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 2025-03-27.

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% 9% 3 7% 5% 4 4 4 6% 7% 38%	109125
	Night	13% 9% 3 8% 5% 4 4 3 5% 6% 40%	110414
Transparency	Day	44% 3 12% 25% 15% 2	112678
	Night	45% 2 11% 24% 16% 2	115001
Seeing	Day	17% 10% 30% 25% 13% 4	16256
	Night	18% 10% 31% 29% 11% 1	26293

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% 16% 7% 7% 5% 3 5% 5% 26%	3617
	11th to 20th	Day	9% 10% 6% 11% 7% 5% 6% 5% 7% 6% 29%	3620
	21st to end	Day	13% 15% 7% 9% 4 4 4 3 4 6% 29%	4458
February	1st to 10th	Day	14% 12% 5% 10% 7% 4 4 3 5% 6% 29%	1800
	11th to 20th	Day	15% 9% 3 8% 5% 5% 4 3 5% 7% 35%	1862
	21st to end	Day	13% 12% 4 9% 5% 4 4 4 6% 6% 34%	1650
March	1st to 10th	Day	16% 12% 4 6% 4 3 3 3 4 6% 40%	3220
	11th to 20th	Day	10% 6% 2 7% 5% 4 5% 4 5% 7% 45%	3585
	21st to end	Day	21% 12% 4 11% 5% 5% 5% 4 5% 6% 23%	4105
April	1st to 10th	Day	16% 12% 4 7% 5% 4 4 3 4 6% 33%	2280
	11th to 20th	Day	16% 10% 3 7% 5% 4 4 4 5% 6% 36%	2254
	21st to end	Day	22% 9% 3 6% 3 5% 4 4 5% 6% 34%	2280
May	1st to 10th	Day	15% 8% 3 7% 5% 5% 5% 4 6% 8% 35%	4195
	11th to 20th	Day	16% 6% 2 7% 4 4 5% 3 5% 7% 41%	4399
	21st to end	Day	12% 7% 2 6% 4 4 4 4 6% 8% 43%	4824
June	1st to 10th	Day	8% 5% 2 6% 4 5% 5% 5% 8% 9% 44%	2660
	11th to 20th	Day	10% 5% 2 5% 4 5% 5% 4 6% 8% 46%	2660
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 5% 7% 9% 40%	2660
July	1st to 10th	Day	9% 7% 3 8% 6% 5% 6% 4 7% 10% 34%	4440
	11th to 20th	Day	7% 4 2 5% 3 4 4 4 6% 10% 51%	4440
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 5% 8% 10% 40%	4813
August	1st to 10th	Day	7% 5% 2 6% 4 5% 5% 5% 8% 10% 43%	4139
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 4 7% 10% 43%	3831
	21st to end	Day	7% 6% 2 5% 4 4 4 3 5% 9% 50%	4180
September	1st to 10th	Day	12% 5% 1 5% 4 5% 3 4 5% 7% 48%	2280
	11th to 20th	Day	11% 4 1 4 3 3 4 4 5% 7% 54%	2256
	21st to end	Day	10% 5% 2 6% 4 4 5% 3 6% 8% 47%	2075
October	1st to 10th	Day	12% 7% 2 5% 3 3 4 3 5% 7% 49%	1900
	11th to 20th	Day	15% 8% 2 6% 4 3 4 3 5% 7% 42%	1900
	21st to end	Day	18% 9% 3 6% 4 3 4 3 5% 6% 41%	1990
November	1st to 10th	Day	12% 8% 3 6% 3 4 5% 3 6% 6% 45%	1762
	11th to 20th	Day	15% 12% 4 6% 5% 5% 4 4 4 7% 34%	1790
	21st to end	Day	15% 12% 4 10% 4 4 3 4 5% 4 36%	1600
December	1st to 10th	Day	9% 9% 3 8% 4 4 3 3 4 6% 47%	2610
	11th to 20th	Day	15% 13% 6% 12% 6% 6% 5% 3 6% 6% 23%	3008
	21st to end	Day	23% 21% 8% 11% 5% 3 3 2 3 4 14%	3982
January	1st to 10th	Night	11% 12% 6% 14% 5% 4 3 3 4 6% 32%	6348
	11th to 20th	Night	11% 13% 6% 12% 7% 6% 5% 5% 6% 5% 25%	6360
	21st to end	Night	14% 14% 6% 10% 5% 4 4 3 4 5% 31%	6518
February	1st to 10th	Night	14% 13% 4 7% 5% 5% 4 3 5% 5% 34%	2600
	11th to 20th	Night	12% 11% 3 7% 5% 4 4 3 5% 5% 40%	2424
