Chamberlin Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chamberlin Observatory clear sky chart from 2005-11-06 to 2025-11-03.

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	5% 5% 2 5% 4 4 4 4 6% 8% 54%	135482
	Night	5% 6% 2 5% 3 3 4 3 5% 7% 57%	132000
Transparency	Day	26% 5% 9% 23% 25% 12%	130443
	Night	26% 4 10% 25% 26% 10%	128043
Seeing	Day	8% 7% 25% 33% 20% 7%	16952
	Night	7% 4 27% 46% 16% 1	26927

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	1 6% 3 6% 3 3 3 3 5% 8% 60%	3638
	11th to 20th	Day	1 5% 3 7% 4 4 4 3 4 7% 57%	3750
	21st to end	Day	1 5% 3 6% 3 3 3 3 5% 7% 61%	4510
February	1st to 10th	Day	3 7% 3 7% 4 3 4 3 5% 6% 55%	1800
	11th to 20th	Day	1 5% 3 6% 4 4 4 4 6% 8% 56%	1934
	21st to end	Day	2 7% 2 7% 3 3 3 3 5% 6% 59%	1650
March	1st to 10th	Day	4 10% 2 4 3 3 3 2 4 6% 59%	4500
	11th to 20th	Day	5% 8% 2 6% 4 4 4 3 6% 8% 50%	4908
	21st to end	Day	4 6% 2 4 4 3 4 3 5% 8% 55%	5832
April	1st to 10th	Day	6% 6% 2 4 3 4 4 3 6% 8% 54%	2400
	11th to 20th	Day	9% 8% 3 5% 3 4 4 4 5% 8% 48%	2371
	21st to end	Day	10% 7% 1 4 4 4 4 4 7% 9% 47%	2400
May	1st to 10th	Day	10% 6% 2 5% 4 4 5% 4 6% 7% 45%	5752
	11th to 20th	Day	8% 6% 2 5% 5% 4 5% 4 7% 8% 46%	6251
	21st to end	Day	7% 6% 2 5% 4 5% 5% 5% 6% 9% 46%	6925
June	1st to 10th	Day	6% 4 2 4 4 4 5% 5% 8% 10% 50%	2800
	11th to 20th	Day	5% 3 1 4 4 5% 5% 6% 6% 9% 52%	2800
	21st to end	Day	5% 3 1 4 5% 4 5% 5% 8% 9% 50%	2800
July	1st to 10th	Day	4 3 1 4 3 4 5% 4 7% 8% 57%	6339
	11th to 20th	Day	3 2 1 5% 5% 5% 6% 6% 9% 12% 46%	6340
	21st to end	Day	5% 4 2 6% 5% 5% 5% 5% 8% 8% 47%	6796
August	1st to 10th	Day	7% 4 1 5% 5% 6% 7% 6% 9% 9% 39%	5746
	11th to 20th	Day	3 2 1 4 4 4 6% 5% 7% 9% 55%	5366
	21st to end	Day	6% 5% 2 6% 5% 5% 5% 5% 7% 8% 46%	5984
September	1st to 10th	Day	5% 3 1 4 3 3 4 3 5% 7% 62%	2400
	11th to 20th	Day	6% 3 1 4 3 3 4 3 6% 7% 59%	2370
	21st to end	Day	5% 5% 1 3 2 3 3 3 4 6% 65%	2200
October	1st to 10th	Day	4 2 1 2 2 2 2 2 3 4 75%	3309
	11th to 20th	Day	4 5% 2 4 3 3 3 3 5% 7% 61%	3250
	21st to end	Day	6% 6% 2 3 2 2 2 2 5% 5% 65%	3420
November	1st to 10th	Day	3 5% 1 3 2 2 3 3 5% 8% 64%	1781
	11th to 20th	Day	2 5% 2 4 3 3 2 2 5% 8% 64%	1800
	21st to end	Day	3 6% 2 5% 2 3 3 3 5% 7% 60%	1710
December	1st to 10th	Day	1 4 2 4 3 3 2 2 3 6% 69%	2620
	11th to 20th	Day	2 4 1 4 2 2 3 3 4 7% 68%	3026
	21st to end	Day	2 6% 3 7% 4 4 4 4 5% 7% 54%	4004
January	1st to 10th	Night	2 9% 4 7% 3 3 3 2 4 8% 55%	6327
	11th to 20th	Night	1 5% 2 7% 4 3 3 2 4 7% 61%	6274
	21st to end	Night	3 6% 3 6% 4 3 3 3 3 5% 60%	6468
February	1st to 10th	Night	3 8% 2 6% 3 3 4 3 4 8% 56%	2600
	11th to 20th	Night	2 8% 4 7% 5% 4 4 3 4 6% 52%	2352
	21st to end	Night	2 8% 3 6% 3 3 3 3 4 5% 60%	1980
March	1st to 10th	Night	5% 9% 2 5% 3 3 4 3 5% 5% 56%	5374
