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 2024-01-16.

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	7% 9% 5% 8% 6% 3 4 4 4 5% 44%	74702
	Night	2 6% 4 7% 5% 3 4 4 5% 7% 53%	75119
Transparency	Day	42% 3 8% 20% 19% 8%	74423
	Night	31% 4 9% 23% 23% 10%	74919
Seeing	Day	19% 20% 27% 20% 11% 3	14116
	Night	10% 11% 24% 31% 20% 3	24388

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	1 6% 6% 9% 6% 3 3 5% 5% 7% 49%	1674
	11th to 20th	Day	7% 5% 12% 6% 2 5% 3 4 5% 51%	1669
	21st to end	Day	1 7% 6% 10% 8% 4 3 4 4 7% 46%	1782
February	1st to 10th	Day	7% 6% 13% 7% 3 4 4 4 5% 47%	1620
	11th to 20th	Day	1 7% 9% 11% 8% 4 4 5% 5% 5% 42%	1623
	21st to end	Day	1 7% 6% 16% 11% 4 4 4 4 5% 39%	1480
March	1st to 10th	Day	1 13% 6% 11% 9% 3 4 3 6% 6% 40%	1975
	11th to 20th	Day	1 13% 6% 10% 5% 3 4 5% 5% 7% 41%	1980
	21st to end	Day	3 11% 8% 13% 7% 3 4 4 4 5% 38%	2184
April	1st to 10th	Day	3 14% 6% 8% 7% 3 3 4 4 6% 42%	2160
	11th to 20th	Day	2 15% 8% 10% 7% 4 4 4 4 5% 38%	2131
	21st to end	Day	2 15% 7% 8% 6% 3 5% 4 4 5% 40%	2160
May	1st to 10th	Day	3 17% 5% 7% 6% 3 4 5% 5% 6% 37%	2272
	11th to 20th	Day	6% 14% 4 7% 6% 4 6% 4 6% 7% 36%	2309
	21st to end	Day	7% 12% 3 5% 5% 4 4 4 5% 7% 45%	2724
June	1st to 10th	Day	5% 5% 2 4 5% 4 5% 5% 6% 7% 52%	2520
	11th to 20th	Day	7% 5% 2 5% 5% 3 4 3 3 4 58%	2520
	21st to end	Day	7% 5% 3 7% 6% 3 5% 5% 5% 6% 50%	2520
July	1st to 10th	Day	14% 7% 3 7% 7% 4 5% 4 5% 6% 36%	2489
	11th to 20th	Day	14% 9% 4 9% 9% 4 7% 5% 5% 5% 29%	2340
	21st to end	Day	19% 10% 5% 9% 8% 4 7% 5% 5% 6% 21%	2545
August	1st to 10th	Day	21% 9% 4 8% 8% 4 5% 5% 5% 5% 27%	2328
	11th to 20th	Day	16% 9% 4 9% 7% 4 6% 6% 4 4 31%	2221
	21st to end	Day	16% 9% 4 8% 8% 4 6% 5% 5% 5% 30%	2376
September	1st to 10th	Day	15% 8% 3 9% 7% 3 4 4 4 4 38%	2160
	11th to 20th	Day	14% 10% 3 6% 6% 2 5% 3 3 5% 42%	2156
	21st to end	Day	10% 9% 2 5% 5% 2 4 3 3 4 52%	1895
October	1st to 10th	Day	12% 14% 3 4 3 2 4 2 3 3 49%	1800
	11th to 20th	Day	5% 8% 3 4 3 2 2 2 2 5% 64%	1800
	21st to end	Day	4 8% 3 6% 3 2 2 3 3 5% 61%	1980
November	1st to 10th	Day	1 6% 3 5% 4 2 3 2 4 5% 64%	1843
	11th to 20th	Day	1 7% 3 5% 2 2 3 3 4 5% 64%	1706
	21st to end	Day	1 7% 5% 7% 6% 3 3 3 3 5% 57%	1520
December	1st to 10th	Day	1 8% 5% 8% 6% 3 5% 4 4 6% 52%	1520
	11th to 20th	Day	10% 7% 11% 6% 4 4 4 5% 6% 44%	1926
	21st to end	Day	6% 5% 9% 6% 2 5% 4 4 6% 52%	2794
January	1st to 10th	Night	5% 4 8% 7% 3 6% 5% 5% 7% 51%	2506
	11th to 20th	Night	5% 5% 9% 6% 3 5% 4 4 8% 53%	2418
	21st to end	Night	1 6% 3 10% 7% 3 5% 4 4 5% 52%	2574
February	1st to 10th	Night	6% 5% 10% 7% 3 3 3 4 8% 50%	2340
	11th to 20th	Night	1 6% 5% 9% 7% 4 5% 5% 5% 6% 48%	2222
	21st to end	Night	6% 5% 11% 9% 5% 4 3 5% 7% 44%	1776
March	1st to 10th	Night	1 8% 4 9% 6% 3 4 5% 5% 7% 49%	1980
