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 2025-05-11.

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	17% 9% 3 7% 5% 4 4 3 5% 6% 36%	118029
	Night	18% 10% 3 8% 5% 4 4 3 5% 6% 34%	119742
Transparency	Day	47% 8% 12% 17% 13% 4	117772
	Night	50% 8% 12% 16% 11% 4	119588
Seeing	Day	20% 11% 21% 27% 17% 4	15240
	Night	24% 9% 17% 30% 19% 2	24334

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	33% 16% 5% 13% 7% 5% 4 3 3 4 8%	3639
	11th to 20th	Day	26% 11% 3 7% 4 5% 5% 4 6% 7% 21%	3640
	21st to end	Day	20% 9% 3 8% 5% 5% 5% 4 5% 5% 29%	4436
February	1st to 10th	Day	26% 13% 4 8% 5% 6% 5% 4 7% 7% 15%	1800
	11th to 20th	Day	24% 13% 4 11% 6% 5% 5% 3 5% 5% 18%	1755
	21st to end	Day	27% 14% 4 10% 5% 4 4 4 4 5% 19%	1570
March	1st to 10th	Day	15% 13% 4 10% 6% 6% 6% 5% 7% 9% 20%	4751
	11th to 20th	Day	21% 14% 4 8% 5% 5% 4 3 4 5% 26%	5000
	21st to end	Day	19% 12% 4 12% 7% 6% 7% 5% 7% 7% 14%	5603
April	1st to 10th	Day	25% 12% 4 10% 6% 6% 5% 3 5% 6% 18%	2400
	11th to 20th	Day	21% 12% 3 8% 6% 4 4 4 5% 6% 27%	2369
	21st to end	Day	20% 10% 3 8% 5% 5% 4 4 6% 7% 26%	2584
May	1st to 10th	Day	20% 9% 3 7% 5% 4 4 4 5% 6% 33%	4463
	11th to 20th	Day	11% 5% 2 6% 4 4 4 3 5% 7% 49%	4380
	21st to end	Day	19% 8% 4 8% 6% 6% 6% 4 6% 7% 27%	4813
June	1st to 10th	Day	20% 9% 3 7% 4 4 3 4 5% 6% 33%	2660
	11th to 20th	Day	18% 8% 3 7% 7% 5% 5% 4 7% 7% 29%	2660
	21st to end	Day	8% 5% 2 5% 4 4 4 3 5% 5% 55%	2660
July	1st to 10th	Day	4 2 1 2 2 2 2 2 3 5% 75%	4420
	11th to 20th	Day	3 2 1 2 2 2 2 2 3 4 76%	4420
	21st to end	Day	5% 2 1 2 2 2 2 1 3 4 76%	4790
August	1st to 10th	Day	2 2 1 3 3 3 3 3 4 6% 69%	4094
	11th to 20th	Day	5% 3 1 5% 3 3 4 3 6% 7% 60%	4026
	21st to end	Day	7% 5% 2 4 3 3 4 3 5% 7% 57%	4287
September	1st to 10th	Day	7% 3 1 3 3 3 3 3 4 7% 64%	2280
	11th to 20th	Day	10% 7% 3 7% 5% 4 4 4 5% 7% 45%	2280
	21st to end	Day	14% 5% 2 6% 4 3 5% 3 5% 7% 45%	1996
October	1st to 10th	Day	15% 5% 2 5% 3 4 3 4 5% 8% 47%	3180
	11th to 20th	Day	16% 6% 3 7% 5% 5% 6% 4 6% 7% 33%	3161
	21st to end	Day	20% 10% 4 8% 6% 6% 5% 4 7% 9% 22%	3391
November	1st to 10th	Day	28% 12% 5% 9% 5% 4 5% 3 5% 7% 18%	1669
	11th to 20th	Day	27% 12% 4 9% 7% 5% 6% 4 6% 4 17%	1600
	21st to end	Day	24% 11% 4 10% 6% 5% 5% 4 6% 6% 19%	1600
December	1st to 10th	Day	20% 8% 3 6% 7% 5% 5% 4 5% 9% 28%	2620
	11th to 20th	Day	24% 13% 5% 12% 8% 7% 7% 5% 6% 5% 8%	3028
	21st to end	Day	31% 17% 5% 11% 7% 5% 5% 4 5% 4 7%	4004
January	1st to 10th	Night	29% 17% 6% 12% 6% 5% 5% 3 3 4 11%	6306
	11th to 20th	Night	27% 11% 3 8% 5% 6% 4 3 4 6% 22%	6320
	21st to end	Night	23% 12% 3 8% 4 4 3 3 4 5% 32%	6534
February	1st to 10th	Night	24% 14% 4 9% 6% 5% 5% 4 6% 5% 17%	2600
	11th to 20th	Night	28% 13% 3 8% 4 3 3 3 5% 5% 25%	2532
	21st to end	Night	25% 16% 4 8% 5% 4 4 3 5% 5% 21%	2060
March	1st to 10th	Night	16% 12% 4 10% 7% 6% 6% 5% 6% 6% 22%	5139
