Chamberlain
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chamberlain clear sky chart from 2005-11-06 to 2026-07-16.

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	9% 7% 3 7% 4 4 4 3 5% 7% 46%	145931
	Night	8% 7% 3 7% 4 4 4 4 6% 7% 46%	144135
Transparency	Day	37% 6% 14% 25% 16% 2	140640
	Night	36% 6% 15% 25% 16% 2	140439
Seeing	Day	13% 10% 31% 30% 13% 2	18291
	Night	12% 8% 34% 36% 10%	28187

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	6% 13% 5% 17% 8% 5% 4 2 4 6% 30%	4720
	11th to 20th	Day	3 10% 5% 12% 7% 7% 7% 4 9% 6% 29%	4740
	21st to end	Day	3 8% 4 12% 8% 6% 5% 3 4 5% 41%	5280
February	1st to 10th	Day	8% 13% 4 10% 6% 5% 5% 3 5% 7% 35%	1890
	11th to 20th	Day	7% 7% 4 10% 6% 6% 4 3 5% 7% 41%	2012
	21st to end	Day	8% 10% 3 9% 5% 4 3 4 5% 7% 43%	1730
March	1st to 10th	Day	8% 9% 3 6% 3 3 4 3 5% 6% 51%	4600
	11th to 20th	Day	6% 6% 2 5% 3 3 4 3 6% 10% 51%	5085
	21st to end	Day	18% 12% 3 9% 5% 4 4 4 5% 6% 28%	5585
April	1st to 10th	Day	15% 9% 3 6% 5% 3 4 4 5% 6% 41%	2504
	11th to 20th	Day	17% 7% 3 6% 5% 4 4 3 5% 6% 39%	2636
	21st to end	Day	16% 8% 3 6% 4 4 4 4 5% 7% 39%	2730
May	1st to 10th	Day	9% 4 2 5% 4 4 5% 4 6% 8% 50%	7655
	11th to 20th	Day	16% 6% 2 5% 3 3 3 3 5% 6% 49%	7627
	21st to end	Day	17% 7% 2 5% 4 4 4 4 5% 7% 41%	8874
June	1st to 10th	Day	9% 4 2 5% 3 4 5% 4 6% 9% 50%	3045
	11th to 20th	Day	10% 4 1 5% 4 4 4 4 7% 8% 48%	3150
	21st to end	Day	7% 4 2 5% 4 4 4 4 6% 7% 52%	3016
July	1st to 10th	Day	9% 5% 2 5% 4 4 4 4 6% 8% 50%	6479
	11th to 20th	Day	8% 4 2 5% 4 4 4 4 5% 7% 54%	6411
	21st to end	Day	6% 4 2 5% 4 4 5% 4 7% 9% 51%	6844
August	1st to 10th	Day	7% 5% 2 5% 4 4 5% 4 6% 8% 50%	5801
	11th to 20th	Day	10% 6% 2 5% 4 4 4 4 6% 7% 48%	5821
	21st to end	Day	7% 5% 2 5% 3 3 4 3 5% 8% 55%	6334
September	1st to 10th	Day	8% 4 2 4 3 4 4 2 5% 6% 58%	2335
	11th to 20th	Day	8% 5% 2 4 3 3 3 3 5% 6% 58%	2200
	21st to end	Day	11% 5% 2 4 2 2 3 2 5% 7% 58%	2200
October	1st to 10th	Day	8% 3 1 3 2 3 2 3 3 5% 66%	3539
	11th to 20th	Day	9% 5% 2 6% 4 3 3 2 4 7% 55%	3185
	21st to end	Day	10% 7% 3 7% 4 4 4 3 5% 7% 45%	3190
November	1st to 10th	Day	9% 7% 2 6% 4 3 5% 4 6% 7% 49%	1853
	11th to 20th	Day	6% 8% 3 8% 6% 3 5% 3 5% 6% 49%	1890
	21st to end	Day	7% 9% 3 7% 5% 4 4 3 5% 7% 45%	1774
December	1st to 10th	Day	6% 11% 4 11% 7% 6% 5% 4 5% 6% 37%	2511
	11th to 20th	Day	6% 10% 4 9% 7% 7% 7% 4 5% 7% 35%	2713
	21st to end	Day	13% 10% 3 12% 5% 4 4 3 4 6% 37%	3972
January	1st to 10th	Night	5% 11% 5% 12% 8% 6% 5% 5% 7% 7% 30%	8201
	11th to 20th	Night	2 8% 4 10% 7% 7% 7% 5% 8% 7% 34%	8250
	21st to end	Night	3 7% 4 12% 5% 4 5% 4 5% 6% 44%	8932
February	1st to 10th	Night	10% 10% 4 8% 5% 5% 5% 4 5% 8% 35%	2730
	11th to 20th	Night	6% 10% 3 8% 5% 5% 5% 4 6% 7% 41%	2515
	21st to end	Night	8% 7% 2 6% 5% 4 4 4 6% 8% 45%	2076
