Quiet Skies Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Quiet Skies Observatory clear sky chart from 2005-11-06 to 2025-12-03.

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	14% 6% 2 6% 4 4 5% 4 6% 8% 42%	126850
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 44%	123848
Transparency	Day	38% 7% 12% 23% 16% 4	130450
	Night	38% 6% 11% 23% 18% 4	128318
Seeing	Day	18% 11% 26% 25% 15% 5%	16380
	Night	17% 8% 29% 31% 13% 1	25994

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	15% 4 2 4 3 4 5% 3 4 5% 50%	3891
	11th to 20th	Day	13% 8% 2 7% 4 3 4 3 4 5% 48%	4060
	21st to end	Day	15% 7% 2 6% 4 4 4 3 4 5% 45%	4466
February	1st to 10th	Day	13% 8% 2 7% 5% 4 5% 3 6% 6% 40%	1800
	11th to 20th	Day	18% 7% 2 6% 3 4 4 3 5% 6% 40%	1774
	21st to end	Day	18% 8% 2 6% 5% 3 3 3 5% 6% 41%	1665
March	1st to 10th	Day	18% 5% 2 5% 4 5% 4 4 4 7% 42%	4935
	11th to 20th	Day	11% 5% 2 6% 4 3 4 3 6% 6% 51%	4980
	21st to end	Day	16% 7% 2 6% 5% 4 4 4 5% 7% 40%	5465
April	1st to 10th	Day	14% 6% 2 6% 4 4 4 4 5% 6% 44%	2400
	11th to 20th	Day	14% 6% 2 5% 5% 3 3 3 5% 6% 47%	2374
	21st to end	Day	17% 6% 2 6% 4 5% 4 4 6% 6% 41%	2400
May	1st to 10th	Day	18% 7% 3 7% 5% 5% 5% 4 7% 9% 31%	5858
	11th to 20th	Day	22% 6% 2 7% 4 4 5% 4 7% 9% 31%	5969
	21st to end	Day	16% 5% 2 6% 4 4 4 4 6% 9% 39%	6934
June	1st to 10th	Day	11% 5% 2 5% 5% 4 4 5% 8% 9% 42%	2800
	11th to 20th	Day	12% 5% 2 6% 4 4 5% 5% 7% 9% 41%	2800
	21st to end	Day	8% 4 1 5% 5% 5% 5% 4 8% 10% 45%	2800
July	1st to 10th	Day	7% 4 2 6% 5% 5% 6% 5% 8% 11% 42%	6360
	11th to 20th	Day	8% 6% 3 7% 5% 6% 7% 7% 10% 12% 30%	5929
	21st to end	Day	9% 5% 2 6% 5% 6% 6% 5% 7% 10% 39%	6424
August	1st to 10th	Day	15% 5% 2 5% 5% 5% 6% 5% 8% 10% 34%	4234
	11th to 20th	Day	9% 4 2 6% 6% 5% 6% 5% 7% 9% 39%	4048
	21st to end	Day	10% 4 2 5% 4 4 4 3 6% 9% 49%	4312
September	1st to 10th	Day	13% 6% 2 5% 5% 5% 4 3 5% 6% 47%	2400
	11th to 20th	Day	12% 6% 2 5% 4 4 4 3 5% 8% 48%	2380
	21st to end	Day	22% 6% 2 6% 4 3 5% 3 5% 7% 36%	2095
October	1st to 10th	Day	16% 5% 2 5% 3 3 4 3 5% 7% 48%	2000
	11th to 20th	Day	16% 4 2 5% 3 4 4 4 5% 5% 50%	2000
	21st to end	Day	14% 5% 2 5% 3 3 4 3 5% 6% 51%	2200
November	1st to 10th	Day	11% 6% 2 4 4 3 4 3 5% 6% 53%	2019
	11th to 20th	Day	13% 5% 2 5% 4 4 4 3 5% 6% 49%	1832
	21st to end	Day	15% 5% 2 5% 3 4 4 4 4 5% 49%	1680
December	1st to 10th	Day	12% 4 2 5% 3 3 4 4 5% 6% 52%	2616
	11th to 20th	Day	19% 8% 3 4 3 3 3 3 4 7% 44%	2990
	21st to end	Day	18% 8% 2 7% 4 4 4 4 5% 7% 38%	3960
January	1st to 10th	Night	12% 6% 2 5% 3 3 3 4 5% 6% 52%	6051
	11th to 20th	Night	14% 7% 2 4 3 2 2 3 4 5% 54%	5900
	21st to end	Night	15% 7% 2 5% 3 3 3 3 4 5% 49%	6490
February	1st to 10th	Night	15% 7% 2 6% 4 4 4 5% 5% 6% 40%	2600
	11th to 20th	Night	20% 7% 2 6% 3 3 3 2 5% 5% 44%	2510
	21st to end	Night	16% 8% 2 6% 4 4 4 3 4 5% 43%	1960
