Glencoe
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Glencoe 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	13% 8% 3 7% 5% 5% 5% 4 6% 7% 38%	126228
	Night	14% 10% 4 8% 5% 4 4 4 5% 6% 37%	123036
Transparency	Day	43% 7% 13% 23% 13% 2	129753
	Night	46% 6% 11% 22% 14% 2	127585
Seeing	Day	15% 7% 25% 32% 17% 4	17048
	Night	18% 8% 27% 30% 15% 2	26826

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	11% 10% 6% 12% 7% 5% 6% 5% 6% 6% 26%	3617
	11th to 20th	Day	10% 11% 6% 11% 4 3 6% 7% 7% 6% 27%	3620
	21st to end	Day	16% 17% 5% 7% 4 3 4 3 4 7% 31%	4028
February	1st to 10th	Day	13% 13% 5% 9% 7% 5% 5% 4 6% 7% 27%	1854
	11th to 20th	Day	13% 11% 4 8% 5% 5% 6% 4 5% 5% 34%	1948
	21st to end	Day	11% 12% 3 12% 6% 4 5% 4 5% 6% 31%	1650
March	1st to 10th	Day	15% 9% 3 7% 4 4 4 3 4 5% 42%	4480
	11th to 20th	Day	9% 6% 2 6% 5% 5% 6% 4 7% 9% 42%	4650
	21st to end	Day	18% 12% 6% 12% 6% 6% 5% 4 5% 5% 22%	5967
April	1st to 10th	Day	16% 11% 3 8% 5% 5% 5% 3 6% 6% 32%	2400
	11th to 20th	Day	14% 7% 2 7% 6% 4 6% 4 6% 7% 36%	2374
	21st to end	Day	17% 8% 3 7% 5% 5% 5% 4 5% 7% 34%	2533
May	1st to 10th	Day	16% 8% 3 6% 5% 4 5% 4 6% 8% 35%	5862
	11th to 20th	Day	14% 7% 3 6% 4 5% 5% 4 6% 7% 39%	6152
	21st to end	Day	14% 6% 2 6% 4 4 4 4 6% 9% 42%	6935
June	1st to 10th	Day	9% 6% 2 6% 4 5% 5% 4 6% 7% 46%	2800
	11th to 20th	Day	10% 5% 2 5% 4 4 4 5% 6% 9% 45%	2800
	21st to end	Day	9% 6% 2 6% 4 5% 5% 4 7% 9% 42%	2800
July	1st to 10th	Day	10% 6% 2 7% 5% 5% 5% 5% 8% 9% 38%	6360
	11th to 20th	Day	8% 5% 2 6% 4 4 5% 4 7% 9% 46%	6360
	21st to end	Day	9% 6% 3 6% 5% 5% 5% 4 7% 8% 42%	6924
August	1st to 10th	Day	11% 4 2 5% 4 4 5% 4 6% 9% 46%	4446
	11th to 20th	Day	9% 5% 2 5% 3 4 4 4 6% 8% 50%	3921
	21st to end	Day	7% 5% 2 6% 5% 4 4 4 6% 8% 50%	4312
September	1st to 10th	Day	11% 5% 3 5% 4 4 4 4 5% 7% 49%	2400
	11th to 20th	Day	11% 5% 1 4 3 3 3 3 6% 9% 53%	2237
	21st to end	Day	12% 5% 2 7% 5% 4 4 4 7% 8% 42%	2090
October	1st to 10th	Day	13% 6% 3 7% 5% 4 4 3 5% 6% 45%	2058
	11th to 20th	Day	18% 8% 3 8% 4 5% 5% 3 5% 7% 35%	1867
	21st to end	Day	19% 8% 3 6% 5% 5% 5% 3 5% 7% 34%	1881
November	1st to 10th	Day	12% 10% 2 7% 5% 4 4 3 5% 7% 43%	1762
	11th to 20th	Day	14% 12% 4 10% 5% 5% 3 4 6% 7% 31%	1800
	21st to end	Day	18% 11% 3 9% 5% 5% 5% 3 3 4 34%	1740
December	1st to 10th	Day	10% 10% 3 8% 6% 8% 5% 5% 4 6% 35%	2610
	11th to 20th	Day	12% 13% 5% 12% 8% 6% 5% 5% 6% 5% 23%	3008
	21st to end	Day	22% 20% 5% 11% 6% 4 4 3 4 5% 14%	3982
January	1st to 10th	Night	13% 12% 6% 12% 6% 5% 5% 3 4 5% 29%	6348
	11th to 20th	Night	14% 15% 5% 12% 6% 6% 5% 5% 7% 6% 19%	6360
	21st to end	Night	17% 15% 6% 8% 4 3 3 3 4 6% 31%	6932
February	1st to 10th	Night	15% 15% 5% 8% 5% 5% 4 2 4 6% 31%	2545
	11th to 20th	Night	13% 10% 4 9% 5% 5% 4 3 5% 6% 36%	2338
	21st to end	Night	19% 10% 3 9% 4 4 4 2 4 5% 36%	1980
