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 2026-09-14.

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% 3 4 4 4 6% 8% 54%	157747
	Night	5% 6% 2 5% 3 3 4 3 5% 7% 56%	150924
Transparency	Day	27% 5% 9% 23% 24% 12%	152710
	Night	26% 5% 10% 25% 25% 9%	146971
Seeing	Day	7% 7% 24% 33% 21% 8%	18166
	Night	7% 3 27% 46% 15% 1	28080

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 7% 3 6% 3 3 3 3 4 8% 59%	4720
	11th to 20th	Day	1 4 3 6% 3 3 4 3 4 6% 62%	4877
	21st to end	Day	1 4 3 7% 4 4 3 3 4 6% 61%	5844
February	1st to 10th	Day	2 8% 3 6% 4 4 3 3 5% 7% 55%	1890
	11th to 20th	Day	1 4 2 6% 4 5% 4 3 6% 8% 57%	2042
	21st to end	Day	2 7% 3 6% 3 3 2 3 5% 6% 61%	1730
March	1st to 10th	Day	3 10% 3 4 3 3 3 2 4 6% 59%	4600
	11th to 20th	Day	5% 8% 3 7% 3 4 4 4 5% 7% 51%	5011
	21st to end	Day	4 7% 2 4 4 4 4 4 5% 8% 54%	5965
April	1st to 10th	Day	7% 6% 2 4 3 4 3 3 7% 7% 53%	2520
	11th to 20th	Day	8% 8% 3 5% 3 4 5% 4 5% 7% 48%	2491
	21st to end	Day	9% 7% 1 4 4 5% 4 4 7% 9% 46%	2520
May	1st to 10th	Day	11% 7% 2 5% 4 4 4 4 6% 7% 46%	7467
	11th to 20th	Day	10% 7% 2 6% 4 4 5% 4 6% 7% 45%	8109
	21st to end	Day	7% 6% 2 5% 4 4 6% 5% 7% 10% 44%	9026
June	1st to 10th	Day	5% 4 2 5% 3 4 6% 5% 7% 9% 49%	2940
	11th to 20th	Day	5% 3 1 4 3 4 6% 5% 7% 7% 54%	2940
	21st to end	Day	4 3 1 5% 4 5% 6% 5% 8% 10% 51%	2940
July	1st to 10th	Day	3 3 1 3 3 4 5% 4 7% 8% 58%	8249
	11th to 20th	Day	2 2 1 4 4 4 5% 4 7% 9% 58%	8250
	21st to end	Day	5% 4 2 6% 5% 5% 6% 5% 8% 10% 46%	8824
August	1st to 10th	Day	7% 4 2 5% 4 5% 6% 5% 8% 9% 46%	7497
	11th to 20th	Day	3 3 1 4 4 5% 6% 5% 7% 10% 53%	7003
	21st to end	Day	6% 4 2 6% 4 5% 6% 5% 8% 11% 44%	7751
September	1st to 10th	Day	4 4 1 4 3 4 3 4 5% 7% 62%	2520
	11th to 20th	Day	6% 3 2 3 3 3 4 3 5% 7% 60%	2404
	21st to end	Day	6% 4 1 3 2 3 3 2 5% 6% 65%	2200
October	1st to 10th	Day	4 3 1 2 2 2 2 2 3 4 75%	3317
	11th to 20th	Day	4 5% 2 5% 3 3 3 3 4 7% 61%	3250
	21st to end	Day	5% 6% 1 4 2 2 3 2 4 6% 65%	3403
November	1st to 10th	Day	3 5% 1 3 3 2 2 3 6% 8% 64%	1853
	11th to 20th	Day	2 6% 1 4 2 3 3 2 5% 7% 63%	1890
	21st to end	Day	4 7% 2 5% 3 2 3 3 5% 7% 60%	1806
December	1st to 10th	Day	1 5% 2 4 3 3 2 2 4 5% 69%	2700
	11th to 20th	Day	2 4 1 4 3 3 3 2 5% 7% 67%	3106
	21st to end	Day	2 7% 3 7% 3 4 4 3 6% 7% 54%	4092
January	1st to 10th	Night	2 9% 4 7% 3 3 3 2 4 7% 56%	8201
	11th to 20th	Night	2 6% 3 7% 4 3 3 2 4 6% 61%	8158
	21st to end	Night	2 5% 3 6% 4 4 3 2 4 5% 61%	8370
February	1st to 10th	Night	2 8% 3 5% 4 4 4 3 4 8% 56%	2730
	11th to 20th	Night	2 8% 4 7% 4 4 3 3 6% 6% 52%	2463
	21st to end	Night	3 8% 2 5% 3 3 2 3 4 6% 61%	2076
March	1st to 10th	Night	5% 10% 3 5% 3 3 3 3 4 5% 57%	5494
	11th to 20th	Night	7% 9% 2 5% 3 3 4 3 4 5% 54%	5200
	21st to end	Night	7% 9% 3 6% 4 3 4 3 5% 7% 50%	5235
