Silver Bell Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Silver Bell Observatory 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	9% 5% 2 6% 5% 5% 5% 5% 7% 9% 41%	114278
	Night	10% 6% 2 6% 4 4 4 4 6% 9% 45%	114675
Transparency	Day	36% 8% 15% 18% 16% 7%	118083
	Night	35% 7% 14% 19% 17% 8%	119009
Seeing	Day	10% 8% 21% 24% 22% 15%	15933
	Night	11% 5% 21% 33% 24% 6%	26037

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	11% 6% 2 4 3 2 3 2 5% 7% 56%	4056
	11th to 20th	Day	19% 9% 3 6% 4 3 4 3 4 6% 38%	4060
	21st to end	Day	9% 5% 2 6% 4 3 4 3 7% 8% 48%	4763
February	1st to 10th	Day	16% 6% 2 6% 4 4 4 2 5% 6% 45%	2000
	11th to 20th	Day	14% 5% 2 5% 4 4 4 3 5% 5% 49%	1948
	21st to end	Day	13% 6% 2 5% 4 3 4 4 4 6% 49%	1650
March	1st to 10th	Day	16% 6% 2 5% 3 3 4 4 7% 8% 41%	4929
	11th to 20th	Day	8% 4 1 4 3 3 4 4 6% 8% 55%	4980
	21st to end	Day	11% 4 2 5% 3 4 4 4 6% 8% 50%	5465
April	1st to 10th	Day	8% 4 2 5% 5% 4 4 3 6% 9% 50%	2200
	11th to 20th	Day	10% 5% 2 6% 5% 5% 3 4 5% 6% 49%	2176
	21st to end	Day	5% 4 2 4 4 4 5% 5% 6% 7% 54%	2536
May	1st to 10th	Day	5% 3 1 4 4 4 5% 4 6% 10% 53%	4212
	11th to 20th	Day	9% 6% 2 8% 6% 4 5% 5% 7% 9% 39%	4081
	21st to end	Day	5% 4 2 5% 4 4 5% 4 7% 11% 51%	4476
June	1st to 10th	Day	10% 6% 2 9% 7% 7% 6% 6% 8% 10% 30%	2470
	11th to 20th	Day	6% 5% 2 7% 7% 7% 8% 7% 11% 11% 30%	2470
	21st to end	Day	8% 5% 2 7% 7% 7% 8% 7% 8% 11% 30%	2470
July	1st to 10th	Day	6% 6% 3 8% 6% 7% 8% 8% 11% 14% 23%	4120
	11th to 20th	Day	5% 4 2 8% 7% 8% 9% 7% 10% 13% 26%	4120
	21st to end	Day	5% 6% 3 8% 7% 8% 9% 8% 10% 14% 23%	4466
August	1st to 10th	Day	11% 5% 2 7% 6% 7% 7% 5% 10% 13% 27%	4104
	11th to 20th	Day	7% 4 3 8% 6% 6% 7% 6% 9% 13% 33%	3759
	21st to end	Day	10% 5% 2 8% 7% 7% 7% 6% 8% 10% 29%	3933
September	1st to 10th	Day	10% 7% 4 11% 7% 8% 7% 6% 8% 10% 21%	2090
	11th to 20th	Day	11% 8% 2 9% 7% 7% 7% 6% 7% 9% 27%	2090
	21st to end	Day	9% 6% 3 8% 6% 6% 5% 4 6% 7% 39%	2090
October	1st to 10th	Day	12% 7% 2 9% 4 5% 5% 4 6% 8% 38%	2090
	11th to 20th	Day	6% 4 2 4 3 4 4 4 5% 8% 57%	1948
	21st to end	Day	7% 5% 2 4 4 4 6% 4 6% 6% 52%	2090
November	1st to 10th	Day	9% 4 2 5% 4 4 4 4 6% 7% 52%	1957
	11th to 20th	Day	10% 5% 3 5% 4 4 4 3 6% 7% 49%	1910
	21st to end	Day	11% 6% 2 5% 4 4 4 3 6% 7% 49%	1800
December	1st to 10th	Day	5% 4 2 5% 4 4 4 3 6% 7% 56%	2930
	11th to 20th	Day	11% 5% 2 6% 4 4 4 4 7% 8% 46%	3373
	21st to end	Day	16% 9% 3 6% 4 4 4 3 4 6% 42%	4466
January	1st to 10th	Night	10% 6% 2 4 2 3 3 2 4 7% 57%	5889
	11th to 20th	Night	17% 8% 3 8% 5% 5% 5% 4 5% 7% 34%	5900
	21st to end	Night	15% 6% 2 6% 3 3 4 3 6% 8% 43%	6189
February	1st to 10th	Night	12% 6% 2 6% 5% 4 4 3 5% 6% 47%	2400
	11th to 20th	Night	12% 7% 2 5% 4 4 4 3 5% 6% 48%	2338
	21st to end	Night	12% 6% 3 6% 4 4 3 3 4 6% 50%	1980
March	1st to 10th	Night	15% 8% 3 7% 5% 4 4 3 4 6% 42%	4923
