Night Skies Network Observatory
Clear Sky Chart
History

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

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	13% 6% 2 6% 4 4 4 4 6% 8% 43%	126764
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 45%	123292
Transparency	Day	38% 5% 12% 23% 18% 5%	130370
	Night	38% 4 10% 22% 20% 6%	127755
Seeing	Day	17% 7% 22% 25% 19% 9%	16721
	Night	17% 6% 24% 33% 18% 3	26783

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% 5% 3 4 3 2 2 2 4 4 56%	4056
	11th to 20th	Day	13% 6% 2 4 2 3 3 2 3 5% 58%	4060
	21st to end	Day	16% 8% 2 6% 3 3 4 2 3 4 48%	4466
February	1st to 10th	Day	14% 5% 2 6% 5% 4 5% 3 5% 8% 43%	1800
	11th to 20th	Day	20% 7% 2 3 4 3 4 3 4 6% 44%	1813
	21st to end	Day	17% 7% 2 6% 4 3 4 3 5% 7% 41%	1750
March	1st to 10th	Day	19% 7% 3 6% 3 4 3 3 5% 7% 41%	4935
	11th to 20th	Day	10% 5% 2 4 3 4 4 3 5% 7% 53%	4980
	21st to end	Day	16% 6% 3 6% 4 3 3 3 4 6% 46%	5465
April	1st to 10th	Day	11% 4 1 4 3 3 4 4 6% 8% 52%	2269
	11th to 20th	Day	13% 6% 2 5% 4 3 3 3 5% 7% 49%	2374
	21st to end	Day	15% 5% 2 5% 4 5% 5% 4 5% 6% 44%	2491
May	1st to 10th	Day	10% 7% 2 7% 6% 6% 6% 5% 7% 11% 34%	5862
	11th to 20th	Day	20% 7% 3 7% 5% 5% 5% 4 7% 8% 29%	5861
	21st to end	Day	13% 5% 2 6% 4 4 5% 5% 7% 9% 40%	6433
June	1st to 10th	Day	8% 6% 2 6% 5% 5% 5% 4 8% 11% 41%	2600
	11th to 20th	Day	8% 5% 2 6% 5% 6% 5% 4 8% 11% 40%	2600
	21st to end	Day	7% 4 2 6% 5% 5% 6% 4 7% 11% 43%	2600
July	1st to 10th	Day	8% 5% 2 7% 5% 6% 6% 5% 7% 10% 38%	5900
	11th to 20th	Day	7% 5% 2 8% 6% 7% 7% 6% 9% 14% 31%	5900
	21st to end	Day	10% 7% 3 8% 6% 6% 6% 5% 8% 10% 32%	6424
August	1st to 10th	Day	17% 7% 3 7% 5% 5% 6% 5% 8% 10% 26%	4234
	11th to 20th	Day	8% 5% 2 7% 6% 6% 6% 5% 8% 12% 35%	4174
	21st to end	Day	6% 4 2 5% 4 4 4 4 6% 10% 51%	4312
September	1st to 10th	Day	11% 4 2 5% 5% 4 4 4 5% 8% 49%	2360
	11th to 20th	Day	14% 5% 2 6% 4 3 4 4 6% 9% 43%	2200
	21st to end	Day	21% 6% 2 7% 4 3 4 3 5% 5% 38%	2200
October	1st to 10th	Day	13% 5% 2 5% 3 2 3 2 4 6% 54%	2011
	11th to 20th	Day	14% 5% 2 5% 3 3 3 2 4 5% 56%	2000
	21st to end	Day	12% 5% 2 5% 4 2 3 2 3 6% 56%	2200
November	1st to 10th	Day	13% 7% 2 5% 2 4 3 2 4 5% 54%	1977
	11th to 20th	Day	14% 4 2 4 4 3 3 2 5% 6% 53%	1888
	21st to end	Day	13% 5% 2 5% 4 3 4 3 6% 6% 50%	1800
December	1st to 10th	Day	13% 6% 2 4 3 3 4 3 5% 6% 53%	2930
	11th to 20th	Day	18% 6% 2 5% 4 3 4 3 5% 6% 45%	3373
	21st to end	Day	20% 6% 2 4 3 3 3 4 5% 8% 41%	4466
January	1st to 10th	Night	12% 6% 2 5% 3 3 3 2 4 5% 56%	5889
	11th to 20th	Night	12% 6% 2 5% 3 3 3 3 3 4 57%	5900
	21st to end	Night	16% 7% 2 5% 3 3 3 3 4 6% 49%	6490
February	1st to 10th	Night	16% 7% 2 5% 4 4 4 4 5% 6% 44%	2600
	11th to 20th	Night	20% 7% 2 3 3 3 4 2 6% 6% 44%	2469
	21st to end	Night	15% 7% 2 6% 3 3 4 4 5% 6% 45%	1879
