Nebula Knoll Observatory
Clear Sky Chart
History

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

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% 8% 2 6% 4 4 4 4 6% 7% 37%	101363
	Night	17% 8% 3 6% 4 4 4 4 5% 7% 38%	97823
Transparency	Day	25% 47% 6% 12% 8% 1	104888
	Night	25% 47% 6% 12% 8% 1	102387
Seeing	Day	22% 16% 31% 20% 8% 2	14953
	Night	21% 12% 31% 27% 8% 1	23793

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	14% 8% 3 6% 4 4 3 3 6% 7% 42%	2517
	11th to 20th	Day	16% 8% 3 7% 5% 4 4 4 5% 6% 39%	2520
	21st to end	Day	17% 11% 3 7% 4 4 4 3 5% 7% 36%	2772
February	1st to 10th	Day	15% 10% 2 7% 5% 5% 5% 4 5% 7% 36%	1691
	11th to 20th	Day	11% 9% 3 7% 5% 4 5% 4 6% 7% 39%	1848
	21st to end	Day	15% 11% 4 7% 5% 5% 5% 4 4 6% 35%	1570
March	1st to 10th	Day	21% 16% 3 7% 5% 4 4 3 4 4 29%	3116
	11th to 20th	Day	15% 7% 2 6% 4 4 5% 4 6% 7% 39%	3350
	21st to end	Day	20% 11% 3 6% 4 3 3 3 5% 7% 34%	4001
April	1st to 10th	Day	19% 10% 3 6% 5% 3 3 4 5% 6% 36%	2280
	11th to 20th	Day	16% 8% 3 7% 4 5% 4 4 6% 7% 38%	2304
	21st to end	Day	21% 8% 3 8% 5% 5% 5% 3 4 5% 33%	2470
May	1st to 10th	Day	23% 13% 3 9% 4 4 4 4 4 6% 26%	4106
	11th to 20th	Day	20% 8% 3 8% 6% 4 4 3 6% 6% 31%	4225
	21st to end	Day	15% 6% 2 5% 4 4 5% 4 6% 8% 40%	4837
June	1st to 10th	Day	21% 8% 2 7% 5% 5% 5% 4 6% 8% 31%	2660
	11th to 20th	Day	17% 6% 2 6% 5% 5% 4 5% 6% 7% 38%	2660
	21st to end	Day	20% 7% 3 6% 5% 5% 5% 4 6% 7% 32%	2660
July	1st to 10th	Day	13% 6% 2 6% 6% 5% 6% 5% 6% 10% 35%	4440
	11th to 20th	Day	11% 6% 2 6% 5% 4 5% 5% 7% 10% 39%	4440
	21st to end	Day	17% 7% 2 6% 5% 5% 4 5% 6% 7% 36%	4814
August	1st to 10th	Day	14% 6% 2 8% 5% 6% 5% 5% 6% 8% 35%	4201
	11th to 20th	Day	16% 8% 3 8% 4 4 5% 4 6% 8% 34%	4045
	21st to end	Day	9% 5% 2 6% 4 4 5% 4 6% 9% 46%	4225
September	1st to 10th	Day	12% 5% 2 5% 4 4 4 4 7% 7% 48%	2271
	11th to 20th	Day	14% 4 2 5% 3 3 3 3 4 6% 54%	2090
	21st to end	Day	15% 6% 2 5% 3 4 4 3 6% 6% 46%	2090
October	1st to 10th	Day	20% 7% 3 7% 4 3 4 3 5% 6% 38%	2090
	11th to 20th	Day	16% 6% 2 6% 4 4 4 3 5% 7% 43%	1824
	21st to end	Day	23% 8% 2 6% 4 4 4 3 4 5% 38%	1881
November	1st to 10th	Day	14% 6% 2 5% 4 3 4 3 5% 7% 48%	1761
	11th to 20th	Day	20% 5% 1 4 3 4 4 4 5% 7% 43%	1710
	21st to end	Day	21% 7% 2 5% 4 3 4 3 5% 6% 41%	1696
December	1st to 10th	Day	20% 10% 2 6% 4 3 5% 4 6% 6% 33%	1520
	11th to 20th	Day	21% 8% 2 6% 3 3 4 3 5% 5% 41%	1906
	21st to end	Day	25% 11% 2 6% 3 3 4 3 6% 6% 31%	2772
January	1st to 10th	Night	17% 7% 2 5% 3 4 4 3 5% 6% 44%	4429
	11th to 20th	Night	12% 8% 3 9% 5% 4 4 4 6% 8% 36%	4440
	21st to end	Night	20% 10% 3 8% 5% 4 4 4 4 5% 33%	4884
February	1st to 10th	Night	14% 10% 3 7% 4 5% 4 4 5% 7% 36%	2486
	11th to 20th	Night	12% 10% 3 5% 5% 5% 5% 4 5% 6% 41%	2218
	21st to end	Night	13% 9% 3 6% 4 5% 5% 4 5% 7% 39%	1884
