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 2024-11-12.

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% 6% 2 6% 5% 5% 5% 4 7% 9% 39%	101007
	Night	12% 6% 2 6% 4 4 4 4 6% 8% 44%	98236
Transparency	Day	36% 9% 16% 20% 13% 5%	104694
	Night	35% 8% 15% 20% 15% 7%	102676
Seeing	Day	11% 4 13% 25% 33% 14%	14745
	Night	14% 4 15% 29% 30% 8%	24593

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	19% 11% 3 7% 4 3 4 3 5% 5% 35%	2847
	11th to 20th	Day	13% 8% 2 5% 3 3 3 3 5% 6% 49%	2850
	21st to end	Day	22% 9% 3 7% 4 4 4 4 6% 7% 32%	3135
February	1st to 10th	Day	20% 8% 3 7% 5% 4 4 4 5% 6% 34%	1819
	11th to 20th	Day	20% 9% 3 7% 4 3 4 3 5% 5% 37%	1899
	21st to end	Day	19% 10% 3 6% 3 4 4 3 5% 6% 37%	1727
March	1st to 10th	Day	19% 10% 3 9% 5% 4 4 5% 6% 8% 26%	3436
	11th to 20th	Day	18% 9% 4 9% 6% 5% 4 4 5% 7% 29%	3480
	21st to end	Day	14% 6% 2 5% 4 4 4 3 5% 7% 46%	3814
April	1st to 10th	Day	15% 8% 3 7% 4 4 4 3 6% 7% 39%	2090
	11th to 20th	Day	14% 8% 3 7% 5% 4 4 4 5% 7% 39%	2206
	21st to end	Day	9% 6% 2 6% 6% 5% 4 4 5% 8% 44%	2280
May	1st to 10th	Day	16% 9% 4 7% 5% 5% 5% 5% 7% 8% 30%	3765
	11th to 20th	Day	13% 6% 3 7% 5% 4 5% 4 6% 8% 38%	4045
	21st to end	Day	7% 6% 2 6% 5% 6% 7% 6% 10% 12% 32%	4477
June	1st to 10th	Day	5% 3 1 5% 4 5% 5% 5% 9% 12% 46%	2470
	11th to 20th	Day	5% 4 1 5% 5% 6% 6% 6% 9% 12% 41%	2470
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 7% 10% 13% 35%	2470
July	1st to 10th	Day	7% 5% 2 6% 5% 4 5% 6% 9% 12% 38%	4120
	11th to 20th	Day	5% 6% 4 11% 7% 6% 7% 6% 10% 12% 27%	4120
	21st to end	Day	9% 7% 3 8% 7% 6% 5% 5% 8% 11% 31%	4465
August	1st to 10th	Day	2 2 1 2 2 3 4 4 6% 11% 63%	4104
	11th to 20th	Day	2 2 1 4 4 5% 6% 6% 10% 15% 46%	3820
	21st to end	Day	5% 3 2 7% 6% 6% 7% 6% 9% 13% 37%	4088
September	1st to 10th	Day	9% 6% 2 7% 5% 6% 6% 6% 8% 11% 33%	2090
	11th to 20th	Day	13% 6% 2 9% 5% 5% 5% 6% 7% 9% 33%	2090
	21st to end	Day	8% 6% 2 6% 4 4 5% 5% 5% 7% 47%	2090
October	1st to 10th	Day	5% 4 1 5% 4 4 4 4 6% 8% 54%	1971
	11th to 20th	Day	9% 6% 2 5% 3 3 4 3 5% 7% 52%	1900
	21st to end	Day	10% 4 2 5% 4 4 2 3 5% 7% 54%	2090
November	1st to 10th	Day	9% 6% 3 6% 5% 4 5% 4 7% 9% 43%	1958
	11th to 20th	Day	15% 6% 2 5% 4 4 5% 3 5% 5% 46%	1910
	21st to end	Day	16% 6% 3 7% 5% 4 3 4 5% 6% 41%	1900
December	1st to 10th	Day	17% 7% 2 6% 4 3 4 4 5% 8% 40%	1715
	11th to 20th	Day	13% 7% 2 7% 5% 4 4 4 6% 8% 40%	2161
	21st to end	Day	16% 7% 2 7% 4 4 4 3 6% 7% 40%	3135
January	1st to 10th	Night	20% 9% 3 7% 4 4 4 3 5% 6% 35%	4108
	11th to 20th	Night	15% 7% 3 6% 4 3 3 2 5% 6% 47%	4120
	21st to end	Night	23% 8% 3 6% 4 3 3 3 6% 7% 33%	4532