	11th to 20th	Night	17% 12% 4 9% 6% 5% 4 3 5% 5% 29%	5000
	21st to end	Night	18% 14% 5% 12% 7% 6% 6% 4 6% 7% 15%	5370
April	1st to 10th	Night	25% 12% 3 9% 6% 5% 4 3 6% 6% 21%	2000
	11th to 20th	Night	19% 12% 3 7% 5% 4 4 4 5% 6% 31%	1979
	21st to end	Night	21% 11% 3 9% 4 5% 4 4 5% 6% 29%	1814
May	1st to 10th	Night	21% 10% 3 8% 4 4 3 3 4 5% 36%	2719
	11th to 20th	Night	10% 6% 2 6% 3 3 4 3 5% 7% 51%	2519
	21st to end	Night	17% 7% 3 8% 6% 5% 5% 4 6% 8% 31%	2762
June	1st to 10th	Night	18% 8% 3 8% 5% 4 4 3 4 7% 36%	1520
	11th to 20th	Night	15% 10% 2 7% 5% 5% 4 4 5% 6% 37%	1520
	21st to end	Night	7% 6% 2 5% 4 3 3 2 4 4 59%	1520
July	1st to 10th	Night	4 1 1 2 2 2 2 2 3 3 78%	2540
	11th to 20th	Night	3 2 1 3 2 2 2 2 3 3 78%	2540
	21st to end	Night	3 2 1 2 1 1 1 2 3 4 79%	2761
August	1st to 10th	Night	2 2 1 2 2 2 2 2 4 7% 75%	2841
	11th to 20th	Night	4 4 2 5% 4 4 4 2 5% 7% 59%	2812
	21st to end	Night	9% 4 2 4 3 3 2 2 3 5% 63%	3388
September	1st to 10th	Night	7% 3 1 3 3 2 3 3 4 6% 66%	1900
	11th to 20th	Night	9% 6% 2 5% 4 5% 5% 3 5% 6% 50%	1900
	21st to end	Night	11% 5% 2 5% 4 3 3 3 5% 7% 52%	2179
October	1st to 10th	Night	13% 5% 2 5% 4 3 3 3 5% 9% 49%	3800
	11th to 20th	Night	14% 7% 3 7% 4 4 5% 4 6% 8% 38%	3777
	21st to end	Night	20% 10% 3 9% 5% 5% 5% 4 6% 7% 27%	4174
November	1st to 10th	Night	27% 9% 3 8% 6% 5% 5% 3 5% 6% 23%	2628
	11th to 20th	Night	30% 9% 3 9% 6% 5% 4 4 4 5% 21%	2800
	21st to end	Night	23% 10% 3 7% 6% 4 5% 4 6% 5% 27%	2800
December	1st to 10th	Night	17% 7% 3 6% 6% 6% 5% 4 5% 9% 32%	4560
	11th to 20th	Night	28% 13% 4 11% 7% 6% 5% 4 5% 5% 12%	5176
	21st to end	Night	27% 14% 5% 11% 6% 5% 5% 4 6% 5% 13%	6952

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	77% 9% 4 5% 3 1	3639
	11th to 20th	Day	60% 6% 6% 18% 10% 1	3640
	21st to end	Day	55% 7% 6% 13% 14% 4	4436
February	1st to 10th	Day	66% 9% 12% 9% 4	1800
	11th to 20th	Day	67% 11% 8% 7% 5% 2	1764
	21st to end	Day	68% 7% 9% 8% 6% 2	1570
March	1st to 10th	Day	57% 10% 13% 14% 5%	4751
	11th to 20th	Day	60% 7% 8% 18% 7%	5000
	21st to end	Day	66% 11% 12% 9% 3	5603
April	1st to 10th	Day	67% 9% 11% 10% 3	2400
	11th to 20th	Day	58% 9% 10% 12% 8% 3	2372
	21st to end	Day	57% 10% 12% 13% 7% 2	2584
May	1st to 10th	Day	47% 14% 11% 16% 12%	4463
	11th to 20th	Day	31% 10% 22% 26% 12% 1	4383
	21st to end	Day	50% 18% 17% 13% 2	4814
June	1st to 10th	Day	51% 8% 16% 18% 6%	2660
	11th to 20th	Day	51% 11% 19% 14% 6%	2660
	21st to end	Day	31% 6% 17% 30% 11% 4	2660
July	1st to 10th	Day	14% 5% 9% 18% 22% 32%	4351
	11th to 20th	Day	14% 3 6% 18% 38% 22%	4345
	21st to end	Day	14% 3 10% 27% 38% 8%	4639
August	1st to 10th	Day	17% 5% 10% 29% 29% 11%	4078
	11th to 20th	Day	23% 4 20% 34% 15% 4	4065
	21st to end	Day	27% 5% 15% 24% 21% 8%	4286
September	1st to 10th	Day	20% 6% 13% 28% 21% 12%	2280
