StonyHill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the StonyHill Observatory clear sky chart from 2005-11-06 to 2026-02-16.

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	16% 7% 2 6% 4 4 4 3 5% 7% 42%	130644
	Night	17% 8% 3 6% 4 4 4 3 5% 6% 42%	130214
Transparency	Day	40% 7% 13% 22% 16% 2	134172
	Night	42% 6% 12% 22% 16% 3	134773
Seeing	Day	18% 7% 24% 30% 16% 4	16611
	Night	20% 8% 29% 30% 11% 1	25809

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	13% 6% 2 5% 4 3 3 3 5% 6% 48%	4661
	11th to 20th	Day	16% 8% 3 6% 3 3 4 3 4 5% 46%	4650
	21st to end	Day	16% 8% 3 6% 3 3 4 3 5% 7% 42%	5421
February	1st to 10th	Day	16% 6% 3 7% 5% 3 4 3 5% 7% 41%	1955
	11th to 20th	Day	14% 6% 2 6% 4 4 4 3 5% 6% 46%	1998
	21st to end	Day	18% 8% 2 6% 4 4 3 3 5% 5% 41%	1650
March	1st to 10th	Day	20% 8% 2 5% 3 3 3 3 4 6% 44%	4476
	11th to 20th	Day	11% 7% 2 5% 4 4 4 4 6% 9% 44%	4688
	21st to end	Day	18% 7% 2 5% 4 3 4 3 5% 6% 42%	5970
April	1st to 10th	Day	18% 8% 2 5% 4 4 4 3 6% 6% 39%	2400
	11th to 20th	Day	16% 7% 1 5% 4 4 4 3 5% 6% 44%	2374
	21st to end	Day	18% 7% 2 5% 3 4 5% 3 5% 7% 43%	2472
May	1st to 10th	Day	27% 8% 2 5% 3 3 4 4 5% 8% 31%	5886
	11th to 20th	Day	21% 8% 2 6% 4 3 3 3 5% 6% 39%	6130
	21st to end	Day	19% 6% 3 5% 3 3 4 3 6% 8% 39%	6949
June	1st to 10th	Day	16% 5% 2 5% 4 3 4 4 5% 9% 42%	2800
	11th to 20th	Day	14% 7% 2 5% 4 4 5% 4 6% 9% 40%	2800
	21st to end	Day	10% 6% 2 5% 5% 4 4 4 6% 9% 44%	2800
July	1st to 10th	Day	10% 6% 3 8% 5% 5% 5% 4 7% 10% 37%	6360
	11th to 20th	Day	10% 6% 2 8% 5% 4 6% 5% 8% 10% 35%	6360
	21st to end	Day	13% 6% 2 6% 4 4 4 4 6% 8% 42%	6926
August	1st to 10th	Day	17% 6% 2 6% 4 5% 4 4 6% 9% 35%	4338
	11th to 20th	Day	11% 6% 2 6% 4 4 4 3 6% 9% 46%	3850
	21st to end	Day	9% 4 2 5% 4 4 4 4 6% 9% 49%	4312
September	1st to 10th	Day	14% 5% 2 5% 4 4 5% 3 5% 7% 47%	2400
	11th to 20th	Day	12% 5% 2 5% 3 3 3 3 5% 7% 52%	2275
	21st to end	Day	18% 7% 2 6% 5% 4 4 3 5% 6% 40%	2200
October	1st to 10th	Day	21% 5% 2 4 2 3 3 3 4 7% 45%	2149
	11th to 20th	Day	15% 5% 2 5% 4 4 3 3 5% 7% 47%	2000
	21st to end	Day	19% 5% 1 5% 4 4 4 3 5% 7% 43%	1995
November	1st to 10th	Day	14% 5% 2 5% 4 3 3 3 4 6% 50%	1851
	11th to 20th	Day	16% 6% 2 5% 3 3 3 2 4 7% 48%	1890
	21st to end	Day	15% 8% 2 5% 3 3 3 4 5% 7% 44%	1864
December	1st to 10th	Day	14% 7% 3 6% 4 3 4 3 7% 7% 43%	2680
	11th to 20th	Day	21% 8% 2 4 2 2 2 2 4 5% 47%	3066
	21st to end	Day	17% 9% 3 8% 4 3 4 2 5% 6% 39%	4048
January	1st to 10th	Night	13% 8% 3 6% 4 3 3 2 4 7% 47%	8234
	11th to 20th	Night	16% 8% 3 7% 4 3 3 3 4 5% 45%	8235
	21st to end	Night	16% 8% 3 7% 4 4 4 3 4 6% 41%	8758
February	1st to 10th	Night	16% 9% 3 5% 5% 4 4 4 5% 6% 41%	2663
	11th to 20th	Night	18% 7% 3 5% 5% 3 4 4 5% 6% 40%	2399
	21st to end	Night	19% 7% 2 5% 4 4 4 3 5% 6% 41%	1980
