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 2025-05-11.

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	4 5% 2 5% 3 4 4 4 6% 8% 55%	117761
	Night	5% 6% 2 5% 3 3 3 3 5% 7% 57%	120068
Transparency	Day	26% 5% 9% 23% 25% 12%	117505
	Night	26% 4 9% 25% 26% 10%	119915
Seeing	Day	8% 7% 24% 33% 21% 8%	16210
	Night	7% 3 26% 47% 16% 1	26355

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 4 9% 59%	3638
	11th to 20th	Day	1 6% 3 8% 4 4 4 3 4 7% 56%	3749
	21st to end	Day	2 5% 3 7% 3 4 3 3 4 6% 61%	4510
February	1st to 10th	Day	3 8% 3 6% 4 4 3 3 5% 7% 55%	1800
	11th to 20th	Day	1 4 2 6% 4 5% 4 3 6% 7% 58%	1942
	21st to end	Day	2 7% 3 6% 3 3 2 3 5% 6% 60%	1650
March	1st to 10th	Day	3 10% 3 4 3 3 3 2 4 6% 59%	4500
	11th to 20th	Day	5% 8% 3 7% 3 4 4 4 5% 7% 50%	4903
	21st to end	Day	4 7% 2 4 4 4 4 4 5% 8% 54%	5836
April	1st to 10th	Day	7% 7% 2 5% 3 4 3 3 7% 7% 53%	2400
	11th to 20th	Day	9% 8% 3 5% 3 4 5% 4 5% 7% 48%	2371
	21st to end	Day	9% 7% 1 4 4 5% 4 4 6% 9% 46%	2400
May	1st to 10th	Day	9% 7% 3 7% 5% 5% 5% 5% 7% 8% 41%	4143
	11th to 20th	Day	10% 6% 2 5% 4 5% 5% 5% 7% 8% 43%	4344
	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	4 3 1 2 1 2 2 2 3 4 76%	3215
	11th to 20th	Day	4 5% 2 5% 3 3 3 3 4 7% 60%	3150
	21st to end	Day	5% 6% 1 4 2 2 3 2 4 6% 65%	3297
November	1st to 10th	Day	3 6% 1 3 3 2 2 3 6% 8% 63%	1763
	11th to 20th	Day	2 6% 1 5% 3 3 3 2 5% 7% 64%	1800
	21st to end	Day	4 7% 2 5% 3 2 3 3 5% 7% 61%	1720
December	1st to 10th	Day	1 5% 2 4 3 3 2 2 4 5% 69%	2620
	11th to 20th	Day	2 4 1 4 3 3 3 2 5% 7% 67%	3026
	21st to end	Day	2 7% 3 7% 4 4 4 3 6% 7% 54%	4004
January	1st to 10th	Night	2 9% 4 8% 3 3 3 2 5% 7% 55%	6327
	11th to 20th	Night	1 6% 2 7% 4 3 4 3 4 7% 59%	6276
	21st to end	Night	3 6% 3 6% 4 4 3 2 4 5% 61%	6468
February	1st to 10th	Night	2 8% 3 5% 4 4 4 3 4 7% 56%	2600
	11th to 20th	Night	2 8% 4 7% 5% 4 3 3 6% 6% 51%	2343
	21st to end	Night	3 8% 2 5% 3 3 2 3 4 7% 60%	1980
March	1st to 10th	Night	5% 10% 3 5% 3 3 3 3 4 5% 56%	5374
	11th to 20th	Night	7% 9% 2 5% 3 3 4 3 4 5% 53%	5088
	21st to end	Night	7% 9% 3 6% 4 3 4 3 5% 6% 50%	5122
April	1st to 10th	Night	6% 8% 2 5% 4 3 4 3 5% 6% 55%	2000
	11th to 20th	Night	12% 8% 3 4 3 4 4 3 5% 6% 50%	1977
	21st to end	Night	12% 8% 2 5% 4 3 3 3 4 6% 48%	2000
May	1st to 10th	Night	12% 7% 2 6% 4 4 3 4 5% 7% 47%	3044
	11th to 20th	Night	12% 6% 2 5% 3 3 4 4 7% 9% 45%	2563
	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	5% 4 1 3 2 2 2 2 4 5% 70%	3751
	11th to 20th	Night	7% 4 1 5% 3 2 3 3 5% 7% 60%	3778
	21st to end	Night	4 8% 2 3 2 2 3 2 3 5% 66%	4228
November	1st to 10th	Night	3 4 2 4 3 3 3 2 5% 6% 66%	2535
	11th to 20th	Night	3 5% 2 4 3 3 3 3 4 5% 66%	2600
	21st to end	Night	4 7% 2 4 4 3 3 3 4 6% 61%	2675
December	1st to 10th	Night	1 4 2 4 2 2 2 2 3 5% 72%	4570
	11th to 20th	Night	1 5% 2 4 3 2 3 3 5% 8% 64%	5181
	21st to end	Night	4 9% 3 6% 4 4 3 3 5% 7% 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 7% 33% 25% 10%	3639
	11th to 20th	Day	28% 4 9% 21% 29% 10%	3749
	21st to end	Day	24% 4 7% 27% 25% 12%	4510
February	1st to 10th	Day	29% 3 10% 27% 21% 10%	1800
	11th to 20th	Day	24% 4 7% 29% 27% 8%	1952
	21st to end	Day	25% 4 6% 23% 27% 16%	1650
March	1st to 10th	Day	27% 3 5% 17% 31% 18%	4500
	11th to 20th	Day	31% 4 9% 22% 27% 6%	4903
	21st to end	Day	27% 5% 10% 36% 20% 3	5836
April	1st to 10th	Day	28% 5% 10% 26% 21% 10%	2400
	11th to 20th	Day	35% 5% 8% 21% 20% 10%	2374
	21st to end	Day	33% 6% 10% 25% 22% 4	2400
May	1st to 10th	Day	34% 10% 7% 24% 17% 8%	4143
	11th to 20th	Day	35% 8% 11% 24% 18% 3	4348