March	1st to 10th	Night	25% 12% 2 5% 4 4 4 3 4 5% 33%	3724
	11th to 20th	Night	12% 8% 3 8% 5% 6% 6% 5% 7% 7% 34%	3608
	21st to end	Night	23% 9% 3 6% 4 3 4 3 6% 8% 32%	3628
April	1st to 10th	Night	18% 9% 3 7% 4 4 4 3 5% 7% 36%	1900
	11th to 20th	Night	18% 8% 3 7% 5% 4 3 3 5% 6% 39%	1824
	21st to end	Night	21% 8% 3 6% 3 4 3 4 5% 6% 37%	1710
May	1st to 10th	Night	24% 12% 3 8% 5% 4 5% 3 5% 6% 25%	2834
	11th to 20th	Night	17% 9% 3 9% 6% 7% 6% 5% 7% 7% 26%	2696
	21st to end	Night	16% 7% 2 6% 4 4 4 4 6% 8% 40%	2738
June	1st to 10th	Night	20% 7% 2 7% 5% 4 4 4 7% 6% 34%	1520
	11th to 20th	Night	17% 8% 2 6% 3 4 4 4 5% 7% 38%	1520
	21st to end	Night	19% 6% 2 5% 5% 4 4 4 6% 7% 37%	1520
July	1st to 10th	Night	11% 5% 3 7% 5% 5% 6% 6% 7% 11% 36%	2520
	11th to 20th	Night	13% 5% 3 6% 4 5% 5% 4 6% 8% 41%	2520
	21st to end	Night	15% 5% 3 6% 4 4 4 4 5% 7% 42%	2739
August	1st to 10th	Night	21% 7% 3 6% 5% 4 4 4 4 7% 33%	2695
	11th to 20th	Night	16% 8% 3 6% 5% 4 4 4 5% 8% 36%	2782
	21st to end	Night	10% 5% 1 5% 4 4 4 4 6% 9% 48%	3429
September	1st to 10th	Night	13% 5% 2 4 3 3 4 4 5% 7% 51%	1909
	11th to 20th	Night	14% 6% 2 4 3 3 3 2 5% 7% 51%	2090
	21st to end	Night	14% 6% 3 5% 3 3 3 3 5% 7% 47%	2090
October	1st to 10th	Night	18% 7% 2 6% 5% 4 4 4 4 6% 38%	2090
	11th to 20th	Night	15% 7% 2 5% 4 5% 4 3 6% 6% 42%	2352
	21st to end	Night	23% 8% 2 6% 4 3 4 3 4 6% 37%	2717
November	1st to 10th	Night	15% 6% 2 6% 3 4 4 3 5% 7% 45%	2537
	11th to 20th	Night	16% 8% 2 6% 4 4 4 3 5% 6% 43%	2477
	21st to end	Night	16% 8% 2 5% 5% 4 4 3 5% 6% 43%	2481
December	1st to 10th	Night	19% 10% 3 6% 5% 4 4 4 5% 5% 35%	2660
	11th to 20th	Night	20% 8% 2 5% 5% 4 4 3 5% 6% 39%	3288
	21st to end	Night	23% 10% 3 7% 4 4 3 2 4 5% 34%	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	25% 39% 5% 23% 7% 1	2518
	11th to 20th	Day	28% 39% 7% 18% 7% 1	2520
	21st to end	Day	32% 42% 7% 13% 6%	2772
February	1st to 10th	Day	18% 65% 5% 7% 4 1	1691
	11th to 20th	Day	17% 64% 2 6% 8% 2	1857
	21st to end	Day	16% 65% 3 8% 7% 2	1570
March	1st to 10th	Day	40% 41% 4 5% 8% 1	3119
	11th to 20th	Day	23% 42% 9% 10% 11% 5%	3350
	21st to end	Day	33% 41% 2 8% 13% 4	4001
April	1st to 10th	Day	19% 65% 2 7% 5% 2	2280
	11th to 20th	Day	16% 66% 4 8% 6% 1	2304
	21st to end	Day	20% 64% 3 5% 7%	2470
May	1st to 10th	Day	38% 44% 6% 9% 4	4120
	11th to 20th	Day	33% 44% 11% 9% 3	4229
	21st to end	Day	24% 43% 8% 14% 11%	4837
June	1st to 10th	Day	16% 65% 3 9% 6%	2660
	11th to 20th	Day	14% 65% 5% 8% 8% 1	2660
	21st to end	Day	16% 65% 3 9% 7%	2660
July	1st to 10th	Day	26% 39% 8% 16% 10%	4370
	11th to 20th	Day	23% 39% 11% 18% 9%	4365
	21st to end	Day	32% 37% 7% 12% 11% 1	4661
August	1st to 10th	Day	29% 39% 9% 16% 6%	4187
	11th to 20th	Day	33% 39% 9% 15% 4	4084
	21st to end	Day	20% 39% 10% 17% 13% 1	4225
