Sunny Valley Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sunny Valley Observatory clear sky chart from 2005-11-06 to 2024-11-30.

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	9% 9% 3 9% 6% 5% 5% 5% 6% 8% 34%	106195
	Night	8% 10% 4 10% 6% 6% 6% 4 6% 7% 33%	101976
Transparency	Day	45% 7% 17% 23% 8%	105912
	Night	47% 7% 16% 21% 8% 1	101905
Seeing	Day	14% 8% 24% 29% 19% 6%	16807
	Night	14% 6% 22% 36% 20% 2	24784

By Month

Percentage Cloud Cover NW (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	7% 16% 7% 15% 8% 9% 7% 5% 7% 6% 14%	1899
	11th to 20th	Day	5% 12% 4 12% 9% 7% 5% 5% 9% 6% 27%	1882
	21st to end	Day	5% 13% 6% 13% 7% 5% 5% 4 6% 6% 29%	2363
February	1st to 10th	Day	7% 12% 4 11% 9% 7% 5% 6% 8% 7% 25%	1520
	11th to 20th	Day	6% 11% 5% 11% 7% 6% 7% 6% 7% 7% 28%	1480
	21st to end	Day	4 13% 3 11% 9% 8% 7% 4 7% 8% 27%	1479
March	1st to 10th	Day	3 8% 5% 13% 7% 7% 7% 5% 7% 8% 29%	3138
	11th to 20th	Day	5% 9% 3 10% 6% 6% 6% 5% 7% 10% 33%	3259
	21st to end	Day	4 9% 4 12% 7% 5% 6% 5% 7% 7% 34%	4151
April	1st to 10th	Day	5% 10% 4 10% 6% 7% 7% 5% 6% 10% 31%	2299
	11th to 20th	Day	8% 11% 4 9% 5% 5% 5% 5% 7% 7% 33%	2471
	21st to end	Day	11% 13% 4 9% 8% 5% 5% 4 6% 7% 29%	2660
May	1st to 10th	Day	9% 8% 3 7% 5% 5% 5% 4 6% 8% 42%	4638
	11th to 20th	Day	11% 9% 4 9% 6% 6% 6% 5% 6% 7% 32%	4776
	21st to end	Day	12% 7% 3 8% 6% 6% 5% 5% 7% 8% 34%	5509
June	1st to 10th	Day	13% 8% 3 8% 6% 5% 6% 4 6% 8% 33%	3040
	11th to 20th	Day	19% 9% 3 9% 6% 6% 6% 5% 7% 7% 23%	3040
	21st to end	Day	10% 6% 2 7% 6% 6% 6% 5% 7% 9% 35%	3040
July	1st to 10th	Day	9% 5% 2 6% 5% 5% 5% 5% 7% 10% 41%	5060
	11th to 20th	Day	7% 5% 2 6% 5% 6% 6% 5% 7% 9% 43%	5060
	21st to end	Day	14% 6% 2 6% 4 4 5% 4 7% 10% 39%	5460
August	1st to 10th	Day	9% 6% 2 5% 4 5% 5% 4 6% 9% 45%	4501
	11th to 20th	Day	9% 8% 3 8% 6% 5% 6% 4 6% 7% 37%	4330
	21st to end	Day	14% 6% 2 7% 5% 5% 5% 5% 7% 8% 36%	4676
September	1st to 10th	Day	12% 6% 2 6% 5% 5% 4 5% 7% 8% 39%	2319
	11th to 20th	Day	8% 9% 3 7% 4 5% 4 5% 7% 10% 38%	2214
	21st to end	Day	10% 6% 2 7% 5% 4 5% 4 6% 9% 41%	2090
October	1st to 10th	Day	12% 10% 3 6% 4 4 4 3 6% 7% 40%	3172
	11th to 20th	Day	10% 9% 3 7% 5% 5% 5% 3 6% 8% 38%	2842
	21st to end	Day	10% 15% 5% 11% 5% 4 4 4 6% 6% 30%	3048
November	1st to 10th	Day	10% 13% 4 10% 6% 5% 5% 4 5% 9% 30%	1386
