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 2026-07-31.

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	12% 8% 3 7% 5% 5% 5% 4 6% 7% 38%	138686
	Night	12% 9% 3 8% 5% 4 4 3 5% 6% 40%	134651
Transparency	Day	44% 3 11% 25% 14% 2	142195
	Night	45% 3 11% 24% 16% 2	139219
Seeing	Day	17% 11% 30% 25% 13% 4	18231
	Night	18% 10% 31% 29% 11% 1	28022

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	8% 12% 6% 15% 7% 6% 6% 4 5% 6% 25%	4691
	11th to 20th	Day	7% 10% 6% 11% 7% 5% 7% 5% 7% 7% 30%	4710
	21st to end	Day	10% 13% 7% 9% 5% 6% 5% 4 5% 6% 30%	5768
February	1st to 10th	Day	14% 13% 5% 10% 7% 4 4 3 5% 7% 29%	1890
	11th to 20th	Day	14% 10% 3 8% 5% 5% 4 3 5% 7% 36%	1958
	21st to end	Day	13% 12% 4 10% 5% 4 4 4 5% 6% 34%	1730
March	1st to 10th	Day	18% 10% 3 5% 3 2 3 2 4 5% 43%	4580
	11th to 20th	Day	9% 6% 2 7% 4 4 5% 4 5% 7% 46%	5085
	21st to end	Day	20% 11% 4 10% 5% 5% 5% 4 5% 6% 26%	5911
April	1st to 10th	Day	16% 12% 4 7% 5% 4 4 3 5% 6% 33%	2520
	11th to 20th	Day	16% 10% 3 7% 5% 4 4 4 5% 6% 35%	2494
	21st to end	Day	20% 9% 3 6% 3 5% 4 4 5% 7% 35%	2520
May	1st to 10th	Day	13% 7% 3 7% 5% 5% 5% 4 6% 8% 38%	7826
	11th to 20th	Day	13% 6% 2 7% 4 4 5% 3 6% 7% 43%	8222
	21st to end	Day	12% 8% 3 7% 5% 5% 4 4 6% 8% 38%	9047
June	1st to 10th	Day	9% 5% 2 6% 4 5% 5% 5% 7% 9% 43%	2940
	11th to 20th	Day	10% 5% 2 5% 4 5% 5% 4 6% 9% 46%	2940
	21st to end	Day	10% 6% 2 7% 5% 4 5% 5% 7% 9% 41%	2940
July	1st to 10th	Day	8% 7% 3 8% 5% 5% 5% 4 7% 10% 38%	6500
	11th to 20th	Day	9% 5% 2 7% 4 5% 5% 4 6% 9% 43%	6500
	21st to end	Day	11% 6% 3 7% 5% 5% 5% 4 7% 10% 37%	7065
August	1st to 10th	Day	7% 5% 2 6% 4 5% 5% 5% 8% 10% 43%	4270
	11th to 20th	Day	10% 5% 2 6% 5% 4 5% 4 7% 10% 43%	3954
	21st to end	Day	7% 6% 2 5% 4 4 4 3 5% 9% 50%	4312
September	1st to 10th	Day	11% 5% 2 5% 4 5% 3 5% 5% 8% 48%	2400
	11th to 20th	Day	10% 4 1 3 3 3 4 4 5% 8% 54%	2375
	21st to end	Day	10% 5% 2 6% 4 4 5% 3 6% 8% 47%	2184
October	1st to 10th	Day	12% 7% 2 5% 3 3 4 3 5% 6% 49%	2000
	11th to 20th	Day	15% 8% 3 6% 4 4 4 4 5% 7% 42%	2000
	21st to end	Day	17% 9% 3 6% 4 3 3 3 5% 6% 41%	2095
November	1st to 10th	Day	11% 8% 3 7% 3 4 4 3 6% 6% 45%	1852
	11th to 20th	Day	15% 11% 4 6% 5% 5% 4 4 4 7% 35%	1879
	21st to end	Day	15% 12% 4 10% 4 4 3 4 5% 4 36%	1680
December	1st to 10th	Day	9% 9% 3 8% 4 4 4 3 4 6% 46%	2690
	11th to 20th	Day	15% 13% 6% 12% 6% 6% 5% 3 6% 6% 23%	3088
	21st to end	Day	23% 21% 8% 11% 5% 4 3 2 3 4 14%	4070
January	1st to 10th	Night	12% 12% 6% 13% 4 4 4 3 4 6% 33%	8232
	11th to 20th	Night	9% 11% 5% 13% 8% 6% 5% 5% 6% 5% 27%	8280
	21st to end	Night	12% 13% 5% 9% 5% 4 4 3 5% 6% 35%	8444
February	1st to 10th	Night	14% 12% 4 7% 5% 5% 4 3 5% 5% 35%	2730
	11th to 20th	Night	12% 11% 3 7% 5% 4 4 3 5% 6% 40%	2548
	21st to end	Night	15% 11% 3 8% 4 4 3 2 5% 6% 39%	2076
