Cluny
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cluny clear sky chart from 2005-11-06 to 2025-11-18.

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	8% 8% 3 8% 5% 5% 5% 4 6% 8% 40%	131180
	Night	6% 9% 4 9% 6% 5% 5% 4 6% 8% 37%	127903
Transparency	Day	38% 8% 16% 25% 11% 2	130877
	Night	41% 8% 15% 23% 11% 2	127783
Seeing	Day	11% 7% 25% 31% 20% 6%	16800
	Night	11% 6% 29% 36% 16% 2	25867

By Month

Percentage Cloud Cover NW (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 11% 6% 15% 9% 7% 5% 4 6% 6% 27%	3179
	11th to 20th	Day	3 9% 5% 11% 7% 4 7% 6% 7% 9% 31%	3159
	21st to end	Day	4 8% 3 10% 5% 4 4 4 6% 8% 44%	3958
February	1st to 10th	Day	6% 12% 5% 12% 8% 6% 6% 4 6% 8% 27%	1645
	11th to 20th	Day	6% 12% 5% 11% 7% 7% 6% 4 7% 9% 25%	1755
	21st to end	Day	4 14% 4 11% 7% 6% 6% 6% 6% 8% 28%	1555
March	1st to 10th	Day	3 10% 4 13% 6% 4 5% 4 5% 9% 37%	4557
	11th to 20th	Day	6% 9% 3 8% 5% 6% 5% 4 6% 8% 39%	4999
	21st to end	Day	9% 17% 6% 13% 7% 5% 5% 4 6% 7% 22%	5486
April	1st to 10th	Day	9% 12% 4 10% 7% 6% 5% 5% 6% 6% 30%	2470
	11th to 20th	Day	9% 12% 4 9% 6% 5% 5% 5% 6% 8% 32%	2570
	21st to end	Day	11% 10% 3 8% 7% 6% 5% 5% 6% 7% 32%	2600
May	1st to 10th	Day	15% 10% 3 6% 4 4 4 4 5% 6% 39%	6178
	11th to 20th	Day	11% 9% 4 10% 5% 5% 6% 5% 8% 9% 28%	6734
	21st to end	Day	10% 6% 2 7% 6% 5% 5% 4 6% 8% 40%	7405
June	1st to 10th	Day	13% 7% 2 6% 5% 5% 6% 5% 8% 8% 35%	3000
	11th to 20th	Day	15% 9% 3 7% 4 6% 6% 5% 7% 8% 31%	3000
	21st to end	Day	8% 4 2 5% 5% 4 6% 5% 7% 9% 45%	3000
July	1st to 10th	Day	6% 4 2 4 3 4 4 4 6% 9% 53%	6780
	11th to 20th	Day	9% 4 2 4 3 4 5% 4 6% 10% 49%	6780
	21st to end	Day	8% 4 1 4 3 4 4 4 6% 8% 56%	7336
August	1st to 10th	Day	7% 6% 2 7% 4 5% 5% 5% 6% 8% 45%	4493
	11th to 20th	Day	5% 5% 2 6% 4 4 5% 4 7% 10% 47%	4158
	21st to end	Day	8% 3 1 4 3 3 4 3 6% 9% 56%	4664
September	1st to 10th	Day	8% 6% 2 6% 3 5% 4 3 4 7% 53%	2494
	11th to 20th	Day	13% 7% 2 6% 4 3 4 3 6% 7% 45%	2200
	21st to end	Day	8% 6% 3 6% 4 4 4 3 6% 9% 47%	2200
October	1st to 10th	Day	7% 7% 3 7% 5% 5% 5% 4 6% 8% 45%	3384
	11th to 20th	Day	5% 6% 2 6% 5% 5% 6% 4 6% 10% 45%	3098
	21st to end	Day	7% 7% 3 7% 4 5% 6% 4 6% 6% 46%	3200
November	1st to 10th	Day	10% 15% 4 9% 5% 5% 4 5% 6% 7% 31%	1672
	11th to 20th	Day	6% 12% 3 9% 7% 6% 5% 3 6% 7% 36%	1641
