Centennial
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Centennial clear sky chart from 2005-11-06 to 2024-11-01.

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% 53%	101285
	Night	5% 6% 2 5% 3 3 4 3 5% 7% 56%	97886
Transparency	Day	27% 6% 10% 23% 24% 11%	101029
	Night	26% 5% 11% 25% 24% 9%	97733
Seeing	Day	8% 7% 24% 33% 22% 8%	15570
	Night	7% 3 25% 47% 16% 1	25561

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 7% 4 3 3 3 5% 8% 60%	2538
	11th to 20th	Day	2 8% 3 6% 4 4 4 4 5% 7% 54%	2607
	21st to end	Day	2 6% 2 5% 3 4 3 3 5% 7% 59%	3146
February	1st to 10th	Day	3 8% 3 6% 4 4 4 3 4 6% 55%	1710
	11th to 20th	Day	1 5% 2 6% 4 5% 4 3 6% 7% 58%	1842
	21st to end	Day	2 8% 3 6% 3 3 2 3 5% 6% 59%	1570
March	1st to 10th	Day	3 10% 3 5% 3 3 3 2 5% 7% 56%	3130
	11th to 20th	Day	6% 9% 3 7% 4 3 4 3 5% 6% 50%	3421
	21st to end	Day	4 7% 2 4 4 4 4 4 6% 8% 52%	4071
April	1st to 10th	Day	7% 6% 2 4 3 3 3 3 7% 7% 54%	2280
	11th to 20th	Day	9% 8% 2 5% 3 4 5% 4 5% 7% 47%	2251
	21st to end	Day	10% 7% 1 4 4 5% 4 4 6% 9% 46%	2280
May	1st to 10th	Day	8% 8% 3 7% 5% 5% 5% 5% 7% 8% 41%	4016
	11th to 20th	Day	10% 6% 2 5% 4 5% 5% 5% 7% 8% 43%	4343
	21st to end	Day	6% 5% 2 5% 4 4 5% 5% 7% 9% 49%	4825
June	1st to 10th	Day	5% 4 2 5% 3 4 6% 6% 7% 9% 50%	2660
	11th to 20th	Day	5% 3 1 4 3 4 6% 5% 7% 8% 52%	2660
	21st to end	Day	4 3 1 5% 4 5% 6% 5% 8% 10% 50%	2660
July	1st to 10th	Day	5% 4 1 4 3 4 4 4 7% 8% 57%	4430
	11th to 20th	Day	3 3 1 4 4 5% 5% 5% 9% 11% 50%	4430
	21st to end	Day	5% 3 2 5% 4 4 5% 5% 7% 8% 54%	4725
August	1st to 10th	Day	9% 5% 2 5% 5% 6% 7% 6% 8% 9% 38%	4065
	11th to 20th	Day	4 3 2 4 4 5% 6% 6% 7% 9% 50%	3726
	21st to end	Day	4 4 1 5% 4 4 5% 5% 7% 9% 51%	4180
September	1st to 10th	Day	5% 4 1 4 3 4 3 4 4 7% 61%	2280
	11th to 20th	Day	6% 3 1 3 4 3 4 3 5% 6% 60%	2249
	21st to end	Day	6% 4 1 3 2 2 3 2 4 6% 66%	2090
October	1st to 10th	Day	7% 4 1 3 2 3 3 3 4 6% 63%	1933
	11th to 20th	Day	3 4 1 4 3 2 3 2 4 7% 67%	1900
	21st to end	Day	6% 7% 1 4 2 2 3 2 3 5% 64%	2010
November	1st to 10th	Day	2 5% 1 3 3 2 2 3 6% 8% 65%	1673
	11th to 20th	Day	2 6% 1 5% 3 3 3 2 5% 7% 63%	1710
	21st to end	Day	4 7% 2 5% 3 2 3 3 5% 7% 60%	1634
December	1st to 10th	Day	2 6% 2 5% 3 4 3 2 6% 8% 59%	1520
	11th to 20th	Day	2 5% 2 6% 3 4 3 3 6% 8% 58%	1926
	21st to end	Day	2 9% 4 7% 4 4 4 3 5% 6% 52%	2794
January	1st to 10th	Night	1 9% 4 8% 4 3 3 3 5% 7% 54%	4417
	11th to 20th	Night	2 5% 2 6% 4 4 4 3 4 7% 60%	4394
	21st to end	Night	3 7% 3 7% 4 4 3 2 4 6% 57%	4521
February	1st to 10th	Night	2 8% 3 6% 4 4 4 3 4 7% 55%	2470
	11th to 20th	Night	2 9% 4 7% 4 3 3 3 5% 6% 53%	2223
	21st to end	Night	3 9% 3 5% 3 2 3 3 4 7% 60%	1884
March	1st to 10th	Night	4 8% 2 5% 3 3 3 3 4 6% 59%	3734
	11th to 20th	Night	9% 11% 3 5% 3 3 3 3 4 5% 51%	3552
