Prettyboy Reservoir
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Prettyboy Reservoir 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	15% 6% 2 6% 4 4 5% 4 6% 8% 40%	131005
	Night	15% 7% 2 6% 4 4 4 4 5% 7% 43%	129843
Transparency	Day	41% 6% 11% 23% 16% 3	134598
	Night	40% 5% 11% 24% 17% 3	134321
Seeing	Day	21% 15% 29% 21% 11% 3	16509
	Night	19% 14% 30% 26% 11% 1	26050

By Month

Percentage Cloud Cover SE (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	13% 6% 2 6% 4 5% 4 4 5% 5% 45%	4661
	11th to 20th	Day	14% 8% 3 8% 5% 4 3 3 4 4 44%	5202
	21st to end	Day	14% 6% 2 6% 4 5% 5% 4 6% 7% 40%	5780
February	1st to 10th	Day	13% 9% 3 7% 4 5% 4 4 7% 7% 38%	1890
	11th to 20th	Day	16% 8% 2 6% 4 5% 4 4 5% 6% 39%	1868
	21st to end	Day	18% 10% 3 6% 4 4 3 3 4 6% 39%	1650
March	1st to 10th	Day	19% 5% 2 5% 3 3 4 4 5% 6% 45%	4750
	11th to 20th	Day	11% 7% 2 6% 4 4 4 4 6% 7% 46%	4980
	21st to end	Day	16% 7% 2 6% 5% 4 4 4 5% 6% 40%	5620
April	1st to 10th	Day	16% 8% 2 5% 5% 5% 4 5% 6% 7% 38%	2400
	11th to 20th	Day	14% 6% 2 6% 5% 4 4 4 5% 7% 43%	2374
	21st to end	Day	17% 5% 2 6% 4 4 4 4 5% 7% 41%	2400
May	1st to 10th	Day	25% 6% 2 6% 4 4 4 4 6% 8% 30%	5741
	11th to 20th	Day	23% 7% 3 6% 4 4 5% 4 6% 8% 30%	6313
	21st to end	Day	16% 5% 2 6% 4 4 5% 4 6% 8% 37%	6934
June	1st to 10th	Day	12% 6% 2 6% 4 5% 5% 4 7% 9% 39%	2800
	11th to 20th	Day	13% 6% 2 7% 5% 4 5% 4 6% 9% 39%	2800
	21st to end	Day	10% 4 2 5% 6% 5% 5% 4 7% 10% 42%	2800
July	1st to 10th	Day	7% 5% 3 6% 5% 5% 5% 5% 7% 10% 42%	6360
	11th to 20th	Day	11% 7% 3 8% 5% 6% 7% 6% 8% 10% 31%	6360
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 5% 7% 9% 40%	6434
August	1st to 10th	Day	18% 5% 2 6% 5% 5% 6% 5% 7% 8% 33%	4234
	11th to 20th	Day	11% 5% 2 6% 5% 5% 6% 5% 7% 8% 39%	3982
	21st to end	Day	12% 5% 2 5% 4 4 4 4 6% 8% 47%	4312
September	1st to 10th	Day	15% 6% 2 5% 4 4 3 4 5% 6% 46%	2400
	11th to 20th	Day	12% 5% 1 5% 4 4 4 3 6% 7% 48%	2400
	21st to end	Day	20% 8% 2 7% 5% 4 4 3 5% 7% 36%	2120
October	1st to 10th	Day	17% 6% 2 5% 3 4 3 3 5% 6% 46%	2000
	11th to 20th	Day	16% 6% 2 5% 3 4 4 3 5% 6% 48%	2000
	21st to end	Day	14% 6% 3 7% 3 3 4 3 5% 6% 47%	2200
November	1st to 10th	Day	12% 6% 2 5% 3 2 3 3 5% 8% 51%	1891
	11th to 20th	Day	14% 7% 1 4 3 3 4 4 5% 6% 48%	1871
	21st to end	Day	15% 7% 3 5% 5% 4 4 3 4 6% 44%	1680
December	1st to 10th	Day	13% 6% 2 7% 4 5% 4 4 5% 7% 43%	2680
	11th to 20th	Day	18% 9% 2 5% 4 4 3 3 5% 6% 40%	3070
	21st to end	Day	17% 8% 2 6% 5% 4 5% 4 5% 8% 36%	4048
January	1st to 10th	Night	11% 7% 2 6% 4 5% 5% 4 5% 7% 43%	8233
	11th to 20th	Night	14% 8% 2 6% 3 3 3 3 4 6% 48%	7742
	21st to end	Night	14% 8% 2 5% 3 3 4 3 5% 7% 46%	8401
February	1st to 10th	Night	15% 8% 3 6% 4 4 3 3 6% 7% 40%	2730
	11th to 20th	Night	17% 8% 2 6% 4 3 4 4 4 5% 41%	2529
	21st to end	Night	16% 8% 3 7% 4 4 4 3 5% 6% 40%	1980
