	Night	58% 7% 11% 16% 8% 1	89488
Seeing	Day	27% 17% 27% 18% 9% 3	14245
	Night	26% 15% 25% 24% 9% 1	23396

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	17% 7% 2 6% 5% 4 4 4 6% 8% 36%	2621
	11th to 20th	Day	16% 6% 2 6% 5% 5% 5% 4 7% 9% 34%	2520
	21st to end	Day	17% 6% 3 6% 5% 5% 5% 5% 7% 9% 32%	2744
August	1st to 10th	Day	16% 7% 2 7% 4 5% 5% 4 6% 9% 36%	2444
	11th to 20th	Day	17% 7% 3 7% 6% 6% 5% 4 7% 9% 30%	2192
	21st to end	Day	18% 7% 3 6% 4 5% 4 4 7% 8% 35%	2376
September	1st to 10th	Day	17% 6% 3 5% 4 4 5% 4 6% 8% 38%	2160
	11th to 20th	Day	19% 6% 2 6% 3 3 4 3 5% 6% 42%	2124
	21st to end	Day	20% 7% 2 6% 4 4 4 3 6% 7% 37%	1980
October	1st to 10th	Day	27% 8% 2 7% 5% 4 4 3 5% 5% 30%	1802
	11th to 20th	Day	23% 10% 3 6% 4 4 4 3 5% 6% 31%	1800
	21st to end	Day	28% 13% 4 7% 4 4 4 3 5% 6% 22%	1836
November	1st to 10th	Day	17% 15% 3 8% 4 4 4 3 5% 6% 31%	1671
	11th to 20th	Day	19% 18% 4 8% 5% 4 3 3 4 5% 27%	1658
	21st to end	Day	21% 19% 6% 9% 4 3 3 3 5% 4 21%	1520
December	1st to 10th	Day	23% 26% 6% 10% 5% 3 4 2 4 3 14%	1520
	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	16% 6% 2 5% 5% 4 5% 5% 6% 7% 39%	1504
	11th to 20th	Night	15% 4 2 6% 5% 5% 4 4 5% 8% 42%	1440
	21st to end	Night	13% 8% 2 6% 4 4 4 3 6% 8% 42%	1574
August	1st to 10th	Night	16% 6% 2 6% 4 3 4 4 6% 7% 41%	1491
	11th to 20th	Night	18% 7% 2 7% 4 5% 4 4 5% 8% 36%	1691
	21st to end	Night	16% 6% 2 4 4 4 4 4 5% 7% 43%	1980
September	1st to 10th	Night	15% 7% 3 5% 4 4 4 4 6% 7% 41%	1800
	11th to 20th	Night	18% 7% 3 5% 2 3 3 3 4 5% 47%	1835
	21st to end	Night	21% 6% 1 5% 4 4 4 3 4 7% 41%	1980
October	1st to 10th	Night	25% 9% 3 7% 5% 3 3 3 5% 5% 31%	2155
	11th to 20th	Night	25% 10% 3 6% 4 4 4 3 5% 6% 31%	2160
	21st to end	Night	28% 13% 3 8% 5% 4 5% 3 4 5% 23%	2520
November	1st to 10th	Night	18% 13% 3 8% 5% 4 4 3 5% 5% 32%	2407
	11th to 20th	Night	22% 16% 3 7% 4 4 4 3 5% 6% 28%	2517
	21st to end	Night	20% 17% 5% 9% 5% 4 5% 3 5% 5% 22%	2660
December	1st to 10th	Night	22% 24% 6% 9% 5% 4 4 3 5% 3 16%	2660
	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	44% 8% 13% 24% 11% 1	2551
	11th to 20th	Day	42% 9% 18% 25% 5%	2445
	21st to end	Day	45% 10% 14% 22% 9% 1	2591
August	1st to 10th	Day	43% 9% 16% 22% 10%	2430
	11th to 20th	Day	47% 10% 15% 21% 6%	2192
	21st to end	Day	44% 8% 14% 25% 9% 1	2376
September	1st to 10th	Day	42% 8% 13% 26% 10% 1	2160
	11th to 20th	Day	41% 6% 13% 25% 14% 1	2124
	21st to end	Day	45% 7% 12% 23% 12% 1	1980
October	1st to 10th	Day	55% 6% 11% 17% 10% 1	1802
	11th to 20th	Day	51% 7% 15% 20% 6% 1	1800
	21st to end	Day	62% 8% 10% 10% 8% 1	1836
November	1st to 10th	Day	51% 8% 13% 17% 10%	1671
