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 2025-02-21.

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	13% 7% 2 6% 4 4 4 4 5% 7% 45%	108609
	Night	14% 7% 2 5% 4 4 4 3 5% 6% 47%	110148
Transparency	Day	37% 6% 11% 24% 18% 4	112245
	Night	36% 5% 11% 25% 19% 4	114653
Seeing	Day	17% 12% 26% 24% 16% 5%	15854
	Night	17% 11% 28% 28% 14% 2	25824

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	16% 9% 2 7% 4 3 4 3 4 4 44%	3774
	11th to 20th	Day	10% 7% 3 6% 4 3 4 3 4 6% 50%	4080
	21st to end	Day	20% 6% 2 5% 3 3 3 3 4 5% 45%	4488
February	1st to 10th	Day	18% 8% 3 7% 5% 4 4 3 4 5% 38%	1800
	11th to 20th	Day	16% 8% 3 7% 4 3 4 4 5% 6% 41%	1918
	21st to end	Day	16% 10% 3 6% 5% 4 5% 3 5% 6% 38%	1570
March	1st to 10th	Day	21% 11% 3 6% 3 3 3 2 4 4 40%	3297
	11th to 20th	Day	14% 7% 2 5% 4 3 3 3 4 5% 52%	3480
	21st to end	Day	16% 7% 2 6% 4 4 5% 3 6% 7% 40%	3957
April	1st to 10th	Day	13% 7% 3 7% 4 3 3 3 6% 6% 47%	2280
	11th to 20th	Day	16% 8% 3 6% 5% 4 4 3 4 7% 40%	2254
	21st to end	Day	18% 7% 3 7% 6% 4 4 4 5% 6% 38%	2280
May	1st to 10th	Day	14% 7% 3 7% 5% 5% 6% 5% 7% 8% 33%	3765
	11th to 20th	Day	16% 6% 3 7% 4 4 4 4 6% 8% 38%	4225
	21st to end	Day	13% 6% 3 7% 5% 4 5% 4 6% 8% 39%	4824
June	1st to 10th	Day	9% 4 2 6% 5% 5% 6% 4 6% 9% 43%	2660
	11th to 20th	Day	5% 4 2 5% 4 5% 5% 5% 7% 10% 49%	2660
	21st to end	Day	5% 3 2 5% 4 4 5% 5% 9% 10% 48%	2660
July	1st to 10th	Day	11% 4 2 6% 5% 5% 6% 4 6% 8% 43%	4440
	11th to 20th	Day	4 3 1 5% 4 4 5% 5% 7% 11% 50%	4262
	21st to end	Day	8% 6% 2 8% 7% 6% 6% 6% 8% 10% 34%	4465
August	1st to 10th	Day	4 3 1 5% 4 4 5% 4 6% 9% 55%	4086
	11th to 20th	Day	11% 5% 2 6% 4 4 5% 4 6% 8% 45%	3729
	21st to end	Day	6% 4 2 4 4 4 4 4 5% 8% 55%	4180
September	1st to 10th	Day	9% 6% 2 5% 4 5% 4 4 5% 8% 50%	2280
	11th to 20th	Day	10% 7% 2 6% 4 3 4 4 5% 7% 47%	2280
	21st to end	Day	10% 6% 2 5% 3 3 4 3 4 7% 54%	2040
October	1st to 10th	Day	10% 4 2 4 3 3 4 3 4 5% 57%	1900
	11th to 20th	Day	13% 7% 2 4 3 3 3 2 4 5% 55%	1900
	21st to end	Day	18% 7% 2 5% 3 3 3 2 4 5% 47%	2090
November	1st to 10th	Day	12% 6% 2 5% 3 3 3 4 4 4 54%	1840
	11th to 20th	Day	13% 7% 2 6% 3 3 3 3 4 6% 51%	1800
	21st to end	Day	20% 8% 2 5% 3 3 3 2 4 4 46%	1745
December	1st to 10th	Day	11% 8% 2 5% 3 3 3 2 5% 6% 52%	2610
	11th to 20th	Day	15% 8% 3 7% 4 4 3 2 4 5% 43%	3008
	21st to end	Day	21% 17% 3 7% 4 4 3 3 4 4 29%	3982
January	1st to 10th	Night	16% 8% 3 7% 4 4 4 3 5% 6% 39%	6187
	11th to 20th	Night	12% 7% 2 8% 4 4 3 3 4 5% 48%	5900
	21st to end	Night	19% 8% 2 6% 3 4 3 3 4 5% 44%	6490
February	1st to 10th	Night	17% 10% 3 6% 3 3 4 3 4 6% 41%	2600
	11th to 20th	Night	16% 7% 4 6% 4 3 3 3 4 6% 44%	2367
	21st to end	Night	19% 7% 3 6% 3 3 4 3 4 5% 43%	1885
March	1st to 10th	Night	22% 8% 3 5% 3 3 3 2 4 4 43%	3586
