Stars at Night Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stars at Night Observatory clear sky chart from 2005-11-06 to 2026-04-02.

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	12% 5% 2 5% 4 4 4 3 5% 7% 50%	131765
	Night	12% 5% 2 4 3 3 3 3 4 6% 55%	130379
Transparency	Day	32% 6% 10% 24% 21% 7%	135443
	Night	30% 5% 9% 22% 24% 11%	134765
Seeing	Day	12% 6% 21% 28% 25% 9%	16961
	Night	13% 7% 27% 34% 18% 2	26955

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	12% 6% 2 5% 3 3 3 3 5% 6% 52%	5289
	11th to 20th	Day	7% 5% 1 3 3 3 3 3 4 6% 62%	5310
	21st to end	Day	23% 8% 3 7% 4 3 3 2 3 3 42%	5812
February	1st to 10th	Day	19% 7% 3 6% 4 3 3 3 3 5% 44%	1953
	11th to 20th	Day	18% 8% 3 6% 4 3 3 2 3 6% 44%	2049
	21st to end	Day	20% 5% 2 4 2 3 1 2 4 4 51%	1806
March	1st to 10th	Day	17% 7% 2 4 3 3 3 3 5% 7% 47%	5046
	11th to 20th	Day	13% 5% 2 4 4 3 4 3 4 5% 54%	5090
	21st to end	Day	14% 8% 2 5% 4 4 4 3 5% 6% 45%	5583
April	1st to 10th	Day	14% 7% 2 5% 4 4 3 3 5% 6% 45%	2211
	11th to 20th	Day	15% 6% 2 5% 4 3 4 3 5% 8% 45%	2324
	21st to end	Day	13% 5% 2 6% 4 4 5% 4 6% 7% 44%	2470
May	1st to 10th	Day	17% 8% 3 7% 4 4 4 3 5% 6% 39%	5862
	11th to 20th	Day	14% 6% 3 7% 5% 4 4 3 5% 6% 43%	5862
	21st to end	Day	14% 7% 3 7% 4 5% 5% 4 6% 8% 38%	6434
June	1st to 10th	Day	7% 4 1 5% 5% 4 6% 5% 7% 9% 48%	2600
	11th to 20th	Day	6% 4 1 6% 4 4 5% 4 7% 9% 51%	2600
	21st to end	Day	4 3 2 4 4 5% 4 4 6% 8% 56%	2600
July	1st to 10th	Day	7% 5% 2 7% 6% 6% 6% 5% 7% 9% 41%	5900
	11th to 20th	Day	3 3 1 3 3 3 3 4 6% 9% 62%	5900
	21st to end	Day	5% 4 2 5% 4 4 4 4 6% 9% 52%	6422
August	1st to 10th	Day	2 1 1 2 3 3 3 3 5% 8% 71%	4234
	11th to 20th	Day	4 2 1 3 2 3 3 4 7% 10% 60%	4171
	21st to end	Day	4 3 1 4 4 4 5% 5% 7% 10% 53%	4263
September	1st to 10th	Day	8% 5% 2 5% 4 3 4 4 5% 8% 53%	2200
	11th to 20th	Day	10% 5% 2 6% 4 4 4 4 6% 6% 50%	2200
	21st to end	Day	7% 4 2 4 2 3 3 4 6% 7% 59%	2200
October	1st to 10th	Day	6% 3 1 5% 2 3 3 3 4 6% 64%	2105
	11th to 20th	Day	13% 4 1 3 2 3 3 2 4 5% 61%	2000
	21st to end	Day	15% 4 2 4 3 2 3 2 4 4 58%	2200
November	1st to 10th	Day	12% 5% 1 5% 3 3 3 3 4 5% 57%	2058
	11th to 20th	Day	8% 4 1 6% 3 3 3 3 4 5% 59%	2023
	21st to end	Day	16% 6% 1 4 2 3 3 2 4 6% 53%	1890
December	1st to 10th	Day	18% 5% 1 5% 4 3 3 2 4 5% 51%	3030
	11th to 20th	Day	19% 8% 2 6% 3 4 3 2 4 4 44%	3481
	21st to end	Day	18% 8% 2 6% 3 3 3 3 4 5% 46%	4587
January	1st to 10th	Night	13% 5% 2 4 3 3 3 3 4 6% 55%	7634
	11th to 20th	Night	7% 3 1 2 2 2 2 2 4 5% 71%	7680
	21st to end	Night	25% 7% 2 5% 3 3 2 2 3 5% 42%	8402
February	1st to 10th	Night	15% 6% 2 5% 3 3 3 3 4 5% 51%	2684
	11th to 20th	Night	17% 6% 3 5% 4 3 3 2 4 5% 48%	2458
	21st to end	Night	17% 5% 1 5% 3 3 3 3 3 5% 52%	1998
March	1st to 10th	Night	18% 7% 2 6% 3 3 4 3 4 6% 45%	5029