February	1st to 10th	Night	18% 9% 2 7% 4 4 3 5% 5% 6% 37%	2375
	11th to 20th	Night	19% 9% 3 5% 3 2 3 3 4 6% 43%	2166
	21st to end	Night	19% 9% 3 6% 4 4 4 4 4 6% 38%	1727
March	1st to 10th	Night	24% 12% 3 6% 3 3 4 3 5% 7% 30%	3419
	11th to 20th	Night	20% 10% 3 7% 5% 5% 5% 4 6% 7% 29%	3480
	21st to end	Night	11% 5% 2 5% 4 4 4 3 5% 7% 50%	3817
April	1st to 10th	Night	13% 8% 2 7% 4 4 4 4 6% 8% 41%	2090
	11th to 20th	Night	15% 5% 2 6% 5% 3 4 3 3 7% 46%	1921
	21st to end	Night	12% 6% 2 7% 5% 4 4 4 5% 7% 45%	1900
May	1st to 10th	Night	22% 9% 2 6% 4 4 4 3 6% 8% 33%	3130
	11th to 20th	Night	12% 6% 2 6% 4 4 4 4 5% 8% 44%	2841
	21st to end	Night	10% 8% 2 6% 4 4 4 5% 8% 12% 37%	3090
June	1st to 10th	Night	6% 3 1 4 3 4 3 3 6% 13% 54%	1710
	11th to 20th	Night	5% 4 2 5% 4 3 4 4 8% 11% 50%	1710
	21st to end	Night	5% 4 2 5% 4 6% 7% 5% 9% 13% 41%	1710
July	1st to 10th	Night	6% 5% 2 6% 5% 4 4 3 7% 11% 47%	2850
	11th to 20th	Night	3 2 2 7% 6% 7% 6% 5% 9% 14% 39%	2850
	21st to end	Night	7% 6% 2 7% 6% 6% 6% 5% 7% 12% 37%	3098
August	1st to 10th	Night	1 1 2 2 2 3 3 5% 8% 73%	2836
	11th to 20th	Night	2 2 1 3 3 3 5% 3 7% 15% 55%	3008
	21st to end	Night	4 3 1 4 4 4 5% 5% 8% 13% 49%	3562
September	1st to 10th	Night	7% 5% 2 6% 5% 5% 5% 5% 8% 12% 39%	2090
	11th to 20th	Night	11% 7% 2 7% 5% 5% 5% 4 6% 8% 41%	2090
	21st to end	Night	7% 5% 2 6% 4 4 3 3 5% 9% 51%	2090
October	1st to 10th	Night	5% 4 2 4 4 3 4 4 5% 9% 57%	2187
	11th to 20th	Night	8% 4 1 4 2 3 3 3 5% 7% 60%	2280
	21st to end	Night	9% 4 2 5% 3 3 4 3 4 5% 59%	2508
November	1st to 10th	Night	8% 5% 2 5% 4 5% 5% 3 5% 8% 51%	2340
	11th to 20th	Night	13% 6% 2 6% 4 4 4 3 5% 6% 48%	2293
	21st to end	Night	14% 7% 2 6% 5% 4 4 3 5% 6% 43%	2280
December	1st to 10th	Night	17% 7% 3 5% 4 4 3 3 5% 8% 41%	2460
	11th to 20th	Night	14% 9% 3 6% 5% 4 4 4 5% 7% 39%	3036
	21st to end	Night	15% 8% 3 8% 4 4 4 3 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	48% 7% 7% 15% 15% 8%	2848
	11th to 20th	Day	34% 10% 10% 21% 16% 10%	2850
	21st to end	Day	51% 5% 6% 14% 16% 7%	3135
February	1st to 10th	Day	50% 6% 9% 13% 13% 10%	1819
	11th to 20th	Day	49% 6% 9% 13% 15% 7%	1909
	21st to end	Day	46% 7% 11% 14% 13% 8%	1727
March	1st to 10th	Day	51% 12% 13% 12% 9% 2	3436
	11th to 20th	Day	51% 11% 11% 11% 12% 4	3480
	21st to end	Day	35% 8% 13% 19% 20% 5%	3814
April	1st to 10th	Day	42% 9% 12% 17% 14% 7%	2090
	11th to 20th	Day	41% 9% 14% 16% 13% 8%	2206
	21st to end	Day	35% 9% 13% 21% 17% 5%	2280
May	1st to 10th	Day	44% 12% 17% 15% 8% 3	3800
	11th to 20th	Day	37% 11% 14% 18% 15% 4	4049
	21st to end	Day	33% 14% 18% 28% 6%	4489
June	1st to 10th	Day	23% 11% 22% 29% 14%	2470
