Sterner Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sterner Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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% 7% 2 6% 5% 5% 5% 4 6% 8% 39%	131192
	Night	14% 7% 3 6% 4 4 4 4 5% 7% 41%	129998
Transparency	Day	41% 6% 12% 23% 15% 3	134784
	Night	40% 6% 11% 23% 16% 3	134477
Seeing	Day	19% 12% 27% 23% 14% 5%	16533
	Night	17% 9% 30% 30% 12% 1	26063

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% 7% 3 7% 5% 5% 5% 4 6% 6% 40%	4661
	11th to 20th	Day	13% 10% 4 8% 5% 4 3 3 4 5% 40%	5160
	21st to end	Day	13% 7% 3 8% 5% 6% 6% 4 6% 6% 36%	5780
February	1st to 10th	Day	13% 10% 3 6% 5% 5% 4 4 7% 7% 37%	1890
	11th to 20th	Day	15% 9% 3 7% 4 6% 3 4 5% 6% 39%	1909
	21st to end	Day	17% 10% 3 7% 4 4 3 4 5% 5% 38%	1730
March	1st to 10th	Day	19% 6% 2 5% 4 4 4 3 4 5% 43%	4768
	11th to 20th	Day	11% 6% 3 7% 4 5% 4 4 6% 7% 44%	4980
	21st to end	Day	16% 7% 2 5% 5% 4 5% 4 5% 6% 41%	5620
April	1st to 10th	Day	15% 9% 3 7% 4 4 5% 4 6% 6% 37%	2400
	11th to 20th	Day	13% 6% 3 6% 5% 4 5% 4 5% 7% 42%	2374
	21st to end	Day	16% 7% 2 6% 4 4 4 4 5% 7% 41%	2400
May	1st to 10th	Day	23% 7% 3 6% 4 4 4 4 7% 8% 30%	5801
	11th to 20th	Day	22% 7% 3 7% 4 5% 4 3 6% 8% 31%	6318
	21st to end	Day	16% 6% 2 6% 4 4 5% 4 7% 9% 37%	6934
June	1st to 10th	Day	12% 5% 2 6% 5% 4 5% 5% 7% 9% 40%	2800
	11th to 20th	Day	12% 6% 3 7% 5% 5% 6% 5% 5% 9% 39%	2800
	21st to end	Day	9% 4 2 5% 5% 5% 5% 4 7% 10% 44%	2800
July	1st to 10th	Day	7% 5% 2 6% 5% 6% 5% 5% 7% 10% 42%	6360
	11th to 20th	Day	10% 6% 3 8% 5% 6% 6% 5% 8% 10% 32%	6360
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 4 7% 10% 39%	6477
August	1st to 10th	Day	16% 6% 3 7% 5% 5% 6% 5% 6% 8% 34%	4234
	11th to 20th	Day	11% 6% 3 7% 4 5% 5% 5% 7% 8% 40%	3988
	21st to end	Day	11% 5% 2 5% 4 4 4 3 5% 7% 49%	4312
September	1st to 10th	Day	14% 6% 2 6% 4 5% 4 3 3 6% 47%	2400
	11th to 20th	Day	12% 5% 2 5% 4 4 4 3 5% 8% 48%	2400
	21st to end	Day	18% 10% 3 6% 5% 4 3 3 5% 7% 37%	2119
October	1st to 10th	Day	16% 6% 2 5% 4 4 3 3 5% 7% 46%	2000
	11th to 20th	Day	15% 7% 2 5% 3 3 3 3 5% 7% 46%	2000
	21st to end	Day	14% 7% 2 5% 4 4 4 4 6% 6% 46%	2200
November	1st to 10th	Day	11% 5% 2 5% 3 3 4 3 5% 7% 52%	1881
	11th to 20th	Day	15% 6% 2 6% 4 3 4 4 5% 7% 45%	1858
	21st to end	Day	15% 7% 2 6% 4 4 4 4 5% 6% 43%	1680
December	1st to 10th	Day	13% 5% 2 7% 6% 5% 4 4 7% 7% 41%	2680
	11th to 20th	Day	18% 9% 3 6% 4 3 4 3 5% 7% 37%	3070
	21st to end	Day	14% 9% 3 8% 5% 5% 5% 4 6% 7% 33%	4048
January	1st to 10th	Night	10% 8% 4 9% 5% 5% 5% 4 5% 7% 37%	8233
	11th to 20th	Night	13% 9% 3 7% 3 3 4 3 5% 6% 45%	7782
	21st to end	Night	13% 8% 2 6% 4 4 4 3 6% 7% 43%	8401
February	1st to 10th	Night	15% 8% 3 7% 4 4 5% 3 5% 7% 39%	2730
	11th to 20th	Night	17% 8% 3 6% 4 4 5% 3 5% 6% 38%	2595
	21st to end	Night	15% 9% 3 7% 5% 4 4 4 5% 6% 39%	2076
