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 2024-11-06.

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%	105160
	Night	5% 6% 2 5% 3 3 4 3 5% 7% 57%	102593
Transparency	Day	27% 6% 9% 23% 24% 11%	104904
	Night	26% 5% 11% 25% 25% 9%	102439
Seeing	Day	8% 7% 24% 33% 21% 7%	15589
	Night	7% 3 26% 46% 16% 1	25596

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 4 3 6% 3 3 3 3 5% 8% 61%	2538
	11th to 20th	Day	2 6% 3 6% 5% 3 4 3 5% 8% 55%	2608
	21st to end	Day	2 6% 2 5% 3 3 4 3 5% 8% 59%	3146
February	1st to 10th	Day	3 7% 3 7% 4 3 4 3 5% 6% 54%	1710
	11th to 20th	Day	1 5% 3 6% 4 4 4 3 5% 8% 57%	1834
	21st to end	Day	2 8% 2 8% 3 3 3 4 5% 6% 58%	1570
March	1st to 10th	Day	4 9% 3 5% 3 3 3 2 4 7% 56%	3130
	11th to 20th	Day	6% 9% 2 6% 4 3 4 3 5% 7% 50%	3426
	21st to end	Day	5% 7% 2 5% 4 3 4 4 5% 8% 53%	4067
April	1st to 10th	Day	6% 6% 2 4 3 3 4 3 6% 8% 54%	2280
	11th to 20th	Day	9% 8% 3 5% 3 4 4 4 5% 7% 47%	2251
	21st to end	Day	10% 7% 1 4 4 4 4 4 6% 8% 47%	2280
May	1st to 10th	Day	10% 7% 2 6% 5% 4 6% 4 7% 8% 40%	4014
	11th to 20th	Day	10% 7% 2 5% 5% 4 5% 5% 7% 8% 43%	4354
	21st to end	Day	5% 5% 2 4 4 5% 5% 5% 6% 10% 50%	4825
June	1st to 10th	Day	5% 3 2 4 4 4 5% 5% 8% 10% 50%	2660
	11th to 20th	Day	5% 3 1 4 4 5% 5% 6% 6% 9% 52%	2660
	21st to end	Day	5% 3 1 4 5% 4 5% 6% 8% 10% 50%	2660
July	1st to 10th	Day	5% 3 1 4 3 4 4 4 6% 8% 57%	4430
	11th to 20th	Day	3 2 1 4 4 5% 6% 5% 9% 12% 48%	4430
	21st to end	Day	5% 4 2 5% 4 4 5% 4 7% 8% 54%	4723
August	1st to 10th	Day	9% 4 1 6% 5% 6% 7% 6% 8% 10% 38%	4071
	11th to 20th	Day	4 3 1 5% 4 5% 6% 5% 7% 9% 51%	3726
	21st to end	Day	4 3 2 5% 4 4 5% 5% 8% 9% 51%	4180
September	1st to 10th	Day	5% 3 1 4 3 3 4 3 5% 7% 61%	2280
	11th to 20th	Day	6% 3 1 4 3 3 3 3 6% 7% 60%	2252
	21st to end	Day	5% 4 1 3 2 3 3 2 4 6% 66%	2090
October	1st to 10th	Day	4 2 1 2 2 2 2 2 3 4 76%	3208
	11th to 20th	Day	4 5% 2 4 3 3 3 3 5% 7% 61%	3150
	21st to end	Day	6% 6% 2 3 2 2 2 2 4 5% 65%	3314
November	1st to 10th	Day	2 5% 1 3 2 2 3 3 5% 8% 65%	1718
	11th to 20th	Day	2 5% 2 4 3 3 2 2 5% 8% 63%	1710
	21st to end	Day	4 6% 2 5% 2 3 3 3 5% 6% 60%	1625
December	1st to 10th	Day	2 6% 2 5% 3 3 3 3 5% 9% 59%	1520
	11th to 20th	Day	3 5% 2 5% 2 3 4 3 6% 9% 59%	1926
	21st to end	Day	3 8% 4 8% 4 4 4 3 5% 6% 53%	2794
January	1st to 10th	Night	2 8% 3 8% 4 3 3 2 4 8% 55%	4417
	11th to 20th	Night	1 5% 2 5% 4 4 3 3 5% 7% 62%	4392
	21st to end	Night	3 7% 3 7% 4 3 4 3 4 6% 57%	4521
February	1st to 10th	Night	3 8% 2 6% 3 3 4 3 5% 7% 56%	2470
	11th to 20th	Night	2 8% 3 7% 5% 4 4 3 4 6% 53%	2232
	21st to end	Night	2 8% 4 6% 3 3 3 3 4 5% 60%	1884
March	1st to 10th	Night	5% 8% 2 4 3 3 3 3 5% 5% 58%	3734
	11th to 20th	Night	10% 11% 2 5% 3 3 3 3 3 5% 52%	3535
	21st to end	Night	6% 9% 2 4 4 3 3 3 5% 8% 53%	3578
