Powders Mill
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Powders Mill clear sky chart from 2005-11-06 to 2026-06-16.

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	12% 7% 2 6% 4 4 4 4 5% 7% 44%	137390
	Night	13% 7% 2 6% 4 4 4 3 5% 6% 46%	134888
Transparency	Day	37% 6% 11% 24% 18% 4	140987
	Night	36% 5% 11% 24% 19% 4	139371
Seeing	Day	17% 12% 27% 24% 15% 4	17683
	Night	17% 11% 28% 28% 14% 2	27518

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	15% 9% 2 6% 4 3 4 3 4 5% 45%	4904
	11th to 20th	Day	8% 6% 2 6% 4 4 4 3 4 6% 53%	5310
	21st to end	Day	18% 7% 2 5% 3 3 3 3 4 5% 47%	5812
February	1st to 10th	Day	18% 8% 3 7% 4 4 4 3 4 5% 39%	1890
	11th to 20th	Day	16% 7% 2 7% 4 3 4 4 5% 6% 42%	2017
	21st to end	Day	15% 9% 3 6% 5% 4 5% 4 5% 6% 39%	1730
March	1st to 10th	Day	17% 11% 3 6% 3 3 3 3 4 4 42%	4832
	11th to 20th	Day	10% 6% 1 4 3 3 3 3 5% 6% 54%	5090
	21st to end	Day	15% 6% 2 6% 4 5% 5% 4 7% 7% 40%	5786
April	1st to 10th	Day	14% 7% 3 7% 4 3 3 3 6% 6% 46%	2520
	11th to 20th	Day	16% 8% 3 6% 5% 4 4 3 4 7% 40%	2494
	21st to end	Day	17% 7% 3 6% 6% 4 4 4 5% 6% 37%	2520
May	1st to 10th	Day	14% 6% 3 7% 5% 6% 6% 5% 7% 8% 34%	7021
	11th to 20th	Day	15% 7% 3 8% 5% 5% 5% 4 6% 7% 36%	7884
	21st to end	Day	21% 8% 2 7% 5% 4 5% 3 5% 7% 32%	9047
June	1st to 10th	Day	9% 5% 2 6% 5% 5% 6% 4 6% 9% 43%	2940
	11th to 20th	Day	5% 4 2 5% 5% 5% 5% 5% 7% 9% 47%	2871
	21st to end	Day	5% 3 1 4 4 4 5% 5% 9% 10% 48%	2800
July	1st to 10th	Day	8% 4 2 6% 5% 5% 6% 4 6% 8% 46%	6360
	11th to 20th	Day	5% 4 1 6% 4 5% 6% 5% 8% 11% 45%	6112
	21st to end	Day	6% 5% 2 6% 5% 5% 5% 5% 8% 11% 41%	6422
August	1st to 10th	Day	4 3 1 4 3 4 5% 4 6% 9% 56%	4216
	11th to 20th	Day	11% 5% 2 6% 4 4 5% 4 6% 8% 45%	3849
	21st to end	Day	6% 4 2 4 4 4 4 4 5% 8% 54%	4312
September	1st to 10th	Day	9% 6% 2 5% 4 5% 4 4 5% 8% 50%	2400
	11th to 20th	Day	10% 7% 2 6% 4 3 4 4 5% 7% 48%	2400
	21st to end	Day	11% 6% 2 5% 3 3 4 3 5% 7% 53%	2147
October	1st to 10th	Day	9% 4 2 4 3 3 4 4 4 5% 58%	2000
	11th to 20th	Day	12% 6% 2 5% 3 3 3 2 4 5% 56%	2000
	21st to end	Day	19% 7% 2 5% 3 3 3 2 4 5% 47%	2200
November	1st to 10th	Day	12% 6% 2 5% 3 3 3 4 4 5% 54%	1934
	11th to 20th	Day	13% 6% 2 6% 3 3 3 3 4 5% 51%	1890
	21st to end	Day	20% 8% 2 5% 3 3 3 2 3 5% 46%	1832
December	1st to 10th	Day	11% 8% 2 5% 3 3 3 2 5% 5% 52%	2690
	11th to 20th	Day	15% 8% 3 7% 5% 4 3 2 4 6% 44%	3088
	21st to end	Day	21% 17% 3 7% 4 4 3 3 4 4 30%	4070
January	1st to 10th	Night	15% 8% 3 7% 5% 4 4 3 4 5% 42%	8015
	11th to 20th	Night	9% 7% 2 7% 4 3 3 3 4 5% 53%	7680
	21st to end	Night	17% 9% 2 6% 4 4 4 3 4 5% 42%	8402
February	1st to 10th	Night	16% 10% 3 6% 3 3 4 3 4 6% 43%	2730
	11th to 20th	Night	17% 7% 3 6% 4 3 3 2 4 6% 44%	2488
	21st to end	Night	17% 7% 3 6% 3 3 4 3 4 5% 45%	2076
