Prineville Reservoir State Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Prineville Reservoir State Park clear sky chart from 2005-11-06 to 2026-01-02.

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	7% 8% 3 6% 4 4 4 3 5% 6% 50%	136383
	Night	8% 10% 3 7% 4 4 4 3 5% 6% 46%	132289
Transparency	Day	34% 6% 10% 21% 19% 11%	131179
	Night	38% 6% 11% 22% 17% 7%	128488
Seeing	Day	12% 12% 25% 27% 18% 6%	16034
	Night	13% 7% 19% 37% 21% 2	25177

By Month

Percentage Cloud Cover SW (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	15% 19% 5% 11% 6% 5% 5% 4 5% 7% 19%	3647
	11th to 20th	Day	12% 13% 3 8% 5% 4 5% 3 4 5% 38%	3640
	21st to end	Day	9% 11% 3 7% 4 4 4 3 5% 7% 42%	4251
February	1st to 10th	Day	10% 14% 4 9% 5% 6% 5% 5% 6% 8% 28%	1800
	11th to 20th	Day	10% 14% 4 8% 6% 5% 6% 3 6% 7% 31%	1790
	21st to end	Day	9% 15% 5% 9% 7% 6% 5% 4 6% 8% 27%	1650
March	1st to 10th	Day	8% 12% 5% 9% 6% 5% 5% 4 6% 8% 32%	4518
	11th to 20th	Day	7% 16% 4 9% 5% 5% 4 3 5% 6% 36%	4973
	21st to end	Day	8% 15% 5% 12% 6% 6% 6% 4 6% 6% 27%	5784
April	1st to 10th	Day	10% 16% 5% 10% 5% 5% 5% 4 6% 5% 30%	2400
	11th to 20th	Day	9% 12% 4 8% 6% 5% 4 3 5% 6% 37%	2369
	21st to end	Day	9% 10% 2 7% 4 4 5% 4 6% 8% 41%	2446
May	1st to 10th	Day	10% 8% 2 4 3 3 4 3 5% 6% 51%	6320
	11th to 20th	Day	8% 8% 3 7% 4 4 3 3 4 6% 49%	6277
	21st to end	Day	7% 7% 2 5% 4 4 4 4 5% 7% 52%	6902
June	1st to 10th	Day	11% 9% 2 6% 4 4 5% 4 5% 7% 44%	2800
	11th to 20th	Day	9% 6% 2 5% 4 3 4 4 5% 7% 53%	2800
	21st to end	Day	4 4 2 3 3 2 3 4 4 5% 66%	2800
July	1st to 10th	Day	2 1 1 2 1 2 2 2 3 4 80%	6319
	11th to 20th	Day	1 1 1 2 2 2 2 2 3 3 83%	6320
	21st to end	Day	2 2 1 2 2 2 2 2 4 5% 77%	6880
August	1st to 10th	Day	3 3 1 4 3 3 3 3 4 7% 66%	6004
	11th to 20th	Day	4 2 1 2 2 2 2 2 3 4 75%	5692
	21st to end	Day	4 4 1 4 3 3 4 3 5% 5% 63%	5984
September	1st to 10th	Day	2 2 1 3 2 2 3 3 4 5% 73%	2400
	11th to 20th	Day	5% 3 2 5% 4 3 4 3 5% 7% 59%	2400
	21st to end	Day	7% 5% 2 5% 3 3 3 3 4 6% 59%	2154
October	1st to 10th	Day	7% 5% 2 4 3 3 3 3 3 5% 63%	3280
	11th to 20th	Day	7% 7% 2 6% 4 4 6% 3 6% 9% 46%	3261
	21st to end	Day	10% 10% 3 8% 5% 5% 5% 4 6% 6% 37%	3349
November	1st to 10th	Day	13% 12% 3 8% 5% 5% 4 4 6% 7% 34%	1854
	11th to 20th	Day	13% 11% 4 8% 5% 5% 6% 5% 7% 7% 29%	1739
	21st to end	Day	10% 13% 3 8% 5% 5% 4 4 6% 7% 35%	1680
December	1st to 10th	Day	8% 8% 2 6% 4 3 4 3 5% 6% 50%	2700
	11th to 20th	Day	11% 14% 5% 11% 7% 7% 5% 4 6% 7% 23%	3108
	21st to end	Day	12% 16% 5% 11% 7% 6% 6% 4 6% 7% 21%	4092
January	1st to 10th	Night	13% 20% 5% 12% 6% 5% 6% 3 6% 5% 19%	6320
	11th to 20th	Night	12% 14% 3 7% 4 4 4 3 4 5% 39%	6320
	21st to end	Night	10% 12% 3 8% 4 4 5% 4 4 6% 40%	6715
February	1st to 10th	Night	11% 16% 5% 9% 6% 4 5% 3 6% 6% 29%	2600
	11th to 20th	Night	8% 13% 5% 9% 5% 4 6% 4 5% 7% 33%	2496
