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 2024-11-06.

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%	101205
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 44%	97817
Transparency	Day	38% 4 12% 23% 17% 5%	104855
	Night	39% 4 10% 22% 19% 6%	102306
Seeing	Day	17% 7% 22% 25% 20% 10%	15375
	Night	17% 6% 24% 33% 19% 3	25455

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	18% 4 2 4 3 3 2 2 5% 6% 51%	2836
	11th to 20th	Day	15% 6% 2 5% 3 3 3 2 4 5% 52%	2840
	21st to end	Day	22% 9% 3 6% 3 3 4 2 3 4 42%	3124
February	1st to 10th	Day	13% 5% 2 6% 5% 4 4 3 5% 7% 44%	1710
	11th to 20th	Day	19% 7% 2 4 4 3 4 3 5% 5% 45%	1720
	21st to end	Day	18% 8% 2 6% 4 3 4 3 5% 7% 40%	1665
March	1st to 10th	Day	23% 6% 2 5% 3 3 4 3 5% 7% 38%	3435
	11th to 20th	Day	11% 4 1 4 3 4 4 3 5% 7% 54%	3480
	21st to end	Day	20% 7% 2 5% 4 3 3 3 4 6% 42%	3815
April	1st to 10th	Day	10% 4 1 4 3 3 4 4 6% 8% 53%	2155
	11th to 20th	Day	13% 6% 2 5% 4 3 3 3 5% 7% 49%	2254
	21st to end	Day	15% 5% 2 5% 4 5% 5% 4 5% 6% 45%	2366
May	1st to 10th	Day	13% 7% 2 7% 6% 6% 5% 5% 7% 9% 34%	4082
	11th to 20th	Day	19% 7% 3 6% 5% 5% 5% 4 6% 8% 34%	4081
	21st to end	Day	8% 4 2 5% 4 4 5% 5% 7% 10% 45%	4476
June	1st to 10th	Day	9% 6% 2 6% 5% 5% 5% 4 8% 11% 40%	2470
	11th to 20th	Day	8% 5% 2 6% 5% 6% 5% 4 8% 11% 40%	2470
	21st to end	Day	7% 5% 2 6% 6% 5% 6% 4 7% 10% 42%	2470
July	1st to 10th	Day	7% 5% 2 7% 5% 6% 7% 6% 8% 10% 38%	4120
	11th to 20th	Day	8% 5% 2 7% 5% 6% 6% 5% 8% 12% 34%	4120
	21st to end	Day	12% 8% 3 9% 6% 6% 6% 5% 7% 9% 28%	4466
August	1st to 10th	Day	17% 7% 3 7% 5% 5% 6% 5% 8% 11% 27%	4104
	11th to 20th	Day	8% 5% 2 7% 6% 6% 6% 5% 8% 12% 35%	4044
	21st to end	Day	5% 4 2 5% 4 4 4 4 6% 10% 52%	4180
September	1st to 10th	Day	11% 4 2 5% 5% 5% 4 4 5% 8% 48%	2242
	11th to 20th	Day	15% 6% 2 6% 4 3 4 4 6% 9% 42%	2090
	21st to end	Day	22% 6% 3 6% 4 3 4 3 5% 5% 39%	2090
October	1st to 10th	Day	14% 5% 3 5% 3 2 3 3 4 6% 54%	1911
	11th to 20th	Day	14% 5% 2 5% 3 3 3 2 4 6% 55%	1900
	21st to end	Day	11% 5% 2 5% 4 2 3 2 3 6% 56%	2090
November	1st to 10th	Day	13% 6% 2 5% 2 4 3 2 4 4 55%	1908
	11th to 20th	Day	13% 4 2 5% 4 3 3 3 5% 6% 53%	1794
	21st to end	Day	13% 5% 2 5% 4 3 4 3 5% 6% 50%	1710
December	1st to 10th	Day	17% 7% 2 4 4 4 4 4 6% 6% 41%	1710
	11th to 20th	Day	15% 4 1 3 3 2 3 3 4 6% 54%	2153
	21st to end	Day	18% 6% 2 5% 4 3 4 5% 6% 8% 40%	3124
January	1st to 10th	Night	14% 7% 2 6% 3 3 4 2 4 6% 48%	4109
	11th to 20th	Night	14% 7% 2 5% 4 3 3 3 4 5% 49%	4120
	21st to end	Night	22% 7% 2 5% 3 3 3 3 4 5% 43%	4532
February	1st to 10th	Night	16% 6% 2 5% 4 4 4 4 5% 6% 44%	2470
	11th to 20th	Night	19% 7% 2 3 3 3 3 2 5% 5% 44%	2343
	21st to end	Night	16% 8% 2 6% 3 3 4 4 5% 6% 43%	1788
March	1st to 10th	Night	23% 7% 3 7% 4 4 4 3 3 4 37%	3418
