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 2024-05-13.

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%	83630
	Night	5% 6% 2 5% 3 3 3 3 5% 7% 57%	85969
Transparency	Day	27% 5% 10% 24% 23% 10%	83337
	Night	26% 5% 10% 25% 25% 9%	85784
Seeing	Day	8% 7% 24% 33% 21% 7%	14854
	Night	7% 3 26% 46% 16% 1	25020

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	11% 7% 2 6% 5% 3 5% 4 7% 7% 41%	2403
	11th to 20th	Day	12% 7% 2 5% 5% 4 4 5% 8% 9% 39%	2487
	21st to end	Day	7% 5% 2 5% 4 5% 5% 6% 7% 10% 45%	2724
June	1st to 10th	Day	6% 3 2 4 4 4 5% 5% 8% 10% 50%	2520
	11th to 20th	Day	6% 3 1 5% 4 5% 5% 5% 6% 9% 52%	2520
	21st to end	Day	5% 3 1 4 5% 4 5% 6% 8% 10% 50%	2520
July	1st to 10th	Day	7% 4 1 5% 4 5% 5% 5% 8% 10% 46%	2520
	11th to 20th	Day	4 3 2 4 4 4 5% 5% 8% 10% 52%	2520
	21st to end	Day	5% 4 2 5% 4 5% 6% 5% 9% 10% 46%	2702
August	1st to 10th	Day	5% 4 2 5% 5% 6% 7% 7% 9% 11% 41%	2320
	11th to 20th	Day	5% 2 1 4 3 4 5% 4 7% 8% 55%	2123
	21st to end	Day	5% 3 1 4 4 4 6% 5% 7% 8% 53%	2376
September	1st to 10th	Day	5% 3 1 4 3 3 4 3 6% 7% 61%	2160
	11th to 20th	Day	6% 3 1 4 3 3 3 3 6% 7% 60%	2134
	21st to end	Day	5% 5% 1 3 2 3 3 3 4 6% 66%	1980
October	1st to 10th	Day	8% 3 1 4 3 3 3 3 5% 5% 62%	1825
	11th to 20th	Day	4 4 1 3 2 2 3 2 4 6% 68%	1800
	21st to end	Day	7% 6% 2 4 2 2 2 3 4 5% 63%	1908
November	1st to 10th	Day	2 5% 1 3 2 2 3 3 5% 9% 65%	1673
	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	13% 7% 2 4 4 5% 5% 3 6% 7% 45%	1775
	11th to 20th	Night	15% 5% 2 5% 3 3 4 4 6% 7% 46%	1459
	21st to end	Night	8% 4 2 5% 3 3 4 3 7% 8% 53%	1552
June	1st to 10th	Night	6% 2 1 4 2 3 3 4 6% 11% 58%	1440
	11th to 20th	Night	6% 3 1 2 3 2 3 4 7% 8% 61%	1440
	21st to end	Night	5% 3 1 4 3 3 5% 4 7% 9% 58%	1440
July	1st to 10th	Night	6% 4 1 4 4 5% 4 3 8% 10% 51%	1440
	11th to 20th	Night	2 2 1 4 3 5% 4 4 8% 9% 58%	1440
	21st to end	Night	6% 4 1 4 4 4 4 5% 9% 11% 48%	1612
August	1st to 10th	Night	4 4 2 6% 4 5% 5% 4 7% 9% 49%	1617
	11th to 20th	Night	5% 3 1 3 4 3 3 4 7% 10% 56%	1778
	21st to end	Night	4 3 2 5% 4 4 5% 3 7% 9% 56%	1980
September	1st to 10th	Night	6% 4 2 5% 4 4 3 4 5% 7% 58%	1800
	11th to 20th	Night	7% 3 1 2 3 3 3 2 4 8% 64%	1825
	21st to end	Night	7% 3 1 3 3 2 3 2 4 6% 65%	1980
October	1st to 10th	Night	9% 5% 1 4 2 2 3 3 5% 5% 61%	2132
	11th to 20th	Night	4 3 1 4 2 2 2 2 4 6% 69%	2160
	21st to end	Night	6% 7% 2 4 3 2 3 2 3 6% 62%	2430
November	1st to 10th	Night	3 3 1 3 3 2 3 3 4 6% 69%	2405
	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	39% 6% 10% 23% 16% 5%	2403
	11th to 20th	Day	39% 8% 12% 25% 12% 4	2491
	21st to end	Day	33% 8% 13% 24% 15% 8%	2724
June	1st to 10th	Day	27% 7% 15% 24% 18% 10%	2520
	11th to 20th	Day	26% 7% 10% 18% 22% 17%	2520
	21st to end	Day	26% 7% 10% 20% 25% 13%	2520
July	1st to 10th	Day	29% 7% 11% 20% 20% 13%	2451
	11th to 20th	Day	25% 9% 10% 23% 22% 11%	2445
	21st to end	Day	30% 10% 14% 24% 15% 7%	2552
August	1st to 10th	Day	32% 9% 14% 23% 14% 7%	2305
	11th to 20th	Day	24% 6% 11% 21% 24% 13%	2123
	21st to end	Day	25% 6% 9% 22% 27% 11%	2376
September	1st to 10th	Day	23% 5% 8% 20% 24% 20%	2160