September	1st to 10th	Day	12% 60% 6% 13% 7% 1	2271
	11th to 20th	Day	15% 60% 3 11% 9% 2	2090
	21st to end	Day	16% 60% 3 12% 7% 1	2090
October	1st to 10th	Day	29% 39% 5% 15% 11% 1	3480
	11th to 20th	Day	18% 37% 7% 19% 16% 2	3001
	21st to end	Day	20% 37% 6% 20% 14% 3	3081
November	1st to 10th	Day	17% 56% 7% 11% 8% 1	1761
	11th to 20th	Day	16% 60% 3 8% 11% 2	1710
	21st to end	Day	17% 59% 6% 8% 8% 1	1696
December	1st to 10th	Day	20% 61% 5% 8% 6%	1520
	11th to 20th	Day	24% 48% 4 10% 10% 4	1906
	21st to end	Day	35% 38% 5% 12% 7% 2	2772
January	1st to 10th	Night	25% 38% 7% 20% 6% 3	4429
	11th to 20th	Night	25% 38% 10% 14% 10% 2	4440
	21st to end	Night	37% 41% 7% 11% 4 1	4884
February	1st to 10th	Night	17% 65% 5% 6% 5% 1	2486
	11th to 20th	Night	15% 64% 3 7% 9% 1	2220
	21st to end	Night	16% 65% 3 9% 5% 3	1884
March	1st to 10th	Night	36% 41% 6% 8% 6% 2	3734
	11th to 20th	Night	27% 43% 6% 10% 11% 3	3609
	21st to end	Night	34% 41% 3 8% 11% 4	3628
April	1st to 10th	Night	19% 65% 2 6% 7% 1	1900
	11th to 20th	Night	18% 66% 2 7% 7% 1	1824
	21st to end	Night	16% 65% 4 6% 7% 1	1710
May	1st to 10th	Night	38% 44% 6% 9% 3	2840
	11th to 20th	Night	34% 46% 9% 9% 3	2697
	21st to end	Night	24% 43% 12% 12% 9% 1	2739
June	1st to 10th	Night	16% 66% 4 8% 6%	1520
	11th to 20th	Night	14% 66% 6% 8% 6% 1	1520
	21st to end	Night	16% 64% 3 11% 4 1	1520
July	1st to 10th	Night	24% 40% 11% 14% 10%	2480
	11th to 20th	Night	26% 38% 8% 15% 12%	2472
	21st to end	Night	28% 37% 6% 19% 9%	2651
August	1st to 10th	Night	35% 39% 8% 12% 5%	2685
	11th to 20th	Night	32% 40% 10% 12% 6%	2809
	21st to end	Night	17% 39% 11% 19% 12% 1	3429
September	1st to 10th	Night	12% 62% 6% 13% 8% 1	1909
	11th to 20th	Night	15% 60% 4 11% 9% 2	2090
	21st to end	Night	14% 60% 5% 13% 7% 1	2090
October	1st to 10th	Night	28% 39% 8% 15% 9% 1	3480
	11th to 20th	Night	15% 37% 6% 18% 20% 4	3914
	21st to end	Night	23% 36% 5% 21% 13% 2	4467
November	1st to 10th	Night	18% 56% 4 14% 7% 1	2537
	11th to 20th	Night	17% 60% 3 9% 11% 1	2477
	21st to end	Night	15% 61% 6% 11% 7%	2481
December	1st to 10th	Night	21% 60% 4 9% 6%	2660
	11th to 20th	Night	25% 48% 5% 9% 13% 1	3288
	21st to end	Night	35% 38% 5% 15% 7% 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% 22% 28% 19% 3 1	180
	11th to 20th	Day	29% 19% 31% 16% 3 2	180
	21st to end	Day	18% 19% 43% 17% 4	198
February	1st to 10th	Day	26% 24% 32% 16% 2	320
	11th to 20th	Day	24% 25% 33% 15% 3	456
	21st to end	Day	30% 18% 30% 18% 4	382
March	1st to 10th	Day	28% 20% 33% 17% 3	460
	11th to 20th	Day	25% 28% 30% 13% 4 1	460
	21st to end	Day	27% 21% 30% 13% 6% 2	506
April	1st to 10th	Day	31% 23% 27% 12% 6% 2	460
	11th to 20th	Day	26% 26% 26% 14% 7% 1	452
	21st to end	Day	29% 22% 28% 12% 7% 1	460
May	1st to 10th	Day	33% 19% 27% 16% 4 1	463