	11th to 20th	Night	10% 5% 2 5% 4 4 4 3 5% 6% 54%	5090
	21st to end	Night	14% 5% 2 5% 4 3 4 3 4 5% 51%	5588
April	1st to 10th	Night	11% 5% 1 5% 3 3 4 4 6% 8% 52%	2211
	11th to 20th	Night	13% 6% 2 5% 3 4 4 3 5% 5% 50%	2024
	21st to end	Night	11% 5% 1 5% 3 3 2 3 6% 7% 53%	1925
May	1st to 10th	Night	17% 8% 3 6% 5% 4 4 3 5% 8% 38%	4053
	11th to 20th	Night	10% 6% 2 6% 4 4 4 3 5% 7% 47%	4053
	21st to end	Night	10% 6% 2 5% 4 4 4 4 7% 9% 46%	4443
June	1st to 10th	Night	6% 3 1 4 4 4 3 3 6% 10% 56%	1800
	11th to 20th	Night	5% 3 1 4 3 3 3 3 5% 8% 63%	1800
	21st to end	Night	3 3 1 4 3 3 4 3 7% 11% 58%	1800
July	1st to 10th	Night	6% 4 2 6% 5% 5% 5% 5% 7% 12% 44%	4078
	11th to 20th	Night	3 2 1 3 2 2 3 3 5% 9% 67%	4077
	21st to end	Night	3 3 1 4 3 4 4 4 7% 9% 59%	4449
August	1st to 10th	Night	1 1 2 2 1 2 2 4 9% 75%	2926
	11th to 20th	Night	4 2 1 3 2 2 3 3 5% 9% 66%	2888
	21st to end	Night	3 2 1 3 3 3 4 3 7% 9% 62%	3632
September	1st to 10th	Night	7% 5% 2 4 3 3 3 4 6% 8% 56%	2200
	11th to 20th	Night	10% 4 1 5% 3 3 4 4 5% 6% 55%	2200
	21st to end	Night	6% 3 1 4 3 3 3 3 5% 6% 63%	2200
October	1st to 10th	Night	5% 3 1 3 2 3 3 2 5% 6% 68%	2275
	11th to 20th	Night	12% 4 1 3 2 2 2 2 4 5% 64%	2400
	21st to end	Night	15% 4 1 4 2 2 1 2 3 4 61%	2640
November	1st to 10th	Night	11% 5% 2 4 2 2 3 2 3 4 61%	2460
	11th to 20th	Night	8% 5% 2 4 3 2 3 3 4 6% 59%	2593
	21st to end	Night	15% 6% 2 4 3 3 3 3 3 5% 53%	2730
December	1st to 10th	Night	17% 6% 2 5% 3 2 2 2 3 4 54%	4380
	11th to 20th	Night	17% 6% 3 6% 3 3 3 2 3 3 51%	4946
	21st to end	Night	15% 7% 2 5% 3 3 3 2 3 5% 52%	6633

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	34% 4 8% 26% 26% 3	5290
	11th to 20th	Day	24% 2 7% 26% 34% 6%	5310
	21st to end	Day	50% 2 3 15% 20% 10%	5812
February	1st to 10th	Day	45% 3 6% 17% 20% 9%	1953
	11th to 20th	Day	45% 3 7% 20% 18% 8%	2059
	21st to end	Day	38% 3 5% 15% 27% 11%	1806
March	1st to 10th	Day	36% 5% 7% 16% 20% 15%	5046
	11th to 20th	Day	32% 6% 6% 18% 18% 21%	5090
	21st to end	Day	38% 6% 10% 22% 19% 5%	5585
April	1st to 10th	Day	38% 6% 10% 19% 18% 9%	2211
	11th to 20th	Day	38% 6% 9% 18% 21% 8%	2324
	21st to end	Day	37% 7% 10% 20% 22% 4	2470
May	1st to 10th	Day	44% 7% 10% 25% 12% 2	5900
	11th to 20th	Day	42% 6% 9% 19% 21% 3	5866
	21st to end	Day	42% 9% 12% 20% 14% 4	6447
June	1st to 10th	Day	29% 9% 16% 27% 16% 2	2600
	11th to 20th	Day	27% 8% 16% 26% 18% 5%	2600
	21st to end	Day	24% 7% 15% 29% 21% 3	2600
July	1st to 10th	Day	36% 9% 20% 27% 8%	5836
	11th to 20th	Day	18% 6% 15% 39% 21% 2	5831
	21st to end	Day	27% 7% 14% 34% 17% 2	6240
August	1st to 10th	Day	12% 5% 13% 33% 33% 4	4220
	11th to 20th	Day	18% 5% 18% 40% 16% 2	4210
	21st to end	Day	23% 9% 20% 33% 13% 2	4262
September	1st to 10th	Day	30% 6% 11% 23% 20% 10%	2200