March	1st to 10th	Night	19% 9% 3 6% 4 3 3 2 4 4 41%	5272
	11th to 20th	Night	12% 7% 2 6% 3 3 4 3 4 6% 49%	5090
	21st to end	Night	14% 7% 2 7% 4 4 4 4 5% 7% 41%	5383
April	1st to 10th	Night	11% 6% 2 5% 3 4 4 3 5% 7% 51%	2100
	11th to 20th	Night	15% 6% 2 5% 4 3 3 4 5% 6% 45%	2075
	21st to end	Night	19% 6% 2 6% 4 3 3 4 5% 6% 42%	2100
May	1st to 10th	Night	15% 6% 2 6% 4 4 4 4 7% 8% 40%	5832
	11th to 20th	Night	13% 7% 3 7% 4 4 4 4 5% 8% 42%	4969
	21st to end	Night	20% 10% 3 8% 5% 4 5% 4 5% 6% 30%	5139
June	1st to 10th	Night	8% 6% 2 5% 4 5% 4 4 7% 8% 48%	1680
	11th to 20th	Night	6% 4 2 3 2 3 3 3 6% 10% 58%	1640
	21st to end	Night	3 3 2 3 3 3 4 5% 6% 10% 59%	1600
July	1st to 10th	Night	8% 4 2 5% 4 4 5% 4 7% 9% 46%	3618
	11th to 20th	Night	4 4 2 5% 4 4 4 4 8% 9% 53%	3847
	21st to end	Night	6% 4 2 6% 4 4 5% 4 7% 10% 48%	4449
August	1st to 10th	Night	3 2 1 4 3 3 4 3 6% 9% 62%	2943
	11th to 20th	Night	8% 5% 2 6% 4 5% 5% 4 6% 8% 50%	3200
	21st to end	Night	5% 4 1 4 3 4 3 4 6% 8% 59%	3586
September	1st to 10th	Night	9% 5% 2 5% 3 3 3 3 6% 7% 54%	2000
	11th to 20th	Night	10% 5% 2 5% 3 4 5% 4 5% 8% 52%	2000
	21st to end	Night	9% 4 2 5% 3 3 4 3 5% 7% 55%	2229
October	1st to 10th	Night	7% 4 2 4 3 2 3 3 4 6% 63%	2400
	11th to 20th	Night	13% 5% 1 4 3 3 3 2 4 5% 58%	2400
	21st to end	Night	19% 6% 2 4 3 3 3 3 3 4 49%	2640
November	1st to 10th	Night	13% 5% 2 3 2 2 3 3 4 5% 58%	2582
	11th to 20th	Night	13% 6% 2 4 3 3 3 2 4 7% 51%	2730
	21st to end	Night	18% 7% 2 4 4 3 2 3 4 6% 46%	2784
December	1st to 10th	Night	13% 7% 2 5% 3 4 4 3 5% 5% 49%	4720
	11th to 20th	Night	18% 7% 3 6% 4 4 3 2 4 6% 45%	5339
	21st to end	Night	25% 12% 3 7% 4 4 4 3 4 5% 29%	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	42% 3 10% 27% 14% 2	4905
	11th to 20th	Day	32% 4 7% 29% 22% 6%	5310
	21st to end	Day	41% 3 9% 19% 17% 10%	5812
February	1st to 10th	Day	47% 4 11% 22% 14% 1	1890
	11th to 20th	Day	44% 5% 10% 21% 16% 4	2027
	21st to end	Day	45% 6% 7% 22% 15% 7%	1730
March	1st to 10th	Day	45% 4 6% 22% 13% 11%	4832
	11th to 20th	Day	30% 4 7% 25% 23% 10%	5090
	21st to end	Day	41% 6% 13% 27% 13% 1	5788
April	1st to 10th	Day	38% 5% 12% 23% 19% 2	2520
	11th to 20th	Day	44% 5% 11% 20% 17% 4	2494
	21st to end	Day	46% 7% 11% 21% 14% 1	2520
May	1st to 10th	Day	44% 8% 12% 23% 13% 1	7056
	11th to 20th	Day	45% 7% 12% 18% 15% 2	7888
	21st to end	Day	50% 7% 9% 20% 12% 2	9061
June	1st to 10th	Day	37% 8% 13% 24% 17% 1	2940
	11th to 20th	Day	29% 9% 14% 27% 18% 3	2871
	21st to end	Day	26% 9% 19% 26% 16% 4	2800
July	1st to 10th	Day	35% 8% 12% 25% 19% 2	6291
	11th to 20th	Day	29% 10% 17% 29% 13% 2	6038
	21st to end	Day	32% 11% 15% 29% 12% 1	6240
August	1st to 10th	Day	22% 8% 17% 30% 23% 1	4202
	11th to 20th	Day	36% 6% 13% 26% 16% 2	3885
	21st to end	Day	27% 7% 15% 35% 14% 3	4311