March	1st to 10th	Night	19% 9% 2 5% 3 3 3 3 4 6% 43%	5364
	11th to 20th	Night	15% 9% 2 6% 5% 4 4 3 4 6% 42%	5267
	21st to end	Night	20% 9% 2 7% 3 4 4 3 5% 5% 38%	4962
April	1st to 10th	Night	19% 7% 3 6% 4 4 4 4 6% 6% 37%	2000
	11th to 20th	Night	19% 7% 2 6% 4 3 4 3 5% 7% 40%	1975
	21st to end	Night	18% 8% 3 6% 4 3 3 3 4 5% 43%	1925
May	1st to 10th	Night	26% 9% 3 8% 4 4 4 3 4 7% 28%	4054
	11th to 20th	Night	22% 9% 3 7% 4 4 4 3 5% 6% 33%	3805
	21st to end	Night	21% 9% 3 6% 3 3 4 3 5% 6% 36%	3926
June	1st to 10th	Night	16% 8% 2 5% 5% 5% 3 3 4 8% 41%	1600
	11th to 20th	Night	16% 7% 2 6% 3 3 4 4 6% 9% 40%	1600
	21st to end	Night	10% 6% 2 7% 5% 5% 5% 5% 6% 10% 40%	1600
July	1st to 10th	Night	11% 9% 3 8% 5% 5% 5% 5% 6% 9% 35%	3600
	11th to 20th	Night	13% 9% 4 8% 5% 4 5% 4 6% 9% 32%	3600
	21st to end	Night	13% 7% 2 6% 4 4 4 4 6% 8% 42%	3926
August	1st to 10th	Night	21% 7% 3 6% 4 5% 5% 4 7% 7% 32%	2774
	11th to 20th	Night	13% 7% 3 7% 4 4 5% 4 7% 8% 39%	3197
	21st to end	Night	9% 5% 1 5% 3 4 4 4 5% 7% 53%	3586
September	1st to 10th	Night	13% 5% 2 6% 4 4 5% 3 6% 6% 45%	2000
	11th to 20th	Night	14% 4 2 6% 4 2 4 3 5% 7% 51%	2123
	21st to end	Night	18% 6% 2 7% 5% 4 3 3 4 7% 42%	2200
October	1st to 10th	Night	19% 8% 2 6% 4 4 3 3 4 6% 41%	2249
	11th to 20th	Night	16% 4 2 5% 4 4 3 3 5% 7% 49%	2400
	21st to end	Night	20% 6% 2 6% 4 4 3 3 4 7% 41%	2845
November	1st to 10th	Night	12% 6% 2 6% 4 3 4 3 5% 6% 49%	2667
	11th to 20th	Night	17% 6% 2 5% 3 3 3 3 4 5% 48%	2730
	21st to end	Night	15% 6% 2 5% 4 3 4 3 5% 6% 46%	2752
December	1st to 10th	Night	16% 7% 2 5% 3 4 4 2 4 6% 47%	4720
	11th to 20th	Night	23% 8% 2 5% 3 3 3 2 4 4 43%	5348
	21st to end	Night	17% 8% 3 7% 4 4 4 3 4 5% 41%	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	36% 5% 11% 29% 18% 2	4662
	11th to 20th	Day	39% 5% 10% 22% 17% 6%	4680
	21st to end	Day	41% 6% 11% 26% 15% 1	5421
February	1st to 10th	Day	43% 5% 14% 23% 14% 1	1955
	11th to 20th	Day	40% 5% 11% 26% 17% 2	2007
	21st to end	Day	45% 6% 11% 21% 14% 3	1650
March	1st to 10th	Day	41% 6% 13% 17% 14% 9%	4479
	11th to 20th	Day	35% 9% 15% 27% 12% 2	4688
	21st to end	Day	43% 6% 10% 14% 22% 5%	5970
April	1st to 10th	Day	44% 6% 11% 20% 15% 4	2400
	11th to 20th	Day	41% 5% 13% 21% 17% 3	2374
	21st to end	Day	40% 7% 11% 22% 17% 4	2472
May	1st to 10th	Day	51% 8% 17% 14% 9% 1	5897
	11th to 20th	Day	46% 8% 12% 19% 13% 2	6134
	21st to end	Day	42% 8% 14% 22% 13% 1	6949
June	1st to 10th	Day	39% 9% 14% 22% 15% 1	2800
	11th to 20th	Day	40% 8% 14% 24% 13% 2	2800
	21st to end	Day	36% 9% 16% 22% 15% 1	2800
July	1st to 10th	Day	42% 10% 13% 20% 14% 1	6290
	11th to 20th	Day	41% 9% 20% 21% 9% 1	6285
	21st to end	Day	39% 7% 12% 21% 19% 2	6728
August	1st to 10th	Day	45% 8% 16% 22% 9% 1	4324
	11th to 20th	Day	36% 7% 15% 32% 9% 1	3886
	21st to end	Day	30% 7% 14% 27% 20% 2	4312
