Charles E. Daniel Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Charles E. Daniel Observatory clear sky chart from 2005-11-06 to 2026-09-29.

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	13% 6% 2 6% 4 4 5% 4 6% 8% 43%	148904
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 43%	142512
Transparency	Day	37% 8% 13% 20% 17% 5%	152602
	Night	38% 7% 11% 18% 19% 6%	146873
Seeing	Day	16% 9% 23% 25% 18% 9%	17665
	Night	16% 8% 24% 30% 19% 3	27671

By Month

Percentage Cloud Cover SE (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	15% 6% 2 5% 3 3 4 3 4 5% 51%	5258
	11th to 20th	Day	11% 5% 2 4 2 3 3 2 3 3 63%	5280
	21st to end	Day	21% 8% 3 6% 4 3 3 3 4 5% 42%	5780
February	1st to 10th	Day	15% 6% 2 5% 4 3 4 4 5% 6% 45%	1985
	11th to 20th	Day	23% 5% 2 5% 3 4 3 3 3 6% 43%	2048
	21st to end	Day	19% 6% 2 6% 4 4 4 3 5% 6% 42%	1730
March	1st to 10th	Day	21% 7% 2 6% 4 4 3 3 4 5% 41%	4990
	11th to 20th	Day	10% 5% 2 5% 3 4 4 3 4 6% 54%	5090
	21st to end	Day	18% 6% 2 5% 4 4 3 3 5% 5% 46%	5586
April	1st to 10th	Day	12% 4 2 6% 3 4 4 4 6% 9% 47%	2310
	11th to 20th	Day	12% 5% 2 5% 4 4 3 4 5% 8% 49%	2471
	21st to end	Day	11% 4 2 5% 3 4 4 4 5% 6% 50%	2730
May	1st to 10th	Day	12% 7% 3 7% 5% 4 5% 4 6% 9% 38%	7616
	11th to 20th	Day	15% 6% 3 7% 4 4 5% 4 6% 9% 38%	7626
	21st to end	Day	16% 7% 3 8% 5% 5% 5% 4 6% 8% 33%	8391
June	1st to 10th	Day	10% 6% 3 6% 5% 5% 6% 5% 8% 12% 34%	2730
	11th to 20th	Day	6% 5% 2 5% 5% 5% 6% 6% 8% 12% 39%	2730
	21st to end	Day	7% 4 2 6% 5% 5% 6% 5% 8% 11% 42%	2730
July	1st to 10th	Day	4 4 2 6% 4 6% 6% 5% 8% 11% 43%	7680
	11th to 20th	Day	7% 5% 2 7% 6% 6% 7% 6% 8% 12% 35%	7680
	21st to end	Day	9% 7% 3 7% 6% 7% 7% 6% 9% 12% 28%	8340
August	1st to 10th	Day	15% 8% 3 8% 6% 6% 6% 5% 9% 11% 23%	6014
	11th to 20th	Day	6% 4 2 5% 4 4 5% 4 7% 13% 45%	5951
	21st to end	Day	7% 4 2 6% 4 4 5% 4 6% 9% 49%	6055
September	1st to 10th	Day	8% 5% 2 6% 4 4 5% 5% 6% 9% 46%	2310
	11th to 20th	Day	14% 6% 2 6% 4 4 4 4 6% 9% 41%	2310
	21st to end	Day	16% 5% 2 6% 4 3 4 4 5% 6% 44%	2288
October	1st to 10th	Day	14% 4 1 4 3 3 3 3 4 6% 54%	2157
	11th to 20th	Day	12% 5% 1 4 3 2 2 2 4 6% 61%	2000
	21st to end	Day	13% 5% 2 4 3 2 3 2 4 4 58%	2200
November	1st to 10th	Day	13% 6% 1 5% 3 3 2 3 4 6% 55%	2010
	11th to 20th	Day	14% 4 1 4 3 3 3 3 5% 6% 54%	1890
