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 2025-05-26.

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 4 4 5% 8% 44%	114525
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 45%	114763
Transparency	Day	36% 7% 12% 19% 19% 6%	118267
	Night	36% 7% 10% 18% 22% 7%	119147
Seeing	Day	16% 9% 23% 25% 18% 9%	15814
	Night	16% 8% 24% 30% 19% 3	25934

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	14% 4 1 3 2 3 4 3 4 5% 57%	4056
	11th to 20th	Day	12% 5% 2 4 3 3 3 2 3 3 60%	4060
	21st to end	Day	19% 6% 2 6% 3 3 3 2 3 5% 47%	4466
February	1st to 10th	Day	15% 6% 2 5% 4 3 4 4 6% 6% 44%	1890
	11th to 20th	Day	23% 5% 2 5% 3 4 3 3 3 5% 43%	1948
	21st to end	Day	19% 6% 2 6% 4 4 3 3 4 6% 42%	1650
March	1st to 10th	Day	21% 7% 2 6% 4 4 3 3 4 5% 41%	4881
	11th to 20th	Day	10% 5% 2 5% 3 4 3 3 4 6% 54%	4980
	21st to end	Day	19% 6% 2 5% 4 4 3 3 5% 5% 46%	5465
April	1st to 10th	Day	13% 4 2 6% 3 4 4 4 6% 8% 48%	2200
	11th to 20th	Day	13% 5% 2 5% 4 4 3 4 5% 8% 48%	2352
	21st to end	Day	12% 4 2 6% 4 5% 4 4 5% 6% 50%	2600
May	1st to 10th	Day	11% 8% 3 7% 5% 4 5% 4 6% 9% 39%	4212
	11th to 20th	Day	17% 8% 3 6% 4 4 4 3 6% 9% 36%	4211
	21st to end	Day	8% 4 2 5% 4 4 5% 5% 7% 11% 45%	4541
June	1st to 10th	Day	10% 6% 3 6% 5% 5% 6% 5% 7% 12% 34%	2470
	11th to 20th	Day	6% 5% 2 5% 5% 5% 6% 6% 8% 11% 40%	2470
	21st to end	Day	7% 4 2 6% 5% 5% 6% 5% 8% 10% 40%	2470
July	1st to 10th	Day	7% 6% 2 7% 6% 7% 6% 6% 8% 11% 35%	4120
	11th to 20th	Day	7% 6% 2 7% 5% 5% 6% 5% 8% 11% 38%	4120
	21st to end	Day	9% 9% 3 9% 7% 7% 8% 6% 9% 11% 22%	4466
August	1st to 10th	Day	15% 7% 3 7% 5% 5% 6% 5% 8% 10% 29%	4104
	11th to 20th	Day	9% 5% 3 6% 5% 6% 6% 5% 8% 11% 37%	4044
	21st to end	Day	7% 4 2 5% 4 4 4 4 6% 8% 52%	4159
September	1st to 10th	Day	8% 5% 2 6% 5% 5% 5% 5% 6% 9% 44%	2090
	11th to 20th	Day	15% 6% 2 6% 4 4 4 4 6% 9% 39%	2090
	21st to end	Day	16% 5% 2 5% 4 3 4 4 5% 6% 44%	2090
October	1st to 10th	Day	15% 4 1 4 3 2 2 3 4 6% 55%	2049
	11th to 20th	Day	12% 5% 2 4 3 2 2 2 4 6% 60%	1900
	21st to end	Day	13% 5% 2 4 3 2 3 2 4 4 58%	2090
November	1st to 10th	Day	14% 6% 1 5% 3 3 2 3 4 6% 54%	1912
	11th to 20th	Day	15% 4 1 4 3 3 3 3 5% 6% 52%	1800
	21st to end	Day	15% 5% 2 4 3 2 4 4 4 5% 52%	1800
December	1st to 10th	Day	18% 5% 2 4 3 3 3 2 3 5% 53%	2930
	11th to 20th	Day	19% 6% 3 6% 4 4 4 3 5% 6% 40%	3373
	21st to end	Day	24% 5% 2 4 2 2 3 2 4 6% 46%	4466
January	1st to 10th	Night	15% 3 2 3 2 3 3 2 3 4 59%	5889
	11th to 20th	Night	12% 6% 2 4 3 3 3 3 4 5% 55%	5900
	21st to end	Night	19% 7% 2 4 3 3 2 2 4 5% 49%	6490
February	1st to 10th	Night	15% 6% 2 5% 4 4 3 4 5% 5% 48%	2505
	11th to 20th	Night	21% 7% 2 5% 4 3 3 2 4 4 44%	2338
	21st to end	Night	16% 7% 2 6% 3 4 3 3 5% 6% 45%	1980
March	1st to 10th	Night	19% 6% 2 6% 4 3 3 2 5% 7% 43%	4970