	11th to 20th	Night	12% 5% 1 5% 4 3 4 3 4 5% 54%	3480
	21st to end	Night	19% 5% 2 4 3 3 3 3 4 6% 48%	3817
April	1st to 10th	Night	12% 6% 2 5% 4 4 4 4 5% 7% 47%	2024
	11th to 20th	Night	12% 5% 2 5% 4 4 3 3 5% 8% 48%	1875
	21st to end	Night	14% 5% 2 5% 4 4 4 3 5% 7% 46%	1829
May	1st to 10th	Night	13% 7% 2 7% 4 5% 4 3 6% 7% 42%	2813
	11th to 20th	Night	17% 8% 3 8% 4 4 4 3 4 5% 40%	2812
	21st to end	Night	9% 5% 2 5% 3 3 4 4 8% 11% 46%	3079
June	1st to 10th	Night	9% 5% 2 5% 5% 5% 5% 5% 8% 10% 40%	1710
	11th to 20th	Night	8% 5% 2 6% 5% 4 6% 5% 7% 10% 43%	1710
	21st to end	Night	8% 5% 2 5% 4 5% 5% 4 9% 11% 42%	1710
July	1st to 10th	Night	6% 7% 3 10% 7% 7% 6% 6% 7% 8% 34%	2840
	11th to 20th	Night	9% 6% 3 8% 5% 6% 6% 5% 9% 9% 35%	2840
	21st to end	Night	15% 9% 3 8% 6% 5% 6% 4 8% 9% 28%	3087
August	1st to 10th	Night	19% 8% 3 8% 4 5% 6% 5% 8% 9% 25%	2825
	11th to 20th	Night	8% 6% 2 7% 4 5% 6% 5% 8% 11% 38%	2785
	21st to end	Night	6% 4 2 6% 4 5% 4 3 6% 9% 51%	3443
September	1st to 10th	Night	11% 6% 2 6% 4 5% 5% 4 6% 8% 45%	1933
	11th to 20th	Night	14% 6% 2 6% 4 4 5% 4 5% 7% 41%	2090
	21st to end	Night	19% 8% 3 6% 4 4 5% 4 5% 5% 38%	2090
October	1st to 10th	Night	15% 6% 2 6% 4 3 3 3 5% 6% 48%	2267
	11th to 20th	Night	13% 5% 2 4 3 3 3 3 4 5% 55%	2280
	21st to end	Night	13% 5% 2 4 3 3 3 2 3 5% 57%	2508
November	1st to 10th	Night	14% 6% 2 5% 3 2 2 2 3 5% 55%	2292
	11th to 20th	Night	13% 5% 2 5% 4 4 3 3 4 6% 53%	2384
	21st to end	Night	14% 5% 2 3 3 3 4 4 5% 6% 51%	2470
December	1st to 10th	Night	18% 7% 2 6% 4 4 4 3 5% 6% 42%	2470
	11th to 20th	Night	17% 6% 1 5% 4 3 3 3 4 5% 49%	3042
	21st to end	Night	19% 5% 2 5% 3 4 4 4 5% 8% 41%	4532

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	35% 3 5% 29% 21% 7%	2837
	11th to 20th	Day	37% 3 5% 21% 30% 5%	2840
	21st to end	Day	48% 2 4 20% 22% 4	3124
February	1st to 10th	Day	40% 3 11% 22% 20% 4	1710
	11th to 20th	Day	42% 3 7% 24% 19% 6%	1729
	21st to end	Day	45% 4 10% 16% 19% 6%	1665
March	1st to 10th	Day	47% 3 7% 21% 15% 8%	3435
	11th to 20th	Day	31% 3 8% 23% 24% 11%	3480
	21st to end	Day	44% 3 8% 22% 19% 4	3815
April	1st to 10th	Day	29% 3 12% 24% 25% 8%	2155
	11th to 20th	Day	36% 3 11% 22% 21% 7%	2254
	21st to end	Day	39% 4 11% 26% 16% 4	2366
May	1st to 10th	Day	46% 4 16% 20% 11% 3	4120
	11th to 20th	Day	48% 4 11% 26% 9% 1	4085
	21st to end	Day	33% 4 17% 24% 19% 3	4489
June	1st to 10th	Day	35% 5% 17% 27% 14% 1	2470
	11th to 20th	Day	35% 7% 18% 24% 14% 2	2470
	21st to end	Day	35% 6% 18% 25% 15% 1	2470
July	1st to 10th	Day	39% 9% 18% 19% 12% 3	4055
	11th to 20th	Day	40% 7% 21% 24% 7% 1	4051
	21st to end	Day	51% 8% 14% 18% 9%	4324
August	1st to 10th	Day	49% 6% 18% 21% 5%	4091
	11th to 20th	Day	38% 8% 21% 23% 10% 1	4083
	21st to end	Day	27% 5% 16% 35% 15% 2	4180