	11th to 20th	Night	7% 9% 2 5% 3 4 4 3 4 6% 53%	5071
	21st to end	Night	8% 9% 3 5% 4 4 3 3 5% 7% 50%	5127
April	1st to 10th	Night	7% 7% 2 5% 3 3 4 3 5% 6% 56%	2000
	11th to 20th	Night	12% 8% 2 5% 3 3 3 3 4 6% 50%	1977
	21st to end	Night	13% 8% 2 5% 3 3 3 3 5% 6% 49%	2000
May	1st to 10th	Night	14% 7% 2 5% 4 4 4 3 5% 8% 46%	4226
	11th to 20th	Night	9% 6% 2 6% 4 4 5% 4 6% 8% 48%	3648
	21st to end	Night	10% 6% 2 4 3 3 3 3 5% 6% 55%	3972
June	1st to 10th	Night	6% 2 1 4 2 3 3 4 6% 10% 59%	1600
	11th to 20th	Night	6% 3 1 2 3 3 3 4 7% 7% 62%	1600
	21st to end	Night	5% 3 1 4 3 3 5% 4 7% 8% 57%	1600
July	1st to 10th	Night	3 2 1 4 3 4 4 3 6% 7% 61%	3640
	11th to 20th	Night	3 2 1 4 3 5% 5% 5% 8% 11% 51%	3640
	21st to end	Night	4 3 1 5% 4 4 4 5% 9% 10% 50%	4060
August	1st to 10th	Night	6% 4 2 5% 4 4 5% 4 7% 9% 49%	4064
	11th to 20th	Night	3 3 1 4 4 4 5% 5% 8% 11% 53%	4483
	21st to end	Night	7% 5% 2 6% 4 5% 6% 5% 7% 9% 45%	4993
September	1st to 10th	Night	6% 4 2 5% 3 4 3 3 5% 7% 59%	2000
	11th to 20th	Night	7% 3 1 3 3 3 3 2 4 8% 63%	2029
	21st to end	Night	8% 3 1 3 3 3 3 2 4 6% 64%	2200
October	1st to 10th	Night	6% 3 1 3 2 2 2 2 4 4 72%	3877
	11th to 20th	Night	6% 3 2 4 3 2 3 3 4 8% 62%	3898
	21st to end	Night	5% 7% 2 3 2 2 2 2 3 5% 67%	4348
November	1st to 10th	Night	3 4 2 3 3 2 3 3 4 6% 67%	2562
	11th to 20th	Night	3 5% 2 4 3 3 3 3 4 5% 66%	2600
	21st to end	Night	4 6% 2 5% 3 3 3 3 4 6% 62%	2685
December	1st to 10th	Night	1 4 2 4 3 1 2 2 3 5% 72%	4570
	11th to 20th	Night	1 5% 2 4 2 3 3 2 5% 8% 65%	5181
	21st to end	Night	5% 8% 3 6% 4 4 4 3 4 6% 53%	6974

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	23% 3 8% 33% 24% 9%	3639
	11th to 20th	Day	25% 6% 9% 22% 28% 10%	3750
	21st to end	Day	23% 4 7% 28% 26% 12%	4510
February	1st to 10th	Day	30% 3 10% 27% 21% 10%	1800
	11th to 20th	Day	25% 3 8% 29% 27% 8%	1944
	21st to end	Day	25% 3 6% 23% 27% 15%	1650
March	1st to 10th	Day	26% 3 5% 17% 31% 17%	4500
	11th to 20th	Day	30% 4 10% 22% 28% 6%	4908
	21st to end	Day	26% 6% 9% 36% 20% 3	5832
April	1st to 10th	Day	28% 5% 10% 25% 22% 11%	2400
	11th to 20th	Day	34% 6% 9% 21% 21% 10%	2374
	21st to end	Day	32% 6% 10% 25% 23% 4	2400
May	1st to 10th	Day	34% 7% 9% 25% 18% 6%	5752
	11th to 20th	Day	32% 7% 10% 20% 21% 9%	6255
	21st to end	Day	30% 8% 13% 24% 19% 7%	6926
June	1st to 10th	Day	27% 7% 15% 24% 19% 9%	2800
	11th to 20th	Day	25% 7% 10% 18% 23% 17%	2800
	21st to end	Day	26% 7% 10% 20% 24% 14%	2800
July	1st to 10th	Day	21% 6% 8% 16% 27% 22%	6271
	11th to 20th	Day	25% 9% 12% 24% 19% 11%	6265
	21st to end	Day	30% 8% 11% 18% 18% 16%	6600
August	1st to 10th	Day	36% 9% 14% 20% 15% 6%	4186
	11th to 20th	Day	27% 7% 13% 16% 22% 15%	3882
	21st to end	Day	26% 6% 11% 19% 25% 13%	4311
September	1st to 10th	Day	22% 5% 8% 21% 26% 19%	2400