	21st to end	Day	15% 5% 2 4 3 2 3 4 4 5% 52%	1890
December	1st to 10th	Day	18% 6% 2 4 3 3 3 2 3 5% 51%	3020
	11th to 20th	Day	19% 6% 3 6% 4 4 4 3 5% 6% 41%	3463
	21st to end	Day	23% 5% 2 4 2 2 3 2 4 7% 46%	4565
January	1st to 10th	Night	16% 4 2 4 2 3 3 2 3 5% 57%	7635
	11th to 20th	Night	11% 6% 2 4 3 3 3 2 4 5% 56%	7680
	21st to end	Night	20% 7% 2 5% 3 3 3 2 4 5% 46%	8401
February	1st to 10th	Night	15% 6% 2 5% 3 4 3 3 5% 5% 48%	2630
	11th to 20th	Night	21% 7% 2 5% 4 3 3 2 4 4 44%	2458
	21st to end	Night	17% 7% 2 6% 3 4 3 3 5% 6% 45%	2076
March	1st to 10th	Night	19% 6% 2 6% 4 3 3 3 5% 7% 43%	5081
	11th to 20th	Night	9% 4 1 5% 3 3 4 3 6% 7% 55%	5090
	21st to end	Night	16% 5% 2 6% 3 4 3 4 5% 6% 47%	5588
April	1st to 10th	Night	13% 5% 2 5% 3 4 4 4 5% 6% 48%	2310
	11th to 20th	Night	10% 6% 2 5% 3 3 3 3 5% 6% 53%	2113
	21st to end	Night	12% 6% 2 5% 4 4 3 3 5% 6% 50%	1890
May	1st to 10th	Night	13% 7% 3 8% 5% 5% 5% 4 6% 9% 35%	5237
	11th to 20th	Night	17% 7% 2 6% 5% 4 4 4 5% 8% 37%	5234
	21st to end	Night	18% 8% 3 7% 4 4 4 4 6% 8% 32%	5761
June	1st to 10th	Night	10% 6% 2 7% 6% 5% 6% 5% 9% 10% 34%	1890
	11th to 20th	Night	9% 7% 2 6% 5% 5% 5% 4 7% 11% 38%	1890
	21st to end	Night	7% 4 3 7% 6% 6% 6% 7% 7% 11% 36%	1890
July	1st to 10th	Night	7% 7% 2 8% 6% 7% 7% 7% 9% 11% 30%	5278
	11th to 20th	Night	9% 6% 3 8% 5% 6% 7% 6% 9% 13% 27%	5277
	21st to end	Night	13% 9% 3 10% 6% 6% 6% 5% 8% 12% 24%	5736
August	1st to 10th	Night	17% 10% 4 9% 6% 6% 6% 5% 7% 10% 20%	4135
	11th to 20th	Night	8% 6% 2 8% 5% 6% 6% 5% 8% 12% 34%	4095
	21st to end	Night	8% 6% 2 7% 5% 5% 6% 4 6% 9% 43%	5019
September	1st to 10th	Night	9% 6% 2 7% 4 5% 4 4 6% 10% 42%	2310
	11th to 20th	Night	15% 7% 3 8% 5% 5% 5% 3 5% 8% 35%	2310
	21st to end	Night	16% 6% 2 6% 4 4 4 4 5% 5% 44%	2289
October	1st to 10th	Night	13% 5% 2 5% 3 3 4 3 4 6% 50%	2240
	11th to 20th	Night	12% 5% 2 5% 2 4 3 3 4 6% 56%	2400
	21st to end	Night	14% 4 1 4 3 2 3 2 3 5% 58%	2640
November	1st to 10th	Night	14% 5% 2 5% 3 3 3 2 3 5% 55%	2504
	11th to 20th	Night	15% 5% 2 4 3 3 3 2 4 7% 53%	2730
	21st to end	Night	15% 5% 2 4 3 3 4 3 5% 5% 50%	2730
December	1st to 10th	Night	18% 5% 1 3 3 3 3 3 4 4 53%	4380
	11th to 20th	Night	19% 6% 2 6% 4 3 4 3 4 5% 45%	4952
	21st to end	Night	21% 5% 2 5% 3 3 3 3 4 5% 46%	6633

