Chamberlain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chamberlain Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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	12% 8% 3 7% 4 4 4 4 6% 8% 42%	131130
	Night	13% 8% 3 7% 4 4 4 3 5% 7% 42%	130329
Transparency	Day	40% 5% 11% 25% 17% 2	134682
	Night	41% 4 11% 25% 17% 3	134843
Seeing	Day	16% 9% 26% 26% 17% 6%	17380
	Night	17% 8% 29% 31% 14% 1	27346

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% 11% 5% 12% 6% 5% 4 3 5% 7% 31%	4691
	11th to 20th	Day	8% 9% 4 9% 7% 7% 4 4 5% 6% 39%	4987
	21st to end	Day	16% 15% 5% 8% 4 3 3 2 4 4 35%	5812
February	1st to 10th	Day	15% 11% 4 8% 4 4 4 3 5% 7% 34%	1890
	11th to 20th	Day	15% 11% 3 7% 5% 5% 4 3 5% 7% 35%	2038
	21st to end	Day	14% 10% 3 8% 4 4 4 3 5% 6% 40%	1730
March	1st to 10th	Day	16% 11% 2 5% 3 3 3 2 3 4 49%	4515
	11th to 20th	Day	9% 7% 3 4 3 4 4 4 6% 8% 50%	4980
	21st to end	Day	15% 9% 3 8% 5% 4 5% 4 6% 7% 34%	5797
April	1st to 10th	Day	13% 7% 2 7% 5% 5% 4 3 5% 8% 40%	2400
	11th to 20th	Day	15% 10% 3 6% 5% 4 4 3 5% 6% 41%	2374
	21st to end	Day	16% 7% 3 7% 5% 5% 4 3 6% 6% 38%	2400
May	1st to 10th	Day	16% 8% 3 8% 5% 5% 5% 4 7% 10% 30%	5625
	11th to 20th	Day	16% 8% 2 6% 5% 4 5% 4 6% 8% 37%	6241
	21st to end	Day	12% 6% 3 7% 4 5% 5% 4 6% 7% 40%	6935
June	1st to 10th	Day	8% 5% 2 5% 4 4 5% 4 6% 9% 48%	2800
	11th to 20th	Day	8% 4 2 5% 4 4 4 5% 6% 9% 48%	2800
	21st to end	Day	5% 4 1 6% 5% 5% 5% 5% 7% 10% 47%	2800
July	1st to 10th	Day	10% 5% 2 6% 4 4 4 3 6% 9% 46%	6360
	11th to 20th	Day	6% 5% 2 7% 5% 5% 6% 5% 7% 11% 39%	6360
	21st to end	Day	6% 5% 2 7% 5% 5% 6% 5% 8% 11% 40%	6697
August	1st to 10th	Day	5% 4 2 5% 4 4 4 4 7% 9% 52%	4206
	11th to 20th	Day	12% 5% 2 6% 5% 5% 5% 4 6% 9% 41%	3849
	21st to end	Day	5% 4 2 5% 4 4 4 4 7% 10% 51%	4312
September	1st to 10th	Day	12% 5% 2 5% 3 4 4 2 5% 8% 51%	2400
	11th to 20th	Day	9% 5% 1 5% 4 3 3 4 5% 8% 52%	2360
	21st to end	Day	8% 5% 2 6% 4 4 4 4 5% 7% 53%	2200
October	1st to 10th	Day	10% 5% 2 4 3 4 4 3 4 6% 56%	2015
	11th to 20th	Day	14% 5% 2 5% 3 3 3 3 4 6% 51%	2000
	21st to end	Day	17% 8% 2 6% 3 3 3 2 5% 5% 46%	2150
November	1st to 10th	Day	11% 5% 2 5% 4 3 3 3 5% 7% 53%	1852
	11th to 20th	Day	14% 9% 2 5% 3 3 3 4 4 6% 47%	1890
	21st to end	Day	16% 12% 4 7% 4 5% 3 2 4 6% 38%	1816
December	1st to 10th	Day	8% 10% 3 7% 3 3 3 3 4 6% 50%	2690
	11th to 20th	Day	14% 8% 5% 9% 5% 4 5% 4 4 5% 36%	3088
	21st to end	Day	23% 17% 5% 10% 4 4 4 3 4 5% 22%	4070
January	1st to 10th	Night	13% 10% 5% 11% 5% 4 4 3 4 5% 36%	8232
	11th to 20th	Night	9% 9% 4 11% 5% 5% 6% 4 4 5% 38%	7999
	21st to end	Night	16% 14% 4 9% 4 4 4 3 4 5% 33%	8402
February	1st to 10th	Night	14% 11% 5% 8% 4 4 4 4 5% 5% 35%	2730
	11th to 20th	Night	16% 8% 3 7% 4 4 4 3 5% 6% 40%	2467
	21st to end	Night	15% 9% 4 7% 5% 3 3 3 4 5% 43%	2076