	11th to 20th	Day	7% 12% 5% 14% 9% 7% 5% 4 5% 6% 25%	1400
	21st to end	Day	6% 14% 6% 12% 6% 6% 4 3 5% 7% 30%	1321
December	1st to 10th	Day	7% 15% 5% 12% 8% 6% 5% 4 4 7% 28%	1140
	11th to 20th	Day	8% 15% 3 11% 7% 6% 7% 5% 6% 8% 24%	1442
	21st to end	Day	3 8% 4 10% 7% 6% 4 4 7% 8% 39%	2090
January	1st to 10th	Night	5% 15% 8% 20% 9% 7% 5% 3 5% 5% 18%	5046
	11th to 20th	Night	5% 11% 4 11% 7% 6% 7% 5% 8% 7% 28%	5016
	21st to end	Night	8% 16% 6% 13% 6% 6% 6% 4 5% 7% 23%	5324
February	1st to 10th	Night	6% 12% 6% 11% 7% 6% 7% 5% 7% 7% 28%	2660
	11th to 20th	Night	6% 12% 5% 11% 8% 5% 5% 4 6% 6% 30%	2586
	21st to end	Night	4 9% 4 13% 9% 7% 7% 5% 6% 7% 29%	1975
March	1st to 10th	Night	3 9% 4 13% 8% 6% 6% 5% 7% 8% 32%	3736
	11th to 20th	Night	4 10% 3 8% 5% 6% 7% 5% 8% 11% 34%	3720
	21st to end	Night	3 10% 4 10% 6% 5% 5% 4 5% 7% 40%	3500
April	1st to 10th	Night	6% 11% 4 8% 6% 6% 6% 5% 7% 7% 35%	1881
	11th to 20th	Night	7% 11% 4 9% 6% 5% 5% 4 7% 8% 33%	1656
	21st to end	Night	11% 12% 4 10% 6% 6% 5% 7% 6% 7% 28%	1520
May	1st to 10th	Night	9% 6% 3 7% 5% 6% 7% 5% 6% 7% 40%	2321
	11th to 20th	Night	11% 9% 3 9% 4 5% 6% 4 7% 8% 34%	2116
	21st to end	Night	11% 8% 3 7% 5% 5% 6% 4 7% 9% 36%	2066
June	1st to 10th	Night	12% 8% 3 9% 5% 5% 5% 4 7% 8% 34%	1140
	11th to 20th	Night	19% 7% 4 6% 6% 6% 5% 4 6% 9% 28%	1140
	21st to end	Night	9% 5% 2 7% 6% 5% 5% 6% 8% 10% 37%	1140
July	1st to 10th	Night	12% 6% 2 6% 4 4 5% 4 7% 8% 42%	1900
	11th to 20th	Night	9% 6% 3 7% 5% 4 6% 5% 7% 9% 40%	1900
	21st to end	Night	12% 5% 1 5% 4 4 5% 5% 8% 11% 42%	2092
August	1st to 10th	Night	9% 6% 1 6% 4 4 4 4 6% 8% 47%	2427
	11th to 20th	Night	7% 6% 2 7% 5% 5% 6% 4 6% 8% 44%	2497
	21st to end	Night	11% 6% 2 6% 5% 4 5% 4 7% 9% 39%	2986
September	1st to 10th	Night	12% 8% 3 7% 4 5% 4 4 6% 8% 38%	1858
	11th to 20th	Night	8% 8% 2 8% 6% 6% 6% 4 6% 8% 39%	1966
	21st to end	Night	9% 8% 3 8% 5% 5% 5% 5% 5% 8% 38%	2090
October	1st to 10th	Night	9% 9% 3 8% 5% 5% 5% 5% 7% 8% 35%	3804
	11th to 20th	Night	8% 8% 3 7% 5% 5% 5% 4 7% 8% 40%	4086
	21st to end	Night	8% 12% 5% 10% 6% 6% 6% 4 6% 7% 31%	4507
November	1st to 10th	Night	10% 16% 5% 11% 6% 5% 4 4 6% 6% 28%	2892
	11th to 20th	Night	9% 14% 4 13% 7% 6% 5% 5% 5% 7% 25%	3000
	21st to end	Night	6% 12% 5% 12% 7% 7% 5% 5% 7% 6% 27%	3072
