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 2024-11-02.

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	16% 8% 3 7% 5% 4 4 3 5% 6% 38%	101538
	Night	19% 10% 3 8% 5% 4 4 3 5% 5% 35%	97595
Transparency	Day	45% 9% 12% 16% 13% 5%	101281
	Night	49% 9% 12% 15% 11% 4	97441
Seeing	Day	20% 11% 21% 27% 17% 4	14671
	Night	24% 9% 17% 30% 20% 2	23600

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% 17% 5% 12% 8% 5% 3 2 3 3 8%	2539
	11th to 20th	Day	37% 13% 4 9% 5% 4 5% 3 3 4 14%	2540
	21st to end	Day	28% 12% 5% 11% 6% 6% 6% 4 5% 5% 13%	3086
February	1st to 10th	Day	27% 13% 4 8% 5% 6% 4 4 7% 7% 15%	1710
	11th to 20th	Day	24% 14% 3 11% 7% 5% 5% 3 5% 5% 18%	1665
	21st to end	Day	27% 15% 4 10% 5% 4 4 3 4 4 19%	1494
March	1st to 10th	Day	16% 13% 4 10% 7% 6% 6% 5% 7% 8% 18%	3306
	11th to 20th	Day	17% 10% 3 7% 5% 5% 4 3 5% 5% 37%	3490
	21st to end	Day	21% 13% 5% 12% 7% 6% 5% 4 5% 5% 16%	3903
April	1st to 10th	Day	26% 12% 4 10% 6% 6% 5% 3 4 6% 18%	2280
	11th to 20th	Day	22% 12% 3 8% 6% 5% 4 4 5% 6% 25%	2249
	21st to end	Day	21% 10% 3 9% 5% 5% 4 4 6% 7% 25%	2455
May	1st to 10th	Day	21% 9% 3 7% 5% 4 4 4 5% 6% 32%	4326
	11th to 20th	Day	11% 5% 2 6% 4 4 4 3 5% 7% 49%	4377
	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	18% 6% 2 6% 3 4 4 5% 5% 7% 40%	1900
	11th to 20th	Day	17% 7% 4 9% 5% 5% 5% 3 6% 6% 33%	1900
	21st to end	Day	22% 10% 4 8% 5% 5% 5% 3 5% 7% 26%	2057
November	1st to 10th	Day	29% 12% 5% 9% 5% 4 5% 3 5% 7% 18%	1593
	11th to 20th	Day	27% 12% 4 8% 6% 5% 6% 4 6% 4 18%	1520
	21st to end	Day	25% 11% 4 9% 6% 5% 5% 4 6% 6% 20%	1520
December	1st to 10th	Day	23% 12% 5% 8% 7% 5% 4 3 5% 5% 23%	1520
	11th to 20th	Day	26% 11% 3 10% 7% 7% 7% 5% 7% 6% 10%	1928
	21st to end	Day	29% 15% 4 12% 7% 5% 5% 4 6% 5% 7%	2794
January	1st to 10th	Night	29% 18% 6% 12% 6% 5% 4 3 3 3 9%	4406
	11th to 20th	Night	36% 14% 4 9% 5% 3 3 2 3 4 16%	4420
	21st to end	Night	31% 15% 4 11% 6% 4 4 3 4 5% 14%	4574
February	1st to 10th	Night	25% 14% 4 9% 6% 5% 5% 4 6% 5% 17%	2470
	11th to 20th	Night	28% 13% 3 8% 4 3 3 3 5% 5% 25%	2402
	21st to end	Night	25% 16% 4 8% 6% 4 4 3 5% 5% 21%	1960
March	1st to 10th	Night	15% 14% 5% 11% 8% 5% 5% 4 5% 6% 22%	3564
	11th to 20th	Night	15% 8% 3 7% 5% 4 4 3 5% 6% 41%	3490
	21st to end	Night	20% 15% 5% 11% 7% 6% 6% 4 5% 5% 16%	3748
April	1st to 10th	Night	26% 13% 3 8% 6% 4 3 3 5% 6% 22%	1900
	11th to 20th	Night	20% 12% 3 7% 5% 4 4 4 5% 6% 29%	1879
	21st to end	Night	21% 11% 3 9% 4 5% 4 4 5% 6% 28%	1724
May	1st to 10th	Night	21% 10% 4 8% 4 3 3 3 4 5% 35%	2636
	11th to 20th	Night	10% 6% 2 6% 3 3 4 3 5% 7% 51%	2516
	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	16% 6% 3 6% 4 3 3 3 5% 6% 45%	2280
	11th to 20th	Night	16% 7% 3 7% 4 4 4 4 5% 7% 41%	2280
	21st to end	Night	21% 9% 3 8% 4 3 4 3 4 5% 36%	2539
November	1st to 10th	Night	28% 9% 3 8% 6% 5% 4 3 5% 6% 23%	2506
	11th to 20th	Night	30% 9% 3 9% 6% 5% 4 4 4 5% 22%	2660
	21st to end	Night	23% 10% 3 7% 6% 4 5% 4 5% 5% 27%	2660
December	1st to 10th	Night	26% 10% 4 7% 6% 4 4 3 4 7% 25%	2660
	11th to 20th	Night	27% 12% 4 9% 6% 5% 5% 4 6% 6% 16%	3276
	21st to end	Night	25% 14% 5% 11% 7% 5% 6% 4 6% 4 13%	4862