	11th to 20th	Night	6% 5% 2 4 3 3 4 3 6% 9% 55%	4980
	21st to end	Night	10% 5% 2 6% 4 4 4 3 6% 7% 50%	5467
April	1st to 10th	Night	10% 6% 2 5% 4 3 4 4 6% 7% 50%	2200
	11th to 20th	Night	7% 6% 2 6% 3 4 4 3 6% 7% 52%	2173
	21st to end	Night	5% 3 2 4 4 4 4 3 6% 9% 57%	1875
May	1st to 10th	Night	6% 4 2 5% 3 3 4 3 6% 9% 55%	2903
	11th to 20th	Night	10% 5% 3 6% 5% 5% 4 4 6% 10% 44%	2818
	21st to end	Night	3 2 1 3 3 4 3 3 6% 10% 62%	3079
June	1st to 10th	Night	9% 6% 2 7% 5% 5% 5% 5% 7% 12% 35%	1710
	11th to 20th	Night	4 5% 2 7% 5% 5% 6% 5% 11% 13% 36%	1710
	21st to end	Night	6% 5% 2 6% 5% 5% 5% 6% 8% 14% 37%	1710
July	1st to 10th	Night	7% 7% 3 9% 7% 8% 8% 8% 9% 12% 23%	2840
	11th to 20th	Night	3 4 2 6% 5% 5% 6% 7% 10% 14% 39%	2840
	21st to end	Night	5% 6% 3 9% 6% 7% 8% 7% 10% 13% 26%	3087
August	1st to 10th	Night	8% 4 3 8% 6% 6% 7% 7% 9% 12% 31%	2825
	11th to 20th	Night	4 3 1 5% 4 4 5% 4 10% 15% 45%	3064
	21st to end	Night	8% 7% 3 9% 7% 6% 7% 5% 8% 11% 31%	3690
September	1st to 10th	Night	9% 7% 3 10% 6% 7% 6% 6% 7% 10% 28%	2090
	11th to 20th	Night	11% 8% 3 8% 5% 6% 6% 5% 7% 10% 31%	2090
	21st to end	Night	10% 6% 3 6% 5% 4 5% 4 6% 8% 42%	2090
October	1st to 10th	Night	9% 7% 3 7% 5% 4 5% 4 5% 6% 44%	2090
	11th to 20th	Night	6% 4 2 5% 3 4 4 3 5% 7% 58%	2229
	21st to end	Night	7% 5% 2 6% 4 4 4 3 5% 7% 53%	2508
November	1st to 10th	Night	7% 4 2 6% 4 4 4 3 6% 8% 52%	2341
	11th to 20th	Night	10% 7% 1 5% 5% 3 3 3 6% 6% 51%	2485
	21st to end	Night	10% 7% 2 5% 4 4 4 3 6% 6% 48%	2600
December	1st to 10th	Night	10% 5% 2 5% 4 3 4 3 4 7% 55%	4250
	11th to 20th	Night	12% 7% 2 6% 4 4 4 3 5% 6% 48%	4822
	21st to end	Night	14% 8% 3 7% 4 4 4 3 5% 6% 41%	6490

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	29% 3 7% 16% 28% 16%	4057
	11th to 20th	Day	48% 4 4 12% 20% 12%	4060
	21st to end	Day	32% 5% 11% 23% 18% 11%	4763
February	1st to 10th	Day	41% 4 9% 14% 20% 12%	2000
	11th to 20th	Day	36% 4 8% 16% 24% 12%	1958
	21st to end	Day	36% 4 10% 16% 21% 14%	1650
March	1st to 10th	Day	39% 7% 10% 15% 20% 11%	4929
	11th to 20th	Day	26% 5% 11% 17% 23% 18%	4980
	21st to end	Day	31% 5% 9% 25% 19% 11%	5465
April	1st to 10th	Day	30% 6% 12% 19% 19% 13%	2200
	11th to 20th	Day	34% 6% 8% 20% 22% 10%	2176
	21st to end	Day	26% 7% 13% 23% 22% 9%	2536
May	1st to 10th	Day	25% 9% 17% 27% 17% 5%	4250
	11th to 20th	Day	38% 10% 16% 22% 13% 2	4085
	21st to end	Day	25% 8% 16% 31% 18% 2	4489
June	1st to 10th	Day	46% 11% 16% 18% 8% 1	2470
	11th to 20th	Day	41% 14% 19% 20% 6%	2470
	21st to end	Day	44% 13% 21% 16% 6%	2470
July	1st to 10th	Day	42% 17% 31% 9% 1	4055
	11th to 20th	Day	40% 17% 34% 7% 1	4051
	21st to end	Day	42% 18% 30% 9% 1	4324
August	1st to 10th	Day	42% 13% 26% 15% 3	4091
	11th to 20th	Day	38% 13% 24% 17% 8%	3796
	21st to end	Day	44% 12% 21% 19% 4	3933
September	1st to 10th	Day	53% 13% 16% 11% 6% 1	2090