September	1st to 10th	Day	34% 6% 14% 23% 18% 5%	2242
	11th to 20th	Day	39% 4 11% 23% 20% 3	2090
	21st to end	Day	47% 4 9% 20% 18% 3	2090
October	1st to 10th	Day	32% 3 7% 20% 27% 10%	3171
	11th to 20th	Day	22% 2 5% 23% 25% 24%	3140
	21st to end	Day	25% 2 8% 26% 23% 15%	3428
November	1st to 10th	Day	34% 2 7% 22% 26% 8%	1908
	11th to 20th	Day	34% 2 7% 25% 24% 8%	1794
	21st to end	Day	35% 4 8% 23% 26% 5%	1710
December	1st to 10th	Day	43% 4 9% 20% 19% 4	1710
	11th to 20th	Day	33% 3 7% 24% 26% 8%	2150
	21st to end	Day	42% 3 9% 25% 19% 2	3124
January	1st to 10th	Night	38% 3 6% 22% 25% 5%	4109
	11th to 20th	Night	39% 3 6% 19% 28% 6%	4120
	21st to end	Night	45% 2 5% 22% 19% 6%	4532
February	1st to 10th	Night	40% 3 8% 24% 20% 4	2470
	11th to 20th	Night	42% 2 8% 19% 21% 7%	2345
	21st to end	Night	42% 3 9% 21% 19% 6%	1788
March	1st to 10th	Night	52% 3 6% 17% 17% 5%	3418
	11th to 20th	Night	33% 3 5% 20% 24% 14%	3480
	21st to end	Night	40% 3 7% 25% 20% 6%	3817
April	1st to 10th	Night	36% 4 10% 20% 25% 6%	2024
	11th to 20th	Night	35% 2 9% 23% 22% 8%	1875
	21st to end	Night	38% 3 12% 25% 18% 4	1829
May	1st to 10th	Night	42% 5% 16% 24% 12% 2	2840
	11th to 20th	Night	47% 4 10% 23% 15% 1	2813
	21st to end	Night	31% 6% 17% 29% 15% 3	3087
June	1st to 10th	Night	36% 6% 16% 25% 14% 2	1710
	11th to 20th	Night	35% 6% 18% 24% 15% 2	1710
	21st to end	Night	34% 7% 18% 23% 15% 3	1710
July	1st to 10th	Night	45% 10% 14% 16% 13% 2	2795
	11th to 20th	Night	42% 9% 22% 19% 6% 2	2786
	21st to end	Night	50% 7% 14% 20% 8%	2988
August	1st to 10th	Night	52% 7% 18% 17% 6%	2814
	11th to 20th	Night	36% 8% 22% 23% 10% 1	2812
	21st to end	Night	31% 5% 12% 34% 16% 2	3443
September	1st to 10th	Night	36% 6% 13% 22% 16% 6%	1933
	11th to 20th	Night	41% 6% 10% 19% 22% 2	2090
	21st to end	Night	48% 5% 9% 19% 16% 4	2090
October	1st to 10th	Night	40% 3 7% 19% 25% 7%	3787
	11th to 20th	Night	23% 2 5% 24% 29% 17%	3779
	21st to end	Night	25% 2 5% 27% 25% 16%	4120
November	1st to 10th	Night	35% 2 7% 21% 26% 9%	2292
	11th to 20th	Night	35% 2 8% 20% 25% 11%	2384
	21st to end	Night	34% 3 6% 25% 26% 6%	2470
December	1st to 10th	Night	44% 3 9% 21% 19% 5%	2470
	11th to 20th	Night	38% 3 7% 20% 22% 9%	3044
	21st to end	Night	42% 3 8% 22% 22% 2	4532

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% 14% 34% 18% 8% 1	270
	11th to 20th	Day	22% 17% 37% 19% 5%	270
	21st to end	Day	22% 15% 40% 19% 4	297
February	1st to 10th	Day	17% 16% 39% 19% 9% 1	270
	11th to 20th	Day	34% 15% 34% 12% 5%	270
	21st to end	Day	28% 15% 35% 19% 3	320
March	1st to 10th	Day	22% 12% 33% 25% 7% 1	460
	11th to 20th	Day	22% 11% 39% 22% 6% 1	460
	21st to end	Day	22% 13% 33% 25% 7%	506
April	1st to 10th	Day	19% 14% 31% 23% 11% 2	460
	11th to 20th	Day	15% 13% 28% 24% 15% 4	452
	21st to end	Day	20% 10% 26% 25% 15% 3	460
