Stoney Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stoney Hill Observatory clear sky chart from 2005-11-06 to 2025-12-03.

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% 3 8% 5% 5% 5% 4 6% 8% 33%	127119
	Night	15% 9% 3 8% 5% 5% 5% 4 6% 7% 34%	123592
Transparency	Day	46% 7% 13% 21% 12% 1	130608
	Night	47% 6% 14% 20% 12% 1	128190
Seeing	Day	20% 17% 30% 21% 9% 2	16185
	Night	20% 15% 32% 25% 7% 1	25253

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	10% 9% 5% 13% 7% 7% 6% 5% 7% 7% 26%	3597
	11th to 20th	Day	12% 13% 4 11% 6% 5% 6% 4 7% 6% 26%	3570
	21st to end	Day	14% 11% 4 9% 5% 6% 5% 4 5% 6% 32%	4123
February	1st to 10th	Day	14% 12% 4 9% 7% 5% 6% 5% 5% 6% 26%	1800
	11th to 20th	Day	12% 10% 4 11% 7% 6% 5% 6% 6% 6% 29%	1917
	21st to end	Day	16% 11% 4 9% 6% 5% 3 3 6% 6% 30%	1650
March	1st to 10th	Day	18% 11% 4 8% 5% 5% 5% 4 5% 5% 31%	4476
	11th to 20th	Day	13% 10% 4 9% 5% 5% 4 3 5% 6% 38%	4740
	21st to end	Day	16% 10% 3 8% 5% 5% 5% 3 5% 5% 35%	5957
April	1st to 10th	Day	17% 10% 3 8% 5% 4 4 3 5% 6% 34%	2400
	11th to 20th	Day	15% 8% 3 8% 5% 5% 5% 4 5% 6% 37%	2374
	21st to end	Day	17% 8% 2 7% 5% 4 5% 4 5% 7% 35%	2480
May	1st to 10th	Day	27% 9% 3 8% 5% 5% 5% 3 5% 6% 24%	6014
	11th to 20th	Day	13% 8% 3 8% 6% 5% 5% 4 7% 8% 32%	6318
	21st to end	Day	17% 7% 2 7% 5% 4 5% 5% 8% 9% 31%	6949
June	1st to 10th	Day	14% 6% 2 8% 6% 6% 6% 4 7% 9% 33%	2800
	11th to 20th	Day	14% 6% 3 6% 5% 5% 6% 5% 7% 10% 34%	2800
	21st to end	Day	14% 6% 2 8% 6% 5% 5% 4 7% 8% 35%	2800
July	1st to 10th	Day	7% 7% 3 8% 5% 7% 6% 5% 8% 10% 34%	6360
	11th to 20th	Day	7% 5% 2 7% 5% 6% 7% 6% 9% 11% 36%	6360
	21st to end	Day	11% 6% 2 6% 5% 5% 5% 4 7% 8% 41%	6926
August	1st to 10th	Day	14% 7% 2 7% 6% 5% 6% 4 7% 8% 34%	4531
	11th to 20th	Day	10% 8% 3 8% 5% 7% 6% 5% 6% 10% 33%	3850
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 5% 7% 12% 40%	4312
September	1st to 10th	Day	12% 5% 2 6% 5% 5% 5% 4 6% 7% 44%	2400
	11th to 20th	Day	11% 5% 2 5% 4 4 4 4 5% 8% 49%	2313
	21st to end	Day	16% 5% 2 6% 4 4 4 5% 7% 8% 40%	2200
October	1st to 10th	Day	16% 8% 3 7% 5% 5% 5% 3 5% 6% 37%	2080
	11th to 20th	Day	13% 7% 3 7% 5% 5% 4 4 5% 7% 39%	2000
	21st to end	Day	20% 8% 2 7% 4 4 5% 4 5% 7% 33%	1997
November	1st to 10th	Day	12% 8% 3 6% 4 4 5% 4 5% 6% 42%	1851
	11th to 20th	Day	13% 7% 2 8% 6% 5% 5% 5% 6% 6% 38%	1890
	21st to end	Day	17% 9% 2 8% 5% 6% 5% 4 5% 7% 32%	1722
December	1st to 10th	Day	12% 12% 5% 12% 7% 6% 5% 5% 6% 7% 23%	2616
	11th to 20th	Day	19% 9% 3 8% 5% 4 5% 4 5% 7% 31%	2986
	21st to end	Day	18% 13% 5% 9% 6% 5% 4 4 5% 7% 25%	3960
January	1st to 10th	Night	12% 10% 4 14% 6% 5% 4 3 6% 7% 29%	6349
	11th to 20th	Night	10% 12% 4 13% 7% 5% 5% 4 5% 7% 28%	6315
	21st to end	Night	15% 10% 4 9% 5% 6% 5% 4 6% 6% 30%	6830
February	1st to 10th	Night	14% 12% 4 9% 7% 5% 5% 4 6% 6% 28%	2600
	11th to 20th	Night	11% 9% 4 11% 6% 6% 5% 5% 7% 7% 29%	2368