	21st to end	Night	5% 9% 3 5% 4 3 4 3 5% 7% 52%	3573
April	1st to 10th	Night	6% 8% 2 4 4 3 4 2 5% 6% 55%	1900
	11th to 20th	Night	12% 8% 3 4 3 4 4 3 5% 5% 50%	1877
	21st to end	Night	12% 8% 2 5% 4 3 3 3 4 7% 48%	1900
May	1st to 10th	Night	12% 7% 2 6% 4 4 3 4 5% 6% 47%	2951
	11th to 20th	Night	12% 6% 2 5% 3 3 4 4 7% 9% 45%	2558
	21st to end	Night	8% 5% 2 4 3 3 3 4 6% 7% 54%	2762
June	1st to 10th	Night	5% 3 1 4 2 4 3 3 8% 10% 57%	1520
	11th to 20th	Night	5% 3 1 3 3 3 3 4 7% 9% 60%	1520
	21st to end	Night	4 3 1 4 3 4 4 5% 7% 8% 56%	1520
July	1st to 10th	Night	4 3 1 4 5% 4 4 4 6% 8% 57%	2540
	11th to 20th	Night	3 3 1 4 4 5% 5% 5% 7% 11% 53%	2540
	21st to end	Night	4 3 1 4 4 3 4 4 8% 9% 56%	2821
August	1st to 10th	Night	7% 5% 2 7% 5% 5% 5% 5% 7% 10% 41%	2872
	11th to 20th	Night	4 3 2 5% 4 4 5% 5% 9% 11% 49%	3113
	21st to end	Night	3 3 1 5% 3 4 5% 5% 9% 10% 50%	3487
September	1st to 10th	Night	7% 3 2 4 3 3 4 4 6% 7% 57%	1900
	11th to 20th	Night	6% 3 1 4 2 2 3 3 6% 7% 63%	1929
	21st to end	Night	7% 4 1 3 3 2 3 2 4 6% 65%	2090
October	1st to 10th	Night	8% 5% 2 4 2 2 3 3 5% 6% 60%	2244
	11th to 20th	Night	4 4 1 4 3 2 3 3 4 5% 69%	2280
	21st to end	Night	5% 8% 2 4 2 3 2 2 3 6% 63%	2567
November	1st to 10th	Night	2 3 2 4 3 3 3 2 5% 7% 68%	2412
	11th to 20th	Night	3 5% 2 4 3 3 3 4 4 5% 65%	2470
	21st to end	Night	4 7% 2 5% 4 3 3 3 4 6% 61%	2541
December	1st to 10th	Night	2 6% 2 6% 3 3 3 3 4 7% 61%	2660
	11th to 20th	Night	2 6% 2 5% 4 3 3 3 6% 9% 57%	3271
	21st to end	Night	5% 8% 3 7% 4 4 3 3 5% 6% 51%	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	21% 4 9% 32% 25% 8%	2539
	11th to 20th	Day	30% 5% 12% 27% 19% 7%	2607
	21st to end	Day	24% 4 10% 31% 25% 6%	3146
February	1st to 10th	Day	30% 3 10% 27% 20% 9%	1710
	11th to 20th	Day	23% 4 7% 29% 29% 9%	1852
	21st to end	Day	26% 4 6% 23% 27% 14%	1570
March	1st to 10th	Day	28% 4 7% 20% 29% 13%	3130
	11th to 20th	Day	33% 5% 12% 29% 16% 4	3421
	21st to end	Day	27% 6% 12% 28% 22% 4	4071
April	1st to 10th	Day	28% 5% 10% 25% 22% 11%	2280
	11th to 20th	Day	35% 5% 9% 21% 20% 10%	2254
	21st to end	Day	33% 6% 10% 24% 22% 4	2280
May	1st to 10th	Day	34% 11% 7% 24% 16% 8%	4016
	11th to 20th	Day	35% 8% 11% 24% 18% 3	4347
	21st to end	Day	27% 8% 12% 23% 21% 9%	4825
June	1st to 10th	Day	26% 8% 14% 24% 18% 10%	2660
	11th to 20th	Day	25% 7% 10% 18% 22% 17%	2660
	21st to end	Day	26% 7% 10% 19% 24% 13%	2660
July	1st to 10th	Day	24% 6% 7% 15% 29% 19%	4361
	11th to 20th	Day	24% 7% 9% 22% 24% 13%	4355
	21st to end	Day	27% 6% 10% 19% 20% 18%	4576
August	1st to 10th	Day	37% 9% 14% 21% 14% 5%	4051
	11th to 20th	Day	28% 8% 11% 17% 22% 15%	3762
	21st to end	Day	26% 6% 10% 19% 26% 13%	4179
September	1st to 10th	Day	24% 4 7% 21% 25% 20%	2280
	11th to 20th	Day	22% 5% 7% 19% 28% 19%	2249
