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 2024-11-05.

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	12% 7% 2 7% 5% 5% 5% 4 6% 8% 39%	100526
	Night	13% 6% 2 6% 4 4 4 4 6% 8% 44%	98503
Transparency	Day	40% 7% 11% 18% 17% 7%	104176
	Night	36% 7% 12% 18% 18% 9%	102993
Seeing	Day	13% 8% 20% 24% 23% 12%	14892
	Night	14% 6% 22% 30% 22% 5%	24597

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	18% 9% 3 7% 4 4 3 3 5% 6% 39%	2847
	11th to 20th	Day	15% 7% 2 5% 3 2 3 2 3 5% 54%	2850
	21st to end	Day	24% 9% 3 6% 4 4 4 2 5% 5% 36%	3162
February	1st to 10th	Day	17% 10% 2 7% 5% 5% 5% 3 4 5% 38%	1900
	11th to 20th	Day	19% 8% 2 6% 4 5% 5% 3 4 5% 41%	1848
	21st to end	Day	18% 9% 3 7% 5% 4 3 3 6% 6% 36%	1575
March	1st to 10th	Day	24% 9% 3 8% 5% 4 4 3 6% 8% 26%	3436
	11th to 20th	Day	15% 8% 3 8% 5% 5% 5% 4 7% 9% 31%	3480
	21st to end	Day	12% 8% 3 7% 5% 5% 5% 4 5% 6% 41%	3899
April	1st to 10th	Day	11% 7% 3 6% 5% 5% 5% 3 6% 7% 41%	2280
	11th to 20th	Day	15% 8% 2 7% 4 4 4 4 6% 6% 38%	2254
	21st to end	Day	9% 7% 2 5% 5% 4 6% 5% 6% 8% 43%	2280
May	1st to 10th	Day	12% 7% 3 8% 6% 6% 7% 4 6% 8% 33%	3765
	11th to 20th	Day	14% 6% 2 6% 4 4 4 4 5% 7% 42%	3764
	21st to end	Day	6% 5% 3 6% 5% 5% 6% 5% 8% 10% 40%	4129
June	1st to 10th	Day	9% 5% 2 8% 6% 7% 7% 7% 9% 9% 32%	2441
	11th to 20th	Day	5% 5% 2 8% 8% 7% 7% 5% 8% 9% 35%	2470
	21st to end	Day	7% 6% 3 7% 8% 7% 8% 6% 9% 10% 30%	2470
July	1st to 10th	Day	6% 6% 3 10% 8% 8% 8% 7% 8% 10% 25%	4120
	11th to 20th	Day	6% 6% 3 9% 8% 7% 8% 7% 11% 12% 24%	4120
	21st to end	Day	12% 10% 4 10% 8% 6% 6% 5% 7% 9% 24%	4345
August	1st to 10th	Day	3 3 1 5% 4 4 5% 3 6% 9% 57%	3785
	11th to 20th	Day	6% 4 1 6% 5% 6% 6% 6% 8% 10% 41%	3729
	21st to end	Day	11% 6% 2 8% 6% 7% 6% 4 7% 8% 36%	4180
September	1st to 10th	Day	10% 6% 2 7% 5% 6% 7% 5% 9% 9% 33%	2280
	11th to 20th	Day	13% 6% 2 5% 5% 5% 5% 5% 6% 8% 41%	2280
	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% 7% 51%	1898
	11th to 20th	Day	10% 6% 2 6% 5% 3 3 3 6% 5% 51%	1900
	21st to end	Day	14% 7% 2 5% 5% 4 4 3 5% 6% 45%	1848
December	1st to 10th	Day	15% 9% 2 6% 4 4 4 4 5% 6% 42%	1767
	11th to 20th	Day	16% 8% 3 7% 5% 5% 4 3 4 5% 39%	2330
	21st to end	Day	17% 8% 2 6% 4 4 4 3 5% 7% 39%	3135
January	1st to 10th	Night	18% 8% 2 5% 4 3 3 3 4 6% 45%	4108
	11th to 20th	Night	16% 7% 2 4 3 3 3 2 4 5% 51%	4120
	21st to end	Night	26% 8% 2 4 3 3 2 2 4 7% 39%	4500
February	1st to 10th	Night	17% 7% 3 6% 4 3 4 3 5% 6% 41%	2280
	11th to 20th	Night	19% 7% 2 7% 4 3 3 3 4 6% 42%	2218
	21st to end	Night	22% 8% 3 5% 4 3 2 3 4 7% 37%	1884
