Dun Glen
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dun Glen clear sky chart from 2005-11-06 to 2025-07-06.

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	3 6% 2 5% 4 3 4 3 5% 7% 59%	123558
	Night	4 6% 3 6% 4 4 4 3 5% 7% 54%	124041
Transparency	Day	24% 5% 8% 18% 24% 21%	123300
	Night	28% 6% 9% 19% 23% 16%	123887
Seeing	Day	7% 8% 22% 31% 23% 8%	15627
	Night	8% 4 16% 40% 27% 6%	25021

By Month

Percentage Cloud Cover SW (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	4 14% 6% 10% 5% 5% 5% 5% 7% 9% 32%	3639
	11th to 20th	Day	4 9% 3 9% 5% 3 4 3 5% 7% 47%	3640
	21st to end	Day	4 11% 5% 9% 5% 5% 5% 4 5% 8% 38%	4160
February	1st to 10th	Day	4 8% 3 8% 6% 5% 6% 3 8% 9% 41%	1901
	11th to 20th	Day	3 8% 3 8% 6% 5% 5% 4 6% 9% 41%	1950
	21st to end	Day	2 8% 3 7% 5% 6% 5% 4 7% 10% 44%	1650
March	1st to 10th	Day	3 14% 4 11% 6% 5% 5% 4 6% 8% 34%	4518
	11th to 20th	Day	2 7% 3 7% 5% 5% 4 3 6% 10% 48%	4647
	21st to end	Day	2 9% 3 9% 6% 5% 6% 4 6% 7% 42%	5969
April	1st to 10th	Day	4 9% 3 8% 5% 5% 5% 4 6% 8% 43%	2400
	11th to 20th	Day	4 8% 2 6% 6% 4 4 4 5% 8% 49%	2369
	21st to end	Day	6% 7% 2 6% 4 4 4 3 6% 8% 50%	2490
May	1st to 10th	Day	5% 5% 1 4 3 2 3 3 4 7% 63%	5880
	11th to 20th	Day	3 4 2 4 3 3 3 3 4 6% 64%	5963
	21st to end	Day	3 3 1 4 3 3 3 2 4 5% 68%	6902
June	1st to 10th	Day	4 4 1 4 4 4 4 5% 6% 7% 57%	2800
	11th to 20th	Day	3 3 1 3 3 3 3 3 4 6% 70%	2800
	21st to end	Day	2 2 1 2 2 2 2 2 4 4 75%	2800
July	1st to 10th	Day	1 1 1 1 2 1 2 2 3 86%	4491
	11th to 20th	Day	1 2 1 3 2 2 3 3 4 6% 73%	4420
	21st to end	Day	2 2 1 3 2 3 3 3 5% 6% 69%	4790
August	1st to 10th	Day	3 2 1 3 2 2 2 2 4 6% 72%	4177
	11th to 20th	Day	1 1 1 1 1 1 2 2 3 85%	3724
	21st to end	Day	2 2 1 2 2 2 2 2 2 3 82%	4180
September	1st to 10th	Day	1 2 1 1 1 1 2 2 3 4 81%	2280
	11th to 20th	Day	3 2 1 2 3 3 3 3 5% 5% 70%	2161
	21st to end	Day	2 3 1 3 2 2 2 2 4 6% 74%	2090
October	1st to 10th	Day	3 2 1 3 3 3 3 2 3 5% 72%	3459
	11th to 20th	Day	4 5% 1 3 3 2 2 2 3 5% 70%	3161
	21st to end	Day	2 3 2 5% 3 4 4 4 5% 8% 59%	3115
November	1st to 10th	Day	4 9% 2 7% 4 4 3 3 5% 6% 53%	1764
	11th to 20th	Day	5% 6% 3 5% 4 3 3 5% 6% 10% 51%	1800
