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 2024-11-10.

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% 4 4 4 3 5% 7% 56%	105130
	Night	6% 6% 2 6% 4 4 4 4 5% 7% 51%	102720
Transparency	Day	28% 5% 9% 21% 21% 15%	104878
	Night	32% 5% 11% 21% 19% 12%	102562
Seeing	Day	8% 9% 24% 29% 22% 8%	14516
	Night	9% 6% 24% 37% 20% 3	23983

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% 4 4 6% 7% 29%	2539
	11th to 20th	Day	8% 14% 6% 13% 7% 6% 5% 3 5% 6% 28%	2540
	21st to end	Day	8% 11% 4 9% 6% 6% 5% 5% 6% 7% 33%	2925
February	1st to 10th	Day	8% 10% 5% 9% 4 5% 3 4 6% 9% 38%	1710
	11th to 20th	Day	8% 11% 4 7% 4 5% 6% 3 5% 7% 41%	1835
	21st to end	Day	5% 12% 4 9% 5% 5% 5% 3 5% 7% 39%	1570
March	1st to 10th	Day	8% 12% 6% 9% 5% 5% 5% 3 6% 7% 34%	3138
	11th to 20th	Day	5% 7% 2 6% 3 3 3 3 5% 6% 55%	3485
	21st to end	Day	9% 8% 3 8% 5% 5% 6% 5% 7% 8% 36%	3825
April	1st to 10th	Day	8% 7% 3 6% 5% 5% 4 3 6% 9% 44%	2265
	11th to 20th	Day	6% 7% 2 6% 5% 4 4 4 6% 8% 49%	2356
	21st to end	Day	7% 6% 2 6% 4 4 4 3 6% 7% 51%	2470
May	1st to 10th	Day	6% 8% 2 6% 4 5% 5% 4 5% 7% 48%	4110
	11th to 20th	Day	4 3 1 3 2 2 3 3 5% 7% 67%	4071
	21st to end	Day	5% 4 2 5% 4 4 4 4 6% 8% 55%	4627
June	1st to 10th	Day	5% 5% 2 4 4 4 4 4 7% 9% 52%	2660
	11th to 20th	Day	5% 3 1 3 3 3 4 4 4 6% 62%	2660
	21st to end	Day	1 2 1 2 2 2 3 3 4 6% 74%	2660
July	1st to 10th	Day	1 1 1 2 1 2 2 2 3 4 80%	4420
	11th to 20th	Day	2 2 1 4 3 3 3 3 5% 6% 70%	4420
	21st to end	Day	3 2 1 3 2 3 4 3 5% 6% 68%	4670
August	1st to 10th	Day	3 3 1 3 3 4 5% 3 6% 7% 63%	4092
	11th to 20th	Day	2 1 3 2 2 3 2 4 5% 76%	4028
	21st to end	Day	2 3 1 3 2 2 2 2 3 4 76%	4348
September	1st to 10th	Day	2 2 1 3 3 2 3 3 3 6% 73%	2214
	11th to 20th	Day	4 4 1 3 2 3 4 2 4 6% 68%	2090
	21st to end	Day	6% 3 1 3 2 3 2 2 3 5% 68%	2090
October	1st to 10th	Day	5% 3 1 3 2 3 3 3 4 5% 68%	3490
	11th to 20th	Day	5% 4 2 3 3 2 3 2 4 7% 66%	3138
	21st to end	Day	4 3 1 5% 3 3 4 4 6% 9% 57%	3101
November	1st to 10th	Day	8% 7% 2 6% 5% 4 3 3 5% 7% 50%	1755
	11th to 20th	Day	9% 6% 3 6% 4 5% 5% 5% 7% 8% 44%	1710
	21st to end	Day	6% 10% 4 7% 5% 5% 4 4 4 6% 45%	1710
December	1st to 10th	Day	10% 10% 3 8% 4 5% 5% 3 4 6% 42%	1686
	11th to 20th	Day	10% 10% 4 8% 5% 4 5% 5% 7% 10% 33%	1928
	21st to end	Day	8% 10% 4 9% 5% 4 4 4 6% 8% 38%	2794
January	1st to 10th	Night	9% 13% 4 10% 5% 5% 5% 4 6% 7% 31%	4406
	11th to 20th	Night	12% 15% 4 10% 6% 5% 6% 4 5% 7% 25%	4420
	21st to end	Night	11% 13% 4 9% 5% 5% 4 4 5% 6% 34%	4734
February	1st to 10th	Night	10% 10% 4 8% 6% 6% 4 4 6% 8% 35%	2470
	11th to 20th	Night	9% 9% 3 7% 4 3 4 3 6% 8% 44%	2228
	21st to end	Night	6% 9% 2 6% 5% 5% 6% 5% 7% 8% 40%	1884
March	1st to 10th	Night	8% 11% 3 8% 4 4 5% 4 6% 8% 38%	3736
	11th to 20th	Night	5% 6% 2 6% 4 5% 5% 4 5% 6% 53%	3495
