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 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	8% 9% 3 9% 6% 5% 6% 5% 7% 8% 35%	134521
	Night	7% 11% 4 10% 6% 5% 6% 4 6% 8% 32%	134983
Transparency	Day	45% 7% 17% 23% 8% 1	134191
	Night	48% 7% 15% 21% 8% 1	134892
Seeing	Day	14% 8% 24% 29% 19% 6%	18334
	Night	14% 6% 23% 36% 19% 2	26539

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	6% 17% 7% 13% 7% 7% 6% 4 7% 7% 20%	3527
	11th to 20th	Day	4 10% 3 9% 7% 5% 5% 5% 8% 10% 35%	3522
	21st to end	Day	4 12% 7% 16% 7% 5% 5% 4 5% 6% 28%	4410
February	1st to 10th	Day	6% 11% 4 11% 8% 7% 6% 6% 8% 7% 26%	1680
	11th to 20th	Day	5% 10% 5% 11% 7% 6% 7% 6% 7% 7% 30%	1600
	21st to end	Day	4 12% 3 11% 9% 8% 7% 4 8% 8% 27%	1555
March	1st to 10th	Day	2 8% 5% 13% 8% 7% 7% 5% 7% 9% 31%	4518
	11th to 20th	Day	4 11% 5% 13% 7% 6% 5% 5% 6% 9% 29%	4665
	21st to end	Day	4 8% 4 11% 6% 5% 5% 5% 7% 8% 38%	5942
April	1st to 10th	Day	5% 10% 4 10% 6% 7% 6% 5% 6% 10% 32%	2420
	11th to 20th	Day	8% 11% 4 9% 6% 5% 6% 5% 7% 7% 33%	2603
	21st to end	Day	11% 12% 4 9% 8% 5% 5% 4 6% 7% 30%	2800
May	1st to 10th	Day	7% 7% 3 6% 5% 5% 6% 5% 7% 9% 41%	6636
	11th to 20th	Day	10% 9% 4 10% 7% 7% 6% 5% 6% 8% 29%	6844
	21st to end	Day	11% 6% 2 7% 5% 6% 5% 5% 7% 9% 37%	7907
June	1st to 10th	Day	13% 8% 3 7% 6% 5% 6% 4 7% 9% 33%	3200
	11th to 20th	Day	19% 8% 3 9% 6% 6% 6% 5% 7% 7% 23%	3200
	21st to end	Day	9% 6% 2 7% 6% 6% 6% 5% 7% 9% 36%	3200
July	1st to 10th	Day	8% 5% 3 8% 5% 5% 6% 6% 7% 10% 36%	7240
	11th to 20th	Day	5% 4 2 6% 5% 6% 6% 5% 7% 10% 43%	7240
	21st to end	Day	12% 6% 2 6% 5% 4 5% 4 6% 9% 42%	7837
August	1st to 10th	Day	9% 6% 2 6% 4 5% 5% 4 6% 9% 44%	4644
	11th to 20th	Day	9% 8% 3 8% 6% 5% 6% 4 6% 7% 37%	4470
	21st to end	Day	14% 6% 2 7% 5% 5% 5% 5% 7% 8% 37%	4824
September	1st to 10th	Day	11% 6% 2 6% 5% 5% 4 5% 6% 8% 40%	2441
	11th to 20th	Day	8% 9% 3 7% 5% 4 4 5% 7% 10% 39%	2330
	21st to end	Day	10% 6% 2 8% 5% 4 5% 4 6% 9% 41%	2200
October	1st to 10th	Day	11% 10% 3 7% 4 4 4 3 6% 7% 40%	3271
	11th to 20th	Day	10% 9% 3 8% 5% 5% 5% 4 6% 8% 38%	2932
	21st to end	Day	10% 15% 5% 11% 5% 4 4 4 6% 6% 30%	3145
November	1st to 10th	Day	10% 13% 3 10% 6% 4 6% 4 5% 9% 30%	1457
	11th to 20th	Day	8% 12% 5% 13% 9% 7% 5% 4 5% 6% 25%	1470
	21st to end	Day	6% 14% 6% 12% 6% 6% 4 4 6% 7% 30%	1391
December	1st to 10th	Day	9% 15% 5% 13% 9% 8% 7% 5% 5% 6% 18%	2020
	11th to 20th	Day	8% 15% 5% 11% 7% 5% 6% 5% 7% 8% 23%	2322
	21st to end	Day	3 8% 5% 15% 8% 6% 4 3 6% 7% 36%	3058
January	1st to 10th	Night	5% 16% 7% 16% 8% 7% 6% 4 5% 6% 20%	9366
	11th to 20th	Night	5% 9% 4 10% 6% 6% 6% 5% 7% 8% 35%	9376
	21st to end	Night	6% 15% 6% 14% 6% 5% 6% 4 5% 7% 25%	9815
February	1st to 10th	Night	6% 11% 6% 11% 7% 7% 7% 5% 6% 7% 28%	2940
	11th to 20th	Night	6% 11% 5% 11% 8% 6% 5% 4 7% 6% 31%	2796
	21st to end	Night	4 9% 4 13% 9% 7% 7% 5% 7% 7% 29%	2075