March	1st to 10th	Night	10% 8% 2 6% 4 4 4 3 4 6% 49%	5494
	11th to 20th	Night	9% 6% 2 5% 3 3 5% 4 6% 9% 49%	5095
	21st to end	Night	14% 11% 3 7% 4 4 5% 3 5% 7% 36%	5588
April	1st to 10th	Night	14% 7% 3 6% 4 3 5% 3 5% 7% 42%	2111
	11th to 20th	Night	14% 7% 3 7% 3 4 3 3 5% 8% 43%	1932
	21st to end	Night	15% 9% 2 6% 4 4 4 3 6% 7% 41%	1890
May	1st to 10th	Night	7% 4 2 5% 3 4 5% 4 6% 8% 52%	5292
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 4 7% 51%	5265
	21st to end	Night	16% 6% 3 6% 4 4 4 4 6% 7% 40%	5316
June	1st to 10th	Night	7% 5% 2 4 4 3 3 4 6% 9% 54%	1566
	11th to 20th	Night	8% 5% 1 6% 3 3 4 4 6% 8% 50%	1470
	21st to end	Night	7% 4 1 3 3 3 4 3 7% 9% 55%	1594
July	1st to 10th	Night	5% 3 1 4 3 4 4 4 7% 10% 54%	3720
	11th to 20th	Night	6% 4 2 5% 3 3 3 3 5% 8% 59%	3680
	21st to end	Night	5% 4 1 5% 4 4 5% 4 7% 10% 51%	4028
August	1st to 10th	Night	7% 4 2 5% 4 4 5% 5% 8% 11% 44%	4011
	11th to 20th	Night	8% 5% 2 6% 3 4 4 4 7% 9% 49%	4028
	21st to end	Night	7% 4 2 6% 3 4 4 3 6% 8% 54%	4640
September	1st to 10th	Night	8% 4 1 5% 3 3 3 2 5% 5% 60%	2065
	11th to 20th	Night	7% 5% 1 4 2 4 3 3 5% 7% 58%	2200
	21st to end	Night	9% 5% 2 4 2 3 4 3 4 5% 60%	2200
October	1st to 10th	Night	8% 3 1 2 2 2 2 2 3 6% 69%	3608
	11th to 20th	Night	10% 5% 1 3 2 2 3 3 5% 9% 57%	3953
	21st to end	Night	8% 6% 2 7% 4 4 5% 4 5% 7% 48%	4610
November	1st to 10th	Night	6% 6% 2 6% 4 5% 4 3 6% 8% 50%	2665
	11th to 20th	Night	7% 7% 2 8% 5% 4 4 4 4 8% 48%	2730
	21st to end	Night	6% 8% 3 6% 4 4 4 4 5% 7% 48%	2841
December	1st to 10th	Night	4 7% 4 8% 6% 5% 5% 4 4 6% 48%	4885
	11th to 20th	Night	5% 9% 3 8% 5% 5% 5% 4 7% 8% 40%	5714
	21st to end	Night	13% 12% 3 10% 4 4 4 3 5% 6% 38%	7240

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	56% 5% 15% 18% 6%	4723
	11th to 20th	Day	50% 5% 15% 16% 13% 1	4740
	21st to end	Day	44% 6% 17% 21% 12%	5280
February	1st to 10th	Day	50% 7% 15% 17% 11%	1890
	11th to 20th	Day	44% 5% 14% 22% 13% 2	2022
	21st to end	Day	42% 6% 17% 21% 12% 2	1730
March	1st to 10th	Day	35% 5% 9% 31% 18% 2	4600
	11th to 20th	Day	28% 5% 14% 29% 23% 1	5085
	21st to end	Day	56% 7% 14% 16% 6% 1	5585
April	1st to 10th	Day	42% 6% 13% 25% 11% 2	2504
	11th to 20th	Day	45% 7% 11% 22% 13% 2	2639
	21st to end	Day	44% 6% 13% 27% 10%	2730
May	1st to 10th	Day	28% 7% 10% 31% 22% 1	7655
	11th to 20th	Day	35% 6% 10% 22% 23% 4	7631
	21st to end	Day	41% 7% 11% 25% 15% 1	8875
June	1st to 10th	Day	30% 8% 17% 31% 11% 3	3045
	11th to 20th	Day	31% 8% 16% 26% 17% 2	3150
	21st to end	Day	29% 7% 15% 28% 19% 3	3016
July	1st to 10th	Day	30% 7% 15% 29% 16% 2	6411
	11th to 20th	Day	29% 7% 11% 29% 22% 3	6336
	21st to end	Day	27% 8% 18% 25% 18% 3	6647
August	1st to 10th	Day	29% 8% 15% 26% 18% 4	4220
	11th to 20th	Day	39% 7% 15% 24% 13% 2	4210
	21st to end	Day	24% 7% 19% 27% 20% 3	4557
