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 2026-10-29.

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 5% 2 5% 3 3 3 3 5% 7% 61%	157828
	Night	4 6% 2 6% 4 4 4 3 5% 7% 55%	151650
Transparency	Day	22% 5% 7% 17% 25% 24%	152740
	Night	26% 6% 8% 18% 24% 18%	147727
Seeing	Day	7% 8% 23% 31% 23% 8%	17374
	Night	7% 4 16% 40% 27% 5%	26691

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	3 12% 5% 9% 4 4 5% 5% 7% 9% 36%	4722
	11th to 20th	Day	3 7% 2 7% 4 3 3 3 5% 8% 55%	4740
	21st to end	Day	3 9% 4 8% 5% 6% 6% 5% 7% 9% 38%	5385
February	1st to 10th	Day	4 7% 3 7% 6% 5% 6% 3 7% 10% 42%	1996
	11th to 20th	Day	3 8% 3 8% 6% 5% 5% 4 6% 9% 41%	2050
	21st to end	Day	2 8% 3 7% 5% 6% 5% 4 7% 10% 44%	1730
March	1st to 10th	Day	3 13% 4 11% 6% 5% 5% 4 6% 8% 35%	4618
	11th to 20th	Day	2 7% 3 7% 5% 5% 4 3 6% 11% 49%	4749
	21st to end	Day	2 9% 3 9% 6% 5% 6% 4 7% 7% 42%	6101
April	1st to 10th	Day	4 8% 3 8% 5% 5% 5% 5% 6% 8% 43%	2520
	11th to 20th	Day	4 8% 2 6% 6% 4 4 4 6% 8% 49%	2489
	21st to end	Day	6% 7% 2 6% 4 4 4 4 6% 8% 50%	2615
May	1st to 10th	Day	4 5% 2 4 3 2 3 3 4 6% 64%	7626
	11th to 20th	Day	3 4 1 4 3 3 3 3 4 5% 67%	7757
	21st to end	Day	3 3 1 4 3 3 3 3 5% 6% 65%	8992
June	1st to 10th	Day	3 4 1 4 4 3 4 4 6% 7% 58%	2940
	11th to 20th	Day	3 3 1 3 3 2 3 2 4 6% 70%	2940
	21st to end	Day	2 2 1 2 2 2 2 2 4 5% 75%	2940
July	1st to 10th	Day	1 1 1 2 1 2 3 3 86%	8219
	11th to 20th	Day	1 2 1 2 2 2 3 2 4 5% 75%	8220
	21st to end	Day	1 2 1 2 2 3 3 3 5% 7% 72%	8922
August	1st to 10th	Day	2 1 1 2 2 2 2 2 3 5% 79%	7711
	11th to 20th	Day	1 1 1 2 2 2 2 2 4 5% 77%	7001
	21st to end	Day	3 3 1 3 3 3 3 3 5% 6% 66%	7750
September	1st to 10th	Day	1 2 1 2 1 2 2 2 3 5% 79%	2520
	11th to 20th	Day	3 2 1 2 3 3 3 3 5% 5% 71%	2388
	21st to end	Day	2 3 1 3 2 2 2 2 4 6% 73%	2310
October	1st to 10th	Day	3 2 1 3 3 3 3 2 3 6% 72%	3677
	11th to 20th	Day	4 5% 1 3 2 2 2 2 3 5% 70%	3361
	21st to end	Day	2 3 2 5% 3 4 4 4 5% 8% 59%	3288
November	1st to 10th	Day	4 8% 2 7% 4 4 4 3 5% 6% 54%	1854
	11th to 20th	Day	5% 6% 3 5% 4 3 4 5% 6% 9% 50%	1890
	21st to end	Day	4 7% 4 6% 5% 4 4 3 5% 8% 51%	1890
December	1st to 10th	Day	3 4 2 5% 4 3 3 2 4 7% 62%	2717
	11th to 20th	Day	4 11% 3 9% 6% 6% 6% 6% 9% 10% 30%	3108
	21st to end	Day	4 10% 4 10% 7% 6% 5% 4 7% 9% 34%	4092
January	1st to 10th	Night	3 10% 4 10% 5% 5% 5% 5% 8% 9% 36%	8172
	11th to 20th	Night	5% 6% 2 6% 4 3 3 3 5% 6% 57%	8220
	21st to end	Night	3 8% 3 7% 5% 4 6% 5% 6% 9% 45%	8791
February	1st to 10th	Night	3 8% 3 7% 4 5% 5% 4 6% 7% 46%	2621
	11th to 20th	Night	3 8% 3 9% 6% 6% 5% 4 6% 8% 44%	2458
	21st to end	Night	2 8% 3 7% 5% 5% 5% 6% 8% 9% 42%	2076
March	1st to 10th	Night	3 13% 6% 10% 5% 4 4 4 5% 6% 40%	5496
	11th to 20th	Night	3 7% 3 8% 5% 5% 4 4 6% 8% 49%	5453