March	1st to 10th	Night	17% 7% 2 5% 4 3 4 3 4 5% 46%	5098
	11th to 20th	Night	10% 5% 3 6% 4 4 4 3 4 6% 51%	4980
	21st to end	Night	13% 8% 2 6% 4 4 4 3 4 6% 46%	5312
April	1st to 10th	Night	16% 8% 3 7% 5% 4 5% 3 6% 7% 38%	2000
	11th to 20th	Night	14% 5% 2 6% 4 4 4 3 5% 9% 42%	1975
	21st to end	Night	17% 8% 2 4 4 3 4 3 6% 7% 43%	2000
May	1st to 10th	Night	25% 8% 3 6% 4 4 4 3 5% 7% 30%	4182
	11th to 20th	Night	24% 8% 3 7% 4 3 3 3 6% 8% 30%	3576
	21st to end	Night	19% 7% 2 6% 4 4 5% 4 6% 8% 37%	3920
June	1st to 10th	Night	12% 6% 2 5% 4 4 4 4 7% 8% 45%	1600
	11th to 20th	Night	12% 6% 2 5% 5% 5% 5% 5% 7% 9% 40%	1600
	21st to end	Night	10% 5% 2 5% 5% 4 5% 4 7% 10% 43%	1600
July	1st to 10th	Night	8% 5% 3 8% 5% 6% 6% 5% 6% 10% 39%	3598
	11th to 20th	Night	11% 7% 3 8% 5% 6% 6% 6% 8% 10% 30%	3597
	21st to end	Night	10% 6% 2 6% 4 4 5% 4 6% 8% 45%	4413
August	1st to 10th	Night	19% 5% 2 6% 4 5% 4 4 7% 9% 35%	2915
	11th to 20th	Night	11% 8% 2 7% 5% 5% 5% 4 6% 7% 39%	3034
	21st to end	Night	13% 5% 2 5% 4 4 4 3 5% 7% 48%	3586
September	1st to 10th	Night	13% 6% 3 5% 4 3 3 3 6% 8% 47%	2000
	11th to 20th	Night	11% 6% 2 6% 4 3 3 3 4 8% 51%	2000
	21st to end	Night	19% 7% 3 6% 4 4 4 3 4 6% 40%	2276
October	1st to 10th	Night	18% 7% 2 6% 3 3 3 3 4 5% 45%	2400
	11th to 20th	Night	16% 5% 2 4 3 2 4 3 5% 6% 51%	2400
	21st to end	Night	16% 5% 2 4 3 3 4 3 5% 6% 49%	2640
November	1st to 10th	Night	11% 5% 2 5% 4 3 4 3 5% 6% 53%	2625
	11th to 20th	Night	14% 8% 2 5% 4 3 3 3 4 6% 47%	2746
	21st to end	Night	13% 5% 2 6% 4 4 4 4 6% 7% 46%	2940
December	1st to 10th	Night	10% 6% 2 6% 4 4 4 4 5% 8% 47%	4720
	11th to 20th	Night	21% 7% 2 5% 4 4 4 4 6% 6% 39%	5345
	21st to end	Night	17% 7% 2 6% 4 5% 5% 4 6% 7% 35%	7150

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	39% 5% 8% 26% 18% 3	4662
	11th to 20th	Day	43% 4 7% 21% 17% 8%	5202
	21st to end	Day	42% 4 11% 28% 12% 2	5780
February	1st to 10th	Day	42% 6% 13% 25% 11% 2	1890
	11th to 20th	Day	45% 5% 9% 24% 14% 2	1877
	21st to end	Day	47% 4 9% 22% 13% 4	1650
March	1st to 10th	Day	39% 5% 10% 23% 17% 7%	4750
	11th to 20th	Day	36% 5% 10% 27% 16% 6%	4980
	21st to end	Day	44% 4 9% 19% 19% 5%	5620
April	1st to 10th	Day	43% 6% 11% 19% 16% 5%	2400
	11th to 20th	Day	39% 6% 11% 22% 18% 4	2374
	21st to end	Day	41% 6% 9% 21% 19% 3	2400
May	1st to 10th	Day	50% 6% 14% 19% 10% 2	5775
	11th to 20th	Day	50% 7% 11% 21% 10% 1	6317
	21st to end	Day	42% 7% 12% 23% 15% 1	6949
June	1st to 10th	Day	39% 7% 14% 23% 15% 2	2800
	11th to 20th	Day	41% 7% 14% 23% 13% 2	2800
	21st to end	Day	34% 8% 14% 26% 17% 1	2800
July	1st to 10th	Day	36% 9% 15% 20% 16% 3	6290
	11th to 20th	Day	43% 10% 16% 22% 8% 1	6285
	21st to end	Day	38% 7% 10% 22% 22% 1	6249
August	1st to 10th	Day	46% 7% 13% 24% 9%	4221
	11th to 20th	Day	39% 8% 11% 27% 14% 1	4021
