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 2026-08-30.

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 5% 2 5% 3 4 4 4 6% 8% 56%	153428
	Night	4 6% 2 6% 4 4 4 4 6% 8% 53%	146380
Transparency	Day	26% 5% 11% 27% 21% 10%	148261
	Night	29% 5% 12% 28% 19% 7%	142556
Seeing	Day	8% 10% 28% 29% 19% 6%	18323
	Night	8% 9% 32% 36% 15% 1	28286

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% 50%	4720
	11th to 20th	Day	1 5% 2 5% 6% 5% 5% 4 6% 8% 52%	4818
	21st to end	Day	1 4 3 5% 2 3 4 3 5% 8% 62%	5844
February	1st to 10th	Day	2 8% 3 7% 4 5% 5% 6% 7% 9% 44%	1890
	11th to 20th	Day	2 9% 3 7% 5% 5% 4 4 6% 7% 48%	1845
	21st to end	Day	2 6% 3 6% 4 3 4 4 7% 7% 54%	1720
March	1st to 10th	Day	3 8% 3 5% 3 2 3 3 5% 8% 57%	4783
	11th to 20th	Day	4 8% 2 5% 3 3 4 4 6% 8% 54%	5110
	21st to end	Day	5% 10% 3 6% 5% 4 4 4 6% 9% 43%	5775
April	1st to 10th	Day	6% 9% 2 5% 4 4 3 4 7% 10% 48%	2520
	11th to 20th	Day	8% 9% 2 8% 4 4 4 4 5% 7% 45%	2491
	21st to end	Day	11% 10% 3 7% 4 4 5% 3 5% 7% 41%	2593
May	1st to 10th	Day	9% 10% 2 6% 3 3 4 3 5% 7% 48%	7979
	11th to 20th	Day	9% 6% 2 4 3 3 4 3 5% 8% 54%	8191
	21st to end	Day	8% 6% 2 5% 3 4 5% 4 7% 9% 47%	9026
June	1st to 10th	Day	5% 4 2 4 4 4 4 4 6% 9% 55%	2940
	11th to 20th	Day	5% 4 1 4 3 4 4 4 7% 9% 57%	2940
	21st to end	Day	3 3 1 3 2 3 4 4 6% 8% 64%	2940
July	1st to 10th	Day	2 2 1 3 3 3 4 4 7% 8% 64%	8249
	11th to 20th	Day	2 2 1 2 2 3 4 3 5% 8% 67%	8250
	21st to end	Day	3 3 1 4 3 4 5% 5% 7% 10% 55%	8842
August	1st to 10th	Day	4 3 1 4 3 4 4 4 6% 7% 61%	5909
	11th to 20th	Day	3 3 1 4 3 4 4 3 6% 8% 61%	5911
	21st to end	Day	3 3 1 4 3 4 5% 4 6% 8% 58%	6093
September	1st to 10th	Day	5% 2 1 4 3 3 3 2 5% 7% 64%	2400
	11th to 20th	Day	5% 4 1 4 3 3 3 3 5% 8% 62%	2400
	21st to end	Day	6% 4 1 3 2 3 2 3 4 6% 64%	2125
October	1st to 10th	Day	5% 4 1 3 2 2 2 2 2 4 72%	3270
	11th to 20th	Day	7% 6% 2 4 2 3 3 2 3 5% 63%	3250
	21st to end	Day	6% 7% 2 5% 3 3 3 3 5% 8% 54%	3467
November	1st to 10th	Day	4 5% 2 6% 3 4 4 4 5% 8% 55%	1853
	11th to 20th	Day	2 5% 3 5% 5% 4 4 4 7% 8% 55%	1706
	21st to end	Day	2 7% 3 5% 3 5% 4 4 6% 9% 51%	1680
December	1st to 10th	Day	1 4 2 6% 3 3 2 3 4 6% 66%	2700
	11th to 20th	Day	1 5% 2 5% 4 4 4 4 7% 9% 54%	3106
	21st to end	Day	2 6% 3 9% 5% 4 5% 4 6% 9% 47%	4092
January	1st to 10th	Night	1 7% 4 10% 5% 5% 5% 4 6% 8% 44%	8201
	11th to 20th	Night	1 6% 3 9% 6% 5% 4 3 6% 8% 50%	8213
	21st to end	Night	1 6% 3 6% 4 4 4 3 5% 7% 58%	8370
February	1st to 10th	Night	2 7% 3 8% 5% 5% 5% 4 6% 7% 47%	2730
	11th to 20th	Night	3 7% 3 7% 5% 5% 5% 4 5% 7% 48%	2662
	21st to end	Night	2 7% 3 7% 5% 4 4 4 6% 8% 50%	2085
March	1st to 10th	Night	4 10% 3 6% 3 3 3 2 5% 7% 55%	5316
	11th to 20th	Night	4 8% 2 6% 3 3 3 3 5% 8% 54%	5110
	21st to end	Night	6% 9% 4 9% 5% 5% 5% 4 7% 8% 38%	5433
