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 2024-12-02.

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	14% 7% 2 6% 4 4 4 4 6% 7% 39%	101547
	Night	15% 7% 3 6% 4 4 4 4 5% 7% 41%	98050
Transparency	Day	41% 6% 12% 23% 15% 2	105183
	Night	41% 6% 12% 24% 16% 3	102554
Seeing	Day	18% 11% 27% 24% 14% 5%	14991
	Night	17% 9% 30% 31% 12% 1	24419

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% 7% 3 6% 4 4 4 3 5% 6% 42%	2517
	11th to 20th	Day	13% 11% 5% 10% 5% 5% 4 4 5% 5% 33%	2776
	21st to end	Day	17% 11% 4 9% 5% 5% 4 4 5% 5% 32%	3124
February	1st to 10th	Day	14% 10% 3 6% 5% 5% 4 4 7% 7% 37%	1710
	11th to 20th	Day	15% 8% 2 6% 4 5% 4 4 5% 6% 40%	1722
	21st to end	Day	18% 10% 3 7% 4 4 3 4 4 4 38%	1570
March	1st to 10th	Day	23% 7% 2 6% 5% 4 4 3 5% 6% 36%	3304
	11th to 20th	Day	10% 6% 3 6% 5% 5% 4 4 6% 7% 44%	3480
	21st to end	Day	20% 8% 2 5% 4 3 3 3 4 5% 42%	3928
April	1st to 10th	Day	14% 8% 3 7% 4 4 6% 4 6% 6% 38%	2280
	11th to 20th	Day	13% 6% 3 6% 5% 4 5% 4 5% 7% 42%	2254
	21st to end	Day	17% 7% 2 6% 4 4 4 4 5% 7% 40%	2280
May	1st to 10th	Day	27% 7% 3 6% 4 4 4 4 6% 7% 29%	4035
	11th to 20th	Day	23% 7% 3 8% 5% 5% 4 4 6% 7% 28%	4398
	21st to end	Day	10% 5% 2 6% 4 4 5% 4 7% 9% 44%	4823
June	1st to 10th	Day	12% 5% 2 6% 5% 4 5% 5% 7% 9% 39%	2660
	11th to 20th	Day	11% 6% 2 7% 5% 5% 6% 5% 5% 9% 39%	2660
	21st to end	Day	9% 4 2 5% 5% 5% 5% 5% 7% 10% 44%	2660
July	1st to 10th	Day	6% 4 2 7% 5% 6% 6% 5% 8% 11% 40%	4440
	11th to 20th	Day	9% 6% 2 6% 4 5% 6% 5% 8% 10% 40%	4440
	21st to end	Day	11% 7% 2 8% 6% 5% 5% 5% 7% 9% 35%	4505
August	1st to 10th	Day	16% 6% 3 7% 5% 5% 6% 5% 7% 8% 34%	4104
	11th to 20th	Day	11% 6% 2 7% 4 5% 5% 5% 7% 8% 41%	3864
	21st to end	Day	11% 5% 2 6% 4 4 4 3 5% 7% 48%	4180
September	1st to 10th	Day	15% 6% 2 6% 4 5% 4 3 3 6% 46%	2280
	11th to 20th	Day	12% 5% 2 5% 4 4 4 3 5% 8% 48%	2280
	21st to end	Day	18% 10% 3 6% 4 3 3 3 5% 7% 38%	2013
October	1st to 10th	Day	16% 6% 2 5% 4 4 3 3 5% 6% 45%	1900
	11th to 20th	Day	15% 7% 2 5% 3 3 3 3 4 7% 47%	1900
	21st to end	Day	15% 7% 2 5% 3 3 4 4 6% 6% 45%	2090
November	1st to 10th	Day	12% 5% 2 5% 3 3 4 3 5% 6% 51%	1790
	11th to 20th	Day	15% 5% 2 5% 4 3 4 4 6% 7% 46%	1770
	21st to end	Day	15% 7% 2 5% 4 4 5% 4 5% 6% 43%	1600
December	1st to 10th	Day	17% 7% 3 7% 6% 4 3 4 7% 8% 34%	1528
	11th to 20th	Day	17% 8% 3 7% 4 4 5% 3 6% 7% 36%	1910
	21st to end	Day	16% 9% 3 8% 5% 5% 5% 4 6% 7% 32%	2772
January	1st to 10th	Night	14% 8% 4 9% 5% 5% 4 4 5% 7% 36%	4429
	11th to 20th	Night	17% 9% 3 7% 3 4 4 3 5% 6% 40%	4180
	21st to end	Night	19% 11% 3 8% 5% 4 3 3 5% 5% 35%	4532
February	1st to 10th	Night	16% 8% 3 7% 4 4 4 3 5% 6% 39%	2470
	11th to 20th	Night	16% 8% 3 6% 4 4 5% 3 5% 6% 39%	2342
	21st to end	Night	16% 9% 2 7% 5% 4 4 4 5% 6% 38%	1884
March	1st to 10th	Night	21% 9% 3 6% 4 4 3 3 5% 6% 38%	3546