	21st to end	Day	4 8% 4 7% 5% 4 4 3 5% 8% 50%	1800
December	1st to 10th	Day	3 4 2 5% 5% 3 3 2 4 7% 62%	2636
	11th to 20th	Day	4 11% 3 9% 6% 6% 6% 6% 9% 10% 30%	3028
	21st to end	Day	4 10% 4 10% 7% 6% 5% 4 7% 9% 34%	4004
January	1st to 10th	Night	3 11% 5% 11% 5% 5% 6% 5% 8% 8% 33%	6306
	11th to 20th	Night	6% 8% 3 8% 5% 4 4 3 5% 6% 48%	6320
	21st to end	Night	4 9% 3 7% 5% 4 5% 4 5% 8% 44%	6800
February	1st to 10th	Night	4 8% 3 7% 4 4 5% 4 6% 7% 45%	2496
	11th to 20th	Night	3 8% 3 9% 5% 6% 5% 4 6% 8% 44%	2338
	21st to end	Night	2 8% 3 7% 5% 5% 5% 6% 7% 8% 42%	1980
March	1st to 10th	Night	3 13% 6% 10% 5% 4 4 4 5% 7% 40%	5376
	11th to 20th	Night	3 7% 3 8% 5% 5% 4 4 6% 8% 48%	5335
	21st to end	Night	4 8% 3 8% 6% 5% 6% 4 7% 9% 40%	5006
April	1st to 10th	Night	4 8% 3 7% 6% 5% 5% 5% 6% 8% 44%	2000
	11th to 20th	Night	5% 7% 3 5% 4 4 4 3 5% 7% 53%	1979
	21st to end	Night	7% 7% 2 5% 4 4 4 3 5% 7% 53%	1907
May	1st to 10th	Night	6% 6% 2 4 2 2 2 2 3 4 66%	4100
	11th to 20th	Night	3 5% 2 5% 3 3 4 2 5% 7% 62%	3941
	21st to end	Night	3 4 1 3 3 3 3 2 5% 7% 65%	3972
June	1st to 10th	Night	6% 5% 2 4 4 4 4 3 6% 7% 56%	1600
	11th to 20th	Night	3 3 1 3 2 3 4 2 4 5% 68%	1600
	21st to end	Night	2 2 1 3 3 3 4 3 4 6% 72%	1600
July	1st to 10th	Night	1 1 2 2 2 2 2 4 3 81%	2580
	11th to 20th	Night	3 4 1 4 3 3 3 3 4 6% 65%	2540
	21st to end	Night	3 3 1 5% 3 4 4 4 6% 8% 60%	2761
August	1st to 10th	Night	3 3 1 4 3 3 3 3 4 6% 65%	2743
	11th to 20th	Night	2 3 1 3 1 2 2 2 3 4 78%	3126
	21st to end	Night	3 2 1 2 2 1 1 2 2 3 80%	3498
September	1st to 10th	Night	3 3 1 3 2 2 2 2 3 4 75%	1900
	11th to 20th	Night	4 3 1 3 3 3 3 3 4 5% 69%	2019
	21st to end	Night	3 2 1 2 2 3 2 2 3 5% 75%	2090
October	1st to 10th	Night	3 3 1 3 2 3 3 2 3 4 73%	3518
	11th to 20th	Night	2 3 1 3 2 2 3 3 4 4 72%	3777
	21st to end	Night	4 2 1 3 2 3 4 4 5% 8% 62%	4442
November	1st to 10th	Night	5% 7% 2 5% 4 4 5% 4 7% 7% 50%	2535
	11th to 20th	Night	4 6% 2 7% 4 3 4 4 6% 8% 52%	2600
	21st to end	Night	4 7% 3 6% 5% 4 5% 4 5% 7% 51%	2600
December	1st to 10th	Night	3 5% 2 5% 4 3 4 3 4 7% 60%	4528
	11th to 20th	Night	4 11% 4 9% 5% 5% 6% 5% 8% 11% 32%	5176
	21st to end	Night	6% 11% 4 11% 7% 7% 6% 5% 7% 8% 27%	6952

