Powder River
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Powder River clear sky chart from 2005-11-06 to 2025-04-26.

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	4 6% 2 5% 4 4 4 4 6% 8% 55%	117827
	Night	4 6% 2 6% 4 4 4 4 6% 8% 53%	119715
Transparency	Day	26% 6% 12% 26% 21% 9%	117572
	Night	28% 6% 13% 27% 19% 7%	119560
Seeing	Day	8% 9% 28% 29% 20% 6%	16351
	Night	8% 8% 32% 36% 15% 1	26549

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	1 4 3 9% 5% 5% 5% 4 6% 8% 49%	3638
	11th to 20th	Day	2 6% 2 6% 7% 6% 6% 4 7% 8% 47%	3704
	21st to end	Day	1 4 2 5% 2 3 3 3 5% 7% 63%	4510
February	1st to 10th	Day	2 8% 3 7% 4 5% 5% 6% 7% 9% 44%	1800
	11th to 20th	Day	2 9% 3 7% 6% 5% 4 4 6% 7% 49%	1755
	21st to end	Day	2 6% 3 6% 4 3 4 4 7% 7% 53%	1640
March	1st to 10th	Day	3 8% 3 5% 3 2 3 3 5% 8% 58%	4679
	11th to 20th	Day	4 8% 2 5% 3 3 4 4 6% 8% 54%	5000
	21st to end	Day	5% 10% 3 7% 5% 4 4 4 6% 9% 42%	5651
April	1st to 10th	Day	6% 9% 2 5% 4 4 3 4 7% 10% 47%	2400
	11th to 20th	Day	8% 9% 2 8% 4 4 4 4 6% 7% 43%	2371
	21st to end	Day	10% 10% 2 7% 4 4 5% 4 5% 7% 42%	2407
May	1st to 10th	Day	11% 13% 3 8% 4 4 5% 4 5% 7% 36%	4297
	11th to 20th	Day	10% 6% 2 4 3 3 4 3 5% 8% 50%	4388
	21st to end	Day	9% 6% 2 4 3 3 4 4 6% 9% 49%	4825
June	1st to 10th	Day	5% 4 2 4 4 4 4 4 6% 9% 54%	2660
	11th to 20th	Day	5% 4 1 4 3 4 4 4 7% 9% 56%	2660
	21st to end	Day	3 2 1 3 2 3 3 4 6% 8% 65%	2660
July	1st to 10th	Day	2 2 1 2 2 3 4 3 6% 8% 68%	4430
	11th to 20th	Day	2 2 1 3 3 3 4 4 6% 9% 64%	4430
	21st to end	Day	2 2 1 3 3 3 4 5% 6% 9% 61%	4743
August	1st to 10th	Day	5% 3 1 4 3 4 4 4 6% 7% 58%	4092
	11th to 20th	Day	3 3 1 4 3 4 4 4 6% 7% 61%	4011
	21st to end	Day	1 2 1 2 3 3 4 3 6% 7% 68%	4180
September	1st to 10th	Day	5% 3 1 4 3 3 3 2 5% 7% 64%	2280
	11th to 20th	Day	5% 4 1 4 3 2 3 3 5% 7% 63%	2280
	21st to end	Day	6% 4 1 3 2 3 3 3 4 6% 65%	2019
October	1st to 10th	Day	5% 4 1 3 2 2 2 2 2 4 72%	3170
	11th to 20th	Day	7% 6% 2 4 2 3 3 2 3 5% 63%	3150
	21st to end	Day	6% 7% 2 5% 4 3 3 3 5% 8% 53%	3359
November	1st to 10th	Day	4 5% 2 6% 3 3 4 4 5% 8% 55%	1763
	11th to 20th	Day	2 5% 3 4 5% 4 4 4 7% 8% 55%	1625
	21st to end	Day	2 7% 3 4 3 5% 4 4 6% 10% 51%	1600
December	1st to 10th	Day	1 4 2 6% 4 3 2 3 4 5% 66%	2620
	11th to 20th	Day	2 6% 2 5% 4 4 4 4 7% 9% 54%	3026
	21st to end	Day	2 7% 3 9% 5% 4 5% 4 6% 9% 46%	4004
January	1st to 10th	Night	1 7% 4 11% 5% 5% 5% 4 6% 8% 44%	6327
	11th to 20th	Night	1 7% 3 10% 6% 6% 4 4 6% 8% 45%	6317
	21st to end	Night	1 5% 2 6% 4 4 4 3 5% 7% 60%	6468
February	1st to 10th	Night	2 7% 4 8% 5% 5% 5% 4 6% 7% 46%	2600
	11th to 20th	Night	3 7% 3 7% 6% 5% 5% 4 5% 7% 47%	2532
	21st to end	Night	2 7% 3 7% 5% 4 4 4 6% 8% 49%	1989
March	1st to 10th	Night	4 9% 2 6% 3 3 3 2 5% 7% 55%	5200
	11th to 20th	Night	4 8% 2 6% 3 3 3 3 5% 9% 54%	5000