	11th to 20th	Day	58% 7% 11% 14% 8% 2	1658
	21st to end	Day	64% 6% 14% 12% 4 1	1520
December	1st to 10th	Day	74% 5% 10% 8% 2	1520
	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	40% 9% 13% 23% 13% 2	1464
	11th to 20th	Night	38% 8% 16% 26% 11% 1	1392
	21st to end	Night	39% 9% 12% 26% 12% 1	1486
August	1st to 10th	Night	38% 8% 18% 26% 10% 1	1481
	11th to 20th	Night	46% 8% 16% 21% 9%	1691
	21st to end	Night	38% 8% 14% 28% 12% 1	1980
September	1st to 10th	Night	40% 7% 16% 24% 11% 1	1800
	11th to 20th	Night	39% 6% 11% 26% 17% 1	1835
	21st to end	Night	41% 8% 13% 26% 11% 2	1980
October	1st to 10th	Night	54% 8% 12% 16% 11% 1	2155
	11th to 20th	Night	53% 8% 14% 17% 7% 1	2160
	21st to end	Night	63% 7% 11% 11% 7% 1	2520
November	1st to 10th	Night	51% 8% 12% 16% 11% 2	2407
	11th to 20th	Night	56% 8% 11% 16% 8% 2	2517
	21st to end	Night	61% 8% 13% 14% 4	2660
December	1st to 10th	Night	71% 6% 11% 10% 2	2660
	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	17% 8% 29% 28% 14% 4	474
	11th to 20th	Day	15% 7% 29% 32% 13% 4	444
	21st to end	Day	15% 9% 36% 25% 12% 3	488
August	1st to 10th	Day	15% 9% 25% 27% 17% 6%	446
	11th to 20th	Day	18% 8% 26% 26% 13% 8%	439
	21st to end	Day	17% 15% 25% 24% 13% 6%	495
September	1st to 10th	Day	18% 10% 25% 21% 18% 8%	450
	11th to 20th	Day	17% 10% 27% 19% 17% 9%	450
	21st to end	Day	19% 14% 26% 23% 14% 5%	450
October	1st to 10th	Day	31% 12% 23% 18% 14% 3	450
	11th to 20th	Day	27% 17% 28% 19% 9% 1	450
	21st to end	Day	36% 17% 24% 17% 6%	423
November	1st to 10th	Day	30% 18% 19% 20% 12%	365
	11th to 20th	Day	32% 24% 22% 19% 3 1	318
	21st to end	Day	41% 17% 29% 12% 1	180
December	1st to 10th	Day	36% 26% 26% 8% 4	180
	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% 4 18% 42% 20% 1	366
	11th to 20th	Night	13% 4 19% 43% 19% 2	348
	21st to end	Night	13% 5% 21% 44% 16% 1	394
August	1st to 10th	Night	15% 4 18% 38% 24% 1	417
	11th to 20th	Night	18% 6% 17% 34% 23% 3	627
	21st to end	Night	16% 6% 19% 36% 21% 1	792
September	1st to 10th	Night	18% 5% 21% 35% 20% 2	720
	11th to 20th	Night	21% 6% 19% 30% 19% 4	720
	21st to end	Night	21% 8% 25% 27% 18% 1	720
October	1st to 10th	Night	29% 10% 21% 24% 15% 2	720
	11th to 20th	Night	28% 12% 24% 29% 6%	720
	21st to end	Night	33% 17% 25% 22% 3	792
November	1st to 10th	Night	24% 16% 24% 28% 8%	740
	11th to 20th	Night	28% 18% 26% 21% 6% 1	760
	21st to end	Night	30% 18% 28% 19% 5%	760
December	1st to 10th	Night	34% 22% 25% 16% 3	760
	11th to 20th	Night	33% 21% 26% 16% 4 1	760
	21st to end	Night	33% 24% 26% 15% 1	836

Page generated at 2024-07-05 11:05:37. Version ac07

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