	11th to 20th	Day	23% 4 8% 19% 28% 18%	2370
	21st to end	Day	21% 4 5% 18% 37% 15%	2200
October	1st to 10th	Day	15% 2 5% 15% 37% 26%	3309
	11th to 20th	Day	23% 4 8% 23% 32% 10%	3250
	21st to end	Day	21% 2 5% 23% 36% 13%	3420
November	1st to 10th	Day	19% 3 8% 25% 28% 18%	1781
	11th to 20th	Day	19% 4 5% 26% 35% 11%	1800
	21st to end	Day	24% 2 7% 21% 31% 14%	1710
December	1st to 10th	Day	18% 2 7% 32% 30% 10%	2620
	11th to 20th	Day	16% 3 6% 30% 34% 10%	3024
	21st to end	Day	28% 3 8% 25% 27% 9%	4004
January	1st to 10th	Night	29% 4 7% 30% 25% 7%	6327
	11th to 20th	Night	23% 5% 9% 25% 24% 14%	6274
	21st to end	Night	26% 3 9% 25% 26% 11%	6468
February	1st to 10th	Night	28% 4 9% 27% 25% 8%	2600
	11th to 20th	Night	31% 3 9% 28% 23% 7%	2352
	21st to end	Night	26% 3 5% 23% 28% 14%	1980
March	1st to 10th	Night	29% 3 5% 18% 35% 10%	5374
	11th to 20th	Night	31% 4 10% 14% 30% 12%	5071
	21st to end	Night	33% 5% 8% 34% 18% 2	5127
April	1st to 10th	Night	27% 6% 10% 24% 23% 10%	2000
	11th to 20th	Night	35% 4 10% 22% 21% 9%	1977
	21st to end	Night	35% 5% 10% 25% 20% 5%	2000
May	1st to 10th	Night	36% 7% 12% 26% 11% 7%	4223
	11th to 20th	Night	33% 6% 15% 21% 18% 8%	3649
	21st to end	Night	29% 6% 18% 26% 17% 5%	3972
June	1st to 10th	Night	20% 7% 19% 32% 15% 8%	1600
	11th to 20th	Night	20% 5% 13% 23% 23% 17%	1600
	21st to end	Night	21% 6% 11% 24% 27% 11%	1600
July	1st to 10th	Night	20% 5% 11% 19% 34% 11%	3600
	11th to 20th	Night	22% 7% 18% 28% 20% 5%	3592
	21st to end	Night	24% 7% 17% 21% 21% 10%	3944
August	1st to 10th	Night	34% 9% 16% 22% 16% 4	2948
	11th to 20th	Night	23% 8% 19% 21% 20% 10%	3243
	21st to end	Night	25% 7% 15% 23% 25% 6%	3597
September	1st to 10th	Night	23% 5% 10% 22% 26% 14%	2000
	11th to 20th	Night	21% 4 9% 24% 26% 16%	2029
	21st to end	Night	23% 4 6% 21% 34% 13%	2200
October	1st to 10th	Night	17% 3 6% 19% 33% 21%	3877
	11th to 20th	Night	21% 4 10% 26% 29% 10%	3898
	21st to end	Night	21% 2 6% 24% 34% 13%	4348
November	1st to 10th	Night	19% 3 8% 24% 30% 16%	2562
	11th to 20th	Night	21% 2 6% 26% 33% 11%	2600
	21st to end	Night	24% 3 8% 23% 31% 11%	2685
December	1st to 10th	Night	16% 2 7% 35% 32% 7%	4570
	11th to 20th	Night	18% 3 6% 31% 36% 5%	5182
	21st to end	Night	31% 3 8% 24% 26% 8%	6974

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	6% 6% 28% 42% 13% 5%	300
	11th to 20th	Day	6% 7% 31% 41% 14% 1	300
	21st to end	Day	3 5% 36% 39% 14% 3	330
February	1st to 10th	Day	4 12% 36% 38% 7% 2	300
	11th to 20th	Day	4 4 42% 40% 8% 3	300
	21st to end	Day	5% 10% 35% 41% 10%	249
March	1st to 10th	Day	4 9% 33% 37% 14% 2	300
	11th to 20th	Day	13% 11% 30% 30% 13% 3	460
	21st to end	Day	14% 16% 29% 30% 9% 2	550
April	1st to 10th	Day	11% 15% 32% 29% 12% 2	500
	11th to 20th	Day	15% 18% 32% 23% 10% 2	492
