Standing Stone Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Standing Stone Observatory clear sky chart from 2005-11-06 to 2025-09-19.

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	13% 9% 3 8% 5% 5% 5% 4 6% 8% 34%	126326
	Night	13% 9% 4 8% 5% 4 4 4 5% 7% 36%	122742
Transparency	Day	45% 8% 12% 20% 13% 3	129958
	Night	45% 8% 11% 20% 13% 2	127179
Seeing	Day	22% 15% 24% 21% 13% 6%	16369
	Night	19% 13% 21% 29% 16% 2	26424

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	14% 16% 6% 10% 6% 4 3 2 4 5% 30%	4056
	11th to 20th	Day	12% 14% 9% 13% 7% 4 3 2 2 3 31%	4060
	21st to end	Day	15% 12% 3 9% 5% 3 3 3 4 5% 37%	4466
February	1st to 10th	Day	14% 12% 4 10% 6% 4 5% 4 5% 7% 32%	1800
	11th to 20th	Day	18% 11% 5% 8% 5% 5% 4 3 4 6% 31%	1833
	21st to end	Day	17% 12% 3 9% 5% 4 3 3 5% 5% 33%	1686
March	1st to 10th	Day	18% 12% 4 6% 4 5% 4 4 5% 6% 32%	4935
	11th to 20th	Day	12% 8% 3 7% 3 3 4 3 5% 7% 45%	4980
	21st to end	Day	14% 9% 3 8% 5% 5% 5% 4 6% 7% 35%	5465
April	1st to 10th	Day	13% 11% 2 7% 5% 4 4 4 5% 7% 39%	2245
	11th to 20th	Day	13% 9% 3 7% 5% 4 5% 3 5% 7% 41%	2375
	21st to end	Day	16% 8% 3 6% 5% 4 5% 3 6% 7% 37%	2600
May	1st to 10th	Day	14% 9% 3 8% 6% 7% 6% 5% 8% 8% 26%	5862
	11th to 20th	Day	20% 8% 3 8% 5% 5% 5% 4 7% 9% 26%	5861
	21st to end	Day	14% 6% 3 7% 5% 5% 6% 5% 7% 9% 32%	6433
June	1st to 10th	Day	11% 6% 2 7% 6% 5% 6% 5% 6% 8% 38%	2600
	11th to 20th	Day	10% 6% 2 7% 6% 6% 6% 5% 8% 10% 35%	2600
	21st to end	Day	8% 6% 2 6% 6% 6% 6% 5% 8% 9% 37%	2600
July	1st to 10th	Day	6% 5% 3 8% 5% 6% 6% 5% 8% 10% 39%	5900
	11th to 20th	Day	8% 7% 3 8% 6% 6% 7% 5% 9% 11% 31%	5900
	21st to end	Day	12% 8% 4 10% 6% 6% 6% 5% 7% 8% 26%	6424
August	1st to 10th	Day	13% 7% 2 8% 6% 6% 6% 6% 8% 11% 27%	4234
	11th to 20th	Day	10% 5% 2 7% 6% 6% 7% 5% 8% 9% 33%	4174
	21st to end	Day	6% 4 1 5% 5% 5% 5% 5% 7% 11% 47%	4421
September	1st to 10th	Day	11% 5% 3 5% 5% 5% 4 5% 7% 7% 44%	2312
	11th to 20th	Day	12% 9% 3 8% 5% 5% 5% 4 5% 6% 38%	2178
	21st to end	Day	20% 7% 2 7% 5% 5% 4 4 5% 6% 35%	2090
October	1st to 10th	Day	15% 6% 2 6% 4 3 4 3 5% 7% 44%	1940
	11th to 20th	Day	16% 9% 2 6% 4 3 3 3 4 6% 43%	1900
	21st to end	Day	15% 10% 3 7% 3 4 3 3 3 6% 43%	2090
November	1st to 10th	Day	11% 8% 3 5% 5% 3 4 3 5% 7% 47%	1937
	11th to 20th	Day	11% 10% 3 8% 5% 5% 5% 3 5% 7% 39%	1800
	21st to end	Day	13% 11% 4 6% 4 5% 3 3 5% 7% 39%	1800
December	1st to 10th	Day	13% 13% 4 9% 4 4 4 4 5% 5% 36%	2930
	11th to 20th	Day	20% 13% 4 7% 4 4 4 3 4 7% 31%	3373
	21st to end	Day	18% 12% 4 10% 5% 5% 5% 4 5% 6% 25%	4466
January	1st to 10th	Night	12% 17% 7% 15% 6% 4 4 3 5% 4 25%	5889
	11th to 20th	Night	14% 11% 5% 14% 6% 3 3 3 4 5% 33%	5900
	21st to end	Night	16% 11% 3 7% 6% 4 4 3 4 4 37%	6490
February	1st to 10th	Night	15% 11% 4 8% 5% 4 5% 3 5% 7% 32%	2600
	11th to 20th	Night	20% 10% 5% 9% 6% 4 5% 4 4 5% 27%	2450