	11th to 20th	Day	38% 8% 14% 19% 14% 6%	2280
	21st to end	Day	37% 8% 15% 19% 15% 6%	1996
October	1st to 10th	Day	35% 6% 8% 23% 22% 5%	3180
	11th to 20th	Day	49% 6% 17% 15% 11% 1	3161
	21st to end	Day	56% 7% 9% 17% 10% 1	3391
November	1st to 10th	Day	64% 9% 12% 10% 4 1	1669
	11th to 20th	Day	66% 10% 10% 7% 6% 2	1600
	21st to end	Day	62% 10% 9% 8% 11% 1	1600
December	1st to 10th	Day	51% 8% 8% 12% 20% 2	2620
	11th to 20th	Day	71% 11% 9% 8% 1	3028
	21st to end	Day	74% 11% 9% 4 2	4004
January	1st to 10th	Night	74% 9% 6% 6% 4 1	6306
	11th to 20th	Night	61% 8% 6% 17% 6% 3	6320
	21st to end	Night	55% 6% 6% 12% 16% 5%	6534
February	1st to 10th	Night	67% 10% 9% 8% 5% 1	2600
	11th to 20th	Night	61% 8% 13% 9% 7% 2	2534
	21st to end	Night	63% 8% 11% 10% 6% 2	2060
March	1st to 10th	Night	57% 11% 13% 12% 6%	5139
	11th to 20th	Night	55% 9% 10% 15% 10% 1	5000
	21st to end	Night	66% 11% 13% 10% 1	5370
April	1st to 10th	Night	64% 9% 12% 11% 4	2000
	11th to 20th	Night	52% 10% 15% 12% 7% 3	1979
	21st to end	Night	55% 8% 15% 13% 6% 3	1814
May	1st to 10th	Night	46% 14% 11% 19% 9% 1	2719
	11th to 20th	Night	29% 10% 21% 30% 9% 2	2519
	21st to end	Night	43% 18% 26% 10% 2	2762
June	1st to 10th	Night	47% 10% 16% 22% 5% 1	1520
	11th to 20th	Night	46% 10% 19% 19% 6%	1520
	21st to end	Night	29% 6% 18% 31% 12% 5%	1520
July	1st to 10th	Night	13% 4 9% 20% 26% 28%	2500
	11th to 20th	Night	14% 3 8% 18% 34% 24%	2492
	21st to end	Night	11% 3 13% 28% 38% 8%	2674
August	1st to 10th	Night	12% 3 10% 36% 29% 9%	2833
	11th to 20th	Night	26% 4 22% 31% 12% 4	2839
	21st to end	Night	25% 4 14% 29% 21% 7%	3388
September	1st to 10th	Night	20% 6% 14% 28% 21% 12%	1900
	11th to 20th	Night	33% 8% 15% 22% 13% 9%	1900
	21st to end	Night	32% 7% 16% 21% 16% 9%	2179
October	1st to 10th	Night	34% 4 8% 28% 23% 3	3800
	11th to 20th	Night	43% 6% 18% 19% 11% 2	3777
	21st to end	Night	56% 7% 10% 16% 10% 2	4174
November	1st to 10th	Night	60% 11% 11% 11% 7%	2628
	11th to 20th	Night	65% 9% 9% 7% 8% 3	2800
	21st to end	Night	58% 8% 9% 10% 12% 3	2800
December	1st to 10th	Night	46% 10% 8% 20% 15% 1	4560
	11th to 20th	Night	72% 8% 10% 7% 3	5176
	21st to end	Night	66% 13% 13% 5% 2	6952

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	42% 15% 17% 16% 10% 2	200
	11th to 20th	Day	37% 9% 21% 17% 13% 5%	200
	21st to end	Day	28% 16% 19% 17% 16% 5%	405
February	1st to 10th	Day	36% 14% 20% 18% 11% 2	400
	11th to 20th	Day	33% 15% 24% 17% 9% 2	395
	21st to end	Day	36% 15% 20% 17% 11% 1	374
March	1st to 10th	Day	30% 16% 25% 21% 8% 1	500
	11th to 20th	Day	28% 14% 23% 22% 10% 2	500
	21st to end	Day	28% 19% 25% 20% 7%	550
April	1st to 10th	Day	32% 15% 23% 17% 11% 2	500
	11th to 20th	Day	26% 19% 24% 20% 8% 4	492
	21st to end	Day	25% 14% 24% 23% 13% 2	500
May	1st to 10th	Day	18% 15% 24% 26% 12% 4	503