	21st to end	Night	15% 10% 3 8% 7% 5% 5% 5% 6% 7% 29%	1980
March	1st to 10th	Night	18% 9% 3 8% 6% 5% 5% 5% 6% 7% 29%	5364
	11th to 20th	Night	13% 10% 3 7% 5% 5% 5% 4 5% 7% 37%	5215
	21st to end	Night	18% 10% 3 8% 5% 5% 5% 4 5% 5% 33%	4973
April	1st to 10th	Night	17% 9% 4 8% 5% 4 5% 4 5% 6% 31%	2000
	11th to 20th	Night	18% 7% 2 7% 6% 4 5% 3 5% 7% 35%	1975
	21st to end	Night	18% 6% 3 6% 5% 3 4 4 5% 7% 39%	1919
May	1st to 10th	Night	27% 8% 3 7% 5% 5% 4 4 5% 5% 27%	3968
	11th to 20th	Night	13% 9% 3 9% 7% 5% 6% 4 6% 7% 30%	3575
	21st to end	Night	17% 8% 3 8% 5% 6% 5% 4 6% 7% 32%	3926
June	1st to 10th	Night	15% 8% 3 6% 5% 4 5% 4 6% 7% 38%	1600
	11th to 20th	Night	13% 6% 2 6% 4 4 5% 4 7% 10% 39%	1600
	21st to end	Night	11% 4 2 6% 5% 4 5% 4 7% 9% 43%	1600
July	1st to 10th	Night	7% 6% 2 8% 5% 6% 5% 5% 6% 9% 41%	3600
	11th to 20th	Night	8% 6% 3 7% 5% 6% 6% 5% 8% 9% 37%	3600
	21st to end	Night	9% 6% 2 7% 4 5% 4 4 5% 7% 47%	3926
August	1st to 10th	Night	16% 7% 2 6% 4 4 5% 4 7% 9% 36%	2613
	11th to 20th	Night	12% 7% 2 7% 5% 5% 5% 4 7% 7% 39%	3165
	21st to end	Night	10% 6% 2 5% 4 4 4 3 6% 9% 48%	3586
September	1st to 10th	Night	12% 7% 2 6% 4 4 4 4 6% 7% 45%	2000
	11th to 20th	Night	13% 4 2 5% 3 3 4 3 5% 7% 53%	2086
	21st to end	Night	13% 6% 2 6% 5% 5% 4 4 5% 7% 44%	2200
October	1st to 10th	Night	16% 9% 2 7% 4 4 4 4 6% 5% 40%	2315
	11th to 20th	Night	15% 6% 2 7% 4 4 5% 3 6% 7% 41%	2400
	21st to end	Night	20% 7% 3 6% 4 4 4 3 6% 7% 35%	2841
November	1st to 10th	Night	14% 8% 3 6% 4 3 4 4 6% 6% 42%	2667
	11th to 20th	Night	15% 8% 3 6% 5% 5% 5% 4 5% 7% 37%	2730
	21st to end	Night	15% 8% 2 7% 5% 5% 6% 4 5% 7% 37%	2893
December	1st to 10th	Night	13% 10% 4 9% 6% 5% 4 4 7% 8% 29%	4609
	11th to 20th	Night	21% 10% 3 7% 5% 5% 4 3 6% 5% 32%	5208
	21st to end	Night	20% 13% 4 9% 5% 4 4 3 5% 5% 27%	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	53% 7% 14% 19% 7% 1	3598
	11th to 20th	Day	52% 7% 12% 15% 10% 3	3600
	21st to end	Day	51% 6% 11% 20% 12% 1	4123
February	1st to 10th	Day	54% 9% 13% 15% 8% 1	1800
	11th to 20th	Day	53% 7% 12% 18% 9% 1	1926
	21st to end	Day	53% 6% 12% 16% 12% 1	1650
March	1st to 10th	Day	53% 5% 12% 15% 10% 5%	4479
	11th to 20th	Day	44% 6% 17% 21% 7% 4	4740
	21st to end	Day	49% 5% 8% 15% 19% 4	5957
April	1st to 10th	Day	50% 5% 10% 21% 12% 2	2400
	11th to 20th	Day	46% 7% 10% 20% 15% 2	2374
	21st to end	Day	48% 7% 11% 18% 13% 3	2480
May	1st to 10th	Day	58% 8% 10% 14% 9% 1	6025
	11th to 20th	Day	47% 7% 16% 20% 9% 1	6322
	21st to end	Day	46% 9% 13% 22% 10% 1	6949
June	1st to 10th	Day	46% 8% 14% 20% 11% 1	2800
	11th to 20th	Day	42% 10% 15% 20% 12% 1	2800
	21st to end	Day	45% 7% 15% 20% 11% 1	2800
July	1st to 10th	Day	41% 10% 14% 21% 13% 1	6290
	11th to 20th	Day	38% 10% 16% 23% 12% 1	6285
	21st to end	Day	38% 8% 14% 28% 13%	6728
August	1st to 10th	Day	46% 9% 15% 20% 10% 1	4517
