Silverton
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Silverton clear sky chart from 2005-11-06 to 2026-09-14.

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	14% 7% 3 7% 4 4 4 3 5% 7% 42%	158536
	Night	16% 9% 3 7% 5% 4 4 3 5% 6% 37%	149943
Transparency	Day	42% 7% 12% 20% 14% 5%	153318
	Night	47% 7% 12% 18% 12% 4	146160
Seeing	Day	20% 11% 21% 27% 16% 4	17008
	Night	24% 9% 17% 30% 19% 2	25895

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	30% 16% 5% 13% 8% 6% 4 4 4 4 7%	4722
	11th to 20th	Day	20% 9% 2 6% 4 4 5% 4 6% 9% 31%	4740
	21st to end	Day	19% 9% 4 9% 6% 6% 6% 4 6% 6% 27%	5741
February	1st to 10th	Day	26% 13% 4 8% 5% 6% 5% 4 7% 8% 15%	1890
	11th to 20th	Day	24% 14% 4 11% 6% 5% 5% 3 5% 5% 18%	1845
	21st to end	Day	27% 14% 4 9% 5% 4 4 3 4 5% 19%	1646
March	1st to 10th	Day	15% 13% 4 10% 6% 6% 6% 4 7% 9% 19%	4856
	11th to 20th	Day	21% 14% 4 8% 5% 5% 4 3 4 5% 26%	5110
	21st to end	Day	19% 12% 4 11% 7% 6% 7% 5% 7% 7% 14%	5726
April	1st to 10th	Day	24% 12% 4 10% 6% 6% 5% 3 5% 6% 19%	2520
	11th to 20th	Day	20% 12% 3 8% 6% 5% 4 4 5% 6% 27%	2489
	21st to end	Day	20% 10% 3 8% 5% 5% 4 4 6% 7% 27%	2713
May	1st to 10th	Day	13% 7% 2 6% 4 4 4 3 5% 7% 44%	8023
	11th to 20th	Day	11% 8% 3 8% 5% 5% 5% 4 6% 9% 36%	8160
	21st to end	Day	14% 6% 3 6% 5% 5% 5% 4 6% 8% 39%	8992
June	1st to 10th	Day	19% 8% 3 7% 4 4 3 4 5% 7% 35%	2940
	11th to 20th	Day	16% 8% 3 7% 6% 5% 5% 4 7% 7% 33%	2940
	21st to end	Day	9% 6% 2 5% 4 4 4 3 5% 5% 53%	2940
July	1st to 10th	Day	3 2 1 2 2 2 2 2 4 6% 74%	8219
	11th to 20th	Day	3 2 1 2 2 2 2 2 3 4 79%	8220
	21st to end	Day	3 2 1 2 2 2 2 2 3 4 77%	8922
August	1st to 10th	Day	3 2 1 3 2 3 3 2 3 5% 72%	7550
	11th to 20th	Day	4 3 1 4 3 3 4 3 5% 8% 61%	7563
	21st to end	Day	6% 5% 1 4 3 3 4 3 6% 8% 57%	7957
September	1st to 10th	Day	7% 3 1 4 3 3 3 3 4 8% 61%	2520
	11th to 20th	Day	10% 7% 3 7% 5% 4 4 3 5% 7% 45%	2437
	21st to end	Day	14% 5% 2 6% 4 3 5% 3 5% 7% 46%	2101
October	1st to 10th	Day	15% 5% 2 5% 3 3 3 4 5% 8% 47%	3280
	11th to 20th	Day	16% 6% 3 8% 5% 5% 6% 4 6% 7% 33%	3261
	21st to end	Day	19% 10% 4 8% 6% 5% 6% 4 7% 9% 22%	3499
November	1st to 10th	Day	27% 11% 5% 10% 5% 4 5% 3 5% 7% 18%	1754
	11th to 20th	Day	26% 12% 4 9% 6% 5% 6% 5% 6% 4 16%	1680
	21st to end	Day	24% 10% 4 9% 6% 5% 5% 4 7% 7% 19%	1680
December	1st to 10th	Day	20% 8% 3 7% 7% 5% 4 4 5% 8% 27%	2700
	11th to 20th	Day	24% 13% 5% 13% 8% 7% 6% 5% 6% 5% 8%	3108
	21st to end	Day	30% 17% 5% 11% 7% 5% 5% 4 5% 4 7%	4092
January	1st to 10th	Night	27% 16% 5% 12% 6% 6% 5% 3 4 5% 11%	8172
	11th to 20th	Night	21% 9% 3 8% 5% 5% 4 3 4 7% 31%	8220
	21st to end	Night	20% 11% 3 10% 5% 4 3 3 4 6% 31%	8450
February	1st to 10th	Night	24% 13% 4 9% 6% 5% 5% 4 6% 6% 18%	2730
	11th to 20th	Night	27% 13% 3 8% 5% 3 3 3 5% 5% 24%	2662
	21st to end	Night	25% 15% 4 8% 5% 4 4 3 5% 5% 21%	2160