March	1st to 10th	Night	19% 11% 3 6% 3 3 3 3 3 4 42%	5398
	11th to 20th	Night	12% 7% 2 6% 3 3 3 2 4 7% 51%	5008
	21st to end	Night	17% 8% 3 8% 5% 5% 5% 4 5% 6% 34%	5130
April	1st to 10th	Night	12% 7% 2 5% 5% 4 4 4 5% 7% 45%	2000
	11th to 20th	Night	14% 7% 2 6% 3 4 4 3 5% 8% 45%	1975
	21st to end	Night	18% 7% 3 8% 4 4 3 3 4 6% 40%	2000
May	1st to 10th	Night	18% 8% 3 7% 4 4 5% 4 6% 8% 34%	4268
	11th to 20th	Night	13% 7% 2 6% 4 4 4 4 6% 8% 43%	3667
	21st to end	Night	13% 6% 3 7% 4 4 5% 4 6% 8% 40%	3942
June	1st to 10th	Night	8% 4 2 6% 3 4 5% 4 6% 10% 49%	1600
	11th to 20th	Night	6% 4 2 5% 4 3 5% 4 7% 8% 52%	1600
	21st to end	Night	5% 3 2 5% 4 4 4 5% 6% 10% 52%	1600
July	1st to 10th	Night	10% 5% 3 6% 4 3 4 4 6% 9% 48%	3618
	11th to 20th	Night	5% 4 2 7% 5% 5% 5% 4 7% 10% 46%	3617
	21st to end	Night	7% 5% 2 6% 5% 5% 6% 5% 7% 10% 41%	4170
August	1st to 10th	Night	5% 3 1 4 3 3 4 4 6% 8% 58%	2952
	11th to 20th	Night	10% 5% 2 6% 3 4 4 4 5% 7% 50%	3200
	21st to end	Night	5% 3 1 3 3 4 4 3 5% 10% 59%	3586
September	1st to 10th	Night	10% 5% 2 5% 4 4 3 3 4 8% 53%	2000
	11th to 20th	Night	8% 4 2 5% 4 4 4 3 7% 8% 51%	2019
	21st to end	Night	9% 4 2 5% 4 3 4 4 5% 6% 54%	2200
October	1st to 10th	Night	8% 3 1 5% 4 3 3 3 4 6% 58%	2380
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 5% 6% 53%	2400
	21st to end	Night	18% 7% 2 5% 3 3 3 3 4 5% 46%	2688
November	1st to 10th	Night	9% 6% 2 4 4 3 4 4 5% 5% 54%	2666
	11th to 20th	Night	12% 7% 2 5% 4 3 3 3 5% 7% 49%	2730
	21st to end	Night	18% 9% 2 7% 3 3 4 2 5% 5% 42%	2800
December	1st to 10th	Night	12% 8% 3 7% 5% 4 4 3 4 5% 45%	4720
	11th to 20th	Night	16% 8% 3 6% 4 4 4 3 5% 7% 40%	5339
	21st to end	Night	25% 19% 4 9% 4 4 4 3 3 4 21%	7150

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	54% 4 10% 22% 8% 1	4692
	11th to 20th	Day	45% 3 12% 22% 15% 3	4987
	21st to end	Day	54% 3 8% 19% 12% 3	5812
February	1st to 10th	Day	50% 4 13% 23% 9% 1	1890
	11th to 20th	Day	49% 4 9% 22% 14% 2	2048
	21st to end	Day	46% 4 9% 23% 15% 3	1730
March	1st to 10th	Day	42% 3 5% 24% 24% 2	4515
	11th to 20th	Day	32% 5% 10% 28% 22% 4	4980
	21st to end	Day	47% 5% 14% 25% 8% 1	5799
April	1st to 10th	Day	43% 4 12% 25% 16% 1	2400
	11th to 20th	Day	45% 3 9% 25% 15% 3	2374
	21st to end	Day	46% 4 12% 24% 12% 1	2400
May	1st to 10th	Day	47% 6% 14% 22% 11%	5660
	11th to 20th	Day	45% 6% 11% 25% 13% 1	6245
	21st to end	Day	41% 6% 10% 23% 19% 1	6949
June	1st to 10th	Day	33% 5% 14% 29% 16% 3	2800
	11th to 20th	Day	31% 6% 13% 25% 23% 2	2800
	21st to end	Day	30% 6% 17% 25% 18% 4	2800
July	1st to 10th	Day	34% 5% 10% 31% 18% 2	6291
	11th to 20th	Day	36% 9% 15% 25% 14% 1	6285
	21st to end	Day	35% 8% 18% 30% 10% 1	6507
August	1st to 10th	Day	27% 5% 15% 39% 14% 1	4192
	11th to 20th	Day	37% 5% 14% 27% 16%	3885
	21st to end	Day	27% 5% 16% 33% 15% 3	4311