March	1st to 10th	Night	24% 11% 3 7% 5% 3 3 3 4 6% 29%	3429
	11th to 20th	Night	17% 8% 3 7% 6% 5% 4 3 6% 7% 35%	3480
	21st to end	Night	15% 6% 2 6% 4 4 4 3 5% 6% 43%	3732
April	1st to 10th	Night	13% 7% 3 5% 4 4 3 4 6% 8% 44%	1900
	11th to 20th	Night	13% 6% 3 7% 4 3 4 4 4 7% 46%	1875
	21st to end	Night	9% 5% 1 5% 3 4 4 3 4 7% 54%	1900
May	1st to 10th	Night	13% 8% 2 7% 4 4 5% 3 6% 8% 41%	3130
	11th to 20th	Night	15% 7% 2 6% 4 3 5% 4 5% 8% 42%	3129
	21st to end	Night	8% 5% 2 6% 4 5% 5% 5% 8% 10% 41%	3426
June	1st to 10th	Night	7% 4 2 6% 5% 5% 5% 5% 8% 10% 44%	1733
	11th to 20th	Night	3 5% 1 4 4 5% 5% 6% 9% 12% 45%	1710
	21st to end	Night	6% 5% 2 6% 6% 5% 5% 6% 10% 9% 38%	1710
July	1st to 10th	Night	4 7% 3 8% 6% 6% 7% 6% 9% 12% 32%	2850
	11th to 20th	Night	5% 5% 2 6% 5% 6% 7% 6% 9% 13% 35%	2850
	21st to end	Night	7% 6% 3 9% 7% 6% 6% 5% 8% 13% 31%	3211
August	1st to 10th	Night	3 2 1 3 3 3 3 4 7% 12% 60%	3144
	11th to 20th	Night	3 3 2 6% 5% 5% 6% 5% 7% 13% 46%	3100
	21st to end	Night	7% 6% 3 7% 4 6% 6% 4 7% 8% 42%	3476
September	1st to 10th	Night	9% 5% 2 6% 4 4 5% 5% 7% 10% 42%	1900
	11th to 20th	Night	9% 7% 3 6% 4 5% 5% 4 6% 7% 43%	1900
	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% 4 2 5% 3 4 3 3 5% 6% 56%	2280
	11th to 20th	Night	12% 6% 2 5% 3 3 3 3 4 5% 54%	2280
	21st to end	Night	15% 7% 2 5% 4 3 3 3 5% 7% 46%	2310
December	1st to 10th	Night	16% 7% 2 7% 3 3 3 2 5% 7% 45%	2390
	11th to 20th	Night	14% 8% 3 7% 4 5% 3 4 5% 5% 42%	2862
	21st to end	Night	19% 10% 3 6% 4 3 3 3 3 5% 41%	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	46% 5% 4 16% 23% 6%	2848
	11th to 20th	Day	35% 3 4 18% 30% 9%	2850
	21st to end	Day	51% 3 5% 13% 20% 9%	3162
February	1st to 10th	Day	47% 5% 8% 12% 17% 10%	1900
	11th to 20th	Day	45% 4 6% 14% 21% 10%	1858
	21st to end	Day	47% 5% 8% 16% 16% 9%	1575
March	1st to 10th	Day	56% 6% 9% 15% 10% 4	3436
	11th to 20th	Day	47% 7% 9% 18% 15% 4	3480
	21st to end	Day	43% 6% 8% 14% 22% 7%	3899
April	1st to 10th	Day	41% 6% 9% 17% 18% 9%	2280
	11th to 20th	Day	43% 6% 9% 18% 16% 9%	2254
	21st to end	Day	37% 6% 11% 19% 23% 5%	2280
May	1st to 10th	Day	45% 9% 11% 20% 11% 3	3800
	11th to 20th	Day	40% 6% 10% 19% 23% 3	3768
	21st to end	Day	34% 10% 14% 30% 11% 1	4141
June	1st to 10th	Day	42% 11% 14% 20% 12% 1	2441
	11th to 20th	Day	40% 11% 13% 21% 13% 2	2470
	21st to end	Day	44% 14% 15% 19% 7% 1	2470
July	1st to 10th	Day	48% 12% 21% 16% 3	4056
	11th to 20th	Day	44% 16% 25% 11% 3 1	4051
	21st to end	Day	55% 10% 14% 16% 5% 1	4207