September	1st to 10th	Day	32% 7% 12% 27% 17% 5%	2400
	11th to 20th	Day	35% 6% 8% 24% 25% 1	2400
	21st to end	Day	31% 6% 9% 24% 27% 3	2147
October	1st to 10th	Day	17% 4 5% 17% 45% 12%	3260
	11th to 20th	Day	22% 3 6% 21% 41% 7%	3240
	21st to end	Day	31% 4 7% 30% 25% 4	3538
November	1st to 10th	Day	33% 4 9% 25% 22% 6%	1934
	11th to 20th	Day	35% 5% 7% 24% 22% 7%	1890
	21st to end	Day	43% 4 8% 25% 18% 2	1832
December	1st to 10th	Day	34% 4 9% 36% 15% 2	2690
	11th to 20th	Day	43% 4 7% 26% 18% 2	3085
	21st to end	Day	58% 4 11% 15% 10% 2	4070
January	1st to 10th	Night	43% 4 12% 26% 13% 1	8016
	11th to 20th	Night	34% 4 6% 25% 26% 4	7680
	21st to end	Night	44% 4 8% 21% 18% 4	8402
February	1st to 10th	Night	43% 4 12% 23% 16% 2	2730
	11th to 20th	Night	42% 4 12% 21% 18% 4	2489
	21st to end	Night	41% 5% 8% 22% 19% 6%	2076
March	1st to 10th	Night	47% 4 7% 20% 15% 8%	5272
	11th to 20th	Night	36% 5% 8% 24% 18% 10%	5090
	21st to end	Night	40% 6% 12% 27% 14% 1	5383
April	1st to 10th	Night	32% 5% 14% 23% 21% 4	2100
	11th to 20th	Night	38% 5% 12% 22% 18% 5%	2075
	21st to end	Night	42% 7% 10% 24% 15% 2	2100
May	1st to 10th	Night	39% 7% 16% 24% 12% 1	5862
	11th to 20th	Night	39% 8% 12% 25% 14% 3	4970
	21st to end	Night	53% 8% 10% 17% 10% 2	5149
June	1st to 10th	Night	31% 8% 15% 23% 21% 2	1680
	11th to 20th	Night	21% 5% 15% 34% 21% 5%	1640
	21st to end	Night	20% 7% 18% 29% 20% 7%	1600
July	1st to 10th	Night	32% 8% 14% 22% 22% 2	3580
	11th to 20th	Night	25% 9% 16% 32% 15% 2	3801
	21st to end	Night	28% 9% 20% 29% 13% 1	4320
August	1st to 10th	Night	18% 7% 18% 34% 22% 1	2933
	11th to 20th	Night	31% 8% 17% 25% 16% 3	3230
	21st to end	Night	22% 6% 14% 36% 19% 3	3586
September	1st to 10th	Night	29% 6% 11% 29% 20% 5%	2000
	11th to 20th	Night	31% 7% 10% 25% 25% 3	2000
	21st to end	Night	29% 5% 11% 22% 29% 4	2229
October	1st to 10th	Night	14% 3 7% 17% 44% 16%	3920
	11th to 20th	Night	22% 3 6% 20% 34% 15%	3899
	21st to end	Night	30% 4 7% 25% 29% 5%	4252
November	1st to 10th	Night	29% 4 10% 28% 24% 5%	2582
	11th to 20th	Night	33% 4 8% 25% 23% 7%	2730
	21st to end	Night	41% 4 8% 24% 21% 2	2784
December	1st to 10th	Night	37% 4 9% 32% 17% 1	4720
	11th to 20th	Night	42% 4 9% 23% 19% 2	5341
	21st to end	Night	56% 6% 11% 18% 8% 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% 19% 33% 15% 3	330
	11th to 20th	Day	29% 28% 31% 12% 1	330
	21st to end	Day	23% 29% 37% 9% 2	363
February	1st to 10th	Day	26% 24% 35% 12% 4	330
	11th to 20th	Day	33% 26% 28% 10% 2 1	330
	21st to end	Day	33% 21% 33% 11% 1	273
March	1st to 10th	Day	28% 22% 35% 13% 2	330
	11th to 20th	Day	32% 23% 36% 7% 2	330
	21st to end	Day	30% 22% 36% 10% 3	450
April	1st to 10th	Day	26% 18% 33% 18% 6%	540
	11th to 20th	Day	26% 21% 31% 15% 6% 1	532
	21st to end	Day	20% 19% 34% 19% 6% 1	540
