Sunny Knoll Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sunny Knoll Observatory clear sky chart from 2005-11-06 to 2024-02-15.

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	14% 8% 4 6% 6% 3 5% 5% 5% 7% 37%	78313
	Night	14% 9% 4 7% 5% 3 4 4 5% 6% 39%	80819
Transparency	Day	32% 35% 9% 15% 8% 1	78013
	Night	33% 33% 8% 16% 9% 1	80639
Seeing	Day	17% 10% 29% 26% 13% 4	14747
	Night	18% 9% 31% 29% 12% 1	24789

By Month

Percentage Cloud Cover SE (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	11% 10% 8% 14% 9% 4 5% 4 5% 5% 25%	2527
	11th to 20th	Day	12% 11% 5% 11% 11% 4 6% 6% 7% 6% 22%	2530
	21st to end	Day	17% 18% 10% 10% 7% 4 3 4 4 5% 19%	3105
February	1st to 10th	Day	15% 11% 6% 9% 7% 3 4 4 4 7% 30%	1710
	11th to 20th	Day	16% 8% 3 6% 6% 4 5% 4 5% 7% 35%	1707
	21st to end	Day	15% 12% 5% 9% 7% 2 4 4 5% 7% 31%	1480
March	1st to 10th	Day	16% 10% 5% 6% 5% 2 4 4 5% 6% 36%	1800
	11th to 20th	Day	14% 8% 3 8% 6% 3 5% 5% 5% 7% 36%	1975
	21st to end	Day	20% 11% 4 7% 6% 3 6% 5% 5% 7% 26%	2304
April	1st to 10th	Day	17% 10% 4 7% 5% 3 5% 4 4 7% 34%	2160
	11th to 20th	Day	17% 10% 4 6% 5% 3 4 5% 5% 6% 35%	2134
	21st to end	Day	22% 9% 3 5% 5% 4 4 5% 5% 6% 34%	2160
May	1st to 10th	Day	16% 8% 3 6% 6% 4 6% 5% 6% 9% 31%	2397
	11th to 20th	Day	19% 7% 3 6% 5% 3 5% 4 5% 8% 34%	2479
	21st to end	Day	14% 8% 3 6% 6% 3 5% 6% 6% 9% 35%	2725
June	1st to 10th	Day	8% 4 2 5% 5% 4 6% 6% 8% 10% 44%	2520
	11th to 20th	Day	10% 5% 3 4 4 4 5% 5% 6% 8% 47%	2520
	21st to end	Day	10% 6% 2 6% 6% 3 6% 6% 7% 9% 40%	2520
July	1st to 10th	Day	8% 4 2 5% 6% 3 4 6% 7% 10% 47%	2520
	11th to 20th	Day	9% 3 2 5% 5% 3 4 5% 6% 10% 47%	2520
	21st to end	Day	6% 4 2 5% 5% 4 5% 5% 7% 11% 47%	2743
August	1st to 10th	Day	6% 5% 2 5% 6% 4 6% 7% 8% 11% 41%	2349
	11th to 20th	Day	9% 5% 2 5% 5% 3 5% 5% 6% 8% 48%	2188
	21st to end	Day	9% 5% 2 4 4 3 5% 5% 5% 8% 52%	2376
September	1st to 10th	Day	13% 5% 2 4 5% 4 4 6% 5% 8% 45%	2160
	11th to 20th	Day	12% 4 1 3 4 2 5% 4 5% 7% 55%	2138
	21st to end	Day	10% 5% 2 5% 5% 4 4 5% 6% 8% 47%	1966
October	1st to 10th	Day	13% 7% 2 5% 4 3 4 4 5% 7% 47%	1800
	11th to 20th	Day	16% 7% 3 5% 5% 3 4 4 5% 7% 40%	1800
	21st to end	Day	19% 9% 3 5% 5% 2 4 4 5% 6% 40%	1886
November	1st to 10th	Day	11% 7% 3 5% 4 3 4 4 5% 7% 46%	1672
	11th to 20th	Day	15% 10% 4 6% 6% 4 4 5% 5% 7% 35%	1701
	21st to end	Day	15% 11% 4 9% 5% 2 3 4 5% 4 37%	1520
December	1st to 10th	Day	15% 12% 4 8% 6% 4 4 4 4 6% 33%	1520
	11th to 20th	Day	14% 11% 4 8% 7% 4 5% 4 6% 6% 30%	1918
	21st to end	Day	24% 19% 9% 10% 6% 3 3 3 3 4 16%	2783
January	1st to 10th	Night	14% 11% 8% 11% 6% 3 4 4 4 6% 29%	4428
	11th to 20th	Night	13% 15% 7% 11% 6% 4 4 5% 7% 4 23%	4440
	21st to end	Night	18% 17% 7% 9% 7% 3 4 4 4 6% 22%	4560
February	1st to 10th	Night	15% 12% 5% 7% 6% 3 4 4 4 6% 34%	2470