	21st to end	Night	7% 8% 3 7% 5% 6% 6% 5% 7% 8% 37%	3828
April	1st to 10th	Night	8% 8% 3 5% 5% 5% 5% 4 6% 7% 45%	1913
	11th to 20th	Night	7% 8% 2 5% 4 5% 5% 4 5% 6% 50%	1772
	21st to end	Night	6% 6% 2 6% 4 4 4 3 6% 8% 53%	1710
May	1st to 10th	Night	10% 5% 1 5% 4 4 4 4 7% 7% 47%	2860
	11th to 20th	Night	3 3 1 4 3 2 3 3 4 6% 67%	2834
	21st to end	Night	5% 5% 1 6% 4 4 5% 4 6% 7% 53%	2950
June	1st to 10th	Night	4 3 1 4 4 5% 5% 6% 7% 8% 53%	1520
	11th to 20th	Night	4 3 2 3 3 3 3 4 5% 6% 64%	1520
	21st to end	Night	2 2 1 3 1 2 3 2 4 6% 74%	1520
July	1st to 10th	Night	1 1 1 2 2 2 3 3 3 5% 79%	2540
	11th to 20th	Night	1 1 1 2 2 2 4 3 5% 5% 74%	2540
	21st to end	Night	2 2 1 3 4 3 4 3 6% 7% 66%	2881
August	1st to 10th	Night	2 3 2 4 3 3 4 3 6% 6% 64%	2847
	11th to 20th	Night	3 2 1 2 3 2 3 3 5% 6% 70%	2811
	21st to end	Night	3 1 1 3 2 3 3 2 4 4 75%	3323
September	1st to 10th	Night	3 2 1 2 2 2 3 2 3 5% 73%	1966
	11th to 20th	Night	4 3 1 4 3 3 3 3 5% 6% 67%	2090
	21st to end	Night	5% 3 1 3 2 2 3 2 4 4 69%	2090
October	1st to 10th	Night	4 2 1 3 2 2 2 2 4 6% 72%	3490
	11th to 20th	Night	6% 5% 2 5% 4 3 3 2 4 7% 59%	3787
	21st to end	Night	5% 4 2 4 4 4 5% 4 6% 8% 55%	4457
November	1st to 10th	Night	10% 7% 2 5% 4 4 4 3 6% 6% 49%	2528
	11th to 20th	Night	9% 6% 2 7% 4 5% 4 4 6% 8% 47%	2470
	21st to end	Night	7% 6% 2 5% 5% 5% 4 4 5% 8% 47%	2470
December	1st to 10th	Night	10% 10% 3 8% 4 4 4 4 5% 7% 40%	2492
	11th to 20th	Night	11% 11% 4 8% 4 5% 6% 4 6% 8% 32%	3276
	21st to end	Night	10% 10% 4 8% 5% 5% 4 4 6% 6% 38%	4862

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	53% 7% 17% 19% 2 1	2539
	11th to 20th	Day	54% 9% 19% 14% 4 1	2540
	21st to end	Day	50% 8% 20% 16% 6%	2925
February	1st to 10th	Day	45% 8% 19% 21% 6% 1	1710
	11th to 20th	Day	44% 7% 15% 23% 10% 1	1845
	21st to end	Day	45% 6% 14% 23% 11% 1	1570
March	1st to 10th	Day	46% 8% 10% 26% 9% 1	3138
	11th to 20th	Day	30% 4 12% 35% 18% 1	3485
	21st to end	Day	44% 8% 14% 24% 9% 1	3825
April	1st to 10th	Day	39% 5% 13% 23% 16% 4	2265
	11th to 20th	Day	33% 6% 10% 28% 18% 4	2359
	21st to end	Day	32% 5% 12% 27% 22% 2	2470
May	1st to 10th	Day	36% 7% 6% 24% 23% 5%	4110
	11th to 20th	Day	17% 5% 7% 28% 33% 9%	4074
	21st to end	Day	27% 7% 10% 27% 27% 3	4627
June	1st to 10th	Day	27% 6% 12% 30% 22% 3	2660
	11th to 20th	Day	23% 4 9% 21% 32% 12%	2660
	21st to end	Day	12% 3 5% 20% 36% 24%	2660
July	1st to 10th	Day	9% 2 2 10% 19% 58%	4351
	11th to 20th	Day	15% 2 2 7% 20% 53%	4345
	21st to end	Day	16% 3 3 9% 27% 43%	4522
August	1st to 10th	Day	19% 3 4 13% 24% 37%	4078
	11th to 20th	Day	11% 2 3 11% 37% 35%	4067
	21st to end	Day	14% 2 2 12% 37% 33%	4347
September	1st to 10th	Day	15% 3 5% 14% 31% 33%	2214
	11th to 20th	Day	19% 3 7% 23% 27% 20%	2090