March	1st to 10th	Night	3 8% 4 11% 7% 6% 5% 5% 7% 9% 35%	5376
	11th to 20th	Night	3 11% 4 11% 7% 6% 7% 5% 7% 11% 28%	5321
	21st to end	Night	4 10% 4 9% 5% 5% 5% 4 5% 9% 41%	5018
April	1st to 10th	Night	6% 10% 3 8% 6% 6% 5% 5% 7% 7% 36%	1980
	11th to 20th	Night	7% 11% 4 9% 6% 6% 6% 4 7% 8% 32%	1744
	21st to end	Night	10% 11% 4 9% 6% 6% 5% 7% 6% 7% 29%	1600
May	1st to 10th	Night	7% 6% 3 7% 5% 5% 6% 4 6% 8% 42%	3323
	11th to 20th	Night	9% 8% 3 8% 5% 6% 7% 5% 8% 9% 31%	3034
	21st to end	Night	9% 7% 3 7% 5% 5% 6% 5% 8% 9% 37%	2968
June	1st to 10th	Night	12% 8% 3 8% 5% 5% 6% 4 7% 8% 34%	1200
	11th to 20th	Night	19% 7% 4 6% 6% 6% 5% 4 6% 9% 27%	1200
	21st to end	Night	9% 5% 2 7% 5% 5% 5% 7% 8% 11% 37%	1200
July	1st to 10th	Night	14% 8% 3 8% 5% 4 5% 5% 7% 7% 34%	2720
	11th to 20th	Night	7% 6% 3 7% 5% 4 5% 5% 7% 8% 43%	2720
	21st to end	Night	11% 4 1 4 4 3 5% 5% 8% 11% 44%	3006
August	1st to 10th	Night	9% 6% 2 6% 4 4 5% 4 6% 8% 46%	2504
	11th to 20th	Night	7% 6% 3 7% 5% 5% 6% 4 6% 8% 44%	2577
	21st to end	Night	11% 6% 2 6% 5% 4 5% 4 7% 9% 40%	3080
September	1st to 10th	Night	11% 7% 3 7% 4 5% 5% 4 6% 9% 38%	1956
	11th to 20th	Night	8% 8% 2 8% 5% 6% 6% 4 6% 7% 40%	2070
	21st to end	Night	9% 8% 3 8% 5% 5% 5% 5% 5% 8% 37%	2200
October	1st to 10th	Night	9% 9% 3 8% 5% 5% 5% 5% 7% 8% 35%	3924
	11th to 20th	Night	8% 8% 3 7% 5% 5% 5% 4 7% 8% 40%	4216
	21st to end	Night	8% 12% 5% 11% 6% 6% 6% 4 6% 7% 31%	4652
November	1st to 10th	Night	10% 16% 5% 11% 6% 5% 4 4 5% 6% 28%	3040
	11th to 20th	Night	9% 14% 4 12% 7% 6% 5% 5% 5% 7% 26%	3150
	21st to end	Night	6% 12% 5% 12% 7% 7% 5% 4 7% 6% 28%	3226
December	1st to 10th	Night	9% 15% 5% 11% 7% 6% 5% 4 6% 8% 23%	5380
	11th to 20th	Night	8% 17% 6% 13% 7% 5% 5% 5% 7% 7% 21%	6090
	21st to end	Night	3 10% 4 14% 8% 6% 5% 4 6% 7% 33%	8140

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	62% 5% 11% 15% 6%	3527
	11th to 20th	Day	42% 5% 18% 19% 14% 1	3540
	21st to end	Day	56% 7% 16% 15% 5%	4411
February	1st to 10th	Day	52% 8% 13% 16% 10% 1	1680
	11th to 20th	Day	50% 8% 18% 18% 6%	1608
	21st to end	Day	53% 7% 15% 18% 7%	1555
March	1st to 10th	Day	48% 6% 14% 24% 8%	4518
	11th to 20th	Day	51% 7% 17% 19% 6%	4665
	21st to end	Day	41% 6% 13% 22% 17% 1	5942
April	1st to 10th	Day	46% 7% 16% 20% 12%	2420
	11th to 20th	Day	46% 7% 14% 20% 11% 2	2603
	21st to end	Day	52% 6% 13% 21% 7% 1	2800
May	1st to 10th	Day	38% 5% 13% 26% 17% 2	6636
	11th to 20th	Day	49% 6% 13% 22% 9% 1	6848
	21st to end	Day	41% 7% 17% 22% 11% 2	7908
June	1st to 10th	Day	46% 8% 14% 24% 8% 1	3200
	11th to 20th	Day	57% 9% 13% 16% 4	3200
	21st to end	Day	41% 10% 17% 25% 8%	3200
July	1st to 10th	Day	39% 8% 23% 24% 6%	7161
	11th to 20th	Day	34% 8% 17% 33% 8%	7153
	21st to end	Day	38% 7% 19% 29% 6%	7616
August	1st to 10th	Day	36% 7% 19% 34% 3	4626
	11th to 20th	Day	43% 8% 17% 22% 9%	4512
