Centennial Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Centennial Observatory clear sky chart from 2005-11-06 to 2025-12-03.

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% 6% 2 5% 3 3 4 3 5% 7% 57%	135583
	Night	6% 6% 2 6% 4 4 4 3 5% 7% 52%	132490
Transparency	Day	28% 4 9% 22% 21% 15%	130357
	Night	31% 5% 11% 22% 19% 12%	128727
Seeing	Day	8% 10% 24% 29% 22% 8%	15873
	Night	9% 7% 24% 37% 20% 3	25337

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	8% 16% 4 11% 6% 5% 5% 5% 6% 8% 27%	3639
	11th to 20th	Day	5% 10% 5% 13% 6% 5% 4 3 5% 6% 39%	3640
	21st to end	Day	6% 9% 3 7% 5% 5% 4 4 5% 5% 48%	4205
February	1st to 10th	Day	8% 10% 5% 9% 4 5% 4 4 7% 9% 37%	1800
	11th to 20th	Day	9% 11% 4 7% 4 5% 5% 3 5% 7% 40%	1935
	21st to end	Day	5% 12% 4 9% 5% 5% 4 3 5% 7% 41%	1650
March	1st to 10th	Day	8% 12% 5% 8% 4 5% 5% 3 5% 6% 38%	4518
	11th to 20th	Day	6% 9% 3 6% 4 4 4 3 6% 6% 51%	4995
	21st to end	Day	8% 7% 3 8% 5% 5% 6% 4 7% 8% 38%	5486
April	1st to 10th	Day	8% 7% 2 6% 5% 5% 4 3 6% 9% 45%	2384
	11th to 20th	Day	6% 7% 2 6% 4 4 4 4 6% 7% 50%	2482
	21st to end	Day	7% 5% 2 6% 4 4 4 4 6% 7% 51%	2600
May	1st to 10th	Day	6% 7% 2 5% 4 4 4 3 5% 6% 54%	5880
	11th to 20th	Day	5% 5% 2 4 3 3 4 4 5% 7% 60%	5841
	21st to end	Day	4 3 1 4 3 3 4 3 5% 7% 63%	6638
June	1st to 10th	Day	5% 5% 2 4 4 4 4 4 7% 9% 54%	2800
	11th to 20th	Day	5% 3 1 3 3 3 4 4 5% 6% 63%	2800
	21st to end	Day	1 2 1 2 2 2 3 3 5% 6% 74%	2800
July	1st to 10th	Day	1 1 1 2 1 2 3 2 3 5% 77%	6319
	11th to 20th	Day	1 2 1 3 2 3 3 2 4 5% 75%	6320
	21st to end	Day	2 2 1 3 2 3 3 3 6% 8% 68%	6708
August	1st to 10th	Day	3 2 1 3 3 3 4 3 5% 7% 65%	5779
	11th to 20th	Day	1 1 1 3 2 2 3 2 4 5% 76%	5798
	21st to end	Day	4 3 1 4 3 3 3 3 4 5% 68%	6217
September	1st to 10th	Day	2 2 1 3 2 2 3 3 4 6% 72%	2330
	11th to 20th	Day	4 4 1 3 2 3 4 2 4 6% 68%	2200
	21st to end	Day	6% 3 1 3 2 3 3 2 3 5% 68%	2200
October	1st to 10th	Day	5% 3 1 3 2 3 3 3 4 5% 68%	3600
	11th to 20th	Day	6% 4 2 4 3 2 3 2 4 7% 65%	3237
	21st to end	Day	5% 3 1 4 3 3 4 4 6% 9% 58%	3200
November	1st to 10th	Day	8% 7% 2 6% 5% 4 3 3 5% 7% 50%	1854
	11th to 20th	Day	8% 6% 3 6% 4 5% 5% 4 7% 8% 44%	1890
	21st to end	Day	6% 9% 4 7% 5% 5% 4 4 5% 6% 45%	1890
December	1st to 10th	Day	6% 7% 3 6% 3 3 3 2 3 6% 57%	2916
	11th to 20th	Day	10% 10% 4 7% 5% 5% 5% 5% 7% 10% 34%	3028
	21st to end	Day	12% 12% 4 8% 5% 4 4 4 6% 7% 34%	4004
January	1st to 10th	Night	10% 12% 4 12% 6% 5% 5% 4 6% 7% 28%	6306
	11th to 20th	Night	9% 12% 4 10% 5% 5% 5% 4 5% 7% 35%	6320
	21st to end	Night	7% 9% 3 7% 4 4 4 3 5% 6% 48%	6759
February	1st to 10th	Night	10% 10% 4 8% 6% 6% 4 4 7% 8% 34%	2600
	11th to 20th	Night	9% 9% 3 7% 4 4 4 3 6% 8% 44%	2348
	21st to end	Night	6% 9% 2 6% 5% 5% 5% 5% 7% 8% 41%	1980
March	1st to 10th	Night	8% 11% 3 6% 4 4 4 4 6% 7% 43%	5376