	21st to end	Night	9% 15% 5% 9% 7% 5% 5% 4 5% 8% 29%	1980
March	1st to 10th	Night	6% 13% 4 8% 5% 5% 5% 5% 8% 9% 34%	5376
	11th to 20th	Night	7% 10% 3 8% 5% 5% 4 3 6% 7% 41%	5012
	21st to end	Night	8% 15% 4 10% 6% 6% 6% 5% 6% 6% 28%	5190
April	1st to 10th	Night	11% 16% 4 8% 5% 4 4 3 5% 7% 33%	2000
	11th to 20th	Night	10% 13% 4 7% 4 4 5% 4 6% 7% 38%	1979
	21st to end	Night	8% 14% 3 7% 4 4 4 4 5% 8% 39%	1949
May	1st to 10th	Night	8% 12% 3 5% 4 3 3 3 4 5% 51%	3640
	11th to 20th	Night	9% 9% 3 6% 4 3 3 2 4 6% 50%	3616
	21st to end	Night	10% 8% 1 4 3 3 2 2 5% 6% 56%	3972
June	1st to 10th	Night	11% 6% 2 5% 4 4 4 3 6% 6% 48%	1600
	11th to 20th	Night	8% 7% 2 5% 3 4 2 3 4 7% 56%	1600
	21st to end	Night	4 4 1 3 3 3 3 3 4 5% 68%	1600
July	1st to 10th	Night	2 2 1 3 2 2 2 2 3 4 77%	3640
	11th to 20th	Night	1 1 1 2 1 1 1 1 2 3 85%	3640
	21st to end	Night	3 3 1 3 2 2 3 2 3 3 75%	3970
August	1st to 10th	Night	4 3 2 5% 3 3 4 3 5% 7% 62%	3796
	11th to 20th	Night	1 2 1 3 2 2 2 2 3 5% 77%	4160
	21st to end	Night	4 4 1 3 3 2 3 2 4 5% 69%	5015
September	1st to 10th	Night	2 2 1 2 2 3 3 2 3 5% 74%	2000
	11th to 20th	Night	4 3 1 5% 3 3 2 4 5% 7% 62%	2000
	21st to end	Night	7% 6% 1 4 3 2 4 3 5% 5% 61%	2240
October	1st to 10th	Night	6% 6% 2 4 2 2 2 2 3 4 68%	3920
	11th to 20th	Night	9% 6% 2 5% 4 3 4 3 5% 8% 51%	3897
	21st to end	Night	9% 8% 3 7% 5% 5% 5% 5% 6% 7% 41%	4442
November	1st to 10th	Night	14% 13% 3 7% 5% 4 5% 4 4 6% 34%	2665
	11th to 20th	Night	15% 12% 4 8% 6% 5% 5% 4 6% 6% 30%	2877
	21st to end	Night	10% 13% 3 6% 4 4 4 4 5% 7% 39%	2940
December	1st to 10th	Night	7% 8% 2 6% 4 3 3 3 4 7% 52%	4700
	11th to 20th	Night	12% 16% 5% 10% 7% 6% 6% 4 6% 6% 22%	5316
	21st to end	Night	14% 15% 5% 10% 5% 5% 5% 4 5% 6% 27%	7106

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	61% 8% 11% 15% 5%	3647
	11th to 20th	Day	46% 6% 11% 21% 13% 2	3640
	21st to end	Day	41% 8% 11% 14% 19% 7%	4251
February	1st to 10th	Day	51% 8% 15% 18% 8%	1800
	11th to 20th	Day	49% 9% 14% 15% 9% 3	1799
	21st to end	Day	52% 10% 15% 13% 8% 2	1650
March	1st to 10th	Day	45% 8% 10% 21% 12% 4	4518
	11th to 20th	Day	47% 6% 11% 26% 10%	4973
	21st to end	Day	53% 10% 13% 18% 6%	5784
April	1st to 10th	Day	52% 7% 13% 17% 9% 1	2400
	11th to 20th	Day	46% 6% 12% 19% 14% 3	2372
	21st to end	Day	39% 7% 14% 24% 13% 2	2446
May	1st to 10th	Day	32% 6% 8% 27% 25% 3	6320
	11th to 20th	Day	33% 7% 9% 24% 23% 3	6280
	21st to end	Day	29% 6% 14% 34% 15% 1	6904
June	1st to 10th	Day	39% 7% 10% 29% 14% 1	2800
	11th to 20th	Day	30% 4 11% 29% 22% 4	2800
	21st to end	Day	20% 4 10% 26% 29% 11%	2800
July	1st to 10th	Day	10% 2 4 15% 28% 40%	6251
	11th to 20th	Day	8% 2 4 8% 22% 55%	6245
	21st to end	Day	11% 2 4 25% 35% 22%	6681
August	1st to 10th	Day	18% 3 5% 15% 33% 26%	4375
	11th to 20th	Day	14% 3 5% 25% 35% 18%	4115
	21st to end	Day	17% 2 7% 26% 26% 22%	4311