	11th to 20th	Day	45% 8% 16% 26% 6%	3886
	21st to end	Day	36% 8% 17% 28% 11% 1	4312
September	1st to 10th	Day	37% 8% 13% 26% 15% 1	2400
	11th to 20th	Day	33% 7% 12% 27% 20% 1	2313
	21st to end	Day	40% 7% 13% 24% 13% 2	2200
October	1st to 10th	Day	41% 6% 11% 21% 20% 1	3392
	11th to 20th	Day	36% 4 13% 28% 17% 1	3240
	21st to end	Day	34% 7% 16% 27% 14% 2	3197
November	1st to 10th	Day	41% 6% 14% 25% 13% 1	1851
	11th to 20th	Day	45% 7% 12% 20% 14% 2	1890
	21st to end	Day	49% 7% 16% 19% 7% 1	1722
December	1st to 10th	Day	57% 6% 14% 18% 5%	2616
	11th to 20th	Day	50% 6% 15% 17% 10% 1	2986
	21st to end	Day	57% 5% 11% 16% 10% 1	3960
January	1st to 10th	Night	53% 6% 15% 16% 10% 1	6349
	11th to 20th	Night	53% 6% 15% 14% 10% 2	6360
	21st to end	Night	52% 6% 12% 13% 13% 4	6830
February	1st to 10th	Night	54% 7% 13% 18% 8% 1	2600
	11th to 20th	Night	51% 7% 13% 18% 10% 1	2370
	21st to end	Night	51% 6% 15% 18% 9% 2	1980
March	1st to 10th	Night	51% 6% 15% 15% 12% 2	5374
	11th to 20th	Night	47% 6% 17% 21% 8% 2	5216
	21st to end	Night	51% 4 8% 16% 17% 3	4973
April	1st to 10th	Night	52% 6% 12% 19% 11% 1	2000
	11th to 20th	Night	47% 7% 11% 18% 16% 1	1975
	21st to end	Night	44% 7% 13% 18% 15% 3	1919
May	1st to 10th	Night	59% 6% 12% 12% 9% 2	3974
	11th to 20th	Night	51% 8% 13% 19% 9% 1	3576
	21st to end	Night	50% 8% 13% 18% 11% 1	3927
June	1st to 10th	Night	43% 7% 13% 24% 13% 2	1600
	11th to 20th	Night	39% 9% 16% 23% 12%	1600
	21st to end	Night	36% 9% 16% 25% 13% 1	1600
July	1st to 10th	Night	38% 7% 16% 22% 16% 1	3560
	11th to 20th	Night	40% 9% 16% 21% 13% 1	3552
	21st to end	Night	35% 6% 13% 31% 14% 1	3809
August	1st to 10th	Night	44% 8% 17% 22% 9% 1	2603
	11th to 20th	Night	41% 7% 17% 26% 9%	3195
	21st to end	Night	32% 7% 14% 35% 12% 1	3586
September	1st to 10th	Night	38% 6% 13% 27% 16% 1	2000
	11th to 20th	Night	31% 5% 14% 26% 23% 1	2086
	21st to end	Night	40% 6% 13% 24% 17% 1	2200
October	1st to 10th	Night	45% 5% 13% 21% 15% 1	3783
	11th to 20th	Night	32% 5% 14% 26% 22% 1	3899
	21st to end	Night	38% 6% 15% 30% 12% 1	4591
November	1st to 10th	Night	41% 6% 12% 22% 17% 2	2667
	11th to 20th	Night	45% 7% 12% 21% 13% 2	2730
	21st to end	Night	46% 7% 17% 22% 8%	2893
December	1st to 10th	Night	50% 7% 15% 22% 6%	4609
	11th to 20th	Night	53% 6% 15% 13% 13% 1	5208
	21st to end	Night	58% 6% 11% 14% 10% 1	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	25% 31% 30% 11% 5%	200
	11th to 20th	Day	31% 30% 25% 13% 2 1	200
	21st to end	Day	23% 27% 33% 12% 5%	257
February	1st to 10th	Day	30% 29% 33% 7% 1	300
	11th to 20th	Day	30% 28% 32% 9% 1	465
	21st to end	Day	27% 23% 34% 14% 2	414
March	1st to 10th	Day	28% 22% 32% 15% 2	500
	11th to 20th	Day	26% 26% 30% 14% 3 1	500
	21st to end	Day	31% 23% 27% 13% 5% 1	550
April	1st to 10th	Day	30% 26% 26% 13% 5% 1	500
	11th to 20th	Day	29% 25% 26% 13% 6% 1	492