March	1st to 10th	Night	17% 8% 2 5% 4 4 3 3 4 6% 45%	5127
	11th to 20th	Night	9% 5% 3 6% 5% 4 4 3 4 6% 50%	4980
	21st to end	Night	12% 8% 3 7% 5% 4 4 4 4 6% 44%	5312
April	1st to 10th	Night	16% 8% 3 7% 5% 4 4 3 6% 6% 36%	2000
	11th to 20th	Night	14% 6% 2 7% 4 3 4 4 5% 8% 43%	1975
	21st to end	Night	17% 6% 2 6% 3 3 3 4 6% 7% 43%	2000
May	1st to 10th	Night	21% 7% 3 7% 5% 5% 4 4 6% 8% 29%	4134
	11th to 20th	Night	21% 8% 3 6% 4 4 4 4 7% 8% 32%	3575
	21st to end	Night	18% 7% 3 7% 4 4 4 4 5% 8% 37%	3920
June	1st to 10th	Night	12% 5% 2 5% 4 4 5% 4 6% 8% 45%	1600
	11th to 20th	Night	11% 6% 2 6% 5% 5% 4 5% 7% 9% 41%	1600
	21st to end	Night	8% 5% 2 5% 5% 5% 6% 4 7% 9% 43%	1600
July	1st to 10th	Night	7% 5% 3 8% 5% 5% 6% 5% 7% 10% 40%	3598
	11th to 20th	Night	9% 6% 3 9% 6% 7% 7% 5% 8% 10% 31%	3597
	21st to end	Night	10% 6% 2 6% 4 4 4 4 6% 9% 45%	4371
August	1st to 10th	Night	17% 6% 3 6% 4 4 5% 4 7% 8% 36%	2915
	11th to 20th	Night	10% 6% 3 8% 5% 4 5% 4 5% 8% 41%	3026
	21st to end	Night	11% 5% 2 6% 4 4 5% 4 5% 8% 47%	3586
September	1st to 10th	Night	12% 5% 2 6% 4 4 5% 3 5% 7% 47%	2000
	11th to 20th	Night	11% 6% 2 4 4 3 4 3 5% 7% 52%	2000
	21st to end	Night	18% 7% 2 7% 4 4 4 3 5% 6% 39%	2278
October	1st to 10th	Night	18% 7% 2 5% 3 3 3 3 5% 6% 46%	2400
	11th to 20th	Night	15% 5% 2 5% 4 3 4 3 5% 6% 50%	2400
	21st to end	Night	16% 5% 2 4 3 3 3 3 5% 7% 48%	2640
November	1st to 10th	Night	11% 5% 2 6% 3 4 3 3 5% 6% 53%	2633
	11th to 20th	Night	13% 7% 2 5% 4 4 4 3 5% 6% 46%	2759
	21st to end	Night	12% 6% 2 6% 5% 4 5% 4 6% 8% 43%	2940
December	1st to 10th	Night	11% 6% 2 7% 5% 4 5% 4 6% 8% 43%	4720
	11th to 20th	Night	19% 7% 2 6% 4 4 4 4 5% 7% 38%	5345
	21st to end	Night	16% 9% 3 8% 5% 5% 5% 4 5% 7% 33%	7150

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	42% 4 12% 25% 15% 2	4662
	11th to 20th	Day	45% 4 9% 20% 17% 6%	5160
	21st to end	Day	44% 6% 11% 26% 12% 1	5780
February	1st to 10th	Day	43% 6% 13% 25% 11% 2	1890
	11th to 20th	Day	45% 6% 10% 23% 13% 2	1918
	21st to end	Day	48% 4 11% 22% 12% 4	1730
March	1st to 10th	Day	42% 4 9% 24% 14% 7%	4768
	11th to 20th	Day	37% 5% 12% 26% 15% 5%	4980
	21st to end	Day	43% 5% 10% 19% 19% 5%	5620
April	1st to 10th	Day	45% 5% 11% 18% 16% 4	2400
	11th to 20th	Day	40% 7% 11% 22% 17% 4	2374
	21st to end	Day	42% 5% 10% 23% 17% 3	2400
May	1st to 10th	Day	50% 7% 13% 19% 10% 2	5835
	11th to 20th	Day	50% 7% 10% 22% 10% 1	6322
	21st to end	Day	41% 8% 12% 22% 15% 1	6949
June	1st to 10th	Day	38% 9% 12% 24% 16% 2	2800
	11th to 20th	Day	40% 8% 14% 23% 13% 2	2800
	21st to end	Day	34% 8% 14% 26% 16% 1	2800
July	1st to 10th	Day	35% 10% 14% 22% 16% 2	6290
	11th to 20th	Day	42% 9% 17% 22% 9% 1	6285
	21st to end	Day	38% 7% 11% 22% 21% 1	6291
August	1st to 10th	Day	46% 9% 12% 24% 9%	4221
	11th to 20th	Day	39% 7% 11% 28% 14% 1	4027