	21st to end	Night	16% 11% 4 8% 6% 4 5% 3 5% 6% 32%	1942
March	1st to 10th	Night	18% 11% 3 6% 4 3 4 3 4 6% 38%	4918
	11th to 20th	Night	12% 6% 5% 5% 3 3 3 3 5% 7% 46%	4980
	21st to end	Night	13% 9% 2 7% 5% 4 5% 4 6% 7% 38%	5467
April	1st to 10th	Night	12% 11% 3 7% 5% 5% 4 3 5% 7% 37%	2155
	11th to 20th	Night	13% 7% 3 6% 5% 4 5% 4 6% 8% 41%	1975
	21st to end	Night	14% 6% 3 6% 4 5% 4 5% 6% 7% 40%	1815
May	1st to 10th	Night	17% 9% 4 8% 5% 5% 4 4 5% 7% 31%	4033
	11th to 20th	Night	18% 9% 4 9% 5% 5% 5% 4 5% 8% 28%	4032
	21st to end	Night	19% 9% 3 8% 5% 5% 5% 4 6% 7% 30%	4421
June	1st to 10th	Night	7% 6% 3 7% 4 5% 4 4 7% 9% 43%	1800
	11th to 20th	Night	9% 6% 1 6% 4 5% 5% 5% 6% 10% 43%	1800
	21st to end	Night	9% 5% 2 5% 5% 4 5% 5% 8% 10% 44%	1800
July	1st to 10th	Night	8% 5% 3 9% 6% 5% 6% 5% 8% 9% 37%	4058
	11th to 20th	Night	7% 7% 3 9% 6% 5% 5% 4 7% 10% 36%	4057
	21st to end	Night	10% 7% 4 11% 6% 5% 5% 5% 6% 9% 31%	4427
August	1st to 10th	Night	12% 7% 3 10% 5% 6% 6% 4 6% 8% 33%	2915
	11th to 20th	Night	11% 6% 3 7% 5% 4 6% 4 7% 9% 38%	2875
	21st to end	Night	6% 4 2 5% 4 4 4 4 7% 7% 52%	3444
September	1st to 10th	Night	10% 6% 2 6% 4 4 3 4 6% 6% 48%	2084
	11th to 20th	Night	11% 6% 3 6% 5% 4 5% 3 5% 8% 44%	2179
	21st to end	Night	14% 8% 3 6% 4 5% 5% 4 5% 6% 39%	2090
October	1st to 10th	Night	13% 6% 2 5% 4 4 5% 3 5% 6% 46%	2238
	11th to 20th	Night	12% 8% 3 7% 4 4 5% 3 4 6% 44%	2280
	21st to end	Night	13% 9% 2 6% 5% 3 4 3 5% 5% 45%	2508
November	1st to 10th	Night	12% 8% 3 6% 4 3 4 3 5% 6% 47%	2358
	11th to 20th	Night	12% 11% 4 7% 4 4 5% 3 5% 6% 40%	2600
	21st to end	Night	12% 11% 3 7% 5% 4 5% 3 5% 5% 39%	2600
December	1st to 10th	Night	12% 11% 4 12% 5% 5% 4 4 5% 6% 32%	4250
	11th to 20th	Night	18% 12% 7% 9% 5% 4 4 3 4 6% 28%	4822
	21st to end	Night	17% 13% 6% 9% 6% 5% 5% 4 5% 6% 25%	6490

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	57% 5% 6% 17% 14% 1	4057
	11th to 20th	Day	60% 4 5% 12% 14% 5%	4060
	21st to end	Day	48% 5% 8% 24% 14% 2	4466
February	1st to 10th	Day	52% 6% 8% 20% 13% 1	1800
	11th to 20th	Day	54% 5% 9% 17% 12% 2	1842
	21st to end	Day	53% 5% 9% 16% 14% 3	1686
March	1st to 10th	Day	50% 6% 9% 18% 11% 5%	4935
	11th to 20th	Day	38% 6% 8% 20% 23% 4	4980
	21st to end	Day	46% 8% 9% 19% 15% 3	5465
April	1st to 10th	Day	45% 7% 10% 19% 14% 4	2245
	11th to 20th	Day	43% 6% 11% 22% 15% 3	2375
	21st to end	Day	46% 8% 12% 22% 11% 2	2600
May	1st to 10th	Day	50% 11% 11% 17% 9% 1	5897
	11th to 20th	Day	53% 9% 12% 19% 7% 1	5865
	21st to end	Day	43% 11% 15% 18% 11% 3	6447
June	1st to 10th	Day	41% 10% 14% 20% 14%	2600
	11th to 20th	Day	41% 11% 16% 18% 14% 1	2600
	21st to end	Day	39% 11% 14% 22% 12% 2	2600
July	1st to 10th	Day	36% 13% 17% 20% 11% 3	5835
	11th to 20th	Day	41% 14% 18% 20% 7%	5831
	21st to end	Day	50% 11% 13% 17% 9%	6240
August	1st to 10th	Day	45% 12% 18% 19% 6%	4221
	11th to 20th	Day	41% 12% 13% 23% 11%	4213