April	1st to 10th	Night	7% 9% 2 5% 3 3 3 4 5% 9% 48%	2100
	11th to 20th	Night	9% 11% 3 6% 6% 4 5% 3 5% 8% 40%	2077
	21st to end	Night	10% 9% 3 6% 4 4 4 4 6% 7% 43%	2025
May	1st to 10th	Night	11% 9% 3 6% 4 3 3 3 4 6% 48%	4966
	11th to 20th	Night	12% 5% 2 5% 4 4 4 3 5% 7% 48%	4701
	21st to end	Night	9% 6% 2 5% 4 5% 5% 5% 7% 8% 44%	5181
June	1st to 10th	Night	5% 4 2 4 4 4 4 4 7% 9% 53%	1680
	11th to 20th	Night	7% 3 1 3 3 3 3 3 6% 8% 59%	1680
	21st to end	Night	4 2 1 4 2 3 4 3 6% 7% 66%	1680
July	1st to 10th	Night	3 3 2 5% 4 4 5% 5% 8% 10% 52%	4740
	11th to 20th	Night	2 2 1 3 3 4 4 4 6% 9% 61%	4740
	21st to end	Night	4 3 2 4 3 4 4 4 7% 10% 55%	5263
August	1st to 10th	Night	6% 3 1 3 3 3 4 4 6% 9% 57%	4123
	11th to 20th	Night	2 3 1 4 4 4 5% 5% 7% 8% 57%	4156
	21st to end	Night	4 4 2 6% 3 4 5% 4 7% 8% 53%	5083
September	1st to 10th	Night	5% 4 1 3 3 3 3 3 5% 7% 64%	2000
	11th to 20th	Night	6% 4 1 3 2 4 4 3 5% 6% 62%	2000
	21st to end	Night	7% 4 1 4 2 2 2 2 3 6% 67%	2270
October	1st to 10th	Night	6% 5% 1 2 2 2 3 2 4 3 71%	3920
	11th to 20th	Night	8% 5% 1 4 3 3 3 3 5% 7% 59%	3898
	21st to end	Night	7% 7% 2 4 3 3 4 3 5% 7% 54%	4301
November	1st to 10th	Night	3 6% 2 5% 3 3 4 4 7% 9% 53%	2665
	11th to 20th	Night	3 6% 3 6% 5% 4 5% 4 6% 7% 52%	2912
	21st to end	Night	3 7% 3 5% 4 4 4 4 6% 7% 53%	2940
December	1st to 10th	Night	1 5% 2 5% 3 3 3 3 5% 7% 62%	4710
	11th to 20th	Night	2 4 2 5% 4 4 5% 4 7% 8% 56%	5321
	21st to end	Night	2 7% 3 7% 4 5% 5% 5% 7% 10% 45%	7128

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% 6% 19% 29% 15% 1	4723
	11th to 20th	Day	27% 7% 18% 28% 16% 4	4818
	21st to end	Day	22% 5% 16% 36% 19% 3	5844
February	1st to 10th	Day	33% 9% 17% 27% 12% 2	1890
	11th to 20th	Day	35% 7% 15% 25% 15% 3	1854
	21st to end	Day	27% 7% 18% 25% 20% 3	1720
March	1st to 10th	Day	25% 4 8% 37% 17% 9%	4783
	11th to 20th	Day	27% 7% 13% 32% 19% 1	5110
	21st to end	Day	36% 7% 17% 29% 11%	5775
April	1st to 10th	Day	32% 6% 18% 26% 16% 2	2520
	11th to 20th	Day	37% 5% 13% 25% 18% 2	2494
	21st to end	Day	41% 7% 10% 28% 13% 1	2593
May	1st to 10th	Day	35% 5% 14% 26% 17% 4	7979
	11th to 20th	Day	28% 5% 8% 27% 24% 8%	8195
	21st to end	Day	30% 6% 11% 33% 18% 2	9027
June	1st to 10th	Day	25% 7% 16% 28% 20% 6%	2940
	11th to 20th	Day	22% 6% 12% 24% 22% 14%	2940
	21st to end	Day	16% 5% 10% 24% 29% 16%	2940
July	1st to 10th	Day	16% 4 5% 22% 32% 22%	8181
	11th to 20th	Day	15% 3 7% 21% 26% 27%	8175
	21st to end	Day	22% 5% 9% 19% 24% 21%	8645
August	1st to 10th	Day	23% 4 6% 24% 22% 20%	4338
	11th to 20th	Day	21% 4 6% 21% 30% 18%	4310
	21st to end	Day	16% 4 8% 20% 30% 23%	4420
September	1st to 10th	Day	20% 4 6% 20% 30% 20%	2400
	11th to 20th	Day	21% 4 8% 23% 29% 15%	2400
	21st to end	Day	22% 4 9% 25% 32% 9%	2125