	11th to 20th	Night	6% 8% 3 7% 4 5% 5% 4 6% 7% 46%	5005
	21st to end	Night	6% 7% 2 6% 5% 5% 6% 5% 7% 9% 42%	5489
April	1st to 10th	Night	7% 7% 3 5% 5% 5% 5% 4 6% 7% 46%	2013
	11th to 20th	Night	6% 8% 2 5% 4 5% 5% 4 5% 6% 51%	1866
	21st to end	Night	5% 6% 2 6% 4 4 4 3 6% 8% 52%	1800
May	1st to 10th	Night	8% 5% 1 5% 3 4 3 3 6% 7% 55%	4100
	11th to 20th	Night	7% 4 1 5% 4 3 4 3 5% 7% 58%	4074
	21st to end	Night	3 4 1 5% 3 4 4 3 6% 7% 60%	4244
June	1st to 10th	Night	4 3 1 4 4 5% 5% 6% 7% 8% 54%	1600
	11th to 20th	Night	4 3 2 3 3 3 3 4 5% 6% 64%	1600
	21st to end	Night	2 2 1 3 1 2 3 2 4 6% 74%	1600
July	1st to 10th	Night	2 1 1 2 2 3 3 3 3 5% 75%	3640
	11th to 20th	Night	1 1 2 2 2 3 2 4 5% 78%	3640
	21st to end	Night	1 2 1 3 3 2 4 4 7% 7% 66%	4146
August	1st to 10th	Night	2 2 1 4 3 3 4 3 5% 6% 67%	4033
	11th to 20th	Night	2 2 1 3 3 3 3 3 5% 7% 71%	4051
	21st to end	Night	3 2 1 4 3 4 4 3 5% 5% 67%	4756
September	1st to 10th	Night	3 2 1 3 2 2 4 3 3 5% 72%	2070
	11th to 20th	Night	4 3 1 4 3 3 3 3 5% 6% 67%	2200
	21st to end	Night	5% 3 1 4 3 2 3 2 4 4 69%	2200
October	1st to 10th	Night	4 2 1 3 2 2 3 2 4 6% 72%	3600
	11th to 20th	Night	6% 5% 2 5% 4 3 3 2 4 7% 59%	3907
	21st to end	Night	5% 4 2 4 4 4 5% 4 6% 7% 55%	4600
November	1st to 10th	Night	9% 6% 2 5% 4 4 4 4 6% 6% 48%	2665
	11th to 20th	Night	8% 6% 2 7% 5% 5% 4 4 6% 8% 45%	2730
	21st to end	Night	7% 6% 2 6% 5% 5% 4 4 5% 9% 46%	2730
December	1st to 10th	Night	6% 6% 2 5% 3 3 3 3 4 7% 58%	4314
	11th to 20th	Night	11% 12% 4 8% 4 5% 5% 4 7% 9% 31%	5176
	21st to end	Night	11% 10% 4 8% 5% 6% 6% 4 7% 7% 32%	6952

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	54% 7% 18% 19% 1	3639
	11th to 20th	Day	44% 8% 18% 22% 6% 1	3640
	21st to end	Day	38% 6% 15% 27% 13% 1	4205
February	1st to 10th	Day	46% 8% 18% 21% 6% 1	1800
	11th to 20th	Day	45% 7% 15% 22% 10% 1	1945
	21st to end	Day	44% 6% 13% 23% 12% 2	1650
March	1st to 10th	Day	44% 6% 8% 29% 9% 4	4518
	11th to 20th	Day	33% 4 10% 33% 19% 1	4995
	21st to end	Day	42% 7% 16% 24% 9% 1	5486
April	1st to 10th	Day	38% 5% 13% 24% 17% 4	2384
	11th to 20th	Day	33% 6% 9% 29% 19% 4	2485
	21st to end	Day	32% 5% 12% 27% 22% 2	2600
May	1st to 10th	Day	31% 6% 6% 22% 31% 5%	5880
	11th to 20th	Day	23% 4 7% 28% 30% 7%	5844
	21st to end	Day	21% 5% 7% 30% 29% 8%	6639
June	1st to 10th	Day	26% 5% 12% 30% 23% 4	2800
	11th to 20th	Day	23% 4 8% 21% 32% 12%	2800
	21st to end	Day	12% 3 5% 21% 37% 23%	2800
July	1st to 10th	Day	11% 2 2 11% 20% 53%	6251
	11th to 20th	Day	13% 2 2 6% 22% 55%	6245
	21st to end	Day	14% 3 3 13% 34% 34%	6513
August	1st to 10th	Day	19% 3 4 14% 24% 36%	4208
	11th to 20th	Day	11% 2 3 11% 38% 35%	4197
	21st to end	Day	15% 2 2 12% 36% 32%	4484
September	1st to 10th	Day	15% 3 5% 15% 30% 32%	2330
	11th to 20th	Day	19% 3 6% 24% 28% 20%	2200