March	1st to 10th	Night	17% 10% 3 7% 4 3 4 3 4 5% 40%	5373
	11th to 20th	Night	10% 7% 2 6% 4 4 4 3 5% 7% 47%	5334
	21st to end	Night	18% 11% 6% 9% 5% 5% 5% 4 4 5% 28%	4967
April	1st to 10th	Night	17% 9% 3 6% 5% 4 5% 3 5% 7% 37%	2000
	11th to 20th	Night	18% 9% 3 6% 4 4 5% 3 5% 5% 38%	1975
	21st to end	Night	18% 8% 3 6% 5% 3 4 4 5% 7% 38%	1865
May	1st to 10th	Night	19% 8% 3 7% 5% 4 4 4 5% 7% 35%	4033
	11th to 20th	Night	13% 7% 4 7% 4 3 4 4 6% 7% 40%	3741
	21st to end	Night	18% 7% 2 7% 4 4 3 3 4 6% 42%	3942
June	1st to 10th	Night	10% 5% 2 5% 5% 5% 5% 3 6% 7% 47%	1600
	11th to 20th	Night	10% 6% 2 6% 3 5% 4 4 6% 9% 44%	1600
	21st to end	Night	9% 6% 3 7% 5% 5% 5% 4 6% 9% 42%	1600
July	1st to 10th	Night	11% 5% 2 6% 5% 6% 5% 5% 6% 9% 40%	3618
	11th to 20th	Night	7% 5% 2 6% 4 4 4 4 6% 8% 51%	3617
	21st to end	Night	9% 5% 2 6% 4 4 5% 5% 7% 8% 46%	3947
August	1st to 10th	Night	7% 5% 1 5% 4 5% 5% 4 6% 9% 49%	2707
	11th to 20th	Night	8% 5% 1 5% 4 4 5% 3 6% 8% 50%	3126
	21st to end	Night	8% 4 2 6% 4 4 4 3 5% 9% 51%	3586
September	1st to 10th	Night	10% 5% 2 6% 4 3 4 3 5% 8% 51%	2000
	11th to 20th	Night	10% 4 2 4 2 3 4 4 6% 8% 53%	2099
	21st to end	Night	13% 5% 3 7% 4 4 4 3 5% 8% 44%	2090
October	1st to 10th	Night	14% 6% 2 5% 3 4 4 4 5% 6% 48%	2118
	11th to 20th	Night	18% 8% 3 6% 4 4 3 3 4 5% 42%	2312
	21st to end	Night	21% 8% 3 6% 4 4 3 3 5% 7% 36%	2717
November	1st to 10th	Night	13% 9% 2 6% 5% 3 4 4 4 6% 44%	2536
	11th to 20th	Night	15% 11% 4 9% 5% 4 4 3 5% 7% 34%	2600
	21st to end	Night	17% 10% 4 8% 5% 4 4 3 5% 6% 34%	2655
December	1st to 10th	Night	11% 11% 3 8% 6% 5% 5% 5% 5% 6% 36%	4580
	11th to 20th	Night	16% 15% 4 10% 6% 5% 4 4 5% 6% 26%	5199
	21st to end	Night	23% 18% 6% 10% 6% 5% 4 3 4 5% 17%	6996

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	56% 6% 13% 17% 8%	3618
	11th to 20th	Day	54% 6% 13% 10% 13% 4	3620
	21st to end	Day	55% 4 11% 21% 9%	4028
February	1st to 10th	Day	56% 6% 13% 13% 9% 1	1854
	11th to 20th	Day	51% 6% 10% 22% 9% 2	1958
	21st to end	Day	52% 8% 12% 17% 10% 2	1650
March	1st to 10th	Day	44% 5% 11% 23% 12% 4	4480
	11th to 20th	Day	37% 7% 14% 31% 10% 1	4650
	21st to end	Day	64% 7% 11% 11% 6% 1	5969
April	1st to 10th	Day	51% 7% 12% 19% 11%	2400
	11th to 20th	Day	46% 7% 11% 21% 12% 3	2374
	21st to end	Day	49% 8% 12% 18% 12% 2	2533
May	1st to 10th	Day	45% 7% 12% 21% 12% 3	5897
	11th to 20th	Day	40% 8% 12% 25% 14% 1	6156
	21st to end	Day	39% 7% 12% 25% 15% 3	6949
June	1st to 10th	Day	35% 7% 14% 23% 18% 2	2800
	11th to 20th	Day	34% 8% 14% 23% 19% 3	2800
	21st to end	Day	38% 8% 15% 22% 14% 3	2800
July	1st to 10th	Day	39% 7% 13% 26% 13% 2	6291
	11th to 20th	Day	34% 7% 15% 28% 16% 1	6285
	21st to end	Day	37% 7% 16% 21% 17% 2	6728
August	1st to 10th	Day	35% 6% 17% 28% 13% 1	4431
	11th to 20th	Day	32% 6% 13% 33% 15% 1	3959