	11th to 20th	Night	11% 6% 3 6% 5% 4 4 4 5% 6% 48%	3480
	21st to end	Night	13% 9% 3 7% 5% 3 3 3 4 5% 46%	3704
April	1st to 10th	Night	15% 8% 3 7% 5% 4 4 4 6% 7% 36%	1900
	11th to 20th	Night	14% 5% 2 7% 5% 3 4 4 5% 8% 44%	1875
	21st to end	Night	18% 6% 2 6% 3 3 3 4 6% 6% 43%	1900
May	1st to 10th	Night	23% 8% 3 7% 5% 4 3 3 5% 7% 31%	2886
	11th to 20th	Night	23% 8% 3 7% 5% 4 4 4 7% 6% 29%	2495
	21st to end	Night	10% 6% 2 6% 4 5% 4 4 6% 10% 41%	2732
June	1st to 10th	Night	12% 5% 2 5% 3 4 5% 4 6% 8% 45%	1520
	11th to 20th	Night	9% 6% 2 6% 5% 5% 4 5% 7% 9% 41%	1520
	21st to end	Night	8% 5% 3 5% 5% 5% 5% 4 7% 10% 43%	1520
July	1st to 10th	Night	6% 4 2 8% 5% 5% 7% 5% 8% 10% 41%	2520
	11th to 20th	Night	7% 4 2 8% 5% 6% 6% 5% 8% 10% 38%	2520
	21st to end	Night	12% 7% 3 6% 5% 4 4 4 6% 8% 40%	3045
August	1st to 10th	Night	17% 6% 3 6% 4 5% 5% 4 7% 8% 35%	2825
	11th to 20th	Night	10% 6% 3 8% 5% 4 5% 4 5% 8% 41%	2931
	21st to end	Night	11% 5% 2 6% 4 4 5% 4 5% 8% 46%	3476
September	1st to 10th	Night	12% 5% 2 6% 4 4 5% 3 5% 7% 47%	1900
	11th to 20th	Night	11% 6% 2 4 3 3 3 3 5% 7% 52%	1900
	21st to end	Night	18% 6% 2 7% 4 4 4 3 5% 6% 41%	2164
October	1st to 10th	Night	18% 7% 2 5% 3 3 3 3 5% 6% 45%	2280
	11th to 20th	Night	14% 5% 2 5% 4 3 4 3 5% 6% 50%	2280
	21st to end	Night	17% 5% 2 4 3 3 3 3 6% 7% 48%	2508
November	1st to 10th	Night	11% 5% 2 6% 3 4 3 3 5% 5% 53%	2504
	11th to 20th	Night	13% 8% 2 5% 4 4 4 3 5% 6% 46%	2627
	21st to end	Night	12% 5% 2 6% 5% 4 4 4 6% 8% 44%	2800
December	1st to 10th	Night	16% 8% 2 7% 5% 4 5% 4 5% 7% 37%	2686
	11th to 20th	Night	16% 6% 2 6% 5% 5% 4 4 5% 6% 39%	3285
	21st to end	Night	16% 8% 3 7% 5% 5% 6% 4 6% 8% 31%	4884

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	41% 4 14% 26% 11% 3	2518
	11th to 20th	Day	51% 5% 11% 18% 13% 3	2776
	21st to end	Day	52% 6% 10% 23% 8% 1	3124
February	1st to 10th	Day	44% 6% 13% 24% 11% 2	1710
	11th to 20th	Day	44% 6% 11% 24% 13% 3	1731
	21st to end	Day	49% 4 11% 20% 12% 4	1570
March	1st to 10th	Day	48% 5% 10% 25% 9% 3	3304
	11th to 20th	Day	36% 5% 11% 26% 17% 3	3480
	21st to end	Day	45% 5% 9% 22% 17% 3	3928
April	1st to 10th	Day	44% 5% 12% 19% 16% 4	2280
	11th to 20th	Day	39% 7% 11% 22% 17% 4	2254
	21st to end	Day	43% 5% 10% 22% 16% 3	2280
May	1st to 10th	Day	53% 6% 9% 20% 10% 1	4072
	11th to 20th	Day	53% 8% 13% 17% 8% 1	4402
	21st to end	Day	34% 9% 14% 25% 17% 2	4837
June	1st to 10th	Day	38% 9% 12% 24% 15% 1	2660
	11th to 20th	Day	39% 9% 14% 23% 13% 3	2660
	21st to end	Day	33% 9% 14% 27% 16% 1	2660
July	1st to 10th	Day	35% 10% 15% 23% 13% 3	4370
	11th to 20th	Day	35% 9% 16% 26% 13% 1	4365
	21st to end	Day	43% 8% 11% 19% 18% 2	4361
August	1st to 10th	Day	46% 9% 12% 24% 9%	4091
	11th to 20th	Day	38% 7% 11% 28% 15% 1	3903
	21st to end	Day	34% 6% 11% 28% 19% 2	4180
