Silver Moonlight Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Silver Moonlight Observatory clear sky chart from 2005-11-06 to 2024-01-13.

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	11% 5% 3 5% 5% 3 5% 6% 7% 9% 41%	74951
	Night	11% 5% 2 5% 5% 3 4 4 5% 8% 46%	74812
Transparency	Day	36% 10% 16% 20% 13% 5%	74676
	Night	35% 7% 14% 20% 15% 7%	74607
Seeing	Day	12% 3 12% 25% 33% 15%	13603
	Night	14% 4 15% 29% 30% 8%	23586

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	17% 7% 4 6% 6% 3 5% 5% 5% 5% 39%	1707
	11th to 20th	Day	17% 7% 3 4 4 2 4 3 6% 7% 43%	1643
	21st to end	Day	18% 8% 3 6% 5% 3 4 3 5% 6% 39%	1782
February	1st to 10th	Day	21% 8% 3 6% 5% 3 4 4 5% 6% 35%	1724
	11th to 20th	Day	21% 8% 4 6% 5% 3 4 4 5% 5% 37%	1797
	21st to end	Day	19% 9% 4 6% 3 3 4 4 5% 6% 36%	1628
March	1st to 10th	Day	17% 8% 3 7% 6% 3 4 5% 4 6% 39%	1980
	11th to 20th	Day	17% 6% 4 6% 5% 3 3 3 4 6% 42%	1980
	21st to end	Day	14% 5% 3 4 5% 3 5% 4 5% 7% 46%	2178
April	1st to 10th	Day	15% 8% 3 6% 4 3 5% 5% 5% 8% 39%	1980
	11th to 20th	Day	15% 7% 3 6% 5% 3 5% 5% 5% 7% 40%	2088
	21st to end	Day	9% 5% 3 5% 6% 3 5% 5% 5% 8% 45%	2160
May	1st to 10th	Day	9% 5% 3 5% 6% 3 5% 5% 7% 9% 42%	2160
	11th to 20th	Day	9% 5% 2 5% 6% 4 5% 7% 7% 9% 41%	2279
	21st to end	Day	7% 4 2 6% 6% 5% 7% 8% 9% 14% 34%	2531
June	1st to 10th	Day	5% 3 2 4 5% 3 5% 6% 9% 13% 46%	2340
	11th to 20th	Day	5% 3 2 5% 5% 4 6% 7% 9% 12% 41%	2340
	21st to end	Day	7% 4 2 5% 6% 4 7% 8% 10% 14% 33%	2340
July	1st to 10th	Day	10% 4 3 6% 5% 3 5% 6% 9% 14% 34%	2340
	11th to 20th	Day	4 4 2 7% 6% 3 5% 6% 10% 15% 39%	2340
	21st to end	Day	4 4 2 5% 6% 3 6% 7% 9% 13% 41%	2546
August	1st to 10th	Day	3 3 1 3 4 3 6% 7% 8% 14% 49%	2328
	11th to 20th	Day	4 3 1 4 5% 4 6% 9% 11% 14% 38%	2177
	21st to end	Day	7% 4 2 6% 7% 4 7% 7% 9% 12% 35%	2326
September	1st to 10th	Day	8% 5% 2 5% 6% 5% 6% 6% 8% 12% 34%	1980
	11th to 20th	Day	14% 6% 3 7% 7% 4 6% 7% 7% 9% 32%	1980
	21st to end	Day	9% 6% 3 5% 5% 4 5% 6% 5% 8% 45%	1980
October	1st to 10th	Day	5% 4 2 5% 5% 3 4 5% 6% 9% 52%	1868
	11th to 20th	Day	9% 6% 3 4 4 2 4 4 6% 7% 51%	1800
	21st to end	Day	10% 4 2 5% 4 3 3 4 5% 7% 54%	1980
November	1st to 10th	Day	9% 5% 3 5% 5% 3 5% 5% 7% 9% 45%	1858
	11th to 20th	Day	15% 6% 3 5% 4 3 4 5% 5% 6% 46%	1900
	21st to end	Day	16% 5% 3 6% 6% 3 4 5% 4 6% 41%	1900
December	1st to 10th	Day	17% 6% 3 5% 5% 3 4 5% 4 8% 40%	1715
	11th to 20th	Day	13% 7% 3 6% 6% 3 4 5% 6% 8% 40%	2161
	21st to end	Day	16% 7% 3 6% 5% 3 5% 4 5% 7% 40%	3135
January	1st to 10th	Night	19% 7% 3 5% 4 3 4 4 4 5% 43%	2458
	11th to 20th	Night	18% 6% 3 6% 4 2 3 3 4 5% 46%	2379
	21st to end	Night	18% 6% 3 5% 5% 2 4 4 5% 6% 41%	2574
February	1st to 10th	Night	19% 8% 2 6% 4 3 4 5% 5% 6% 38%	2250
	11th to 20th	Night	20% 8% 4 4 3 2 2 3 4 6% 43%	2048
