Dunceath Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dunceath Observatory clear sky chart from 2005-11-06 to 2025-09-19.

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	11% 6% 2 7% 5% 5% 5% 4 6% 8% 40%	125397
	Night	13% 6% 2 6% 4 4 4 4 5% 8% 44%	123724
Transparency	Day	40% 7% 12% 18% 17% 7%	129008
	Night	37% 6% 12% 18% 18% 9%	128188
Seeing	Day	13% 8% 20% 24% 23% 12%	16076
	Night	14% 7% 22% 30% 22% 5%	25706

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% 8% 3 8% 3 4 3 3 4 6% 43%	4077
	11th to 20th	Day	13% 7% 3 6% 4 3 3 2 3 5% 51%	4080
	21st to end	Day	22% 9% 3 6% 4 4 4 3 4 5% 35%	4528
February	1st to 10th	Day	16% 10% 2 7% 4 5% 5% 3 5% 5% 39%	2000
	11th to 20th	Day	18% 8% 3 6% 4 5% 5% 3 4 5% 40%	1948
	21st to end	Day	18% 9% 3 7% 5% 4 3 3 6% 6% 36%	1655
March	1st to 10th	Day	20% 9% 3 7% 5% 4 4 3 5% 7% 32%	4936
	11th to 20th	Day	12% 7% 2 6% 4 5% 4 3 6% 7% 43%	4980
	21st to end	Day	14% 8% 3 8% 5% 5% 5% 3 5% 6% 40%	5589
April	1st to 10th	Day	11% 7% 3 6% 5% 5% 5% 3 6% 7% 41%	2400
	11th to 20th	Day	15% 7% 2 7% 4 4 4 4 6% 6% 40%	2374
	21st to end	Day	9% 7% 2 5% 6% 4 6% 5% 6% 8% 42%	2400
May	1st to 10th	Day	16% 8% 3 7% 5% 5% 6% 4 6% 8% 31%	5405
	11th to 20th	Day	11% 6% 2 5% 4 5% 5% 5% 6% 9% 41%	5404
	21st to end	Day	6% 5% 2 6% 5% 5% 6% 5% 8% 11% 40%	5932
June	1st to 10th	Day	8% 5% 2 8% 6% 6% 7% 7% 9% 9% 33%	2570
	11th to 20th	Day	5% 5% 3 8% 8% 8% 7% 5% 8% 9% 35%	2600
	21st to end	Day	6% 6% 3 7% 8% 7% 8% 6% 9% 10% 30%	2600
July	1st to 10th	Day	5% 5% 2 9% 8% 8% 8% 7% 9% 12% 28%	5900
	11th to 20th	Day	5% 4 2 8% 7% 7% 7% 6% 10% 13% 31%	5900
	21st to end	Day	8% 7% 3 8% 7% 6% 6% 5% 8% 11% 29%	6247
August	1st to 10th	Day	3 3 1 5% 4 4 5% 3 6% 9% 56%	3905
	11th to 20th	Day	6% 4 1 6% 5% 6% 6% 6% 8% 11% 42%	3849
	21st to end	Day	11% 6% 2 8% 6% 6% 6% 4 7% 7% 36%	4312
September	1st to 10th	Day	10% 6% 2 7% 5% 6% 7% 5% 9% 9% 35%	2400
	11th to 20th	Day	12% 5% 2 5% 5% 5% 5% 5% 6% 8% 41%	2376
	21st to end	Day	9% 6% 2 6% 4 3 4 3 5% 6% 51%	2069
October	1st to 10th	Day	6% 4 2 5% 4 4 4 3 6% 7% 54%	1900
	11th to 20th	Day	9% 4 2 5% 3 3 3 3 5% 7% 57%	1900
	21st to end	Day	11% 5% 2 4 3 2 4 3 4 7% 55%	2090
November	1st to 10th	Day	10% 4 2 5% 4 4 4 3 6% 8% 49%	1958
	11th to 20th	Day	10% 6% 2 6% 5% 3 3 3 6% 5% 51%	2000
	21st to end	Day	13% 7% 2 5% 5% 4 4 3 5% 6% 45%	1945
December	1st to 10th	Day	16% 10% 3 6% 4 4 3 4 5% 5% 40%	3029
	11th to 20th	Day	12% 7% 3 7% 5% 5% 4 4 4 5% 44%	3651
	21st to end	Day	17% 7% 2 6% 4 4 3 3 4 5% 45%	4488
January	1st to 10th	Night	19% 7% 2 4 3 3 3 2 4 5% 49%	5888
	11th to 20th	Night	18% 8% 3 5% 3 3 2 2 3 5% 48%	5900
	21st to end	Night	25% 8% 2 5% 3 3 3 3 5% 6% 36%	6444
February	1st to 10th	Night	17% 8% 3 6% 4 3 4 3 5% 6% 41%	2400
	11th to 20th	Night	20% 7% 3 7% 4 3 3 3 4 6% 41%	2338
	21st to end	Night	22% 8% 3 5% 4 4 3 3 5% 7% 37%	1980