December	1st to 10th	Night	8% 14% 5% 10% 8% 6% 5% 4 6% 6% 27%	3040
	11th to 20th	Night	8% 15% 6% 11% 7% 6% 5% 4 6% 7% 26%	3750
	21st to end	Night	3 8% 3 9% 7% 6% 6% 5% 6% 7% 39%	5566

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	69% 8% 10% 10% 4	1899
	11th to 20th	Day	53% 6% 10% 15% 15% 2	1900
	21st to end	Day	54% 8% 17% 16% 5%	2363
February	1st to 10th	Day	53% 8% 13% 16% 9% 1	1520
	11th to 20th	Day	52% 8% 18% 17% 5%	1488
	21st to end	Day	53% 7% 14% 18% 8%	1479
March	1st to 10th	Day	49% 8% 14% 24% 5%	3138
	11th to 20th	Day	44% 9% 20% 20% 7%	3259
	21st to end	Day	46% 7% 15% 21% 11%	4151
April	1st to 10th	Day	46% 7% 16% 20% 11%	2299
	11th to 20th	Day	46% 7% 13% 21% 10% 2	2471
	21st to end	Day	53% 6% 12% 21% 7% 1	2660
May	1st to 10th	Day	39% 6% 13% 25% 17% 1	4638
	11th to 20th	Day	48% 6% 10% 24% 10% 1	4780
	21st to end	Day	44% 8% 13% 23% 11% 1	5510
June	1st to 10th	Day	46% 8% 14% 24% 7% 1	3040
	11th to 20th	Day	57% 9% 13% 16% 4	3040
	21st to end	Day	42% 10% 16% 24% 8%	3040
July	1st to 10th	Day	36% 8% 21% 28% 7%	4981
	11th to 20th	Day	36% 9% 17% 28% 10%	4973
	21st to end	Day	39% 8% 22% 25% 5%	5287
August	1st to 10th	Day	36% 7% 19% 35% 3	4483
	11th to 20th	Day	43% 8% 17% 22% 10%	4372
	21st to end	Day	43% 7% 27% 19% 3	4675
September	1st to 10th	Day	40% 8% 23% 23% 4	2319
	11th to 20th	Day	40% 8% 23% 25% 4	2214
	21st to end	Day	39% 9% 19% 24% 9%	2090
October	1st to 10th	Day	42% 6% 21% 24% 7%	3172
	11th to 20th	Day	43% 5% 20% 26% 5%	2842
	21st to end	Day	54% 7% 16% 21% 3	3048
November	1st to 10th	Day	51% 8% 14% 24% 4	1386
	11th to 20th	Day	59% 8% 15% 16% 3	1400
	21st to end	Day	54% 6% 16% 18% 4 2	1321
December	1st to 10th	Day	56% 6% 13% 15% 9% 1	1140
	11th to 20th	Day	56% 10% 11% 19% 4	1444
	21st to end	Day	41% 6% 16% 26% 8% 2	2090
January	1st to 10th	Night	67% 7% 10% 10% 5%	5046
	11th to 20th	Night	52% 6% 12% 12% 18% 1	5060
	21st to end	Night	58% 10% 15% 13% 4	5324
February	1st to 10th	Night	54% 7% 12% 19% 8% 1	2660
	11th to 20th	Night	53% 6% 16% 18% 7% 1	2588
	21st to end	Night	51% 7% 15% 20% 7%	1975
March	1st to 10th	Night	48% 6% 15% 25% 7%	3736
	11th to 20th	Night	40% 8% 20% 27% 5%	3721
	21st to end	Night	41% 6% 15% 26% 12%	3500
April	1st to 10th	Night	45% 7% 12% 21% 14% 1	1881
	11th to 20th	Night	45% 7% 12% 18% 15% 2	1656