	21st to end	Night	4 8% 3 8% 6% 5% 6% 4 7% 9% 41%	5116
April	1st to 10th	Night	4 7% 3 7% 6% 5% 5% 5% 6% 8% 44%	2100
	11th to 20th	Night	5% 7% 3 5% 4 4 4 3 5% 7% 52%	2079
	21st to end	Night	7% 7% 2 5% 4 4 4 3 5% 6% 53%	2002
May	1st to 10th	Night	5% 6% 2 4 3 3 3 2 4 5% 64%	5321
	11th to 20th	Night	3 4 1 4 3 3 3 2 4 6% 66%	5121
	21st to end	Night	4 4 1 4 3 3 3 3 5% 8% 61%	5181
June	1st to 10th	Night	6% 5% 2 4 4 4 4 3 6% 7% 56%	1680
	11th to 20th	Night	3 3 1 3 2 3 3 2 4 5% 69%	1680
	21st to end	Night	2 2 1 3 3 3 4 3 4 6% 71%	1680
July	1st to 10th	Night	2 2 1 2 2 2 2 2 3 4 79%	4740
	11th to 20th	Night	3 3 1 4 3 3 3 3 4 6% 66%	4740
	21st to end	Night	3 3 1 4 3 4 4 4 6% 9% 59%	5153
August	1st to 10th	Night	2 2 1 3 2 2 2 2 4 5% 75%	5086
	11th to 20th	Night	2 3 1 4 2 3 3 3 4 5% 69%	5886
	21st to end	Night	5% 4 2 5% 4 3 4 3 5% 6% 62%	6499
September	1st to 10th	Night	3 3 1 3 2 2 2 2 4 5% 73%	2100
	11th to 20th	Night	3 4 1 3 3 3 3 3 4 5% 70%	2231
	21st to end	Night	3 2 1 3 2 3 2 2 3 5% 74%	2310
October	1st to 10th	Night	3 3 1 3 2 3 3 2 4 4 73%	3740
	11th to 20th	Night	2 3 1 3 2 2 3 2 4 4 71%	4017
	21st to end	Night	4 2 1 4 2 3 4 4 5% 8% 62%	4687
November	1st to 10th	Night	5% 6% 2 5% 4 4 5% 4 7% 7% 50%	2665
	11th to 20th	Night	4 6% 2 7% 4 3 4 4 6% 8% 51%	2730
	21st to end	Night	4 6% 3 6% 5% 4 4 4 5% 7% 52%	2730
December	1st to 10th	Night	3 5% 2 5% 4 3 4 3 4 7% 60%	4667
	11th to 20th	Night	4 10% 4 9% 5% 5% 6% 5% 8% 11% 33%	5316
	21st to end	Night	5% 11% 4 11% 7% 7% 6% 5% 7% 8% 27%	7106

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	40% 9% 20% 24% 7%	4723
	11th to 20th	Day	28% 6% 11% 20% 29% 6%	4740
	21st to end	Day	39% 7% 9% 16% 23% 7%	5385
February	1st to 10th	Day	35% 7% 13% 28% 14% 4	1996
	11th to 20th	Day	36% 8% 14% 23% 17% 3	2060
	21st to end	Day	33% 8% 15% 23% 15% 5%	1730
March	1st to 10th	Day	43% 6% 10% 20% 16% 5%	4618
	11th to 20th	Day	28% 6% 12% 34% 18% 2	4749
	21st to end	Day	35% 9% 13% 23% 17% 3	6101
April	1st to 10th	Day	37% 6% 10% 24% 17% 6%	2520
	11th to 20th	Day	31% 5% 11% 23% 22% 7%	2492
	21st to end	Day	31% 5% 10% 28% 23% 3	2615
May	1st to 10th	Day	20% 5% 6% 23% 35% 11%	7626
	11th to 20th	Day	18% 5% 5% 17% 40% 15%	7760
	21st to end	Day	19% 4 5% 19% 40% 13%	8994
June	1st to 10th	Day	23% 5% 7% 19% 31% 15%	2940
	11th to 20th	Day	17% 2 5% 15% 31% 30%	2940
	21st to end	Day	13% 2 3 11% 32% 39%	2940
July	1st to 10th	Day	6% 1 2 5% 17% 69%	8151
	11th to 20th	Day	10% 3 3 7% 13% 64%	8145
	21st to end	Day	11% 3 3 9% 26% 47%	8723
August	1st to 10th	Day	11% 2 2 6% 19% 61%	6103
	11th to 20th	Day	12% 3 4 11% 24% 46%	5517
	21st to end	Day	14% 3 4 16% 28% 36%	6077
September	1st to 10th	Day	9% 2 3 10% 28% 48%	2520
	11th to 20th	Day	15% 3 5% 13% 31% 33%	2388
	21st to end	Day	15% 2 5% 18% 31% 29%	2310