September	1st to 10th	Day	37% 7% 14% 21% 19% 2	2400
	11th to 20th	Day	33% 6% 12% 25% 24% 2	2275
	21st to end	Day	45% 6% 11% 18% 18% 2	2200
October	1st to 10th	Day	34% 6% 12% 21% 23% 5%	3500
	11th to 20th	Day	26% 5% 11% 23% 31% 4	3240
	21st to end	Day	29% 5% 15% 26% 22% 2	3195
November	1st to 10th	Day	36% 6% 14% 22% 21% 2	1851
	11th to 20th	Day	37% 5% 10% 23% 22% 3	1890
	21st to end	Day	39% 6% 14% 27% 11% 2	1864
December	1st to 10th	Day	39% 5% 17% 27% 12% 1	2680
	11th to 20th	Day	41% 3 14% 22% 17% 3	3066
	21st to end	Day	45% 8% 10% 20% 14% 3	4048
January	1st to 10th	Night	38% 4 12% 24% 20% 2	8234
	11th to 20th	Night	41% 4 11% 21% 17% 6%	8280
	21st to end	Night	44% 5% 9% 21% 18% 4	8758
February	1st to 10th	Night	44% 6% 12% 23% 13% 2	2663
	11th to 20th	Night	43% 5% 10% 21% 16% 4	2401
	21st to end	Night	44% 4 9% 22% 16% 4	1980
March	1st to 10th	Night	44% 4 11% 19% 13% 8%	5374
	11th to 20th	Night	42% 6% 13% 24% 11% 4	5268
	21st to end	Night	46% 5% 7% 13% 23% 5%	4962
April	1st to 10th	Night	46% 6% 11% 17% 16% 3	2000
	11th to 20th	Night	44% 6% 10% 18% 20% 2	1975
	21st to end	Night	43% 6% 10% 20% 17% 5%	1925
May	1st to 10th	Night	56% 8% 13% 13% 9% 1	4060
	11th to 20th	Night	51% 7% 13% 14% 13% 2	3806
	21st to end	Night	48% 7% 14% 20% 10% 1	3927
June	1st to 10th	Night	43% 7% 10% 20% 19% 1	1600
	11th to 20th	Night	39% 9% 13% 25% 12% 1	1600
	21st to end	Night	39% 9% 14% 24% 13% 2	1600
July	1st to 10th	Night	47% 9% 12% 21% 11% 1	3560
	11th to 20th	Night	48% 8% 14% 20% 9% 1	3552
	21st to end	Night	40% 7% 14% 24% 14% 1	3809
August	1st to 10th	Night	48% 9% 16% 17% 9% 1	2764
	11th to 20th	Night	39% 7% 15% 29% 9% 1	3227
	21st to end	Night	28% 7% 12% 33% 18% 1	3586
September	1st to 10th	Night	39% 7% 10% 25% 18% 1	2000
	11th to 20th	Night	33% 6% 10% 25% 22% 3	2123
	21st to end	Night	43% 7% 9% 20% 18% 3	2200
October	1st to 10th	Night	38% 6% 11% 23% 19% 3	3678
	11th to 20th	Night	25% 4 11% 26% 30% 4	3899
	21st to end	Night	31% 5% 13% 28% 21% 2	4595
November	1st to 10th	Night	35% 6% 13% 23% 20% 3	2667
	11th to 20th	Night	39% 4 10% 23% 19% 5%	2730
	21st to end	Night	37% 6% 13% 29% 14% 1	2752
December	1st to 10th	Night	40% 3 13% 29% 12% 2	4720
	11th to 20th	Night	46% 4 9% 24% 15% 2	5348
	21st to end	Night	44% 5% 11% 20% 16% 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	24% 17% 31% 21% 7%	220
	11th to 20th	Day	20% 19% 37% 19% 4	220
	21st to end	Day	22% 12% 37% 21% 7% 1	297
February	1st to 10th	Day	23% 11% 36% 27% 4	395
	11th to 20th	Day	21% 12% 37% 27% 3	516
	21st to end	Day	23% 13% 32% 25% 7%	414
March	1st to 10th	Day	17% 11% 36% 28% 7%	500
	11th to 20th	Day	16% 8% 39% 29% 6% 1	500
	21st to end	Day	23% 10% 32% 26% 7% 2	550
April	1st to 10th	Day	23% 11% 32% 24% 10% 1	500
	11th to 20th	Day	20% 8% 26% 33% 12% 1	492
	21st to end	Day	23% 8% 25% 33% 10% 1	500