	11th to 20th	Night	1 7% 5% 8% 6% 3 5% 5% 5% 8% 48%	1980
	21st to end	Night	1 8% 6% 10% 7% 3 5% 4 6% 7% 44%	2170
April	1st to 10th	Night	2 10% 6% 7% 5% 4 3 4 5% 7% 47%	1800
	11th to 20th	Night	2 11% 5% 8% 6% 3 5% 4 5% 6% 46%	1777
	21st to end	Night	1 8% 5% 7% 6% 3 4 4 5% 7% 50%	1800
May	1st to 10th	Night	2 8% 3 5% 5% 3 6% 4 5% 7% 52%	1684
	11th to 20th	Night	3 9% 3 7% 4 2 5% 4 4 7% 52%	1583
	21st to end	Night	4 7% 2 4 4 2 4 5% 5% 7% 56%	1552
June	1st to 10th	Night	2 2 1 3 3 2 3 4 6% 7% 65%	1440
	11th to 20th	Night	4 3 1 3 3 2 4 4 4 5% 69%	1440
	21st to end	Night	2 3 1 4 4 3 5% 5% 5% 8% 60%	1440
July	1st to 10th	Night	5% 5% 2 5% 5% 4 6% 5% 7% 8% 50%	1466
	11th to 20th	Night	4 5% 2 5% 5% 3 6% 7% 7% 10% 46%	1620
	21st to end	Night	7% 6% 2 6% 5% 5% 7% 7% 9% 11% 34%	1773
August	1st to 10th	Night	7% 4 2 5% 5% 3 4 5% 7% 8% 48%	1611
	11th to 20th	Night	7% 4 2 4 4 2 4 6% 5% 11% 50%	1678
	21st to end	Night	6% 5% 2 5% 5% 3 5% 6% 6% 9% 47%	1980
September	1st to 10th	Night	6% 4 3 6% 5% 4 4 4 6% 9% 50%	1800
	11th to 20th	Night	6% 7% 2 4 4 3 5% 4 5% 6% 56%	1800
	21st to end	Night	6% 6% 2 4 4 2 3 3 3 4 64%	2064
October	1st to 10th	Night	8% 9% 2 4 3 3 3 4 5% 6% 55%	2160
	11th to 20th	Night	2 5% 2 4 3 2 3 4 4 5% 68%	2160
	21st to end	Night	2 8% 3 4 3 2 2 3 4 6% 62%	2376
November	1st to 10th	Night	1 8% 2 3 3 2 3 4 5% 7% 61%	2220
	11th to 20th	Night	1 6% 3 4 3 2 3 4 4 7% 63%	2465
	21st to end	Night	1 6% 5% 7% 5% 2 4 4 5% 5% 56%	2660
December	1st to 10th	Night	1 6% 4 7% 6% 4 4 4 5% 7% 54%	2660
	11th to 20th	Night	8% 5% 7% 6% 4 5% 5% 6% 9% 45%	3271
	21st to end	Night	5% 5% 11% 7% 2 4 4 4 7% 51%	4873

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	33% 3 13% 24% 20% 7%	1675
	11th to 20th	Day	36% 2 11% 23% 21% 7%	1669
	21st to end	Day	38% 3 8% 25% 19% 7%	1782
February	1st to 10th	Day	39% 3 13% 18% 16% 10%	1620
	11th to 20th	Day	43% 4 5% 18% 21% 8%	1632
	21st to end	Day	48% 3 4 15% 21% 9%	1480
March	1st to 10th	Day	45% 2 6% 21% 18% 7%	1975
	11th to 20th	Day	42% 3 9% 25% 15% 5%	1980
	21st to end	Day	47% 3 8% 21% 16% 4	2184
April	1st to 10th	Day	43% 4 8% 19% 19% 8%	2160
	11th to 20th	Day	50% 3 7% 19% 17% 5%	2134
	21st to end	Day	46% 3 7% 27% 15% 2	2160
May	1st to 10th	Day	47% 3 6% 26% 15% 2	2272
	11th to 20th	Day	48% 3 10% 24% 13% 2	2312
	21st to end	Day	40% 2 7% 25% 19% 6%	2724
June	1st to 10th	Day	31% 3 10% 29% 19% 8%	2520
	11th to 20th	Day	31% 2 5% 19% 28% 14%	2520
	21st to end	Day	36% 3 7% 17% 24% 13%	2520
July	1st to 10th	Day	49% 4 8% 13% 16% 10%	2421
	11th to 20th	Day	57% 4 13% 12% 8% 6%	2271
	21st to end	Day	63% 5% 13% 10% 7% 1	2404
August	1st to 10th	Day	60% 4 8% 9% 14% 4	2313
	11th to 20th	Day	57% 4 8% 11% 15% 5%	2221
	21st to end	Day	56% 4 9% 15% 14% 3	2376
September	1st to 10th	Day	52% 3 5% 13% 20% 7%	2160