	11th to 20th	Day	33% 7% 12% 20% 21% 6%	2200
	21st to end	Day	24% 6% 10% 22% 27% 11%	2200
October	1st to 10th	Day	13% 3 7% 13% 33% 31%	3430
	11th to 20th	Day	18% 2 5% 23% 31% 21%	3240
	21st to end	Day	24% 4 5% 30% 28% 9%	3538
November	1st to 10th	Day	32% 4 7% 23% 26% 9%	2058
	11th to 20th	Day	28% 4 9% 19% 28% 12%	2023
	21st to end	Day	35% 3 6% 22% 25% 9%	1890
December	1st to 10th	Day	36% 4 6% 27% 23% 4	3030
	11th to 20th	Day	43% 4 5% 14% 25% 10%	3479
	21st to end	Day	41% 5% 5% 22% 21% 6%	4587
January	1st to 10th	Night	31% 4 7% 23% 29% 5%	7635
	11th to 20th	Night	18% 3 7% 20% 36% 16%	7680
	21st to end	Night	47% 3 5% 14% 20% 11%	8402
February	1st to 10th	Night	37% 4 8% 18% 23% 11%	2684
	11th to 20th	Night	40% 4 7% 17% 23% 9%	2458
	21st to end	Night	36% 4 7% 15% 23% 15%	1998
March	1st to 10th	Night	41% 5% 6% 12% 20% 17%	5029
	11th to 20th	Night	31% 5% 8% 15% 19% 22%	5090
	21st to end	Night	36% 5% 8% 19% 27% 6%	5588
April	1st to 10th	Night	31% 6% 11% 21% 18% 13%	2211
	11th to 20th	Night	35% 6% 8% 19% 20% 12%	2024
	21st to end	Night	30% 6% 11% 21% 26% 6%	1925
May	1st to 10th	Night	43% 8% 11% 21% 13% 4	4080
	11th to 20th	Night	36% 7% 15% 17% 23% 3	4054
	21st to end	Night	31% 9% 15% 24% 11% 8%	4452
June	1st to 10th	Night	23% 8% 16% 29% 20% 4	1800
	11th to 20th	Night	20% 5% 13% 34% 20% 7%	1800
	21st to end	Night	19% 8% 14% 30% 25% 5%	1800
July	1st to 10th	Night	31% 7% 20% 32% 10% 1	4035
	11th to 20th	Night	15% 4 15% 40% 22% 3	4026
	21st to end	Night	21% 7% 17% 34% 18% 3	4320
August	1st to 10th	Night	8% 3 11% 32% 39% 7%	2916
	11th to 20th	Night	15% 4 14% 42% 20% 4	2915
	21st to end	Night	18% 7% 20% 31% 21% 2	3632
September	1st to 10th	Night	25% 7% 11% 23% 23% 11%	2200
	11th to 20th	Night	28% 5% 12% 24% 21% 10%	2200
	21st to end	Night	21% 5% 9% 21% 28% 15%	2200
October	1st to 10th	Night	11% 3 6% 15% 28% 38%	3717
	11th to 20th	Night	17% 2 5% 18% 33% 26%	3899
	21st to end	Night	23% 3 4 25% 28% 16%	4252
November	1st to 10th	Night	28% 4 7% 21% 27% 14%	2460
	11th to 20th	Night	27% 4 7% 19% 27% 15%	2593
	21st to end	Night	35% 3 8% 16% 27% 10%	2730
December	1st to 10th	Night	37% 3 6% 20% 27% 8%	4380
	11th to 20th	Night	40% 3 6% 13% 24% 13%	4947
	21st to end	Night	36% 4 6% 19% 26% 9%	6633

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% 13% 35% 26% 8%	330
	11th to 20th	Day	14% 16% 40% 22% 7% 1	330
	21st to end	Day	24% 10% 35% 23% 7% 1	363
February	1st to 10th	Day	21% 13% 37% 23% 7%	330
	11th to 20th	Day	24% 14% 41% 16% 4 1	330
	21st to end	Day	26% 14% 39% 17% 4	273
March	1st to 10th	Day	22% 12% 34% 27% 5%	330
	11th to 20th	Day	18% 19% 37% 21% 4 1	330
	21st to end	Day	17% 12% 37% 26% 8% 1	363
April	1st to 10th	Day	20% 14% 33% 26% 6% 1	303
	11th to 20th	Day	21% 11% 35% 24% 9%	442
	21st to end	Day	17% 13% 33% 26% 10% 2	570