May	1st to 10th	Day	18% 11% 26% 27% 14% 4	463
	11th to 20th	Day	17% 4 17% 29% 23% 10%	487
	21st to end	Day	18% 2 14% 27% 24% 15%	529
June	1st to 10th	Day	12% 1 16% 32% 26% 13%	490
	11th to 20th	Day	11% 2 14% 28% 26% 19%	490
	21st to end	Day	11% 1 11% 31% 33% 13%	490
July	1st to 10th	Day	12% 1 8% 27% 33% 21%	485
	11th to 20th	Day	6% 1 7% 25% 39% 23%	484
	21st to end	Day	8% 1 9% 27% 35% 19%	532
August	1st to 10th	Day	16% 1 12% 23% 30% 18%	486
	11th to 20th	Day	9% 1 8% 29% 31% 21%	479
	21st to end	Day	11% 6% 24% 35% 23%	539
September	1st to 10th	Day	13% 2 11% 21% 30% 23%	490
	11th to 20th	Day	17% 2 12% 21% 28% 20%	490
	21st to end	Day	27% 4 16% 27% 20% 7%	490
October	1st to 10th	Day	18% 3 14% 32% 23% 10%	490
	11th to 20th	Day	16% 5% 22% 28% 23% 6%	490
	21st to end	Day	14% 6% 23% 32% 18% 6%	539
November	1st to 10th	Day	18% 5% 25% 30% 18% 4	476
	11th to 20th	Day	15% 11% 34% 22% 15% 3	354
	21st to end	Day	13% 13% 31% 25% 14% 4	270
December	1st to 10th	Day	21% 9% 36% 26% 6% 1	270
	11th to 20th	Day	18% 9% 33% 29% 10% 2	270
	21st to end	Day	25% 14% 28% 20% 10% 2	297
January	1st to 10th	Night	20% 10% 35% 29% 5%	760
	11th to 20th	Night	21% 9% 38% 28% 4	760
	21st to end	Night	19% 8% 40% 30% 3	836
February	1st to 10th	Night	20% 10% 38% 29% 4	760
	11th to 20th	Night	24% 12% 38% 23% 2	743
	21st to end	Night	20% 12% 36% 28% 4	628
March	1st to 10th	Night	18% 11% 34% 30% 6%	760
	11th to 20th	Night	21% 11% 34% 27% 7% 1	760
	21st to end	Night	19% 8% 35% 32% 5%	836
April	1st to 10th	Night	16% 12% 35% 30% 8%	760
	11th to 20th	Night	16% 9% 32% 31% 12%	751
	21st to end	Night	16% 5% 31% 38% 10%	689
May	1st to 10th	Night	13% 7% 28% 36% 15% 1	570
	11th to 20th	Night	17% 3 20% 36% 22% 2	561
	21st to end	Night	15% 1 9% 38% 34% 4	617
June	1st to 10th	Night	12% 1 12% 41% 32% 2	570
	11th to 20th	Night	10% 1 12% 36% 34% 7%	570
	21st to end	Night	11% 1 10% 38% 37% 3	570
July	1st to 10th	Night	12% 2 5% 33% 41% 7%	555
	11th to 20th	Night	7% 1 5% 33% 44% 9%	552
	21st to end	Night	11% 1 5% 39% 37% 6%	624
August	1st to 10th	Night	15% 1 5% 34% 37% 8%	567
	11th to 20th	Night	13% 1 6% 30% 43% 7%	564
	21st to end	Night	12% 1 5% 32% 42% 8%	816
September	1st to 10th	Night	14% 1 6% 33% 38% 8%	760
	11th to 20th	Night	18% 2 8% 30% 34% 7%	760
	21st to end	Night	24% 1 13% 33% 26% 3	760
October	1st to 10th	Night	17% 2 16% 31% 28% 6%	760
	11th to 20th	Night	17% 4 24% 33% 19% 3	760
	21st to end	Night	15% 6% 24% 36% 18% 1	836
November	1st to 10th	Night	16% 5% 22% 38% 17% 2	764
	11th to 20th	Night	15% 7% 30% 37% 11%	760
	21st to end	Night	16% 6% 30% 36% 13% 1	760
December	1st to 10th	Night	22% 7% 31% 33% 8%	760
	11th to 20th	Night	19% 6% 34% 34% 7%	760
	21st to end	Night	22% 8% 28% 34% 8%	836

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