	21st to end	Day	34% 6% 10% 28% 19% 2	4312
September	1st to 10th	Day	39% 5% 11% 20% 20% 4	2400
	11th to 20th	Day	34% 6% 9% 26% 21% 4	2400
	21st to end	Day	47% 5% 8% 18% 19% 2	2119
October	1st to 10th	Day	36% 5% 8% 21% 26% 5%	3260
	11th to 20th	Day	28% 4 12% 28% 21% 7%	3240
	21st to end	Day	26% 4 11% 28% 29% 3	3538
November	1st to 10th	Day	33% 3 14% 25% 21% 4	1881
	11th to 20th	Day	38% 5% 11% 24% 20% 3	1858
	21st to end	Day	41% 6% 11% 28% 12% 2	1680
December	1st to 10th	Day	41% 5% 17% 26% 9% 2	2680
	11th to 20th	Day	45% 3 15% 27% 9% 1	3066
	21st to end	Day	47% 5% 16% 19% 12% 1	4048
January	1st to 10th	Night	43% 5% 13% 24% 13% 2	8234
	11th to 20th	Night	40% 4 11% 21% 18% 6%	7782
	21st to end	Night	38% 3 11% 29% 17% 2	8401
February	1st to 10th	Night	45% 5% 13% 23% 13% 1	2730
	11th to 20th	Night	45% 4 10% 21% 17% 2	2597
	21st to end	Night	45% 5% 10% 21% 16% 4	2076
March	1st to 10th	Night	41% 5% 8% 26% 12% 8%	5127
	11th to 20th	Night	33% 6% 13% 30% 13% 6%	4980
	21st to end	Night	41% 4 8% 19% 24% 3	5312
April	1st to 10th	Night	46% 5% 10% 19% 16% 4	2000
	11th to 20th	Night	40% 6% 11% 20% 19% 4	1975
	21st to end	Night	40% 5% 11% 22% 18% 5%	2000
May	1st to 10th	Night	53% 6% 10% 16% 11% 3	4163
	11th to 20th	Night	49% 9% 11% 20% 10% 1	3576
	21st to end	Night	45% 7% 11% 23% 11% 2	3927
June	1st to 10th	Night	36% 8% 10% 26% 18% 2	1600
	11th to 20th	Night	36% 10% 14% 22% 16% 1	1600
	21st to end	Night	35% 8% 13% 26% 17% 2	1600
July	1st to 10th	Night	39% 9% 16% 19% 14% 3	3560
	11th to 20th	Night	47% 10% 14% 18% 10% 1	3552
	21st to end	Night	36% 7% 12% 23% 21% 1	4243
August	1st to 10th	Night	45% 7% 17% 22% 8% 1	2904
	11th to 20th	Night	40% 8% 10% 26% 15% 1	3053
	21st to end	Night	35% 7% 11% 29% 16% 2	3586
September	1st to 10th	Night	37% 6% 11% 21% 21% 4	2000
	11th to 20th	Night	32% 6% 10% 24% 25% 3	2000
	21st to end	Night	45% 6% 9% 18% 19% 3	2278
October	1st to 10th	Night	40% 5% 10% 21% 22% 2	3920
	11th to 20th	Night	26% 4 11% 27% 26% 5%	3899
	21st to end	Night	26% 3 10% 33% 24% 3	4252
November	1st to 10th	Night	33% 5% 11% 25% 22% 4	2633
	11th to 20th	Night	40% 4 8% 24% 19% 5%	2759
	21st to end	Night	38% 5% 12% 27% 16% 2	2940
December	1st to 10th	Night	38% 6% 15% 29% 11% 1	4720
	11th to 20th	Night	44% 4 14% 22% 14% 3	5348
	21st to end	Night	49% 6% 11% 19% 15% 1	7150

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% 25% 31% 14% 5%	220
	11th to 20th	Day	33% 22% 30% 13% 2	308
	21st to end	Day	22% 24% 37% 11% 5% 1	363
February	1st to 10th	Day	26% 20% 36% 15% 3	330
	11th to 20th	Day	30% 22% 37% 10% 2	396
	21st to end	Day	27% 19% 35% 17% 2	446
March	1st to 10th	Day	23% 19% 31% 21% 5%	508
	11th to 20th	Day	23% 19% 33% 19% 4 1	500
	21st to end	Day	24% 19% 32% 17% 6% 1	550
April	1st to 10th	Day	24% 18% 34% 17% 6% 1	500
	11th to 20th	Day	20% 18% 29% 18% 10% 4	492
	21st to end	Day	25% 12% 31% 21% 8% 3	500