March	1st to 10th	Night	17% 6% 2 6% 4 4 4 3 4 5% 45%	4918
	11th to 20th	Night	10% 4 1 5% 3 3 3 4 5% 6% 56%	4980
	21st to end	Night	15% 5% 2 5% 4 4 4 4 5% 7% 47%	5467
April	1st to 10th	Night	13% 6% 2 5% 4 4 4 4 5% 7% 47%	2130
	11th to 20th	Night	11% 5% 2 5% 4 4 3 3 6% 8% 48%	1975
	21st to end	Night	14% 5% 2 5% 4 4 5% 3 6% 7% 46%	1925
May	1st to 10th	Night	13% 7% 2 7% 5% 5% 5% 4 6% 8% 38%	4033
	11th to 20th	Night	18% 8% 3 8% 4 4 4 4 5% 7% 35%	4032
	21st to end	Night	15% 6% 2 6% 3 3 4 3 7% 10% 42%	4421
June	1st to 10th	Night	9% 6% 2 5% 5% 5% 5% 5% 8% 9% 41%	1800
	11th to 20th	Night	8% 5% 2 6% 5% 5% 6% 5% 7% 10% 42%	1800
	21st to end	Night	7% 4 2 5% 4 5% 6% 4 9% 11% 43%	1800
July	1st to 10th	Night	10% 7% 2 9% 6% 6% 6% 6% 7% 8% 33%	4058
	11th to 20th	Night	9% 7% 3 9% 6% 6% 6% 6% 9% 10% 28%	4057
	21st to end	Night	11% 8% 3 7% 5% 6% 7% 5% 8% 10% 30%	4427
August	1st to 10th	Night	19% 8% 3 8% 4 5% 6% 5% 8% 9% 25%	2915
	11th to 20th	Night	8% 6% 2 7% 5% 5% 6% 5% 8% 11% 38%	2875
	21st to end	Night	6% 4 2 6% 4 5% 4 3 6% 9% 50%	3552
September	1st to 10th	Night	10% 6% 2 5% 4 5% 5% 4 6% 8% 47%	2035
	11th to 20th	Night	14% 6% 2 7% 4 4 5% 4 5% 7% 42%	2200
	21st to end	Night	19% 8% 3 7% 4 5% 5% 4 5% 5% 37%	2200
October	1st to 10th	Night	15% 6% 2 6% 4 3 3 3 5% 6% 48%	2387
	11th to 20th	Night	14% 5% 2 4 3 3 3 3 4 5% 55%	2400
	21st to end	Night	13% 5% 1 4 3 3 3 2 3 5% 57%	2640
November	1st to 10th	Night	14% 6% 2 5% 3 2 3 2 3 5% 54%	2366
	11th to 20th	Night	13% 5% 2 5% 3 4 3 3 4 6% 52%	2510
	21st to end	Night	13% 5% 2 3 3 3 4 4 6% 6% 52%	2600
December	1st to 10th	Night	12% 5% 2 5% 3 4 4 3 4 6% 52%	4250
	11th to 20th	Night	21% 9% 2 7% 4 3 3 3 4 4 41%	4822
	21st to end	Night	19% 4 2 4 3 4 4 3 5% 7% 45%	6490

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	33% 2 4 22% 29% 9%	4057
	11th to 20th	Day	31% 2 4 20% 35% 8%	4060
	21st to end	Day	42% 2 4 24% 23% 5%	4466
February	1st to 10th	Day	40% 3 11% 22% 19% 5%	1800
	11th to 20th	Day	43% 3 7% 23% 19% 6%	1822
	21st to end	Day	44% 4 10% 17% 20% 7%	1750
March	1st to 10th	Day	44% 2 6% 18% 18% 11%	4935
	11th to 20th	Day	31% 3 6% 20% 24% 16%	4980
	21st to end	Day	42% 3 7% 23% 20% 6%	5465
April	1st to 10th	Day	30% 3 12% 24% 24% 8%	2269
	11th to 20th	Day	36% 2 11% 23% 21% 7%	2374
	21st to end	Day	39% 4 11% 26% 15% 3	2491
May	1st to 10th	Day	43% 5% 17% 21% 11% 2	5897
	11th to 20th	Day	52% 5% 11% 22% 8% 1	5865
	21st to end	Day	39% 4 14% 22% 17% 3	6447
June	1st to 10th	Day	35% 6% 17% 27% 14% 1	2600
	11th to 20th	Day	36% 7% 18% 23% 14% 2	2600
	21st to end	Day	34% 6% 19% 25% 15% 1	2600
July	1st to 10th	Day	39% 8% 17% 20% 13% 2	5835
	11th to 20th	Day	40% 11% 25% 19% 5% 1	5831
	21st to end	Day	43% 9% 16% 22% 9%	6240
August	1st to 10th	Day	49% 6% 18% 21% 5%	4221
	11th to 20th	Day	38% 8% 21% 23% 10% 1	4213