March	1st to 10th	Night	19% 10% 3 7% 4 3 4 3 5% 6% 37%	4929
	11th to 20th	Night	16% 7% 2 6% 4 4 3 2 4 6% 46%	4980
	21st to end	Night	18% 6% 3 6% 4 4 4 3 4 5% 42%	5340
April	1st to 10th	Night	14% 7% 3 5% 4 4 3 4 6% 8% 43%	2000
	11th to 20th	Night	13% 6% 3 7% 4 3 4 3 4 6% 47%	1975
	21st to end	Night	9% 5% 1 6% 3 4 4 4 4 7% 53%	2000
May	1st to 10th	Night	15% 8% 3 7% 4 4 4 3 6% 8% 37%	4490
	11th to 20th	Night	15% 8% 2 5% 4 3 4 3 5% 8% 42%	4489
	21st to end	Night	9% 6% 3 7% 4 5% 5% 4 7% 10% 39%	4922
June	1st to 10th	Night	7% 4 2 6% 5% 5% 5% 5% 8% 10% 44%	1824
	11th to 20th	Night	3 5% 2 4 5% 5% 6% 6% 9% 12% 45%	1800
	21st to end	Night	6% 5% 2 6% 6% 5% 5% 6% 10% 10% 38%	1800
July	1st to 10th	Night	3 6% 3 8% 6% 7% 8% 6% 9% 12% 33%	4078
	11th to 20th	Night	4 4 2 6% 5% 5% 6% 6% 9% 14% 40%	4077
	21st to end	Night	5% 5% 3 8% 6% 6% 6% 5% 8% 13% 35%	4614
August	1st to 10th	Night	3 2 1 3 3 3 3 4 7% 12% 59%	3244
	11th to 20th	Night	3 3 2 6% 5% 5% 6% 6% 7% 13% 46%	3200
	21st to end	Night	7% 6% 3 8% 4 6% 6% 4 7% 8% 42%	3586
September	1st to 10th	Night	9% 5% 2 6% 4 5% 5% 6% 7% 10% 42%	2000
	11th to 20th	Night	9% 7% 3 6% 5% 5% 5% 4 6% 8% 43%	1980
	21st to end	Night	7% 5% 2 5% 4 4 4 3 6% 6% 54%	2086
October	1st to 10th	Night	6% 5% 1 4 3 3 3 3 5% 7% 60%	2280
	11th to 20th	Night	7% 5% 2 5% 3 3 3 3 5% 5% 59%	2280
	21st to end	Night	10% 5% 2 5% 3 3 3 2 4 5% 57%	2508
November	1st to 10th	Night	10% 5% 2 5% 3 4 3 3 5% 6% 54%	2340
	11th to 20th	Night	12% 6% 2 5% 3 3 3 3 4 5% 55%	2400
	21st to end	Night	15% 7% 2 5% 4 3 3 3 4 7% 47%	2432
December	1st to 10th	Night	20% 7% 2 6% 4 4 3 2 4 5% 43%	4100
	11th to 20th	Night	11% 9% 3 7% 4 5% 4 4 5% 6% 44%	4530
	21st to end	Night	20% 10% 2 6% 3 3 2 2 3 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	44% 4 4 19% 24% 5%	4078
	11th to 20th	Day	38% 3 3 16% 30% 10%	4080
	21st to end	Day	53% 3 5% 11% 16% 12%	4528
February	1st to 10th	Day	47% 5% 8% 13% 18% 10%	2000
	11th to 20th	Day	46% 4 7% 14% 20% 9%	1958
	21st to end	Day	47% 5% 7% 16% 16% 9%	1655
March	1st to 10th	Day	51% 4 7% 14% 14% 10%	4936
	11th to 20th	Day	40% 5% 7% 14% 17% 17%	4980
	21st to end	Day	45% 5% 9% 15% 19% 7%	5591
April	1st to 10th	Day	41% 6% 9% 17% 17% 9%	2400
	11th to 20th	Day	42% 5% 9% 18% 17% 9%	2374
	21st to end	Day	37% 6% 11% 19% 22% 4	2400
May	1st to 10th	Day	48% 7% 11% 22% 9% 2	5440
	11th to 20th	Day	36% 6% 14% 24% 18% 2	5408
	21st to end	Day	34% 8% 15% 28% 13% 1	5945
June	1st to 10th	Day	41% 11% 14% 21% 12% 1	2570
	11th to 20th	Day	40% 11% 13% 21% 13% 2	2600
	21st to end	Day	44% 14% 16% 20% 6% 1	2600
July	1st to 10th	Day	43% 12% 24% 17% 4	5836
	11th to 20th	Day	38% 15% 31% 13% 2	5831
	21st to end	Day	46% 11% 18% 22% 3	6067
August	1st to 10th	Day	24% 8% 14% 32% 22%	3893
	11th to 20th	Day	33% 10% 21% 27% 8% 1	3885
	21st to end	Day	44% 9% 15% 15% 16% 2	4311