April	1st to 10th	Night	7% 7% 2 5% 2 3 4 3 5% 6% 56%	1900
	11th to 20th	Night	12% 8% 2 5% 4 3 3 4 4 6% 50%	1877
	21st to end	Night	13% 8% 2 5% 3 3 3 3 5% 6% 49%	1900
May	1st to 10th	Night	12% 7% 2 5% 4 4 4 3 5% 7% 48%	2954
	11th to 20th	Night	11% 4 2 5% 4 3 4 4 5% 9% 47%	2535
	21st to end	Night	9% 4 2 4 3 3 3 3 6% 7% 55%	2762
June	1st to 10th	Night	6% 2 1 3 2 3 3 4 6% 10% 59%	1520
	11th to 20th	Night	6% 3 1 2 3 3 3 4 7% 8% 62%	1520
	21st to end	Night	5% 3 1 4 3 3 5% 4 7% 8% 57%	1520
July	1st to 10th	Night	4 3 1 4 3 5% 4 3 6% 8% 59%	2540
	11th to 20th	Night	2 3 1 5% 3 5% 5% 5% 8% 10% 54%	2540
	21st to end	Night	4 3 1 4 4 3 4 5% 8% 9% 56%	2823
August	1st to 10th	Night	8% 5% 2 6% 4 5% 5% 5% 8% 10% 42%	2866
	11th to 20th	Night	3 3 1 4 4 4 5% 5% 8% 12% 51%	3113
	21st to end	Night	3 3 2 5% 4 4 6% 5% 8% 11% 51%	3487
September	1st to 10th	Night	6% 3 1 4 3 4 3 3 5% 7% 58%	1900
	11th to 20th	Night	7% 3 1 2 3 3 3 2 4 8% 65%	1927
	21st to end	Night	7% 3 1 3 3 2 3 2 4 6% 65%	2090
October	1st to 10th	Night	5% 3 1 3 2 2 2 2 4 4 72%	3758
	11th to 20th	Night	6% 3 2 4 2 2 3 3 4 8% 62%	3778
	21st to end	Night	5% 7% 2 3 2 2 2 2 3 5% 66%	4213
November	1st to 10th	Night	3 4 2 3 3 2 3 4 4 6% 68%	2483
	11th to 20th	Night	3 6% 2 4 3 3 3 3 4 5% 65%	2470
	21st to end	Night	4 6% 2 5% 3 3 3 3 4 6% 62%	2550
December	1st to 10th	Night	2 6% 2 5% 4 2 3 2 5% 8% 61%	2660
	11th to 20th	Night	2 7% 2 5% 3 3 3 3 6% 9% 58%	3271
	21st to end	Night	5% 8% 3 6% 4 4 4 3 5% 6% 52%	4873

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	22% 3 10% 32% 25% 8%	2539
	11th to 20th	Day	27% 8% 11% 28% 19% 7%	2608
	21st to end	Day	24% 4 9% 32% 25% 5%	3146
February	1st to 10th	Day	31% 3 10% 26% 21% 9%	1710
	11th to 20th	Day	25% 3 7% 29% 28% 8%	1844
	21st to end	Day	26% 3 7% 23% 27% 13%	1570
March	1st to 10th	Day	28% 4 7% 20% 29% 12%	3130
	11th to 20th	Day	33% 5% 11% 30% 16% 4	3426
	21st to end	Day	27% 6% 11% 29% 22% 4	4067
April	1st to 10th	Day	27% 5% 10% 24% 23% 11%	2280
	11th to 20th	Day	34% 6% 8% 21% 20% 10%	2254
	21st to end	Day	33% 6% 10% 24% 22% 4	2280
May	1st to 10th	Day	37% 8% 8% 23% 17% 7%	4014
	11th to 20th	Day	35% 8% 11% 25% 17% 3	4358
	21st to end	Day	27% 8% 12% 24% 21% 9%	4825
June	1st to 10th	Day	27% 7% 14% 24% 18% 10%	2660
	11th to 20th	Day	25% 7% 10% 18% 23% 17%	2660
	21st to end	Day	26% 7% 10% 20% 25% 13%	2660
July	1st to 10th	Day	24% 5% 8% 16% 29% 19%	4361
	11th to 20th	Day	23% 8% 10% 22% 23% 13%	4355
	21st to end	Day	25% 7% 10% 19% 21% 18%	4574
August	1st to 10th	Day	37% 9% 14% 21% 15% 5%	4057
	11th to 20th	Day	28% 7% 13% 16% 21% 15%	3762
	21st to end	Day	26% 6% 10% 19% 25% 13%	4179
September	1st to 10th	Day	22% 5% 8% 20% 25% 20%	2280
	11th to 20th	Day	22% 4 9% 18% 28% 19%	2252