	11th to 20th	Day	22% 4 9% 17% 28% 19%	2134
	21st to end	Day	21% 4 5% 18% 38% 14%	1980
October	1st to 10th	Day	24% 4 8% 20% 30% 14%	1825
	11th to 20th	Day	18% 3 8% 22% 33% 16%	1800
	21st to end	Day	24% 3 6% 26% 25% 15%	1908
November	1st to 10th	Day	17% 3 8% 25% 29% 18%	1673
	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	36% 7% 12% 23% 15% 5%	1775
	11th to 20th	Night	36% 6% 20% 21% 12% 4	1460
	21st to end	Night	29% 6% 17% 27% 13% 8%	1552
June	1st to 10th	Night	19% 7% 19% 32% 15% 8%	1440
	11th to 20th	Night	20% 5% 13% 22% 22% 18%	1440
	21st to end	Night	21% 6% 11% 25% 28% 10%	1440
July	1st to 10th	Night	27% 8% 15% 22% 20% 7%	1400
	11th to 20th	Night	19% 6% 12% 28% 24% 10%	1392
	21st to end	Night	26% 8% 19% 28% 16% 4	1522
August	1st to 10th	Night	29% 9% 18% 24% 14% 6%	1608
	11th to 20th	Night	23% 7% 10% 26% 24% 10%	1778
	21st to end	Night	24% 6% 12% 24% 26% 8%	1980
September	1st to 10th	Night	24% 5% 11% 21% 25% 15%	1800
	11th to 20th	Night	20% 4 9% 23% 26% 17%	1825
	21st to end	Night	23% 4 6% 21% 35% 12%	1980
October	1st to 10th	Night	25% 5% 10% 19% 28% 13%	2132
	11th to 20th	Night	17% 3 10% 22% 30% 18%	2160
	21st to end	Night	26% 2 7% 23% 28% 13%	2430
November	1st to 10th	Night	17% 3 9% 25% 29% 17%	2405
	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	18% 7% 22% 28% 19% 6%	637
	21st to end	Day	11% 9% 22% 30% 21% 7%	679
June	1st to 10th	Day	7% 5% 19% 32% 26% 13%	630
	11th to 20th	Day	6% 6% 25% 34% 22% 6%	630
	21st to end	Day	6% 6% 21% 32% 26% 8%	630
July	1st to 10th	Day	7% 3 14% 26% 35% 14%	621
	11th to 20th	Day	5% 2 11% 31% 39% 11%	618
	21st to end	Day	5% 3 11% 28% 40% 13%	644
August	1st to 10th	Day	6% 4 19% 29% 27% 15%	447
	11th to 20th	Day	4 4 19% 34% 27% 13%	437
	21st to end	Day	7% 4 17% 33% 30% 10%	495
September	1st to 10th	Day	6% 3 20% 34% 25% 12%	450
	11th to 20th	Day	5% 4 20% 36% 24% 11%	424
	21st to end	Day	6% 6% 19% 34% 23% 13%	270
October	1st to 10th	Day	10% 6% 22% 25% 25% 13%	270
	11th to 20th	Day	3 7% 27% 38% 19% 6%	270
	21st to end	Day	12% 8% 24% 35% 17% 4	297
November	1st to 10th	Day	5% 7% 25% 40% 16% 6%	270
	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% 4 14% 44% 19% 1	540
	21st to end	Night	11% 5% 22% 39% 23% 1	584
June	1st to 10th	Night	6% 3 14% 47% 27% 2	540
	11th to 20th	Night	8% 2 21% 51% 18% 1	540
	21st to end	Night	5% 2 15% 47% 29% 1	540
July	1st to 10th	Night	7% 2 11% 41% 36% 3	525
	11th to 20th	Night	3 1 11% 44% 39% 2	522
	21st to end	Night	5% 2 7% 42% 40% 4	591
August	1st to 10th	Night	5% 2 10% 44% 37% 2	537
	11th to 20th	Night	5% 2 14% 47% 31% 1	534
	21st to end	Night	5% 1 14% 45% 33% 3	594
September	1st to 10th	Night	6% 3 14% 45% 28% 5%	540
	11th to 20th	Night	7% 1 13% 49% 27% 3	540
	21st to end	Night	7% 2 17% 49% 22% 3	540
October	1st to 10th	Night	13% 3 18% 44% 19% 3	692
	11th to 20th	Night	5% 2 25% 50% 16% 2	720
	21st to end	Night	9% 4 23% 46% 15% 2	846
November	1st to 10th	Night	4 3 29% 49% 13% 2	925
	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 2024-05-10 10:39:00. Version ac07

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