October	1st to 10th	Day	16% 2 4 11% 33% 35%	3677
	11th to 20th	Day	19% 3 4 19% 41% 15%	3361
	21st to end	Day	19% 6% 8% 31% 24% 12%	3288
November	1st to 10th	Day	31% 6% 9% 24% 22% 8%	1854
	11th to 20th	Day	29% 6% 13% 29% 20% 3	1890
	21st to end	Day	32% 5% 11% 27% 21% 3	1890
December	1st to 10th	Day	22% 4 8% 24% 35% 6%	2717
	11th to 20th	Day	41% 11% 14% 21% 11% 2	3108
	21st to end	Day	39% 11% 18% 23% 9%	4092
January	1st to 10th	Night	37% 11% 19% 27% 7%	8172
	11th to 20th	Night	29% 4 9% 17% 28% 11%	8220
	21st to end	Night	33% 7% 9% 18% 27% 7%	8791
February	1st to 10th	Night	34% 6% 13% 26% 17% 3	2621
	11th to 20th	Night	36% 6% 12% 26% 17% 4	2459
	21st to end	Night	33% 8% 14% 23% 16% 6%	2076
March	1st to 10th	Night	41% 6% 9% 20% 15% 9%	5496
	11th to 20th	Night	30% 7% 8% 29% 24% 2	5453
	21st to end	Night	35% 9% 15% 26% 13% 2	5116
April	1st to 10th	Night	35% 7% 10% 25% 18% 5%	2100
	11th to 20th	Night	30% 5% 9% 26% 22% 9%	2079
	21st to end	Night	31% 5% 10% 25% 24% 5%	2002
May	1st to 10th	Night	22% 4 5% 21% 36% 12%	5319
	11th to 20th	Night	17% 5% 5% 16% 42% 14%	5122
	21st to end	Night	20% 5% 5% 18% 41% 10%	5182
June	1st to 10th	Night	26% 5% 6% 22% 28% 13%	1680
	11th to 20th	Night	18% 3 5% 15% 34% 25%	1680
	21st to end	Night	14% 3 5% 11% 30% 38%	1680
July	1st to 10th	Night	11% 2 2 5% 17% 64%	4700
	11th to 20th	Night	18% 3 3 5% 12% 59%	4692
	21st to end	Night	17% 6% 5% 13% 20% 39%	5040
August	1st to 10th	Night	14% 2 3 6% 17% 58%	4020
	11th to 20th	Night	19% 4 4 7% 24% 43%	4636
	21st to end	Night	18% 3 3 13% 24% 39%	5092
September	1st to 10th	Night	15% 2 3 8% 27% 45%	2100
	11th to 20th	Night	18% 3 4 12% 28% 35%	2231
	21st to end	Night	15% 2 4 15% 30% 35%	2310
October	1st to 10th	Night	16% 3 4 12% 30% 36%	3740
	11th to 20th	Night	16% 4 4 18% 40% 20%	4017
	21st to end	Night	17% 5% 7% 28% 32% 11%	4687
November	1st to 10th	Night	30% 6% 11% 22% 22% 9%	2665
	11th to 20th	Night	29% 5% 13% 24% 25% 3	2730
	21st to end	Night	31% 4 9% 26% 24% 5%	2730
December	1st to 10th	Night	25% 4 9% 20% 35% 7%	4667
	11th to 20th	Night	39% 10% 14% 20% 13% 4	5316
	21st to end	Night	47% 10% 14% 18% 11% 1	7106

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	8% 11% 29% 26% 22% 5%	220
	11th to 20th	Day	5% 4 23% 29% 27% 13%	220
	21st to end	Day	10% 3 21% 31% 26% 9%	310
February	1st to 10th	Day	8% 7% 22% 36% 20% 6%	486
	11th to 20th	Day	9% 11% 26% 36% 16% 2	535
	21st to end	Day	7% 9% 27% 36% 19% 3	446
March	1st to 10th	Day	11% 11% 25% 31% 19% 3	540
	11th to 20th	Day	8% 13% 27% 28% 19% 5%	540
	21st to end	Day	10% 14% 26% 32% 16% 3	594
April	1st to 10th	Day	9% 12% 24% 28% 21% 7%	540
	11th to 20th	Day	8% 14% 28% 28% 17% 5%	532
	21st to end	Day	9% 13% 23% 27% 21% 7%	540
May	1st to 10th	Day	6% 15% 30% 26% 17% 6%	543