September	1st to 10th	Day	27% 6% 13% 29% 21% 4	2335
	11th to 20th	Day	28% 5% 15% 30% 21% 2	2200
	21st to end	Day	27% 5% 10% 33% 24% 2	2200
October	1st to 10th	Day	22% 4 9% 24% 36% 6%	3539
	11th to 20th	Day	31% 4 11% 34% 18% 2	3185
	21st to end	Day	39% 5% 12% 24% 18% 2	3190
November	1st to 10th	Day	34% 6% 17% 26% 14% 3	1853
	11th to 20th	Day	36% 5% 17% 25% 14% 3	1890
	21st to end	Day	39% 5% 16% 25% 13% 1	1774
December	1st to 10th	Day	47% 6% 17% 22% 7% 1	2511
	11th to 20th	Day	47% 7% 16% 19% 10% 1	2710
	21st to end	Day	49% 4 12% 30% 4	3972
January	1st to 10th	Night	50% 6% 16% 19% 8% 1	8202
	11th to 20th	Night	44% 6% 17% 19% 13% 1	8250
	21st to end	Night	39% 7% 16% 24% 14%	8932
February	1st to 10th	Night	47% 6% 16% 19% 11% 1	2730
	11th to 20th	Night	41% 6% 14% 23% 14% 3	2515
	21st to end	Night	36% 7% 15% 27% 11% 4	2076
March	1st to 10th	Night	36% 5% 11% 28% 18% 2	5494
	11th to 20th	Night	30% 6% 11% 29% 21% 1	5095
	21st to end	Night	47% 7% 13% 24% 9% 1	5588
April	1st to 10th	Night	40% 6% 15% 25% 13% 1	2111
	11th to 20th	Night	39% 6% 11% 24% 17% 2	1932
	21st to end	Night	42% 6% 12% 25% 13% 2	1890
May	1st to 10th	Night	27% 7% 12% 28% 24% 3	5293
	11th to 20th	Night	33% 5% 9% 19% 28% 5%	5266
	21st to end	Night	40% 8% 15% 23% 12% 1	5318
June	1st to 10th	Night	27% 7% 17% 28% 17% 3	1566
	11th to 20th	Night	31% 7% 16% 23% 20% 3	1470
	21st to end	Night	25% 6% 17% 31% 18% 3	1594
July	1st to 10th	Night	23% 7% 20% 31% 16% 2	3680
	11th to 20th	Night	23% 6% 13% 31% 22% 5%	3632
	21st to end	Night	26% 8% 22% 26% 15% 3	3913
August	1st to 10th	Night	29% 9% 17% 28% 12% 6%	2916
	11th to 20th	Night	35% 8% 16% 26% 14% 2	2915
	21st to end	Night	25% 6% 15% 27% 22% 5%	3348
September	1st to 10th	Night	26% 5% 12% 29% 22% 7%	2065
	11th to 20th	Night	25% 6% 14% 32% 20% 3	2200
	21st to end	Night	26% 5% 8% 29% 28% 4	2200
October	1st to 10th	Night	20% 3 7% 19% 45% 6%	3608
	11th to 20th	Night	26% 4 11% 33% 21% 6%	3953
	21st to end	Night	34% 5% 15% 24% 20% 2	4610
November	1st to 10th	Night	31% 5% 14% 26% 19% 5%	2665
	11th to 20th	Night	35% 5% 14% 23% 17% 4	2730
	21st to end	Night	35% 5% 16% 28% 15% 2	2841
December	1st to 10th	Night	36% 5% 16% 29% 12% 1	4885
	11th to 20th	Night	39% 7% 18% 27% 8%	5716
	21st to end	Night	47% 4 14% 26% 8%	7240

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	11% 10% 28% 36% 13% 2	330
	11th to 20th	Day	12% 14% 37% 29% 7% 2	330
	21st to end	Day	14% 9% 40% 24% 9% 3	363
February	1st to 10th	Day	12% 14% 43% 25% 6%	330
	11th to 20th	Day	13% 11% 40% 25% 10% 2	330
	21st to end	Day	15% 12% 40% 26% 6%	273
March	1st to 10th	Day	16% 10% 39% 26% 7% 2	330
	11th to 20th	Day	13% 11% 33% 31% 11% 2	330
	21st to end	Day	21% 15% 31% 19% 12% 3	363
April	1st to 10th	Day	23% 15% 35% 19% 7%	524
	11th to 20th	Day	22% 16% 31% 24% 6%	677
	21st to end	Day	20% 15% 29% 26% 9% 1	750