	21st to end	Day	4 13% 4 9% 6% 5% 6% 4 7% 8% 34%	1400
December	1st to 10th	Day	7% 12% 4 10% 7% 5% 6% 4 6% 7% 30%	2290
	11th to 20th	Day	6% 11% 5% 13% 8% 5% 5% 5% 6% 8% 29%	2642
	21st to end	Day	3 9% 4 8% 5% 5% 5% 4 6% 7% 45%	3498
January	1st to 10th	Night	4 14% 6% 13% 8% 7% 5% 4 6% 8% 25%	6766
	11th to 20th	Night	3 9% 4 12% 7% 6% 7% 5% 6% 8% 34%	6739
	21st to end	Night	3 9% 4 10% 6% 5% 4 4 5% 7% 43%	7027
February	1st to 10th	Night	5% 10% 5% 12% 8% 6% 6% 5% 6% 8% 29%	2754
	11th to 20th	Night	5% 12% 5% 11% 7% 6% 6% 5% 8% 7% 29%	2532
	21st to end	Night	4 9% 5% 12% 6% 5% 6% 4 7% 8% 34%	2071
March	1st to 10th	Night	3 7% 4 12% 8% 6% 5% 5% 7% 9% 33%	5337
	11th to 20th	Night	5% 8% 4 9% 5% 6% 6% 5% 7% 9% 36%	5000
	21st to end	Night	9% 18% 6% 14% 6% 5% 6% 4 6% 7% 19%	5489
April	1st to 10th	Night	8% 11% 4 10% 7% 5% 5% 5% 6% 9% 31%	1926
	11th to 20th	Night	7% 12% 4 9% 7% 5% 6% 5% 7% 8% 31%	1781
	21st to end	Night	9% 9% 3 8% 6% 6% 7% 4 8% 7% 33%	1800
May	1st to 10th	Night	13% 12% 3 8% 5% 5% 5% 4 5% 6% 34%	3795
	11th to 20th	Night	12% 9% 4 8% 5% 6% 6% 5% 7% 10% 29%	3159
	21st to end	Night	10% 8% 3 9% 6% 5% 5% 5% 6% 7% 36%	3470
June	1st to 10th	Night	14% 8% 3 7% 5% 5% 5% 5% 8% 7% 33%	1400
	11th to 20th	Night	11% 8% 3 8% 7% 7% 6% 6% 7% 7% 30%	1400
	21st to end	Night	7% 5% 1 6% 5% 5% 5% 5% 6% 8% 46%	1400
July	1st to 10th	Night	6% 4 1 6% 5% 5% 5% 5% 7% 9% 48%	3180
	11th to 20th	Night	6% 4 2 5% 5% 5% 5% 5% 8% 12% 44%	3180
	21st to end	Night	10% 4 2 5% 4 4 4 3 6% 9% 51%	3503
August	1st to 10th	Night	7% 5% 3 7% 5% 5% 5% 4 6% 9% 45%	2658
	11th to 20th	Night	5% 4 2 7% 5% 5% 5% 4 8% 11% 44%	2901
	21st to end	Night	7% 3 1 4 3 4 4 3 6% 10% 56%	3245
September	1st to 10th	Night	9% 5% 1 6% 4 4 4 5% 5% 7% 50%	1903
	11th to 20th	Night	12% 6% 2 5% 3 3 4 3 5% 8% 48%	2200
	21st to end	Night	7% 5% 2 5% 5% 4 5% 4 6% 9% 50%	2200
October	1st to 10th	Night	7% 6% 2 6% 4 4 5% 4 6% 9% 48%	3809
	11th to 20th	Night	7% 7% 2 6% 4 4 5% 4 6% 8% 47%	4046
	21st to end	Night	5% 7% 3 8% 4 5% 5% 4 7% 8% 43%	4600
November	1st to 10th	Night	8% 13% 3 8% 5% 5% 4 4 7% 8% 33%	2825
	11th to 20th	Night	5% 10% 3 8% 7% 6% 6% 5% 8% 9% 35%	2909
	21st to end	Night	6% 11% 4 9% 7% 6% 5% 5% 7% 9% 33%	3000
December	1st to 10th	Night	5% 11% 4 12% 7% 7% 6% 6% 7% 8% 28%	4890
	11th to 20th	Night	5% 12% 6% 13% 7% 6% 6% 5% 7% 8% 25%	5550