	21st to end	Day	21% 3 5% 22% 35% 15%	2090
October	1st to 10th	Day	20% 3 6% 18% 33% 19%	3490
	11th to 20th	Day	22% 2 6% 34% 28% 8%	3138
	21st to end	Day	24% 3 11% 34% 22% 6%	3101
November	1st to 10th	Day	35% 6% 12% 28% 18% 2	1755
	11th to 20th	Day	36% 6% 18% 30% 8% 1	1710
	21st to end	Day	40% 5% 17% 25% 12%	1710
December	1st to 10th	Day	44% 8% 17% 22% 8% 2	1686
	11th to 20th	Day	45% 7% 26% 19% 3	1928
	21st to end	Day	44% 6% 19% 21% 10%	2794
January	1st to 10th	Night	50% 7% 17% 23% 4	4406
	11th to 20th	Night	57% 8% 17% 15% 4	4420
	21st to end	Night	51% 7% 18% 20% 4	4734
February	1st to 10th	Night	47% 8% 18% 22% 5% 1	2470
	11th to 20th	Night	38% 6% 17% 26% 12% 1	2229
	21st to end	Night	38% 7% 16% 28% 10% 1	1884
March	1st to 10th	Night	40% 8% 13% 27% 11% 1	3736
	11th to 20th	Night	31% 5% 12% 31% 20% 1	3495
	21st to end	Night	40% 8% 15% 27% 9% 1	3828
April	1st to 10th	Night	37% 6% 11% 25% 15% 6%	1913
	11th to 20th	Night	34% 5% 9% 28% 20% 3	1772
	21st to end	Night	30% 5% 11% 28% 26% 2	1710
May	1st to 10th	Night	32% 7% 7% 22% 24% 8%	2860
	11th to 20th	Night	18% 4 7% 23% 37% 12%	2835
	21st to end	Night	28% 6% 9% 24% 32% 2	2950
June	1st to 10th	Night	25% 6% 15% 27% 24% 4	1520
	11th to 20th	Night	21% 5% 7% 21% 33% 14%	1520
	21st to end	Night	13% 2 4 17% 31% 33%	1520
July	1st to 10th	Night	9% 3 2 7% 19% 59%	2500
	11th to 20th	Night	12% 2 2 6% 17% 60%	2492
	21st to end	Night	17% 3 3 8% 28% 41%	2791
August	1st to 10th	Night	20% 2 5% 10% 21% 42%	2838
	11th to 20th	Night	14% 4 2 13% 29% 37%	2838
	21st to end	Night	14% 2 2 11% 31% 39%	3323
September	1st to 10th	Night	15% 3 3 11% 33% 35%	1966
	11th to 20th	Night	18% 4 5% 22% 28% 23%	2090
	21st to end	Night	18% 4 5% 17% 36% 20%	2090
October	1st to 10th	Night	16% 2 5% 16% 34% 27%	3490
	11th to 20th	Night	27% 3 5% 25% 30% 10%	3787
	21st to end	Night	27% 4 11% 31% 23% 4	4457
November	1st to 10th	Night	36% 4 12% 25% 19% 4	2528
	11th to 20th	Night	35% 5% 15% 30% 14% 1	2470
	21st to end	Night	34% 6% 16% 30% 12% 1	2470
December	1st to 10th	Night	44% 7% 20% 21% 7% 1	2492
	11th to 20th	Night	49% 6% 18% 24% 4	3276
	21st to end	Night	44% 6% 18% 22% 9%	4862

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	18% 14% 26% 22% 19% 2	180
	11th to 20th	Day	9% 10% 28% 28% 18% 7%	180
	21st to end	Day	16% 10% 21% 28% 19% 6%	236
February	1st to 10th	Day	19% 14% 32% 21% 12% 1	270
	11th to 20th	Day	15% 16% 30% 24% 13% 3	440
	21st to end	Day	10% 20% 33% 27% 10% 1	382
March	1st to 10th	Day	12% 13% 35% 26% 12% 1	460
	11th to 20th	Day	8% 13% 32% 28% 16% 3	460
	21st to end	Day	16% 16% 31% 26% 9% 2	506
April	1st to 10th	Day	13% 18% 28% 20% 16% 5%	460
	11th to 20th	Day	10% 20% 29% 25% 13% 4	452
	21st to end	Day	10% 16% 32% 22% 16% 3	460
May	1st to 10th	Day	5% 16% 26% 30% 18% 4	463
	11th to 20th	Day	11% 9% 22% 30% 18% 9%	488