August	1st to 10th	Day	24% 8% 13% 32% 23%	3773
	11th to 20th	Day	34% 10% 20% 27% 8% 1	3765
	21st to end	Day	45% 9% 15% 15% 16% 2	4179
September	1st to 10th	Day	42% 12% 16% 14% 12% 4	2280
	11th to 20th	Day	40% 9% 13% 16% 17% 6%	2280
	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	32% 4 8% 19% 26% 10%	1898
	11th to 20th	Day	35% 4 6% 16% 26% 13%	1900
	21st to end	Day	40% 5% 7% 16% 22% 11%	1848
December	1st to 10th	Day	42% 5% 8% 19% 17% 8%	1767
	11th to 20th	Day	47% 5% 6% 13% 23% 7%	2328
	21st to end	Day	44% 6% 7% 17% 23% 3	3135
January	1st to 10th	Night	40% 4 6% 14% 26% 9%	4108
	11th to 20th	Night	35% 4 5% 19% 25% 12%	4120
	21st to end	Night	47% 3 5% 17% 20% 8%	4500
February	1st to 10th	Night	44% 4 7% 16% 19% 9%	2280
	11th to 20th	Night	43% 5% 5% 15% 19% 12%	2219
	21st to end	Night	47% 4 9% 15% 14% 12%	1884
March	1st to 10th	Night	54% 6% 9% 13% 12% 6%	3429
	11th to 20th	Night	47% 7% 9% 16% 16% 5%	3480
	21st to end	Night	41% 5% 9% 19% 20% 6%	3732
April	1st to 10th	Night	37% 7% 9% 18% 18% 11%	1900
	11th to 20th	Night	39% 5% 9% 18% 18% 11%	1875
	21st to end	Night	28% 7% 11% 21% 25% 7%	1900
May	1st to 10th	Night	39% 9% 13% 21% 14% 4	3160
	11th to 20th	Night	39% 7% 11% 22% 19% 2	3130
	21st to end	Night	32% 10% 17% 27% 12% 1	3435
June	1st to 10th	Night	31% 12% 18% 24% 13% 1	1733
	11th to 20th	Night	28% 11% 20% 25% 14% 3	1710
	21st to end	Night	35% 12% 19% 24% 9%	1710
July	1st to 10th	Night	40% 14% 27% 14% 4 1	2805
	11th to 20th	Night	34% 16% 34% 13% 2 1	2796
	21st to end	Night	43% 17% 18% 18% 4	3109
August	1st to 10th	Night	16% 8% 21% 30% 24% 1	3132
	11th to 20th	Night	28% 11% 25% 30% 6% 1	3130
	21st to end	Night	38% 9% 17% 18% 14% 4	3476
September	1st to 10th	Night	34% 11% 20% 17% 12% 6%	1900
	11th to 20th	Night	38% 10% 15% 16% 15% 6%	1900
	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	31% 4 9% 19% 25% 13%	2280
	11th to 20th	Night	33% 4 7% 14% 26% 16%	2280
	21st to end	Night	39% 4 6% 18% 21% 12%	2310
December	1st to 10th	Night	40% 5% 8% 21% 20% 6%	2390
	11th to 20th	Night	44% 4 6% 12% 20% 13%	2863
	21st to end	Night	47% 4 7% 16% 19% 7%	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	25% 18% 33% 17% 6% 1	270
	11th to 20th	Day	21% 19% 36% 19% 5%	270
	21st to end	Day	28% 20% 36% 13% 2	297
February	1st to 10th	Day	26% 23% 27% 19% 5%	270
	11th to 20th	Day	30% 19% 31% 15% 4	270
	21st to end	Day	20% 29% 40% 9% 2	225
March	1st to 10th	Day	24% 21% 31% 20% 4	270
	11th to 20th	Day	25% 19% 35% 16% 4	270
	21st to end	Day	16% 21% 34% 21% 8% 1	344
April	1st to 10th	Day	17% 16% 34% 24% 9% 1	460
