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 2024-11-02.

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	14% 6% 2 6% 4 4 4 4 6% 8% 43%	101098
	Night	15% 6% 2 6% 4 4 4 4 5% 7% 43%	97820
Transparency	Day	37% 8% 13% 19% 18% 4	100911
	Night	39% 8% 11% 18% 19% 6%	97677
Seeing	Day	16% 9% 22% 25% 19% 9%	15108
	Night	16% 7% 24% 30% 19% 3	25151

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	17% 4 1 3 3 3 5% 3 4 7% 50%	2836
	11th to 20th	Day	15% 6% 2 4 4 3 4 3 3 3 53%	2840
	21st to end	Day	26% 5% 2 4 2 2 2 2 3 4 48%	3124
February	1st to 10th	Day	15% 7% 2 6% 4 3 4 4 6% 6% 44%	1795
	11th to 20th	Day	22% 5% 2 5% 3 4 3 3 3 5% 44%	1848
	21st to end	Day	20% 6% 2 6% 4 4 3 3 4 6% 41%	1570
March	1st to 10th	Day	25% 8% 2 5% 3 3 3 2 4 5% 40%	3395
	11th to 20th	Day	12% 5% 2 5% 4 4 4 3 5% 5% 52%	3480
	21st to end	Day	21% 6% 2 4 4 4 3 4 5% 6% 41%	3815
April	1st to 10th	Day	12% 4 2 6% 3 3 4 4 6% 8% 48%	2090
	11th to 20th	Day	13% 5% 2 5% 4 4 3 4 5% 8% 46%	2232
	21st to end	Day	11% 4 2 5% 3 4 4 4 5% 6% 51%	2470
May	1st to 10th	Day	11% 8% 3 7% 5% 4 5% 4 6% 9% 39%	4082
	11th to 20th	Day	17% 8% 3 6% 4 4 4 3 5% 8% 37%	4081
	21st to end	Day	8% 4 2 5% 4 4 5% 5% 7% 11% 45%	4476
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 59%	2090
November	1st to 10th	Day	13% 5% 1 4 3 3 2 3 4 5% 56%	1825
	11th to 20th	Day	14% 4 1 5% 3 4 3 3 5% 6% 52%	1710
	21st to end	Day	16% 5% 2 4 3 3 4 4 4 5% 50%	1710
December	1st to 10th	Day	20% 8% 3 5% 4 4 3 2 4 6% 40%	1710
	11th to 20th	Day	18% 5% 2 4 3 2 3 3 4 6% 49%	2153
	21st to end	Day	22% 5% 2 4 3 3 3 3 4 6% 43%	3124
January	1st to 10th	Night	18% 4 2 4 3 3 3 3 4 5% 51%	4109
	11th to 20th	Night	12% 6% 2 5% 4 3 4 3 5% 6% 50%	4120
	21st to end	Night	25% 7% 2 5% 3 3 2 2 3 4 44%	4532
February	1st to 10th	Night	15% 6% 2 5% 3 4 3 4 5% 5% 47%	2380
	11th to 20th	Night	21% 7% 2 5% 5% 3 3 2 4 5% 45%	2218
	21st to end	Night	17% 7% 2 6% 4 4 3 3 5% 5% 43%	1884
March	1st to 10th	Night	22% 7% 2 7% 4 3 4 3 4 6% 37%	3456
	11th to 20th	Night	11% 5% 1 5% 3 3 4 3 5% 7% 52%	3480
	21st to end	Night	20% 6% 2 6% 3 4 3 4 5% 5% 43%	3817
April	1st to 10th	Night	13% 6% 2 5% 3 4 4 4 5% 7% 49%	2090
	11th to 20th	Night	11% 7% 2 5% 3 4 3 3 5% 6% 51%	1910
	21st to end	Night	12% 6% 2 5% 4 3 4 3 5% 6% 50%	1710
May	1st to 10th	Night	14% 7% 2 6% 5% 5% 4 3 6% 8% 39%	2813
	11th to 20th	Night	18% 6% 2 6% 5% 3 4 4 5% 7% 38%	2812
	21st to end	Night	9% 4 2 6% 5% 4 5% 4 7% 11% 43%	3079
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% 5% 1 4 3 2 3 2 4 5% 58%	2508
November	1st to 10th	Night	14% 4 2 4 3 3 3 2 3 5% 55%	2272
	11th to 20th	Night	15% 5% 2 4 3 3 3 2 4 7% 52%	2470
	21st to end	Night	16% 5% 2 4 3 3 4 3 5% 5% 49%	2470
December	1st to 10th	Night	20% 7% 2 5% 4 3 4 3 5% 5% 42%	2470
	11th to 20th	Night	17% 5% 2 5% 3 2 3 2 3 5% 53%	3042
	21st to end	Night	21% 6% 2 6% 3 4 4 3 5% 6% 40%	4532