September	1st to 10th	Day	40% 6% 11% 19% 20% 4	2280
	11th to 20th	Day	34% 6% 10% 26% 21% 4	2280
	21st to end	Day	46% 5% 8% 19% 19% 2	2013
October	1st to 10th	Day	36% 5% 8% 21% 26% 4	3160
	11th to 20th	Day	27% 4 12% 28% 21% 7%	3140
	21st to end	Day	26% 4 11% 28% 28% 3	3428
November	1st to 10th	Day	34% 3 13% 25% 20% 4	1790
	11th to 20th	Day	38% 5% 11% 24% 19% 3	1770
	21st to end	Day	40% 6% 11% 29% 12% 2	1600
December	1st to 10th	Day	47% 6% 14% 21% 11% 1	1528
	11th to 20th	Day	44% 4 13% 29% 8% 1	1906
	21st to end	Day	49% 5% 16% 19% 10% 1	2772
January	1st to 10th	Night	43% 5% 12% 26% 11% 3	4429
	11th to 20th	Night	44% 4 14% 24% 12% 2	4180
	21st to end	Night	49% 3 10% 27% 9% 2	4532
February	1st to 10th	Night	45% 5% 13% 23% 12% 1	2470
	11th to 20th	Night	45% 4 10% 21% 18% 3	2344
	21st to end	Night	46% 5% 10% 20% 15% 4	1884
March	1st to 10th	Night	47% 6% 9% 25% 12% 1	3546
	11th to 20th	Night	34% 6% 11% 32% 15% 2	3480
	21st to end	Night	41% 4 7% 21% 22% 4	3704
April	1st to 10th	Night	45% 6% 11% 19% 17% 4	1900
	11th to 20th	Night	39% 6% 11% 21% 19% 4	1875
	21st to end	Night	40% 6% 11% 22% 17% 5%	1900
May	1st to 10th	Night	52% 7% 8% 18% 12% 2	2915
	11th to 20th	Night	53% 10% 11% 14% 10% 1	2496
	21st to end	Night	36% 9% 14% 25% 13% 3	2739
June	1st to 10th	Night	35% 8% 10% 26% 18% 2	1520
	11th to 20th	Night	35% 10% 14% 22% 17% 2	1520
	21st to end	Night	35% 8% 13% 27% 16% 2	1520
July	1st to 10th	Night	37% 9% 19% 18% 12% 5%	2480
	11th to 20th	Night	39% 9% 14% 23% 14% 1	2472
	21st to end	Night	41% 8% 14% 19% 19% 1	2948
August	1st to 10th	Night	45% 7% 17% 22% 8% 1	2814
	11th to 20th	Night	40% 7% 10% 26% 16% 1	2958
	21st to end	Night	35% 7% 11% 29% 16% 2	3476
September	1st to 10th	Night	38% 6% 11% 20% 21% 4	1900
	11th to 20th	Night	31% 6% 10% 25% 25% 3	1900
	21st to end	Night	44% 6% 9% 19% 19% 3	2164
October	1st to 10th	Night	41% 5% 10% 20% 21% 2	3800
	11th to 20th	Night	26% 4 11% 27% 27% 5%	3779
	21st to end	Night	26% 3 10% 33% 24% 3	4120
November	1st to 10th	Night	33% 5% 11% 25% 22% 4	2504
	11th to 20th	Night	40% 4 8% 24% 19% 5%	2627
	21st to end	Night	38% 5% 12% 27% 17% 2	2800
December	1st to 10th	Night	46% 6% 12% 23% 13% 1	2686
	11th to 20th	Night	43% 5% 13% 22% 15% 3	3288
	21st to end	Night	48% 7% 14% 21% 9% 1	4884

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% 23% 32% 16% 5% 1	180
	11th to 20th	Day	31% 21% 31% 16% 2	252
	21st to end	Day	21% 23% 37% 13% 5% 1	297
February	1st to 10th	Day	26% 19% 36% 16% 3	270
	11th to 20th	Day	26% 23% 40% 9% 2	329
	21st to end	Day	29% 18% 32% 18% 3	382
March	1st to 10th	Day	22% 19% 31% 23% 5%	460
	11th to 20th	Day	23% 20% 33% 19% 4 1	460
	21st to end	Day	25% 18% 32% 18% 6% 2	506
April	1st to 10th	Day	21% 18% 36% 17% 7% 1	460
	11th to 20th	Day	17% 18% 30% 19% 11% 4	452
	21st to end	Day	26% 12% 31% 21% 8% 3	460