	21st to end	Night	15% 11% 3 7% 4 4 3 2 4 6% 40%	1980
March	1st to 10th	Night	19% 9% 3 5% 4 3 3 3 4 4 43%	3853
	11th to 20th	Night	10% 6% 3 6% 4 4 4 3 5% 6% 49%	3624
	21st to end	Night	22% 12% 5% 10% 5% 4 4 3 5% 5% 24%	3650
April	1st to 10th	Night	16% 10% 3 7% 4 4 3 3 4 6% 40%	1900
	11th to 20th	Night	17% 9% 3 5% 4 4 4 4 4 5% 41%	1875
	21st to end	Night	17% 9% 4 7% 4 4 4 4 5% 6% 37%	1900
May	1st to 10th	Night	18% 8% 3 7% 5% 5% 4 4 7% 6% 34%	2702
	11th to 20th	Night	13% 6% 2 7% 4 5% 5% 4 5% 8% 40%	2506
	21st to end	Night	7% 6% 2 4 5% 5% 5% 4 6% 8% 48%	2743
June	1st to 10th	Night	7% 6% 2 6% 3 4 5% 3 6% 8% 50%	1520
	11th to 20th	Night	8% 4 2 7% 4 4 4 4 8% 8% 47%	1520
	21st to end	Night	10% 5% 1 6% 4 6% 6% 4 6% 7% 45%	1520
July	1st to 10th	Night	14% 6% 2 7% 5% 5% 5% 4 6% 8% 40%	2530
	11th to 20th	Night	5% 3 1 4 4 4 4 4 6% 8% 57%	2530
	21st to end	Night	5% 4 2 6% 5% 5% 5% 5% 7% 8% 47%	2750
August	1st to 10th	Night	5% 3 2 5% 4 4 4 4 6% 10% 54%	2797
	11th to 20th	Night	6% 4 2 5% 4 4 5% 4 6% 8% 53%	2999
	21st to end	Night	6% 4 2 6% 4 4 4 3 5% 8% 54%	3476
September	1st to 10th	Night	9% 6% 1 5% 3 2 2 3 4 6% 57%	1900
	11th to 20th	Night	9% 4 1 5% 4 3 5% 3 6% 8% 52%	1905
	21st to end	Night	8% 5% 2 5% 5% 4 4 3 6% 7% 50%	2102
October	1st to 10th	Night	10% 5% 2 6% 4 3 3 3 4 6% 54%	2280
	11th to 20th	Night	14% 7% 3 5% 4 3 4 3 4 6% 47%	2280
	21st to end	Night	18% 8% 3 7% 4 5% 3 3 5% 6% 38%	2606
November	1st to 10th	Night	12% 6% 3 5% 3 3 3 3 4 6% 51%	2536
	11th to 20th	Night	15% 9% 3 7% 5% 4 4 3 4 6% 39%	2605
	21st to end	Night	16% 10% 3 6% 5% 4 4 3 4 6% 39%	2800
December	1st to 10th	Night	9% 9% 3 7% 4 4 4 3 5% 6% 47%	4580
	11th to 20th	Night	16% 14% 4 9% 4 4 4 3 5% 5% 33%	5199
	21st to end	Night	23% 17% 6% 11% 6% 5% 4 3 4 4 15%	6996

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	61% 2 10% 19% 7% 1	3618
	11th to 20th	Day	56% 2 9% 13% 16% 3	3620
	21st to end	Day	57% 2 12% 21% 8% 1	4458
February	1st to 10th	Day	56% 2 12% 22% 8% 1	1800
	11th to 20th	Day	50% 2 11% 24% 11% 2	1871
	21st to end	Day	50% 3 11% 24% 10% 2	1650
March	1st to 10th	Day	48% 1 11% 24% 14% 2	3220
	11th to 20th	Day	38% 2 12% 31% 15% 1	3585
	21st to end	Day	61% 2 14% 19% 4 1	4105
April	1st to 10th	Day	52% 3 11% 23% 11% 1	2280
	11th to 20th	Day	50% 2 8% 23% 14% 2	2254
	21st to end	Day	53% 2 10% 20% 14% 2	2280
May	1st to 10th	Day	48% 2 11% 23% 16%	4233
	11th to 20th	Day	45% 2 10% 31% 12%	4403
	21st to end	Day	40% 3 10% 26% 18% 3	4837
June	1st to 10th	Day	36% 3 14% 27% 17% 2	2660
	11th to 20th	Day	36% 4 11% 23% 23% 3	2660
	21st to end	Day	39% 5% 14% 25% 15% 2	2660
July	1st to 10th	Day	45% 5% 11% 27% 11% 2	4371
	11th to 20th	Day	29% 4 15% 32% 18% 2	4365
	21st to end	Day	36% 7% 16% 21% 18% 2	4662
August	1st to 10th	Day	34% 5% 16% 30% 14% 1	4124
	11th to 20th	Day	36% 5% 15% 27% 16% 1	3869
