Stony Creek Ranch Resort Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stony Creek Ranch Resort Observatory clear sky chart from 2005-11-06 to 2025-05-11.

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	15% 9% 3 8% 5% 5% 5% 4 6% 7% 32%	113710
	Night	15% 9% 3 8% 5% 5% 5% 4 5% 6% 34%	115215
Transparency	Day	47% 8% 13% 19% 11% 2	117235
	Night	47% 8% 13% 19% 12% 1	119853
Seeing	Day	22% 17% 30% 20% 9% 3	15335
	Night	20% 14% 30% 27% 9% 1	24504

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% 11% 5% 17% 6% 6% 6% 5% 5% 5% 25%	3597
	11th to 20th	Day	12% 14% 6% 14% 8% 6% 5% 3 5% 5% 24%	3570
	21st to end	Day	12% 12% 5% 10% 6% 7% 6% 4 5% 6% 27%	4051
February	1st to 10th	Day	12% 13% 4 10% 7% 6% 6% 5% 5% 6% 26%	1800
	11th to 20th	Day	11% 11% 5% 11% 7% 5% 6% 4 5% 5% 30%	1898
	21st to end	Day	18% 11% 5% 8% 6% 4 4 3 5% 5% 31%	1650
March	1st to 10th	Day	18% 13% 4 10% 5% 5% 4 3 4 5% 30%	4476
	11th to 20th	Day	15% 11% 4 7% 5% 4 4 3 5% 6% 35%	4722
	21st to end	Day	19% 12% 3 7% 5% 4 4 3 4 4 35%	5960
April	1st to 10th	Day	19% 12% 3 7% 4 4 4 3 4 6% 35%	2400
	11th to 20th	Day	17% 10% 2 6% 5% 6% 4 4 5% 6% 36%	2374
	21st to end	Day	20% 10% 3 8% 5% 5% 5% 3 5% 6% 31%	2505
May	1st to 10th	Day	23% 12% 3 7% 4 4 4 3 5% 6% 28%	4385
	11th to 20th	Day	17% 7% 3 7% 4 6% 5% 4 7% 8% 31%	4398
	21st to end	Day	12% 6% 2 6% 5% 5% 5% 4 7% 9% 39%	4837
June	1st to 10th	Day	16% 7% 3 7% 6% 5% 5% 5% 7% 8% 30%	2660
	11th to 20th	Day	17% 5% 3 6% 6% 4 5% 5% 7% 8% 33%	2660
	21st to end	Day	17% 7% 2 7% 5% 5% 5% 4 7% 9% 30%	2660
July	1st to 10th	Day	10% 5% 2 7% 6% 6% 6% 5% 7% 9% 36%	4440
	11th to 20th	Day	10% 5% 2 6% 5% 6% 6% 5% 9% 12% 33%	4440
	21st to end	Day	13% 6% 2 6% 5% 6% 6% 5% 7% 7% 36%	4814
August	1st to 10th	Day	15% 5% 2 7% 5% 6% 6% 5% 7% 8% 35%	4423
	11th to 20th	Day	14% 7% 2 7% 5% 6% 6% 5% 8% 10% 31%	3777
	21st to end	Day	12% 6% 3 6% 4 5% 6% 5% 8% 9% 37%	4180
September	1st to 10th	Day	13% 4 2 6% 4 5% 5% 4 6% 8% 43%	2280
	11th to 20th	Day	14% 5% 2 5% 4 4 4 3 5% 8% 47%	2199
	21st to end	Day	17% 5% 2 6% 4 4 4 4 6% 6% 40%	2090
October	1st to 10th	Day	21% 8% 3 9% 5% 5% 5% 3 5% 5% 32%	1971
	11th to 20th	Day	15% 8% 3 7% 4 4 5% 4 6% 7% 36%	1900
	21st to end	Day	22% 10% 3 7% 5% 4 4 3 5% 6% 29%	1881
November	1st to 10th	Day	11% 9% 3 7% 5% 4 5% 4 5% 7% 38%	1761
	11th to 20th	Day	14% 9% 3 6% 5% 5% 5% 4 5% 7% 36%	1800
	21st to end	Day	17% 10% 4 7% 5% 4 5% 4 5% 7% 32%	1605
December	1st to 10th	Day	17% 14% 8% 13% 7% 5% 5% 3 4 5% 20%	2600
	11th to 20th	Day	19% 12% 4 8% 5% 5% 4 4 5% 5% 29%	2986
	21st to end	Day	19% 14% 5% 9% 5% 4 3 3 4 6% 27%	3960
January	1st to 10th	Night	12% 11% 6% 11% 6% 5% 5% 4 5% 6% 29%	6349
	11th to 20th	Night	8% 13% 7% 15% 7% 6% 6% 4 6% 6% 22%	6315
	21st to end	Night	13% 11% 5% 10% 6% 6% 5% 4 5% 6% 28%	6904
February	1st to 10th	Night	12% 11% 4 11% 8% 6% 6% 4 6% 6% 26%	2600