May	1st to 10th	Day	14% 9% 30% 35% 11% 1	703
	11th to 20th	Day	16% 5% 28% 33% 15% 3	725
	21st to end	Day	13% 6% 24% 33% 21% 4	789
June	1st to 10th	Day	6% 2 14% 37% 31% 10%	730
	11th to 20th	Day	7% 2 14% 39% 30% 8%	730
	21st to end	Day	3 2 11% 31% 38% 14%	730
July	1st to 10th	Day	6% 1 9% 29% 40% 15%	721
	11th to 20th	Day	3 1 5% 30% 47% 15%	718
	21st to end	Day	4 1 4 26% 47% 18%	794
August	1st to 10th	Day	2 1 4 26% 50% 18%	726
	11th to 20th	Day	8% 1 4 26% 42% 18%	715
	21st to end	Day	6% 2 4 20% 40% 27%	560
September	1st to 10th	Day	8% 1 4 18% 40% 29%	330
	11th to 20th	Day	6% 1 5% 22% 35% 30%	330
	21st to end	Day	8% 4 7% 23% 37% 22%	330
October	1st to 10th	Day	2 2 10% 28% 42% 15%	330
	11th to 20th	Day	11% 4 22% 31% 24% 8%	330
	21st to end	Day	23% 9% 19% 26% 19% 4	363
November	1st to 10th	Day	16% 6% 29% 25% 19% 5%	330
	11th to 20th	Day	12% 8% 28% 31% 18% 3	330
	21st to end	Day	18% 9% 32% 28% 10% 2	330
December	1st to 10th	Day	19% 8% 34% 26% 11% 2	330
	11th to 20th	Day	19% 12% 35% 25% 8% 1	330
	21st to end	Day	21% 11% 34% 25% 9% 1	363
January	1st to 10th	Night	17% 9% 36% 31% 7%	1050
	11th to 20th	Night	16% 15% 35% 28% 5%	1050
	21st to end	Night	18% 10% 36% 30% 6%	1155
February	1st to 10th	Night	18% 11% 37% 29% 5%	1004
	11th to 20th	Night	22% 12% 38% 25% 2	824
	21st to end	Night	22% 14% 39% 21% 3	614
March	1st to 10th	Night	20% 11% 34% 29% 6%	630
	11th to 20th	Night	16% 12% 37% 31% 4	630
	21st to end	Night	14% 11% 37% 33% 5%	693
April	1st to 10th	Night	12% 10% 44% 28% 5%	603
	11th to 20th	Night	16% 12% 38% 26% 8%	593
	21st to end	Night	13% 7% 41% 29% 10% 1	600
May	1st to 10th	Night	11% 8% 37% 35% 8% 1	600
	11th to 20th	Night	10% 4 33% 36% 16% 1	591
	21st to end	Night	8% 4 30% 40% 16% 3	650
June	1st to 10th	Night	6% 2 17% 42% 31% 3	600
	11th to 20th	Night	5% 2 17% 51% 24% 2	600
	21st to end	Night	4 1 13% 40% 37% 7%	600
July	1st to 10th	Night	4 1 10% 37% 41% 7%	585
	11th to 20th	Night	3 1 5% 42% 45% 5%	582
	21st to end	Night	2 3 43% 45% 7%	657
August	1st to 10th	Night	2 3 40% 49% 5%	597
	11th to 20th	Night	7% 4 34% 47% 7%	594
	21st to end	Night	4 1 4 34% 49% 7%	660
September	1st to 10th	Night	10% 1 8% 34% 43% 5%	600
	11th to 20th	Night	12% 2 5% 33% 45% 4	600
	21st to end	Night	7% 1 11% 38% 38% 5%	600
October	1st to 10th	Night	7% 1 16% 42% 31% 4	675
	11th to 20th	Night	13% 4 24% 39% 19% 2	800
	21st to end	Night	16% 7% 27% 31% 17% 1	880
November	1st to 10th	Night	11% 5% 28% 39% 15% 1	820
	11th to 20th	Night	9% 11% 29% 39% 11% 1	913
	21st to end	Night	18% 10% 32% 30% 10% 1	1050
December	1st to 10th	Night	17% 10% 35% 32% 6%	1050
	11th to 20th	Night	20% 9% 32% 34% 6%	1050
	21st to end	Night	19% 9% 33% 31% 6%	1155

Page generated at 2026-03-30 01:18:43. Version ac07

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