	11th to 20th	Night	10% 4 1 5% 3 3 4 3 6% 7% 55%	4980
	21st to end	Night	16% 5% 2 6% 3 4 3 3 5% 6% 47%	5467
April	1st to 10th	Night	13% 6% 2 5% 3 4 4 4 5% 6% 49%	2200
	11th to 20th	Night	11% 7% 2 5% 3 4 3 3 5% 6% 52%	2011
	21st to end	Night	12% 6% 2 5% 4 4 4 3 5% 6% 50%	1800
May	1st to 10th	Night	13% 7% 2 6% 5% 5% 5% 3 6% 8% 39%	2903
	11th to 20th	Night	18% 6% 2 6% 5% 4 4 4 5% 7% 38%	2902
	21st to end	Night	9% 4 2 6% 5% 4 5% 4 7% 11% 43%	3130
June	1st to 10th	Night	11% 5% 2 7% 6% 4 7% 5% 9% 11% 33%	1710
	11th to 20th	Night	9% 6% 2 6% 5% 6% 5% 4 8% 10% 38%	1710
	21st to end	Night	8% 5% 3 7% 7% 6% 6% 7% 7% 10% 36%	1710
July	1st to 10th	Night	10% 8% 2 8% 7% 7% 7% 7% 8% 9% 26%	2840
	11th to 20th	Night	8% 5% 2 6% 5% 6% 6% 6% 9% 13% 32%	2840
	21st to end	Night	13% 10% 4 9% 6% 6% 6% 5% 8% 11% 21%	3087
August	1st to 10th	Night	15% 10% 4 8% 6% 5% 5% 4 7% 9% 26%	2825
	11th to 20th	Night	11% 8% 3 9% 6% 7% 6% 5% 7% 9% 29%	2785
	21st to end	Night	7% 5% 2 7% 5% 5% 5% 4 6% 8% 46%	3461
September	1st to 10th	Night	9% 7% 3 7% 4 5% 4 4 6% 10% 40%	2090
	11th to 20th	Night	16% 7% 3 8% 5% 4 5% 3 5% 8% 35%	2090
	21st to end	Night	16% 6% 2 7% 4 4 3 3 4 5% 45%	2090
October	1st to 10th	Night	14% 5% 2 5% 3 3 4 3 4 6% 51%	2128
	11th to 20th	Night	12% 5% 2 5% 2 4 3 3 4 6% 55%	2280
	21st to end	Night	13% 4 1 4 3 2 3 2 4 5% 58%	2508
November	1st to 10th	Night	15% 5% 2 5% 3 3 3 2 3 5% 54%	2382
	11th to 20th	Night	15% 5% 2 4 3 3 3 2 4 6% 52%	2600
	21st to end	Night	15% 5% 2 4 3 3 3 3 5% 5% 50%	2600
December	1st to 10th	Night	17% 5% 1 3 3 3 3 3 4 4 53%	4250
	11th to 20th	Night	19% 6% 2 6% 4 3 4 3 4 5% 44%	4822
	21st to end	Night	21% 5% 2 5% 3 3 3 3 4 5% 45%	6490

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	30% 5% 5% 23% 30% 7%	4057
	11th to 20th	Day	30% 4 5% 15% 39% 8%	4060
	21st to end	Day	42% 3 4 16% 25% 11%	4466
February	1st to 10th	Day	39% 5% 9% 21% 20% 5%	1890
	11th to 20th	Day	44% 5% 7% 18% 21% 6%	1958
	21st to end	Day	42% 5% 9% 15% 22% 7%	1650
March	1st to 10th	Day	45% 5% 6% 15% 18% 11%	4881
	11th to 20th	Day	30% 5% 7% 13% 26% 20%	4980
	21st to end	Day	40% 5% 7% 21% 21% 5%	5465
April	1st to 10th	Day	34% 6% 12% 20% 24% 6%	2200
	11th to 20th	Day	34% 6% 10% 20% 21% 9%	2352
	21st to end	Day	33% 6% 12% 25% 19% 4	2600
May	1st to 10th	Day	40% 9% 15% 19% 14% 2	4250
	11th to 20th	Day	43% 9% 13% 20% 14% 1	4215
	21st to end	Day	29% 10% 18% 23% 19% 2	4554
June	1st to 10th	Day	40% 11% 18% 22% 8% 2	2470
	11th to 20th	Day	33% 13% 19% 23% 12% 1	2470
	21st to end	Day	35% 12% 17% 19% 14% 3	2470
July	1st to 10th	Day	39% 13% 20% 18% 7% 1	4055
	11th to 20th	Day	35% 14% 28% 18% 4 1	4051
	21st to end	Day	51% 14% 18% 12% 5%	4324
August	1st to 10th	Day	47% 10% 17% 20% 5%	4091
	11th to 20th	Day	35% 12% 21% 17% 14% 2	4083
	21st to end	Day	28% 9% 16% 25% 19% 3	4159
September	1st to 10th	Day	35% 11% 13% 19% 19% 3	2090
