Stelle
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stelle clear sky chart from 2005-11-06 to 2025-09-19.

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% 9% 3 8% 5% 5% 4 4 6% 8% 36%	126224
	Night	14% 9% 4 8% 4 4 4 3 5% 6% 38%	123059
Transparency	Day	44% 6% 12% 22% 14% 2	129758
	Night	45% 5% 11% 22% 15% 2	127586
Seeing	Day	16% 10% 28% 25% 15% 6%	16968
	Night	17% 10% 31% 30% 12% 1	26735

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	13% 12% 7% 14% 6% 5% 4 3 5% 6% 23%	3617
	11th to 20th	Day	11% 10% 5% 15% 8% 8% 4 5% 6% 4 23%	3620
	21st to end	Day	17% 18% 5% 9% 6% 3 3 3 4 5% 27%	4188
February	1st to 10th	Day	17% 12% 5% 8% 6% 7% 5% 3 4 6% 28%	1868
	11th to 20th	Day	14% 12% 4 8% 5% 6% 4 3 5% 6% 33%	1948
	21st to end	Day	15% 13% 5% 10% 4 4 3 3 5% 6% 31%	1650
March	1st to 10th	Day	19% 14% 4 6% 3 3 3 2 3 5% 38%	4480
	11th to 20th	Day	8% 9% 4 6% 4 5% 5% 4 6% 8% 42%	4675
	21st to end	Day	19% 12% 5% 10% 5% 5% 5% 3 6% 6% 25%	5967
April	1st to 10th	Day	16% 12% 3 7% 5% 4 4 3 5% 6% 35%	2400
	11th to 20th	Day	16% 10% 3 7% 6% 5% 4 4 4 7% 36%	2374
	21st to end	Day	19% 8% 2 7% 5% 4 4 4 5% 7% 34%	2482
May	1st to 10th	Day	19% 8% 2 6% 5% 5% 5% 4 6% 9% 31%	5862
	11th to 20th	Day	15% 6% 2 6% 5% 4 5% 4 6% 8% 40%	6079
	21st to end	Day	13% 6% 3 7% 5% 5% 5% 4 6% 9% 38%	6935
June	1st to 10th	Day	10% 5% 2 6% 5% 4 5% 5% 6% 8% 43%	2800
	11th to 20th	Day	9% 5% 2 6% 4 5% 5% 4 7% 9% 44%	2800
	21st to end	Day	10% 5% 2 7% 5% 4 6% 4 8% 9% 40%	2800
July	1st to 10th	Day	10% 5% 2 7% 5% 5% 5% 5% 7% 11% 38%	6360
	11th to 20th	Day	9% 5% 2 7% 5% 6% 5% 5% 7% 10% 38%	6360
	21st to end	Day	11% 6% 2 6% 5% 5% 6% 5% 7% 9% 38%	6924
August	1st to 10th	Day	7% 5% 2 6% 4 4 5% 4 6% 9% 48%	4360
	11th to 20th	Day	10% 6% 2 5% 4 3 5% 5% 7% 11% 43%	3849
	21st to end	Day	6% 5% 2 6% 4 4 5% 4 6% 9% 49%	4312
September	1st to 10th	Day	11% 5% 2 6% 4 4 4 4 6% 7% 48%	2400
	11th to 20th	Day	11% 5% 1 5% 3 3 5% 3 5% 7% 52%	2248
	21st to end	Day	11% 5% 2 6% 5% 4 4 3 6% 8% 45%	2090
October	1st to 10th	Day	11% 7% 3 6% 4 4 4 4 5% 7% 46%	2047
	11th to 20th	Day	16% 7% 2 6% 4 4 5% 3 5% 6% 41%	1900
	21st to end	Day	18% 10% 2 7% 5% 4 4 3 5% 7% 35%	1887
November	1st to 10th	Day	12% 7% 2 6% 4 3 4 3 5% 7% 48%	1762
	11th to 20th	Day	16% 11% 3 7% 5% 4 4 3 5% 5% 36%	1800
	21st to end	Day	17% 12% 3 9% 5% 4 3 3 4 5% 35%	1780
December	1st to 10th	Day	9% 12% 3 8% 5% 4 3 3 4 5% 43%	2610
	11th to 20th	Day	12% 16% 6% 11% 6% 5% 4 4 6% 5% 23%	3008
	21st to end	Day	26% 19% 7% 14% 5% 4 4 3 3 4 12%	3982
January	1st to 10th	Night	14% 14% 6% 12% 6% 4 4 3 5% 5% 28%	6348
	11th to 20th	Night	13% 16% 7% 13% 7% 5% 5% 3 5% 4 21%	6360
	21st to end	Night	20% 13% 6% 9% 4 4 4 3 3 3 31%	6789
February	1st to 10th	Night	17% 13% 4 9% 6% 4 3 3 4 6% 30%	2530
	11th to 20th	Night	15% 10% 4 8% 5% 5% 4 3 5% 5% 35%	2338