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	45% 9% 16% 21% 9%	3639
	11th to 20th	Day	35% 7% 13% 23% 20% 2	3640
	21st to end	Day	43% 7% 8% 13% 22% 6%	4160
February	1st to 10th	Day	36% 7% 13% 27% 13% 4	1901
	11th to 20th	Day	36% 7% 14% 23% 17% 3	1960
	21st to end	Day	32% 7% 16% 24% 15% 6%	1650
March	1st to 10th	Day	44% 6% 9% 20% 16% 5%	4518
	11th to 20th	Day	29% 6% 12% 34% 18% 1	4647
	21st to end	Day	35% 9% 13% 23% 17% 3	5969
April	1st to 10th	Day	38% 5% 10% 24% 17% 6%	2400
	11th to 20th	Day	32% 5% 11% 23% 21% 8%	2372
	21st to end	Day	31% 5% 10% 28% 23% 3	2490
May	1st to 10th	Day	21% 5% 7% 24% 37% 7%	5880
	11th to 20th	Day	19% 6% 6% 20% 40% 10%	5966
	21st to end	Day	18% 4 4 17% 42% 15%	6904
June	1st to 10th	Day	24% 5% 7% 19% 31% 14%	2800
	11th to 20th	Day	17% 3 5% 15% 32% 29%	2800
	21st to end	Day	13% 2 3 10% 32% 40%	2800
July	1st to 10th	Day	6% 1 2 5% 13% 72%	4422
	11th to 20th	Day	11% 2 4 6% 15% 62%	4345
	21st to end	Day	14% 4 4 8% 24% 47%	4639
August	1st to 10th	Day	14% 2 2 6% 18% 57%	4161
	11th to 20th	Day	7% 1 3 5% 20% 65%	3760
	21st to end	Day	9% 2 2 8% 30% 49%	4179
September	1st to 10th	Day	9% 2 2 8% 28% 52%	2280
	11th to 20th	Day	16% 3 5% 13% 30% 33%	2161
	21st to end	Day	14% 2 5% 17% 31% 31%	2090
October	1st to 10th	Day	16% 2 4 10% 33% 35%	3459
	11th to 20th	Day	19% 3 4 18% 41% 15%	3161
	21st to end	Day	19% 6% 8% 30% 25% 12%	3115
November	1st to 10th	Day	32% 6% 9% 23% 22% 8%	1764
	11th to 20th	Day	28% 6% 13% 29% 21% 3	1800
	21st to end	Day	33% 5% 11% 26% 22% 4	1800
December	1st to 10th	Day	23% 4 8% 23% 36% 6%	2636
	11th to 20th	Day	41% 11% 14% 21% 11% 1	3028
	21st to end	Day	39% 11% 18% 23% 9%	4004
January	1st to 10th	Night	40% 11% 16% 24% 8% 1	6306
	11th to 20th	Night	37% 6% 12% 20% 21% 4	6320
	21st to end	Night	36% 7% 8% 17% 26% 6%	6800
February	1st to 10th	Night	35% 6% 13% 26% 16% 4	2496
	11th to 20th	Night	35% 5% 12% 26% 18% 4	2339
	21st to end	Night	34% 7% 14% 22% 16% 7%	1980
March	1st to 10th	Night	41% 6% 9% 19% 15% 10%	5376
	11th to 20th	Night	30% 7% 8% 29% 24% 2	5335
	21st to end	Night	35% 9% 15% 26% 13% 2	5006
April	1st to 10th	Night	36% 7% 10% 25% 17% 5%	2000
	11th to 20th	Night	30% 5% 9% 26% 21% 9%	1979
	21st to end	Night	31% 5% 10% 26% 24% 4	1907
May	1st to 10th	Night	22% 3 5% 24% 35% 10%	4097
	11th to 20th	Night	20% 6% 6% 19% 39% 10%	3942
	21st to end	Night	18% 5% 5% 16% 43% 12%	3972
June	1st to 10th	Night	26% 5% 6% 23% 28% 12%	1600
	11th to 20th	Night	19% 3 5% 16% 35% 22%	1600
	21st to end	Night	13% 3 5% 11% 29% 39%	1600
July	1st to 10th	Night	9% 2 3 5% 13% 68%	2540
	11th to 20th	Night	19% 3 3 5% 13% 57%	2492
	21st to end	Night	19% 6% 4 9% 21% 41%	2674
August	1st to 10th	Night	19% 3 3 7% 17% 51%	2735
	11th to 20th	Night	12% 2 3 5% 19% 60%	3156
	21st to end	Night	12% 2 2 7% 27% 51%	3498
September	1st to 10th	Night	15% 2 2 6% 25% 50%	1900
	11th to 20th	Night	18% 3 4 12% 27% 36%	2019
	21st to end	Night	14% 2 4 15% 29% 36%	2090
October	1st to 10th	Night	16% 3 3 12% 30% 36%	3518
	11th to 20th	Night	15% 4 4 18% 40% 19%	3777
	21st to end	Night	17% 4 7% 28% 32% 12%	4442
November	1st to 10th	Night	30% 6% 11% 21% 22% 9%	2535
	11th to 20th	Night	29% 5% 13% 24% 26% 4	2600
	21st to end	Night	32% 4 9% 24% 25% 6%	2600
December	1st to 10th	Night	25% 4 8% 19% 36% 7%	4528
	11th to 20th	Night	40% 10% 14% 20% 13% 3	5176
	21st to end	Night	47% 10% 14% 17% 10% 1	6952