April	1st to 10th	Night	6% 7% 2 4 4 3 4 3 5% 6% 55%	2100
	11th to 20th	Night	11% 8% 3 4 3 4 4 3 5% 6% 50%	2077
	21st to end	Night	12% 8% 2 5% 4 3 3 3 4 6% 48%	2100
May	1st to 10th	Night	15% 8% 2 5% 3 4 4 3 5% 6% 44%	5477
	11th to 20th	Night	11% 7% 2 5% 3 4 4 3 5% 8% 47%	4779
	21st to end	Night	10% 7% 3 5% 4 4 4 3 6% 6% 50%	5181
June	1st to 10th	Night	6% 3 1 4 2 4 3 3 8% 10% 56%	1680
	11th to 20th	Night	5% 3 1 3 3 3 3 4 6% 8% 61%	1680
	21st to end	Night	4 3 1 5% 3 4 4 4 7% 8% 57%	1680
July	1st to 10th	Night	3 3 1 4 4 3 4 4 6% 8% 62%	4740
	11th to 20th	Night	3 3 1 4 3 4 5% 4 7% 9% 57%	4740
	21st to end	Night	4 3 1 6% 5% 5% 7% 5% 10% 10% 44%	5268
August	1st to 10th	Night	6% 4 2 5% 4 4 4 4 6% 8% 53%	5323
	11th to 20th	Night	4 3 2 5% 4 4 6% 5% 8% 11% 47%	5853
	21st to end	Night	6% 4 2 6% 4 5% 6% 5% 9% 11% 43%	6465
September	1st to 10th	Night	6% 4 2 4 3 3 4 4 5% 7% 58%	2100
	11th to 20th	Night	6% 3 1 5% 2 2 3 3 5% 7% 62%	2061
	21st to end	Night	7% 4 1 4 3 2 3 2 4 6% 64%	2200
October	1st to 10th	Night	5% 4 1 3 2 2 2 2 4 5% 69%	3869
	11th to 20th	Night	6% 4 1 5% 3 2 3 3 5% 7% 60%	3898
	21st to end	Night	4 7% 2 3 2 2 2 2 3 5% 66%	4363
November	1st to 10th	Night	3 4 2 4 3 3 3 2 4 6% 67%	2665
	11th to 20th	Night	3 5% 2 4 3 3 3 3 4 5% 66%	2730
	21st to end	Night	4 7% 2 5% 4 3 2 3 4 6% 60%	2809
December	1st to 10th	Night	1 4 2 4 2 2 2 2 3 5% 72%	4710
	11th to 20th	Night	1 5% 2 4 3 2 3 3 5% 8% 65%	5321
	21st to end	Night	4 9% 3 6% 4 4 3 3 4 7% 53%	7128

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	25% 3 6% 30% 27% 9%	4723
	11th to 20th	Day	24% 3 9% 25% 30% 10%	4877
	21st to end	Day	25% 3 7% 27% 27% 12%	5844
February	1st to 10th	Day	29% 3 9% 28% 21% 10%	1890
	11th to 20th	Day	24% 4 7% 29% 27% 9%	2052
	21st to end	Day	24% 3 5% 23% 29% 16%	1730
March	1st to 10th	Day	27% 3 5% 16% 31% 18%	4600
	11th to 20th	Day	31% 4 9% 22% 27% 7%	5011
	21st to end	Day	27% 5% 10% 35% 20% 3	5965
April	1st to 10th	Day	28% 5% 9% 26% 22% 10%	2520
	11th to 20th	Day	34% 5% 8% 21% 21% 11%	2494
	21st to end	Day	33% 6% 10% 24% 21% 5%	2520
May	1st to 10th	Day	34% 8% 8% 26% 18% 6%	7467
	11th to 20th	Day	36% 6% 9% 19% 22% 8%	8113
	21st to end	Day	32% 8% 13% 24% 17% 7%	9027
June	1st to 10th	Day	26% 8% 14% 23% 19% 10%	2940
	11th to 20th	Day	24% 7% 9% 19% 23% 18%	2940
	21st to end	Day	26% 7% 10% 19% 24% 14%	2940
July	1st to 10th	Day	21% 5% 6% 16% 29% 23%	8181
	11th to 20th	Day	20% 6% 9% 20% 22% 23%	8175
	21st to end	Day	31% 7% 14% 21% 15% 12%	8628
August	1st to 10th	Day	31% 7% 11% 18% 18% 15%	5937
	11th to 20th	Day	27% 8% 13% 20% 20% 10%	5519
	21st to end	Day	27% 7% 15% 24% 20% 9%	6078
September	1st to 10th	Day	23% 4 7% 21% 26% 19%	2520
	11th to 20th	Day	23% 5% 7% 20% 28% 17%	2404