	21st to end	Day	34% 6% 12% 28% 18% 2	4312
September	1st to 10th	Day	39% 6% 11% 20% 20% 4	2400
	11th to 20th	Day	34% 5% 10% 27% 20% 3	2400
	21st to end	Day	48% 5% 8% 18% 18% 2	2120
October	1st to 10th	Day	37% 4 7% 21% 27% 5%	3260
	11th to 20th	Day	27% 5% 10% 26% 22% 10%	3240
	21st to end	Day	26% 3 10% 28% 29% 4	3538
November	1st to 10th	Day	33% 4 13% 25% 21% 4	1891
	11th to 20th	Day	36% 4 11% 24% 21% 3	1871
	21st to end	Day	41% 4 11% 29% 13% 2	1680
December	1st to 10th	Day	41% 4 15% 27% 11% 3	2680
	11th to 20th	Day	45% 5% 11% 29% 9% 2	3066
	21st to end	Day	45% 5% 16% 19% 14% 2	4048
January	1st to 10th	Night	39% 4 11% 26% 17% 3	8234
	11th to 20th	Night	38% 3 8% 23% 20% 7%	7742
	21st to end	Night	38% 3 8% 29% 20% 2	8401
February	1st to 10th	Night	43% 4 14% 24% 14% 1	2730
	11th to 20th	Night	44% 4 9% 21% 18% 4	2531
	21st to end	Night	45% 4 10% 20% 16% 5%	1980
March	1st to 10th	Night	41% 4 8% 27% 11% 10%	5098
	11th to 20th	Night	33% 5% 12% 30% 13% 6%	4980
	21st to end	Night	40% 4 9% 20% 24% 4	5312
April	1st to 10th	Night	45% 5% 10% 19% 17% 3	2000
	11th to 20th	Night	39% 5% 12% 21% 19% 5%	1975
	21st to end	Night	40% 6% 10% 22% 18% 5%	2000
May	1st to 10th	Night	53% 6% 10% 16% 11% 3	4211
	11th to 20th	Night	51% 6% 12% 20% 10% 1	3577
	21st to end	Night	44% 7% 13% 24% 10% 2	3927
June	1st to 10th	Night	36% 8% 11% 25% 18% 2	1600
	11th to 20th	Night	39% 7% 14% 23% 16% 1	1600
	21st to end	Night	35% 7% 14% 26% 17% 2	1600
July	1st to 10th	Night	40% 9% 16% 18% 14% 3	3560
	11th to 20th	Night	45% 11% 15% 19% 8% 1	3552
	21st to end	Night	35% 7% 12% 24% 20% 1	4284
August	1st to 10th	Night	45% 7% 18% 21% 8% 1	2904
	11th to 20th	Night	42% 6% 10% 28% 13% 1	3061
	21st to end	Night	34% 5% 11% 30% 17% 2	3586
September	1st to 10th	Night	36% 6% 12% 22% 20% 5%	2000
	11th to 20th	Night	33% 6% 9% 25% 25% 3	2000
	21st to end	Night	46% 5% 9% 19% 18% 3	2276
October	1st to 10th	Night	42% 4 10% 20% 21% 3	3920
	11th to 20th	Night	25% 4 11% 26% 28% 6%	3899
	21st to end	Night	25% 3 11% 32% 24% 5%	4252
November	1st to 10th	Night	32% 5% 10% 26% 22% 5%	2625
	11th to 20th	Night	38% 4 8% 25% 20% 5%	2746
	21st to end	Night	37% 5% 12% 26% 18% 2	2940
December	1st to 10th	Night	35% 4 15% 30% 14% 1	4720
	11th to 20th	Night	45% 4 12% 19% 16% 3	5348
	21st to end	Night	45% 6% 13% 19% 16% 1	7150

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	30% 30% 26% 10% 4	220
	11th to 20th	Day	38% 23% 29% 9% 1	316
	21st to end	Day	25% 33% 30% 10% 2	363
February	1st to 10th	Day	28% 25% 37% 8% 2 1	330
	11th to 20th	Day	38% 26% 31% 4 1	385
	21st to end	Day	32% 23% 33% 11% 2	414
March	1st to 10th	Day	26% 26% 30% 16% 2	500
	11th to 20th	Day	27% 24% 31% 13% 3 1	500
	21st to end	Day	25% 24% 32% 14% 4 1	550
April	1st to 10th	Day	26% 23% 33% 12% 4 1	500
	11th to 20th	Day	22% 22% 28% 17% 8% 2	492
	21st to end	Day	25% 16% 37% 14% 7% 1	500