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% 12% 5% 3 1 1	2539
	11th to 20th	Day	73% 6% 6% 9% 4 2	2540
	21st to end	Day	70% 10% 8% 7% 5% 1	3086
February	1st to 10th	Day	66% 9% 12% 8% 4	1710
	11th to 20th	Day	66% 12% 8% 8% 4 2	1674
	21st to end	Day	68% 7% 10% 8% 5% 2	1494
March	1st to 10th	Day	58% 14% 13% 11% 3	3306
	11th to 20th	Day	49% 9% 9% 23% 10% 1	3490
	21st to end	Day	66% 14% 10% 7% 4	3903
April	1st to 10th	Day	67% 9% 11% 10% 3	2280
	11th to 20th	Day	60% 10% 10% 10% 7% 3	2252
	21st to end	Day	58% 10% 12% 12% 7% 2	2455
May	1st to 10th	Day	48% 14% 12% 15% 11%	4326
	11th to 20th	Day	31% 10% 22% 26% 12% 1	4380
	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	41% 9% 12% 18% 16% 4	1900
	11th to 20th	Day	50% 8% 14% 13% 13% 2	1900
	21st to end	Day	55% 9% 10% 12% 12% 1	2057
November	1st to 10th	Day	65% 9% 12% 10% 4 1	1593
	11th to 20th	Day	65% 10% 10% 7% 6% 2	1520
	21st to end	Day	61% 10% 8% 8% 12% 1	1520
December	1st to 10th	Day	60% 9% 8% 11% 9% 2	1520
	11th to 20th	Day	65% 15% 10% 8% 2	1928
	21st to end	Day	69% 16% 10% 3 2	2794
January	1st to 10th	Night	75% 11% 6% 4 3 1	4406
	11th to 20th	Night	71% 8% 5% 10% 4 2	4420
	21st to end	Night	71% 8% 8% 6% 6% 1	4574
February	1st to 10th	Night	66% 10% 9% 8% 5% 1	2470
	11th to 20th	Night	61% 8% 13% 10% 6% 2	2404
	21st to end	Night	64% 8% 11% 9% 6% 2	1960
March	1st to 10th	Night	58% 14% 14% 11% 4	3564
	11th to 20th	Night	42% 12% 13% 19% 13% 1	3490
	21st to end	Night	67% 13% 10% 9% 1	3748
April	1st to 10th	Night	63% 9% 12% 10% 5%	1900
	11th to 20th	Night	54% 11% 15% 11% 6% 3	1879
	21st to end	Night	55% 9% 15% 13% 6% 3	1724
May	1st to 10th	Night	47% 14% 11% 18% 9% 1	2636
	11th to 20th	Night	29% 10% 21% 30% 9% 2	2516
	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	40% 6% 10% 22% 18% 5%	2280
	11th to 20th	Night	42% 9% 12% 17% 16% 4	2280
	21st to end	Night	50% 9% 9% 13% 16% 3	2539
November	1st to 10th	Night	60% 11% 10% 12% 7%	2506
	11th to 20th	Night	64% 10% 9% 7% 8% 3	2660
	21st to end	Night	57% 9% 9% 9% 12% 3	2660
December	1st to 10th	Night	58% 9% 10% 10% 10% 2	2660
	11th to 20th	Night	64% 12% 11% 9% 4	3276
	21st to end	Night	63% 17% 12% 5% 2 1	4862