	21st to end	Night	20% 8% 3 6% 5% 3 4 4 4 5% 37%	1628
March	1st to 10th	Night	17% 8% 3 5% 3 2 3 4 5% 7% 42%	1980
	11th to 20th	Night	14% 8% 3 5% 5% 3 5% 5% 6% 7% 40%	1980
	21st to end	Night	13% 5% 2 4 4 2 4 3 4 7% 51%	2178
April	1st to 10th	Night	13% 7% 2 6% 4 3 5% 4 6% 8% 42%	1980
	11th to 20th	Night	15% 4 2 5% 5% 2 4 4 3 8% 47%	1819
	21st to end	Night	12% 5% 3 5% 7% 3 4 5% 6% 7% 45%	1800
May	1st to 10th	Night	9% 6% 3 5% 5% 3 5% 4 6% 10% 44%	1800
	11th to 20th	Night	8% 3 2 4 4 2 4 5% 6% 10% 50%	1611
	21st to end	Night	6% 4 2 4 5% 3 5% 6% 8% 12% 45%	1745
June	1st to 10th	Night	6% 3 1 4 4 3 3 4 6% 12% 54%	1620
	11th to 20th	Night	5% 4 2 5% 4 2 4 5% 8% 12% 49%	1620
	21st to end	Night	5% 4 3 5% 4 4 8% 6% 9% 13% 39%	1620
July	1st to 10th	Night	6% 5% 3 6% 6% 4 5% 5% 8% 12% 40%	1620
	11th to 20th	Night	2 2 2 4 5% 3 5% 5% 8% 13% 53%	1620
	21st to end	Night	3 3 2 4 4 3 5% 5% 8% 13% 50%	1772
August	1st to 10th	Night	1 1 1 2 4 2 4 6% 6% 12% 61%	1611
	11th to 20th	Night	4 4 2 4 4 3 4 4 8% 15% 49%	1720
	21st to end	Night	6% 4 2 4 5% 3 4 6% 6% 14% 47%	2024
September	1st to 10th	Night	6% 4 2 5% 6% 4 5% 6% 8% 13% 41%	1980
	11th to 20th	Night	11% 7% 3 6% 6% 3 6% 5% 5% 8% 40%	1980
	21st to end	Night	7% 5% 2 5% 5% 3 3 4 5% 9% 50%	1980
October	1st to 10th	Night	5% 4 2 4 4 2 4 5% 5% 9% 57%	2071
	11th to 20th	Night	9% 3 1 4 3 2 3 4 5% 7% 58%	2160
	21st to end	Night	9% 4 2 4 4 2 4 4 4 5% 58%	2376
November	1st to 10th	Night	8% 4 2 4 5% 3 5% 4 5% 8% 53%	2220
	11th to 20th	Night	13% 5% 2 5% 5% 3 3 4 5% 6% 48%	2280
	21st to end	Night	14% 6% 2 5% 6% 3 4 4 5% 6% 44%	2280
December	1st to 10th	Night	18% 6% 3 5% 4 3 4 4 5% 9% 41%	2460
	11th to 20th	Night	15% 8% 3 6% 5% 3 4 4 5% 7% 39%	3036
	21st to end	Night	15% 8% 3 6% 6% 3 4 4 4 6% 41%	4532

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	46% 5% 9% 15% 18% 7%	1708
	11th to 20th	Day	40% 7% 8% 17% 17% 11%	1643
	21st to end	Day	46% 5% 7% 17% 15% 10%	1782
February	1st to 10th	Day	50% 6% 8% 13% 13% 10%	1724
	11th to 20th	Day	50% 6% 9% 13% 15% 7%	1807
	21st to end	Day	47% 7% 11% 14% 13% 8%	1628
March	1st to 10th	Day	45% 7% 13% 15% 15% 4	1980
	11th to 20th	Day	45% 6% 11% 14% 17% 7%	1980
	21st to end	Day	37% 8% 14% 15% 19% 6%	2178
April	1st to 10th	Day	42% 9% 12% 17% 14% 6%	1980
	11th to 20th	Day	41% 9% 13% 16% 13% 8%	2088
	21st to end	Day	35% 8% 13% 22% 17% 5%	2160
May	1st to 10th	Day	35% 9% 16% 20% 14% 6%	2160
	11th to 20th	Day	33% 9% 18% 22% 16% 2	2283
	21st to end	Day	33% 12% 19% 25% 11% 1	2531
June	1st to 10th	Day	24% 11% 22% 29% 14%	2340
	11th to 20th	Day	27% 12% 22% 28% 11% 1	2340
	21st to end	Day	34% 14% 22% 25% 5%	2340
July	1st to 10th	Day	34% 11% 27% 24% 4	2276
	11th to 20th	Day	28% 13% 31% 23% 5%	2271
	21st to end	Day	27% 15% 29% 25% 4	2405
August	1st to 10th	Day	21% 10% 31% 31% 7%	2314
	11th to 20th	Day	26% 17% 30% 23% 4	2177