September	1st to 10th	Day	40% 11% 16% 15% 13% 4	2400
	11th to 20th	Day	39% 9% 13% 16% 17% 6%	2376
	21st to end	Day	34% 6% 10% 22% 19% 10%	2069
October	1st to 10th	Day	24% 4 6% 13% 22% 31%	3160
	11th to 20th	Day	17% 3 5% 18% 26% 30%	3140
	21st to end	Day	23% 3 4 11% 40% 18%	3428
November	1st to 10th	Day	34% 4 8% 19% 25% 10%	1958
	11th to 20th	Day	34% 4 6% 16% 27% 14%	2000
	21st to end	Day	39% 5% 7% 16% 23% 11%	1945
December	1st to 10th	Day	46% 4 6% 15% 22% 7%	3029
	11th to 20th	Day	41% 5% 9% 14% 22% 9%	3649
	21st to end	Day	42% 5% 5% 19% 25% 4	4488
January	1st to 10th	Night	39% 4 5% 21% 26% 7%	5888
	11th to 20th	Night	40% 3 4 15% 28% 11%	5900
	21st to end	Night	50% 3 6% 14% 17% 11%	6444
February	1st to 10th	Night	44% 4 7% 15% 20% 10%	2400
	11th to 20th	Night	45% 5% 5% 15% 19% 11%	2339
	21st to end	Night	47% 3 8% 15% 14% 12%	1980
March	1st to 10th	Night	47% 5% 8% 11% 18% 11%	4929
	11th to 20th	Night	40% 5% 7% 14% 17% 16%	4980
	21st to end	Night	43% 4 8% 18% 18% 8%	5340
April	1st to 10th	Night	38% 7% 9% 18% 18% 11%	2000
	11th to 20th	Night	38% 5% 9% 19% 19% 11%	1975
	21st to end	Night	29% 7% 11% 21% 24% 7%	2000
May	1st to 10th	Night	43% 8% 13% 21% 14% 3	4520
	11th to 20th	Night	39% 6% 14% 24% 15% 1	4490
	21st to end	Night	35% 8% 17% 28% 10% 1	4931
June	1st to 10th	Night	31% 12% 19% 23% 14% 1	1824
	11th to 20th	Night	29% 11% 20% 25% 14% 2	1800
	21st to end	Night	34% 12% 21% 24% 9%	1800
July	1st to 10th	Night	38% 14% 28% 15% 4	4035
	11th to 20th	Night	31% 16% 39% 12% 2 1	4026
	21st to end	Night	38% 15% 19% 24% 4	4481
August	1st to 10th	Night	16% 8% 21% 30% 23% 1	3232
	11th to 20th	Night	28% 11% 25% 29% 6%	3230
	21st to end	Night	39% 9% 17% 18% 14% 4	3586
September	1st to 10th	Night	34% 11% 20% 18% 12% 5%	2000
	11th to 20th	Night	37% 9% 16% 16% 16% 6%	1980
	21st to end	Night	29% 7% 11% 24% 17% 12%	2086
October	1st to 10th	Night	23% 4 8% 14% 21% 31%	3800
	11th to 20th	Night	17% 3 4 20% 26% 29%	3779
	21st to end	Night	20% 3 5% 12% 35% 24%	4120
November	1st to 10th	Night	32% 4 8% 19% 24% 13%	2340
	11th to 20th	Night	33% 3 7% 14% 26% 17%	2400
	21st to end	Night	39% 4 6% 18% 22% 12%	2432
December	1st to 10th	Night	45% 3 5% 18% 25% 5%	4100
	11th to 20th	Night	40% 5% 8% 16% 20% 10%	4531
	21st to end	Night	45% 3 6% 16% 22% 7%	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	25% 18% 34% 16% 6% 1	300
	11th to 20th	Day	21% 20% 36% 18% 5%	300
	21st to end	Day	27% 20% 36% 15% 2	330
February	1st to 10th	Day	24% 21% 28% 21% 5%	300
	11th to 20th	Day	30% 21% 31% 14% 4	300
	21st to end	Day	19% 27% 40% 10% 2	249
March	1st to 10th	Day	25% 21% 31% 19% 4	300
	11th to 20th	Day	25% 20% 35% 15% 4	300
	21st to end	Day	16% 20% 34% 22% 7% 1	380
April	1st to 10th	Day	18% 16% 34% 23% 9% 1	500
	11th to 20th	Day	20% 15% 31% 22% 11% 1	492
	21st to end	Day	11% 13% 28% 29% 16% 3	500