	11th to 20th	Day	23% 10% 19% 26% 18% 3	489
	21st to end	Day	18% 9% 21% 27% 21% 5%	529
June	1st to 10th	Day	20% 10% 19% 28% 19% 4	490
	11th to 20th	Day	19% 12% 23% 30% 14% 1	490
	21st to end	Day	9% 6% 19% 42% 20% 5%	490
July	1st to 10th	Day	6% 5% 19% 40% 24% 5%	486
	11th to 20th	Day	4 4 20% 43% 24% 5%	484
	21st to end	Day	4 2 18% 41% 28% 7%	531
August	1st to 10th	Day	3 3 14% 41% 31% 7%	487
	11th to 20th	Day	4 2 13% 40% 32% 8%	477
	21st to end	Day	4 6% 23% 37% 23% 7%	539
September	1st to 10th	Day	8% 3 26% 31% 28% 4	490
	11th to 20th	Day	13% 7% 21% 30% 25% 5%	490
	21st to end	Day	16% 11% 22% 31% 15% 5%	490
October	1st to 10th	Day	19% 8% 22% 27% 18% 6%	490
	11th to 20th	Day	20% 10% 19% 27% 18% 6%	490
	21st to end	Day	26% 14% 25% 19% 10% 5%	506
November	1st to 10th	Day	29% 17% 21% 17% 12% 4	253
	11th to 20th	Day	29% 15% 22% 15% 13% 7%	200
	21st to end	Day	31% 13% 18% 16% 19% 5%	200
December	1st to 10th	Day	35% 13% 10% 21% 13% 9%	200
	11th to 20th	Day	41% 15% 20% 17% 6% 3	200
	21st to end	Day	37% 17% 20% 12% 12% 1	220
January	1st to 10th	Night	35% 14% 18% 19% 13% 1	800
	11th to 20th	Night	34% 10% 16% 21% 17% 2	800
	21st to end	Night	29% 10% 15% 28% 15% 2	880
February	1st to 10th	Night	31% 11% 19% 25% 13% 1	800
	11th to 20th	Night	33% 11% 19% 23% 13% 1	784
	21st to end	Night	33% 14% 18% 26% 9%	660
March	1st to 10th	Night	28% 13% 20% 27% 11% 1	800
	11th to 20th	Night	26% 12% 19% 29% 13% 1	800
	21st to end	Night	30% 14% 21% 23% 11% 1	880
April	1st to 10th	Night	31% 10% 19% 25% 14% 1	800
	11th to 20th	Night	24% 11% 23% 26% 15% 1	791
	21st to end	Night	25% 9% 20% 29% 15% 1	614
May	1st to 10th	Night	18% 9% 23% 34% 15% 1	460
	11th to 20th	Night	18% 8% 18% 30% 26%	375
	21st to end	Night	21% 7% 16% 33% 23% 1	411
June	1st to 10th	Night	19% 5% 17% 37% 21% 1	380
	11th to 20th	Night	19% 6% 20% 41% 13% 1	380
	21st to end	Night	12% 2 17% 46% 23% 1	380
July	1st to 10th	Night	6% 3 8% 53% 29% 1	370
	11th to 20th	Night	6% 2 16% 47% 28% 1	368
	21st to end	Night	2 2 10% 50% 35% 1	416
August	1st to 10th	Night	4 3 6% 39% 44% 5%	560
	11th to 20th	Night	5% 2 8% 34% 47% 4	564
	21st to end	Night	6% 4 14% 42% 31% 3	765
September	1st to 10th	Night	8% 3 14% 42% 31% 2	760
	11th to 20th	Night	12% 4 10% 34% 36% 4	760
	21st to end	Night	14% 6% 14% 35% 28% 3	760
October	1st to 10th	Night	18% 5% 19% 30% 25% 3	760
	11th to 20th	Night	19% 7% 14% 31% 26% 3	760
	21st to end	Night	26% 10% 20% 25% 15% 4	836
November	1st to 10th	Night	31% 10% 17% 28% 13%	780
	11th to 20th	Night	35% 14% 17% 21% 12% 1	800
	21st to end	Night	27% 10% 17% 29% 17% 1	800
December	1st to 10th	Night	30% 8% 16% 28% 15% 3	800
	11th to 20th	Night	37% 12% 16% 22% 13% 1	800
	21st to end	Night	39% 14% 17% 20% 9% 1	880

Page generated at 2025-05-08 06:33:55. Version ac07

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