	11th to 20th	Day	47% 2 4 17% 22% 8%	2156
	21st to end	Day	38% 2 3 23% 26% 8%	1895
October	1st to 10th	Day	43% 2 3 19% 21% 12%	1800
	11th to 20th	Day	26% 2 2 25% 32% 13%	1800
	21st to end	Day	28% 2 7% 27% 29% 8%	1980
November	1st to 10th	Day	23% 3 9% 28% 27% 10%	1843
	11th to 20th	Day	23% 3 7% 29% 28% 10%	1706
	21st to end	Day	32% 2 7% 23% 22% 14%	1520
December	1st to 10th	Day	33% 4 12% 23% 20% 8%	1520
	11th to 20th	Day	42% 3 7% 20% 22% 6%	1924
	21st to end	Day	35% 4 6% 15% 22% 18%	2794
January	1st to 10th	Night	30% 3 8% 26% 22% 10%	2506
	11th to 20th	Night	30% 3 10% 23% 23% 10%	2418
	21st to end	Night	33% 4 4 26% 25% 9%	2574
February	1st to 10th	Night	33% 3 11% 20% 22% 11%	2340
	11th to 20th	Night	35% 4 6% 21% 24% 11%	2223
	21st to end	Night	40% 3 4 20% 23% 10%	1776
March	1st to 10th	Night	36% 2 7% 22% 27% 6%	1980
	11th to 20th	Night	35% 3 9% 27% 21% 5%	1980
	21st to end	Night	38% 4 12% 22% 20% 5%	2170
April	1st to 10th	Night	36% 4 11% 23% 19% 8%	1800
	11th to 20th	Night	39% 3 8% 23% 23% 4	1777
	21st to end	Night	32% 4 11% 33% 18% 3	1800
May	1st to 10th	Night	33% 4 10% 31% 20% 2	1684
	11th to 20th	Night	33% 4 12% 33% 16% 2	1584
	21st to end	Night	30% 3 11% 27% 25% 6%	1552
June	1st to 10th	Night	18% 3 10% 33% 27% 8%	1440
	11th to 20th	Night	21% 2 6% 20% 35% 16%	1440
	21st to end	Night	23% 3 6% 24% 28% 16%	1440
July	1st to 10th	Night	33% 5% 12% 14% 24% 12%	1425
	11th to 20th	Night	31% 8% 19% 20% 12% 10%	1566
	21st to end	Night	39% 13% 25% 13% 9% 2	1674
August	1st to 10th	Night	33% 9% 16% 18% 17% 7%	1602
	11th to 20th	Night	30% 5% 18% 20% 20% 6%	1678
	21st to end	Night	34% 5% 15% 24% 18% 4	1980
September	1st to 10th	Night	32% 5% 12% 19% 26% 7%	1800
	11th to 20th	Night	31% 3 10% 22% 24% 10%	1800
	21st to end	Night	26% 3 4 30% 29% 8%	2064
October	1st to 10th	Night	33% 3 8% 21% 23% 12%	2160
	11th to 20th	Night	19% 3 5% 25% 35% 12%	2160
	21st to end	Night	25% 2 6% 29% 28% 10%	2376
November	1st to 10th	Night	24% 3 11% 25% 26% 12%	2220
	11th to 20th	Night	21% 3 7% 28% 28% 12%	2465
	21st to end	Night	30% 2 6% 23% 25% 14%	2660
December	1st to 10th	Night	31% 4 13% 19% 22% 12%	2660
	11th to 20th	Night	35% 4 7% 20% 24% 9%	3272
	21st to end	Night	34% 3 6% 16% 22% 20%	4873

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	20% 21% 31% 20% 8%	270
	11th to 20th	Day	23% 29% 21% 21% 5%	257
	21st to end	Day	14% 27% 34% 19% 6% 2	264
February	1st to 10th	Day	25% 30% 30% 11% 3 1	240
	11th to 20th	Day	24% 29% 27% 14% 5%	240
	21st to end	Day	33% 28% 21% 15% 3	198
March	1st to 10th	Day	29% 25% 29% 13% 3	415
	11th to 20th	Day	27% 25% 26% 16% 6%	420
	21st to end	Day	30% 30% 29% 10% 1	462
April	1st to 10th	Day	21% 28% 33% 13% 3 1	420
	11th to 20th	Day	29% 31% 26% 9% 3 1	412
	21st to end	Day	25% 31% 28% 11% 4	420
May	1st to 10th	Day	23% 28% 34% 11% 4	423