	21st to end	Day	43% 7% 26% 20% 4	4823
September	1st to 10th	Day	39% 8% 23% 24% 5%	2441
	11th to 20th	Day	39% 8% 22% 26% 5%	2330
	21st to end	Day	39% 9% 19% 24% 9%	2200
October	1st to 10th	Day	43% 6% 21% 24% 7%	3271
	11th to 20th	Day	43% 5% 19% 27% 5%	2932
	21st to end	Day	53% 7% 16% 21% 3	3145
November	1st to 10th	Day	51% 8% 14% 23% 5%	1457
	11th to 20th	Day	59% 7% 15% 16% 3	1470
	21st to end	Day	54% 6% 16% 19% 5% 2	1391
December	1st to 10th	Day	65% 8% 12% 10% 5%	2020
	11th to 20th	Day	57% 7% 16% 17% 3	2324
	21st to end	Day	47% 5% 16% 25% 6% 2	3058
January	1st to 10th	Night	64% 5% 10% 13% 7%	9367
	11th to 20th	Night	45% 5% 16% 18% 14% 2	9420
	21st to end	Night	56% 8% 17% 15% 3	9815
February	1st to 10th	Night	54% 6% 12% 19% 8% 1	2940
	11th to 20th	Night	51% 6% 15% 18% 8% 1	2798
	21st to end	Night	51% 7% 15% 20% 7%	2075
March	1st to 10th	Night	43% 5% 15% 27% 9%	5376
	11th to 20th	Night	48% 7% 17% 24% 4	5322
	21st to end	Night	38% 5% 13% 28% 15% 2	5018
April	1st to 10th	Night	44% 6% 12% 22% 15% 2	1980
	11th to 20th	Night	46% 7% 12% 18% 14% 2	1744
	21st to end	Night	51% 7% 10% 23% 8% 2	1600
May	1st to 10th	Night	39% 5% 13% 25% 16% 2	3320
	11th to 20th	Night	44% 7% 13% 24% 11% 1	3034
	21st to end	Night	39% 7% 19% 26% 9% 1	2968
June	1st to 10th	Night	45% 8% 13% 24% 10% 1	1200
	11th to 20th	Night	53% 9% 13% 20% 5%	1200
	21st to end	Night	37% 9% 18% 24% 11% 1	1200
July	1st to 10th	Night	45% 8% 17% 21% 9%	2690
	11th to 20th	Night	36% 7% 15% 31% 10% 1	2684
	21st to end	Night	32% 7% 22% 30% 8%	2922
August	1st to 10th	Night	34% 9% 18% 36% 3	2497
	11th to 20th	Night	36% 8% 17% 26% 13%	2601
	21st to end	Night	38% 9% 25% 22% 6%	3080
September	1st to 10th	Night	40% 9% 19% 25% 7% 1	1956
	11th to 20th	Night	41% 7% 18% 26% 7%	2070
	21st to end	Night	43% 7% 17% 21% 10% 1	2200
October	1st to 10th	Night	45% 7% 17% 22% 9%	3924
	11th to 20th	Night	40% 6% 19% 28% 7%	4216
	21st to end	Night	51% 7% 17% 21% 4	4652
November	1st to 10th	Night	57% 6% 13% 19% 5%	3040
	11th to 20th	Night	56% 8% 13% 18% 5%	3150
	21st to end	Night	53% 8% 16% 16% 6% 1	3226
December	1st to 10th	Night	58% 7% 14% 15% 5% 1	5380
	11th to 20th	Night	60% 7% 14% 14% 4	6087
	21st to end	Night	49% 6% 17% 21% 6% 1	8140

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	17% 5% 23% 32% 17% 5%	220
	11th to 20th	Day	18% 8% 24% 31% 15% 4	220
	21st to end	Day	12% 8% 29% 37% 12% 3	242
February	1st to 10th	Day	11% 6% 36% 28% 12% 7%	220
	11th to 20th	Day	10% 5% 35% 32% 14% 4	210
	21st to end	Day	12% 11% 27% 28% 19% 3	351
March	1st to 10th	Day	8% 11% 27% 31% 16% 8%	500
	11th to 20th	Day	9% 10% 34% 26% 18% 3	500
	21st to end	Day	12% 12% 27% 29% 15% 5%	550
April	1st to 10th	Day	11% 11% 30% 30% 14% 3	500
	11th to 20th	Day	13% 16% 26% 26% 15% 4	492
	21st to end	Day	18% 13% 26% 24% 16% 4	500
May	1st to 10th	Day	14% 13% 26% 26% 16% 5%	637