March	1st to 10th	Night	16% 12% 4 10% 7% 6% 6% 5% 6% 6% 22%	5254
	11th to 20th	Night	18% 12% 4 9% 6% 5% 4 3 5% 5% 29%	5110
	21st to end	Night	18% 14% 5% 11% 7% 6% 6% 4 6% 7% 16%	5489
April	1st to 10th	Night	24% 12% 3 9% 6% 5% 4 3 6% 6% 22%	2100
	11th to 20th	Night	19% 12% 3 7% 5% 5% 4 4 5% 6% 30%	2079
	21st to end	Night	20% 11% 3 9% 4 5% 4 4 5% 6% 30%	1905
May	1st to 10th	Night	15% 6% 2 6% 3 3 3 3 6% 7% 46%	4903
	11th to 20th	Night	15% 9% 3 8% 4 4 4 3 4 7% 38%	4700
	21st to end	Night	14% 6% 3 7% 5% 4 4 4 6% 9% 39%	5181
June	1st to 10th	Night	17% 7% 3 8% 5% 4 4 3 4 6% 38%	1680
	11th to 20th	Night	14% 9% 2 7% 5% 5% 4 3 5% 6% 40%	1680
	21st to end	Night	8% 6% 2 5% 4 3 3 2 4 5% 58%	1680
July	1st to 10th	Night	5% 2 1 2 2 2 2 2 4 5% 73%	4740
	11th to 20th	Night	2 2 1 3 2 2 2 2 3 3 79%	4740
	21st to end	Night	3 2 1 3 2 2 2 2 4 5% 76%	5153
August	1st to 10th	Night	4 3 1 2 2 2 3 2 4 6% 71%	5268
	11th to 20th	Night	7% 4 2 5% 3 4 4 2 5% 6% 58%	5292
	21st to end	Night	6% 3 2 4 3 3 3 3 5% 6% 62%	6281
September	1st to 10th	Night	7% 4 1 4 3 2 3 3 4 6% 64%	2100
	11th to 20th	Night	9% 6% 2 6% 4 5% 5% 3 5% 6% 50%	2030
	21st to end	Night	11% 5% 2 5% 4 3 3 3 5% 7% 52%	2294
October	1st to 10th	Night	13% 5% 2 5% 4 3 3 3 5% 9% 49%	3920
	11th to 20th	Night	14% 7% 3 7% 4 4 5% 4 6% 8% 38%	3897
	21st to end	Night	20% 10% 3 9% 5% 5% 5% 4 6% 7% 27%	4308
November	1st to 10th	Night	27% 9% 3 8% 6% 5% 5% 3 5% 6% 22%	2763
	11th to 20th	Night	30% 9% 3 10% 5% 5% 4 3 4 5% 21%	2940
	21st to end	Night	23% 10% 4 8% 6% 4 5% 4 5% 6% 26%	2940
December	1st to 10th	Night	17% 8% 3 6% 6% 6% 5% 4 5% 9% 31%	4700
	11th to 20th	Night	28% 13% 4 11% 7% 6% 5% 4 5% 5% 12%	5316
	21st to end	Night	27% 14% 5% 11% 6% 5% 5% 3 6% 5% 13%	7106

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	78% 9% 5% 6% 3	4723
	11th to 20th	Day	48% 6% 6% 16% 17% 9%	4740
	21st to end	Day	56% 6% 6% 13% 15% 5%	5741
February	1st to 10th	Day	65% 9% 12% 9% 5%	1890
	11th to 20th	Day	67% 11% 8% 8% 5% 2	1854
	21st to end	Day	67% 7% 9% 8% 7% 2	1646
March	1st to 10th	Day	57% 10% 13% 14% 5%	4856
	11th to 20th	Day	61% 7% 8% 17% 7%	5110
	21st to end	Day	66% 11% 12% 9% 3	5726
April	1st to 10th	Day	65% 9% 12% 11% 3	2520
	11th to 20th	Day	58% 9% 10% 12% 8% 2	2492
	21st to end	Day	56% 9% 12% 14% 7% 2	2713
May	1st to 10th	Day	37% 8% 12% 28% 15%	8023
	11th to 20th	Day	43% 7% 21% 22% 7%	8163
	21st to end	Day	41% 11% 22% 21% 5%	8994
June	1st to 10th	Day	49% 8% 15% 20% 7% 1	2940
	11th to 20th	Day	48% 10% 17% 16% 8% 1	2940
	21st to end	Day	32% 6% 16% 29% 11% 4	2940
July	1st to 10th	Day	14% 3 13% 34% 19% 17%	8151
	11th to 20th	Day	13% 2 6% 19% 41% 20%	8145
	21st to end	Day	13% 2 14% 37% 28% 4	8723
August	1st to 10th	Day	14% 3 7% 27% 31% 17%	5978
	11th to 20th	Day	21% 3 19% 37% 16% 3	5962
	21st to end	Day	24% 4 14% 34% 18% 6%	6236