	21st to end	Day	20% 4 5% 19% 36% 15%	2090
October	1st to 10th	Day	23% 4 7% 21% 30% 16%	1933
	11th to 20th	Day	18% 3 7% 23% 33% 15%	1900
	21st to end	Day	23% 3 6% 27% 26% 15%	2010
November	1st to 10th	Day	17% 3 9% 25% 28% 18%	1673
	11th to 20th	Day	21% 3 7% 25% 34% 10%	1710
	21st to end	Day	24% 3 7% 21% 29% 16%	1634
December	1st to 10th	Day	23% 4 11% 29% 24% 9%	1520
	11th to 20th	Day	23% 3 10% 25% 31% 7%	1924
	21st to end	Day	31% 4 8% 22% 26% 9%	2794
January	1st to 10th	Night	27% 5% 8% 27% 28% 5%	4417
	11th to 20th	Night	23% 4 13% 26% 23% 11%	4394
	21st to end	Night	28% 5% 10% 32% 18% 6%	4521
February	1st to 10th	Night	28% 4 10% 27% 24% 7%	2470
	11th to 20th	Night	30% 3 9% 28% 23% 7%	2223
	21st to end	Night	25% 3 5% 26% 27% 14%	1884
March	1st to 10th	Night	25% 4 6% 20% 37% 9%	3734
	11th to 20th	Night	34% 6% 11% 19% 26% 4	3552
	21st to end	Night	29% 6% 10% 32% 20% 3	3573
April	1st to 10th	Night	28% 5% 11% 22% 22% 11%	1900
	11th to 20th	Night	35% 4 10% 24% 20% 8%	1877
	21st to end	Night	37% 4 10% 26% 19% 5%	1900
May	1st to 10th	Night	34% 8% 10% 24% 14% 10%	2951
	11th to 20th	Night	33% 7% 16% 25% 14% 5%	2559
	21st to end	Night	25% 7% 19% 24% 18% 7%	2762
June	1st to 10th	Night	20% 5% 20% 31% 15% 8%	1520
	11th to 20th	Night	20% 5% 14% 22% 22% 17%	1520
	21st to end	Night	22% 6% 12% 25% 26% 10%	1520
July	1st to 10th	Night	24% 7% 11% 18% 31% 9%	2500
	11th to 20th	Night	22% 7% 14% 29% 22% 6%	2492
	21st to end	Night	22% 5% 15% 24% 20% 14%	2733
August	1st to 10th	Night	37% 8% 15% 23% 14% 4	2863
	11th to 20th	Night	26% 8% 18% 20% 19% 10%	3143
	21st to end	Night	26% 5% 13% 26% 24% 6%	3487
September	1st to 10th	Night	25% 4 10% 22% 25% 14%	1900
	11th to 20th	Night	20% 4 9% 24% 26% 16%	1929
	21st to end	Night	22% 3 7% 21% 34% 12%	2090
October	1st to 10th	Night	25% 4 10% 20% 27% 14%	2244
	11th to 20th	Night	18% 4 9% 24% 29% 16%	2280
	21st to end	Night	25% 3 6% 24% 28% 14%	2567
November	1st to 10th	Night	17% 4 8% 25% 29% 17%	2412
	11th to 20th	Night	21% 3 6% 28% 32% 10%	2470
	21st to end	Night	25% 3 7% 22% 31% 12%	2541
December	1st to 10th	Night	23% 4 11% 27% 26% 9%	2660
	11th to 20th	Night	23% 4 9% 26% 33% 5%	3272
	21st to end	Night	32% 3 8% 25% 26% 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	2 7% 24% 47% 17% 4	270
	11th to 20th	Day	8% 7% 31% 43% 10% 1	270
	21st to end	Day	4 6% 30% 43% 14% 3	297
February	1st to 10th	Day	4 9% 41% 36% 8% 1	270
	11th to 20th	Day	3 6% 41% 38% 11% 1	270
	21st to end	Day	4 11% 36% 38% 10% 1	225
March	1st to 10th	Day	4 9% 31% 36% 16% 4	270
	11th to 20th	Day	13% 7% 33% 32% 12% 3	417
	21st to end	Day	13% 18% 30% 26% 11% 2	506
April	1st to 10th	Day	10% 15% 33% 28% 12% 3	460
	11th to 20th	Day	14% 21% 27% 25% 10% 3	452
	21st to end	Day	17% 10% 31% 25% 16% 2	460