	21st to end	Day	17% 11% 31% 26% 12% 3	500
May	1st to 10th	Day	15% 12% 26% 29% 15% 3	633
	11th to 20th	Day	16% 8% 23% 29% 19% 6%	725
	21st to end	Day	11% 9% 23% 31% 20% 7%	789
June	1st to 10th	Day	6% 5% 19% 32% 26% 11%	730
	11th to 20th	Day	6% 7% 25% 34% 21% 7%	730
	21st to end	Day	6% 6% 22% 34% 25% 8%	730
July	1st to 10th	Day	7% 3 15% 29% 33% 13%	721
	11th to 20th	Day	4 3 12% 31% 38% 11%	718
	21st to end	Day	5% 3 12% 29% 38% 12%	749
August	1st to 10th	Day	7% 5% 21% 29% 25% 13%	527
	11th to 20th	Day	3 4 20% 33% 26% 13%	517
	21st to end	Day	7% 5% 17% 33% 30% 10%	583
September	1st to 10th	Day	5% 3 19% 33% 26% 13%	530
	11th to 20th	Day	5% 5% 22% 37% 22% 10%	500
	21st to end	Day	6% 6% 18% 33% 22% 15%	330
October	1st to 10th	Day	9% 6% 22% 27% 24% 12%	330
	11th to 20th	Day	5% 8% 28% 36% 18% 5%	330
	21st to end	Day	12% 7% 24% 37% 16% 4	363
November	1st to 10th	Day	7% 7% 26% 40% 14% 6%	306
	11th to 20th	Day	6% 6% 30% 41% 13% 4	300
	21st to end	Day	8% 7% 27% 42% 14% 3	300
December	1st to 10th	Day	6% 7% 29% 38% 17% 3	300
	11th to 20th	Day	5% 4 40% 41% 10% 1	300
	21st to end	Day	6% 6% 32% 43% 10% 2	330
January	1st to 10th	Night	4 3 34% 47% 10% 2	1000
	11th to 20th	Night	4 4 38% 44% 9% 1	973
	21st to end	Night	5% 3 33% 49% 10% 1	880
February	1st to 10th	Night	6% 2 41% 44% 6%	800
	11th to 20th	Night	4 3 40% 45% 7%	784
	21st to end	Night	7% 5% 38% 47% 4	660
March	1st to 10th	Night	6% 3 37% 45% 9%	800
	11th to 20th	Night	11% 4 27% 47% 11%	800
	21st to end	Night	10% 6% 36% 42% 7%	725
April	1st to 10th	Night	11% 7% 37% 40% 6%	600
	11th to 20th	Night	15% 8% 30% 38% 9%	593
	21st to end	Night	16% 6% 29% 40% 10% 1	600
May	1st to 10th	Night	15% 5% 24% 43% 13% 1	600
	11th to 20th	Night	16% 3 17% 45% 18% 1	591
	21st to end	Night	11% 5% 23% 39% 21% 1	650
June	1st to 10th	Night	6% 4 16% 47% 25% 2	600
	11th to 20th	Night	7% 3 22% 50% 17% 1	600
	21st to end	Night	5% 2 17% 48% 27% 1	600
July	1st to 10th	Night	7% 3 13% 42% 33% 2	585
	11th to 20th	Night	3 2 11% 45% 37% 2	582
	21st to end	Night	5% 2 8% 44% 38% 4	657
August	1st to 10th	Night	5% 3 11% 45% 34% 2	597
	11th to 20th	Night	4 2 15% 47% 30% 1	594
	21st to end	Night	5% 1 16% 45% 31% 2	660
September	1st to 10th	Night	6% 2 13% 45% 28% 5%	600
	11th to 20th	Night	7% 2 14% 50% 25% 3	600
	21st to end	Night	7% 2 18% 49% 21% 2	600
October	1st to 10th	Night	12% 4 18% 46% 18% 3	770
	11th to 20th	Night	5% 3 26% 49% 15% 2	800
	21st to end	Night	8% 4 25% 46% 15% 2	940
November	1st to 10th	Night	5% 3 30% 48% 12% 2	986
	11th to 20th	Night	5% 5% 31% 49% 9% 1	1000
	21st to end	Night	7% 3 28% 49% 12% 1	1000
December	1st to 10th	Night	5% 4 31% 51% 9%	1000
	11th to 20th	Night	5% 4 34% 47% 9% 1	1000
	21st to end	Night	7% 4 35% 45% 8% 1	1100

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