May	1st to 10th	Day	13% 10% 30% 30% 14% 3	503
	11th to 20th	Day	11% 5% 27% 28% 21% 7%	527
	21st to end	Day	6% 3 20% 33% 26% 11%	573
June	1st to 10th	Day	9% 4 14% 32% 28% 14%	700
	11th to 20th	Day	6% 2 14% 29% 38% 11%	730
	21st to end	Day	8% 2 11% 25% 32% 23%	730
July	1st to 10th	Day	7% 2 8% 22% 40% 21%	721
	11th to 20th	Day	6% 2 5% 22% 39% 26%	718
	21st to end	Day	9% 2 7% 21% 39% 23%	719
August	1st to 10th	Day	3 1 5% 19% 39% 33%	526
	11th to 20th	Day	7% 1 4 13% 38% 37%	519
	21st to end	Day	12% 3 5% 23% 30% 26%	583
September	1st to 10th	Day	10% 2 8% 23% 40% 18%	530
	11th to 20th	Day	11% 2 11% 21% 38% 17%	522
	21st to end	Day	11% 4 12% 26% 32% 15%	464
October	1st to 10th	Day	6% 4 23% 38% 22% 7%	300
	11th to 20th	Day	7% 7% 22% 35% 19% 10%	300
	21st to end	Day	16% 11% 21% 31% 16% 5%	330
November	1st to 10th	Day	15% 4 28% 36% 15% 3	300
	11th to 20th	Day	15% 12% 32% 24% 15% 1	300
	21st to end	Day	14% 19% 34% 23% 8% 3	300
December	1st to 10th	Day	20% 12% 33% 24% 9% 2	300
	11th to 20th	Day	21% 13% 37% 22% 6%	300
	21st to end	Day	25% 16% 32% 21% 6% 1	330
January	1st to 10th	Night	23% 9% 37% 25% 6%	1000
	11th to 20th	Night	21% 11% 32% 28% 8%	1000
	21st to end	Night	24% 11% 33% 26% 6%	1080
February	1st to 10th	Night	20% 12% 36% 28% 5%	800
	11th to 20th	Night	23% 14% 30% 27% 5%	783
	21st to end	Night	25% 16% 35% 21% 3	660
March	1st to 10th	Night	17% 13% 32% 32% 5%	610
	11th to 20th	Night	18% 10% 33% 31% 8%	600
	21st to end	Night	16% 9% 35% 31% 10%	660
April	1st to 10th	Night	19% 9% 33% 28% 10% 1	600
	11th to 20th	Night	16% 10% 33% 31% 10% 1	593
	21st to end	Night	10% 7% 28% 39% 16% 1	600
May	1st to 10th	Night	12% 4 27% 41% 16% 1	600
	11th to 20th	Night	13% 4 22% 39% 21% 2	591
	21st to end	Night	8% 2 17% 37% 31% 5%	650
June	1st to 10th	Night	7% 2 10% 37% 39% 5%	600
	11th to 20th	Night	3 1 9% 36% 42% 9%	600
	21st to end	Night	6% 1 7% 27% 43% 16%	600
July	1st to 10th	Night	4 1 5% 24% 48% 18%	585
	11th to 20th	Night	5% 1 4 20% 47% 23%	582
	21st to end	Night	5% 2 5% 20% 52% 16%	657
August	1st to 10th	Night	5% 1 3 21% 51% 20%	597
	11th to 20th	Night	4 3 20% 51% 22%	594
	21st to end	Night	10% 1 5% 23% 43% 18%	660
September	1st to 10th	Night	10% 2 7% 28% 42% 12%	600
	11th to 20th	Night	10% 2 8% 27% 43% 10%	594
	21st to end	Night	8% 2 10% 34% 40% 6%	735
October	1st to 10th	Night	7% 2 17% 37% 33% 4	760
	11th to 20th	Night	8% 4 22% 38% 26% 3	760
	21st to end	Night	13% 6% 22% 35% 21% 2	836
November	1st to 10th	Night	12% 6% 20% 40% 19% 3	780
	11th to 20th	Night	13% 8% 29% 34% 14% 1	800
	21st to end	Night	19% 8% 27% 31% 15% 1	800
December	1st to 10th	Night	20% 9% 27% 32% 12%	800
	11th to 20th	Night	19% 9% 32% 32% 8%	839
	21st to end	Night	26% 11% 29% 27% 8%	1100

Page generated at 2025-09-16 00:49:12. Version ac07

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