March	1st to 10th	Night	18% 9% 3 5% 3 3 3 3 4 4 44%	5493
	11th to 20th	Night	10% 7% 3 6% 4 4 4 3 4 7% 49%	5138
	21st to end	Night	17% 12% 5% 11% 5% 5% 4 3 5% 5% 27%	5246
April	1st to 10th	Night	17% 10% 3 7% 4 4 4 3 4 5% 39%	2100
	11th to 20th	Night	17% 9% 3 6% 4 5% 4 4 4 6% 40%	2075
	21st to end	Night	16% 9% 4 7% 4 4 4 4 5% 6% 37%	2100
May	1st to 10th	Night	14% 7% 3 7% 5% 4 4 4 6% 6% 39%	5039
	11th to 20th	Night	11% 5% 2 7% 4 5% 5% 4 6% 8% 42%	4670
	21st to end	Night	11% 8% 3 6% 5% 5% 5% 4 5% 7% 42%	5139
June	1st to 10th	Night	7% 6% 2 6% 3 4 5% 3 6% 8% 50%	1680
	11th to 20th	Night	9% 5% 2 7% 4 4 4 4 7% 8% 46%	1680
	21st to end	Night	10% 5% 1 6% 4 6% 6% 4 6% 7% 45%	1680
July	1st to 10th	Night	12% 5% 2 7% 4 4 5% 4 6% 9% 42%	3698
	11th to 20th	Night	8% 5% 2 5% 4 4 4 3 5% 8% 51%	3697
	21st to end	Night	9% 6% 2 7% 5% 5% 5% 4 6% 7% 43%	4027
August	1st to 10th	Night	5% 3 2 5% 4 4 4 4 6% 10% 53%	2886
	11th to 20th	Night	6% 4 2 6% 4 4 5% 4 6% 8% 53%	3096
	21st to end	Night	6% 4 2 6% 4 4 4 3 5% 8% 54%	3586
September	1st to 10th	Night	9% 5% 1 5% 3 2 3 3 4 7% 58%	2000
	11th to 20th	Night	9% 4 1 4 3 3 5% 4 6% 8% 52%	2005
	21st to end	Night	8% 5% 2 5% 4 4 4 3 6% 7% 51%	2213
October	1st to 10th	Night	10% 5% 2 6% 4 3 3 3 4 6% 54%	2400
	11th to 20th	Night	15% 7% 3 5% 4 3 4 3 4 6% 47%	2400
	21st to end	Night	18% 8% 3 7% 4 5% 4 3 4 6% 39%	2743
November	1st to 10th	Night	12% 6% 2 5% 3 3 3 3 5% 6% 51%	2666
	11th to 20th	Night	15% 9% 3 7% 5% 4 4 3 4 6% 39%	2735
	21st to end	Night	16% 10% 3 6% 5% 4 4 3 4 5% 39%	2940
December	1st to 10th	Night	9% 9% 3 7% 4 4 4 3 5% 6% 46%	4720
	11th to 20th	Night	15% 14% 4 9% 5% 4 4 3 5% 5% 33%	5339
	21st to end	Night	23% 17% 6% 11% 6% 5% 4 3 4 4 15%	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	59% 3 13% 18% 7% 1	4692
	11th to 20th	Day	54% 2 8% 19% 14% 2	4710
	21st to end	Day	56% 2 12% 22% 7%	5768
February	1st to 10th	Day	55% 2 13% 22% 8% 1	1890
	11th to 20th	Day	50% 2 11% 24% 10% 2	1967
	21st to end	Day	51% 3 11% 24% 9% 2	1730
March	1st to 10th	Day	46% 1 9% 28% 13% 3	4580
	11th to 20th	Day	38% 2 10% 34% 15% 1	5085
	21st to end	Day	59% 2 13% 20% 6% 1	5913
April	1st to 10th	Day	53% 3 11% 22% 11%	2520
	11th to 20th	Day	50% 2 9% 23% 13% 2	2494
	21st to end	Day	51% 2 10% 21% 13% 2	2520
May	1st to 10th	Day	44% 3 9% 24% 19%	7862
	11th to 20th	Day	41% 4 10% 27% 15% 3	8226
	21st to end	Day	44% 4 10% 23% 17% 2	9061
June	1st to 10th	Day	37% 4 13% 27% 17% 2	2940
	11th to 20th	Day	36% 4 11% 24% 23% 3	2940
	21st to end	Day	39% 5% 14% 26% 15% 2	2940
July	1st to 10th	Day	40% 5% 12% 28% 13% 1	6431
	11th to 20th	Day	37% 5% 13% 29% 15% 1	6425
	21st to end	Day	41% 6% 16% 21% 14% 1	6869
August	1st to 10th	Day	34% 5% 17% 30% 14% 1	4255
	11th to 20th	Day	36% 5% 15% 27% 16% 1	3992
	21st to end	Day	32% 4 12% 33% 17% 2	4311