September	1st to 10th	Day	33% 4 9% 29% 22% 5%	2400
	11th to 20th	Day	30% 4 9% 28% 27% 3	2360
	21st to end	Day	31% 5% 8% 29% 24% 3	2200
October	1st to 10th	Day	19% 3 6% 19% 47% 7%	3275
	11th to 20th	Day	24% 2 7% 21% 37% 7%	3240
	21st to end	Day	35% 2 6% 28% 26% 2	3452
November	1st to 10th	Day	32% 3 12% 28% 21% 4	1852
	11th to 20th	Day	39% 3 9% 25% 19% 5%	1890
	21st to end	Day	49% 4 9% 25% 13%	1816
December	1st to 10th	Day	38% 3 15% 31% 14%	2690
	11th to 20th	Day	50% 3 10% 22% 12% 2	3085
	21st to end	Day	67% 3 12% 12% 6% 1	4070
January	1st to 10th	Night	50% 4 13% 24% 8%	8233
	11th to 20th	Night	48% 4 6% 23% 16% 3	7999
	21st to end	Night	53% 3 10% 19% 13% 2	8402
February	1st to 10th	Night	51% 3 10% 23% 12% 1	2730
	11th to 20th	Night	46% 4 10% 23% 16% 2	2468
	21st to end	Night	45% 2 10% 25% 15% 3	2076
March	1st to 10th	Night	47% 3 5% 22% 20% 2	5398
	11th to 20th	Night	35% 3 9% 26% 21% 6%	5008
	21st to end	Night	50% 5% 9% 25% 10% 1	5130
April	1st to 10th	Night	38% 5% 10% 26% 17% 4	2000
	11th to 20th	Night	38% 4 11% 25% 19% 3	1975
	21st to end	Night	46% 4 8% 24% 16% 2	2000
May	1st to 10th	Night	47% 7% 14% 20% 12% 1	4298
	11th to 20th	Night	39% 5% 13% 26% 15% 2	3668
	21st to end	Night	41% 6% 12% 23% 17% 2	3950
June	1st to 10th	Night	31% 5% 14% 28% 19% 3	1600
	11th to 20th	Night	28% 5% 12% 27% 25% 4	1600
	21st to end	Night	27% 5% 15% 28% 18% 7%	1600
July	1st to 10th	Night	34% 4 10% 30% 20% 2	3580
	11th to 20th	Night	30% 8% 15% 31% 14% 2	3572
	21st to end	Night	34% 7% 20% 27% 11% 1	4050
August	1st to 10th	Night	22% 6% 17% 38% 16% 1	2942
	11th to 20th	Night	33% 5% 12% 27% 22% 1	3230
	21st to end	Night	23% 5% 15% 35% 19% 3	3586
September	1st to 10th	Night	32% 2 9% 27% 23% 7%	2000
	11th to 20th	Night	30% 3 9% 28% 26% 5%	2019
	21st to end	Night	30% 4 9% 23% 28% 5%	2200
October	1st to 10th	Night	18% 2 6% 20% 48% 6%	3887
	11th to 20th	Night	24% 3 7% 23% 37% 7%	3899
	21st to end	Night	32% 3 8% 26% 25% 5%	4336
November	1st to 10th	Night	31% 4 11% 25% 22% 6%	2666
	11th to 20th	Night	37% 2 9% 28% 21% 4	2730
	21st to end	Night	45% 3 10% 26% 16% 1	2800
December	1st to 10th	Night	42% 3 10% 30% 15% 1	4720
	11th to 20th	Night	44% 4 12% 24% 15% 1	5341
	21st to end	Night	67% 4 10% 12% 6% 1	7150

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	22% 17% 37% 19% 5%	330
	11th to 20th	Day	26% 21% 36% 15% 3	330
	21st to end	Day	21% 18% 45% 13% 3	363
February	1st to 10th	Day	25% 14% 43% 14% 4	330
	11th to 20th	Day	26% 23% 30% 16% 4 1	330
	21st to end	Day	24% 17% 42% 16% 1	273
March	1st to 10th	Day	20% 17% 42% 18% 2	306
	11th to 20th	Day	28% 20% 30% 17% 5%	300
	21st to end	Day	31% 14% 34% 16% 5%	471
April	1st to 10th	Day	20% 13% 38% 20% 8% 1	500
	11th to 20th	Day	22% 14% 29% 23% 11% 1	492
	21st to end	Day	23% 13% 33% 21% 8% 1	500