	11th to 20th	Night	15% 5% 2 5% 3 3 4 3 4 6% 50%	3480
	21st to end	Night	15% 8% 2 7% 4 4 5% 4 5% 7% 40%	3672
April	1st to 10th	Night	10% 6% 2 5% 3 4 4 4 5% 7% 52%	1900
	11th to 20th	Night	14% 6% 2 6% 4 3 3 4 5% 6% 45%	1875
	21st to end	Night	19% 6% 2 6% 4 3 3 3 5% 6% 42%	1900
May	1st to 10th	Night	17% 5% 2 4 4 3 3 4 5% 8% 43%	3130
	11th to 20th	Night	14% 7% 2 6% 3 4 4 3 5% 8% 43%	2659
	21st to end	Night	13% 8% 2 6% 5% 4 5% 4 6% 8% 39%	2743
June	1st to 10th	Night	7% 6% 2 5% 3 5% 4 4 8% 8% 49%	1520
	11th to 20th	Night	5% 3 1 3 2 3 3 3 6% 11% 60%	1520
	21st to end	Night	3 3 2 3 3 3 4 5% 7% 10% 58%	1520
July	1st to 10th	Night	9% 4 2 4 3 4 5% 4 7% 10% 47%	2530
	11th to 20th	Night	4 3 1 4 3 3 4 3 7% 8% 60%	2690
	21st to end	Night	8% 5% 3 8% 5% 5% 5% 5% 7% 8% 42%	3098
August	1st to 10th	Night	3 2 1 4 3 3 4 3 6% 9% 62%	2853
	11th to 20th	Night	8% 5% 2 6% 4 5% 5% 4 6% 8% 49%	3100
	21st to end	Night	5% 4 1 4 3 4 3 4 6% 8% 59%	3476
September	1st to 10th	Night	9% 5% 2 5% 3 3 3 3 6% 7% 54%	1900
	11th to 20th	Night	10% 5% 2 5% 3 4 5% 4 5% 7% 52%	1900
	21st to end	Night	9% 4 2 5% 4 3 4 3 5% 7% 54%	2117
October	1st to 10th	Night	7% 4 2 4 3 2 3 3 4 6% 63%	2280
	11th to 20th	Night	13% 5% 1 4 3 3 3 2 4 5% 57%	2280
	21st to end	Night	18% 6% 2 4 3 3 3 3 3 5% 50%	2508
November	1st to 10th	Night	13% 5% 2 3 3 3 3 3 4 5% 57%	2456
	11th to 20th	Night	13% 6% 2 5% 3 3 3 2 4 8% 51%	2600
	21st to end	Night	18% 7% 2 4 4 3 2 3 4 6% 47%	2651
December	1st to 10th	Night	14% 7% 2 5% 3 4 4 3 5% 5% 49%	4580
	11th to 20th	Night	18% 7% 3 6% 4 4 3 2 4 6% 44%	5199
	21st to end	Night	25% 13% 3 7% 4 4 4 3 4 5% 28%	6996

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	44% 4 10% 25% 16% 2	3775
	11th to 20th	Day	34% 4 8% 27% 20% 7%	4080
	21st to end	Day	41% 3 9% 17% 18% 11%	4488
February	1st to 10th	Day	48% 4 11% 22% 13% 2	1800
	11th to 20th	Day	44% 5% 11% 20% 16% 5%	1928
	21st to end	Day	46% 6% 7% 21% 13% 6%	1570
March	1st to 10th	Day	48% 4 7% 21% 12% 8%	3297
	11th to 20th	Day	35% 5% 8% 27% 19% 7%	3480
	21st to end	Day	42% 7% 15% 22% 13% 1	3957
April	1st to 10th	Day	38% 5% 11% 24% 20% 2	2280
	11th to 20th	Day	44% 5% 10% 20% 17% 4	2254
	21st to end	Day	47% 7% 10% 21% 15% 1	2280
May	1st to 10th	Day	44% 9% 14% 20% 12% 1	3800
	11th to 20th	Day	42% 8% 13% 23% 12% 1	4229
	21st to end	Day	41% 8% 10% 23% 16% 1	4837
June	1st to 10th	Day	36% 8% 14% 23% 17% 1	2660
	11th to 20th	Day	28% 9% 14% 28% 18% 3	2660
	21st to end	Day	26% 9% 19% 26% 15% 5%	2660
July	1st to 10th	Day	37% 7% 13% 27% 13% 3	4371
	11th to 20th	Day	25% 8% 14% 32% 18% 2	4188
	21st to end	Day	40% 12% 15% 24% 8% 1	4325
August	1st to 10th	Day	22% 8% 17% 29% 24% 1	4072
	11th to 20th	Day	37% 6% 13% 25% 17% 2	3765
	21st to end	Day	26% 7% 15% 35% 14% 3	4179
September	1st to 10th	Day	32% 7% 12% 27% 17% 5%	2280