May	1st to 10th	Day	27% 5% 27% 30% 9% 2	503
	11th to 20th	Day	20% 5% 21% 36% 17% 1	527
	21st to end	Day	19% 3 14% 32% 28% 5%	573
June	1st to 10th	Day	19% 3 15% 35% 25% 3	530
	11th to 20th	Day	17% 4 18% 37% 21% 3	530
	21st to end	Day	12% 4 17% 36% 27% 5%	530
July	1st to 10th	Day	11% 4 14% 32% 30% 8%	525
	11th to 20th	Day	10% 4 12% 34% 32% 8%	524
	21st to end	Day	13% 3 15% 35% 30% 4	576
August	1st to 10th	Day	12% 3 12% 39% 26% 8%	526
	11th to 20th	Day	14% 3 11% 35% 30% 7%	519
	21st to end	Day	10% 3 16% 37% 24% 9%	583
September	1st to 10th	Day	18% 3 18% 28% 22% 11%	530
	11th to 20th	Day	15% 2 14% 32% 25% 11%	530
	21st to end	Day	22% 3 22% 31% 17% 5%	530
October	1st to 10th	Day	23% 5% 19% 26% 18% 8%	530
	11th to 20th	Day	16% 8% 27% 29% 16% 4	530
	21st to end	Day	23% 9% 30% 24% 12% 2	470
November	1st to 10th	Day	16% 10% 25% 31% 15% 3	425
	11th to 20th	Day	19% 16% 33% 25% 7%	430
	21st to end	Day	20% 14% 30% 26% 9% 1	404
December	1st to 10th	Day	20% 14% 35% 21% 10%	220
	11th to 20th	Day	30% 15% 26% 23% 5%	220
	21st to end	Day	24% 12% 34% 21% 8%	242
January	1st to 10th	Night	20% 16% 39% 21% 4	840
	11th to 20th	Night	26% 13% 33% 26% 2	840
	21st to end	Night	20% 15% 38% 23% 2	924
February	1st to 10th	Night	21% 12% 37% 27% 3	840
	11th to 20th	Night	24% 10% 43% 21% 2	804
	21st to end	Night	23% 12% 37% 24% 5%	660
March	1st to 10th	Night	21% 10% 38% 25% 6%	800
	11th to 20th	Night	19% 9% 40% 26% 6% 1	800
	21st to end	Night	22% 8% 38% 25% 6% 1	880
April	1st to 10th	Night	23% 7% 35% 29% 6% 1	800
	11th to 20th	Night	20% 8% 31% 31% 9%	791
	21st to end	Night	21% 5% 30% 35% 8% 1	725
May	1st to 10th	Night	24% 6% 28% 31% 10% 2	600
	11th to 20th	Night	22% 5% 29% 33% 10% 1	494
	21st to end	Night	22% 3 13% 36% 24% 2	433
June	1st to 10th	Night	21% 3 21% 36% 19% 2	400
	11th to 20th	Night	18% 3 15% 43% 19% 2	400
	21st to end	Night	10% 4 17% 49% 19% 2	400
July	1st to 10th	Night	12% 3 16% 41% 26% 2	390
	11th to 20th	Night	13% 3 12% 45% 23% 4	388
	21st to end	Night	14% 2 15% 46% 21% 1	438
August	1st to 10th	Night	10% 1 14% 45% 27% 2	508
	11th to 20th	Night	15% 3 14% 34% 31% 3	790
	21st to end	Night	13% 4 15% 36% 27% 5%	880
September	1st to 10th	Night	16% 3 21% 35% 21% 5%	800
	11th to 20th	Night	16% 3 16% 37% 22% 6%	800
	21st to end	Night	22% 3 22% 32% 18% 3	800
October	1st to 10th	Night	23% 7% 24% 26% 16% 5%	800
	11th to 20th	Night	20% 9% 26% 32% 13% 1	800
	21st to end	Night	24% 9% 32% 26% 8%	880
November	1st to 10th	Night	16% 9% 32% 29% 12% 2	820
	11th to 20th	Night	21% 13% 33% 24% 8%	840
	21st to end	Night	19% 13% 33% 27% 7% 1	840
December	1st to 10th	Night	24% 14% 34% 21% 5% 1	840
	11th to 20th	Night	24% 13% 37% 23% 4	840
	21st to end	Night	23% 14% 36% 22% 5%	924

Page generated at 2026-02-13 01:42:13. Version ac07

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