	21st to end	Day	20% 4 6% 19% 36% 15%	2200
October	1st to 10th	Day	15% 2 4 16% 36% 27%	3317
	11th to 20th	Day	24% 3 6% 23% 33% 10%	3250
	21st to end	Day	22% 2 4 22% 37% 13%	3403
November	1st to 10th	Day	18% 3 8% 25% 30% 17%	1853
	11th to 20th	Day	21% 3 6% 25% 34% 11%	1890
	21st to end	Day	24% 2 8% 22% 29% 14%	1806
December	1st to 10th	Day	19% 2 7% 32% 30% 10%	2700
	11th to 20th	Day	17% 2 7% 30% 33% 11%	3104
	21st to end	Day	28% 3 6% 26% 27% 10%	4092
January	1st to 10th	Night	28% 3 5% 30% 27% 6%	8202
	11th to 20th	Night	25% 3 8% 25% 25% 14%	8158
	21st to end	Night	25% 4 8% 23% 28% 12%	8370
February	1st to 10th	Night	27% 3 9% 27% 27% 8%	2730
	11th to 20th	Night	30% 3 10% 27% 22% 8%	2463
	21st to end	Night	24% 3 4 26% 28% 15%	2076
March	1st to 10th	Night	29% 3 5% 16% 35% 12%	5494
	11th to 20th	Night	31% 4 9% 15% 28% 13%	5200
	21st to end	Night	33% 5% 8% 34% 18% 2	5235
April	1st to 10th	Night	28% 5% 12% 23% 22% 10%	2100
	11th to 20th	Night	34% 4 9% 23% 22% 8%	2077
	21st to end	Night	37% 4 10% 25% 18% 6%	2100
May	1st to 10th	Night	39% 7% 10% 27% 12% 5%	5475
	11th to 20th	Night	34% 6% 12% 24% 17% 6%	4780
	21st to end	Night	32% 7% 17% 25% 15% 4	5182
June	1st to 10th	Night	21% 6% 19% 31% 15% 8%	1680
	11th to 20th	Night	20% 5% 13% 22% 23% 17%	1680
	21st to end	Night	22% 6% 11% 25% 25% 11%	1680
July	1st to 10th	Night	20% 5% 9% 22% 37% 8%	4700
	11th to 20th	Night	21% 6% 13% 23% 23% 15%	4692
	21st to end	Night	29% 8% 19% 19% 17% 8%	5153
August	1st to 10th	Night	29% 6% 12% 25% 21% 6%	4207
	11th to 20th	Night	28% 10% 20% 21% 14% 7%	4613
	21st to end	Night	26% 8% 20% 25% 17% 4	5069
September	1st to 10th	Night	24% 4 10% 22% 26% 13%	2100
	11th to 20th	Night	21% 4 9% 24% 26% 16%	2061
	21st to end	Night	23% 3 7% 21% 33% 13%	2200
October	1st to 10th	Night	18% 3 6% 20% 31% 21%	3869
	11th to 20th	Night	24% 3 8% 26% 28% 10%	3898
	21st to end	Night	22% 2 4 25% 33% 14%	4363
November	1st to 10th	Night	18% 3 7% 25% 29% 17%	2665
	11th to 20th	Night	20% 3 5% 27% 33% 11%	2730
	21st to end	Night	25% 3 8% 23% 30% 11%	2809
December	1st to 10th	Night	17% 3 6% 34% 32% 8%	4710
	11th to 20th	Night	19% 3 6% 30% 36% 6%	5322
	21st to end	Night	31% 3 7% 24% 27% 8%	7128

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	3 9% 24% 47% 15% 3	330
	11th to 20th	Day	7% 7% 32% 42% 11% 2	330
	21st to end	Day	3 7% 31% 44% 12% 2	363
February	1st to 10th	Day	4 8% 42% 37% 8% 1	330
	11th to 20th	Day	2 6% 42% 38% 10% 1	330
	21st to end	Day	4 10% 37% 38% 10% 1	273
March	1st to 10th	Day	4 8% 33% 35% 16% 4	330
	11th to 20th	Day	12% 9% 35% 30% 11% 3	493
	21st to end	Day	11% 18% 31% 26% 12% 2	594
April	1st to 10th	Day	9% 15% 33% 28% 12% 3	540
	11th to 20th	Day	13% 20% 28% 26% 9% 3	532
	21st to end	Day	15% 12% 33% 24% 14% 2	540