September	1st to 10th	Day	14% 3 5% 24% 32% 23%	2400
	11th to 20th	Day	25% 6% 9% 22% 25% 14%	2400
	21st to end	Day	27% 4 9% 25% 23% 12%	2154
October	1st to 10th	Day	25% 4 9% 19% 26% 17%	3280
	11th to 20th	Day	33% 5% 15% 25% 18% 4	3261
	21st to end	Day	44% 7% 9% 22% 17% 2	3349
November	1st to 10th	Day	48% 7% 13% 20% 11% 1	1854
	11th to 20th	Day	50% 11% 15% 15% 9%	1739
	21st to end	Day	46% 7% 15% 16% 14% 1	1680
December	1st to 10th	Day	35% 7% 10% 23% 23% 2	2700
	11th to 20th	Day	59% 8% 14% 14% 5%	3108
	21st to end	Day	59% 8% 12% 17% 4	4092
January	1st to 10th	Night	64% 8% 11% 12% 4	6320
	11th to 20th	Night	45% 6% 10% 24% 13% 2	6320
	21st to end	Night	44% 7% 10% 14% 18% 7%	6715
February	1st to 10th	Night	54% 8% 16% 14% 9%	2600
	11th to 20th	Night	47% 10% 14% 16% 9% 3	2498
	21st to end	Night	52% 9% 11% 19% 8% 1	1980
March	1st to 10th	Night	42% 7% 10% 25% 12% 4	5376
	11th to 20th	Night	40% 7% 13% 27% 13%	5012
	21st to end	Night	52% 12% 13% 19% 5%	5190
April	1st to 10th	Night	49% 7% 17% 18% 8% 1	2000
	11th to 20th	Night	43% 7% 15% 21% 13% 2	1979
	21st to end	Night	41% 8% 15% 23% 11% 1	1949
May	1st to 10th	Night	33% 7% 11% 25% 24% 1	3637
	11th to 20th	Night	34% 6% 13% 29% 16% 1	3616
	21st to end	Night	28% 6% 24% 32% 10%	3972
June	1st to 10th	Night	36% 7% 12% 30% 15% 1	1600
	11th to 20th	Night	29% 6% 14% 33% 16% 3	1600
	21st to end	Night	19% 4 12% 31% 27% 8%	1600
July	1st to 10th	Night	12% 3 5% 19% 28% 33%	3600
	11th to 20th	Night	8% 1 5% 13% 25% 48%	3592
	21st to end	Night	16% 2 3 27% 31% 21%	3857
August	1st to 10th	Night	18% 3 6% 18% 33% 21%	2753
	11th to 20th	Night	12% 2 6% 39% 30% 11%	3011
	21st to end	Night	14% 3 8% 29% 28% 19%	3608
September	1st to 10th	Night	14% 3 6% 24% 35% 19%	2000
	11th to 20th	Night	21% 5% 10% 25% 26% 13%	2000
	21st to end	Night	25% 4 9% 24% 23% 15%	2240
October	1st to 10th	Night	23% 3 7% 21% 32% 14%	3920
	11th to 20th	Night	32% 5% 18% 23% 19% 3	3897
	21st to end	Night	39% 7% 10% 26% 15% 2	4442
November	1st to 10th	Night	49% 6% 13% 17% 13% 2	2665
	11th to 20th	Night	52% 8% 13% 18% 8%	2877
	21st to end	Night	42% 8% 13% 18% 17% 2	2940
December	1st to 10th	Night	32% 7% 10% 24% 26% 1	4700
	11th to 20th	Night	60% 8% 12% 14% 6%	5316
	21st to end	Night	55% 9% 13% 16% 6% 1	7106

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% 16% 26% 19% 13%	202
	11th to 20th	Day	20% 13% 30% 22% 14% 3	200
	21st to end	Day	15% 10% 29% 27% 16% 2	324
February	1st to 10th	Day	20% 17% 23% 28% 13% 1	400
	11th to 20th	Day	16% 16% 33% 23% 11%	412
	21st to end	Day	16% 14% 29% 31% 9% 1	414
March	1st to 10th	Day	16% 15% 30% 29% 9% 1	500
	11th to 20th	Day	18% 15% 26% 25% 13% 2	500
	21st to end	Day	18% 19% 26% 27% 9% 1	550
April	1st to 10th	Day	17% 19% 25% 24% 11% 4	500
	11th to 20th	Day	13% 17% 30% 24% 12% 4	492
	21st to end	Day	12% 17% 28% 24% 15% 4	500