	21st to end	Day	29% 11% 17% 26% 15% 1	4421
September	1st to 10th	Day	38% 9% 13% 18% 18% 4	2312
	11th to 20th	Day	45% 7% 9% 21% 16% 1	2178
	21st to end	Day	49% 7% 11% 18% 14% 1	2090
October	1st to 10th	Day	30% 7% 10% 20% 26% 7%	3226
	11th to 20th	Day	31% 5% 11% 24% 16% 13%	3140
	21st to end	Day	30% 4 9% 31% 16% 11%	3428
November	1st to 10th	Day	36% 5% 10% 22% 23% 3	1937
	11th to 20th	Day	45% 5% 8% 19% 20% 3	1800
	21st to end	Day	45% 5% 11% 21% 16% 2	1800
December	1st to 10th	Day	48% 6% 8% 19% 16% 2	2930
	11th to 20th	Day	55% 4 10% 16% 14% 1	3370
	21st to end	Day	56% 8% 11% 17% 8%	4466
January	1st to 10th	Night	60% 7% 6% 15% 11% 1	5889
	11th to 20th	Night	54% 5% 6% 16% 14% 5%	5900
	21st to end	Night	49% 5% 8% 22% 13% 1	6490
February	1st to 10th	Night	52% 6% 11% 17% 13% 1	2600
	11th to 20th	Night	58% 6% 7% 15% 11% 2	2452
	21st to end	Night	53% 5% 10% 17% 10% 4	1942
March	1st to 10th	Night	46% 6% 8% 21% 14% 4	4918
	11th to 20th	Night	37% 6% 8% 23% 23% 3	4980
	21st to end	Night	43% 8% 11% 20% 16% 2	5467
April	1st to 10th	Night	46% 8% 11% 19% 14% 3	2155
	11th to 20th	Night	39% 6% 14% 21% 16% 4	1975
	21st to end	Night	41% 8% 14% 24% 12% 1	1815
May	1st to 10th	Night	49% 12% 13% 17% 9% 1	4060
	11th to 20th	Night	51% 12% 14% 17% 6%	4033
	21st to end	Night	48% 12% 15% 18% 4 3	4429
June	1st to 10th	Night	35% 12% 17% 22% 14% 1	1800
	11th to 20th	Night	34% 11% 19% 22% 13% 1	1800
	21st to end	Night	31% 12% 19% 23% 14% 1	1800
July	1st to 10th	Night	38% 13% 17% 20% 10% 2	4015
	11th to 20th	Night	40% 13% 18% 23% 5% 1	4006
	21st to end	Night	45% 12% 14% 19% 9%	4297
August	1st to 10th	Night	45% 11% 18% 21% 5%	2904
	11th to 20th	Night	40% 10% 15% 26% 10%	2902
	21st to end	Night	27% 10% 15% 33% 14% 1	3444
September	1st to 10th	Night	36% 9% 14% 19% 17% 5%	2084
	11th to 20th	Night	36% 8% 12% 24% 18% 2	2179
	21st to end	Night	43% 8% 12% 22% 13% 1	2090
October	1st to 10th	Night	33% 8% 9% 21% 25% 4	3732
	11th to 20th	Night	32% 6% 9% 23% 20% 10%	3779
	21st to end	Night	26% 4 11% 29% 21% 10%	4120
November	1st to 10th	Night	37% 6% 11% 21% 22% 4	2358
	11th to 20th	Night	44% 6% 8% 19% 19% 4	2600
	21st to end	Night	45% 6% 9% 21% 16% 3	2600
December	1st to 10th	Night	50% 7% 10% 23% 10% 1	4250
	11th to 20th	Night	58% 6% 10% 16% 9% 1	4824
	21st to end	Night	56% 9% 12% 15% 9%	6490

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	42% 26% 20% 8% 3 1	300
	11th to 20th	Day	41% 25% 26% 7% 1	300
	21st to end	Day	36% 27% 25% 9% 3	330
February	1st to 10th	Day	32% 28% 26% 11% 3	300
	11th to 20th	Day	53% 21% 18% 6% 1	300
	21st to end	Day	42% 22% 27% 6% 2	285
March	1st to 10th	Day	33% 21% 22% 18% 4 1	500
	11th to 20th	Day	31% 26% 24% 14% 5% 1	500
	21st to end	Day	29% 23% 27% 14% 6%	550
April	1st to 10th	Day	28% 25% 24% 14% 8% 1	500
	11th to 20th	Day	25% 17% 28% 17% 10% 2	492
	21st to end	Day	27% 20% 24% 16% 10% 2	500