May	1st to 10th	Day	17% 22% 34% 19% 6% 1	543
	11th to 20th	Day	19% 14% 32% 21% 11% 3	733
	21st to end	Day	14% 11% 32% 27% 13% 4	844
June	1st to 10th	Day	10% 6% 27% 28% 22% 6%	780
	11th to 20th	Day	7% 5% 23% 39% 22% 5%	756
	21st to end	Day	5% 5% 27% 38% 20% 5%	730
July	1st to 10th	Day	10% 3 16% 30% 30% 10%	721
	11th to 20th	Day	5% 3 19% 34% 30% 8%	718
	21st to end	Day	8% 3 15% 35% 31% 9%	794
August	1st to 10th	Day	7% 3 17% 37% 27% 8%	721
	11th to 20th	Day	14% 5% 17% 28% 29% 8%	519
	21st to end	Day	11% 1 15% 29% 30% 14%	583
September	1st to 10th	Day	8% 4 17% 35% 27% 8%	530
	11th to 20th	Day	10% 4 16% 28% 30% 13%	530
	21st to end	Day	16% 8% 22% 29% 18% 8%	467
October	1st to 10th	Day	11% 8% 28% 34% 15% 4	330
	11th to 20th	Day	18% 15% 35% 23% 8% 1	330
	21st to end	Day	27% 19% 31% 17% 5%	363
November	1st to 10th	Day	18% 15% 37% 23% 7% 1	330
	11th to 20th	Day	22% 15% 37% 18% 8% 1	330
	21st to end	Day	33% 16% 28% 16% 5% 2	330
December	1st to 10th	Day	20% 23% 33% 17% 5% 2	330
	11th to 20th	Day	25% 20% 34% 14% 7%	330
	21st to end	Day	32% 22% 31% 12% 3	363
January	1st to 10th	Night	22% 19% 35% 20% 4	1050
	11th to 20th	Night	23% 21% 35% 17% 3	1050
	21st to end	Night	20% 18% 39% 21% 2	1155
February	1st to 10th	Night	24% 15% 39% 21% 1	1050
	11th to 20th	Night	25% 19% 37% 17% 2	853
	21st to end	Night	23% 22% 34% 19% 2	692
March	1st to 10th	Night	25% 16% 33% 24% 1	749
	11th to 20th	Night	24% 15% 32% 25% 3	630
	21st to end	Night	24% 14% 34% 23% 5%	693
April	1st to 10th	Night	15% 17% 39% 25% 4	630
	11th to 20th	Night	20% 16% 31% 25% 8%	623
	21st to end	Night	21% 11% 38% 22% 8% 1	630
May	1st to 10th	Night	13% 10% 37% 30% 9%	630
	11th to 20th	Night	19% 7% 32% 27% 14% 2	621
	21st to end	Night	14% 6% 27% 34% 16% 3	683
June	1st to 10th	Night	8% 6% 19% 36% 29% 2	630
	11th to 20th	Night	7% 2 20% 45% 24% 2	615
	21st to end	Night	3 2 21% 41% 30% 3	600
July	1st to 10th	Night	10% 2 12% 32% 36% 9%	585
	11th to 20th	Night	4 2 11% 42% 36% 6%	582
	21st to end	Night	5% 1 13% 37% 38% 5%	657
August	1st to 10th	Night	5% 3 11% 39% 38% 4	597
	11th to 20th	Night	7% 2 13% 37% 38% 3	594
	21st to end	Night	9% 1 10% 35% 39% 7%	660
September	1st to 10th	Night	10% 2 14% 35% 35% 4	600
	11th to 20th	Night	13% 1 13% 30% 37% 6%	600
	21st to end	Night	11% 6% 21% 30% 28% 4	770
October	1st to 10th	Night	9% 4 23% 35% 25% 3	800
	11th to 20th	Night	15% 11% 29% 30% 15% 1	800
	21st to end	Night	23% 13% 29% 28% 8%	880
November	1st to 10th	Night	16% 9% 34% 30% 11%	820
	11th to 20th	Night	18% 12% 33% 29% 9%	840
	21st to end	Night	24% 14% 31% 24% 8%	894
December	1st to 10th	Night	22% 14% 33% 26% 5%	1050
	11th to 20th	Night	26% 12% 31% 26% 4	1050
	21st to end	Night	29% 15% 32% 21% 3	1155

Page generated at 2026-06-13 01:31:39. Version ac07

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