	11th to 20th	Day	14% 11% 25% 26% 17% 7%	568
	21st to end	Day	10% 9% 20% 28% 24% 9%	617
June	1st to 10th	Day	4 11% 22% 31% 23% 9%	570
	11th to 20th	Day	6% 14% 25% 33% 16% 6%	570
	21st to end	Day	4 7% 22% 34% 24% 9%	570
July	1st to 10th	Day	1 4 22% 37% 27% 9%	566
	11th to 20th	Day	1 3 20% 37% 30% 10%	564
	21st to end	Day	2 3 18% 36% 26% 14%	619
August	1st to 10th	Day	4 3 21% 32% 26% 13%	567
	11th to 20th	Day	2 4 18% 32% 27% 16%	557
	21st to end	Day	3 5% 17% 36% 29% 10%	627
September	1st to 10th	Day	2 4 20% 32% 28% 15%	570
	11th to 20th	Day	6% 7% 19% 34% 24% 11%	570
	21st to end	Day	4 5% 24% 30% 27% 10%	570
October	1st to 10th	Day	5% 8% 23% 27% 26% 11%	570
	11th to 20th	Day	8% 9% 19% 28% 28% 9%	570
	21st to end	Day	8% 7% 22% 29% 25% 9%	403
November	1st to 10th	Day	8% 8% 25% 27% 24% 8%	330
	11th to 20th	Day	6% 5% 23% 32% 25% 8%	330
	21st to end	Day	10% 8% 18% 33% 25% 6%	330
December	1st to 10th	Day	5% 5% 23% 32% 21% 14%	228
	11th to 20th	Day	12% 7% 21% 30% 19% 10%	220
	21st to end	Day	19% 7% 15% 32% 21% 5%	242
January	1st to 10th	Night	9% 5% 21% 36% 21% 7%	840
	11th to 20th	Night	6% 4 18% 40% 25% 5%	840
	21st to end	Night	12% 3 14% 42% 24% 6%	924
February	1st to 10th	Night	8% 5% 17% 43% 23% 4	840
	11th to 20th	Night	8% 4 21% 41% 21% 4	824
	21st to end	Night	6% 5% 23% 44% 19% 2	692
March	1st to 10th	Night	10% 5% 23% 41% 18% 3	840
	11th to 20th	Night	7% 6% 19% 40% 25% 3	840
	21st to end	Night	10% 7% 22% 42% 16% 3	924
April	1st to 10th	Night	9% 5% 21% 40% 22% 3	840
	11th to 20th	Night	9% 9% 18% 40% 19% 5%	831
	21st to end	Night	11% 5% 19% 36% 24% 5%	742
May	1st to 10th	Night	7% 8% 23% 37% 21% 3	630
	11th to 20th	Night	8% 5% 20% 38% 24% 5%	564
	21st to end	Night	8% 6% 14% 36% 32% 5%	455
June	1st to 10th	Night	7% 3 18% 40% 27% 5%	420
	11th to 20th	Night	5% 4 17% 43% 29% 2	420
	21st to end	Night	2 3 13% 50% 29% 3	420
July	1st to 10th	Night	2 1 12% 50% 32% 3	410
	11th to 20th	Night	3 1 6% 52% 37% 1	408
	21st to end	Night	3 2 6% 44% 40% 5%	460
August	1st to 10th	Night	5% 2 11% 44% 37% 2	555
	11th to 20th	Night	3 2 9% 40% 39% 7%	832
	21st to end	Night	3 1 6% 42% 41% 6%	924
September	1st to 10th	Night	3 3 8% 39% 38% 8%	840
	11th to 20th	Night	4 2 11% 35% 36% 11%	840
	21st to end	Night	4 3 9% 39% 33% 12%	840
October	1st to 10th	Night	7% 3 12% 39% 33% 6%	840
	11th to 20th	Night	5% 4 13% 40% 31% 7%	840
	21st to end	Night	6% 4 13% 40% 27% 9%	912
November	1st to 10th	Night	8% 4 18% 36% 28% 7%	820
	11th to 20th	Night	8% 4 15% 45% 23% 4	840
	21st to end	Night	8% 3 19% 39% 26% 5%	840
December	1st to 10th	Night	9% 4 18% 43% 20% 7%	840
	11th to 20th	Night	14% 5% 17% 38% 21% 5%	840
	21st to end	Night	15% 7% 21% 38% 16% 2	924

Page generated at 2026-10-26 01:37:50. Version ac07

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