	21st to end	Day	27% 5% 16% 35% 15% 2	4312
September	1st to 10th	Day	34% 5% 14% 23% 18% 5%	2360
	11th to 20th	Day	39% 4 11% 24% 20% 3	2200
	21st to end	Day	47% 4 10% 19% 18% 3	2200
October	1st to 10th	Day	32% 3 7% 21% 27% 10%	3271
	11th to 20th	Day	22% 2 5% 23% 26% 23%	3240
	21st to end	Day	25% 2 7% 26% 24% 15%	3538
November	1st to 10th	Day	35% 3 7% 22% 25% 8%	1977
	11th to 20th	Day	34% 2 7% 24% 25% 7%	1888
	21st to end	Day	35% 4 8% 23% 26% 4	1800
December	1st to 10th	Day	33% 3 7% 23% 27% 8%	2930
	11th to 20th	Day	42% 3 9% 17% 23% 5%	3370
	21st to end	Day	42% 3 10% 27% 17% 1	4466
January	1st to 10th	Night	32% 2 4 20% 31% 11%	5889
	11th to 20th	Night	32% 2 4 18% 35% 8%	5900
	21st to end	Night	39% 2 5% 24% 25% 5%	6490
February	1st to 10th	Night	41% 3 9% 24% 20% 4	2600
	11th to 20th	Night	43% 2 9% 19% 21% 7%	2471
	21st to end	Night	40% 3 9% 21% 20% 7%	1879
March	1st to 10th	Night	43% 3 5% 17% 23% 10%	4918
	11th to 20th	Night	30% 3 5% 19% 26% 17%	4980
	21st to end	Night	38% 3 6% 26% 22% 4	5467
April	1st to 10th	Night	36% 4 10% 20% 24% 6%	2130
	11th to 20th	Night	35% 2 10% 23% 22% 8%	1975
	21st to end	Night	38% 3 12% 25% 18% 4	1925
May	1st to 10th	Night	43% 6% 15% 21% 13% 1	4060
	11th to 20th	Night	49% 5% 12% 20% 13% 1	4033
	21st to end	Night	38% 5% 13% 27% 13% 3	4429
June	1st to 10th	Night	36% 6% 16% 24% 15% 2	1800
	11th to 20th	Night	35% 6% 18% 23% 14% 2	1800
	21st to end	Night	33% 7% 18% 24% 15% 3	1800
July	1st to 10th	Night	46% 9% 15% 17% 12% 2	4015
	11th to 20th	Night	48% 12% 21% 14% 4 1	4006
	21st to end	Night	45% 8% 16% 22% 8%	4297
August	1st to 10th	Night	52% 6% 18% 17% 6%	2904
	11th to 20th	Night	36% 8% 22% 23% 10% 1	2902
	21st to end	Night	31% 5% 12% 34% 16% 2	3552
September	1st to 10th	Night	35% 6% 14% 23% 16% 6%	2035
	11th to 20th	Night	41% 6% 10% 20% 22% 2	2200
	21st to end	Night	48% 4 9% 19% 15% 4	2200
October	1st to 10th	Night	39% 3 6% 20% 25% 7%	3907
	11th to 20th	Night	24% 2 5% 23% 29% 16%	3899
	21st to end	Night	26% 2 5% 26% 26% 15%	4252
November	1st to 10th	Night	35% 2 7% 21% 25% 9%	2366
	11th to 20th	Night	35% 2 8% 20% 25% 10%	2510
	21st to end	Night	33% 3 6% 25% 26% 6%	2600
December	1st to 10th	Night	35% 2 7% 22% 30% 4	4250
	11th to 20th	Night	49% 3 7% 18% 15% 8%	4824
	21st to end	Night	39% 3 12% 23% 21% 2	6490

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	25% 17% 34% 17% 7% 1	300
	11th to 20th	Day	22% 19% 36% 18% 5%	300
	21st to end	Day	21% 17% 40% 19% 4	330
February	1st to 10th	Day	17% 16% 41% 17% 8% 1	300
	11th to 20th	Day	36% 15% 33% 11% 5%	300
	21st to end	Day	26% 16% 35% 20% 4	349
March	1st to 10th	Day	23% 13% 32% 24% 7% 1	500
	11th to 20th	Day	22% 11% 38% 22% 6% 1	500
	21st to end	Day	22% 14% 34% 24% 6%	550
April	1st to 10th	Day	20% 15% 31% 22% 10% 2	500
	11th to 20th	Day	16% 13% 29% 24% 15% 4	492