May	1st to 10th	Day	19% 9% 32% 27% 11% 1	503
	11th to 20th	Day	18% 8% 28% 25% 17% 4	686
	21st to end	Day	14% 6% 30% 28% 15% 7%	789
June	1st to 10th	Day	10% 5% 22% 32% 22% 9%	730
	11th to 20th	Day	10% 4 20% 35% 24% 7%	730
	21st to end	Day	6% 4 27% 35% 21% 7%	730
July	1st to 10th	Day	12% 1 15% 28% 29% 16%	721
	11th to 20th	Day	8% 3 15% 36% 29% 10%	718
	21st to end	Day	9% 2 15% 33% 31% 10%	794
August	1st to 10th	Day	8% 2 15% 37% 27% 10%	711
	11th to 20th	Day	12% 4 13% 29% 31% 11%	519
	21st to end	Day	8% 2 13% 31% 34% 12%	583
September	1st to 10th	Day	12% 4 15% 33% 24% 12%	530
	11th to 20th	Day	9% 2 12% 31% 33% 13%	530
	21st to end	Day	11% 5% 23% 27% 25% 9%	530
October	1st to 10th	Day	10% 7% 23% 38% 16% 6%	345
	11th to 20th	Day	14% 16% 30% 26% 12% 2	330
	21st to end	Day	29% 15% 29% 20% 6%	363
November	1st to 10th	Day	15% 12% 36% 21% 12% 4	330
	11th to 20th	Day	20% 17% 35% 16% 10% 2	330
	21st to end	Day	30% 11% 29% 23% 6% 1	330
December	1st to 10th	Day	15% 15% 45% 18% 5% 3	330
	11th to 20th	Day	21% 15% 37% 22% 4	330
	21st to end	Day	26% 16% 39% 16% 4	363
January	1st to 10th	Night	17% 14% 38% 25% 5%	1050
	11th to 20th	Night	21% 16% 37% 22% 5%	1050
	21st to end	Night	20% 14% 39% 25% 2	1155
February	1st to 10th	Night	21% 14% 38% 24% 3	1050
	11th to 20th	Night	21% 15% 38% 25% 2	832
	21st to end	Night	22% 15% 37% 25% 2	692
March	1st to 10th	Night	23% 13% 37% 22% 5%	809
	11th to 20th	Night	25% 10% 33% 26% 6%	615
	21st to end	Night	26% 11% 31% 28% 4	660
April	1st to 10th	Night	17% 12% 38% 27% 6%	600
	11th to 20th	Night	17% 10% 35% 28% 9% 1	593
	21st to end	Night	23% 5% 38% 27% 8%	600
May	1st to 10th	Night	18% 7% 31% 34% 10% 1	600
	11th to 20th	Night	17% 4 24% 38% 16% 1	591
	21st to end	Night	11% 5% 24% 40% 18% 3	650
June	1st to 10th	Night	10% 2 21% 40% 27% 1	600
	11th to 20th	Night	10% 2 18% 46% 21% 3	600
	21st to end	Night	6% 3 20% 43% 26% 2	600
July	1st to 10th	Night	12% 1 8% 34% 37% 8%	585
	11th to 20th	Night	5% 1 13% 43% 34% 3	582
	21st to end	Night	7% 1 12% 39% 37% 4	657
August	1st to 10th	Night	8% 2 10% 44% 33% 3	597
	11th to 20th	Night	8% 2 10% 43% 33% 4	594
	21st to end	Night	5% 1 10% 39% 39% 6%	660
September	1st to 10th	Night	12% 2 14% 35% 32% 5%	600
	11th to 20th	Night	9% 3 14% 35% 35% 5%	619
	21st to end	Night	12% 5% 22% 31% 27% 4	800
October	1st to 10th	Night	11% 2 25% 34% 24% 4	800
	11th to 20th	Night	17% 8% 30% 31% 14% 1	800
	21st to end	Night	22% 9% 33% 29% 6%	880
November	1st to 10th	Night	13% 7% 39% 31% 9% 1	820
	11th to 20th	Night	16% 12% 36% 29% 6%	840
	21st to end	Night	25% 11% 33% 25% 6%	910
December	1st to 10th	Night	22% 10% 36% 27% 4 1	1050
	11th to 20th	Night	23% 9% 34% 28% 5%	1050
	21st to end	Night	29% 13% 33% 23% 2	1155

Page generated at 2026-02-28 00:40:25. Version ac07

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