	21st to end	Night	4 8% 4 9% 5% 5% 5% 4 5% 8% 42%	7458

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	53% 8% 12% 19% 7% 1	3179
	11th to 20th	Day	43% 9% 14% 16% 16% 2	3180
	21st to end	Day	36% 7% 15% 30% 11%	3958
February	1st to 10th	Day	54% 8% 14% 14% 9% 1	1645
	11th to 20th	Day	51% 9% 15% 17% 7% 1	1764
	21st to end	Day	50% 9% 12% 17% 12%	1555
March	1st to 10th	Day	41% 7% 15% 22% 15%	4557
	11th to 20th	Day	40% 7% 16% 22% 15% 1	5000
	21st to end	Day	56% 9% 16% 14% 5%	5486
April	1st to 10th	Day	49% 9% 14% 18% 9% 1	2470
	11th to 20th	Day	46% 7% 14% 22% 9% 1	2570
	21st to end	Day	47% 8% 14% 20% 10% 1	2600
May	1st to 10th	Day	43% 7% 10% 21% 14% 5%	6178
	11th to 20th	Day	43% 12% 16% 21% 7%	6738
	21st to end	Day	37% 10% 16% 26% 10% 1	7406
June	1st to 10th	Day	41% 10% 16% 21% 10% 1	3000
	11th to 20th	Day	47% 10% 15% 19% 8% 2	3000
	21st to end	Day	32% 8% 21% 27% 9% 2	3000
July	1st to 10th	Day	25% 8% 18% 29% 18% 3	6706
	11th to 20th	Day	29% 8% 21% 29% 12% 1	6699
	21st to end	Day	25% 7% 16% 27% 18% 7%	7129
August	1st to 10th	Day	32% 9% 15% 33% 9% 1	4476
	11th to 20th	Day	29% 8% 22% 33% 8% 1	4197
	21st to end	Day	24% 6% 20% 35% 14% 1	4663
September	1st to 10th	Day	30% 7% 14% 28% 15% 5%	2494
	11th to 20th	Day	37% 7% 14% 27% 13% 2	2200
	21st to end	Day	32% 8% 15% 29% 12% 4	2200
October	1st to 10th	Day	35% 6% 18% 30% 10% 1	3384
	11th to 20th	Day	33% 7% 17% 29% 13% 2	3098
	21st to end	Day	36% 7% 11% 32% 13% 1	3200
November	1st to 10th	Day	50% 7% 13% 19% 8% 2	1672
	11th to 20th	Day	46% 7% 15% 23% 9% 1	1641
	21st to end	Day	45% 8% 17% 20% 9% 2	1400
December	1st to 10th	Day	50% 8% 16% 20% 5% 2	2290
	11th to 20th	Day	50% 8% 16% 20% 5% 1	2644
	21st to end	Day	36% 6% 12% 26% 17% 3	3498
January	1st to 10th	Night	54% 8% 12% 19% 5% 1	6766
	11th to 20th	Night	45% 7% 14% 16% 15% 2	6780
	21st to end	Night	38% 7% 15% 29% 11% 1	7027
February	1st to 10th	Night	51% 8% 12% 18% 10% 1	2754
	11th to 20th	Night	48% 8% 13% 18% 11% 1	2533
	21st to end	Night	44% 7% 15% 20% 11% 2	2071
March	1st to 10th	Night	41% 9% 19% 20% 11% 1	5337
	11th to 20th	Night	39% 8% 17% 24% 10% 1	5000
	21st to end	Night	60% 9% 13% 11% 6% 1	5489
April	1st to 10th	Night	47% 8% 14% 18% 12%	1926
	11th to 20th	Night	45% 8% 14% 21% 10% 2	1781
	21st to end	Night	42% 10% 15% 22% 9% 2	1800