May	1st to 10th	Day	27% 18% 29% 17% 7% 1	503
	11th to 20th	Day	25% 14% 31% 21% 7% 2	527
	21st to end	Day	18% 8% 27% 26% 18% 3	573
June	1st to 10th	Day	14% 14% 31% 21% 15% 5%	530
	11th to 20th	Day	18% 9% 30% 28% 12% 3	530
	21st to end	Day	12% 8% 32% 29% 15% 4	530
July	1st to 10th	Day	13% 4 23% 32% 19% 8%	525
	11th to 20th	Day	6% 4 22% 37% 24% 6%	524
	21st to end	Day	11% 5% 29% 34% 18% 4	576
August	1st to 10th	Day	12% 6% 24% 31% 20% 7%	526
	11th to 20th	Day	11% 6% 23% 35% 19% 7%	519
	21st to end	Day	11% 5% 25% 32% 18% 9%	583
September	1st to 10th	Day	17% 10% 22% 22% 20% 9%	530
	11th to 20th	Day	12% 6% 24% 31% 20% 8%	530
	21st to end	Day	27% 12% 23% 24% 12% 3	530
October	1st to 10th	Day	22% 9% 25% 23% 14% 7%	530
	11th to 20th	Day	19% 17% 27% 23% 11% 3	530
	21st to end	Day	16% 19% 37% 19% 8% 2	583
November	1st to 10th	Day	19% 15% 34% 22% 9% 2	447
	11th to 20th	Day	21% 22% 33% 17% 7%	411
	21st to end	Day	32% 18% 33% 14% 4	220
December	1st to 10th	Day	26% 25% 35% 10% 2 1	220
	11th to 20th	Day	31% 24% 30% 12% 3	220
	21st to end	Day	28% 22% 31% 13% 6%	242
January	1st to 10th	Night	20% 22% 39% 17% 2	840
	11th to 20th	Night	26% 20% 33% 18% 3	840
	21st to end	Night	19% 22% 41% 17% 1	924
February	1st to 10th	Night	21% 19% 37% 20% 3	840
	11th to 20th	Night	27% 25% 34% 14%	804
	21st to end	Night	24% 21% 37% 15% 3	660
March	1st to 10th	Night	21% 21% 37% 19% 3	800
	11th to 20th	Night	21% 21% 34% 19% 5%	800
	21st to end	Night	21% 19% 39% 17% 4 1	880
April	1st to 10th	Night	22% 24% 34% 18% 3	800
	11th to 20th	Night	18% 17% 36% 22% 6%	791
	21st to end	Night	22% 16% 32% 23% 7%	800
May	1st to 10th	Night	22% 10% 31% 27% 9% 1	673
	11th to 20th	Night	23% 7% 31% 30% 8% 1	395
	21st to end	Night	17% 6% 23% 36% 17% 1	433
June	1st to 10th	Night	11% 4 32% 33% 20%	400
	11th to 20th	Night	15% 5% 27% 39% 15% 1	400
	21st to end	Night	12% 4 25% 44% 16%	400
July	1st to 10th	Night	8% 4 20% 40% 25% 3	390
	11th to 20th	Night	8% 3 19% 40% 28% 3	388
	21st to end	Night	10% 3 20% 39% 27% 2	643
August	1st to 10th	Night	12% 4 13% 45% 23% 3	597
	11th to 20th	Night	12% 6% 17% 39% 25% 2	688
	21st to end	Night	14% 4 18% 37% 25% 4	880
September	1st to 10th	Night	15% 4 22% 33% 22% 3	800
	11th to 20th	Night	13% 4 18% 37% 25% 4	800
	21st to end	Night	23% 6% 22% 31% 17% 2	800
October	1st to 10th	Night	21% 8% 22% 26% 19% 4	800
	11th to 20th	Night	18% 12% 28% 30% 11% 1	800
	21st to end	Night	19% 14% 32% 23% 12%	880
November	1st to 10th	Night	15% 12% 34% 28% 10% 1	820
	11th to 20th	Night	20% 20% 30% 23% 7% 1	840
	21st to end	Night	18% 18% 34% 23% 6% 1	840
December	1st to 10th	Night	22% 19% 36% 17% 6%	840
	11th to 20th	Night	20% 17% 38% 20% 4	840
	21st to end	Night	21% 19% 32% 24% 3	924

Page generated at 2026-02-13 01:42:15. Version ac07

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