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 2024-01-15.

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	10% 9% 4 8% 7% 4 6% 6% 6% 8% 33%	74571
	Night	8% 10% 4 9% 7% 4 6% 6% 6% 8% 32%	75207
Transparency	Day	47% 8% 17% 21% 6% 1	74228
	Night	49% 8% 16% 20% 7% 1	75068
Seeing	Day	14% 8% 24% 29% 19% 6%	15523
	Night	14% 6% 22% 36% 20% 3	23716

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% 13% 7% 11% 9% 5% 7% 5% 7% 7% 23%	1139
	11th to 20th	Day	8% 14% 5% 10% 10% 5% 5% 5% 6% 8% 24%	1104
	21st to end	Day	5% 13% 7% 12% 9% 4 7% 6% 6% 6% 26%	1349
February	1st to 10th	Day	7% 10% 5% 10% 9% 7% 6% 7% 8% 8% 25%	1440
	11th to 20th	Day	6% 11% 6% 10% 9% 5% 7% 7% 7% 6% 26%	1400
	21st to end	Day	5% 12% 4 10% 10% 5% 8% 6% 7% 8% 26%	1395
March	1st to 10th	Day	4 10% 6% 9% 8% 5% 8% 7% 7% 9% 28%	1800
	11th to 20th	Day	7% 11% 4 10% 8% 4 6% 6% 5% 8% 30%	1840
	21st to end	Day	7% 13% 5% 9% 8% 4 6% 6% 6% 7% 28%	2362
April	1st to 10th	Day	5% 8% 5% 9% 8% 5% 7% 7% 6% 10% 30%	2178
	11th to 20th	Day	8% 11% 4 8% 6% 3 7% 6% 7% 7% 32%	2339
	21st to end	Day	11% 11% 5% 8% 8% 5% 5% 5% 6% 7% 29%	2520
May	1st to 10th	Day	10% 10% 4 6% 7% 4 6% 5% 5% 8% 35%	2640
	11th to 20th	Day	10% 6% 3 6% 6% 4 6% 5% 7% 8% 38%	2694
	21st to end	Day	13% 5% 3 6% 6% 4 5% 5% 7% 8% 37%	3111
June	1st to 10th	Day	13% 7% 3 6% 6% 3 6% 5% 6% 9% 34%	2880
	11th to 20th	Day	19% 8% 4 8% 7% 4 7% 6% 6% 7% 23%	2880
	21st to end	Day	10% 6% 2 6% 7% 5% 6% 7% 7% 9% 35%	2880
July	1st to 10th	Day	13% 5% 3 6% 6% 4 6% 6% 7% 9% 35%	2880
	11th to 20th	Day	10% 6% 4 7% 6% 4 7% 6% 7% 9% 35%	2880
	21st to end	Day	12% 6% 3 6% 6% 3 6% 6% 6% 9% 38%	3122
August	1st to 10th	Day	12% 7% 2 5% 5% 4 6% 6% 6% 10% 36%	2565
	11th to 20th	Day	9% 7% 3 7% 7% 4 6% 5% 7% 8% 39%	2475
	21st to end	Day	13% 6% 3 6% 7% 4 6% 6% 8% 8% 34%	2664
September	1st to 10th	Day	12% 7% 2 5% 6% 3 5% 6% 6% 8% 39%	2197
	11th to 20th	Day	9% 9% 3 5% 5% 3 5% 6% 7% 10% 39%	2098
	21st to end	Day	10% 5% 2 6% 5% 3 6% 5% 6% 9% 41%	1980
October	1st to 10th	Day	13% 9% 4 7% 6% 3 5% 5% 5% 6% 36%	1807
	11th to 20th	Day	11% 11% 3 8% 7% 4 5% 5% 6% 8% 32%	1620
	21st to end	Day	12% 15% 5% 10% 6% 3 4 5% 6% 6% 28%	1755
November	1st to 10th	Day	11% 11% 5% 9% 7% 4 5% 5% 6% 8% 30%	1316
	11th to 20th	Day	8% 10% 6% 11% 12% 5% 6% 5% 5% 7% 25%	1330
	21st to end	Day	6% 13% 7% 10% 8% 5% 4 4 6% 7% 31%	1259
December	1st to 10th	Day	7% 14% 5% 10% 9% 4 6% 4 4 8% 28%	1140
	11th to 20th	Day	8% 15% 3 8% 9% 4 7% 6% 6% 8% 24%	1442
	21st to end	Day	3 8% 4 10% 8% 4 5% 5% 7% 8% 39%	2090
January	1st to 10th	Night	6% 13% 6% 12% 10% 5% 6% 5% 7% 7% 23%	3026
	11th to 20th	Night	8% 12% 6% 11% 8% 5% 6% 5% 7% 8% 25%	2945
	21st to end	Night	8% 13% 6% 12% 9% 5% 7% 5% 5% 7% 24%	3024