May	1st to 10th	Day	14% 10% 30% 31% 13% 3	588
	11th to 20th	Day	15% 7% 22% 29% 22% 6%	675
	21st to end	Day	10% 9% 22% 31% 20% 7%	734
June	1st to 10th	Day	5% 5% 20% 31% 25% 13%	680
	11th to 20th	Day	6% 6% 24% 35% 22% 6%	680
	21st to end	Day	5% 4 21% 35% 27% 8%	680
July	1st to 10th	Day	8% 3 15% 25% 35% 14%	671
	11th to 20th	Day	4 1 12% 28% 40% 13%	668
	21st to end	Day	5% 2 12% 27% 38% 16%	698
August	1st to 10th	Day	8% 3 16% 30% 28% 16%	487
	11th to 20th	Day	5% 2 17% 34% 27% 15%	477
	21st to end	Day	5% 3 20% 31% 28% 12%	539
September	1st to 10th	Day	6% 5% 16% 32% 29% 12%	490
	11th to 20th	Day	5% 5% 22% 31% 26% 11%	459
	21st to end	Day	6% 3 21% 29% 24% 17%	300
October	1st to 10th	Day	10% 3 19% 23% 28% 17%	300
	11th to 20th	Day	2 5% 23% 39% 21% 9%	300
	21st to end	Day	10% 9% 22% 37% 19% 4	330
November	1st to 10th	Day	4 9% 23% 39% 19% 7%	270
	11th to 20th	Day	6% 4 22% 44% 20% 4	270
	21st to end	Day	8% 4 30% 40% 14% 4	270
December	1st to 10th	Day	4 6% 29% 44% 16% 2	270
	11th to 20th	Day	6% 4 30% 45% 14%	270
	21st to end	Day	5% 5% 31% 48% 9% 3	297
January	1st to 10th	Night	4 3 32% 49% 11% 2	950
	11th to 20th	Night	4 3 34% 48% 9% 2	927
	21st to end	Night	5% 3 30% 51% 11% 1	836
February	1st to 10th	Night	6% 4 39% 44% 7%	760
	11th to 20th	Night	5% 3 37% 46% 8% 1	744
	21st to end	Night	5% 4 37% 50% 3	628
March	1st to 10th	Night	5% 3 37% 45% 9%	760
	11th to 20th	Night	10% 3 26% 50% 11%	760
	21st to end	Night	9% 5% 36% 43% 8%	684
April	1st to 10th	Night	10% 6% 37% 41% 6%	570
	11th to 20th	Night	17% 8% 27% 39% 9%	563
	21st to end	Night	17% 5% 29% 39% 11%	570
May	1st to 10th	Night	15% 5% 25% 42% 12% 1	570
	11th to 20th	Night	17% 4 14% 45% 19% 1	561
	21st to end	Night	10% 3 25% 41% 20% 1	617
June	1st to 10th	Night	5% 3 16% 47% 26% 2	570
	11th to 20th	Night	5% 3 22% 51% 18% 1	570
	21st to end	Night	5% 2 16% 50% 26% 1	570
July	1st to 10th	Night	8% 2 10% 42% 35% 3	555
	11th to 20th	Night	2 1 11% 45% 38% 3	552
	21st to end	Night	6% 2 8% 42% 40% 3	624
August	1st to 10th	Night	5% 1 9% 46% 36% 2	567
	11th to 20th	Night	7% 2 11% 52% 26% 2	564
	21st to end	Night	5% 3 12% 49% 29% 2	627
September	1st to 10th	Night	7% 2 11% 48% 29% 3	570
	11th to 20th	Night	7% 1 15% 50% 25% 2	570
	21st to end	Night	8% 2 16% 50% 22% 1	570
October	1st to 10th	Night	10% 3 20% 47% 18% 3	724
	11th to 20th	Night	5% 1 25% 48% 19% 1	760
	21st to end	Night	8% 3 23% 47% 16% 2	895
November	1st to 10th	Night	4 3 26% 52% 14% 1	928
	11th to 20th	Night	5% 3 29% 52% 10% 1	950
	21st to end	Night	6% 3 27% 49% 14% 1	950
December	1st to 10th	Night	4 3 30% 52% 11%	950
	11th to 20th	Night	5% 3 31% 49% 12% 1	950
	21st to end	Night	7% 4 34% 46% 8% 1	1045

Page generated at 2024-10-29 09:20:46. Version ac07

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