	11th to 20th	Day	39% 8% 15% 15% 20% 2	2090
	21st to end	Day	39% 7% 11% 19% 18% 6%	2090
October	1st to 10th	Day	26% 4 9% 18% 33% 10%	3400
	11th to 20th	Day	18% 3 7% 16% 30% 26%	3140
	21st to end	Day	25% 4 5% 24% 26% 16%	3428
November	1st to 10th	Day	33% 5% 7% 19% 27% 9%	1912
	11th to 20th	Day	33% 4 7% 22% 25% 9%	1800
	21st to end	Day	34% 5% 8% 21% 24% 7%	1800
December	1st to 10th	Day	36% 3 5% 18% 28% 10%	2930
	11th to 20th	Day	43% 6% 9% 17% 18% 7%	3370
	21st to end	Day	39% 4 8% 26% 21% 2	4466
January	1st to 10th	Night	29% 4 5% 20% 34% 8%	5889
	11th to 20th	Night	31% 5% 6% 15% 35% 8%	5900
	21st to end	Night	38% 3 6% 16% 28% 10%	6490
February	1st to 10th	Night	38% 5% 9% 21% 21% 6%	2505
	11th to 20th	Night	45% 4 6% 16% 23% 7%	2339
	21st to end	Night	39% 6% 9% 16% 22% 8%	1980
March	1st to 10th	Night	39% 5% 9% 12% 27% 7%	4970
	11th to 20th	Night	27% 5% 8% 13% 29% 19%	4980
	21st to end	Night	38% 5% 7% 20% 21% 9%	5467
April	1st to 10th	Night	35% 5% 10% 19% 25% 7%	2200
	11th to 20th	Night	33% 5% 8% 22% 24% 9%	2011
	21st to end	Night	33% 5% 11% 26% 20% 5%	1800
May	1st to 10th	Night	41% 8% 14% 21% 15% 1	2930
	11th to 20th	Night	43% 9% 14% 19% 14% 1	2903
	21st to end	Night	29% 11% 18% 26% 15% 2	3138
June	1st to 10th	Night	40% 11% 17% 19% 10% 2	1710
	11th to 20th	Night	36% 11% 19% 21% 12% 1	1710
	21st to end	Night	37% 15% 17% 14% 12% 4	1710
July	1st to 10th	Night	47% 15% 17% 14% 6% 1	2795
	11th to 20th	Night	37% 17% 24% 18% 3 1	2786
	21st to end	Night	53% 14% 17% 11% 4 1	2988
August	1st to 10th	Night	50% 11% 20% 14% 5%	2814
	11th to 20th	Night	46% 13% 18% 14% 8%	2812
	21st to end	Night	31% 11% 14% 23% 19% 2	3461
September	1st to 10th	Night	38% 9% 15% 19% 15% 4	2090
	11th to 20th	Night	45% 9% 13% 16% 16% 2	2090
	21st to end	Night	40% 7% 11% 17% 18% 6%	2090
October	1st to 10th	Night	27% 7% 11% 15% 31% 8%	3544
	11th to 20th	Night	20% 4 6% 17% 27% 26%	3779
	21st to end	Night	27% 4 5% 20% 27% 17%	4120
November	1st to 10th	Night	34% 4 7% 18% 25% 11%	2382
	11th to 20th	Night	34% 4 6% 18% 27% 11%	2600
	21st to end	Night	35% 5% 7% 19% 27% 8%	2600
December	1st to 10th	Night	35% 4 5% 19% 30% 8%	4250
	11th to 20th	Night	43% 5% 8% 17% 18% 9%	4824
	21st to end	Night	40% 5% 9% 24% 19% 3	6490

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	22% 18% 35% 21% 3	300
	11th to 20th	Day	26% 16% 35% 18% 5%	300
	21st to end	Day	23% 19% 40% 14% 3	330
February	1st to 10th	Day	19% 21% 32% 21% 7%	300
	11th to 20th	Day	39% 13% 31% 15% 2	300
	21st to end	Day	26% 21% 35% 14% 4	249
March	1st to 10th	Day	20% 16% 34% 22% 7% 1	470
	11th to 20th	Day	18% 17% 38% 21% 5% 1	500
	21st to end	Day	21% 17% 34% 21% 6% 1	550
April	1st to 10th	Day	18% 14% 31% 26% 9% 2	500
	11th to 20th	Day	14% 13% 33% 25% 13% 2	492
	21st to end	Day	15% 10% 29% 23% 16% 6%	500