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	33% 6% 7% 24% 23% 7%	2837
	11th to 20th	Day	36% 5% 6% 16% 32% 6%	2840
	21st to end	Day	42% 3 5% 17% 27% 7%	3124
February	1st to 10th	Day	39% 5% 9% 20% 20% 6%	1795
	11th to 20th	Day	43% 5% 7% 18% 21% 6%	1858
	21st to end	Day	44% 5% 9% 15% 20% 7%	1570
March	1st to 10th	Day	46% 6% 8% 17% 14% 10%	3395
	11th to 20th	Day	32% 6% 8% 14% 29% 11%	3480
	21st to end	Day	43% 7% 8% 19% 21% 3	3815
April	1st to 10th	Day	33% 6% 11% 19% 25% 6%	2090
	11th to 20th	Day	35% 6% 10% 19% 21% 10%	2232
	21st to end	Day	33% 6% 12% 26% 20% 4	2470
May	1st to 10th	Day	40% 9% 15% 19% 14% 3	4120
	11th to 20th	Day	43% 9% 13% 21% 14% 1	4085
	21st to end	Day	29% 10% 18% 23% 18% 2	4489
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	30% 5% 11% 19% 29% 6%	2049
	11th to 20th	Day	27% 4 11% 23% 25% 10%	1900
	21st to end	Day	30% 3 5% 22% 28% 11%	2090
November	1st to 10th	Day	31% 5% 7% 18% 28% 10%	1825
	11th to 20th	Day	33% 5% 7% 21% 25% 9%	1710
	21st to end	Day	35% 5% 8% 20% 24% 7%	1710
December	1st to 10th	Day	46% 5% 7% 20% 17% 5%	1710
	11th to 20th	Day	37% 4 8% 18% 22% 11%	2150
	21st to end	Day	40% 5% 8% 20% 23% 3	3124
January	1st to 10th	Night	35% 5% 7% 19% 27% 7%	4109
	11th to 20th	Night	34% 6% 8% 17% 28% 8%	4120
	21st to end	Night	44% 4 6% 15% 24% 7%	4532
February	1st to 10th	Night	38% 5% 9% 19% 22% 6%	2380
	11th to 20th	Night	44% 4 6% 16% 23% 7%	2219
	21st to end	Night	41% 6% 9% 16% 21% 8%	1884
March	1st to 10th	Night	46% 6% 12% 14% 15% 7%	3456
	11th to 20th	Night	30% 5% 9% 16% 26% 14%	3480
	21st to end	Night	42% 6% 7% 16% 20% 8%	3817
April	1st to 10th	Night	35% 5% 9% 18% 26% 7%	2090
	11th to 20th	Night	33% 5% 8% 21% 24% 9%	1910
	21st to end	Night	33% 5% 11% 25% 21% 5%	1710
May	1st to 10th	Night	41% 8% 13% 21% 15% 2	2840
	11th to 20th	Night	43% 9% 14% 19% 15% 1	2813
	21st to end	Night	28% 11% 18% 26% 15% 2	3087
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	33% 7% 11% 16% 28% 6%	2128
	11th to 20th	Night	31% 5% 9% 21% 22% 12%	2280
	21st to end	Night	30% 4 6% 20% 28% 13%	2508
November	1st to 10th	Night	33% 4 7% 18% 26% 12%	2272
	11th to 20th	Night	34% 4 6% 17% 27% 12%	2470
	21st to end	Night	36% 5% 7% 19% 26% 7%	2470
December	1st to 10th	Night	44% 5% 7% 18% 20% 6%	2470
	11th to 20th	Night	36% 4 7% 17% 26% 10%	3044
	21st to end	Night	43% 6% 9% 20% 19% 4	4532