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	35% 5% 8% 24% 23% 5%	5259
	11th to 20th	Day	28% 4 4 12% 40% 13%	5280
	21st to end	Day	46% 3 4 15% 21% 11%	5780
February	1st to 10th	Day	38% 5% 9% 22% 21% 5%	1985
	11th to 20th	Day	43% 5% 7% 19% 21% 6%	2058
	21st to end	Day	42% 5% 9% 14% 22% 7%	1730
March	1st to 10th	Day	45% 5% 7% 15% 18% 11%	4990
	11th to 20th	Day	30% 5% 7% 13% 25% 19%	5090
	21st to end	Day	40% 5% 7% 22% 21% 5%	5586
April	1st to 10th	Day	33% 5% 12% 20% 24% 5%	2310
	11th to 20th	Day	33% 6% 10% 20% 22% 9%	2471
	21st to end	Day	33% 6% 12% 25% 19% 4	2730
May	1st to 10th	Day	41% 9% 13% 22% 13% 2	7651
	11th to 20th	Day	41% 9% 16% 20% 13% 2	7630
	21st to end	Day	46% 10% 13% 16% 14% 2	8405
June	1st to 10th	Day	39% 11% 18% 22% 8% 2	2730
	11th to 20th	Day	32% 12% 20% 23% 12% 1	2730
	21st to end	Day	33% 12% 19% 20% 13% 3	2730
July	1st to 10th	Day	31% 13% 23% 26% 7% 1	7615
	11th to 20th	Day	37% 15% 28% 17% 3 1	7611
	21st to end	Day	43% 13% 18% 17% 8% 1	8156
August	1st to 10th	Day	50% 13% 19% 15% 4	6001
	11th to 20th	Day	26% 11% 26% 24% 12% 1	5990
	21st to end	Day	29% 10% 16% 26% 17% 2	6055
September	1st to 10th	Day	33% 10% 14% 21% 19% 3	2310
	11th to 20th	Day	38% 8% 16% 17% 19% 2	2310
	21st to end	Day	40% 7% 12% 18% 18% 6%	2288
October	1st to 10th	Day	26% 4 9% 18% 32% 10%	3508
	11th to 20th	Day	19% 3 7% 16% 30% 26%	3240
	21st to end	Day	26% 4 5% 24% 26% 15%	3538
November	1st to 10th	Day	32% 5% 7% 19% 27% 10%	2010
	11th to 20th	Day	32% 4 7% 23% 25% 9%	1890
	21st to end	Day	34% 5% 8% 21% 24% 7%	1890
December	1st to 10th	Day	37% 3 5% 18% 27% 9%	3020
	11th to 20th	Day	43% 6% 9% 17% 19% 7%	3460
	21st to end	Day	39% 4 8% 26% 21% 2	4565
January	1st to 10th	Night	31% 4 9% 24% 26% 6%	7636
	11th to 20th	Night	31% 4 5% 15% 35% 10%	7680
	21st to end	Night	41% 3 6% 16% 24% 11%	8401
February	1st to 10th	Night	38% 5% 9% 21% 21% 7%	2630
	11th to 20th	Night	45% 4 6% 17% 23% 6%	2459
	21st to end	Night	40% 6% 8% 16% 22% 8%	2076
March	1st to 10th	Night	39% 5% 10% 13% 26% 7%	5081
	11th to 20th	Night	27% 5% 8% 14% 29% 18%	5090
	21st to end	Night	37% 5% 7% 20% 21% 9%	5588
April	1st to 10th	Night	35% 6% 10% 19% 25% 6%	2310
	11th to 20th	Night	31% 5% 8% 23% 25% 9%	2113
	21st to end	Night	33% 5% 11% 26% 21% 4	1890
May	1st to 10th	Night	43% 8% 14% 21% 13% 2	5264
	11th to 20th	Night	43% 8% 15% 21% 12% 1	5235
	21st to end	Night	46% 9% 14% 18% 11% 2	5771
June	1st to 10th	Night	41% 11% 16% 19% 11% 2	1890
	11th to 20th	Night	36% 11% 19% 21% 11% 1	1890
	21st to end	Night	36% 14% 18% 16% 13% 4	1890
July	1st to 10th	Night	40% 14% 22% 19% 4 1	5235
	11th to 20th	Night	43% 16% 24% 14% 2 1	5226
	21st to end	Night	50% 13% 18% 13% 5% 1	5606
August	1st to 10th	Night	55% 13% 17% 11% 4	4124
	11th to 20th	Night	37% 13% 24% 20% 6%	4122
	21st to end	Night	35% 11% 14% 24% 15% 1	5019
September	1st to 10th	Night	36% 9% 15% 20% 16% 3	2310
	11th to 20th	Night	44% 9% 13% 17% 16% 2	2310
	21st to end	Night	40% 8% 11% 17% 18% 6%	2289
October	1st to 10th	Night	27% 7% 11% 16% 31% 8%	3656
	11th to 20th	Night	20% 4 6% 18% 27% 25%	3899
	21st to end	Night	27% 4 5% 19% 27% 17%	4252
November	1st to 10th	Night	34% 4 7% 18% 26% 11%	2504
	11th to 20th	Night	32% 4 6% 18% 28% 11%	2730
	21st to end	Night	35% 5% 7% 19% 26% 8%	2730
December	1st to 10th	Night	35% 4 6% 19% 29% 7%	4380
	11th to 20th	Night	43% 5% 8% 17% 19% 9%	4954
	21st to end	Night	40% 5% 9% 25% 19% 3	6633