May	1st to 10th	Day	16% 10% 32% 27% 13% 2	753
	11th to 20th	Day	20% 10% 29% 25% 13% 3	775
	21st to end	Day	17% 9% 33% 25% 13% 3	844
June	1st to 10th	Day	8% 8% 27% 33% 18% 6%	780
	11th to 20th	Day	11% 9% 37% 29% 13% 1	780
	21st to end	Day	10% 9% 32% 35% 14% 1	780
July	1st to 10th	Day	6% 4 24% 40% 21% 4	771
	11th to 20th	Day	6% 5% 28% 42% 17% 1	744
	21st to end	Day	5% 3 29% 44% 16% 3	794
August	1st to 10th	Day	7% 5% 25% 41% 21% 2	726
	11th to 20th	Day	11% 4 26% 39% 17% 3	715
	21st to end	Day	6% 5% 26% 38% 22% 3	738
September	1st to 10th	Day	10% 6% 28% 34% 19% 3	465
	11th to 20th	Day	12% 5% 28% 30% 19% 5%	330
	21st to end	Day	16% 15% 25% 24% 17% 3	330
October	1st to 10th	Day	15% 6% 31% 30% 14% 5%	330
	11th to 20th	Day	18% 14% 37% 20% 10% 1	330
	21st to end	Day	18% 18% 31% 19% 13% 1	363
November	1st to 10th	Day	18% 10% 32% 27% 11% 2	330
	11th to 20th	Day	12% 14% 37% 26% 10%	330
	21st to end	Day	15% 15% 29% 28% 10% 3	330
December	1st to 10th	Day	17% 9% 37% 27% 9% 2	330
	11th to 20th	Day	15% 15% 35% 28% 7% 1	330
	21st to end	Day	18% 12% 33% 26% 10% 1	363
January	1st to 10th	Night	9% 9% 36% 35% 9% 1	1050
	11th to 20th	Night	9% 14% 34% 34% 7% 1	1050
	21st to end	Night	12% 8% 40% 33% 7%	1155
February	1st to 10th	Night	14% 10% 38% 31% 6%	1050
	11th to 20th	Night	10% 8% 37% 36% 8%	880
	21st to end	Night	12% 8% 37% 33% 9% 1	692
March	1st to 10th	Night	14% 9% 34% 37% 6%	840
	11th to 20th	Night	16% 9% 35% 34% 6%	635
	21st to end	Night	15% 11% 35% 30% 8% 1	693
April	1st to 10th	Night	17% 10% 38% 31% 3	630
	11th to 20th	Night	18% 11% 38% 30% 4	623
	21st to end	Night	17% 11% 31% 31% 8% 1	630
May	1st to 10th	Night	11% 8% 37% 35% 10%	630
	11th to 20th	Night	17% 8% 31% 33% 10% 1	621
	21st to end	Night	15% 8% 33% 34% 9% 1	683
June	1st to 10th	Night	9% 5% 31% 37% 17% 1	630
	11th to 20th	Night	9% 6% 39% 39% 8%	630
	21st to end	Night	9% 4 29% 47% 10%	630
July	1st to 10th	Night	6% 3 27% 46% 18%	615
	11th to 20th	Night	5% 2 31% 48% 13%	597
	21st to end	Night	7% 2 27% 51% 12% 1	657
August	1st to 10th	Night	6% 3 28% 47% 15% 1	597
	11th to 20th	Night	8% 3 26% 46% 18%	594
	21st to end	Night	7% 4 28% 44% 16%	660
September	1st to 10th	Night	9% 5% 26% 46% 12% 1	600
	11th to 20th	Night	10% 4 33% 38% 15% 1	600
	21st to end	Night	10% 9% 33% 34% 13% 1	600
October	1st to 10th	Night	16% 7% 30% 35% 11%	605
	11th to 20th	Night	16% 10% 31% 32% 10%	830
	21st to end	Night	15% 12% 32% 33% 8%	1100
November	1st to 10th	Night	11% 9% 36% 35% 9% 1	1025
	11th to 20th	Night	11% 10% 36% 33% 9%	1050
	21st to end	Night	9% 11% 34% 37% 9%	1050
December	1st to 10th	Night	11% 10% 37% 33% 8%	1050
	11th to 20th	Night	11% 10% 36% 35% 8%	1050
	21st to end	Night	18% 10% 32% 34% 6% 1	1155

Page generated at 2026-07-13 02:14:47. Version ac07

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