	21st to end	Night	18% 12% 4 8% 3 4 4 4 4 6% 35%	1980
March	1st to 10th	Night	15% 12% 4 6% 3 4 4 3 4 5% 40%	5373
	11th to 20th	Night	11% 7% 2 6% 5% 5% 4 3 5% 7% 45%	5313
	21st to end	Night	18% 10% 5% 10% 5% 5% 5% 4 5% 6% 28%	4962
April	1st to 10th	Night	13% 10% 3 7% 4 5% 4 4 5% 7% 39%	2000
	11th to 20th	Night	17% 9% 2 5% 5% 4 4 3 4 6% 41%	1975
	21st to end	Night	19% 7% 2 6% 5% 5% 4 4 7% 8% 34%	1916
May	1st to 10th	Night	19% 9% 3 8% 4 4 4 4 5% 7% 33%	4033
	11th to 20th	Night	13% 7% 2 6% 3 4 4 3 6% 9% 41%	3820
	21st to end	Night	13% 8% 3 7% 5% 4 5% 3 5% 8% 38%	3942
June	1st to 10th	Night	10% 6% 2 5% 4 4 4 3 5% 7% 51%	1600
	11th to 20th	Night	8% 5% 3 6% 5% 4 3 4 5% 10% 49%	1600
	21st to end	Night	9% 5% 2 6% 5% 5% 5% 4 6% 9% 45%	1600
July	1st to 10th	Night	10% 5% 2 6% 4 5% 5% 4 7% 9% 42%	3618
	11th to 20th	Night	8% 4 2 6% 5% 4 4 4 6% 9% 48%	3617
	21st to end	Night	9% 6% 2 7% 5% 5% 5% 4 6% 8% 44%	3947
August	1st to 10th	Night	6% 4 1 6% 4 4 4 4 8% 8% 50%	2787
	11th to 20th	Night	6% 4 2 5% 4 4 5% 4 6% 9% 51%	3200
	21st to end	Night	7% 4 2 5% 3 4 4 3 5% 9% 54%	3586
September	1st to 10th	Night	11% 4 2 4 4 3 4 3 5% 7% 54%	2000
	11th to 20th	Night	7% 5% 2 4 3 3 4 3 5% 7% 56%	2089
	21st to end	Night	10% 5% 2 6% 4 5% 5% 3 6% 8% 46%	2090
October	1st to 10th	Night	11% 5% 2 5% 3 3 4 3 5% 5% 54%	2128
	11th to 20th	Night	14% 6% 2 6% 3 3 3 3 4 7% 49%	2280
	21st to end	Night	19% 8% 3 6% 4 4 3 3 5% 6% 39%	2709
November	1st to 10th	Night	12% 7% 2 6% 4 3 4 3 5% 6% 48%	2536
	11th to 20th	Night	14% 9% 3 8% 5% 4 4 4 4 6% 40%	2600
	21st to end	Night	17% 11% 3 8% 4 4 3 3 4 5% 37%	2618
December	1st to 10th	Night	12% 8% 3 7% 4 4 3 3 5% 6% 44%	4580
	11th to 20th	Night	17% 13% 4 10% 5% 4 4 3 4 5% 31%	5199
	21st to end	Night	25% 18% 6% 10% 5% 4 4 3 4 4 16%	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	61% 4 12% 15% 7% 1	3618
	11th to 20th	Day	63% 4 9% 10% 12% 3	3620
	21st to end	Day	62% 3 11% 16% 7%	4188
February	1st to 10th	Day	58% 5% 11% 17% 7% 2	1868
	11th to 20th	Day	52% 5% 11% 22% 9% 1	1958
	21st to end	Day	54% 4 11% 22% 7% 3	1650
March	1st to 10th	Day	50% 4 7% 22% 14% 2	4480
	11th to 20th	Day	38% 6% 11% 34% 11%	4675
	21st to end	Day	58% 6% 12% 15% 7% 1	5969
April	1st to 10th	Day	49% 6% 12% 20% 13%	2400
	11th to 20th	Day	47% 6% 10% 22% 11% 3	2374
	21st to end	Day	50% 6% 12% 19% 10% 2	2482
May	1st to 10th	Day	48% 7% 11% 20% 12% 1	5897
	11th to 20th	Day	42% 7% 11% 25% 13% 2	6083
	21st to end	Day	40% 7% 11% 22% 16% 3	6949
June	1st to 10th	Day	36% 8% 11% 25% 17% 3	2800
	11th to 20th	Day	35% 7% 10% 24% 20% 4	2800
	21st to end	Day	38% 8% 15% 21% 16% 3	2800
July	1st to 10th	Day	38% 7% 12% 26% 15% 2	6291
	11th to 20th	Day	38% 7% 15% 25% 13% 1	6285
	21st to end	Day	40% 7% 17% 21% 13% 2	6728
August	1st to 10th	Day	30% 7% 18% 30% 14% 1	4345
	11th to 20th	Day	33% 6% 14% 29% 16% 1	3885