	21st to end	Day	32% 4 12% 33% 17% 2	4179
September	1st to 10th	Day	35% 3 10% 26% 21% 4	2280
	11th to 20th	Day	30% 2 11% 30% 23% 4	2256
	21st to end	Day	36% 4 9% 30% 20% 2	2075
October	1st to 10th	Day	23% 2 5% 32% 37% 2	3160
	11th to 20th	Day	28% 2 11% 24% 31% 4	3140
	21st to end	Day	41% 2 8% 30% 15% 4	3254
November	1st to 10th	Day	40% 2 14% 27% 14% 3	1762
	11th to 20th	Day	49% 2 13% 22% 12% 1	1790
	21st to end	Day	51% 3 13% 23% 10%	1600
December	1st to 10th	Day	40% 2 14% 35% 8% 1	2610
	11th to 20th	Day	62% 2 11% 16% 6% 2	3005
	21st to end	Day	75% 2 9% 11% 4	3982
January	1st to 10th	Night	56% 2 10% 23% 9%	6348
	11th to 20th	Night	61% 2 10% 12% 14% 1	6360
	21st to end	Night	57% 2 9% 20% 12% 1	6518
February	1st to 10th	Night	53% 2 11% 21% 13% 1	2600
	11th to 20th	Night	48% 2 11% 25% 14% 1	2425
	21st to end	Night	47% 2 9% 24% 16% 2	1980
March	1st to 10th	Night	47% 2 11% 25% 15% 2	3853
	11th to 20th	Night	38% 2 10% 30% 18% 3	3624
	21st to end	Night	64% 2 12% 17% 6% 1	3650
April	1st to 10th	Night	47% 2 9% 25% 14% 2	1900
	11th to 20th	Night	46% 2 7% 23% 18% 4	1875
	21st to end	Night	48% 3 10% 22% 14% 3	1900
May	1st to 10th	Night	51% 3 9% 20% 15% 2	2729
	11th to 20th	Night	43% 2 15% 27% 14% 1	2507
	21st to end	Night	34% 3 11% 29% 21% 2	2751
June	1st to 10th	Night	33% 3 14% 28% 19% 3	1520
	11th to 20th	Night	33% 4 11% 24% 24% 4	1520
	21st to end	Night	37% 3 15% 23% 19% 3	1520
July	1st to 10th	Night	43% 4 13% 24% 13% 3	2490
	11th to 20th	Night	24% 4 13% 34% 23% 2	2482
	21st to end	Night	32% 7% 16% 23% 18% 4	2662
August	1st to 10th	Night	25% 4 17% 34% 17% 2	2788
	11th to 20th	Night	30% 5% 15% 31% 18% 1	3027
	21st to end	Night	28% 4 13% 31% 19% 4	3476
September	1st to 10th	Night	29% 2 10% 27% 27% 4	1900
	11th to 20th	Night	31% 3 9% 30% 22% 5%	1905
	21st to end	Night	32% 4 9% 29% 23% 3	2102
October	1st to 10th	Night	24% 2 6% 31% 33% 3	3800
	11th to 20th	Night	29% 2 7% 23% 34% 5%	3779
	21st to end	Night	45% 2 7% 23% 19% 4	4292
November	1st to 10th	Night	35% 3 13% 27% 19% 4	2536
	11th to 20th	Night	45% 2 11% 25% 15% 2	2605
	21st to end	Night	49% 2 10% 26% 13% 1	2800
December	1st to 10th	Night	40% 2 11% 35% 11% 1	4580
	11th to 20th	Night	55% 2 11% 22% 8% 1	5201
	21st to end	Night	74% 2 8% 12% 5%	6996

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% 16% 36% 21% 6% 1	300
	11th to 20th	Day	22% 22% 38% 14% 3	300
	21st to end	Day	24% 19% 38% 14% 5%	330
February	1st to 10th	Day	22% 21% 38% 15% 3	300
	11th to 20th	Day	27% 20% 34% 15% 3 1	300
	21st to end	Day	26% 19% 44% 8% 2	249
March	1st to 10th	Day	24% 21% 35% 17% 3	300
	11th to 20th	Day	25% 17% 37% 14% 6% 1	300
	21st to end	Day	32% 16% 31% 15% 4 1	450
April	1st to 10th	Day	26% 15% 33% 20% 5% 1	460
	11th to 20th	Day	21% 17% 30% 21% 8% 2	452
	21st to end	Day	28% 12% 33% 19% 8% 1	460