March	1st to 10th	Night	16% 5% 2 5% 4 4 4 3 5% 6% 47%	4918
	11th to 20th	Night	11% 6% 2 4 3 4 4 4 5% 7% 50%	4980
	21st to end	Night	11% 6% 2 7% 4 4 4 4 6% 7% 45%	5453
April	1st to 10th	Night	15% 8% 2 7% 5% 3 4 3 5% 6% 43%	2000
	11th to 20th	Night	13% 5% 2 5% 5% 4 3 3 6% 6% 48%	1975
	21st to end	Night	15% 5% 2 5% 3 4 4 3 7% 7% 44%	2000
May	1st to 10th	Night	20% 9% 3 8% 4 4 5% 4 6% 8% 30%	4079
	11th to 20th	Night	25% 7% 2 6% 4 4 5% 3 5% 7% 31%	3923
	21st to end	Night	16% 6% 3 7% 5% 5% 5% 4 5% 8% 36%	3920
June	1st to 10th	Night	10% 5% 3 5% 5% 4 3 5% 8% 8% 43%	1600
	11th to 20th	Night	12% 6% 2 5% 4 5% 5% 4 6% 8% 42%	1600
	21st to end	Night	9% 4 2 5% 5% 4 6% 5% 6% 9% 45%	1600
July	1st to 10th	Night	9% 5% 2 7% 4 5% 6% 5% 8% 10% 38%	3598
	11th to 20th	Night	10% 8% 4 9% 7% 6% 6% 6% 7% 10% 28%	4009
	21st to end	Night	7% 7% 3 7% 5% 5% 5% 5% 8% 9% 39%	4427
August	1st to 10th	Night	15% 5% 3 8% 5% 5% 6% 5% 7% 9% 32%	2915
	11th to 20th	Night	11% 6% 3 9% 5% 5% 5% 4 6% 8% 37%	2965
	21st to end	Night	11% 6% 2 5% 4 4 4 3 5% 8% 48%	3586
September	1st to 10th	Night	15% 6% 1 5% 3 4 5% 3 5% 7% 45%	2000
	11th to 20th	Night	10% 6% 2 7% 3 3 4 4 5% 7% 49%	2015
	21st to end	Night	21% 8% 2 6% 4 4 4 3 5% 6% 38%	2303
October	1st to 10th	Night	18% 6% 2 5% 4 3 3 3 4 5% 47%	2400
	11th to 20th	Night	16% 5% 2 5% 3 3 3 3 4 6% 51%	2400
	21st to end	Night	13% 4 2 4 4 3 4 3 5% 6% 51%	2640
November	1st to 10th	Night	12% 6% 2 5% 3 3 3 2 4 5% 54%	2497
	11th to 20th	Night	14% 7% 2 4 3 3 4 3 6% 6% 49%	2784
	21st to end	Night	13% 5% 1 5% 3 4 4 4 5% 7% 49%	2940
December	1st to 10th	Night	11% 5% 2 4 3 3 3 3 5% 7% 53%	4609
	11th to 20th	Night	20% 6% 2 5% 4 4 3 4 5% 6% 42%	5205
	21st to end	Night	17% 6% 2 6% 4 4 4 4 5% 7% 39%	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	36% 4 7% 25% 20% 8%	3892
	11th to 20th	Day	38% 5% 8% 21% 18% 11%	4060
	21st to end	Day	41% 5% 9% 26% 16% 3	4466
February	1st to 10th	Day	42% 6% 12% 23% 14% 2	1800
	11th to 20th	Day	43% 5% 11% 21% 17% 3	1783
	21st to end	Day	44% 5% 13% 18% 16% 4	1665
March	1st to 10th	Day	41% 5% 8% 20% 17% 9%	4935
	11th to 20th	Day	32% 5% 10% 25% 21% 6%	4980
	21st to end	Day	42% 6% 11% 17% 17% 7%	5465
April	1st to 10th	Day	38% 7% 11% 19% 18% 6%	2400
	11th to 20th	Day	36% 6% 11% 21% 20% 5%	2374
	21st to end	Day	42% 8% 10% 22% 14% 4	2400
May	1st to 10th	Day	46% 10% 15% 18% 10% 2	5893
	11th to 20th	Day	47% 8% 11% 24% 9% 1	5973
	21st to end	Day	39% 8% 14% 21% 16% 2	6949
June	1st to 10th	Day	36% 8% 15% 24% 17% 1	2800
	11th to 20th	Day	37% 10% 14% 24% 13% 2	2800
	21st to end	Day	31% 10% 16% 24% 17% 1	2800
July	1st to 10th	Day	33% 12% 18% 19% 14% 4	6290
	11th to 20th	Day	41% 11% 18% 22% 7% 1	5859
	21st to end	Day	37% 8% 10% 24% 20% 1	6240
August	1st to 10th	Day	42% 8% 18% 24% 7%	4221
	11th to 20th	Day	37% 9% 13% 27% 14% 1	4087
	21st to end	Day	32% 7% 15% 28% 17% 2	4312