	11th to 20th	Day	50% 12% 17% 13% 7% 1	2090
	21st to end	Day	42% 7% 12% 19% 15% 5%	2090
October	1st to 10th	Day	50% 8% 7% 12% 14% 8%	3480
	11th to 20th	Day	18% 4 6% 17% 39% 16%	3214
	21st to end	Day	20% 4 8% 21% 37% 10%	3428
November	1st to 10th	Day	30% 5% 11% 20% 20% 14%	1957
	11th to 20th	Day	35% 4 8% 18% 23% 12%	1910
	21st to end	Day	35% 4 9% 20% 23% 8%	1800
December	1st to 10th	Day	27% 4 12% 19% 24% 13%	2930
	11th to 20th	Day	35% 6% 9% 22% 20% 8%	3370
	21st to end	Day	43% 4 8% 18% 22% 5%	4466
January	1st to 10th	Night	29% 3 7% 15% 32% 15%	5889
	11th to 20th	Night	49% 4 6% 16% 19% 7%	5900
	21st to end	Night	39% 5% 10% 18% 17% 10%	6189
February	1st to 10th	Night	38% 5% 9% 15% 20% 12%	2400
	11th to 20th	Night	36% 5% 8% 19% 19% 13%	2339
	21st to end	Night	35% 5% 9% 18% 21% 12%	1980
March	1st to 10th	Night	44% 6% 8% 15% 17% 10%	4923
	11th to 20th	Night	26% 5% 12% 17% 21% 18%	4980
	21st to end	Night	33% 5% 9% 21% 20% 11%	5467
April	1st to 10th	Night	33% 6% 13% 19% 19% 11%	2200
	11th to 20th	Night	30% 6% 8% 23% 22% 10%	2173
	21st to end	Night	24% 6% 13% 29% 20% 8%	1875
May	1st to 10th	Night	24% 10% 17% 29% 15% 5%	2930
	11th to 20th	Night	34% 9% 19% 23% 12% 2	2819
	21st to end	Night	18% 7% 16% 37% 21% 1	3087
June	1st to 10th	Night	39% 10% 20% 24% 6%	1710
	11th to 20th	Night	33% 14% 25% 23% 5%	1710
	21st to end	Night	34% 13% 26% 20% 5% 1	1710
July	1st to 10th	Night	48% 17% 25% 8% 2	2795
	11th to 20th	Night	30% 18% 41% 9% 1	2786
	21st to end	Night	44% 16% 29% 10% 2	2988
August	1st to 10th	Night	40% 15% 29% 13% 3	2814
	11th to 20th	Night	26% 12% 33% 22% 8%	3093
	21st to end	Night	44% 13% 23% 16% 3	3690
September	1st to 10th	Night	46% 14% 20% 13% 5% 1	2090
	11th to 20th	Night	46% 13% 18% 16% 6% 1	2090
	21st to end	Night	37% 9% 16% 19% 15% 4	2090
October	1st to 10th	Night	50% 8% 10% 12% 12% 8%	3480
	11th to 20th	Night	20% 4 7% 21% 32% 16%	3702
	21st to end	Night	21% 4 7% 22% 35% 11%	4120
November	1st to 10th	Night	29% 6% 10% 22% 19% 13%	2341
	11th to 20th	Night	33% 5% 8% 19% 22% 14%	2485
	21st to end	Night	35% 5% 11% 18% 21% 11%	2600
December	1st to 10th	Night	31% 4 10% 19% 19% 17%	4250
	11th to 20th	Night	36% 6% 10% 21% 18% 10%	4824
	21st to end	Night	42% 5% 9% 19% 19% 6%	6490

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	8% 19% 51% 16% 4 1	300
	11th to 20th	Day	13% 15% 39% 23% 8% 2	300
	21st to end	Day	16% 22% 43% 15% 5%	330
February	1st to 10th	Day	15% 13% 37% 23% 10% 1	300
	11th to 20th	Day	19% 15% 38% 20% 7%	300
	21st to end	Day	11% 15% 41% 23% 9% 1	249
March	1st to 10th	Day	11% 18% 30% 28% 12% 1	494
	11th to 20th	Day	16% 12% 31% 26% 14% 2	500
	21st to end	Day	12% 14% 29% 29% 14% 1	550
April	1st to 10th	Day	11% 15% 28% 27% 16% 3	500
	11th to 20th	Day	11% 11% 30% 27% 18% 4	492
	21st to end	Day	5% 7% 24% 32% 23% 8%	500