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% 34% 22% 4	270
	11th to 20th	Day	27% 17% 33% 17% 5%	270
	21st to end	Day	23% 20% 40% 14% 3	297
February	1st to 10th	Day	20% 22% 32% 19% 6%	270
	11th to 20th	Day	37% 14% 32% 15% 2	270
	21st to end	Day	28% 22% 36% 12% 2	225
March	1st to 10th	Day	21% 15% 34% 23% 6% 1	431
	11th to 20th	Day	18% 17% 38% 21% 5% 1	460
	21st to end	Day	21% 17% 35% 20% 6% 1	506
April	1st to 10th	Day	18% 15% 31% 26% 9% 2	460
	11th to 20th	Day	15% 13% 31% 26% 13% 2	452
	21st to end	Day	16% 11% 28% 24% 15% 7%	460
May	1st to 10th	Day	11% 14% 33% 23% 14% 5%	463
	11th to 20th	Day	18% 5% 20% 27% 22% 8%	487
	21st to end	Day	14% 5% 18% 28% 23% 12%	529
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% 4	539
November	1st to 10th	Day	20% 9% 22% 28% 15% 5%	417
	11th to 20th	Day	17% 13% 36% 25% 7% 1	270
	21st to end	Day	10% 11% 36% 29% 11% 3	270
December	1st to 10th	Day	29% 13% 30% 23% 5%	270
	11th to 20th	Day	24% 13% 31% 23% 8% 1	270
	21st to end	Day	27% 13% 31% 20% 7% 2	297
January	1st to 10th	Night	22% 13% 34% 25% 6%	760
	11th to 20th	Night	22% 12% 35% 25% 6%	760
	21st to end	Night	20% 13% 37% 27% 3	836
February	1st to 10th	Night	19% 12% 33% 32% 4	760
	11th to 20th	Night	24% 13% 36% 25% 2	743
	21st to end	Night	20% 14% 38% 25% 3	628
March	1st to 10th	Night	18% 12% 34% 30% 5%	760
	11th to 20th	Night	17% 12% 37% 25% 8% 1	760
	21st to end	Night	17% 11% 35% 27% 9%	836
April	1st to 10th	Night	15% 16% 32% 28% 9%	760
	11th to 20th	Night	13% 11% 29% 32% 14% 1	698
	21st to end	Night	14% 4 29% 34% 18% 1	570
May	1st to 10th	Night	11% 8% 28% 33% 19% 1	570
	11th to 20th	Night	14% 6% 20% 36% 21% 2	561
	21st to end	Night	14% 2 12% 34% 32% 5%	617
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% 7% 24% 33% 20% 3	836
November	1st to 10th	Night	17% 6% 23% 33% 18% 3	745
	11th to 20th	Night	16% 9% 31% 31% 12% 1	760
	21st to end	Night	17% 8% 27% 35% 13% 1	760
December	1st to 10th	Night	22% 11% 25% 33% 9%	760
	11th to 20th	Night	19% 8% 30% 33% 10%	760
	21st to end	Night	26% 9% 26% 31% 8%	836

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