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% 18% 15% 9% 2	180
	11th to 20th	Day	41% 9% 18% 17% 13% 3	180
	21st to end	Day	29% 16% 17% 18% 16% 3	373
February	1st to 10th	Day	35% 13% 21% 18% 12% 2	370
	11th to 20th	Day	32% 16% 23% 17% 10% 2	365
	21st to end	Day	35% 16% 21% 16% 11% 1	346
March	1st to 10th	Day	31% 16% 23% 22% 8% 1	460
	11th to 20th	Day	25% 14% 24% 23% 11% 3	460
	21st to end	Day	28% 20% 24% 20% 8%	506
April	1st to 10th	Day	32% 16% 23% 17% 11% 2	460
	11th to 20th	Day	27% 20% 23% 18% 8% 4	452
	21st to end	Day	26% 15% 24% 21% 12% 1	460
May	1st to 10th	Day	19% 14% 24% 26% 13% 4	463
	11th to 20th	Day	23% 10% 19% 26% 18% 3	488
	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	30% 18% 19% 18% 11% 3	231
	11th to 20th	Day	25% 14% 22% 16% 14% 8%	180
	21st to end	Day	32% 12% 17% 16% 18% 4	180
December	1st to 10th	Day	37% 13% 11% 21% 12% 6%	180
	11th to 20th	Day	39% 15% 18% 18% 7% 3	180
	21st to end	Day	36% 18% 20% 12% 13% 2	198
January	1st to 10th	Night	35% 14% 19% 18% 13% 1	760
	11th to 20th	Night	36% 11% 15% 20% 17% 2	760
	21st to end	Night	30% 11% 15% 28% 14% 2	836
February	1st to 10th	Night	32% 10% 18% 25% 13% 1	760
	11th to 20th	Night	33% 11% 19% 23% 14% 1	744
	21st to end	Night	33% 14% 18% 25% 9%	628
March	1st to 10th	Night	29% 14% 19% 26% 11% 1	760
	11th to 20th	Night	25% 11% 19% 31% 13% 1	760
	21st to end	Night	30% 14% 21% 23% 11% 1	836
April	1st to 10th	Night	32% 10% 19% 25% 14% 1	760
	11th to 20th	Night	25% 11% 23% 25% 15% 1	751
	21st to end	Night	26% 9% 20% 28% 15% 1	584
May	1st to 10th	Night	18% 10% 24% 32% 15% 1	437
	11th to 20th	Night	17% 8% 18% 30% 26%	374
	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% 19% 25% 15% 4	836
November	1st to 10th	Night	31% 10% 17% 29% 13%	744
	11th to 20th	Night	33% 14% 17% 22% 13% 1	760
	21st to end	Night	26% 10% 17% 29% 16% 1	760
December	1st to 10th	Night	31% 8% 17% 28% 14% 2	760
	11th to 20th	Night	37% 12% 16% 22% 13% 1	760
	21st to end	Night	39% 13% 18% 20% 9% 1	836

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