May	1st to 10th	Day	15% 9% 29% 30% 14% 4	682
	11th to 20th	Day	14% 7% 23% 30% 21% 5%	775
	21st to end	Day	9% 9% 22% 32% 20% 7%	844
June	1st to 10th	Day	5% 5% 20% 33% 24% 13%	780
	11th to 20th	Day	6% 6% 25% 35% 22% 7%	780
	21st to end	Day	5% 5% 22% 35% 25% 7%	780
July	1st to 10th	Day	7% 3 15% 27% 34% 13%	771
	11th to 20th	Day	4 2 12% 28% 40% 14%	768
	21st to end	Day	6% 3 11% 27% 38% 16%	804
August	1st to 10th	Day	7% 3 18% 31% 27% 14%	567
	11th to 20th	Day	5% 3 17% 33% 27% 16%	557
	21st to end	Day	5% 4 19% 31% 29% 13%	627
September	1st to 10th	Day	6% 5% 15% 35% 28% 12%	570
	11th to 20th	Day	6% 5% 22% 31% 26% 11%	510
	21st to end	Day	6% 3 20% 28% 26% 17%	330
October	1st to 10th	Day	10% 4 19% 23% 27% 16%	330
	11th to 20th	Day	4 8% 23% 38% 19% 8%	330
	21st to end	Day	9% 9% 23% 37% 19% 3	363
November	1st to 10th	Day	6% 9% 23% 39% 17% 6%	330
	11th to 20th	Day	5% 5% 22% 45% 19% 4	330
	21st to end	Day	8% 4 31% 39% 14% 5%	330
December	1st to 10th	Day	4 6% 28% 44% 15% 3	330
	11th to 20th	Day	5% 5% 33% 43% 13% 1	330
	21st to end	Day	4 6% 33% 45% 9% 2	363
January	1st to 10th	Night	4 3 34% 47% 10% 2	1050
	11th to 20th	Night	4 3 36% 47% 8% 1	1023
	21st to end	Night	4 3 33% 50% 10% 1	924
February	1st to 10th	Night	6% 3 41% 43% 7%	840
	11th to 20th	Night	5% 3 39% 45% 7%	824
	21st to end	Night	5% 4 39% 48% 3	692
March	1st to 10th	Night	5% 3 37% 45% 8%	840
	11th to 20th	Night	10% 4 29% 47% 10%	840
	21st to end	Night	9% 5% 36% 43% 8%	756
April	1st to 10th	Night	9% 5% 38% 41% 6%	630
	11th to 20th	Night	15% 7% 29% 39% 9%	623
	21st to end	Night	17% 5% 30% 38% 10%	630
May	1st to 10th	Night	16% 5% 25% 41% 12% 1	630
	11th to 20th	Night	17% 4 15% 45% 18% 1	621
	21st to end	Night	11% 3 27% 40% 19% 1	683
June	1st to 10th	Night	6% 3 17% 46% 25% 2	630
	11th to 20th	Night	5% 3 23% 50% 17% 1	630
	21st to end	Night	5% 2 18% 49% 25% 1	630
July	1st to 10th	Night	8% 2 12% 44% 32% 2	615
	11th to 20th	Night	3 1 12% 46% 36% 3	612
	21st to end	Night	6% 1 8% 42% 39% 3	690
August	1st to 10th	Night	5% 1 11% 48% 34% 2	627
	11th to 20th	Night	6% 2 12% 51% 27% 2	624
	21st to end	Night	5% 3 12% 49% 28% 2	693
September	1st to 10th	Night	7% 2 12% 49% 28% 3	630
	11th to 20th	Night	7% 1 15% 50% 24% 2	609
	21st to end	Night	9% 2 16% 50% 23% 1	600
October	1st to 10th	Night	10% 3 20% 47% 17% 3	762
	11th to 20th	Night	5% 2 27% 47% 18% 1	800
	21st to end	Night	8% 3 24% 48% 15% 2	942
November	1st to 10th	Night	5% 3 28% 50% 13% 1	1025
	11th to 20th	Night	5% 3 31% 51% 9% 1	1050
	21st to end	Night	6% 3 29% 48% 13% 1	1050
December	1st to 10th	Night	4 3 32% 50% 10%	1050
	11th to 20th	Night	4 3 35% 46% 11% 1	1050
	21st to end	Night	6% 4 37% 44% 8% 1	1155

Page generated at 2026-09-11 02:11:57. Version ac07

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