	21st to end	Night	52% 7% 10% 22% 8% 2	1520
May	1st to 10th	Night	39% 6% 11% 27% 14% 2	2321
	11th to 20th	Night	44% 7% 11% 24% 13% 1	2116
	21st to end	Night	40% 8% 15% 27% 9% 1	2066
June	1st to 10th	Night	45% 8% 12% 24% 9% 1	1140
	11th to 20th	Night	52% 9% 13% 20% 5%	1140
	21st to end	Night	38% 9% 17% 24% 11% 1	1140
July	1st to 10th	Night	37% 8% 16% 27% 11%	1870
	11th to 20th	Night	38% 8% 18% 22% 12% 2	1864
	21st to end	Night	34% 8% 24% 25% 9% 1	2026
August	1st to 10th	Night	34% 9% 18% 37% 3	2420
	11th to 20th	Night	35% 8% 17% 26% 13%	2521
	21st to end	Night	39% 9% 26% 21% 5%	2986
September	1st to 10th	Night	41% 9% 18% 25% 7% 1	1858
	11th to 20th	Night	41% 7% 19% 26% 7%	1966
	21st to end	Night	43% 7% 18% 22% 10% 1	2090
October	1st to 10th	Night	45% 7% 17% 22% 9%	3804
	11th to 20th	Night	39% 6% 19% 29% 7%	4086
	21st to end	Night	51% 7% 18% 20% 4	4507
November	1st to 10th	Night	56% 7% 13% 20% 4	2892
	11th to 20th	Night	57% 8% 14% 17% 3	3000
	21st to end	Night	54% 9% 16% 15% 4 1	3072
December	1st to 10th	Night	56% 7% 13% 15% 8% 1	3040
	11th to 20th	Night	57% 8% 12% 18% 5%	3747
	21st to end	Night	42% 7% 19% 23% 8% 1	5566

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	16% 4 22% 35% 18% 5%	180
	11th to 20th	Day	16% 8% 21% 32% 18% 5%	180
	21st to end	Day	12% 6% 26% 39% 13% 4	198
February	1st to 10th	Day	12% 5% 36% 27% 12% 7%	180
	11th to 20th	Day	11% 5% 33% 31% 14% 5%	180
	21st to end	Day	13% 11% 25% 29% 20% 3	323
March	1st to 10th	Day	7% 10% 27% 31% 17% 8%	460
	11th to 20th	Day	9% 10% 34% 27% 18% 3	460
	21st to end	Day	13% 11% 27% 29% 15% 4	506
April	1st to 10th	Day	12% 11% 30% 30% 14% 4	460
	11th to 20th	Day	12% 16% 25% 27% 15% 4	452
	21st to end	Day	18% 13% 26% 23% 15% 4	460
May	1st to 10th	Day	15% 13% 25% 26% 16% 5%	590
	11th to 20th	Day	15% 8% 20% 32% 20% 5%	717
	21st to end	Day	15% 7% 19% 30% 22% 7%	940
June	1st to 10th	Day	16% 7% 25% 30% 18% 4	870
	11th to 20th	Day	21% 6% 21% 27% 20% 5%	870
	21st to end	Day	10% 4 17% 30% 26% 11%	870
July	1st to 10th	Day	13% 5% 20% 29% 24% 8%	856
	11th to 20th	Day	11% 5% 18% 28% 27% 11%	852
	21st to end	Day	14% 4 16% 31% 26% 8%	933
August	1st to 10th	Day	17% 5% 15% 26% 26% 11%	548
	11th to 20th	Day	13% 7% 29% 32% 15% 5%	477
	21st to end	Day	17% 7% 27% 27% 15% 7%	539