	11th to 20th	Night	10% 10% 4 10% 8% 4 5% 5% 6% 6% 33%	2388
	21st to end	Night	13% 9% 3 9% 6% 5% 5% 4 6% 7% 32%	1980
March	1st to 10th	Night	20% 10% 3 8% 5% 5% 5% 4 5% 6% 29%	5364
	11th to 20th	Night	13% 11% 3 7% 5% 4 4 4 5% 7% 37%	5220
	21st to end	Night	20% 11% 3 8% 5% 4 4 3 5% 5% 32%	4972
April	1st to 10th	Night	19% 8% 3 7% 5% 4 5% 4 5% 5% 35%	2000
	11th to 20th	Night	18% 9% 3 6% 5% 3 5% 4 5% 7% 36%	1975
	21st to end	Night	18% 9% 2 7% 4 4 3 3 6% 6% 37%	1895
May	1st to 10th	Night	23% 11% 3 8% 5% 4 3 4 5% 7% 27%	2805
	11th to 20th	Night	18% 9% 4 9% 6% 4 4 3 4 7% 31%	2501
	21st to end	Night	13% 5% 2 6% 5% 5% 5% 5% 7% 8% 40%	2738
June	1st to 10th	Night	18% 6% 1 5% 4 5% 5% 4 6% 7% 39%	1520
	11th to 20th	Night	16% 7% 2 6% 4 4 5% 3 6% 9% 39%	1520
	21st to end	Night	16% 5% 2 6% 5% 4 6% 4 5% 7% 40%	1520
July	1st to 10th	Night	9% 5% 3 7% 5% 5% 5% 5% 7% 8% 40%	2520
	11th to 20th	Night	13% 4 2 5% 5% 4 4 4 6% 8% 45%	2520
	21st to end	Night	13% 6% 2 6% 5% 5% 4 3 6% 7% 42%	2739
August	1st to 10th	Night	17% 7% 2 7% 5% 4 5% 3 5% 6% 39%	2506
	11th to 20th	Night	16% 6% 2 7% 4 4 4 4 6% 7% 41%	3002
	21st to end	Night	9% 5% 2 5% 4 4 5% 5% 6% 9% 46%	3476
September	1st to 10th	Night	11% 5% 2 5% 4 3 4 3 7% 8% 47%	1900
	11th to 20th	Night	12% 5% 2 5% 3 3 4 3 4 5% 53%	1981
	21st to end	Night	15% 6% 2 4 4 5% 4 4 4 6% 46%	2090
October	1st to 10th	Night	19% 6% 3 8% 4 4 5% 4 5% 6% 34%	2205
	11th to 20th	Night	15% 6% 3 7% 5% 4 6% 3 6% 6% 40%	2280
	21st to end	Night	22% 9% 3 7% 4 4 4 4 6% 7% 31%	2717
November	1st to 10th	Night	12% 8% 4 7% 5% 5% 4 3 5% 6% 41%	2537
	11th to 20th	Night	18% 7% 3 7% 4 4 4 3 6% 7% 37%	2600
	21st to end	Night	15% 9% 3 7% 5% 4 5% 4 6% 7% 34%	2792
December	1st to 10th	Night	15% 13% 5% 10% 6% 7% 5% 4 6% 7% 24%	4580
	11th to 20th	Night	23% 11% 3 8% 5% 5% 4 3 4 5% 30%	5208
	21st to end	Night	20% 14% 5% 10% 5% 4 4 3 4 4 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	56% 9% 11% 17% 6% 1	3598
	11th to 20th	Day	60% 7% 10% 12% 9% 3	3600
	21st to end	Day	54% 7% 12% 14% 10% 2	4051
February	1st to 10th	Day	55% 10% 11% 16% 7% 2	1800
	11th to 20th	Day	55% 8% 11% 15% 10% 2	1907
	21st to end	Day	54% 6% 11% 16% 12% 1	1650
March	1st to 10th	Day	55% 6% 10% 12% 10% 7%	4479
	11th to 20th	Day	46% 10% 18% 14% 8% 4	4722
	21st to end	Day	51% 6% 7% 14% 18% 4	5960
April	1st to 10th	Day	51% 6% 9% 19% 12% 3	2400
	11th to 20th	Day	48% 7% 11% 19% 14% 2	2374
	21st to end	Day	52% 7% 10% 15% 12% 4	2505
May	1st to 10th	Day	52% 10% 10% 17% 10% 1	4399
	11th to 20th	Day	46% 13% 14% 17% 8% 1	4402
	21st to end	Day	35% 11% 16% 25% 11% 1	4837
June	1st to 10th	Day	47% 11% 14% 16% 10% 1	2660
	11th to 20th	Day	45% 9% 14% 18% 12% 1	2660
	21st to end	Day	48% 10% 14% 19% 10% 1	2660
July	1st to 10th	Day	39% 9% 16% 23% 13%	4370
	11th to 20th	Day	39% 9% 17% 27% 7%	4365
	21st to end	Day	43% 8% 16% 22% 10%	4661