May	1st to 10th	Night	47% 8% 8% 22% 13% 2	3792
	11th to 20th	Night	42% 12% 17% 22% 7% 1	3159
	21st to end	Night	40% 13% 16% 22% 8% 1	3470
June	1st to 10th	Night	46% 9% 15% 21% 9% 1	1400
	11th to 20th	Night	46% 11% 17% 17% 7% 2	1400
	21st to end	Night	30% 10% 21% 29% 9% 1	1400
July	1st to 10th	Night	29% 9% 19% 22% 19% 2	3145
	11th to 20th	Night	30% 9% 19% 32% 9% 2	3138
	21st to end	Night	29% 7% 15% 28% 15% 6%	3405
August	1st to 10th	Night	32% 9% 16% 29% 13% 2	2650
	11th to 20th	Night	30% 10% 20% 30% 9% 1	2928
	21st to end	Night	22% 6% 18% 35% 16% 3	3245
September	1st to 10th	Night	32% 8% 15% 24% 15% 6%	1903
	11th to 20th	Night	33% 8% 14% 27% 13% 4	2200
	21st to end	Night	30% 7% 17% 27% 15% 3	2200
October	1st to 10th	Night	31% 6% 16% 29% 15% 2	3809
	11th to 20th	Night	33% 7% 17% 27% 15% 2	4046
	21st to end	Night	36% 7% 14% 31% 12% 1	4600
November	1st to 10th	Night	46% 8% 15% 20% 10% 1	2825
	11th to 20th	Night	41% 9% 17% 20% 11% 1	2909
	21st to end	Night	45% 9% 17% 18% 8% 2	3000
December	1st to 10th	Night	50% 8% 17% 18% 5% 1	4890
	11th to 20th	Night	54% 9% 19% 14% 3	5547
	21st to end	Night	38% 6% 11% 25% 17% 3	7458

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	4 5% 29% 32% 20% 12%	200
	11th to 20th	Day	6% 8% 29% 35% 17% 6%	200
	21st to end	Day	8% 7% 25% 37% 19% 5%	220
February	1st to 10th	Day	11% 9% 29% 33% 14% 4	223
	11th to 20th	Day	9% 7% 24% 37% 20% 4	300
	21st to end	Day	7% 7% 29% 35% 16% 6%	319
March	1st to 10th	Day	10% 9% 31% 28% 16% 5%	500
	11th to 20th	Day	9% 7% 28% 35% 16% 4	500
	21st to end	Day	12% 9% 31% 26% 15% 6%	550
April	1st to 10th	Day	12% 14% 29% 27% 14% 4	500
	11th to 20th	Day	11% 12% 29% 27% 16% 5%	492
	21st to end	Day	15% 10% 25% 28% 17% 5%	500
May	1st to 10th	Day	15% 9% 23% 27% 20% 6%	595
	11th to 20th	Day	13% 6% 22% 32% 21% 6%	725
	21st to end	Day	16% 6% 19% 29% 22% 8%	789
June	1st to 10th	Day	13% 7% 26% 28% 21% 4	730
	11th to 20th	Day	16% 7% 30% 28% 15% 3	730
	21st to end	Day	9% 4 22% 34% 23% 7%	730
July	1st to 10th	Day	8% 5% 18% 33% 26% 10%	721
	11th to 20th	Day	11% 3 19% 35% 24% 8%	718
	21st to end	Day	6% 3 19% 34% 28% 10%	779
August	1st to 10th	Day	7% 4 22% 32% 23% 11%	527
	11th to 20th	Day	7% 4 20% 30% 30% 9%	517
	21st to end	Day	9% 7% 18% 31% 24% 12%	583
September	1st to 10th	Day	11% 6% 23% 29% 23% 7%	530