	21st to end	Day	21% 4 5% 18% 37% 15%	2090
October	1st to 10th	Day	14% 2 5% 15% 36% 26%	3208
	11th to 20th	Day	23% 4 8% 23% 32% 9%	3150
	21st to end	Day	21% 2 5% 23% 36% 13%	3314
November	1st to 10th	Day	17% 3 8% 25% 29% 18%	1718
	11th to 20th	Day	20% 4 5% 27% 34% 10%	1710
	21st to end	Day	25% 2 7% 21% 30% 15%	1625
December	1st to 10th	Day	23% 4 11% 29% 24% 9%	1520
	11th to 20th	Day	21% 4 9% 26% 32% 7%	1924
	21st to end	Day	32% 2 9% 21% 27% 8%	2794
January	1st to 10th	Night	28% 4 8% 27% 27% 5%	4417
	11th to 20th	Night	21% 6% 12% 27% 24% 10%	4392
	21st to end	Night	29% 4 11% 31% 19% 6%	4521
February	1st to 10th	Night	29% 4 9% 27% 24% 7%	2470
	11th to 20th	Night	30% 3 8% 28% 24% 7%	2232
	21st to end	Night	26% 3 6% 24% 28% 14%	1884
March	1st to 10th	Night	25% 3 6% 21% 35% 9%	3734
	11th to 20th	Night	34% 5% 12% 18% 26% 4	3535
	21st to end	Night	30% 5% 10% 33% 20% 3	3578
April	1st to 10th	Night	27% 6% 10% 23% 24% 11%	1900
	11th to 20th	Night	35% 4 10% 22% 20% 9%	1877
	21st to end	Night	35% 5% 9% 25% 20% 5%	1900
May	1st to 10th	Night	34% 8% 10% 22% 15% 10%	2954
	11th to 20th	Night	34% 6% 16% 25% 14% 5%	2536
	21st to end	Night	26% 6% 19% 24% 18% 7%	2762
June	1st to 10th	Night	19% 7% 20% 32% 15% 8%	1520
	11th to 20th	Night	20% 5% 13% 22% 22% 18%	1520
	21st to end	Night	21% 6% 11% 24% 28% 10%	1520
July	1st to 10th	Night	22% 6% 13% 19% 31% 10%	2500
	11th to 20th	Night	20% 7% 15% 29% 23% 7%	2492
	21st to end	Night	20% 7% 14% 24% 21% 15%	2734
August	1st to 10th	Night	34% 9% 15% 22% 15% 4	2857
	11th to 20th	Night	23% 9% 19% 20% 19% 10%	3143
	21st to end	Night	24% 7% 14% 24% 25% 6%	3487
September	1st to 10th	Night	24% 5% 10% 21% 25% 15%	1900
	11th to 20th	Night	20% 4 9% 23% 27% 17%	1927
	21st to end	Night	22% 4 6% 21% 35% 13%	2090
October	1st to 10th	Night	17% 3 6% 20% 32% 21%	3758
	11th to 20th	Night	21% 4 10% 27% 28% 10%	3778
	21st to end	Night	22% 2 6% 25% 33% 13%	4213
November	1st to 10th	Night	18% 3 9% 24% 29% 16%	2483
	11th to 20th	Night	21% 2 7% 28% 32% 10%	2470
	21st to end	Night	24% 3 8% 22% 31% 12%	2550
December	1st to 10th	Night	23% 3 12% 27% 26% 9%	2660
	11th to 20th	Night	23% 4 8% 27% 33% 5%	3272
	21st to end	Night	32% 3 9% 25% 25% 6%	4873

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% 41% 14% 5%	270
	11th to 20th	Day	7% 7% 32% 39% 14% 1	270
	21st to end	Day	3 4 36% 38% 15% 3	297
February	1st to 10th	Day	5% 12% 36% 37% 8% 2	270
	11th to 20th	Day	3 4 40% 40% 9% 3	270
	21st to end	Day	5% 8% 36% 41% 10%	225
March	1st to 10th	Day	4 9% 32% 36% 15% 3	270
	11th to 20th	Day	13% 11% 29% 31% 14% 3	422
	21st to end	Day	15% 16% 29% 29% 10% 1	506
April	1st to 10th	Day	11% 14% 32% 29% 12% 2	460
	11th to 20th	Day	16% 18% 31% 22% 10% 3	452
	21st to end	Day	17% 11% 30% 26% 13% 3	460
May	1st to 10th	Day	15% 12% 27% 30% 14% 2	586