	21st to end	Day	19% 9% 26% 27% 15% 3	500
May	1st to 10th	Day	17% 12% 27% 27% 13% 4	503
	11th to 20th	Day	18% 5% 19% 27% 22% 9%	527
	21st to end	Day	19% 4 15% 26% 22% 13%	573
June	1st to 10th	Day	12% 1 18% 32% 25% 13%	530
	11th to 20th	Day	11% 2 14% 29% 26% 18%	530
	21st to end	Day	11% 1 10% 31% 34% 14%	530
July	1st to 10th	Day	11% 1 8% 27% 33% 20%	525
	11th to 20th	Day	6% 1 7% 24% 38% 24%	524
	21st to end	Day	8% 1 9% 27% 35% 20%	576
August	1st to 10th	Day	18% 2 11% 23% 29% 17%	526
	11th to 20th	Day	9% 1 8% 29% 31% 21%	519
	21st to end	Day	11% 1 8% 25% 34% 21%	583
September	1st to 10th	Day	13% 2 12% 24% 28% 22%	530
	11th to 20th	Day	16% 2 12% 22% 28% 20%	530
	21st to end	Day	26% 4 18% 26% 20% 6%	530
October	1st to 10th	Day	17% 3 15% 31% 23% 11%	530
	11th to 20th	Day	16% 5% 23% 28% 22% 6%	530
	21st to end	Day	16% 8% 23% 31% 17% 5%	583
November	1st to 10th	Day	19% 5% 26% 29% 17% 4	503
	11th to 20th	Day	16% 10% 34% 22% 14% 3	388
	21st to end	Day	13% 14% 31% 25% 13% 4	300
December	1st to 10th	Day	22% 12% 37% 24% 5% 1	300
	11th to 20th	Day	18% 9% 34% 28% 9% 2	300
	21st to end	Day	25% 15% 28% 20% 11% 2	330
January	1st to 10th	Night	20% 11% 36% 28% 5%	800
	11th to 20th	Night	20% 10% 39% 28% 4	800
	21st to end	Night	18% 8% 41% 30% 3	880
February	1st to 10th	Night	19% 10% 39% 29% 3	800
	11th to 20th	Night	25% 13% 38% 22% 2	783
	21st to end	Night	19% 13% 37% 28% 4	660
March	1st to 10th	Night	18% 12% 35% 30% 6%	800
	11th to 20th	Night	21% 11% 34% 27% 7% 1	800
	21st to end	Night	19% 9% 36% 31% 5%	880
April	1st to 10th	Night	16% 13% 35% 30% 7%	800
	11th to 20th	Night	15% 9% 33% 31% 12%	791
	21st to end	Night	16% 5% 31% 38% 10%	725
May	1st to 10th	Night	13% 7% 29% 36% 15% 1	600
	11th to 20th	Night	17% 3 20% 36% 21% 2	591
	21st to end	Night	16% 1 10% 37% 32% 4	650
June	1st to 10th	Night	12% 1 13% 41% 31% 2	600
	11th to 20th	Night	10% 1 12% 37% 33% 7%	600
	21st to end	Night	11% 1 10% 37% 38% 4	600
July	1st to 10th	Night	13% 2 5% 33% 40% 6%	585
	11th to 20th	Night	8% 1 5% 33% 45% 9%	582
	21st to end	Night	11% 1 5% 37% 38% 7%	657
August	1st to 10th	Night	16% 2 5% 34% 36% 7%	597
	11th to 20th	Night	13% 1 6% 30% 43% 8%	594
	21st to end	Night	12% 1 6% 33% 41% 8%	859
September	1st to 10th	Night	14% 1 6% 34% 37% 7%	800
	11th to 20th	Night	18% 2 8% 31% 35% 7%	800
	21st to end	Night	24% 2 13% 34% 26% 3	800
October	1st to 10th	Night	17% 2 16% 31% 28% 6%	800
	11th to 20th	Night	17% 5% 24% 34% 18% 3	800
	21st to end	Night	15% 6% 24% 35% 18% 1	880
November	1st to 10th	Night	16% 5% 23% 38% 16% 2	789
	11th to 20th	Night	15% 7% 30% 37% 11%	800
	21st to end	Night	15% 7% 30% 35% 12% 1	800
December	1st to 10th	Night	22% 7% 31% 32% 7%	800
	11th to 20th	Night	20% 6% 34% 33% 7%	800
	21st to end	Night	22% 8% 27% 35% 8%	880

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