September	1st to 10th	Day	18% 7% 27% 29% 14% 5%	490
	11th to 20th	Day	14% 6% 23% 30% 18% 8%	490
	21st to end	Day	14% 14% 29% 27% 12% 3	490
October	1st to 10th	Day	20% 11% 26% 29% 13% 2	465
	11th to 20th	Day	19% 11% 27% 27% 13% 3	300
	21st to end	Day	21% 9% 29% 25% 13% 5%	314
November	1st to 10th	Day	14% 11% 30% 24% 19% 3	200
	11th to 20th	Day	12% 6% 34% 32% 14% 3	200
	21st to end	Day	17% 7% 30% 33% 11% 2	199
December	1st to 10th	Day	8% 7% 33% 31% 17% 4	180
	11th to 20th	Day	13% 11% 27% 31% 16% 3	180
	21st to end	Day	8% 6% 25% 34% 22% 5%	198
January	1st to 10th	Night	13% 6% 25% 38% 15% 3	1040
	11th to 20th	Night	14% 6% 24% 38% 14% 2	1040
	21st to end	Night	15% 9% 27% 34% 13% 2	1025
February	1st to 10th	Night	13% 6% 20% 38% 20% 3	760
	11th to 20th	Night	13% 5% 24% 39% 17% 2	744
	21st to end	Night	8% 4 26% 35% 23% 4	628
March	1st to 10th	Night	9% 6% 22% 40% 21% 4	760
	11th to 20th	Night	11% 7% 22% 42% 17% 1	760
	21st to end	Night	12% 5% 20% 38% 23% 2	836
April	1st to 10th	Night	11% 5% 23% 36% 23% 1	741
	11th to 20th	Night	13% 7% 23% 33% 22% 2	528
	21st to end	Night	17% 7% 21% 35% 18% 1	380
May	1st to 10th	Night	14% 6% 18% 34% 24% 4	380
	11th to 20th	Night	11% 7% 22% 36% 24% 2	374
	21st to end	Night	15% 3 15% 39% 26% 2	411
June	1st to 10th	Night	15% 4 17% 38% 23% 2	380
	11th to 20th	Night	23% 3 11% 30% 31% 2	380
	21st to end	Night	11% 3 16% 36% 32% 4	380
July	1st to 10th	Night	14% 2 16% 30% 35% 2	370
	11th to 20th	Night	13% 4 10% 33% 36% 4	368
	21st to end	Night	12% 2 11% 35% 35% 5%	416
August	1st to 10th	Night	13% 4 10% 30% 39% 6%	378
	11th to 20th	Night	9% 3 14% 45% 26% 2	376
	21st to end	Night	17% 2 15% 38% 25% 2	532
September	1st to 10th	Night	17% 3 17% 33% 25% 3	718
	11th to 20th	Night	13% 5% 17% 38% 25% 2	760
	21st to end	Night	14% 7% 29% 33% 15% 2	760
October	1st to 10th	Night	17% 8% 23% 33% 17% 2	760
	11th to 20th	Night	15% 6% 27% 36% 15% 2	760
	21st to end	Night	20% 8% 22% 33% 15% 2	836
November	1st to 10th	Night	19% 8% 23% 33% 16% 2	942
	11th to 20th	Night	15% 8% 27% 38% 11% 2	1000
	21st to end	Night	13% 7% 29% 36% 13% 2	1037
December	1st to 10th	Night	16% 7% 25% 35% 14% 2	1040
	11th to 20th	Night	16% 8% 25% 33% 16% 2	1040
	21st to end	Night	12% 5% 25% 37% 17% 5%	1144

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