February	1st to 10th	Night	6% 10% 7% 10% 8% 5% 6% 6% 7% 7% 28%	2520
	11th to 20th	Night	7% 11% 6% 11% 9% 5% 6% 5% 6% 7% 28%	2446
	21st to end	Night	4 8% 4 11% 9% 5% 7% 7% 6% 7% 30%	1861
March	1st to 10th	Night	5% 9% 4 8% 8% 4 6% 6% 7% 8% 35%	2160
	11th to 20th	Night	7% 13% 5% 9% 7% 4 5% 6% 6% 8% 32%	2119
	21st to end	Night	4 13% 5% 8% 7% 3 6% 6% 6% 8% 32%	1992
April	1st to 10th	Night	6% 10% 4 8% 7% 4 6% 7% 6% 7% 35%	1782
	11th to 20th	Night	7% 10% 4 8% 7% 4 6% 6% 7% 9% 33%	1568
	21st to end	Night	10% 11% 4 8% 7% 5% 5% 8% 6% 7% 28%	1440
May	1st to 10th	Night	9% 6% 3 7% 7% 4 8% 8% 6% 9% 32%	1319
	11th to 20th	Night	7% 6% 3 7% 4 3 6% 6% 8% 9% 41%	1198
	21st to end	Night	14% 7% 3 5% 5% 3 5% 5% 6% 8% 38%	1164
June	1st to 10th	Night	12% 7% 3 8% 6% 4 5% 5% 7% 8% 35%	1080
	11th to 20th	Night	19% 6% 4 5% 8% 5% 5% 5% 6% 10% 27%	1080
	21st to end	Night	9% 5% 2 5% 6% 4 6% 7% 8% 10% 37%	1080
July	1st to 10th	Night	11% 7% 3 6% 5% 3 6% 5% 6% 8% 41%	1080
	11th to 20th	Night	10% 6% 3 6% 5% 3 5% 8% 8% 8% 39%	1080
	21st to end	Night	11% 4 2 4 5% 3 5% 6% 8% 11% 41%	1195
August	1st to 10th	Night	11% 5% 2 5% 6% 3 5% 5% 7% 10% 42%	1372
	11th to 20th	Night	6% 4 2 5% 6% 4 6% 6% 7% 10% 44%	1424
	21st to end	Night	13% 7% 3 5% 6% 3 4 5% 8% 9% 39%	1692
September	1st to 10th	Night	12% 8% 3 7% 5% 4 5% 4 6% 8% 38%	1760
	11th to 20th	Night	9% 7% 3 6% 7% 4 6% 5% 6% 7% 40%	1862
	21st to end	Night	10% 7% 3 7% 6% 4 5% 6% 5% 8% 39%	1980
October	1st to 10th	Night	12% 10% 4 8% 6% 4 5% 5% 5% 6% 36%	2151
	11th to 20th	Night	10% 9% 3 6% 7% 3 5% 6% 6% 9% 35%	2340
	21st to end	Night	11% 13% 5% 9% 7% 3 6% 5% 6% 6% 30%	2600
November	1st to 10th	Night	11% 15% 5% 10% 7% 3 5% 4 5% 6% 28%	2743
	11th to 20th	Night	9% 13% 5% 11% 8% 4 6% 6% 5% 7% 26%	2850
	21st to end	Night	7% 11% 6% 11% 8% 5% 6% 6% 7% 6% 28%	2918
December	1st to 10th	Night	8% 13% 5% 9% 9% 5% 6% 5% 6% 6% 28%	3040
	11th to 20th	Night	8% 15% 6% 9% 8% 4 6% 5% 5% 7% 26%	3750
	21st to end	Night	3 8% 4 8% 8% 5% 6% 6% 6% 8% 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	58% 9% 14% 14% 6%	1139
	11th to 20th	Day	58% 7% 12% 17% 5% 2	1104
	21st to end	Day	57% 8% 14% 16% 5%	1349
February	1st to 10th	Day	52% 9% 13% 17% 9%	1440
	11th to 20th	Day	55% 8% 18% 16% 3	1408
	21st to end	Day	53% 7% 14% 18% 7%	1395
March	1st to 10th	Day	50% 9% 17% 18% 6%	1800
	11th to 20th	Day	51% 8% 15% 21% 6%	1840
	21st to end	Day	53% 8% 16% 18% 5%	2362
April	1st to 10th	Day	47% 7% 16% 20% 10%	2178
	11th to 20th	Day	47% 7% 14% 21% 8% 3	2339
	21st to end	Day	53% 6% 12% 21% 7% 1	2520
May	1st to 10th	Day	46% 6% 13% 22% 12% 1	2640
	11th to 20th	Day	41% 7% 11% 25% 15% 2	2698
	21st to end	Day	43% 7% 13% 22% 13% 1	3112
June	1st to 10th	Day	45% 8% 14% 24% 8% 1	2880