September	1st to 10th	Day	37% 8% 12% 18% 20% 5%	2400
	11th to 20th	Day	34% 6% 9% 27% 20% 3	2380
	21st to end	Day	46% 7% 11% 18% 16% 2	2095
October	1st to 10th	Day	35% 5% 8% 19% 29% 5%	3260
	11th to 20th	Day	26% 4 8% 27% 25% 10%	3240
	21st to end	Day	21% 4 10% 30% 25% 10%	3538
November	1st to 10th	Day	32% 5% 11% 24% 23% 5%	2019
	11th to 20th	Day	35% 5% 10% 21% 22% 6%	1832
	21st to end	Day	36% 5% 11% 26% 20% 3	1680
December	1st to 10th	Day	33% 5% 11% 26% 19% 5%	2616
	11th to 20th	Day	41% 4 14% 26% 12% 3	2986
	21st to end	Day	42% 7% 15% 20% 15% 1	3960
January	1st to 10th	Night	32% 5% 7% 24% 26% 5%	6051
	11th to 20th	Night	33% 3 8% 24% 23% 9%	5900
	21st to end	Night	38% 4 8% 27% 19% 3	6490
February	1st to 10th	Night	42% 6% 13% 22% 15% 2	2600
	11th to 20th	Night	43% 4 9% 22% 18% 6%	2512
	21st to end	Night	42% 4 11% 21% 16% 5%	1960
March	1st to 10th	Night	37% 5% 8% 24% 18% 8%	4918
	11th to 20th	Night	31% 6% 11% 23% 22% 7%	4980
	21st to end	Night	37% 6% 9% 22% 20% 6%	5453
April	1st to 10th	Night	41% 6% 11% 18% 20% 4	2000
	11th to 20th	Night	35% 7% 11% 22% 20% 5%	1975
	21st to end	Night	37% 8% 12% 25% 14% 5%	2000
May	1st to 10th	Night	51% 8% 13% 17% 8% 4	4106
	11th to 20th	Night	50% 7% 11% 21% 10% 1	3924
	21st to end	Night	44% 8% 13% 23% 10% 2	3927
June	1st to 10th	Night	35% 9% 13% 26% 16% 2	1600
	11th to 20th	Night	37% 10% 14% 23% 14% 1	1600
	21st to end	Night	34% 8% 16% 20% 19% 3	1600
July	1st to 10th	Night	38% 11% 17% 17% 14% 2	3560
	11th to 20th	Night	48% 11% 18% 15% 7% 1	3960
	21st to end	Night	37% 9% 15% 22% 17%	4297
August	1st to 10th	Night	46% 9% 15% 22% 7% 1	2904
	11th to 20th	Night	43% 8% 11% 23% 14% 1	2992
	21st to end	Night	33% 6% 13% 29% 17% 1	3586
September	1st to 10th	Night	37% 8% 11% 21% 18% 6%	2000
	11th to 20th	Night	34% 6% 10% 26% 21% 3	2015
	21st to end	Night	46% 7% 10% 18% 16% 3	2303
October	1st to 10th	Night	41% 4 9% 20% 21% 5%	3920
	11th to 20th	Night	24% 3 11% 22% 31% 9%	3899
	21st to end	Night	22% 4 9% 31% 26% 8%	4252
November	1st to 10th	Night	33% 4 9% 25% 23% 6%	2497
	11th to 20th	Night	35% 5% 8% 24% 22% 7%	2784
	21st to end	Night	34% 5% 11% 24% 21% 4	2940
December	1st to 10th	Night	32% 4 12% 28% 22% 3	4609
	11th to 20th	Night	43% 4 12% 19% 17% 5%	5208
	21st to end	Night	41% 8% 15% 21% 15% 1	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	27% 23% 30% 16% 3	263
	11th to 20th	Day	29% 24% 28% 17% 2	300
	21st to end	Day	20% 23% 39% 14% 3 1	330
February	1st to 10th	Day	23% 25% 35% 13% 4	300
	11th to 20th	Day	35% 24% 32% 7% 2	321
	21st to end	Day	25% 21% 30% 21% 3	414
March	1st to 10th	Day	20% 20% 34% 21% 4	500
	11th to 20th	Day	23% 20% 32% 17% 6% 1	500
	21st to end	Day	21% 19% 34% 19% 6% 1	550
April	1st to 10th	Day	25% 22% 25% 22% 6% 1	500
	11th to 20th	Day	17% 18% 26% 21% 14% 2	492
	21st to end	Day	22% 11% 30% 23% 12% 1	500