	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	14% 2 4 16% 36% 27%	3215
	11th to 20th	Day	25% 3 7% 23% 33% 9%	3150
	21st to end	Day	22% 3 4 22% 36% 13%	3297
November	1st to 10th	Day	19% 3 8% 25% 29% 17%	1763
	11th to 20th	Day	21% 3 6% 24% 34% 12%	1800
	21st to end	Day	24% 2 7% 22% 30% 15%	1720
December	1st to 10th	Day	19% 2 7% 32% 30% 10%	2620
	11th to 20th	Day	18% 2 7% 29% 33% 11%	3024
	21st to end	Day	29% 3 6% 26% 27% 9%	4004
January	1st to 10th	Night	28% 4 6% 29% 26% 7%	6327
	11th to 20th	Night	26% 3 10% 24% 24% 14%	6276
	21st to end	Night	25% 5% 9% 25% 25% 12%	6468
February	1st to 10th	Night	27% 3 9% 27% 25% 8%	2600
	11th to 20th	Night	31% 3 9% 28% 22% 7%	2343
	21st to end	Night	25% 3 4 26% 27% 15%	1980
March	1st to 10th	Night	29% 3 5% 16% 35% 12%	5374
	11th to 20th	Night	32% 4 8% 14% 29% 13%	5088
	21st to end	Night	33% 5% 8% 34% 18% 2	5122
April	1st to 10th	Night	29% 5% 11% 23% 22% 10%	2000
	11th to 20th	Night	35% 4 9% 24% 20% 8%	1977
	21st to end	Night	37% 4 10% 26% 18% 5%	2000
May	1st to 10th	Night	34% 8% 10% 24% 14% 10%	3044
	11th to 20th	Night	33% 7% 16% 24% 14% 5%	2564
	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	18% 3 6% 20% 31% 21%	3751
	11th to 20th	Night	24% 3 8% 26% 28% 10%	3778
	21st to end	Night	22% 2 4 25% 32% 14%	4228
November	1st to 10th	Night	19% 4 7% 25% 29% 16%	2535
	11th to 20th	Night	20% 3 5% 27% 33% 12%	2600
	21st to end	Night	25% 3 7% 23% 31% 11%	2675
December	1st to 10th	Night	16% 3 6% 34% 33% 8%	4570
	11th to 20th	Night	19% 3 6% 30% 36% 6%	5182
	21st to end	Night	32% 3 7% 24% 27% 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	2 7% 24% 47% 15% 3	300
	11th to 20th	Day	7% 7% 30% 43% 11% 2	300
	21st to end	Day	4 6% 31% 43% 14% 2	330
February	1st to 10th	Day	4 8% 43% 36% 7% 1	300
	11th to 20th	Day	3 6% 42% 38% 10% 1	300
	21st to end	Day	4 11% 36% 37% 11% 1	249
March	1st to 10th	Day	4 9% 31% 35% 16% 4	300
	11th to 20th	Day	13% 8% 34% 31% 12% 3	455
	21st to end	Day	12% 18% 30% 27% 11% 2	550
April	1st to 10th	Day	10% 14% 34% 28% 12% 3	500
	11th to 20th	Day	14% 20% 27% 26% 10% 3	492
	21st to end	Day	15% 11% 32% 25% 15% 2	500
May	1st to 10th	Day	14% 9% 29% 30% 14% 3	635
	11th to 20th	Day	14% 7% 22% 29% 22% 6%	676
	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	6% 9% 22% 38% 18% 6%	300
	11th to 20th	Day	6% 6% 22% 45% 18% 4	300
	21st to end	Day	8% 4 30% 40% 14% 5%	300
December	1st to 10th	Day	4 6% 27% 44% 16% 4	300
	11th to 20th	Day	6% 5% 31% 45% 13%	300
	21st to end	Day	5% 6% 32% 46% 9% 2	330
January	1st to 10th	Night	4 3 34% 48% 10% 2	1000
	11th to 20th	Night	4 3 35% 47% 9% 2	975
	21st to end	Night	5% 3 31% 51% 10% 1	880
February	1st to 10th	Night	6% 3 41% 42% 7%	800
	11th to 20th	Night	5% 3 39% 46% 7% 1	784
	21st to end	Night	6% 4 38% 49% 3	660
March	1st to 10th	Night	5% 4 37% 45% 8%	800
	11th to 20th	Night	10% 4 27% 49% 10%	800
	21st to end	Night	9% 5% 36% 42% 8%	720
April	1st to 10th	Night	10% 6% 38% 41% 6%	600
	11th to 20th	Night	16% 7% 28% 39% 9%	593
	21st to end	Night	16% 5% 30% 39% 10%	600
May	1st to 10th	Night	15% 5% 25% 41% 13% 1	600
	11th to 20th	Night	17% 4 14% 44% 19% 1	563
	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% 48% 16% 2	895
November	1st to 10th	Night	5% 3 27% 51% 14% 1	975
	11th to 20th	Night	5% 3 30% 51% 9% 1	1000
	21st to end	Night	6% 3 28% 49% 14% 1	1000
December	1st to 10th	Night	4 3 30% 52% 11%	1000
	11th to 20th	Night	5% 3 33% 47% 11% 1	1000
	21st to end	Night	6% 4 35% 46% 8% 1	1100

Page generated at 2025-05-08 07:33:57. Version ac07

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