	21st to end	Day	35% 15% 22% 21% 6% 2	2326
September	1st to 10th	Day	35% 16% 18% 18% 11% 2	1980
	11th to 20th	Day	43% 13% 14% 16% 13% 1	1980
	21st to end	Day	35% 9% 14% 19% 16% 6%	1980
October	1st to 10th	Day	27% 7% 14% 25% 19% 8%	1868
	11th to 20th	Day	31% 7% 10% 22% 18% 11%	1800
	21st to end	Day	30% 6% 9% 19% 22% 14%	1980
November	1st to 10th	Day	33% 9% 12% 21% 20% 6%	1858
	11th to 20th	Day	38% 7% 12% 17% 17% 10%	1900
	21st to end	Day	43% 8% 10% 17% 15% 9%	1900
December	1st to 10th	Day	41% 8% 12% 19% 14% 5%	1715
	11th to 20th	Day	40% 8% 10% 15% 18% 9%	2159
	21st to end	Day	41% 8% 10% 14% 17% 10%	3135
January	1st to 10th	Night	43% 6% 8% 13% 20% 10%	2458
	11th to 20th	Night	42% 5% 7% 14% 19% 12%	2379
	21st to end	Night	44% 5% 7% 15% 18% 11%	2574
February	1st to 10th	Night	47% 6% 10% 10% 16% 12%	2250
	11th to 20th	Night	42% 5% 10% 16% 16% 11%	2048
	21st to end	Night	47% 6% 9% 13% 16% 9%	1628
March	1st to 10th	Night	40% 7% 11% 19% 15% 8%	1980
	11th to 20th	Night	42% 8% 12% 14% 16% 9%	1980
	21st to end	Night	33% 6% 15% 18% 20% 8%	2178
April	1st to 10th	Night	40% 6% 12% 19% 16% 8%	1980
	11th to 20th	Night	37% 6% 13% 18% 14% 12%	1819
	21st to end	Night	37% 7% 12% 21% 16% 7%	1800
May	1st to 10th	Night	34% 8% 14% 20% 15% 8%	1800
	11th to 20th	Night	29% 7% 16% 27% 18% 3	1612
	21st to end	Night	28% 9% 20% 28% 13% 1	1746
June	1st to 10th	Night	23% 7% 19% 32% 18%	1620
	11th to 20th	Night	26% 7% 18% 33% 14% 2	1620
	21st to end	Night	32% 12% 24% 24% 8%	1620
July	1st to 10th	Night	35% 10% 25% 26% 4	1575
	11th to 20th	Night	22% 9% 34% 28% 6%	1566
	21st to end	Night	22% 11% 30% 31% 5%	1673
August	1st to 10th	Night	14% 10% 29% 40% 6%	1601
	11th to 20th	Night	24% 10% 30% 31% 6%	1720
	21st to end	Night	27% 12% 25% 27% 8% 1	2024
September	1st to 10th	Night	33% 11% 22% 19% 12% 3	1980
	11th to 20th	Night	42% 9% 14% 20% 13% 2	1980
	21st to end	Night	32% 8% 15% 21% 18% 6%	1980
October	1st to 10th	Night	24% 8% 13% 27% 19% 10%	2071
	11th to 20th	Night	26% 6% 10% 25% 22% 11%	2160
	21st to end	Night	28% 6% 9% 17% 24% 17%	2376
November	1st to 10th	Night	30% 7% 13% 22% 20% 8%	2220
	11th to 20th	Night	36% 6% 13% 18% 18% 9%	2280
	21st to end	Night	41% 6% 11% 19% 16% 7%	2280
December	1st to 10th	Night	41% 7% 11% 20% 13% 7%	2460
	11th to 20th	Night	43% 6% 10% 13% 14% 14%	3037
	21st to end	Night	43% 7% 7% 15% 20% 8%	4532

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	19% 7% 31% 31% 10% 2	270
	11th to 20th	Day	17% 10% 23% 34% 16% 1	248
	21st to end	Day	20% 8% 32% 31% 9%	264
February	1st to 10th	Day	30% 4 24% 32% 8% 1	240
	11th to 20th	Day	25% 7% 23% 32% 11% 1	240
	21st to end	Day	18% 9% 33% 36% 4	198
March	1st to 10th	Day	19% 6% 30% 35% 10%	240
	11th to 20th	Day	22% 5% 30% 33% 8% 2	240
	21st to end	Day	15% 5% 28% 42% 9%	264
April	1st to 10th	Day	17% 10% 28% 35% 11%	240