May	1st to 10th	Day	20% 15% 29% 21% 12% 3	503
	11th to 20th	Day	19% 10% 28% 24% 15% 3	527
	21st to end	Day	20% 4 25% 23% 20% 7%	573
June	1st to 10th	Day	10% 7% 25% 29% 22% 7%	530
	11th to 20th	Day	13% 6% 23% 31% 17% 9%	530
	21st to end	Day	9% 3 22% 37% 22% 7%	530
July	1st to 10th	Day	11% 4 14% 34% 27% 11%	525
	11th to 20th	Day	5% 2 15% 31% 34% 13%	524
	21st to end	Day	11% 3 16% 32% 27% 10%	576
August	1st to 10th	Day	14% 3 17% 28% 27% 11%	526
	11th to 20th	Day	12% 5% 17% 29% 25% 13%	519
	21st to end	Day	9% 3 17% 32% 23% 16%	583
September	1st to 10th	Day	14% 4 22% 28% 18% 14%	530
	11th to 20th	Day	14% 3 18% 28% 25% 12%	530
	21st to end	Day	27% 8% 21% 25% 15% 4	530
October	1st to 10th	Day	19% 5% 20% 29% 19% 8%	530
	11th to 20th	Day	17% 12% 25% 29% 15% 3	530
	21st to end	Day	17% 12% 34% 25% 10% 3	583
November	1st to 10th	Day	15% 11% 33% 26% 11% 4	515
	11th to 20th	Day	19% 13% 34% 22% 10% 2	372
	21st to end	Day	20% 17% 34% 22% 6% 1	220
December	1st to 10th	Day	18% 18% 40% 19% 5% 1	204
	11th to 20th	Day	20% 13% 36% 23% 9%	200
	21st to end	Day	24% 17% 31% 20% 8%	220
January	1st to 10th	Night	20% 15% 41% 22% 3	800
	11th to 20th	Night	21% 13% 37% 26% 4	800
	21st to end	Night	18% 13% 42% 25% 2	880
February	1st to 10th	Night	20% 12% 41% 25% 3	800
	11th to 20th	Night	25% 17% 40% 17% 2	783
	21st to end	Night	22% 14% 39% 22% 3	660
March	1st to 10th	Night	17% 16% 39% 25% 3	800
	11th to 20th	Night	21% 15% 36% 23% 5% 1	800
	21st to end	Night	17% 14% 40% 23% 5%	880
April	1st to 10th	Night	17% 16% 36% 26% 4	800
	11th to 20th	Night	15% 11% 35% 30% 8% 1	791
	21st to end	Night	18% 8% 32% 33% 8% 1	800
May	1st to 10th	Night	20% 7% 29% 31% 12% 1	620
	11th to 20th	Night	21% 4 29% 32% 13% 1	547
	21st to end	Night	15% 3 19% 42% 21% 1	433
June	1st to 10th	Night	10% 3 21% 46% 20% 1	400
	11th to 20th	Night	13% 5% 15% 43% 23% 1	400
	21st to end	Night	10% 2 18% 49% 21% 2	400
July	1st to 10th	Night	11% 3 18% 38% 27% 4	390
	11th to 20th	Night	7% 1 9% 41% 36% 5%	562
	21st to end	Night	9% 2 15% 41% 30% 4	657
August	1st to 10th	Night	10% 3 12% 41% 32% 2	597
	11th to 20th	Night	12% 2 13% 39% 31% 2	645
	21st to end	Night	11% 3 13% 40% 28% 5%	880
September	1st to 10th	Night	17% 2 14% 33% 29% 4	800
	11th to 20th	Night	12% 2 16% 37% 30% 3	800
	21st to end	Night	25% 3 20% 32% 19% 1	800
October	1st to 10th	Night	20% 3 22% 29% 22% 4	800
	11th to 20th	Night	18% 8% 26% 34% 15% 1	800
	21st to end	Night	16% 9% 32% 30% 12% 1	880
November	1st to 10th	Night	14% 9% 31% 34% 12% 1	820
	11th to 20th	Night	18% 8% 35% 29% 8% 1	840
	21st to end	Night	15% 11% 36% 29% 8% 1	840
December	1st to 10th	Night	20% 10% 37% 26% 7%	809
	11th to 20th	Night	18% 8% 38% 31% 4	800
	21st to end	Night	20% 10% 32% 33% 4	880

Page generated at 2025-11-30 01:00:50. Version ac07

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