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	9% 9% 29% 26% 23% 5%	200
	11th to 20th	Day	5% 5% 25% 30% 27% 10%	200
	21st to end	Day	11% 3 21% 29% 26% 10%	285
February	1st to 10th	Day	9% 7% 21% 37% 20% 6%	451
	11th to 20th	Day	9% 10% 24% 37% 17% 2	495
	21st to end	Day	6% 8% 26% 37% 20% 3	414
March	1st to 10th	Day	11% 10% 24% 32% 20% 3	500
	11th to 20th	Day	8% 13% 27% 28% 20% 5%	500
	21st to end	Day	10% 14% 25% 32% 16% 3	550
April	1st to 10th	Day	9% 12% 25% 27% 21% 6%	500
	11th to 20th	Day	9% 13% 28% 27% 17% 6%	492
	21st to end	Day	9% 14% 22% 26% 22% 7%	500
May	1st to 10th	Day	6% 16% 29% 25% 17% 6%	503
	11th to 20th	Day	14% 10% 25% 26% 18% 7%	528
	21st to end	Day	10% 9% 19% 28% 24% 10%	573
June	1st to 10th	Day	4 10% 21% 32% 24% 10%	530
	11th to 20th	Day	6% 15% 25% 32% 16% 6%	530
	21st to end	Day	3 7% 22% 35% 24% 9%	530
July	1st to 10th	Day	1 3 23% 36% 27% 9%	507
	11th to 20th	Day	1 3 19% 37% 30% 10%	484
	21st to end	Day	2 2 17% 37% 27% 14%	531
August	1st to 10th	Day	5% 3 20% 32% 28% 13%	487
	11th to 20th	Day	3 5% 19% 31% 28% 15%	477
	21st to end	Day	2 5% 16% 37% 30% 10%	539
September	1st to 10th	Day	1 3 19% 32% 29% 16%	490
	11th to 20th	Day	6% 7% 17% 34% 24% 12%	490
	21st to end	Day	4 5% 25% 29% 28% 10%	490
October	1st to 10th	Day	5% 8% 22% 27% 26% 11%	490
	11th to 20th	Day	7% 8% 18% 28% 29% 10%	490
	21st to end	Day	6% 8% 23% 30% 24% 9%	344
November	1st to 10th	Day	7% 9% 25% 27% 24% 8%	300
	11th to 20th	Day	6% 5% 24% 31% 26% 8%	300
	21st to end	Day	11% 8% 17% 33% 24% 7%	300
December	1st to 10th	Day	5% 5% 20% 33% 22% 14%	207
	11th to 20th	Day	11% 6% 22% 33% 20% 9%	200
	21st to end	Day	18% 6% 15% 35% 21% 5%	220
January	1st to 10th	Night	9% 5% 20% 37% 22% 7%	800
	11th to 20th	Night	7% 4 19% 41% 24% 6%	800
	21st to end	Night	12% 3 14% 41% 24% 6%	880
February	1st to 10th	Night	9% 4 18% 42% 23% 4	800
	11th to 20th	Night	8% 4 20% 42% 22% 4	784
	21st to end	Night	6% 5% 22% 44% 20% 3	660
March	1st to 10th	Night	10% 5% 22% 41% 19% 4	800
	11th to 20th	Night	8% 7% 18% 39% 25% 3	800
	21st to end	Night	10% 7% 22% 41% 17% 3	880
April	1st to 10th	Night	9% 6% 22% 40% 21% 3	800
	11th to 20th	Night	9% 8% 18% 39% 19% 6%	791
	21st to end	Night	11% 5% 18% 36% 25% 5%	707
May	1st to 10th	Night	7% 9% 23% 37% 22% 3	600
	11th to 20th	Night	8% 5% 19% 38% 24% 6%	537
	21st to end	Night	8% 5% 14% 35% 32% 5%	433
June	1st to 10th	Night	7% 3 17% 39% 28% 5%	400
	11th to 20th	Night	5% 4 18% 42% 29% 2	400
	21st to end	Night	2 3 13% 50% 30% 3	400
July	1st to 10th	Night	2 1 12% 49% 32% 3	380
	11th to 20th	Night	2 1 7% 50% 38% 2	368
	21st to end	Night	3 1 6% 42% 42% 6%	416
August	1st to 10th	Night	5% 2 10% 43% 38% 3	501
	11th to 20th	Night	3 2 9% 39% 40% 7%	752
	21st to end	Night	3 1 6% 41% 42% 7%	836
September	1st to 10th	Night	3 3 7% 38% 40% 9%	760
	11th to 20th	Night	5% 2 10% 34% 37% 12%	760
	21st to end	Night	4 3 9% 38% 33% 13%	760
October	1st to 10th	Night	7% 3 12% 38% 34% 7%	760
	11th to 20th	Night	6% 4 11% 39% 33% 7%	760
	21st to end	Night	6% 3 13% 41% 27% 10%	836
November	1st to 10th	Night	8% 4 18% 36% 28% 7%	780
	11th to 20th	Night	8% 4 15% 45% 23% 5%	800
	21st to end	Night	9% 3 19% 38% 27% 5%	800
December	1st to 10th	Night	9% 4 17% 43% 21% 7%	800
	11th to 20th	Night	15% 5% 17% 38% 21% 4	800
	21st to end	Night	15% 7% 21% 40% 16% 2	880

Page generated at 2025-07-02 22:30:12. Version ac07

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