	11th to 20th	Day	20% 7% 27% 25% 19% 2	366
	21st to end	Day	11% 5% 22% 42% 18% 1	420
May	1st to 10th	Day	13% 4 20% 44% 18% 1	423
	11th to 20th	Day	9% 2 14% 39% 32% 4	602
	21st to end	Day	5% 2 10% 41% 38% 4	679
June	1st to 10th	Day	7% 2 3 27% 50% 11%	630
	11th to 20th	Day	8% 3 25% 52% 11%	630
	21st to end	Day	9% 1 6% 16% 47% 21%	630
July	1st to 10th	Day	8% 2 1 14% 46% 29%	621
	11th to 20th	Day	5% 1 7% 51% 36%	618
	21st to end	Day	4 2 8% 54% 31%	684
August	1st to 10th	Day	3 2 7% 46% 42%	626
	11th to 20th	Day	4 1 1 5% 43% 46%	491
	21st to end	Day	11% 1 3 10% 35% 40%	445
September	1st to 10th	Day	10% 1 4 10% 42% 34%	270
	11th to 20th	Day	11% 1 1 17% 42% 28%	270
	21st to end	Day	10% 1 4 11% 57% 17%	270
October	1st to 10th	Day	4 4 6% 26% 47% 13%	270
	11th to 20th	Day	11% 2 8% 25% 40% 14%	270
	21st to end	Day	18% 5% 15% 27% 32% 3	297
November	1st to 10th	Day	13% 1 13% 31% 35% 6%	270
	11th to 20th	Day	16% 4 16% 34% 25% 4	270
	21st to end	Day	20% 7% 24% 31% 17% 1	270
December	1st to 10th	Day	16% 8% 16% 36% 20% 4	270
	11th to 20th	Day	21% 9% 24% 31% 15%	270
	21st to end	Day	21% 4 25% 30% 18% 2	297
January	1st to 10th	Night	21% 8% 24% 33% 15% 1	950
	11th to 20th	Night	20% 6% 25% 36% 13% 1	915
	21st to end	Night	20% 7% 25% 36% 12%	990
February	1st to 10th	Night	23% 7% 23% 34% 12%	810
	11th to 20th	Night	25% 6% 23% 33% 14% 1	654
	21st to end	Night	24% 8% 27% 33% 8%	444
March	1st to 10th	Night	19% 5% 26% 34% 16% 1	540
	11th to 20th	Night	17% 7% 20% 39% 16% 1	540
	21st to end	Night	14% 4 21% 42% 17% 2	594
April	1st to 10th	Night	15% 3 23% 42% 16%	540
	11th to 20th	Night	16% 5% 25% 34% 19%	533
	21st to end	Night	14% 4 23% 39% 19% 1	540
May	1st to 10th	Night	11% 6% 20% 43% 20%	540
	11th to 20th	Night	10% 2 14% 46% 26% 3	531
	21st to end	Night	6% 2 10% 37% 41% 4	584
June	1st to 10th	Night	5% 1 3 27% 56% 8%	540
	11th to 20th	Night	7% 1 3 25% 54% 11%	540
	21st to end	Night	8% 4 18% 49% 21%	540
July	1st to 10th	Night	6% 2 4 12% 48% 28%	525
	11th to 20th	Night	2 2 7% 52% 36%	522
	21st to end	Night	4 1 2 8% 58% 26%	591
August	1st to 10th	Night	1 1 6% 59% 33%	537
	11th to 20th	Night	5% 1 7% 53% 34%	534
	21st to end	Night	7% 1 3 10% 49% 31%	594
September	1st to 10th	Night	7% 1 2 13% 49% 27%	540
	11th to 20th	Night	17% 1 4 14% 47% 17%	540
	21st to end	Night	11% 1 4 17% 53% 13%	540
October	1st to 10th	Night	7% 2 5% 23% 50% 13%	631
	11th to 20th	Night	11% 2 10% 31% 39% 7%	720
	21st to end	Night	11% 4 14% 30% 34% 6%	792
November	1st to 10th	Night	11% 2 12% 31% 37% 6%	740
	11th to 20th	Night	16% 4 16% 32% 31% 2	760
	21st to end	Night	16% 5% 20% 36% 19% 3	760
December	1st to 10th	Night	21% 6% 18% 29% 24% 3	940
	11th to 20th	Night	20% 7% 20% 37% 15% 1	950
	21st to end	Night	21% 7% 24% 32% 14% 1	1045

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