May	1st to 10th	Day	25% 19% 28% 15% 9% 4	503
	11th to 20th	Day	22% 15% 27% 19% 12% 5%	527
	21st to end	Day	18% 11% 25% 22% 16% 8%	573
June	1st to 10th	Day	12% 12% 27% 24% 16% 8%	530
	11th to 20th	Day	10% 11% 28% 25% 18% 8%	530
	21st to end	Day	11% 10% 25% 27% 16% 11%	530
July	1st to 10th	Day	11% 6% 20% 29% 22% 13%	525
	11th to 20th	Day	6% 6% 21% 35% 20% 13%	524
	21st to end	Day	13% 6% 20% 28% 20% 12%	576
August	1st to 10th	Day	16% 6% 21% 23% 21% 12%	526
	11th to 20th	Day	13% 6% 21% 23% 23% 14%	519
	21st to end	Day	10% 5% 19% 27% 24% 14%	583
September	1st to 10th	Day	11% 6% 21% 27% 23% 13%	530
	11th to 20th	Day	13% 4 17% 28% 26% 11%	522
	21st to end	Day	22% 12% 21% 27% 15% 3	490
October	1st to 10th	Day	17% 9% 23% 27% 19% 6%	490
	11th to 20th	Day	20% 16% 21% 24% 16% 3	490
	21st to end	Day	23% 14% 24% 22% 14% 3	539
November	1st to 10th	Day	20% 13% 29% 25% 11% 3	475
	11th to 20th	Day	30% 18% 27% 18% 7%	300
	21st to end	Day	29% 19% 29% 18% 4 1	300
December	1st to 10th	Day	34% 25% 26% 10% 4 1	300
	11th to 20th	Day	33% 19% 26% 16% 5% 1	300
	21st to end	Day	32% 21% 23% 15% 8% 1	330
January	1st to 10th	Night	31% 23% 25% 18% 3	800
	11th to 20th	Night	34% 21% 25% 19% 2	800
	21st to end	Night	30% 21% 30% 18% 1	880
February	1st to 10th	Night	27% 18% 29% 24% 2	800
	11th to 20th	Night	34% 25% 24% 17%	783
	21st to end	Night	26% 25% 29% 19% 1	660
March	1st to 10th	Night	26% 20% 28% 23% 4	800
	11th to 20th	Night	28% 21% 23% 21% 7%	800
	21st to end	Night	23% 19% 29% 23% 7%	880
April	1st to 10th	Night	20% 22% 27% 25% 6%	800
	11th to 20th	Night	18% 16% 26% 28% 11%	791
	21st to end	Night	16% 16% 25% 34% 8%	615
May	1st to 10th	Night	18% 13% 24% 31% 13% 1	600
	11th to 20th	Night	16% 10% 23% 34% 16% 1	591
	21st to end	Night	13% 6% 17% 33% 27% 5%	650
June	1st to 10th	Night	7% 6% 19% 38% 27% 3	600
	11th to 20th	Night	11% 4 13% 35% 35% 2	600
	21st to end	Night	9% 4 14% 38% 33% 3	600
July	1st to 10th	Night	10% 2 10% 39% 34% 5%	585
	11th to 20th	Night	6% 3 9% 33% 45% 5%	582
	21st to end	Night	8% 3 10% 35% 39% 5%	657
August	1st to 10th	Night	12% 3 11% 35% 36% 3	597
	11th to 20th	Night	13% 3 11% 32% 36% 5%	594
	21st to end	Night	10% 3 9% 35% 38% 5%	790
September	1st to 10th	Night	11% 5% 12% 34% 34% 5%	800
	11th to 20th	Night	11% 4 13% 32% 35% 4	793
	21st to end	Night	16% 7% 19% 30% 26% 1	760
October	1st to 10th	Night	14% 9% 19% 30% 25% 3	760
	11th to 20th	Night	17% 15% 20% 29% 18% 1	760
	21st to end	Night	20% 12% 20% 33% 13% 2	836
November	1st to 10th	Night	17% 9% 23% 35% 14% 1	780
	11th to 20th	Night	24% 16% 20% 31% 9% 1	800
	21st to end	Night	20% 18% 22% 31% 8% 1	800
December	1st to 10th	Night	30% 19% 26% 20% 6%	800
	11th to 20th	Night	26% 18% 26% 25% 5%	800
	21st to end	Night	26% 16% 26% 28% 4	880

Page generated at 2025-09-16 01:49:10. Version ac07

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