August	1st to 10th	Day	43% 9% 16% 19% 12% 1	4409
	11th to 20th	Day	43% 10% 16% 25% 6%	3814
	21st to end	Day	39% 8% 16% 26% 10%	4180
September	1st to 10th	Day	37% 10% 13% 29% 11% 1	2280
	11th to 20th	Day	34% 7% 13% 28% 17% 1	2199
	21st to end	Day	41% 8% 15% 22% 13% 1	2090
October	1st to 10th	Day	47% 6% 9% 23% 15% 1	3270
	11th to 20th	Day	39% 6% 13% 24% 15% 2	3140
	21st to end	Day	38% 7% 14% 28% 12% 1	3081
November	1st to 10th	Day	42% 10% 16% 20% 11%	1761
	11th to 20th	Day	46% 8% 14% 15% 15% 2	1800
	21st to end	Day	50% 9% 16% 16% 7% 1	1605
December	1st to 10th	Day	65% 7% 10% 15% 4	2600
	11th to 20th	Day	55% 7% 12% 15% 10% 1	2986
	21st to end	Day	58% 6% 8% 13% 14% 1	3960
January	1st to 10th	Night	52% 9% 14% 15% 10% 1	6349
	11th to 20th	Night	59% 9% 12% 10% 9% 2	6360
	21st to end	Night	54% 6% 13% 11% 13% 3	6904
February	1st to 10th	Night	55% 9% 12% 16% 7% 1	2600
	11th to 20th	Night	49% 9% 12% 17% 11% 2	2390
	21st to end	Night	48% 8% 14% 20% 8% 2	1980
March	1st to 10th	Night	52% 7% 11% 13% 14% 3	5374
	11th to 20th	Night	45% 8% 17% 17% 10% 2	5221
	21st to end	Night	51% 5% 7% 17% 17% 2	4972
April	1st to 10th	Night	49% 6% 10% 19% 13% 2	2000
	11th to 20th	Night	46% 8% 11% 20% 14% 1	1975
	21st to end	Night	45% 8% 11% 19% 14% 3	1895
May	1st to 10th	Night	52% 12% 11% 13% 10% 1	2811
	11th to 20th	Night	51% 11% 12% 16% 9% 1	2502
	21st to end	Night	37% 12% 15% 19% 15% 2	2739
June	1st to 10th	Night	43% 9% 15% 20% 12% 2	1520
	11th to 20th	Night	40% 8% 17% 21% 13% 1	1520
	21st to end	Night	41% 10% 14% 22% 12% 1	1520
July	1st to 10th	Night	38% 10% 16% 22% 14% 1	2480
	11th to 20th	Night	36% 8% 18% 24% 14%	2472
	21st to end	Night	39% 8% 14% 27% 11% 1	2651
August	1st to 10th	Night	43% 8% 15% 23% 10%	2496
	11th to 20th	Night	41% 7% 17% 26% 9%	3030
	21st to end	Night	32% 8% 16% 33% 11%	3476
September	1st to 10th	Night	33% 9% 16% 28% 13% 1	1900
	11th to 20th	Night	34% 6% 13% 28% 19% 1	1981
	21st to end	Night	38% 8% 14% 23% 15% 2	2090
October	1st to 10th	Night	46% 8% 11% 21% 13% 1	3686
	11th to 20th	Night	35% 6% 14% 25% 17% 4	3779
	21st to end	Night	42% 7% 13% 28% 9% 1	4467
November	1st to 10th	Night	43% 7% 14% 21% 13% 2	2537
	11th to 20th	Night	44% 8% 12% 21% 13% 2	2600
	21st to end	Night	47% 9% 16% 18% 9% 1	2792
December	1st to 10th	Night	55% 8% 15% 17% 4	4580
	11th to 20th	Night	56% 7% 10% 13% 13% 1	5208
	21st to end	Night	58% 8% 8% 12% 11% 2	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	23% 24% 32% 15% 8%	200
	11th to 20th	Day	25% 29% 30% 14% 2 1	200
	21st to end	Day	20% 27% 35% 13% 4 1	240
February	1st to 10th	Day	27% 34% 31% 7% 1	300
	11th to 20th	Day	25% 31% 31% 12% 1	446
	21st to end	Day	31% 22% 30% 15% 2	414
March	1st to 10th	Day	29% 16% 34% 17% 3	500
	11th to 20th	Day	27% 25% 29% 14% 4 1	500
	21st to end	Day	30% 25% 24% 13% 6% 2	550
April	1st to 10th	Day	35% 22% 24% 13% 6% 1	500