May	1st to 10th	Day	14% 16% 25% 24% 16% 4	503
	11th to 20th	Day	20% 14% 23% 19% 16% 8%	528
	21st to end	Day	13% 8% 25% 24% 20% 9%	573
June	1st to 10th	Day	9% 12% 24% 24% 21% 10%	530
	11th to 20th	Day	11% 16% 26% 25% 17% 5%	530
	21st to end	Day	5% 10% 22% 27% 25% 12%	530
July	1st to 10th	Day	3 6% 25% 33% 23% 10%	526
	11th to 20th	Day	2 6% 26% 28% 26% 11%	524
	21st to end	Day	3 6% 26% 28% 25% 13%	575
August	1st to 10th	Day	3 6% 24% 28% 27% 13%	527
	11th to 20th	Day	2 6% 20% 29% 28% 15%	517
	21st to end	Day	6% 7% 22% 26% 28% 12%	583
September	1st to 10th	Day	3 5% 20% 34% 28% 11%	530
	11th to 20th	Day	6% 8% 19% 30% 28% 8%	530
	21st to end	Day	11% 9% 20% 26% 27% 7%	530
October	1st to 10th	Day	14% 9% 21% 28% 21% 8%	530
	11th to 20th	Day	13% 12% 20% 29% 20% 6%	530
	21st to end	Day	16% 12% 26% 26% 16% 4	462
November	1st to 10th	Day	18% 13% 22% 33% 13% 1	330
	11th to 20th	Day	18% 14% 26% 28% 11% 2	250
	21st to end	Day	15% 13% 25% 36% 10% 1	220
December	1st to 10th	Day	17% 13% 29% 25% 10% 5%	220
	11th to 20th	Day	21% 17% 26% 28% 7%	220
	21st to end	Day	21% 16% 23% 28% 10% 2	242
January	1st to 10th	Night	18% 12% 26% 28% 14% 2	804
	11th to 20th	Night	16% 12% 24% 27% 19% 2	800
	21st to end	Night	17% 7% 20% 36% 17% 3	880
February	1st to 10th	Night	16% 10% 25% 34% 15% 1	800
	11th to 20th	Night	14% 8% 25% 36% 15% 2	784
	21st to end	Night	16% 11% 23% 37% 12% 1	660
March	1st to 10th	Night	14% 12% 26% 34% 13% 1	800
	11th to 20th	Night	16% 8% 22% 36% 17% 2	800
	21st to end	Night	16% 10% 24% 34% 14%	880
April	1st to 10th	Night	17% 10% 22% 31% 18% 2	800
	11th to 20th	Night	15% 9% 25% 32% 17% 3	791
	21st to end	Night	15% 9% 20% 37% 17% 1	749
May	1st to 10th	Night	11% 6% 20% 42% 20% 1	400
	11th to 20th	Night	14% 4 16% 39% 25% 2	394
	21st to end	Night	17% 4 14% 39% 24% 2	433
June	1st to 10th	Night	13% 5% 16% 43% 23% 1	400
	11th to 20th	Night	11% 4 23% 44% 17% 1	400
	21st to end	Night	5% 2 16% 49% 28% 1	400
July	1st to 10th	Night	2 2 12% 52% 32% 1	390
	11th to 20th	Night	1 1 13% 59% 24% 2	388
	21st to end	Night	1 13% 54% 30% 1	438
August	1st to 10th	Night	2 1 9% 49% 35% 4	485
	11th to 20th	Night	2 1 8% 40% 42% 7%	637
	21st to end	Night	3 2 14% 46% 32% 3	880
September	1st to 10th	Night	4 2 12% 44% 35% 4	800
	11th to 20th	Night	6% 3 12% 39% 33% 7%	800
	21st to end	Night	9% 4 13% 37% 33% 5%	800
October	1st to 10th	Night	12% 3 14% 40% 27% 4	800
	11th to 20th	Night	13% 4 16% 40% 24% 4	800
	21st to end	Night	15% 6% 19% 37% 18% 5%	880
November	1st to 10th	Night	18% 8% 20% 39% 14% 1	820
	11th to 20th	Night	19% 10% 22% 34% 14% 2	840
	21st to end	Night	16% 10% 19% 35% 18% 3	840
December	1st to 10th	Night	16% 8% 22% 32% 20% 2	840
	11th to 20th	Night	18% 10% 25% 33% 12% 1	840
	21st to end	Night	22% 14% 26% 27% 10%	924

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