	21st to end	Day	31% 6% 14% 26% 20% 2	4311
September	1st to 10th	Day	34% 6% 14% 25% 18% 3	2400
	11th to 20th	Day	28% 6% 13% 31% 19% 3	2237
	21st to end	Day	38% 8% 12% 24% 17% 1	2090
October	1st to 10th	Day	27% 4 7% 38% 23% 1	3422
	11th to 20th	Day	36% 6% 11% 27% 20%	3083
	21st to end	Day	44% 4 10% 26% 12% 3	3081
November	1st to 10th	Day	41% 5% 14% 22% 16% 2	1762
	11th to 20th	Day	52% 7% 12% 19% 9% 1	1800
	21st to end	Day	54% 6% 11% 22% 8%	1740
December	1st to 10th	Day	45% 8% 18% 23% 6%	2610
	11th to 20th	Day	59% 7% 13% 16% 5% 1	3005
	21st to end	Day	74% 6% 7% 10% 4	3982
January	1st to 10th	Night	56% 6% 10% 19% 8% 1	6348
	11th to 20th	Night	64% 5% 11% 8% 12% 1	6360
	21st to end	Night	56% 3 14% 16% 10% 1	6932
February	1st to 10th	Night	54% 6% 12% 16% 12% 1	2545
	11th to 20th	Night	50% 5% 12% 20% 12% 1	2339
	21st to end	Night	51% 4 11% 20% 10% 4	1980
March	1st to 10th	Night	47% 4 11% 24% 11% 3	5373
	11th to 20th	Night	36% 5% 12% 30% 15% 2	5334
	21st to end	Night	58% 5% 10% 17% 10% 1	4967
April	1st to 10th	Night	46% 6% 12% 22% 13% 1	2000
	11th to 20th	Night	47% 6% 8% 19% 17% 3	1975
	21st to end	Night	45% 6% 11% 22% 13% 2	1865
May	1st to 10th	Night	48% 6% 11% 15% 15% 3	4060
	11th to 20th	Night	40% 7% 13% 22% 14% 3	3742
	21st to end	Night	44% 5% 9% 22% 16% 4	3950
June	1st to 10th	Night	34% 7% 13% 25% 19% 3	1600
	11th to 20th	Night	36% 6% 12% 24% 22% 2	1600
	21st to end	Night	36% 9% 14% 22% 17% 2	1600
July	1st to 10th	Night	38% 7% 14% 27% 10% 4	3580
	11th to 20th	Night	31% 7% 14% 29% 18% 2	3572
	21st to end	Night	34% 7% 14% 23% 20% 2	3832
August	1st to 10th	Night	31% 7% 15% 28% 17% 1	2698
	11th to 20th	Night	31% 7% 13% 30% 19% 1	3154
	21st to end	Night	30% 5% 11% 32% 18% 3	3586
September	1st to 10th	Night	32% 6% 12% 27% 21% 3	2000
	11th to 20th	Night	28% 5% 8% 31% 25% 2	2099
	21st to end	Night	39% 6% 10% 28% 16% 2	2090
October	1st to 10th	Night	23% 4 8% 38% 25% 2	3534
	11th to 20th	Night	37% 5% 9% 22% 25% 2	3835
	21st to end	Night	41% 5% 9% 25% 18% 2	4467
November	1st to 10th	Night	40% 6% 10% 25% 16% 3	2536
	11th to 20th	Night	48% 7% 12% 21% 10% 1	2600
	21st to end	Night	51% 5% 14% 21% 9%	2655
December	1st to 10th	Night	44% 8% 15% 24% 8% 2	4580
	11th to 20th	Night	57% 6% 12% 15% 9% 1	5201
	21st to end	Night	69% 6% 9% 11% 5%	6996

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	17% 10% 36% 30% 5% 1	300
	11th to 20th	Day	19% 13% 37% 25% 5% 1	300
	21st to end	Day	22% 8% 35% 28% 6%	330
February	1st to 10th	Day	19% 9% 42% 26% 4 1	300
	11th to 20th	Day	20% 15% 34% 26% 5%	300
	21st to end	Day	16% 12% 42% 25% 4	249
March	1st to 10th	Day	18% 11% 38% 30% 3	300
	11th to 20th	Day	19% 13% 34% 26% 7% 1	340
	21st to end	Day	24% 11% 32% 24% 7% 1	550
April	1st to 10th	Day	21% 11% 32% 28% 7%	500
	11th to 20th	Day	16% 10% 36% 25% 11% 3	492
	21st to end	Day	20% 7% 32% 28% 12% 2	500