May	1st to 10th	Day	12% 14% 33% 24% 13% 4	503
	11th to 20th	Day	18% 6% 21% 26% 20% 8%	527
	21st to end	Day	15% 5% 18% 27% 23% 12%	549
June	1st to 10th	Day	10% 5% 15% 34% 25% 11%	490
	11th to 20th	Day	8% 5% 14% 29% 28% 17%	490
	21st to end	Day	10% 3 15% 32% 25% 16%	490
July	1st to 10th	Day	8% 3 10% 21% 34% 24%	485
	11th to 20th	Day	8% 2 9% 26% 35% 21%	484
	21st to end	Day	5% 3 9% 33% 33% 17%	532
August	1st to 10th	Day	16% 5% 13% 21% 27% 19%	486
	11th to 20th	Day	11% 3 9% 32% 28% 18%	479
	21st to end	Day	11% 2 10% 24% 27% 25%	539
September	1st to 10th	Day	9% 4 12% 27% 30% 17%	490
	11th to 20th	Day	19% 4 13% 20% 29% 15%	490
	21st to end	Day	20% 5% 18% 25% 25% 7%	490
October	1st to 10th	Day	18% 5% 18% 28% 21% 10%	490
	11th to 20th	Day	12% 7% 24% 29% 22% 6%	490
	21st to end	Day	16% 9% 21% 28% 21% 5%	539
November	1st to 10th	Day	21% 10% 22% 28% 15% 4	450
	11th to 20th	Day	18% 13% 35% 25% 7% 1	300
	21st to end	Day	10% 13% 35% 29% 11% 2	300
December	1st to 10th	Day	28% 14% 31% 22% 5%	300
	11th to 20th	Day	25% 13% 31% 22% 8% 1	300
	21st to end	Day	27% 13% 30% 20% 8% 2	330
January	1st to 10th	Night	21% 14% 35% 25% 6%	800
	11th to 20th	Night	22% 12% 36% 24% 6%	800
	21st to end	Night	20% 14% 37% 27% 3	880
February	1st to 10th	Night	19% 12% 34% 32% 4	800
	11th to 20th	Night	24% 13% 36% 25% 2	783
	21st to end	Night	19% 14% 38% 26% 3	660
March	1st to 10th	Night	18% 13% 34% 30% 5%	800
	11th to 20th	Night	17% 12% 38% 25% 8% 1	800
	21st to end	Night	17% 11% 36% 28% 8%	880
April	1st to 10th	Night	16% 16% 32% 28% 8%	800
	11th to 20th	Night	13% 11% 30% 33% 13% 1	735
	21st to end	Night	14% 4 29% 34% 19% 1	600
May	1st to 10th	Night	11% 8% 29% 33% 18% 1	600
	11th to 20th	Night	15% 7% 20% 36% 20% 2	591
	21st to end	Night	15% 2 13% 34% 32% 5%	634
June	1st to 10th	Night	11% 1 15% 36% 31% 5%	570
	11th to 20th	Night	11% 2 16% 27% 32% 11%	570
	21st to end	Night	8% 1 13% 35% 36% 7%	570
July	1st to 10th	Night	10% 2 8% 29% 40% 11%	555
	11th to 20th	Night	11% 2 5% 28% 42% 12%	552
	21st to end	Night	8% 2 10% 28% 42% 9%	624
August	1st to 10th	Night	15% 3 10% 24% 38% 10%	567
	11th to 20th	Night	13% 2 12% 27% 38% 8%	564
	21st to end	Night	13% 2 10% 24% 43% 9%	703
September	1st to 10th	Night	12% 2 11% 29% 38% 9%	760
	11th to 20th	Night	18% 3 14% 26% 32% 7%	760
	21st to end	Night	18% 4 18% 32% 27% 2	760
October	1st to 10th	Night	15% 4 19% 28% 27% 6%	760
	11th to 20th	Night	15% 6% 23% 34% 19% 3	760
	21st to end	Night	14% 6% 24% 33% 20% 3	836
November	1st to 10th	Night	17% 6% 23% 33% 18% 3	780
	11th to 20th	Night	16% 9% 30% 32% 12% 1	800
	21st to end	Night	16% 8% 28% 35% 13% 1	800
December	1st to 10th	Night	22% 11% 26% 33% 9%	800
	11th to 20th	Night	20% 8% 29% 33% 10%	800
	21st to end	Night	26% 9% 25% 31% 8%	880

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