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	23% 18% 35% 21% 3	330
	11th to 20th	Day	25% 18% 34% 17% 5%	330
	21st to end	Day	23% 20% 40% 14% 3	363
February	1st to 10th	Day	19% 21% 33% 21% 6%	330
	11th to 20th	Day	39% 13% 32% 15% 2	330
	21st to end	Day	26% 22% 34% 12% 5%	273
March	1st to 10th	Day	20% 16% 34% 22% 7% 1	509
	11th to 20th	Day	18% 17% 37% 21% 5% 1	540
	21st to end	Day	20% 17% 33% 23% 6% 1	594
April	1st to 10th	Day	18% 14% 30% 27% 10% 2	540
	11th to 20th	Day	14% 13% 33% 26% 13% 2	532
	21st to end	Day	15% 10% 29% 25% 15% 6%	540
May	1st to 10th	Day	12% 14% 34% 24% 13% 4	543
	11th to 20th	Day	17% 6% 20% 27% 22% 7%	567
	21st to end	Day	16% 6% 18% 26% 22% 11%	617
June	1st to 10th	Day	10% 5% 16% 34% 24% 11%	570
	11th to 20th	Day	7% 5% 16% 29% 28% 16%	570
	21st to end	Day	8% 2 14% 31% 26% 17%	570
July	1st to 10th	Day	7% 3 10% 22% 34% 24%	565
	11th to 20th	Day	7% 2 8% 25% 35% 23%	564
	21st to end	Day	6% 4 10% 31% 32% 17%	620
August	1st to 10th	Day	18% 6% 12% 22% 25% 17%	566
	11th to 20th	Day	10% 2 8% 30% 28% 22%	559
	21st to end	Day	11% 3 10% 25% 27% 24%	627
September	1st to 10th	Day	8% 4 12% 28% 31% 18%	570
	11th to 20th	Day	17% 4 12% 20% 31% 17%	570
	21st to end	Day	21% 5% 18% 25% 24% 7%	562
October	1st to 10th	Day	17% 6% 18% 29% 21% 9%	530
	11th to 20th	Day	13% 7% 23% 30% 22% 5%	530
	21st to end	Day	17% 9% 23% 27% 19% 4	583
November	1st to 10th	Day	19% 10% 24% 28% 14% 4	488
	11th to 20th	Day	17% 13% 37% 25% 8% 1	330
	21st to end	Day	11% 14% 34% 28% 10% 2	330
December	1st to 10th	Day	30% 13% 31% 21% 5%	330
	11th to 20th	Day	24% 13% 32% 22% 7% 1	330
	21st to end	Day	26% 13% 31% 20% 8% 2	363
January	1st to 10th	Night	20% 14% 34% 26% 6%	840
	11th to 20th	Night	21% 13% 36% 24% 5%	840
	21st to end	Night	19% 14% 38% 26% 3	924
February	1st to 10th	Night	18% 13% 34% 32% 4	840
	11th to 20th	Night	24% 14% 36% 25% 2	823
	21st to end	Night	19% 14% 38% 25% 3	692
March	1st to 10th	Night	18% 13% 34% 30% 5%	840
	11th to 20th	Night	16% 12% 38% 25% 7% 1	840
	21st to end	Night	16% 11% 36% 28% 8%	924
April	1st to 10th	Night	15% 15% 31% 29% 9%	840
	11th to 20th	Night	12% 11% 30% 33% 13% 1	772
	21st to end	Night	13% 4 29% 34% 19% 1	630
May	1st to 10th	Night	11% 8% 30% 33% 17% 1	630
	11th to 20th	Night	14% 7% 21% 36% 21% 2	621
	21st to end	Night	16% 2 14% 34% 29% 5%	683
June	1st to 10th	Night	11% 2 16% 36% 31% 5%	630
	11th to 20th	Night	11% 3 17% 28% 31% 10%	630
	21st to end	Night	7% 2 12% 34% 38% 7%	630
July	1st to 10th	Night	10% 2 8% 29% 40% 11%	615
	11th to 20th	Night	11% 2 5% 27% 42% 13%	612
	21st to end	Night	9% 3 11% 28% 41% 8%	690
August	1st to 10th	Night	17% 4 10% 23% 37% 9%	627
	11th to 20th	Night	13% 2 11% 26% 39% 9%	624
	21st to end	Night	12% 2 11% 25% 42% 8%	777
September	1st to 10th	Night	11% 2 10% 29% 38% 9%	840
	11th to 20th	Night	17% 3 13% 26% 34% 7%	840
	21st to end	Night	18% 4 17% 32% 26% 3	833
October	1st to 10th	Night	15% 4 18% 29% 27% 6%	800
	11th to 20th	Night	15% 6% 23% 34% 19% 3	800
	21st to end	Night	15% 7% 24% 33% 20% 2	880
November	1st to 10th	Night	17% 7% 23% 34% 17% 3	820
	11th to 20th	Night	15% 10% 31% 31% 11% 1	840
	21st to end	Night	16% 8% 28% 35% 13% 1	840
December	1st to 10th	Night	22% 11% 25% 33% 8%	840
	11th to 20th	Night	20% 8% 30% 32% 9%	840
	21st to end	Night	25% 9% 26% 32% 8%	924

Page generated at 2026-09-26 04:18:44. Version ac07

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