	11th to 20th	Day	27% 11% 21% 28% 12% 1	2470
	21st to end	Day	32% 13% 23% 27% 5%	2470
July	1st to 10th	Day	35% 8% 35% 20% 2	4056
	11th to 20th	Day	45% 12% 25% 15% 3	4051
	21st to end	Day	43% 11% 25% 18% 2	4325
August	1st to 10th	Day	14% 7% 22% 47% 11%	4090
	11th to 20th	Day	20% 11% 42% 24% 3	3858
	21st to end	Day	34% 13% 21% 22% 8% 1	4087
September	1st to 10th	Day	37% 15% 17% 17% 11% 2	2090
	11th to 20th	Day	42% 12% 14% 17% 14% 1	2090
	21st to end	Day	33% 9% 14% 20% 16% 8%	2090
October	1st to 10th	Day	21% 5% 9% 16% 18% 31%	3270
	11th to 20th	Day	22% 5% 8% 24% 30% 11%	3140
	21st to end	Day	23% 5% 8% 21% 33% 10%	3428
November	1st to 10th	Day	35% 8% 12% 20% 19% 6%	1958
	11th to 20th	Day	38% 7% 12% 17% 17% 10%	1910
	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	47% 8% 7% 11% 14% 12%	4108
	11th to 20th	Night	37% 7% 8% 15% 21% 11%	4120
	21st to end	Night	50% 5% 7% 13% 17% 9%	4532
February	1st to 10th	Night	46% 6% 10% 10% 16% 12%	2375
	11th to 20th	Night	42% 5% 9% 16% 17% 11%	2166
	21st to end	Night	46% 6% 9% 13% 17% 8%	1727
March	1st to 10th	Night	50% 9% 12% 15% 9% 4	3419
	11th to 20th	Night	52% 9% 11% 13% 9% 5%	3480
	21st to end	Night	31% 7% 14% 16% 26% 6%	3817
April	1st to 10th	Night	40% 6% 12% 18% 15% 8%	2090
	11th to 20th	Night	38% 6% 13% 17% 13% 12%	1921
	21st to end	Night	37% 7% 12% 22% 16% 7%	1900
May	1st to 10th	Night	44% 10% 15% 16% 10% 5%	3160
	11th to 20th	Night	35% 9% 13% 22% 14% 7%	2842
	21st to end	Night	32% 12% 18% 28% 8% 1	3099
June	1st to 10th	Night	23% 7% 20% 32% 18%	1710
	11th to 20th	Night	25% 7% 17% 33% 16% 2	1710
	21st to end	Night	30% 11% 24% 26% 9%	1710
July	1st to 10th	Night	31% 7% 34% 26% 2	2805
	11th to 20th	Night	33% 9% 33% 21% 4	2796
	21st to end	Night	39% 12% 27% 20% 3	2999
August	1st to 10th	Night	10% 7% 18% 52% 14%	2826
	11th to 20th	Night	19% 6% 39% 32% 4	3036
	21st to end	Night	25% 14% 25% 25% 10% 1	3562
September	1st to 10th	Night	35% 11% 22% 18% 11% 3	2090
	11th to 20th	Night	41% 9% 13% 20% 14% 2	2090
	21st to end	Night	31% 8% 14% 21% 18% 8%	2090
October	1st to 10th	Night	19% 6% 9% 17% 18% 31%	3655
	11th to 20th	Night	17% 4 8% 24% 32% 14%	3779
	21st to end	Night	19% 4 7% 20% 36% 14%	4120
November	1st to 10th	Night	31% 7% 13% 22% 19% 8%	2340
	11th to 20th	Night	36% 6% 13% 18% 18% 9%	2293
	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% 30% 31% 10% 2	270
	11th to 20th	Day	17% 10% 24% 34% 14% 1	270
	21st to end	Day	20% 9% 31% 30% 10%	297
February	1st to 10th	Day	27% 5% 26% 33% 8% 1	270
	11th to 20th	Day	25% 7% 24% 34% 10% 1	270
	21st to end	Day	18% 9% 33% 36% 4	225
March	1st to 10th	Day	18% 7% 30% 36% 9%	270