May	1st to 10th	Day	26% 16% 27% 19% 9% 3	503
	11th to 20th	Day	21% 12% 28% 22% 12% 4	527
	21st to end	Day	18% 5% 22% 25% 20% 8%	573
June	1st to 10th	Day	14% 9% 29% 25% 16% 8%	530
	11th to 20th	Day	17% 7% 25% 26% 18% 7%	530
	21st to end	Day	12% 5% 24% 32% 19% 8%	530
July	1st to 10th	Day	12% 3 17% 30% 26% 12%	525
	11th to 20th	Day	6% 3 20% 31% 29% 11%	524
	21st to end	Day	12% 4 20% 35% 23% 6%	576
August	1st to 10th	Day	12% 3 19% 31% 25% 10%	526
	11th to 20th	Day	13% 3 18% 31% 24% 11%	519
	21st to end	Day	11% 5% 18% 30% 22% 13%	583
September	1st to 10th	Day	16% 7% 17% 24% 22% 14%	530
	11th to 20th	Day	12% 4 19% 30% 23% 12%	530
	21st to end	Day	25% 8% 24% 24% 13% 5%	530
October	1st to 10th	Day	18% 9% 22% 23% 18% 9%	530
	11th to 20th	Day	18% 12% 27% 27% 14% 3	530
	21st to end	Day	17% 11% 38% 24% 9% 2	583
November	1st to 10th	Day	17% 12% 32% 24% 13% 2	441
	11th to 20th	Day	20% 14% 32% 24% 9% 1	398
	21st to end	Day	25% 20% 34% 15% 7%	220
December	1st to 10th	Day	24% 18% 39% 14% 4 2	220
	11th to 20th	Day	28% 19% 31% 16% 6%	220
	21st to end	Day	21% 20% 33% 18% 7%	242
January	1st to 10th	Night	17% 16% 43% 20% 3	840
	11th to 20th	Night	22% 16% 34% 25% 3	840
	21st to end	Night	17% 14% 45% 22% 2	924
February	1st to 10th	Night	20% 14% 37% 27% 2	840
	11th to 20th	Night	23% 18% 40% 18% 1	823
	21st to end	Night	21% 18% 40% 19% 3	692
March	1st to 10th	Night	19% 15% 39% 24% 4	809
	11th to 20th	Night	20% 16% 36% 23% 5%	800
	21st to end	Night	21% 12% 38% 23% 5% 1	880
April	1st to 10th	Night	21% 14% 40% 21% 4	800
	11th to 20th	Night	19% 11% 33% 30% 7% 1	791
	21st to end	Night	20% 10% 34% 28% 8%	800
May	1st to 10th	Night	20% 7% 29% 32% 11%	651
	11th to 20th	Night	19% 6% 30% 36% 9%	394
	21st to end	Night	15% 4 20% 41% 19% 1	433
June	1st to 10th	Night	14% 2 25% 38% 21% 1	400
	11th to 20th	Night	11% 5% 24% 43% 17% 1	400
	21st to end	Night	11% 2 22% 49% 17%	400
July	1st to 10th	Night	9% 2 18% 42% 26% 2	390
	11th to 20th	Night	5% 2 14% 45% 30% 3	388
	21st to end	Night	11% 1 15% 46% 25% 2	627
August	1st to 10th	Night	11% 2 11% 47% 26% 2	597
	11th to 20th	Night	11% 3 14% 40% 29% 3	680
	21st to end	Night	12% 3 15% 38% 28% 4	880
September	1st to 10th	Night	14% 2 21% 34% 26% 3	800
	11th to 20th	Night	13% 2 16% 37% 27% 4	800
	21st to end	Night	21% 4 20% 33% 19% 2	800
October	1st to 10th	Night	20% 5% 21% 30% 21% 3	800
	11th to 20th	Night	16% 9% 28% 33% 13% 1	800
	21st to end	Night	19% 8% 33% 28% 11%	880
November	1st to 10th	Night	15% 6% 33% 35% 11% 1	820
	11th to 20th	Night	18% 12% 34% 27% 8% 1	840
	21st to end	Night	16% 13% 35% 28% 7%	840
December	1st to 10th	Night	21% 13% 38% 22% 6% 1	840
	11th to 20th	Night	19% 12% 39% 26% 4	840
	21st to end	Night	20% 12% 36% 30% 2	924

Page generated at 2026-02-28 01:40:24. Version ac07

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