	21st to end	Night	6% 9% 4 9% 5% 5% 5% 4 7% 8% 37%	5316
April	1st to 10th	Night	8% 9% 2 5% 3 3 3 3 6% 9% 48%	2000
	11th to 20th	Night	9% 11% 3 7% 6% 4 5% 3 6% 8% 38%	1977
	21st to end	Night	10% 8% 3 6% 4 4 4 4 6% 7% 43%	1888
May	1st to 10th	Night	10% 12% 3 8% 5% 3 4 4 5% 7% 39%	2671
	11th to 20th	Night	11% 5% 2 4 4 4 4 3 5% 8% 50%	2517
	21st to end	Night	10% 6% 2 5% 3 4 4 4 6% 8% 47%	2762
June	1st to 10th	Night	5% 4 2 4 4 4 4 4 7% 9% 53%	1520
	11th to 20th	Night	8% 3 1 3 3 3 3 3 6% 8% 58%	1520
	21st to end	Night	3 2 1 4 2 3 4 3 6% 7% 66%	1520
July	1st to 10th	Night	3 3 1 4 3 3 4 4 7% 9% 58%	2540
	11th to 20th	Night	2 1 1 3 3 4 4 4 6% 10% 61%	2540
	21st to end	Night	3 2 1 2 3 3 4 4 7% 10% 62%	2816
August	1st to 10th	Night	8% 3 1 4 3 3 4 4 6% 10% 54%	2847
	11th to 20th	Night	3 3 1 4 4 5% 6% 5% 7% 8% 55%	2826
	21st to end	Night	2 2 1 3 2 3 4 4 6% 9% 65%	3487
September	1st to 10th	Night	6% 4 1 3 3 3 3 3 4 7% 64%	1900
	11th to 20th	Night	5% 4 1 3 2 3 4 3 5% 6% 63%	1900
	21st to end	Night	7% 4 1 4 2 2 2 2 3 6% 67%	2156
October	1st to 10th	Night	6% 5% 1 2 2 2 3 2 4 3 72%	3800
	11th to 20th	Night	8% 5% 1 4 3 3 2 3 5% 7% 59%	3778
	21st to end	Night	7% 7% 2 4 3 3 4 3 5% 7% 54%	4168
November	1st to 10th	Night	3 7% 2 5% 3 3 4 4 7% 8% 53%	2535
	11th to 20th	Night	3 6% 3 6% 5% 4 5% 3 6% 7% 53%	2773
	21st to end	Night	3 7% 3 5% 4 4 4 4 6% 7% 54%	2800
December	1st to 10th	Night	1 5% 2 5% 3 3 3 3 5% 7% 63%	4570
	11th to 20th	Night	2 4 2 5% 4 4 5% 4 7% 7% 55%	5181
	21st to end	Night	2 7% 3 7% 5% 5% 5% 5% 7% 10% 45%	6974

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	31% 7% 15% 28% 17% 1	3639
	11th to 20th	Day	32% 8% 16% 23% 18% 2	3704
	21st to end	Day	21% 6% 16% 34% 20% 4	4510
February	1st to 10th	Day	33% 9% 18% 27% 11% 2	1800
	11th to 20th	Day	35% 8% 16% 24% 15% 3	1764
	21st to end	Day	27% 7% 18% 25% 19% 3	1640
March	1st to 10th	Day	25% 5% 8% 37% 16% 9%	4679
	11th to 20th	Day	27% 7% 14% 32% 19% 1	5000
	21st to end	Day	36% 7% 17% 29% 10%	5651
April	1st to 10th	Day	32% 6% 18% 25% 17% 3	2400
	11th to 20th	Day	39% 5% 13% 25% 16% 2	2374
	21st to end	Day	41% 7% 10% 29% 12% 1	2407
May	1st to 10th	Day	44% 7% 12% 20% 12% 5%	4297
	11th to 20th	Day	31% 7% 11% 30% 16% 4	4392
	21st to end	Day	29% 7% 12% 30% 18% 4	4825
June	1st to 10th	Day	25% 7% 16% 27% 19% 5%	2660
	11th to 20th	Day	23% 7% 12% 23% 22% 13%	2660
	21st to end	Day	15% 5% 9% 24% 29% 17%	2660
July	1st to 10th	Day	14% 4 6% 21% 35% 20%	4361
	11th to 20th	Day	16% 4 5% 18% 32% 25%	4355
	21st to end	Day	18% 5% 6% 15% 31% 25%	4593
August	1st to 10th	Day	24% 4 7% 24% 22% 19%	4078
	11th to 20th	Day	21% 4 6% 21% 30% 19%	4050
	21st to end	Day	14% 4 7% 19% 31% 24%	4179
September	1st to 10th	Day	20% 4 6% 18% 30% 21%	2280
	11th to 20th	Day	20% 4 8% 22% 30% 16%	2280