	21st to end	Day	21% 3 5% 21% 35% 16%	2200
October	1st to 10th	Day	20% 3 7% 18% 34% 19%	3600
	11th to 20th	Day	22% 2 6% 34% 28% 8%	3237
	21st to end	Day	24% 3 10% 35% 22% 6%	3200
November	1st to 10th	Day	34% 6% 12% 29% 17% 2	1854
	11th to 20th	Day	36% 7% 18% 31% 9% 1	1890
	21st to end	Day	39% 5% 18% 27% 11%	1890
December	1st to 10th	Day	30% 5% 17% 40% 7% 1	2916
	11th to 20th	Day	45% 6% 25% 22% 2	3028
	21st to end	Day	49% 6% 16% 21% 8%	4004
January	1st to 10th	Night	53% 7% 18% 20% 3	6306
	11th to 20th	Night	49% 6% 17% 22% 6% 1	6320
	21st to end	Night	38% 5% 15% 28% 13% 1	6759
February	1st to 10th	Night	47% 8% 18% 21% 6% 1	2600
	11th to 20th	Night	39% 5% 17% 25% 12% 1	2349
	21st to end	Night	37% 7% 15% 28% 11% 1	1980
March	1st to 10th	Night	38% 6% 9% 29% 15% 2	5376
	11th to 20th	Night	36% 5% 10% 27% 22% 1	5005
	21st to end	Night	35% 6% 18% 27% 12% 1	5489
April	1st to 10th	Night	35% 6% 11% 26% 17% 5%	2013
	11th to 20th	Night	34% 5% 9% 27% 21% 3	1866
	21st to end	Night	30% 5% 11% 28% 25% 2	1800
May	1st to 10th	Night	27% 6% 8% 20% 32% 7%	4097
	11th to 20th	Night	26% 4 6% 24% 32% 9%	4075
	21st to end	Night	23% 4 7% 23% 36% 7%	4244
June	1st to 10th	Night	24% 6% 14% 26% 25% 5%	1600
	11th to 20th	Night	20% 4 7% 21% 33% 15%	1600
	21st to end	Night	13% 2 4 16% 32% 32%	1600
July	1st to 10th	Night	12% 2 2 8% 20% 56%	3600
	11th to 20th	Night	10% 2 2 5% 17% 65%	3592
	21st to end	Night	14% 3 3 10% 34% 35%	4029
August	1st to 10th	Night	19% 2 5% 11% 22% 41%	2928
	11th to 20th	Night	14% 4 2 13% 30% 37%	2928
	21st to end	Night	15% 2 3 11% 31% 38%	3427
September	1st to 10th	Night	15% 3 3 12% 32% 34%	2070
	11th to 20th	Night	18% 4 5% 23% 28% 23%	2200
	21st to end	Night	19% 4 5% 17% 36% 20%	2200
October	1st to 10th	Night	16% 2 5% 16% 34% 26%	3600
	11th to 20th	Night	27% 3 5% 26% 30% 10%	3907
	21st to end	Night	27% 4 11% 31% 23% 4	4600
November	1st to 10th	Night	35% 5% 13% 25% 19% 4	2665
	11th to 20th	Night	36% 5% 15% 30% 13% 1	2730
	21st to end	Night	34% 6% 17% 31% 12% 1	2730
December	1st to 10th	Night	29% 4 18% 40% 7% 1	4314
	11th to 20th	Night	49% 6% 20% 23% 3	5176
	21st to end	Night	48% 6% 18% 20% 8%	6952

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	19% 14% 25% 24% 18% 2	200
	11th to 20th	Day	10% 11% 28% 28% 18% 7%	200
	21st to end	Day	14% 10% 22% 27% 21% 5%	263
February	1st to 10th	Day	19% 14% 33% 22% 11% 1	300
	11th to 20th	Day	15% 16% 31% 24% 11% 2	480
	21st to end	Day	9% 19% 32% 28% 11% 1	414
March	1st to 10th	Day	12% 14% 34% 26% 13% 2	500
	11th to 20th	Day	10% 13% 31% 28% 15% 3	500
	21st to end	Day	15% 17% 31% 26% 10% 2	550
April	1st to 10th	Day	12% 18% 28% 22% 16% 5%	500
	11th to 20th	Day	10% 20% 29% 25% 13% 3	492
	21st to end	Day	9% 15% 33% 23% 16% 4	500
May	1st to 10th	Day	5% 15% 25% 30% 19% 5%	503