	11th to 20th	Day	15% 8% 20% 31% 20% 5%	769
	21st to end	Day	15% 8% 19% 30% 21% 7%	1006
June	1st to 10th	Day	16% 8% 25% 29% 18% 4	930
	11th to 20th	Day	22% 6% 20% 26% 20% 6%	930
	21st to end	Day	10% 4 18% 30% 26% 12%	930
July	1st to 10th	Day	13% 5% 21% 29% 24% 8%	916
	11th to 20th	Day	11% 5% 18% 28% 27% 11%	912
	21st to end	Day	14% 4 16% 31% 27% 8%	998
August	1st to 10th	Day	17% 5% 15% 27% 26% 10%	591
	11th to 20th	Day	13% 7% 29% 32% 15% 4	517
	21st to end	Day	16% 7% 26% 27% 16% 8%	583
September	1st to 10th	Day	17% 7% 26% 30% 16% 5%	530
	11th to 20th	Day	13% 6% 24% 30% 19% 8%	530
	21st to end	Day	15% 16% 29% 26% 12% 3	530
October	1st to 10th	Day	19% 11% 27% 29% 12% 2	503
	11th to 20th	Day	19% 12% 27% 27% 12% 2	330
	21st to end	Day	20% 10% 29% 24% 12% 4	345
November	1st to 10th	Day	15% 13% 28% 23% 17% 3	220
	11th to 20th	Day	13% 6% 33% 32% 14% 2	220
	21st to end	Day	15% 7% 30% 33% 12% 3	220
December	1st to 10th	Day	10% 8% 33% 32% 15% 3	220
	11th to 20th	Day	15% 9% 28% 32% 15% 2	220
	21st to end	Day	8% 6% 27% 34% 21% 5%	242
January	1st to 10th	Night	13% 6% 25% 37% 16% 3	1160
	11th to 20th	Night	14% 8% 26% 37% 13% 2	1160
	21st to end	Night	15% 10% 27% 34% 12% 2	1139
February	1st to 10th	Night	13% 6% 20% 38% 20% 3	840
	11th to 20th	Night	12% 5% 25% 40% 17% 2	804
	21st to end	Night	8% 4 27% 35% 22% 4	660
March	1st to 10th	Night	9% 6% 22% 39% 20% 4	800
	11th to 20th	Night	11% 7% 21% 42% 18% 1	800
	21st to end	Night	11% 5% 21% 38% 23% 2	880
April	1st to 10th	Night	11% 5% 23% 37% 22% 1	780
	11th to 20th	Night	13% 7% 24% 32% 21% 2	556
	21st to end	Night	17% 7% 21% 35% 19% 1	400
May	1st to 10th	Night	13% 6% 20% 35% 23% 4	400
	11th to 20th	Night	11% 7% 21% 35% 24% 2	394
	21st to end	Night	15% 3 16% 39% 25% 2	433
June	1st to 10th	Night	15% 5% 18% 38% 22% 2	400
	11th to 20th	Night	23% 3 11% 30% 31% 2	400
	21st to end	Night	10% 3 16% 36% 32% 4	400
July	1st to 10th	Night	15% 2 16% 30% 35% 2	390
	11th to 20th	Night	13% 4 10% 33% 36% 4	388
	21st to end	Night	12% 3 11% 34% 36% 5%	438
August	1st to 10th	Night	13% 4 10% 31% 37% 5%	398
	11th to 20th	Night	9% 4 14% 45% 26% 2	396
	21st to end	Night	16% 3 15% 39% 25% 3	560
September	1st to 10th	Night	16% 3 17% 34% 26% 3	756
	11th to 20th	Night	13% 5% 17% 39% 24% 2	800
	21st to end	Night	14% 8% 30% 32% 14% 2	800
October	1st to 10th	Night	17% 8% 24% 33% 17% 2	800
	11th to 20th	Night	15% 6% 27% 36% 14% 2	800
	21st to end	Night	20% 9% 23% 33% 14% 2	880
November	1st to 10th	Night	19% 8% 23% 32% 15% 2	990
	11th to 20th	Night	15% 8% 28% 38% 11% 2	1050
	21st to end	Night	13% 8% 28% 36% 13% 2	1091
December	1st to 10th	Night	16% 8% 27% 34% 13% 2	1160
	11th to 20th	Night	16% 8% 26% 34% 16% 2	1160
	21st to end	Night	12% 6% 25% 36% 17% 5%	1276

Page generated at 2026-02-12 23:42:14. Version ac07

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