	21st to end	Day	21% 4 9% 25% 31% 10%	2019
October	1st to 10th	Day	18% 3 7% 18% 30% 23%	3170
	11th to 20th	Day	26% 4 9% 28% 26% 7%	3150
	21st to end	Day	30% 4 10% 32% 20% 4	3359
November	1st to 10th	Day	27% 7% 12% 29% 21% 4	1763
	11th to 20th	Day	25% 6% 17% 31% 18% 4	1625
	21st to end	Day	27% 6% 15% 29% 19% 3	1600
December	1st to 10th	Day	21% 4 14% 38% 21% 1	2620
	11th to 20th	Day	26% 5% 25% 30% 15%	3024
	21st to end	Day	34% 6% 11% 30% 17% 2	4004
January	1st to 10th	Night	36% 6% 13% 28% 15% 1	6327
	11th to 20th	Night	35% 6% 17% 23% 15% 3	6317
	21st to end	Night	24% 5% 19% 29% 19% 4	6468
February	1st to 10th	Night	35% 7% 17% 28% 11% 3	2600
	11th to 20th	Night	36% 6% 15% 25% 14% 4	2533
	21st to end	Night	31% 7% 16% 23% 20% 3	1989
March	1st to 10th	Night	29% 4 8% 29% 22% 8%	5200
	11th to 20th	Night	28% 6% 13% 33% 20% 1	5000
	21st to end	Night	42% 8% 17% 24% 9%	5316
April	1st to 10th	Night	33% 7% 16% 25% 15% 3	2000
	11th to 20th	Night	44% 7% 10% 21% 16% 2	1977
	21st to end	Night	38% 7% 10% 28% 14% 1	1888
May	1st to 10th	Night	42% 8% 10% 23% 15% 2	2671
	11th to 20th	Night	30% 7% 13% 32% 11% 6%	2518
	21st to end	Night	30% 8% 16% 31% 12% 2	2762
June	1st to 10th	Night	26% 9% 18% 25% 18% 5%	1520
	11th to 20th	Night	24% 6% 13% 21% 25% 12%	1520
	21st to end	Night	17% 5% 9% 24% 28% 17%	1520
July	1st to 10th	Night	20% 6% 7% 25% 31% 11%	2500
	11th to 20th	Night	16% 5% 7% 23% 29% 20%	2492
	21st to end	Night	16% 5% 7% 22% 30% 19%	2728
August	1st to 10th	Night	25% 5% 9% 27% 18% 16%	2838
	11th to 20th	Night	26% 5% 10% 25% 22% 13%	2853
	21st to end	Night	16% 4 10% 19% 27% 24%	3487
September	1st to 10th	Night	21% 4 8% 19% 29% 19%	1900
	11th to 20th	Night	21% 5% 9% 22% 27% 16%	1900
	21st to end	Night	21% 3 9% 25% 30% 12%	2156
October	1st to 10th	Night	19% 4 6% 19% 32% 20%	3800
	11th to 20th	Night	26% 3 10% 27% 25% 10%	3778
	21st to end	Night	30% 4 8% 31% 19% 7%	4168
November	1st to 10th	Night	27% 7% 14% 27% 21% 5%	2535
	11th to 20th	Night	29% 7% 11% 30% 19% 4	2773
	21st to end	Night	28% 6% 14% 29% 20% 4	2800
December	1st to 10th	Night	22% 5% 15% 36% 20% 2	4570
	11th to 20th	Night	25% 6% 19% 38% 12% 1	5182
	21st to end	Night	32% 6% 13% 31% 15% 3	6974

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	6% 13% 33% 30% 13% 5%	300
	11th to 20th	Day	7% 11% 37% 31% 13% 2	300
	21st to end	Day	4 8% 39% 37% 11% 2	330
February	1st to 10th	Day	7% 18% 47% 22% 5%	300
	11th to 20th	Day	4 15% 44% 27% 9% 1	300
	21st to end	Day	8% 12% 38% 29% 12% 1	249
March	1st to 10th	Day	7% 16% 38% 27% 12% 1	375
	11th to 20th	Day	10% 12% 32% 30% 13% 3	500
	21st to end	Day	12% 12% 32% 30% 12% 2	550
April	1st to 10th	Day	10% 15% 32% 31% 11% 2	500
	11th to 20th	Day	13% 13% 35% 30% 10% 1	492
	21st to end	Day	17% 12% 23% 31% 13% 5%	481
May	1st to 10th	Day	17% 11% 26% 26% 16% 5%	572