	21st to end	Day	30% 7% 13% 30% 17% 4	4311
September	1st to 10th	Day	35% 6% 12% 25% 18% 4	2400
	11th to 20th	Day	30% 5% 10% 26% 23% 5%	2248
	21st to end	Day	36% 6% 10% 27% 20% 2	2090
October	1st to 10th	Day	26% 5% 7% 26% 34% 2	3398
	11th to 20th	Day	30% 4 11% 23% 30% 1	3140
	21st to end	Day	41% 4 9% 24% 19% 2	3087
November	1st to 10th	Day	36% 5% 14% 25% 17% 3	1762
	11th to 20th	Day	49% 5% 12% 21% 13% 1	1800
	21st to end	Day	52% 4 11% 22% 10% 1	1780
December	1st to 10th	Day	44% 4 13% 30% 9%	2610
	11th to 20th	Day	61% 5% 12% 17% 5%	3005
	21st to end	Day	77% 4 8% 7% 4	3982
January	1st to 10th	Night	59% 5% 9% 18% 9%	6348
	11th to 20th	Night	66% 4 9% 9% 11% 2	6360
	21st to end	Night	59% 3 9% 17% 11% 1	6789
February	1st to 10th	Night	55% 5% 11% 18% 10% 1	2530
	11th to 20th	Night	51% 4 11% 21% 12% 1	2339
	21st to end	Night	50% 5% 12% 18% 12% 3	1980
March	1st to 10th	Night	47% 5% 8% 25% 10% 4	5373
	11th to 20th	Night	38% 5% 12% 25% 17% 3	5313
	21st to end	Night	55% 6% 9% 21% 7% 1	4962
April	1st to 10th	Night	45% 6% 12% 23% 13% 2	2000
	11th to 20th	Night	44% 6% 9% 22% 16% 4	1975
	21st to end	Night	46% 6% 13% 21% 12% 3	1916
May	1st to 10th	Night	50% 6% 11% 17% 13% 3	4060
	11th to 20th	Night	38% 7% 14% 25% 13% 3	3821
	21st to end	Night	42% 8% 11% 21% 15% 4	3950
June	1st to 10th	Night	33% 6% 12% 27% 20% 3	1600
	11th to 20th	Night	32% 6% 12% 25% 23% 3	1600
	21st to end	Night	34% 8% 15% 23% 16% 4	1600
July	1st to 10th	Night	37% 6% 12% 28% 13% 4	3580
	11th to 20th	Night	32% 6% 16% 25% 20% 1	3572
	21st to end	Night	37% 7% 16% 22% 15% 3	3832
August	1st to 10th	Night	28% 7% 19% 27% 18% 1	2778
	11th to 20th	Night	30% 6% 16% 28% 20% 1	3230
	21st to end	Night	28% 5% 13% 32% 19% 3	3586
September	1st to 10th	Night	30% 6% 10% 27% 22% 5%	2000
	11th to 20th	Night	26% 6% 8% 27% 28% 5%	2089
	21st to end	Night	36% 7% 11% 24% 20% 3	2090
October	1st to 10th	Night	20% 4 7% 32% 34% 3	3544
	11th to 20th	Night	27% 4 8% 25% 33% 2	3779
	21st to end	Night	37% 3 8% 25% 22% 4	4459
November	1st to 10th	Night	36% 5% 11% 27% 18% 4	2536
	11th to 20th	Night	45% 5% 12% 21% 15% 2	2600
	21st to end	Night	50% 4 10% 24% 12% 1	2618
December	1st to 10th	Night	41% 3 14% 29% 11% 1	4580
	11th to 20th	Night	55% 4 12% 17% 11% 1	5201
	21st to end	Night	72% 4 9% 10% 5%	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	23% 15% 34% 22% 7% 1	300
	11th to 20th	Day	21% 19% 41% 14% 5%	300
	21st to end	Day	22% 18% 35% 20% 6%	330
February	1st to 10th	Day	24% 14% 40% 17% 4	300
	11th to 20th	Day	24% 17% 35% 20% 4	300
	21st to end	Day	27% 14% 43% 14% 2	249
March	1st to 10th	Day	23% 20% 37% 19% 1	300
	11th to 20th	Day	25% 18% 35% 18% 4	324
	21st to end	Day	28% 16% 33% 16% 6% 1	550
April	1st to 10th	Day	24% 15% 32% 22% 6% 1	500
	11th to 20th	Day	20% 18% 30% 19% 10% 3	492
	21st to end	Day	23% 14% 32% 21% 8% 2	500