	11th to 20th	Day	15% 5% 19% 30% 25% 6%	530
	21st to end	Day	13% 10% 29% 29% 14% 4	530
October	1st to 10th	Day	14% 7% 24% 27% 21% 7%	405
	11th to 20th	Day	10% 7% 33% 28% 16% 6%	330
	21st to end	Day	12% 14% 31% 29% 10% 4	363
November	1st to 10th	Day	18% 7% 29% 32% 13% 2	330
	11th to 20th	Day	12% 11% 33% 29% 14% 2	314
	21st to end	Day	8% 7% 34% 28% 16% 9%	200
December	1st to 10th	Day	11% 7% 31% 31% 14% 8%	200
	11th to 20th	Day	10% 8% 34% 33% 14% 2	200
	21st to end	Day	8% 6% 22% 34% 25% 5%	220
January	1st to 10th	Night	8% 8% 32% 36% 15% 2	1000
	11th to 20th	Night	10% 6% 35% 35% 12% 2	1000
	21st to end	Night	10% 9% 33% 32% 14% 2	916
February	1st to 10th	Night	8% 7% 31% 34% 17% 4	800
	11th to 20th	Night	9% 6% 33% 36% 15% 1	784
	21st to end	Night	7% 6% 26% 36% 21% 4	660
March	1st to 10th	Night	8% 5% 31% 37% 16% 4	800
	11th to 20th	Night	8% 7% 31% 38% 14% 2	800
	21st to end	Night	13% 7% 28% 34% 16% 2	880
April	1st to 10th	Night	12% 8% 27% 36% 15% 2	705
	11th to 20th	Night	12% 8% 31% 36% 12% 1	593
	21st to end	Night	13% 7% 28% 38% 13% 1	600
May	1st to 10th	Night	15% 8% 26% 34% 16% 1	570
	11th to 20th	Night	12% 5% 27% 37% 19% 2	394
	21st to end	Night	18% 6% 25% 32% 18% 2	433
June	1st to 10th	Night	18% 7% 24% 37% 12% 2	400
	11th to 20th	Night	16% 6% 30% 35% 14% 1	400
	21st to end	Night	9% 3 24% 40% 22% 2	400
July	1st to 10th	Night	8% 4 17% 45% 25% 1	390
	11th to 20th	Night	10% 3 22% 42% 21% 1	388
	21st to end	Night	6% 2 23% 42% 25% 2	438
August	1st to 10th	Night	8% 5% 20% 40% 26% 3	421
	11th to 20th	Night	7% 3 20% 46% 22% 2	594
	21st to end	Night	9% 5% 20% 44% 21% 1	660
September	1st to 10th	Night	11% 3 22% 42% 20% 2	645
	11th to 20th	Night	15% 4 20% 37% 23% 2	800
	21st to end	Night	9% 6% 30% 36% 16% 3	800
October	1st to 10th	Night	13% 5% 26% 37% 16% 2	800
	11th to 20th	Night	12% 5% 31% 36% 13% 2	800
	21st to end	Night	9% 9% 34% 35% 13% 1	880
November	1st to 10th	Night	14% 8% 35% 32% 10% 1	980
	11th to 20th	Night	10% 9% 38% 30% 11% 3	1036
	21st to end	Night	9% 6% 33% 37% 12% 2	1000
December	1st to 10th	Night	10% 6% 33% 33% 15% 2	1000
	11th to 20th	Night	9% 7% 35% 36% 12% 2	1000
	21st to end	Night	11% 5% 31% 38% 13% 3	1100

Page generated at 2025-11-14 22:55:59. Version ac07

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