May	1st to 10th	Day	27% 14% 27% 19% 10% 4	463
	11th to 20th	Day	21% 11% 27% 24% 13% 4	487
	21st to end	Day	16% 5% 22% 27% 22% 9%	529
June	1st to 10th	Day	14% 8% 29% 26% 16% 8%	490
	11th to 20th	Day	15% 7% 25% 27% 19% 7%	490
	21st to end	Day	12% 5% 25% 31% 19% 8%	490
July	1st to 10th	Day	12% 3 17% 30% 26% 12%	485
	11th to 20th	Day	5% 3 19% 32% 30% 11%	484
	21st to end	Day	12% 4 20% 34% 23% 6%	532
August	1st to 10th	Day	12% 3 19% 30% 27% 10%	486
	11th to 20th	Day	12% 3 18% 32% 23% 12%	479
	21st to end	Day	11% 4 18% 30% 23% 14%	539
September	1st to 10th	Day	17% 7% 17% 23% 22% 14%	490
	11th to 20th	Day	12% 4 18% 30% 23% 12%	490
	21st to end	Day	26% 8% 23% 23% 14% 6%	490
October	1st to 10th	Day	20% 9% 23% 23% 18% 8%	490
	11th to 20th	Day	17% 10% 27% 28% 14% 3	490
	21st to end	Day	17% 10% 38% 24% 10% 2	539
November	1st to 10th	Day	17% 11% 32% 25% 14% 2	410
	11th to 20th	Day	18% 15% 32% 25% 10% 1	370
	21st to end	Day	25% 20% 32% 17% 8%	200
December	1st to 10th	Day	24% 16% 38% 15% 4 2	182
	11th to 20th	Day	27% 18% 31% 16% 7% 1	180
	21st to end	Day	21% 18% 34% 19% 8%	198
January	1st to 10th	Night	17% 14% 44% 21% 3	760
	11th to 20th	Night	22% 14% 33% 27% 3	760
	21st to end	Night	17% 13% 45% 22% 2	836
February	1st to 10th	Night	20% 14% 37% 28% 2	760
	11th to 20th	Night	22% 17% 41% 19% 1	743
	21st to end	Night	21% 18% 39% 18% 3	628
March	1st to 10th	Night	19% 14% 39% 24% 4	760
	11th to 20th	Night	20% 15% 36% 23% 5%	760
	21st to end	Night	21% 11% 38% 24% 5% 1	836
April	1st to 10th	Night	20% 14% 40% 21% 4	760
	11th to 20th	Night	18% 10% 34% 31% 7% 1	751
	21st to end	Night	21% 10% 34% 28% 8%	760
May	1st to 10th	Night	20% 7% 29% 32% 11%	619
	11th to 20th	Night	18% 6% 30% 36% 10%	374
	21st to end	Night	14% 3 19% 42% 20% 1	411
June	1st to 10th	Night	13% 2 24% 38% 22% 1	380
	11th to 20th	Night	10% 4 25% 43% 17% 1	380
	21st to end	Night	10% 2 22% 51% 15%	380
July	1st to 10th	Night	9% 2 19% 41% 27% 2	370
	11th to 20th	Night	5% 2 14% 45% 31% 3	368
	21st to end	Night	11% 1 15% 46% 24% 2	595
August	1st to 10th	Night	11% 2 12% 47% 26% 2	567
	11th to 20th	Night	11% 3 14% 39% 30% 3	645
	21st to end	Night	12% 2 14% 39% 29% 4	836
September	1st to 10th	Night	14% 2 20% 34% 25% 3	760
	11th to 20th	Night	13% 2 16% 36% 28% 4	760
	21st to end	Night	21% 4 20% 33% 20% 2	760
October	1st to 10th	Night	20% 6% 21% 30% 20% 3	760
	11th to 20th	Night	16% 9% 28% 34% 13% 1	760
	21st to end	Night	19% 7% 33% 28% 12%	836
November	1st to 10th	Night	13% 5% 34% 35% 11% 1	780
	11th to 20th	Night	18% 12% 34% 27% 8% 1	800
	21st to end	Night	16% 13% 35% 28% 7%	800
December	1st to 10th	Night	22% 12% 38% 22% 6% 1	768
	11th to 20th	Night	18% 11% 40% 27% 4	760
	21st to end	Night	19% 11% 36% 30% 2	836

Page generated at 2024-11-29 10:43:11. Version ac07

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