	11th to 20th	Day	24% 23% 28% 19% 5% 1	456
	21st to end	Day	21% 22% 24% 20% 9% 4	679
June	1st to 10th	Day	11% 9% 29% 27% 21% 3	630
	11th to 20th	Day	11% 17% 31% 22% 16% 3	630
	21st to end	Day	8% 12% 30% 26% 18% 5%	630
July	1st to 10th	Day	12% 10% 17% 30% 26% 5%	591
	11th to 20th	Day	17% 12% 18% 31% 17% 5%	444
	21st to end	Day	22% 5% 20% 27% 21% 5%	487
August	1st to 10th	Day	20% 11% 23% 30% 12% 5%	447
	11th to 20th	Day	16% 10% 27% 29% 13% 6%	437
	21st to end	Day	17% 10% 25% 28% 15% 5%	495
September	1st to 10th	Day	16% 12% 24% 25% 17% 7%	450
	11th to 20th	Day	15% 14% 35% 21% 10% 4	450
	21st to end	Day	20% 17% 26% 22% 9% 6%	365
October	1st to 10th	Day	27% 17% 19% 18% 12% 7%	270
	11th to 20th	Day	13% 22% 27% 19% 15% 3	270
	21st to end	Day	19% 22% 28% 19% 10% 3	297
November	1st to 10th	Day	19% 23% 27% 19% 9% 3	270
	11th to 20th	Day	14% 26% 27% 22% 8% 2	270
	21st to end	Day	18% 20% 28% 21% 11% 1	270
December	1st to 10th	Day	17% 28% 27% 18% 7% 1	270
	11th to 20th	Day	21% 23% 30% 16% 9%	270
	21st to end	Day	25% 28% 27% 14% 5% 1	297
January	1st to 10th	Night	9% 14% 28% 33% 14% 3	809
	11th to 20th	Night	10% 13% 31% 31% 13% 2	744
	21st to end	Night	9% 15% 30% 34% 10% 3	792
February	1st to 10th	Night	11% 17% 30% 32% 8% 1	720
	11th to 20th	Night	12% 13% 37% 28% 9% 1	704
	21st to end	Night	12% 15% 38% 26% 8%	592
March	1st to 10th	Night	14% 17% 30% 28% 10% 1	720
	11th to 20th	Night	9% 14% 27% 32% 16% 1	720
	21st to end	Night	12% 18% 32% 28% 9% 1	784
April	1st to 10th	Night	12% 20% 31% 28% 9%	540
	11th to 20th	Night	16% 17% 28% 28% 11%	533
	21st to end	Night	10% 17% 32% 27% 12% 2	540
May	1st to 10th	Night	10% 13% 27% 34% 14% 1	540
	11th to 20th	Night	8% 8% 25% 32% 22% 5%	531
	21st to end	Night	10% 13% 26% 29% 19% 4	584
June	1st to 10th	Night	4 6% 21% 36% 30% 4	540
	11th to 20th	Night	8% 7% 22% 36% 23% 3	540
	21st to end	Night	4 5% 17% 34% 36% 4	540
July	1st to 10th	Night	7% 2 6% 35% 44% 6%	525
	11th to 20th	Night	5% 4 6% 29% 47% 9%	522
	21st to end	Night	9% 3 4 25% 48% 11%	591
August	1st to 10th	Night	9% 4 5% 28% 48% 6%	537
	11th to 20th	Night	8% 3 7% 33% 41% 8%	534
	21st to end	Night	8% 4 6% 34% 40% 9%	594
September	1st to 10th	Night	7% 4 10% 37% 36% 6%	540
	11th to 20th	Night	9% 6% 12% 36% 33% 5%	540
	21st to end	Night	12% 7% 15% 30% 29% 7%	720
October	1st to 10th	Night	17% 8% 21% 30% 21% 3	720
	11th to 20th	Night	7% 10% 23% 37% 19% 3	720
	21st to end	Night	10% 11% 22% 34% 18% 4	792
November	1st to 10th	Night	11% 8% 24% 36% 19% 3	740
	11th to 20th	Night	9% 11% 30% 34% 13% 3	945
	21st to end	Night	11% 13% 25% 33% 14% 3	950
December	1st to 10th	Night	10% 13% 33% 33% 10% 2	950
	11th to 20th	Night	12% 16% 32% 27% 11% 1	950
	21st to end	Night	12% 14% 35% 30% 7% 1	1045

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