	11th to 20th	Day	21% 15% 31% 21% 11% 1	452
	21st to end	Day	10% 13% 29% 29% 16% 2	460
May	1st to 10th	Day	11% 11% 29% 31% 15% 3	463
	11th to 20th	Day	11% 5% 25% 29% 23% 8%	487
	21st to end	Day	5% 3 19% 33% 27% 12%	529
June	1st to 10th	Day	9% 4 14% 31% 28% 14%	651
	11th to 20th	Day	7% 2 14% 29% 37% 11%	680
	21st to end	Day	8% 2 11% 25% 31% 22%	680
July	1st to 10th	Day	7% 2 8% 22% 40% 21%	671
	11th to 20th	Day	6% 2 5% 22% 38% 27%	668
	21st to end	Day	10% 2 7% 21% 38% 23%	668
August	1st to 10th	Day	3 1 5% 20% 37% 35%	486
	11th to 20th	Day	7% 1 5% 13% 39% 35%	479
	21st to end	Day	12% 3 6% 23% 30% 26%	539
September	1st to 10th	Day	11% 1 7% 21% 40% 19%	490
	11th to 20th	Day	12% 2 11% 22% 37% 16%	490
	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 27% 36% 15% 3	282
	11th to 20th	Day	16% 12% 31% 24% 16% 1	270
	21st to end	Day	14% 19% 33% 23% 8% 3	270
December	1st to 10th	Day	20% 12% 31% 25% 10% 2	270
	11th to 20th	Day	23% 13% 37% 22% 5%	270
	21st to end	Day	25% 16% 32% 20% 6% 1	297
January	1st to 10th	Night	23% 9% 36% 26% 6%	950
	11th to 20th	Night	21% 10% 32% 29% 9%	950
	21st to end	Night	23% 12% 33% 26% 6%	1026
February	1st to 10th	Night	20% 12% 36% 27% 5%	760
	11th to 20th	Night	23% 14% 30% 28% 6%	743
	21st to end	Night	25% 16% 35% 22% 2	628
March	1st to 10th	Night	17% 13% 32% 32% 6%	580
	11th to 20th	Night	18% 9% 33% 32% 8%	570
	21st to end	Night	16% 9% 35% 30% 10%	627
April	1st to 10th	Night	18% 9% 34% 29% 10% 1	570
	11th to 20th	Night	16% 10% 33% 30% 10% 1	563
	21st to end	Night	10% 6% 28% 39% 16% 1	570
May	1st to 10th	Night	12% 3 27% 40% 17% 1	570
	11th to 20th	Night	12% 3 20% 40% 22% 2	561
	21st to end	Night	7% 2 17% 37% 31% 5%	617
June	1st to 10th	Night	7% 2 10% 36% 40% 5%	570
	11th to 20th	Night	3 1 9% 35% 41% 9%	570
	21st to end	Night	6% 1 7% 27% 42% 16%	570
July	1st to 10th	Night	4 1 5% 24% 47% 19%	555
	11th to 20th	Night	5% 1 4 20% 47% 23%	552
	21st to end	Night	5% 1 5% 20% 52% 16%	624
August	1st to 10th	Night	5% 1 3 21% 50% 20%	567
	11th to 20th	Night	4 3 20% 51% 22%	564
	21st to end	Night	10% 1 5% 22% 44% 18%	627
September	1st to 10th	Night	10% 2 6% 27% 43% 13%	570
	11th to 20th	Night	11% 2 9% 27% 42% 10%	570
	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	11% 5% 21% 40% 20% 3	760
	11th to 20th	Night	13% 8% 28% 35% 14% 1	760
	21st to end	Night	19% 8% 27% 30% 15% 1	760
December	1st to 10th	Night	19% 9% 26% 32% 13%	760
	11th to 20th	Night	20% 9% 32% 31% 8%	797
	21st to end	Night	26% 11% 28% 27% 8%	1045

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