October	1st to 10th	Day	19% 3 7% 18% 30% 23%	3270
	11th to 20th	Day	26% 4 9% 29% 26% 7%	3250
	21st to end	Day	29% 4 10% 33% 20% 4	3467
November	1st to 10th	Day	27% 7% 12% 30% 21% 4	1853
	11th to 20th	Day	25% 6% 16% 32% 17% 4	1706
	21st to end	Day	28% 6% 15% 30% 19% 3	1680
December	1st to 10th	Day	21% 4 14% 38% 21% 1	2700
	11th to 20th	Day	25% 5% 25% 30% 14% 1	3104
	21st to end	Day	34% 6% 11% 30% 17% 2	4092
January	1st to 10th	Night	36% 5% 15% 29% 14% 1	8202
	11th to 20th	Night	32% 5% 18% 27% 14% 4	8213
	21st to end	Night	26% 5% 16% 30% 19% 3	8370
February	1st to 10th	Night	34% 7% 16% 28% 12% 2	2730
	11th to 20th	Night	35% 6% 14% 26% 15% 4	2663
	21st to end	Night	30% 6% 16% 24% 21% 3	2085
March	1st to 10th	Night	30% 4 8% 29% 22% 8%	5316
	11th to 20th	Night	28% 6% 13% 33% 19% 1	5110
	21st to end	Night	42% 8% 16% 24% 9% 1	5433
April	1st to 10th	Night	33% 7% 16% 25% 15% 3	2100
	11th to 20th	Night	43% 6% 10% 21% 17% 2	2077
	21st to end	Night	39% 7% 10% 27% 15% 2	2025
May	1st to 10th	Night	36% 5% 9% 31% 17% 1	4964
	11th to 20th	Night	34% 5% 9% 27% 18% 7%	4702
	21st to end	Night	34% 6% 13% 36% 10% 1	5182
June	1st to 10th	Night	26% 8% 17% 25% 18% 6%	1680
	11th to 20th	Night	23% 5% 13% 23% 25% 12%	1680
	21st to end	Night	18% 5% 9% 25% 28% 16%	1680
July	1st to 10th	Night	24% 5% 6% 27% 25% 13%	4700
	11th to 20th	Night	18% 4 7% 23% 24% 23%	4692
	21st to end	Night	24% 4 11% 21% 23% 17%	5148
August	1st to 10th	Night	24% 5% 8% 26% 19% 17%	3018
	11th to 20th	Night	26% 5% 10% 25% 22% 13%	3033
	21st to end	Night	17% 4 10% 19% 26% 23%	3687
September	1st to 10th	Night	20% 4 8% 21% 29% 18%	2000
	11th to 20th	Night	23% 5% 9% 22% 27% 15%	2000
	21st to end	Night	21% 3 8% 25% 30% 13%	2270
October	1st to 10th	Night	19% 4 6% 19% 32% 20%	3920
	11th to 20th	Night	26% 4 10% 27% 24% 9%	3898
	21st to end	Night	30% 4 8% 32% 19% 7%	4301
November	1st to 10th	Night	26% 7% 13% 27% 22% 5%	2665
	11th to 20th	Night	29% 7% 12% 31% 18% 4	2912
	21st to end	Night	28% 6% 14% 29% 20% 3	2940
December	1st to 10th	Night	23% 5% 15% 36% 20% 2	4710
	11th to 20th	Night	25% 6% 19% 37% 12% 1	5322
	21st to end	Night	32% 6% 13% 31% 15% 3	7128

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% 15% 32% 28% 13% 5%	330
	11th to 20th	Day	8% 12% 38% 29% 12% 2	330
	21st to end	Day	4 9% 41% 35% 10% 1	363
February	1st to 10th	Day	8% 17% 47% 22% 5%	330
	11th to 20th	Day	4 15% 44% 27% 9% 1	330
	21st to end	Day	8% 12% 39% 28% 11% 1	273
March	1st to 10th	Day	8% 17% 37% 25% 12%	409
	11th to 20th	Day	10% 14% 33% 28% 12% 3	540
	21st to end	Day	11% 13% 34% 29% 11% 2	594
April	1st to 10th	Day	10% 14% 31% 31% 10% 3	540
	11th to 20th	Day	12% 13% 36% 29% 10% 1	532
	21st to end	Day	17% 11% 23% 31% 13% 5%	540
May	1st to 10th	Day	17% 10% 25% 25% 17% 6%	664