	11th to 20th	Day	58% 9% 14% 15% 4	2880
	21st to end	Day	42% 10% 17% 24% 7%	2880
July	1st to 10th	Day	43% 10% 17% 24% 5%	2801
	11th to 20th	Day	44% 12% 24% 18% 2	2793
	21st to end	Day	40% 10% 24% 22% 4	2948
August	1st to 10th	Day	43% 10% 25% 20% 2	2547
	11th to 20th	Day	41% 9% 21% 24% 5%	2475
	21st to end	Day	44% 10% 19% 22% 5%	2664
September	1st to 10th	Day	41% 9% 23% 23% 4	2197
	11th to 20th	Day	40% 8% 23% 25% 4	2098
	21st to end	Day	38% 9% 20% 25% 8%	1980
October	1st to 10th	Day	47% 7% 18% 21% 7%	1807
	11th to 20th	Day	49% 8% 17% 21% 5%	1620
	21st to end	Day	56% 9% 15% 16% 5%	1755
November	1st to 10th	Day	52% 8% 13% 24% 3	1316
	11th to 20th	Day	58% 8% 16% 15% 3	1330
	21st to end	Day	53% 6% 16% 18% 4 2	1259
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	58% 8% 14% 12% 7% 1	3026
	11th to 20th	Night	57% 8% 16% 13% 5% 1	2945
	21st to end	Night	60% 8% 12% 12% 7% 1	3024
February	1st to 10th	Night	53% 7% 13% 19% 7% 1	2520
	11th to 20th	Night	55% 6% 16% 17% 6%	2448
	21st to end	Night	50% 8% 15% 20% 8%	1861
March	1st to 10th	Night	43% 6% 17% 24% 8%	2160
	11th to 20th	Night	50% 7% 12% 25% 7%	2120
	21st to end	Night	47% 8% 15% 23% 7%	1992
April	1st to 10th	Night	45% 7% 12% 21% 14% 1	1782
	11th to 20th	Night	45% 8% 13% 19% 13% 2	1568
	21st to end	Night	51% 7% 10% 22% 8% 1	1440
May	1st to 10th	Night	44% 8% 10% 24% 11% 2	1319
	11th to 20th	Night	37% 8% 10% 25% 18% 3	1198
	21st to end	Night	42% 6% 13% 24% 13% 1	1164
June	1st to 10th	Night	45% 8% 12% 25% 9% 1	1080
	11th to 20th	Night	52% 9% 14% 19% 5%	1080
	21st to end	Night	38% 9% 18% 25% 9% 1	1080
July	1st to 10th	Night	39% 9% 18% 27% 6%	1050
	11th to 20th	Night	38% 11% 27% 21% 3	1044
	21st to end	Night	34% 11% 28% 22% 4 1	1129
August	1st to 10th	Night	35% 12% 25% 24% 4	1365
	11th to 20th	Night	33% 10% 24% 27% 6%	1424
	21st to end	Night	39% 11% 18% 23% 8%	1692
September	1st to 10th	Night	42% 9% 18% 24% 7% 1	1760
	11th to 20th	Night	40% 8% 19% 26% 7%	1862
	21st to end	Night	42% 7% 18% 22% 10% 1	1980
October	1st to 10th	Night	48% 7% 15% 22% 8% 1	2151
	11th to 20th	Night	44% 9% 17% 24% 6%	2340
	21st to end	Night	53% 8% 17% 16% 6%	2600
November	1st to 10th	Night	57% 7% 13% 20% 4	2743
	11th to 20th	Night	57% 8% 14% 17% 4	2850
	21st to end	Night	53% 9% 17% 16% 5% 1	2918
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	17% 8% 21% 32% 18% 5%	168
	21st to end	Day	13% 5% 24% 41% 15% 3	176
February	1st to 10th	Day	13% 6% 35% 28% 11% 8%	160
	11th to 20th	Day	13% 6% 36% 29% 12% 6%	160
	21st to end	Day	12% 10% 25% 29% 20% 3	293
March	1st to 10th	Day	8% 10% 26% 31% 17% 9%	420
	11th to 20th	Day	9% 9% 34% 26% 19% 3	420
	21st to end	Day	14% 11% 28% 27% 15% 4	462