September	1st to 10th	Day	35% 3 10% 27% 21% 4	2400
	11th to 20th	Day	30% 2 12% 30% 23% 3	2375
	21st to end	Day	35% 4 10% 29% 20% 2	2184
October	1st to 10th	Day	23% 2 5% 31% 37% 2	3260
	11th to 20th	Day	29% 2 11% 24% 31% 4	3240
	21st to end	Day	40% 2 8% 30% 15% 3	3359
November	1st to 10th	Day	40% 2 14% 27% 14% 3	1852
	11th to 20th	Day	49% 2 13% 23% 12% 1	1879
	21st to end	Day	52% 3 13% 23% 10%	1680
December	1st to 10th	Day	41% 2 14% 35% 8% 1	2690
	11th to 20th	Day	62% 2 11% 16% 6% 2	3085
	21st to end	Day	75% 2 9% 11% 4	4070
January	1st to 10th	Night	55% 2 14% 21% 8%	8233
	11th to 20th	Night	59% 2 10% 14% 14% 1	8280
	21st to end	Night	52% 2 12% 21% 11% 1	8444
February	1st to 10th	Night	52% 2 11% 22% 12% 1	2730
	11th to 20th	Night	47% 2 11% 26% 13% 1	2549
	21st to end	Night	48% 2 9% 24% 16% 2	2076
March	1st to 10th	Night	46% 2 9% 25% 15% 3	5493
	11th to 20th	Night	38% 2 9% 30% 18% 4	5138
	21st to end	Night	59% 3 11% 18% 8% 1	5246
April	1st to 10th	Night	48% 2 9% 25% 14% 2	2100
	11th to 20th	Night	46% 3 8% 23% 17% 4	2075
	21st to end	Night	48% 3 10% 23% 14% 3	2100
May	1st to 10th	Night	45% 3 8% 21% 21% 2	5066
	11th to 20th	Night	39% 3 14% 23% 17% 4	4671
	21st to end	Night	42% 4 9% 24% 18% 3	5149
June	1st to 10th	Night	33% 3 14% 27% 19% 3	1680
	11th to 20th	Night	34% 4 10% 23% 25% 4	1680
	21st to end	Night	37% 3 14% 24% 19% 3	1680
July	1st to 10th	Night	39% 4 12% 27% 16% 2	3660
	11th to 20th	Night	31% 5% 13% 30% 21% 1	3652
	21st to end	Night	38% 6% 15% 24% 14% 3	3912
August	1st to 10th	Night	26% 4 17% 34% 17% 2	2877
	11th to 20th	Night	30% 5% 15% 31% 18% 1	3124
	21st to end	Night	28% 4 13% 32% 19% 4	3586
September	1st to 10th	Night	28% 3 10% 28% 27% 4	2000
	11th to 20th	Night	31% 3 10% 31% 21% 5%	2005
	21st to end	Night	32% 3 9% 29% 24% 3	2213
October	1st to 10th	Night	24% 2 6% 31% 34% 3	3920
	11th to 20th	Night	30% 2 7% 23% 34% 4	3899
	21st to end	Night	45% 2 7% 23% 20% 4	4429
November	1st to 10th	Night	35% 3 12% 28% 19% 4	2666
	11th to 20th	Night	46% 2 11% 26% 15% 2	2735
	21st to end	Night	49% 2 9% 26% 12% 1	2940
December	1st to 10th	Night	41% 2 11% 34% 11% 1	4720
	11th to 20th	Night	55% 2 11% 22% 8% 1	5341
	21st to end	Night	73% 2 8% 12% 4	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	21% 15% 37% 21% 5% 1	330
	11th to 20th	Day	22% 23% 39% 13% 3	330
	21st to end	Day	22% 21% 39% 13% 4	363
February	1st to 10th	Day	21% 22% 38% 15% 3	330
	11th to 20th	Day	26% 22% 34% 15% 3 1	330
	21st to end	Day	25% 21% 44% 8% 2	273
March	1st to 10th	Day	25% 20% 35% 17% 3	330
	11th to 20th	Day	26% 17% 38% 13% 5% 1	330
	21st to end	Day	31% 18% 32% 15% 4 1	510
April	1st to 10th	Day	27% 17% 32% 18% 5% 1	540
	11th to 20th	Day	22% 20% 29% 20% 7% 2	532
	21st to end	Day	25% 13% 34% 19% 8% 1	540
May	1st to 10th	Day	19% 11% 36% 23% 9% 2	643