September	1st to 10th	Day	22% 6% 14% 27% 20% 11%	2520
	11th to 20th	Day	38% 8% 14% 20% 14% 6%	2437
	21st to end	Day	36% 8% 15% 19% 16% 6%	2101
October	1st to 10th	Day	35% 6% 8% 23% 22% 5%	3280
	11th to 20th	Day	49% 6% 17% 16% 11% 1	3261
	21st to end	Day	56% 7% 10% 17% 9% 1	3499
November	1st to 10th	Day	65% 9% 12% 10% 4 1	1754
	11th to 20th	Day	66% 10% 10% 7% 6% 2	1680
	21st to end	Day	61% 10% 9% 8% 11% 1	1680
December	1st to 10th	Day	52% 8% 8% 12% 19% 2	2700
	11th to 20th	Day	71% 10% 9% 8% 1	3108
	21st to end	Day	73% 11% 9% 5% 2	4092
January	1st to 10th	Night	74% 8% 7% 7% 3 1	8172
	11th to 20th	Night	51% 7% 5% 16% 14% 7%	8220
	21st to end	Night	56% 5% 5% 12% 17% 6%	8450
February	1st to 10th	Night	66% 10% 9% 9% 5% 1	2730
	11th to 20th	Night	61% 8% 12% 10% 6% 2	2664
	21st to end	Night	63% 8% 10% 10% 7% 2	2160
March	1st to 10th	Night	57% 10% 13% 12% 6%	5254
	11th to 20th	Night	55% 9% 10% 15% 9% 1	5110
	21st to end	Night	65% 11% 13% 10% 1	5489
April	1st to 10th	Night	62% 9% 13% 11% 4	2100
	11th to 20th	Night	53% 10% 15% 12% 7% 3	2079
	21st to end	Night	54% 8% 15% 14% 6% 3	1905
May	1st to 10th	Night	35% 9% 12% 33% 11% 1	4901
	11th to 20th	Night	45% 7% 18% 24% 5% 1	4700
	21st to end	Night	38% 11% 25% 22% 4	5182
June	1st to 10th	Night	46% 9% 16% 23% 6% 1	1680
	11th to 20th	Night	43% 9% 18% 22% 8% 1	1680
	21st to end	Night	30% 6% 17% 30% 12% 4	1680
July	1st to 10th	Night	16% 3 11% 35% 20% 15%	4700
	11th to 20th	Night	13% 2 8% 17% 42% 17%	4692
	21st to end	Night	13% 2 20% 35% 26% 4	5040
August	1st to 10th	Night	10% 2 7% 33% 32% 16%	4163
	11th to 20th	Night	25% 3 21% 37% 11% 3	4169
	21st to end	Night	23% 3 16% 36% 16% 5%	4926
September	1st to 10th	Night	22% 6% 15% 27% 20% 11%	2100
	11th to 20th	Night	33% 8% 15% 22% 13% 9%	2030
	21st to end	Night	31% 7% 16% 22% 16% 8%	2294
October	1st to 10th	Night	34% 4 8% 28% 23% 3	3920
	11th to 20th	Night	43% 6% 18% 19% 12% 2	3897
	21st to end	Night	55% 7% 10% 17% 10% 2	4308
November	1st to 10th	Night	61% 10% 11% 11% 7%	2763
	11th to 20th	Night	66% 9% 8% 7% 7% 3	2940
	21st to end	Night	59% 8% 9% 10% 11% 3	2940
December	1st to 10th	Night	46% 10% 8% 20% 15% 1	4700
	11th to 20th	Night	72% 8% 10% 7% 3	5316
	21st to end	Night	66% 13% 13% 6% 2	7106

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	41% 15% 17% 16% 10% 1	220
	11th to 20th	Day	33% 8% 20% 18% 14% 7%	220
	21st to end	Day	27% 15% 19% 18% 16% 5%	436
February	1st to 10th	Day	35% 13% 20% 19% 11% 2	430
	11th to 20th	Day	33% 14% 25% 17% 9% 2	425
	21st to end	Day	36% 14% 21% 18% 10%	402
March	1st to 10th	Day	30% 17% 25% 20% 7% 1	540
	11th to 20th	Day	29% 16% 23% 21% 10% 2	540
	21st to end	Day	28% 19% 26% 20% 7%	594
April	1st to 10th	Day	31% 15% 23% 18% 11% 3	540
	11th to 20th	Day	26% 19% 24% 19% 9% 4	532
	21st to end	Day	24% 14% 25% 23% 13% 2	540
May	1st to 10th	Day	17% 14% 25% 27% 13% 4	543