	11th to 20th	Day	36% 6% 8% 23% 25% 1	2280
	21st to end	Day	30% 6% 9% 25% 26% 3	2040
October	1st to 10th	Day	17% 4 5% 17% 45% 12%	3160
	11th to 20th	Day	22% 3 6% 21% 41% 7%	3140
	21st to end	Day	30% 4 7% 30% 25% 4	3428
November	1st to 10th	Day	34% 4 10% 24% 22% 6%	1840
	11th to 20th	Day	35% 5% 6% 24% 22% 7%	1800
	21st to end	Day	43% 4 8% 25% 19% 2	1745
December	1st to 10th	Day	34% 4 8% 36% 15% 2	2610
	11th to 20th	Day	43% 4 7% 26% 18% 2	3005
	21st to end	Day	58% 4 11% 15% 10% 2	3982
January	1st to 10th	Night	45% 5% 10% 24% 15% 2	6187
	11th to 20th	Night	38% 5% 8% 25% 22% 3	5900
	21st to end	Night	43% 4 7% 22% 18% 5%	6490
February	1st to 10th	Night	45% 4 11% 22% 15% 2	2600
	11th to 20th	Night	42% 4 12% 20% 18% 4	2368
	21st to end	Night	43% 5% 8% 21% 17% 6%	1885
March	1st to 10th	Night	45% 5% 7% 22% 14% 7%	3586
	11th to 20th	Night	36% 6% 10% 25% 18% 5%	3480
	21st to end	Night	41% 7% 12% 28% 11%	3672
April	1st to 10th	Night	32% 5% 13% 23% 22% 4	1900
	11th to 20th	Night	37% 5% 11% 23% 17% 6%	1875
	21st to end	Night	42% 6% 10% 24% 15% 2	1900
May	1st to 10th	Night	38% 7% 16% 23% 14% 2	3160
	11th to 20th	Night	36% 10% 14% 22% 16% 1	2660
	21st to end	Night	40% 9% 12% 23% 15% 1	2751
June	1st to 10th	Night	31% 8% 16% 23% 21% 2	1520
	11th to 20th	Night	19% 5% 14% 36% 21% 5%	1520
	21st to end	Night	20% 7% 18% 29% 19% 8%	1520
July	1st to 10th	Night	30% 8% 17% 26% 17% 3	2490
	11th to 20th	Night	21% 6% 13% 37% 21% 2	2641
	21st to end	Night	36% 10% 16% 26% 10% 1	2999
August	1st to 10th	Night	18% 7% 18% 34% 22% 1	2843
	11th to 20th	Night	31% 8% 17% 25% 16% 3	3130
	21st to end	Night	22% 6% 14% 36% 18% 3	3476
September	1st to 10th	Night	29% 6% 11% 29% 20% 6%	1900
	11th to 20th	Night	31% 6% 10% 24% 26% 3	1900
	21st to end	Night	29% 5% 11% 23% 28% 4	2117
October	1st to 10th	Night	14% 3 7% 17% 44% 16%	3800
	11th to 20th	Night	22% 3 6% 19% 35% 16%	3779
	21st to end	Night	29% 4 7% 26% 29% 5%	4120
November	1st to 10th	Night	30% 4 10% 27% 23% 5%	2456
	11th to 20th	Night	34% 4 7% 24% 24% 8%	2600
	21st to end	Night	40% 4 8% 24% 22% 2	2651
December	1st to 10th	Night	37% 4 9% 32% 17% 1	4580
	11th to 20th	Night	43% 4 9% 23% 19% 2	5201
	21st to end	Night	57% 6% 11% 18% 8% 1	6996

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	29% 21% 32% 15% 2	300
	11th to 20th	Day	30% 27% 30% 12% 1	300
	21st to end	Day	22% 29% 37% 10% 2	330
February	1st to 10th	Day	28% 23% 34% 11% 3	300
	11th to 20th	Day	34% 27% 26% 11% 2 1	300
	21st to end	Day	36% 22% 32% 9% 1	225
March	1st to 10th	Day	27% 23% 35% 13% 2	270
	11th to 20th	Day	33% 22% 34% 8% 3	270
	21st to end	Day	31% 21% 35% 11% 3	376
April	1st to 10th	Day	24% 18% 33% 18% 6%	460
	11th to 20th	Day	26% 20% 30% 16% 7% 1	452
	21st to end	Day	20% 20% 34% 19% 5% 2	460
May	1st to 10th	Day	18% 21% 33% 20% 7% 1	463