	11th to 20th	Day	33% 23% 23% 13% 7% 2	492
	21st to end	Day	28% 21% 30% 14% 5% 2	500
May	1st to 10th	Day	32% 18% 29% 12% 7% 2	503
	11th to 20th	Day	17% 17% 35% 22% 6% 2	487
	21st to end	Day	18% 15% 31% 23% 11% 2	529
June	1st to 10th	Day	19% 16% 32% 22% 10% 2	490
	11th to 20th	Day	16% 16% 32% 25% 10% 2	490
	21st to end	Day	20% 14% 33% 21% 8% 3	490
July	1st to 10th	Day	15% 9% 31% 28% 15% 3	485
	11th to 20th	Day	15% 8% 31% 30% 14% 3	484
	21st to end	Day	14% 9% 36% 27% 10% 3	532
August	1st to 10th	Day	12% 10% 28% 28% 16% 5%	486
	11th to 20th	Day	18% 8% 25% 28% 13% 8%	479
	21st to end	Day	12% 12% 30% 26% 14% 6%	539
September	1st to 10th	Day	15% 9% 27% 25% 16% 9%	490
	11th to 20th	Day	14% 10% 26% 22% 18% 11%	490
	21st to end	Day	18% 10% 31% 23% 11% 7%	490
October	1st to 10th	Day	26% 14% 25% 17% 13% 4	490
	11th to 20th	Day	20% 18% 29% 23% 9% 2	490
	21st to end	Day	30% 14% 30% 19% 6% 1	429
November	1st to 10th	Day	24% 17% 26% 20% 12% 1	395
	11th to 20th	Day	24% 23% 27% 19% 7% 1	400
	21st to end	Day	31% 19% 33% 15% 2	205
December	1st to 10th	Day	27% 24% 30% 15% 4 1	200
	11th to 20th	Day	31% 24% 31% 12% 3	200
	21st to end	Day	26% 21% 34% 15% 3	220
January	1st to 10th	Night	19% 19% 36% 23% 3	800
	11th to 20th	Night	21% 19% 34% 23% 3	800
	21st to end	Night	14% 20% 38% 24% 4	880
February	1st to 10th	Night	18% 19% 40% 21% 2	800
	11th to 20th	Night	19% 21% 39% 19% 2	783
	21st to end	Night	22% 18% 38% 20% 3	660
March	1st to 10th	Night	25% 18% 33% 22% 2	800
	11th to 20th	Night	24% 19% 33% 21% 3	800
	21st to end	Night	23% 19% 31% 20% 6%	880
April	1st to 10th	Night	26% 18% 30% 22% 4	800
	11th to 20th	Night	23% 19% 32% 19% 6%	791
	21st to end	Night	25% 17% 28% 23% 6%	695
May	1st to 10th	Night	26% 15% 28% 22% 8%	533
	11th to 20th	Night	18% 16% 32% 26% 7% 1	376
	21st to end	Night	15% 9% 27% 35% 14% 1	411
June	1st to 10th	Night	19% 6% 27% 34% 12% 1	380
	11th to 20th	Night	18% 7% 25% 36% 13%	380
	21st to end	Night	16% 6% 28% 39% 11% 1	380
July	1st to 10th	Night	11% 5% 24% 40% 19% 1	370
	11th to 20th	Night	13% 2 24% 40% 20% 1	368
	21st to end	Night	11% 4 25% 43% 16% 1	416
August	1st to 10th	Night	10% 2 24% 44% 19% 1	378
	11th to 20th	Night	14% 6% 21% 36% 21% 3	691
	21st to end	Night	15% 6% 19% 39% 21% 1	836
September	1st to 10th	Night	13% 5% 25% 36% 18% 2	760
	11th to 20th	Night	15% 7% 24% 27% 22% 5%	760
	21st to end	Night	16% 6% 30% 30% 16% 2	760
October	1st to 10th	Night	23% 11% 25% 26% 14% 1	760
	11th to 20th	Night	19% 12% 28% 31% 10% 1	760
	21st to end	Night	25% 14% 30% 25% 6%	836
November	1st to 10th	Night	17% 15% 33% 27% 8%	780
	11th to 20th	Night	22% 19% 29% 23% 6% 1	800
	21st to end	Night	20% 17% 34% 23% 6% 1	800
December	1st to 10th	Night	24% 16% 36% 20% 4	800
	11th to 20th	Night	25% 17% 31% 23% 4	800
	21st to end	Night	25% 18% 33% 22% 3	880

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