May	1st to 10th	Day	21% 7% 27% 33% 11% 2	503
	11th to 20th	Day	19% 4 23% 37% 15% 2	649
	21st to end	Day	16% 4 22% 33% 21% 6%	789
June	1st to 10th	Day	12% 3 22% 37% 22% 4	730
	11th to 20th	Day	12% 3 22% 37% 22% 3	730
	21st to end	Day	11% 3 22% 40% 19% 4	730
July	1st to 10th	Day	9% 2 13% 38% 26% 12%	721
	11th to 20th	Day	11% 2 16% 40% 23% 8%	718
	21st to end	Day	8% 3 14% 42% 27% 5%	794
August	1st to 10th	Day	8% 2 12% 38% 30% 9%	661
	11th to 20th	Day	12% 3 14% 35% 29% 8%	519
	21st to end	Day	7% 4 13% 38% 31% 8%	583
September	1st to 10th	Day	13% 5% 16% 32% 28% 6%	530
	11th to 20th	Day	9% 3 12% 33% 31% 12%	522
	21st to end	Day	15% 7% 22% 28% 21% 7%	490
October	1st to 10th	Day	17% 5% 22% 32% 18% 5%	458
	11th to 20th	Day	21% 13% 25% 28% 12% 1	300
	21st to end	Day	28% 10% 27% 31% 5%	330
November	1st to 10th	Day	14% 13% 26% 32% 13% 2	300
	11th to 20th	Day	21% 15% 33% 21% 8% 1	300
	21st to end	Day	24% 10% 33% 29% 3	300
December	1st to 10th	Day	16% 16% 37% 26% 6%	300
	11th to 20th	Day	16% 15% 39% 23% 6%	300
	21st to end	Day	27% 15% 35% 19% 4	330
January	1st to 10th	Night	18% 15% 38% 24% 5%	1000
	11th to 20th	Night	21% 14% 36% 23% 5% 1	1000
	21st to end	Night	21% 11% 36% 27% 5%	1100
February	1st to 10th	Night	22% 10% 37% 24% 7%	945
	11th to 20th	Night	19% 11% 35% 30% 5%	783
	21st to end	Night	23% 9% 39% 25% 4	660
March	1st to 10th	Night	21% 8% 35% 27% 9%	800
	11th to 20th	Night	16% 8% 32% 34% 9% 1	800
	21st to end	Night	23% 7% 29% 30% 10% 1	665
April	1st to 10th	Night	21% 9% 31% 30% 8% 1	600
	11th to 20th	Night	23% 6% 29% 29% 12% 1	593
	21st to end	Night	21% 6% 26% 33% 14% 1	600
May	1st to 10th	Night	19% 4 25% 38% 14% 1	600
	11th to 20th	Night	17% 4 19% 34% 23% 3	591
	21st to end	Night	15% 3 16% 36% 26% 4	650
June	1st to 10th	Night	12% 2 15% 42% 27% 3	600
	11th to 20th	Night	13% 2 18% 41% 24% 2	600
	21st to end	Night	10% 3 17% 38% 29% 3	600
July	1st to 10th	Night	8% 3 7% 37% 36% 8%	585
	11th to 20th	Night	9% 2 11% 39% 31% 7%	582
	21st to end	Night	7% 2 11% 41% 35% 4	657
August	1st to 10th	Night	6% 2 10% 41% 35% 7%	597
	11th to 20th	Night	9% 3 11% 35% 34% 9%	594
	21st to end	Night	9% 2 14% 34% 35% 5%	660
September	1st to 10th	Night	13% 3 16% 31% 30% 7%	600
	11th to 20th	Night	11% 4 14% 32% 30% 9%	713
	21st to end	Night	14% 6% 22% 28% 22% 7%	760
October	1st to 10th	Night	16% 6% 21% 31% 21% 4	760
	11th to 20th	Night	21% 9% 29% 28% 10% 1	760
	21st to end	Night	24% 11% 29% 26% 9% 1	836
November	1st to 10th	Night	17% 11% 35% 24% 13% 1	780
	11th to 20th	Night	20% 17% 32% 24% 7% 1	800
	21st to end	Night	20% 14% 35% 25% 6%	855
December	1st to 10th	Night	22% 14% 37% 22% 5%	1000
	11th to 20th	Night	22% 14% 35% 25% 4	1000
	21st to end	Night	30% 12% 35% 21% 2	1100

Page generated at 2025-09-16 00:49:13. Version ac07

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