	11th to 20th	Day	17% 13% 32% 22% 12% 4	637
	21st to end	Day	11% 10% 34% 27% 14% 4	734
June	1st to 10th	Day	10% 5% 25% 29% 24% 6%	680
	11th to 20th	Day	6% 4 21% 41% 23% 5%	680
	21st to end	Day	5% 6% 28% 36% 20% 5%	680
July	1st to 10th	Day	11% 3 16% 30% 31% 10%	671
	11th to 20th	Day	5% 3 19% 34% 30% 8%	668
	21st to end	Day	8% 4 15% 34% 30% 9%	739
August	1st to 10th	Day	8% 3 17% 37% 27% 9%	671
	11th to 20th	Day	15% 5% 18% 28% 28% 6%	479
	21st to end	Day	12% 1 14% 28% 31% 15%	539
September	1st to 10th	Day	9% 4 17% 34% 28% 8%	490
	11th to 20th	Day	10% 4 17% 28% 29% 12%	490
	21st to end	Day	15% 8% 22% 30% 18% 7%	430
October	1st to 10th	Day	12% 9% 27% 33% 15% 4	300
	11th to 20th	Day	19% 15% 36% 23% 7% 1	300
	21st to end	Day	25% 19% 31% 18% 5%	330
November	1st to 10th	Day	18% 13% 37% 23% 7% 1	300
	11th to 20th	Day	23% 15% 36% 17% 8% 1	300
	21st to end	Day	32% 15% 30% 16% 6% 2	300
December	1st to 10th	Day	21% 23% 33% 16% 6% 2	300
	11th to 20th	Day	26% 19% 33% 14% 7%	300
	21st to end	Day	33% 23% 29% 12% 3	330
January	1st to 10th	Night	23% 19% 34% 19% 4	1000
	11th to 20th	Night	24% 20% 35% 17% 3	1000
	21st to end	Night	20% 18% 39% 21% 2	1100
February	1st to 10th	Night	25% 16% 38% 20% 1	1000
	11th to 20th	Night	24% 20% 38% 17% 2	812
	21st to end	Night	24% 22% 33% 18% 2	629
March	1st to 10th	Night	25% 15% 33% 26% 2	678
	11th to 20th	Night	24% 14% 32% 26% 3	570
	21st to end	Night	25% 13% 35% 23% 5%	627
April	1st to 10th	Night	14% 17% 39% 25% 5%	570
	11th to 20th	Night	18% 17% 31% 26% 9%	563
	21st to end	Night	21% 11% 39% 22% 7% 1	570
May	1st to 10th	Night	14% 10% 37% 29% 10%	570
	11th to 20th	Night	18% 6% 32% 27% 15% 2	561
	21st to end	Night	13% 6% 27% 35% 17% 4	617
June	1st to 10th	Night	7% 6% 18% 36% 31% 2	570
	11th to 20th	Night	6% 1 19% 47% 25% 2	570
	21st to end	Night	3 2 22% 41% 30% 3	570
July	1st to 10th	Night	10% 1 12% 33% 36% 8%	555
	11th to 20th	Night	5% 2 11% 41% 35% 6%	552
	21st to end	Night	5% 1 14% 37% 37% 5%	624
August	1st to 10th	Night	5% 2 11% 38% 39% 4	567
	11th to 20th	Night	8% 2 14% 37% 36% 4	564
	21st to end	Night	9% 1 9% 34% 40% 7%	627
September	1st to 10th	Night	10% 2 14% 34% 36% 4	570
	11th to 20th	Night	13% 1 14% 30% 36% 6%	570
	21st to end	Night	11% 6% 22% 30% 28% 3	731
October	1st to 10th	Night	9% 4 24% 34% 25% 3	760
	11th to 20th	Night	15% 11% 31% 29% 14% 1	760
	21st to end	Night	22% 12% 29% 28% 8%	836
November	1st to 10th	Night	16% 9% 34% 30% 11% 1	780
	11th to 20th	Night	18% 12% 33% 28% 9% 1	800
	21st to end	Night	23% 14% 31% 24% 8%	851
December	1st to 10th	Night	23% 14% 32% 26% 5%	1000
	11th to 20th	Night	26% 12% 30% 26% 5%	1000
	21st to end	Night	29% 15% 32% 21% 3	1100

Page generated at 2025-02-18 02:26:33. Version ac07

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