	11th to 20th	Day	17% 9% 22% 26% 21% 5%	775
	21st to end	Day	12% 9% 25% 25% 21% 7%	844
June	1st to 10th	Day	5% 8% 24% 26% 26% 11%	780
	11th to 20th	Day	7% 9% 32% 29% 19% 3	780
	21st to end	Day	5% 7% 24% 32% 26% 6%	780
July	1st to 10th	Day	4 3 18% 31% 34% 9%	771
	11th to 20th	Day	3 3 17% 34% 32% 11%	768
	21st to end	Day	3 3 15% 35% 34% 10%	810
August	1st to 10th	Day	4 5% 19% 32% 30% 10%	567
	11th to 20th	Day	4 3 18% 37% 27% 11%	557
	21st to end	Day	3 7% 21% 33% 24% 13%	620
September	1st to 10th	Day	5% 4 21% 31% 26% 13%	530
	11th to 20th	Day	7% 8% 28% 27% 21% 9%	530
	21st to end	Day	9% 6% 26% 29% 21% 10%	400
October	1st to 10th	Day	10% 8% 24% 28% 21% 9%	330
	11th to 20th	Day	10% 12% 30% 25% 18% 4	330
	21st to end	Day	7% 11% 35% 32% 12% 2	363
November	1st to 10th	Day	8% 14% 34% 32% 10% 2	330
	11th to 20th	Day	6% 14% 41% 25% 12% 2	330
	21st to end	Day	10% 9% 38% 29% 12% 2	330
December	1st to 10th	Day	4 12% 45% 28% 11% 2	330
	11th to 20th	Day	9% 18% 43% 23% 7% 1	330
	21st to end	Day	12% 20% 29% 26% 10% 2	363
January	1st to 10th	Night	4 14% 36% 31% 13% 1	1050
	11th to 20th	Night	5% 14% 42% 28% 10% 1	1039
	21st to end	Night	5% 8% 45% 31% 10% 1	924
February	1st to 10th	Night	6% 10% 43% 32% 8% 1	840
	11th to 20th	Night	7% 10% 37% 35% 10% 1	824
	21st to end	Night	6% 15% 38% 31% 9% 1	692
March	1st to 10th	Night	8% 11% 36% 32% 11% 1	840
	11th to 20th	Night	8% 10% 33% 35% 14% 1	840
	21st to end	Night	12% 9% 30% 36% 13% 1	850
April	1st to 10th	Night	13% 10% 33% 37% 8%	630
	11th to 20th	Night	14% 8% 33% 36% 9%	623
	21st to end	Night	14% 8% 25% 35% 16% 1	630
May	1st to 10th	Night	14% 8% 25% 34% 17% 2	630
	11th to 20th	Night	17% 5% 24% 33% 20% 2	621
	21st to end	Night	12% 8% 24% 37% 18% 1	683
June	1st to 10th	Night	6% 7% 24% 39% 22% 2	630
	11th to 20th	Night	9% 8% 34% 38% 11%	630
	21st to end	Night	4 5% 25% 44% 20%	630
July	1st to 10th	Night	6% 1 22% 43% 27% 1	615
	11th to 20th	Night	3 3 21% 47% 24% 1	612
	21st to end	Night	5% 2 16% 49% 27% 1	690
August	1st to 10th	Night	3 3 20% 48% 26%	627
	11th to 20th	Night	3 3 19% 46% 28% 1	624
	21st to end	Night	3 3 22% 47% 23% 2	687
September	1st to 10th	Night	7% 3 21% 43% 25% 2	600
	11th to 20th	Night	7% 5% 24% 41% 20% 3	600
	21st to end	Night	10% 4 22% 38% 23% 2	740
October	1st to 10th	Night	13% 6% 26% 35% 18% 1	800
	11th to 20th	Night	12% 6% 29% 37% 15% 1	800
	21st to end	Night	9% 8% 31% 36% 13% 2	905
November	1st to 10th	Night	6% 9% 39% 34% 11% 1	1025
	11th to 20th	Night	6% 13% 42% 30% 8%	1050
	21st to end	Night	7% 9% 39% 34% 11% 1	1050
December	1st to 10th	Night	6% 12% 41% 31% 9%	1050
	11th to 20th	Night	8% 12% 41% 32% 7%	1050
	21st to end	Night	8% 15% 40% 28% 8% 1	1155

Page generated at 2026-08-27 02:07:16. Version ac07

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