	11th to 20th	Day	21% 12% 29% 24% 12% 2	775
	21st to end	Day	17% 8% 29% 26% 13% 7%	844
June	1st to 10th	Day	10% 7% 27% 34% 17% 5%	780
	11th to 20th	Day	11% 7% 32% 31% 15% 5%	780
	21st to end	Day	12% 8% 32% 29% 14% 4	780
July	1st to 10th	Day	11% 2 22% 30% 24% 11%	771
	11th to 20th	Day	11% 5% 22% 33% 20% 9%	768
	21st to end	Day	8% 2 25% 39% 20% 5%	845
August	1st to 10th	Day	8% 3 22% 36% 23% 7%	726
	11th to 20th	Day	12% 4 19% 31% 24% 10%	588
	21st to end	Day	7% 3 22% 34% 25% 9%	583
September	1st to 10th	Day	13% 6% 21% 32% 21% 7%	530
	11th to 20th	Day	9% 2 20% 35% 24% 10%	530
	21st to end	Day	12% 9% 27% 26% 20% 6%	514
October	1st to 10th	Day	18% 8% 28% 29% 13% 5%	330
	11th to 20th	Day	20% 19% 34% 20% 7%	330
	21st to end	Day	28% 15% 35% 17% 5% 1	363
November	1st to 10th	Day	16% 16% 41% 18% 6% 3	330
	11th to 20th	Day	25% 17% 33% 17% 6% 1	330
	21st to end	Day	25% 15% 37% 20% 4	330
December	1st to 10th	Day	17% 17% 42% 18% 5% 2	330
	11th to 20th	Day	21% 18% 35% 22% 4	330
	21st to end	Day	30% 18% 36% 14% 2	363
January	1st to 10th	Night	19% 15% 36% 23% 7%	1050
	11th to 20th	Night	19% 16% 35% 24% 5% 1	1050
	21st to end	Night	21% 13% 35% 26% 5%	1155
February	1st to 10th	Night	21% 15% 36% 23% 4	1050
	11th to 20th	Night	19% 15% 37% 25% 4	913
	21st to end	Night	22% 15% 40% 18% 3 1	692
March	1st to 10th	Night	24% 14% 35% 23% 5%	840
	11th to 20th	Night	18% 12% 34% 29% 7%	652
	21st to end	Night	26% 11% 35% 23% 5%	693
April	1st to 10th	Night	23% 13% 36% 23% 4	630
	11th to 20th	Night	22% 15% 29% 28% 6% 1	623
	21st to end	Night	20% 10% 32% 33% 5% 1	630
May	1st to 10th	Night	18% 6% 40% 28% 7% 1	630
	11th to 20th	Night	19% 6% 32% 32% 11%	621
	21st to end	Night	14% 5% 30% 33% 17% 1	683
June	1st to 10th	Night	10% 4 29% 41% 14% 1	630
	11th to 20th	Night	10% 4 31% 39% 14% 1	630
	21st to end	Night	13% 4 28% 40% 15% 1	630
July	1st to 10th	Night	11% 2 16% 39% 28% 4	615
	11th to 20th	Night	7% 2 22% 40% 26% 3	612
	21st to end	Night	9% 2 19% 43% 26% 1	687
August	1st to 10th	Night	7% 2 15% 43% 30% 4	597
	11th to 20th	Night	7% 2 14% 43% 29% 5%	594
	21st to end	Night	10% 1 18% 43% 26% 3	660
September	1st to 10th	Night	11% 2 22% 38% 23% 4	600
	11th to 20th	Night	11% 2 21% 36% 25% 5%	605
	21st to end	Night	10% 7% 24% 34% 23% 2	800
October	1st to 10th	Night	13% 6% 31% 31% 19% 1	800
	11th to 20th	Night	19% 12% 32% 27% 10%	800
	21st to end	Night	24% 12% 33% 24% 7%	880
November	1st to 10th	Night	17% 11% 37% 26% 7% 1	820
	11th to 20th	Night	21% 16% 36% 22% 5%	845
	21st to end	Night	23% 13% 34% 24% 6%	1050
December	1st to 10th	Night	21% 14% 37% 23% 5%	1050
	11th to 20th	Night	23% 12% 34% 26% 5%	1050
	21st to end	Night	30% 14% 31% 22% 3	1155

Page generated at 2026-07-28 02:25:25. Version ac07

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