April	1st to 10th	Day	10% 11% 30% 31% 15% 4	420
	11th to 20th	Day	13% 17% 25% 26% 16% 4	412
	21st to end	Day	18% 14% 27% 22% 15% 4	420
May	1st to 10th	Day	15% 13% 25% 27% 16% 5%	543
	11th to 20th	Day	14% 8% 19% 32% 21% 6%	664
	21st to end	Day	14% 8% 18% 31% 22% 7%	874
June	1st to 10th	Day	15% 7% 24% 30% 19% 4	810
	11th to 20th	Day	21% 6% 20% 27% 20% 5%	810
	21st to end	Day	10% 4 18% 30% 26% 12%	810
July	1st to 10th	Day	14% 5% 21% 28% 24% 9%	796
	11th to 20th	Day	11% 5% 18% 28% 26% 11%	792
	21st to end	Day	14% 3 16% 32% 27% 8%	868
August	1st to 10th	Day	18% 4 15% 26% 25% 11%	505
	11th to 20th	Day	13% 7% 29% 31% 15% 5%	437
	21st to end	Day	18% 7% 25% 28% 16% 7%	495
September	1st to 10th	Day	19% 6% 27% 29% 15% 5%	450
	11th to 20th	Day	15% 6% 24% 30% 17% 7%	450
	21st to end	Day	13% 13% 29% 29% 14% 3	450
October	1st to 10th	Day	20% 10% 25% 30% 13% 2	427
	11th to 20th	Day	20% 10% 26% 28% 14% 3	270
	21st to end	Day	22% 9% 27% 24% 13% 5%	283
November	1st to 10th	Day	13% 12% 29% 26% 17% 3	180
	11th to 20th	Day	12% 5% 34% 33% 14% 3	180
	21st to end	Day	18% 7% 32% 32% 9% 2	180
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	15% 7% 24% 38% 14% 2	1005
	21st to end	Night	15% 9% 26% 34% 13% 2	966
February	1st to 10th	Night	12% 5% 20% 39% 20% 3	720
	11th to 20th	Night	13% 5% 24% 39% 17% 2	704
	21st to end	Night	8% 4 25% 36% 24% 4	592
March	1st to 10th	Night	9% 5% 21% 40% 21% 4	720
	11th to 20th	Night	11% 6% 22% 42% 17% 1	720
	21st to end	Night	12% 5% 20% 37% 24% 2	792
April	1st to 10th	Night	11% 5% 22% 37% 24% 2	702
	11th to 20th	Night	12% 7% 23% 33% 23% 2	500
	21st to end	Night	18% 8% 21% 34% 18% 1	360
May	1st to 10th	Night	13% 6% 19% 34% 24% 4	360
	11th to 20th	Night	10% 7% 21% 36% 25% 1	354
	21st to end	Night	15% 3 15% 39% 26% 2	389
June	1st to 10th	Night	16% 4 17% 37% 24% 3	360
	11th to 20th	Night	23% 3 11% 29% 32% 2	360
	21st to end	Night	11% 3 16% 36% 31% 4	360
July	1st to 10th	Night	14% 2 17% 30% 35% 2	350
	11th to 20th	Night	13% 4 9% 34% 36% 5%	348
	21st to end	Night	12% 3 10% 35% 36% 5%	394
August	1st to 10th	Night	13% 3 9% 30% 39% 6%	358
	11th to 20th	Night	9% 3 14% 45% 27% 3	356
	21st to end	Night	17% 2 15% 37% 26% 2	504
September	1st to 10th	Night	18% 3 16% 33% 26% 4	680
	11th to 20th	Night	13% 5% 17% 38% 24% 2	720
	21st to end	Night	13% 7% 28% 33% 16% 2	720
October	1st to 10th	Night	18% 7% 22% 34% 18% 2	720
	11th to 20th	Night	15% 6% 25% 36% 16% 3	720
	21st to end	Night	21% 8% 22% 33% 15% 2	792
November	1st to 10th	Night	19% 7% 23% 33% 16% 2	893
	11th to 20th	Night	15% 8% 27% 38% 11% 2	950
	21st to end	Night	14% 8% 29% 36% 13% 2	983
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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