	11th to 20th	Day	12% 12% 22% 29% 17% 8%	528
	21st to end	Day	6% 9% 21% 26% 24% 14%	573
June	1st to 10th	Day	5% 9% 21% 31% 19% 14%	530
	11th to 20th	Day	7% 9% 30% 31% 17% 7%	530
	21st to end	Day	2 5% 24% 34% 25% 9%	530
July	1st to 10th	Day	2 2 18% 39% 29% 9%	526
	11th to 20th	Day	1 2 18% 42% 31% 6%	524
	21st to end	Day	3 2 13% 35% 34% 13%	575
August	1st to 10th	Day	5% 2 20% 34% 30% 9%	527
	11th to 20th	Day	3 4 17% 30% 32% 15%	517
	21st to end	Day	2 4 19% 29% 31% 15%	583
September	1st to 10th	Day	2 3 16% 30% 29% 19%	530
	11th to 20th	Day	5% 5% 18% 31% 26% 15%	530
	21st to end	Day	7% 6% 19% 29% 27% 11%	530
October	1st to 10th	Day	9% 6% 21% 27% 24% 14%	530
	11th to 20th	Day	9% 10% 20% 21% 30% 10%	343
	21st to end	Day	10% 9% 22% 33% 22% 4	363
November	1st to 10th	Day	13% 8% 28% 25% 22% 3	330
	11th to 20th	Day	10% 11% 20% 30% 23% 6%	330
	21st to end	Day	12% 10% 19% 28% 25% 6%	330
December	1st to 10th	Day	16% 11% 23% 26% 17% 8%	292
	11th to 20th	Day	16% 11% 26% 30% 15% 4	200
	21st to end	Day	20% 13% 22% 26% 14% 5%	220
January	1st to 10th	Night	16% 9% 23% 32% 19% 1	800
	11th to 20th	Night	14% 10% 21% 34% 18% 2	800
	21st to end	Night	14% 7% 22% 33% 21% 2	880
February	1st to 10th	Night	15% 10% 30% 31% 13%	800
	11th to 20th	Night	14% 8% 31% 34% 13% 1	784
	21st to end	Night	11% 10% 31% 33% 14% 1	660
March	1st to 10th	Night	11% 9% 32% 32% 15% 2	800
	11th to 20th	Night	9% 9% 32% 35% 14% 2	800
	21st to end	Night	13% 10% 33% 33% 10% 1	880
April	1st to 10th	Night	10% 10% 29% 35% 15% 2	800
	11th to 20th	Night	8% 11% 34% 33% 12% 1	678
	21st to end	Night	7% 9% 32% 36% 15% 2	600
May	1st to 10th	Night	9% 8% 29% 34% 18% 2	600
	11th to 20th	Night	8% 7% 20% 39% 23% 4	591
	21st to end	Night	9% 5% 24% 34% 24% 4	550
June	1st to 10th	Night	6% 5% 22% 39% 25% 4	400
	11th to 20th	Night	4 4 28% 47% 17% 1	400
	21st to end	Night	2 2 21% 50% 22% 3	400
July	1st to 10th	Night	2 1 12% 60% 23% 2	390
	11th to 20th	Night	1 20% 53% 25% 1	388
	21st to end	Night	2 1 10% 50% 34% 3	513
August	1st to 10th	Night	3 2 14% 50% 29% 2	597
	11th to 20th	Night	2 2 14% 49% 30% 3	594
	21st to end	Night	3 2 16% 46% 29% 3	764
September	1st to 10th	Night	4 1 14% 44% 31% 5%	800
	11th to 20th	Night	4 3 15% 41% 31% 6%	800
	21st to end	Night	6% 4 16% 39% 27% 8%	800
October	1st to 10th	Night	8% 5% 21% 40% 24% 4	800
	11th to 20th	Night	9% 6% 22% 35% 25% 5%	800
	21st to end	Night	7% 7% 25% 36% 21% 3	880
November	1st to 10th	Night	11% 6% 25% 34% 22% 2	820
	11th to 20th	Night	11% 8% 26% 38% 15% 2	840
	21st to end	Night	10% 8% 26% 33% 20% 4	840
December	1st to 10th	Night	13% 7% 27% 33% 17% 3	808
	11th to 20th	Night	19% 6% 23% 35% 15% 3	800
	21st to end	Night	19% 11% 28% 31% 10% 1	880

Page generated at 2025-11-29 23:00:55. Version ac07

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