	11th to 20th	Night	13% 9% 3 7% 5% 3 4 4 5% 6% 40%	2234
	21st to end	Night	17% 11% 4 6% 5% 3 4 3 4 6% 38%	1776
March	1st to 10th	Night	18% 9% 3 5% 5% 3 4 4 4 5% 41%	2160
	11th to 20th	Night	13% 8% 5% 6% 5% 4 4 4 5% 7% 40%	1999
	21st to end	Night	20% 11% 4 7% 6% 3 4 4 5% 6% 29%	2052
April	1st to 10th	Night	16% 9% 4 6% 5% 3 4 4 4 6% 40%	1800
	11th to 20th	Night	18% 9% 3 5% 5% 3 4 4 4 5% 40%	1775
	21st to end	Night	18% 9% 4 7% 5% 3 4 4 4 6% 37%	1800
May	1st to 10th	Night	15% 6% 3 5% 5% 4 5% 6% 6% 8% 37%	1560
	11th to 20th	Night	18% 6% 2 6% 5% 3 4 5% 5% 8% 38%	1416
	21st to end	Night	11% 6% 3 5% 6% 3 5% 5% 6% 9% 41%	1551
June	1st to 10th	Night	7% 5% 2 5% 4 2 5% 4 5% 8% 51%	1440
	11th to 20th	Night	9% 4 2 6% 5% 3 5% 4 7% 8% 46%	1440
	21st to end	Night	10% 4 2 5% 5% 4 6% 5% 6% 8% 47%	1440
July	1st to 10th	Night	9% 3 1 4 3 2 4 5% 7% 9% 52%	1440
	11th to 20th	Night	6% 3 2 4 3 3 3 4 5% 9% 58%	1440
	21st to end	Night	6% 3 2 4 5% 3 5% 5% 6% 9% 51%	1575
August	1st to 10th	Night	5% 3 2 5% 5% 3 4 5% 6% 10% 52%	1586
	11th to 20th	Night	6% 4 2 4 4 3 4 5% 6% 8% 55%	1711
	21st to end	Night	7% 3 2 4 4 2 3 4 4 7% 58%	1980
September	1st to 10th	Night	10% 6% 2 4 4 2 3 4 4 7% 56%	1800
	11th to 20th	Night	10% 4 1 4 4 2 5% 4 5% 8% 53%	1804
	21st to end	Night	8% 5% 3 4 5% 3 4 4 7% 7% 50%	1991
October	1st to 10th	Night	11% 4 2 5% 6% 2 3 4 4 6% 53%	2160
	11th to 20th	Night	15% 7% 3 4 4 2 4 4 4 6% 46%	2160
	21st to end	Night	19% 8% 3 6% 5% 3 4 4 4 6% 39%	2468
November	1st to 10th	Night	11% 6% 3 5% 4 2 4 4 4 6% 52%	2406
	11th to 20th	Night	15% 8% 4 6% 6% 3 4 3 4 6% 40%	2474
	21st to end	Night	17% 9% 3 5% 5% 3 4 4 3 6% 40%	2660
December	1st to 10th	Night	16% 11% 4 7% 6% 3 5% 4 4 6% 34%	2660
	11th to 20th	Night	15% 11% 5% 7% 5% 4 5% 4 5% 6% 34%	3279
	21st to end	Night	21% 15% 6% 10% 7% 4 5% 4 4 4 19%	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	49% 24% 10% 13% 3	2528
	11th to 20th	Day	50% 25% 8% 9% 7%	2530
	21st to end	Day	57% 21% 9% 10% 3	3105
February	1st to 10th	Day	37% 33% 9% 15% 5% 1	1710
	11th to 20th	Day	32% 36% 10% 16% 5% 1	1716
	21st to end	Day	35% 36% 9% 14% 5%	1480
March	1st to 10th	Day	33% 35% 10% 16% 6%	1800
	11th to 20th	Day	33% 37% 12% 14% 5%	1975
	21st to end	Day	40% 36% 8% 13% 3	2304
April	1st to 10th	Day	32% 36% 8% 16% 8%	2160
	11th to 20th	Day	34% 35% 6% 15% 9% 1	2134
	21st to end	Day	35% 37% 7% 13% 7% 1	2160
May	1st to 10th	Day	33% 36% 9% 15% 6%	2397
	11th to 20th	Day	31% 37% 7% 15% 10%	2483
	21st to end	Day	31% 39% 8% 12% 9% 1	2725
June	1st to 10th	Day	28% 37% 10% 16% 9% 1	2520
	11th to 20th	Day	23% 36% 7% 15% 16% 2	2520
	21st to end	Day	27% 37% 9% 15% 11% 1	2520
July	1st to 10th	Day	23% 37% 9% 19% 10% 2	2451
	11th to 20th	Day	22% 38% 10% 17% 12% 3	2445
	21st to end	Day	22% 39% 10% 18% 10% 1	2591
August	1st to 10th	Day	24% 37% 11% 18% 9% 1	2334