	21st to end	Day	25% 23% 32% 14% 5% 2	500
May	1st to 10th	Day	30% 18% 30% 14% 6% 2	503
	11th to 20th	Day	17% 18% 34% 21% 7% 2	527
	21st to end	Day	16% 12% 33% 26% 11% 3	573
June	1st to 10th	Day	15% 16% 32% 24% 10% 2	530
	11th to 20th	Day	16% 13% 32% 25% 12% 2	530
	21st to end	Day	16% 14% 31% 27% 10% 3	530
July	1st to 10th	Day	11% 11% 29% 32% 14% 4	525
	11th to 20th	Day	10% 6% 33% 31% 16% 3	524
	21st to end	Day	11% 6% 36% 29% 15% 3	576
August	1st to 10th	Day	9% 8% 28% 33% 17% 5%	526
	11th to 20th	Day	13% 6% 28% 30% 17% 6%	519
	21st to end	Day	10% 12% 29% 29% 16% 5%	583
September	1st to 10th	Day	15% 9% 27% 24% 18% 6%	530
	11th to 20th	Day	11% 10% 25% 22% 19% 12%	530
	21st to end	Day	19% 11% 25% 26% 14% 5%	530
October	1st to 10th	Day	21% 10% 27% 23% 14% 5%	530
	11th to 20th	Day	18% 18% 31% 22% 8% 2	530
	21st to end	Day	26% 15% 32% 20% 6% 1	470
November	1st to 10th	Day	23% 19% 26% 21% 10% 1	425
	11th to 20th	Day	23% 23% 31% 17% 5% 1	430
	21st to end	Day	27% 26% 26% 19% 2	262
December	1st to 10th	Day	24% 27% 29% 14% 5% 1	204
	11th to 20th	Day	30% 23% 37% 9% 2	200
	21st to end	Day	21% 23% 40% 14% 2	220
January	1st to 10th	Night	21% 21% 38% 19% 2	800
	11th to 20th	Night	25% 20% 33% 19% 3	800
	21st to end	Night	18% 21% 40% 19% 2	880
February	1st to 10th	Night	25% 19% 37% 18% 2	800
	11th to 20th	Night	21% 23% 39% 16%	783
	21st to end	Night	25% 22% 34% 17% 3	660
March	1st to 10th	Night	24% 22% 35% 17% 2	800
	11th to 20th	Night	23% 24% 33% 18% 2	800
	21st to end	Night	23% 23% 33% 16% 4	880
April	1st to 10th	Night	25% 22% 33% 18% 2	800
	11th to 20th	Night	24% 20% 33% 18% 5%	791
	21st to end	Night	22% 20% 32% 23% 4	719
May	1st to 10th	Night	23% 18% 31% 21% 7%	566
	11th to 20th	Night	16% 17% 36% 26% 5% 1	394
	21st to end	Night	12% 12% 32% 36% 8%	433
June	1st to 10th	Night	18% 8% 33% 30% 10% 1	400
	11th to 20th	Night	16% 7% 34% 33% 10%	400
	21st to end	Night	10% 7% 34% 37% 11% 1	400
July	1st to 10th	Night	12% 5% 24% 43% 15% 1	390
	11th to 20th	Night	12% 6% 23% 41% 16% 1	388
	21st to end	Night	11% 4 25% 47% 12%	438
August	1st to 10th	Night	11% 3 23% 44% 18% 1	412
	11th to 20th	Night	12% 5% 24% 36% 20% 3	770
	21st to end	Night	14% 7% 24% 35% 19% 1	880
September	1st to 10th	Night	15% 6% 28% 34% 16% 1	800
	11th to 20th	Night	15% 6% 25% 31% 19% 5%	800
	21st to end	Night	16% 7% 29% 35% 13% 2	800
October	1st to 10th	Night	20% 10% 27% 27% 15% 2	800
	11th to 20th	Night	17% 13% 32% 30% 8%	800
	21st to end	Night	25% 14% 32% 24% 4	880
November	1st to 10th	Night	20% 16% 33% 24% 7%	820
	11th to 20th	Night	20% 18% 35% 20% 6% 1	840
	21st to end	Night	20% 19% 32% 23% 5%	840
December	1st to 10th	Night	24% 19% 36% 17% 3	809
	11th to 20th	Night	25% 18% 33% 22% 2	800
	21st to end	Night	28% 20% 33% 18% 2	880

Page generated at 2025-11-30 01:00:55. Version ac07

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