May	1st to 10th	Day	19% 13% 29% 26% 10% 2	503
	11th to 20th	Day	19% 9% 29% 23% 14% 5%	615
	21st to end	Day	13% 8% 31% 23% 17% 7%	789
June	1st to 10th	Day	10% 7% 27% 32% 19% 5%	730
	11th to 20th	Day	10% 5% 29% 33% 19% 4	730
	21st to end	Day	11% 8% 31% 30% 16% 5%	730
July	1st to 10th	Day	10% 4 17% 30% 23% 17%	721
	11th to 20th	Day	9% 3 20% 32% 22% 13%	718
	21st to end	Day	9% 3 20% 35% 26% 8%	794
August	1st to 10th	Day	6% 2 16% 34% 29% 13%	642
	11th to 20th	Day	9% 4 18% 29% 27% 14%	519
	21st to end	Day	5% 3 16% 31% 32% 13%	583
September	1st to 10th	Day	10% 8% 17% 29% 27% 10%	530
	11th to 20th	Day	9% 4 13% 29% 31% 14%	522
	21st to end	Day	13% 7% 28% 21% 22% 9%	490
October	1st to 10th	Day	12% 7% 28% 29% 19% 6%	447
	11th to 20th	Day	14% 20% 32% 22% 10% 1	300
	21st to end	Day	29% 16% 31% 17% 5% 1	330
November	1st to 10th	Day	14% 13% 37% 22% 9% 4	300
	11th to 20th	Day	24% 15% 36% 16% 8% 1	300
	21st to end	Day	25% 11% 35% 25% 4	300
December	1st to 10th	Day	15% 19% 38% 18% 8% 1	300
	11th to 20th	Day	19% 19% 35% 22% 4 1	300
	21st to end	Day	26% 18% 37% 15% 4	330
January	1st to 10th	Night	20% 16% 38% 21% 5%	1000
	11th to 20th	Night	23% 16% 37% 20% 5%	1000
	21st to end	Night	21% 14% 36% 25% 3	1100
February	1st to 10th	Night	24% 13% 39% 21% 3	930
	11th to 20th	Night	20% 16% 36% 25% 4	783
	21st to end	Night	24% 14% 40% 20% 3	660
March	1st to 10th	Night	20% 15% 37% 24% 4	800
	11th to 20th	Night	18% 12% 39% 26% 5% 1	776
	21st to end	Night	23% 11% 37% 24% 5%	660
April	1st to 10th	Night	18% 14% 37% 25% 6%	600
	11th to 20th	Night	20% 13% 34% 26% 7%	593
	21st to end	Night	22% 10% 34% 27% 7% 1	600
May	1st to 10th	Night	20% 7% 35% 31% 7%	600
	11th to 20th	Night	18% 6% 29% 33% 13% 1	591
	21st to end	Night	11% 6% 31% 34% 18%	650
June	1st to 10th	Night	11% 5% 26% 45% 13%	600
	11th to 20th	Night	10% 5% 26% 45% 14% 1	600
	21st to end	Night	9% 4 27% 43% 16% 1	600
July	1st to 10th	Night	8% 3 15% 41% 30% 4	585
	11th to 20th	Night	7% 1 18% 41% 29% 3	582
	21st to end	Night	6% 2 14% 47% 29% 2	657
August	1st to 10th	Night	5% 3 14% 43% 31% 3	597
	11th to 20th	Night	7% 2 12% 43% 32% 4	594
	21st to end	Night	6% 2 15% 38% 38% 2	660
September	1st to 10th	Night	13% 3 16% 40% 26% 3	600
	11th to 20th	Night	8% 3 16% 39% 31% 3	703
	21st to end	Night	11% 7% 25% 32% 23% 2	760
October	1st to 10th	Night	13% 6% 23% 38% 19% 2	760
	11th to 20th	Night	17% 12% 33% 28% 10%	760
	21st to end	Night	23% 13% 33% 23% 8%	836
November	1st to 10th	Night	16% 10% 38% 26% 9% 1	780
	11th to 20th	Night	18% 15% 35% 26% 6% 1	800
	21st to end	Night	23% 13% 35% 24% 6%	818
December	1st to 10th	Night	20% 13% 39% 24% 4	1000
	11th to 20th	Night	24% 11% 34% 26% 5%	1000
	21st to end	Night	30% 14% 32% 22% 2	1100

Page generated at 2025-09-16 00:49:09. Version ac07

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