	11th to 20th	Day	18% 8% 19% 26% 23% 6%	675
	21st to end	Day	12% 9% 25% 24% 22% 7%	734
June	1st to 10th	Day	5% 8% 23% 25% 27% 12%	680
	11th to 20th	Day	7% 10% 33% 28% 19% 3	680
	21st to end	Day	5% 5% 24% 32% 28% 7%	680
July	1st to 10th	Day	4 3 19% 30% 35% 9%	671
	11th to 20th	Day	2 3 17% 35% 32% 11%	668
	21st to end	Day	3 3 13% 34% 36% 11%	704
August	1st to 10th	Day	4 5% 18% 32% 31% 10%	487
	11th to 20th	Day	5% 4 18% 35% 27% 12%	477
	21st to end	Day	3 8% 20% 32% 24% 13%	539
September	1st to 10th	Day	4 4 21% 31% 25% 13%	490
	11th to 20th	Day	7% 8% 28% 27% 21% 9%	490
	21st to end	Day	8% 7% 27% 29% 21% 9%	367
October	1st to 10th	Day	10% 6% 24% 28% 21% 10%	300
	11th to 20th	Day	10% 11% 28% 26% 20% 5%	300
	21st to end	Day	8% 11% 35% 32% 11% 2	330
November	1st to 10th	Day	9% 13% 33% 33% 11% 2	300
	11th to 20th	Day	6% 14% 42% 23% 12% 2	300
	21st to end	Day	10% 10% 38% 29% 10% 2	300
December	1st to 10th	Day	3 11% 44% 30% 11% 2	300
	11th to 20th	Day	7% 17% 45% 23% 7% 1	300
	21st to end	Day	10% 18% 30% 28% 12% 2	330
January	1st to 10th	Night	4 13% 36% 32% 14% 2	1000
	11th to 20th	Night	5% 14% 41% 29% 10% 1	990
	21st to end	Night	5% 8% 44% 31% 10% 1	880
February	1st to 10th	Night	6% 11% 42% 32% 8% 1	800
	11th to 20th	Night	7% 9% 38% 34% 11% 1	784
	21st to end	Night	6% 14% 38% 32% 10% 1	660
March	1st to 10th	Night	8% 11% 36% 33% 12% 1	800
	11th to 20th	Night	8% 8% 32% 37% 15% 1	800
	21st to end	Night	12% 9% 29% 36% 13% 1	810
April	1st to 10th	Night	13% 10% 32% 37% 8%	600
	11th to 20th	Night	14% 8% 32% 36% 9%	593
	21st to end	Night	13% 8% 26% 35% 16% 1	587
May	1st to 10th	Night	14% 9% 26% 32% 17% 2	570
	11th to 20th	Night	17% 4 22% 33% 21% 2	561
	21st to end	Night	12% 8% 25% 36% 18% 1	617
June	1st to 10th	Night	5% 7% 23% 39% 23% 2	570
	11th to 20th	Night	10% 7% 33% 39% 11%	570
	21st to end	Night	4 4 25% 46% 22%	570
July	1st to 10th	Night	5% 1 23% 41% 29% 1	555
	11th to 20th	Night	3 3 22% 46% 24% 1	552
	21st to end	Night	4 2 14% 50% 29% 1	624
August	1st to 10th	Night	4 3 19% 48% 27% 1	567
	11th to 20th	Night	3 3 19% 46% 28% 1	564
	21st to end	Night	3 4 21% 46% 25% 2	627
September	1st to 10th	Night	7% 3 22% 41% 25% 2	570
	11th to 20th	Night	7% 5% 25% 40% 21% 3	570
	21st to end	Night	10% 4 22% 38% 23% 3	703
October	1st to 10th	Night	13% 6% 26% 34% 18% 1	760
	11th to 20th	Night	12% 6% 28% 38% 16% 1	760
	21st to end	Night	10% 8% 31% 36% 13% 2	860
November	1st to 10th	Night	6% 9% 37% 35% 12% 1	975
	11th to 20th	Night	6% 12% 43% 30% 8%	1000
	21st to end	Night	6% 9% 39% 33% 11% 1	1000
December	1st to 10th	Night	6% 12% 40% 32% 10%	1000
	11th to 20th	Night	7% 11% 42% 33% 7%	1000
	21st to end	Night	8% 14% 40% 29% 8% 1	1100

Page generated at 2025-04-23 07:25:07. Version ac07

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