	21st to end	Day	6% 9% 22% 26% 24% 13%	529
June	1st to 10th	Day	6% 9% 22% 30% 20% 14%	490
	11th to 20th	Day	7% 9% 29% 30% 17% 7%	490
	21st to end	Day	2 5% 24% 34% 26% 9%	490
July	1st to 10th	Day	2 2 18% 39% 29% 9%	486
	11th to 20th	Day	1 2 18% 40% 33% 6%	484
	21st to end	Day	3 2 13% 36% 33% 13%	531
August	1st to 10th	Day	5% 2 18% 35% 30% 10%	487
	11th to 20th	Day	3 4 18% 29% 32% 15%	477
	21st to end	Day	1 4 19% 29% 31% 15%	539
September	1st to 10th	Day	3 3 17% 29% 29% 20%	490
	11th to 20th	Day	5% 4 18% 30% 27% 15%	490
	21st to end	Day	7% 6% 20% 29% 27% 11%	490
October	1st to 10th	Day	9% 6% 20% 27% 24% 14%	490
	11th to 20th	Day	8% 11% 19% 19% 32% 12%	313
	21st to end	Day	9% 10% 23% 32% 22% 5%	330
November	1st to 10th	Day	14% 9% 29% 25% 21% 4	297
	11th to 20th	Day	11% 10% 19% 29% 24% 7%	270
	21st to end	Day	13% 9% 18% 26% 27% 6%	270
December	1st to 10th	Day	17% 10% 25% 26% 16% 5%	258
	11th to 20th	Day	17% 11% 23% 30% 15% 4	180
	21st to end	Day	19% 12% 21% 28% 15% 6%	198
January	1st to 10th	Night	16% 9% 23% 32% 19% 1	760
	11th to 20th	Night	14% 9% 21% 34% 19% 2	760
	21st to end	Night	15% 8% 22% 32% 21% 2	836
February	1st to 10th	Night	14% 10% 30% 32% 14%	760
	11th to 20th	Night	14% 8% 30% 34% 14% 1	744
	21st to end	Night	11% 11% 31% 33% 14% 1	628
March	1st to 10th	Night	11% 9% 32% 32% 15% 2	760
	11th to 20th	Night	8% 8% 32% 35% 15% 2	760
	21st to end	Night	13% 10% 33% 33% 10% 2	836
April	1st to 10th	Night	10% 10% 30% 34% 15% 2	760
	11th to 20th	Night	8% 11% 35% 32% 12% 1	644
	21st to end	Night	7% 9% 31% 36% 15% 2	570
May	1st to 10th	Night	9% 8% 29% 34% 18% 2	570
	11th to 20th	Night	7% 6% 20% 40% 24% 4	561
	21st to end	Night	9% 5% 24% 33% 24% 4	522
June	1st to 10th	Night	6% 4 22% 39% 25% 4	380
	11th to 20th	Night	4 4 28% 46% 17% 1	380
	21st to end	Night	2 2 22% 49% 22% 3	380
July	1st to 10th	Night	2 1 11% 60% 24% 2	370
	11th to 20th	Night	1 20% 52% 26% 1	368
	21st to end	Night	2 1 10% 49% 34% 3	487
August	1st to 10th	Night	3 1 14% 50% 30% 2	567
	11th to 20th	Night	2 2 15% 48% 30% 3	564
	21st to end	Night	3 2 17% 46% 29% 3	726
September	1st to 10th	Night	4 1 15% 44% 31% 6%	760
	11th to 20th	Night	4 3 14% 42% 31% 6%	760
	21st to end	Night	6% 4 16% 38% 27% 8%	760
October	1st to 10th	Night	8% 5% 20% 40% 24% 4	760
	11th to 20th	Night	9% 6% 20% 36% 26% 5%	760
	21st to end	Night	7% 7% 25% 36% 21% 4	836
November	1st to 10th	Night	11% 6% 25% 33% 22% 3	778
	11th to 20th	Night	11% 8% 26% 38% 15% 3	760
	21st to end	Night	10% 7% 26% 32% 21% 4	760
December	1st to 10th	Night	14% 7% 28% 33% 16% 3	760
	11th to 20th	Night	19% 6% 22% 34% 15% 3	760
	21st to end	Night	19% 11% 27% 31% 10% 1	836

Page generated at 2024-11-07 04:55:43. Version ac07

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