	11th to 20th	Day	17% 7% 21% 29% 19% 6%	675
	21st to end	Day	10% 9% 23% 31% 20% 7%	734
June	1st to 10th	Day	6% 5% 19% 32% 25% 12%	680
	11th to 20th	Day	6% 7% 25% 35% 21% 6%	680
	21st to end	Day	5% 6% 22% 34% 25% 8%	680
July	1st to 10th	Day	7% 3 15% 28% 34% 13%	671
	11th to 20th	Day	4 2 12% 31% 39% 12%	668
	21st to end	Day	5% 3 12% 28% 39% 13%	696
August	1st to 10th	Day	7% 4 19% 29% 27% 14%	487
	11th to 20th	Day	4 4 20% 34% 27% 12%	477
	21st to end	Day	7% 4 17% 33% 29% 10%	539
September	1st to 10th	Day	6% 3 19% 33% 26% 13%	490
	11th to 20th	Day	5% 5% 22% 36% 23% 10%	462
	21st to end	Day	6% 6% 20% 33% 23% 13%	300
October	1st to 10th	Day	9% 6% 22% 26% 25% 12%	300
	11th to 20th	Day	3 7% 27% 38% 20% 6%	300
	21st to end	Day	13% 8% 23% 36% 16% 4	330
November	1st to 10th	Day	5% 8% 26% 41% 15% 6%	285
	11th to 20th	Day	6% 6% 28% 42% 13% 5%	270
	21st to end	Day	8% 7% 27% 41% 14% 3	270
December	1st to 10th	Day	6% 7% 29% 39% 16% 2	270
	11th to 20th	Day	6% 4 37% 41% 11% 1	270
	21st to end	Day	5% 7% 30% 46% 9% 2	297
January	1st to 10th	Night	3 4 33% 47% 11% 2	950
	11th to 20th	Night	4 4 36% 45% 10% 2	925
	21st to end	Night	5% 3 32% 49% 10% 1	836
February	1st to 10th	Night	6% 2 40% 45% 6%	760
	11th to 20th	Night	5% 3 39% 46% 7%	744
	21st to end	Night	7% 5% 36% 48% 4	628
March	1st to 10th	Night	6% 3 37% 45% 9%	760
	11th to 20th	Night	11% 4 26% 48% 11%	760
	21st to end	Night	9% 5% 36% 43% 7%	689
April	1st to 10th	Night	10% 7% 36% 41% 6%	570
	11th to 20th	Night	15% 7% 30% 38% 10%	563
	21st to end	Night	16% 5% 29% 40% 10% 1	570
May	1st to 10th	Night	15% 5% 24% 43% 12% 1	570
	11th to 20th	Night	17% 3 15% 45% 19% 1	561
	21st to end	Night	11% 5% 22% 39% 22% 1	617
June	1st to 10th	Night	6% 4 16% 47% 26% 2	570
	11th to 20th	Night	7% 3 21% 51% 17% 1	570
	21st to end	Night	5% 2 16% 48% 28% 1	570
July	1st to 10th	Night	7% 3 12% 41% 35% 3	555
	11th to 20th	Night	3 2 11% 44% 38% 2	552
	21st to end	Night	5% 2 7% 43% 39% 4	624
August	1st to 10th	Night	5% 3 11% 44% 35% 2	567
	11th to 20th	Night	5% 2 15% 46% 30% 1	564
	21st to end	Night	5% 1 15% 46% 31% 2	627
September	1st to 10th	Night	6% 2 14% 45% 28% 5%	570
	11th to 20th	Night	6% 2 14% 49% 26% 3	570
	21st to end	Night	7% 2 18% 50% 21% 2	570
October	1st to 10th	Night	12% 3 18% 45% 19% 3	731
	11th to 20th	Night	5% 2 25% 50% 16% 2	760
	21st to end	Night	9% 5% 24% 46% 15% 2	893
November	1st to 10th	Night	4 3 30% 48% 13% 2	955
	11th to 20th	Night	5% 5% 30% 50% 10% 1	950
	21st to end	Night	7% 3 27% 49% 12% 1	950
December	1st to 10th	Night	5% 4 32% 51% 9%	950
	11th to 20th	Night	5% 4 32% 49% 9% 1	950
	21st to end	Night	7% 3 35% 46% 8% 1	1045

Page generated at 2024-11-03 11:19:05. Version ac07

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