May	1st to 10th	Day	20% 10% 36% 24% 8% 2	553
	11th to 20th	Day	21% 10% 29% 25% 13% 2	675
	21st to end	Day	17% 8% 27% 26% 14% 7%	734
June	1st to 10th	Day	9% 6% 27% 35% 17% 5%	680
	11th to 20th	Day	10% 5% 31% 33% 15% 5%	680
	21st to end	Day	12% 8% 32% 28% 15% 5%	680
July	1st to 10th	Day	11% 3 22% 28% 24% 12%	671
	11th to 20th	Day	12% 4 23% 33% 19% 9%	668
	21st to end	Day	7% 1 24% 41% 22% 6%	739
August	1st to 10th	Day	8% 3 23% 36% 23% 7%	676
	11th to 20th	Day	12% 4 19% 31% 23% 10%	545
	21st to end	Day	8% 3 22% 34% 24% 9%	539
September	1st to 10th	Day	14% 5% 21% 32% 21% 6%	490
	11th to 20th	Day	9% 2 21% 35% 22% 10%	490
	21st to end	Day	12% 10% 28% 25% 20% 5%	475
October	1st to 10th	Day	19% 8% 27% 29% 12% 4	300
	11th to 20th	Day	22% 18% 34% 18% 7%	300
	21st to end	Day	29% 14% 33% 18% 5% 1	330
November	1st to 10th	Day	16% 15% 41% 18% 7% 3	300
	11th to 20th	Day	25% 17% 34% 17% 6% 1	300
	21st to end	Day	24% 15% 37% 20% 3	300
December	1st to 10th	Day	17% 16% 43% 18% 5% 2	300
	11th to 20th	Day	20% 18% 35% 23% 4	300
	21st to end	Day	29% 17% 37% 14% 2	330
January	1st to 10th	Night	19% 15% 36% 23% 7%	1000
	11th to 20th	Night	19% 16% 35% 25% 5% 1	1000
	21st to end	Night	21% 13% 34% 26% 5%	1100
February	1st to 10th	Night	22% 15% 36% 23% 4	1000
	11th to 20th	Night	19% 15% 38% 25% 4	869
	21st to end	Night	23% 15% 40% 19% 3 1	660
March	1st to 10th	Night	24% 13% 35% 23% 5%	800
	11th to 20th	Night	17% 12% 34% 30% 7%	621
	21st to end	Night	27% 10% 34% 23% 5%	647
April	1st to 10th	Night	22% 13% 37% 24% 4 1	570
	11th to 20th	Night	23% 14% 29% 28% 6% 1	563
	21st to end	Night	20% 10% 32% 32% 5% 1	570
May	1st to 10th	Night	19% 6% 39% 28% 8% 1	570
	11th to 20th	Night	19% 5% 32% 33% 11%	561
	21st to end	Night	14% 3 31% 33% 17% 1	617
June	1st to 10th	Night	10% 3 28% 42% 15% 2	570
	11th to 20th	Night	10% 3 31% 40% 15% 1	570
	21st to end	Night	13% 4 27% 40% 16% 1	570
July	1st to 10th	Night	12% 2 14% 39% 28% 5%	555
	11th to 20th	Night	7% 1 24% 40% 26% 2	552
	21st to end	Night	9% 1 19% 43% 27% 1	624
August	1st to 10th	Night	7% 2 15% 43% 30% 4	567
	11th to 20th	Night	7% 2 15% 43% 29% 5%	564
	21st to end	Night	10% 1 18% 42% 26% 3	627
September	1st to 10th	Night	12% 2 22% 38% 23% 4	570
	11th to 20th	Night	11% 2 21% 36% 25% 5%	575
	21st to end	Night	10% 8% 24% 34% 22% 2	760
October	1st to 10th	Night	13% 6% 31% 30% 20% 1	760
	11th to 20th	Night	19% 11% 31% 28% 11%	760
	21st to end	Night	24% 12% 33% 24% 8%	836
November	1st to 10th	Night	17% 11% 37% 27% 8% 1	780
	11th to 20th	Night	21% 15% 36% 22% 5%	805
	21st to end	Night	22% 12% 35% 24% 6%	1000
December	1st to 10th	Night	21% 14% 37% 23% 6%	1000
	11th to 20th	Night	23% 11% 34% 26% 6%	1000
	21st to end	Night	30% 14% 30% 23% 3	1100

Page generated at 2025-03-24 08:04:31. Version ac07

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