May	1st to 10th	Day	6% 8% 23% 30% 22% 10%	503
	11th to 20th	Day	6% 6% 17% 30% 25% 16%	487
	21st to end	Day	9% 3 11% 23% 32% 22%	529
June	1st to 10th	Day	11% 2 10% 26% 28% 23%	490
	11th to 20th	Day	8% 5% 11% 19% 27% 31%	490
	21st to end	Day	6% 4 3 22% 31% 34%	490
July	1st to 10th	Day	7% 1 6% 13% 28% 45%	485
	11th to 20th	Day	7% 1 2 12% 30% 48%	484
	21st to end	Day	7% 3 5% 15% 34% 37%	532
August	1st to 10th	Day	12% 2 5% 15% 31% 35%	486
	11th to 20th	Day	8% 2 5% 13% 28% 44%	479
	21st to end	Day	12% 1 5% 20% 32% 30%	539
September	1st to 10th	Day	13% 3 7% 22% 27% 30%	490
	11th to 20th	Day	13% 2 9% 27% 31% 18%	490
	21st to end	Day	10% 2 11% 25% 32% 19%	490
October	1st to 10th	Day	14% 6% 23% 27% 21% 8%	490
	11th to 20th	Day	5% 6% 19% 34% 27% 8%	490
	21st to end	Day	7% 6% 21% 39% 22% 5%	539
November	1st to 10th	Day	12% 5% 24% 33% 22% 4	495
	11th to 20th	Day	13% 9% 31% 30% 11% 4	410
	21st to end	Day	10% 9% 39% 29% 11% 1	300
December	1st to 10th	Day	7% 16% 36% 23% 15% 3	300
	11th to 20th	Day	15% 12% 34% 26% 11% 1	300
	21st to end	Day	14% 12% 33% 28% 12% 1	330
January	1st to 10th	Night	16% 10% 34% 32% 9%	800
	11th to 20th	Night	13% 11% 26% 37% 13% 1	800
	21st to end	Night	18% 12% 33% 33% 5%	880
February	1st to 10th	Night	14% 8% 32% 35% 11% 1	800
	11th to 20th	Night	15% 10% 35% 32% 8% 1	783
	21st to end	Night	15% 8% 34% 38% 5%	660
March	1st to 10th	Night	11% 10% 33% 37% 10%	800
	11th to 20th	Night	11% 9% 28% 36% 15%	800
	21st to end	Night	11% 8% 32% 34% 15%	880
April	1st to 10th	Night	11% 10% 28% 36% 15% 1	800
	11th to 20th	Night	9% 8% 30% 39% 14% 1	791
	21st to end	Night	5% 5% 25% 40% 24% 2	645
May	1st to 10th	Night	7% 4 18% 45% 25% 3	600
	11th to 20th	Night	6% 2 18% 42% 29% 3	563
	21st to end	Night	5% 1 12% 36% 40% 6%	617
June	1st to 10th	Night	11% 2 10% 34% 34% 9%	570
	11th to 20th	Night	5% 1 9% 26% 40% 18%	570
	21st to end	Night	8% 1 5% 23% 46% 18%	570
July	1st to 10th	Night	6% 2 5% 16% 45% 25%	555
	11th to 20th	Night	5% 1 2 10% 55% 27%	552
	21st to end	Night	5% 2 4 16% 54% 20%	624
August	1st to 10th	Night	10% 1 3 16% 49% 21%	567
	11th to 20th	Night	5% 1 4 12% 53% 25%	564
	21st to end	Night	11% 2 7% 21% 41% 18%	750
September	1st to 10th	Night	11% 3 7% 23% 42% 15%	760
	11th to 20th	Night	14% 3 11% 29% 35% 9%	760
	21st to end	Night	12% 2 11% 31% 37% 6%	760
October	1st to 10th	Night	13% 3 19% 37% 25% 3	760
	11th to 20th	Night	6% 4 21% 40% 27% 2	760
	21st to end	Night	9% 4 21% 41% 24% 2	836
November	1st to 10th	Night	9% 4 26% 38% 21% 2	780
	11th to 20th	Night	13% 7% 23% 41% 15% 1	800
	21st to end	Night	12% 7% 26% 40% 14% 1	800
December	1st to 10th	Night	13% 6% 26% 37% 17% 2	800
	11th to 20th	Night	15% 7% 27% 36% 15% 1	800
	21st to end	Night	15% 8% 29% 35% 14%	880

Page generated at 2025-05-08 09:33:57. Version ac07

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