	11th to 20th	Day	21% 35% 9% 19% 15% 1	2188
	21st to end	Day	18% 35% 7% 22% 15% 3	2376
September	1st to 10th	Day	24% 36% 6% 18% 13% 3	2160
	11th to 20th	Day	23% 35% 7% 18% 16% 1	2138
	21st to end	Day	23% 36% 6% 19% 14% 1	1966
October	1st to 10th	Day	20% 35% 6% 23% 15% 1	1800
	11th to 20th	Day	31% 35% 8% 16% 8% 1	1800
	21st to end	Day	29% 37% 8% 17% 8% 2	1886
November	1st to 10th	Day	22% 40% 10% 17% 9% 2	1672
	11th to 20th	Day	31% 39% 10% 13% 6%	1701
	21st to end	Day	30% 39% 9% 16% 7%	1520
December	1st to 10th	Day	35% 39% 9% 13% 4	1520
	11th to 20th	Day	37% 32% 11% 14% 4 2	1915
	21st to end	Day	61% 24% 7% 6% 2	2783
January	1st to 10th	Night	48% 25% 9% 15% 3	4428
	11th to 20th	Night	50% 23% 8% 11% 7%	4440
	21st to end	Night	56% 22% 8% 11% 3	4560
February	1st to 10th	Night	35% 33% 7% 16% 8% 1	2470
	11th to 20th	Night	30% 36% 9% 17% 7% 1	2235
	21st to end	Night	34% 35% 8% 15% 7% 1	1776
March	1st to 10th	Night	30% 35% 11% 17% 7%	2160
	11th to 20th	Night	31% 36% 10% 15% 7% 2	1999
	21st to end	Night	39% 36% 8% 12% 4	2052
April	1st to 10th	Night	29% 36% 7% 16% 12% 1	1800
	11th to 20th	Night	32% 34% 5% 14% 12% 3	1775
	21st to end	Night	34% 36% 8% 16% 7% 1	1800
May	1st to 10th	Night	30% 38% 8% 16% 9% 1	1560
	11th to 20th	Night	30% 36% 6% 15% 12% 1	1417
	21st to end	Night	28% 37% 8% 14% 10% 2	1552
June	1st to 10th	Night	23% 36% 11% 17% 12% 2	1440
	11th to 20th	Night	22% 36% 8% 14% 17% 4	1440
	21st to end	Night	25% 35% 11% 13% 13% 3	1440
July	1st to 10th	Night	19% 37% 10% 20% 12% 3	1400
	11th to 20th	Night	18% 36% 8% 22% 14% 3	1392
	21st to end	Night	21% 38% 9% 18% 10% 3	1487
August	1st to 10th	Night	19% 36% 11% 22% 9% 2	1577
	11th to 20th	Night	18% 35% 9% 21% 16% 2	1711
	21st to end	Night	16% 35% 7% 19% 18% 6%	1980
September	1st to 10th	Night	19% 36% 7% 16% 18% 4	1800
	11th to 20th	Night	23% 35% 6% 21% 15% 1	1804
	21st to end	Night	21% 36% 6% 20% 14% 2	1991
October	1st to 10th	Night	17% 35% 7% 21% 17% 3	2160
	11th to 20th	Night	29% 35% 6% 16% 13% 1	2160
	21st to end	Night	31% 36% 6% 16% 9% 2	2468
November	1st to 10th	Night	18% 41% 9% 18% 13% 2	2406
	11th to 20th	Night	29% 38% 8% 16% 7% 1	2474
	21st to end	Night	29% 38% 8% 17% 8%	2660
December	1st to 10th	Night	34% 38% 9% 15% 4	2660
	11th to 20th	Night	36% 32% 8% 18% 4 1	3281
	21st to end	Night	56% 25% 8% 9% 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	22% 15% 35% 21% 6% 1	270
	11th to 20th	Day	24% 21% 37% 14% 3	270
	21st to end	Day	25% 17% 37% 15% 5%	297
February	1st to 10th	Day	22% 20% 39% 14% 4	270
	11th to 20th	Day	27% 19% 32% 17% 4 1	252
	21st to end	Day	27% 22% 40% 8% 2 1	198
March	1st to 10th	Day	24% 22% 34% 17% 4	240
	11th to 20th	Day	24% 16% 38% 15% 7% 1	240
	21st to end	Day	32% 16% 30% 16% 5% 1	390
April	1st to 10th	Day	25% 13% 34% 20% 6% 1	420
	11th to 20th	Day	21% 16% 31% 20% 9% 2	412