	11th to 20th	Day	23% 12% 21% 25% 16% 3	568
	21st to end	Day	17% 9% 22% 29% 20% 4	617
June	1st to 10th	Day	20% 11% 20% 27% 18% 4	570
	11th to 20th	Day	17% 11% 26% 31% 13% 1	570
	21st to end	Day	10% 6% 20% 41% 19% 4	570
July	1st to 10th	Day	6% 4 19% 43% 24% 4	566
	11th to 20th	Day	4 5% 23% 42% 22% 5%	564
	21st to end	Day	4 2 19% 41% 27% 7%	619
August	1st to 10th	Day	4 4 15% 40% 30% 7%	567
	11th to 20th	Day	4 2 13% 43% 29% 8%	557
	21st to end	Day	5% 6% 23% 37% 22% 7%	627
September	1st to 10th	Day	9% 3 26% 30% 28% 4	570
	11th to 20th	Day	12% 7% 22% 30% 24% 5%	543
	21st to end	Day	16% 10% 23% 31% 15% 5%	530
October	1st to 10th	Day	19% 8% 23% 27% 18% 5%	530
	11th to 20th	Day	20% 11% 19% 27% 17% 5%	530
	21st to end	Day	26% 14% 24% 20% 10% 5%	548
November	1st to 10th	Day	29% 18% 21% 17% 12% 4	278
	11th to 20th	Day	28% 14% 23% 16% 13% 6%	220
	21st to end	Day	32% 12% 18% 17% 18% 4	220
December	1st to 10th	Day	37% 13% 10% 20% 12% 8%	220
	11th to 20th	Day	41% 15% 19% 16% 6% 2	220
	21st to end	Day	38% 17% 19% 13% 12% 2	242
January	1st to 10th	Night	36% 14% 18% 19% 12% 1	840
	11th to 20th	Night	33% 10% 16% 21% 18% 2	840
	21st to end	Night	29% 10% 15% 28% 15% 2	924
February	1st to 10th	Night	30% 11% 19% 26% 13% 1	840
	11th to 20th	Night	33% 11% 19% 24% 13% 1	824
	21st to end	Night	33% 14% 17% 27% 9%	692
March	1st to 10th	Night	29% 13% 20% 26% 11% 1	840
	11th to 20th	Night	27% 12% 20% 28% 12% 1	840
	21st to end	Night	30% 14% 21% 23% 10% 1	924
April	1st to 10th	Night	30% 9% 19% 26% 15% 1	840
	11th to 20th	Night	24% 11% 23% 26% 15% 1	831
	21st to end	Night	25% 8% 20% 31% 15% 1	645
May	1st to 10th	Night	17% 9% 22% 34% 16% 1	483
	11th to 20th	Night	19% 8% 19% 29% 24%	414
	21st to end	Night	20% 7% 16% 34% 22% 1	455
June	1st to 10th	Night	18% 6% 18% 37% 20% 1	420
	11th to 20th	Night	18% 6% 20% 41% 14% 1	420
	21st to end	Night	13% 1 15% 46% 23%	420
July	1st to 10th	Night	6% 3 9% 53% 29%	410
	11th to 20th	Night	5% 2 17% 47% 28% 1	408
	21st to end	Night	2 2 9% 50% 36% 1	460
August	1st to 10th	Night	5% 2 6% 40% 43% 5%	620
	11th to 20th	Night	6% 2 8% 35% 46% 4	624
	21st to end	Night	6% 4 15% 42% 30% 3	845
September	1st to 10th	Night	8% 3 15% 42% 30% 2	840
	11th to 20th	Night	12% 4 11% 34% 35% 4	812
	21st to end	Night	13% 6% 15% 36% 28% 3	800
October	1st to 10th	Night	18% 5% 19% 30% 24% 3	800
	11th to 20th	Night	19% 6% 16% 31% 25% 3	800
	21st to end	Night	26% 11% 20% 25% 15% 4	880
November	1st to 10th	Night	31% 11% 17% 28% 13%	820
	11th to 20th	Night	35% 14% 17% 22% 12% 1	840
	21st to end	Night	27% 10% 17% 29% 16% 1	840
December	1st to 10th	Night	31% 8% 16% 28% 15% 3	840
	11th to 20th	Night	38% 12% 16% 21% 13% 1	840
	21st to end	Night	39% 14% 17% 20% 9% 1	924

Page generated at 2026-09-11 01:11:56. Version ac07

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