	11th to 20th	Day	21% 7% 30% 33% 7% 2	270
	21st to end	Day	14% 6% 28% 42% 9%	297
April	1st to 10th	Day	17% 10% 29% 35% 10%	270
	11th to 20th	Day	20% 8% 27% 27% 17% 1	404
	21st to end	Day	11% 6% 23% 41% 17% 2	460
May	1st to 10th	Day	13% 5% 20% 43% 17% 1	463
	11th to 20th	Day	10% 3 14% 40% 30% 4	651
	21st to end	Day	5% 2 12% 41% 36% 4	734
June	1st to 10th	Day	6% 2 4 28% 49% 10%	680
	11th to 20th	Day	8% 1 4 25% 51% 11%	680
	21st to end	Day	8% 1 6% 18% 47% 20%	680
July	1st to 10th	Day	8% 2 2 13% 45% 31%	671
	11th to 20th	Day	5% 1 2 6% 50% 36%	668
	21st to end	Day	6% 1 3 9% 53% 30%	739
August	1st to 10th	Day	3 2 7% 47% 42%	676
	11th to 20th	Day	4 1 1 4 43% 47%	534
	21st to end	Day	10% 1 3 10% 36% 39%	486
September	1st to 10th	Day	12% 2 5% 12% 39% 31%	300
	11th to 20th	Day	10% 1 2 16% 43% 28%	300
	21st to end	Day	9% 1 3 12% 57% 18%	300
October	1st to 10th	Day	5% 4 5% 27% 46% 12%	300
	11th to 20th	Day	10% 2 8% 26% 41% 13%	300
	21st to end	Day	16% 5% 13% 27% 34% 5%	330
November	1st to 10th	Day	13% 4 13% 32% 32% 5%	300
	11th to 20th	Day	16% 4 16% 33% 26% 4	273
	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	19% 6% 26% 35% 13% 1	950
	21st to end	Night	21% 7% 25% 36% 12%	1045
February	1st to 10th	Night	22% 8% 23% 34% 13%	855
	11th to 20th	Night	24% 6% 23% 33% 13% 1	692
	21st to end	Night	23% 8% 28% 34% 8%	471
March	1st to 10th	Night	20% 6% 25% 33% 15% 1	570
	11th to 20th	Night	17% 7% 21% 39% 15% 1	570
	21st to end	Night	13% 5% 22% 41% 17% 2	627
April	1st to 10th	Night	15% 3 24% 41% 16%	570
	11th to 20th	Night	17% 6% 25% 34% 18%	563
	21st to end	Night	14% 4 23% 38% 19% 1	570
May	1st to 10th	Night	12% 7% 20% 42% 19%	570
	11th to 20th	Night	10% 2 15% 46% 25% 3	561
	21st to end	Night	6% 2 12% 37% 39% 3	617
June	1st to 10th	Night	5% 1 3 28% 56% 8%	570
	11th to 20th	Night	7% 1 4 25% 54% 10%	570
	21st to end	Night	7% 4 19% 48% 21%	570
July	1st to 10th	Night	6% 2 4 12% 48% 29%	555
	11th to 20th	Night	2 1 1 7% 52% 37%	552
	21st to end	Night	5% 1 2 9% 58% 25%	624
August	1st to 10th	Night	1 1 5% 60% 32%	567
	11th to 20th	Night	5% 1 7% 53% 34%	564
	21st to end	Night	7% 1 4 10% 49% 30%	627
September	1st to 10th	Night	9% 2 3 13% 48% 27%	570
	11th to 20th	Night	16% 1 4 15% 47% 16%	570
	21st to end	Night	11% 1 4 18% 53% 13%	570
October	1st to 10th	Night	6% 2 5% 24% 50% 13%	667
	11th to 20th	Night	10% 2 10% 32% 39% 7%	760
	21st to end	Night	11% 4 14% 30% 35% 6%	836
November	1st to 10th	Night	11% 3 12% 32% 36% 6%	780
	11th to 20th	Night	16% 4 16% 32% 30% 2	765
	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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