	21st to end	Day	28% 10% 33% 19% 9% 1	420
May	1st to 10th	Day	20% 10% 35% 24% 9% 2	508
	11th to 20th	Day	22% 11% 28% 25% 12% 2	625
	21st to end	Day	17% 7% 27% 26% 14% 8%	679
June	1st to 10th	Day	9% 6% 26% 35% 18% 6%	630
	11th to 20th	Day	10% 5% 30% 34% 16% 6%	630
	21st to end	Day	13% 8% 30% 29% 16% 5%	630
July	1st to 10th	Day	11% 2 20% 28% 26% 13%	621
	11th to 20th	Day	12% 4 22% 34% 19% 8%	618
	21st to end	Day	7% 1 25% 40% 22% 5%	684
August	1st to 10th	Day	8% 2 22% 37% 24% 8%	626
	11th to 20th	Day	12% 4 18% 30% 25% 11%	502
	21st to end	Day	8% 3 23% 33% 23% 10%	495
September	1st to 10th	Day	16% 5% 21% 31% 20% 6%	450
	11th to 20th	Day	10% 3 23% 34% 20% 10%	450
	21st to end	Day	12% 10% 27% 25% 21% 5%	436
October	1st to 10th	Day	21% 8% 26% 29% 11% 4	270
	11th to 20th	Day	24% 19% 36% 16% 6%	270
	21st to end	Day	30% 14% 34% 17% 5% 1	297
November	1st to 10th	Day	14% 16% 42% 17% 7% 3	270
	11th to 20th	Day	24% 17% 33% 18% 6% 1	270
	21st to end	Day	26% 13% 39% 19% 3	270
December	1st to 10th	Day	19% 16% 39% 19% 5% 2	270
	11th to 20th	Day	19% 16% 36% 24% 4	270
	21st to end	Day	28% 17% 39% 14% 2	297
January	1st to 10th	Night	19% 14% 37% 22% 7%	950
	11th to 20th	Night	20% 15% 35% 25% 5% 1	950
	21st to end	Night	22% 12% 34% 27% 5%	1045
February	1st to 10th	Night	23% 14% 36% 24% 4	950
	11th to 20th	Night	18% 14% 38% 26% 4	803
	21st to end	Night	24% 15% 38% 19% 3 1	592
March	1st to 10th	Night	24% 13% 35% 23% 5%	720
	11th to 20th	Night	18% 10% 33% 31% 8%	559
	21st to end	Night	27% 10% 33% 24% 6% 1	594
April	1st to 10th	Night	22% 12% 37% 24% 4 1	540
	11th to 20th	Night	24% 13% 28% 28% 6% 1	533
	21st to end	Night	19% 10% 32% 33% 5% 1	540
May	1st to 10th	Night	18% 6% 40% 27% 8% 1	540
	11th to 20th	Night	19% 5% 32% 32% 11%	531
	21st to end	Night	14% 3 30% 33% 18% 1	584
June	1st to 10th	Night	10% 3 28% 42% 16% 2	540
	11th to 20th	Night	10% 3 30% 40% 16% 1	540
	21st to end	Night	13% 4 25% 41% 16% 1	540
July	1st to 10th	Night	10% 2 14% 39% 29% 5%	525
	11th to 20th	Night	7% 1 24% 40% 25% 2	522
	21st to end	Night	9% 1 19% 43% 27% 1	591
August	1st to 10th	Night	7% 1 14% 43% 31% 4	537
	11th to 20th	Night	6% 2 14% 43% 30% 5%	534
	21st to end	Night	10% 1 18% 42% 26% 3	594
September	1st to 10th	Night	13% 2 22% 37% 22% 4	540
	11th to 20th	Night	12% 2 21% 35% 25% 5%	544
	21st to end	Night	9% 8% 24% 33% 23% 2	720
October	1st to 10th	Night	13% 6% 32% 29% 20% 1	720
	11th to 20th	Night	19% 11% 31% 28% 10%	720
	21st to end	Night	24% 11% 33% 23% 8% 1	792
November	1st to 10th	Night	16% 11% 38% 27% 8% 1	740
	11th to 20th	Night	20% 15% 36% 23% 5%	764
	21st to end	Night	22% 12% 35% 25% 6%	950
December	1st to 10th	Night	21% 14% 36% 23% 6%	950
	11th to 20th	Night	23% 10% 34% 27% 6%	950
	21st to end	Night	29% 14% 31% 23% 3	1045

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