	11th to 20th	Day	21% 19% 35% 18% 5% 2	487
	21st to end	Day	21% 17% 32% 23% 7% 1	529
June	1st to 10th	Day	21% 15% 38% 18% 8% 1	490
	11th to 20th	Day	17% 17% 34% 23% 7% 2	490
	21st to end	Day	22% 15% 34% 17% 10% 2	490
July	1st to 10th	Day	18% 9% 33% 26% 10% 3	485
	11th to 20th	Day	13% 8% 35% 29% 12% 3	484
	21st to end	Day	15% 8% 36% 30% 9% 2	532
August	1st to 10th	Day	14% 10% 26% 31% 15% 3	486
	11th to 20th	Day	16% 11% 28% 25% 15% 4	479
	21st to end	Day	12% 10% 30% 28% 16% 4	539
September	1st to 10th	Day	15% 11% 27% 26% 14% 6%	490
	11th to 20th	Day	19% 12% 27% 19% 14% 10%	490
	21st to end	Day	17% 11% 33% 23% 11% 5%	490
October	1st to 10th	Day	26% 12% 24% 22% 12% 4	490
	11th to 20th	Day	20% 14% 29% 23% 12% 2	417
	21st to end	Day	32% 10% 31% 20% 7%	429
November	1st to 10th	Day	21% 17% 32% 23% 6% 1	395
	11th to 20th	Day	30% 16% 28% 21% 5% 1	370
	21st to end	Day	28% 19% 30% 17% 5% 1	356
December	1st to 10th	Day	25% 27% 24% 16% 8%	180
	11th to 20th	Day	29% 24% 33% 12% 3	180
	21st to end	Day	28% 23% 30% 13% 5%	198
January	1st to 10th	Night	20% 15% 39% 23% 3	760
	11th to 20th	Night	26% 17% 33% 22% 4	760
	21st to end	Night	17% 18% 36% 24% 4	836
February	1st to 10th	Night	22% 16% 38% 22% 2	760
	11th to 20th	Night	19% 19% 40% 18% 4	743
	21st to end	Night	22% 15% 36% 23% 4	628
March	1st to 10th	Night	25% 15% 36% 21% 3	760
	11th to 20th	Night	21% 17% 35% 22% 5%	760
	21st to end	Night	26% 15% 33% 19% 7%	836
April	1st to 10th	Night	24% 18% 33% 23% 4	760
	11th to 20th	Night	23% 16% 35% 22% 5%	700
	21st to end	Night	25% 15% 30% 25% 5%	570
May	1st to 10th	Night	26% 11% 29% 28% 4 1	570
	11th to 20th	Night	23% 11% 34% 25% 7%	497
	21st to end	Night	22% 9% 26% 33% 9% 1	411
June	1st to 10th	Night	21% 8% 33% 28% 10%	380
	11th to 20th	Night	20% 7% 29% 30% 13% 1	380
	21st to end	Night	21% 4 31% 36% 9%	380
July	1st to 10th	Night	16% 3 24% 42% 14%	370
	11th to 20th	Night	14% 4 22% 41% 18% 1	368
	21st to end	Night	15% 3 22% 45% 15% 1	416
August	1st to 10th	Night	17% 4 21% 40% 17%	489
	11th to 20th	Night	15% 7% 27% 33% 16% 2	562
	21st to end	Night	15% 5% 25% 36% 17% 2	802
September	1st to 10th	Night	16% 4 28% 34% 17% 1	760
	11th to 20th	Night	17% 7% 23% 29% 20% 4	760
	21st to end	Night	18% 8% 28% 28% 17% 2	760
October	1st to 10th	Night	23% 10% 23% 30% 13% 2	760
	11th to 20th	Night	19% 11% 32% 25% 12% 1	760
	21st to end	Night	29% 12% 27% 26% 6%	836
November	1st to 10th	Night	20% 12% 33% 26% 8% 1	780
	11th to 20th	Night	23% 17% 29% 24% 5% 1	763
	21st to end	Night	22% 18% 31% 24% 5%	760
December	1st to 10th	Night	25% 12% 39% 20% 5%	760
	11th to 20th	Night	24% 18% 31% 24% 3	760
	21st to end	Night	26% 14% 34% 24% 3	836

Page generated at 2024-11-08 06:44:47. Version ac07

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