Skyline Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Skyline Observatory 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	14% 7% 3 6% 4 4 5% 4 6% 8% 39%	125967
	Night	15% 7% 2 6% 4 4 4 3 5% 7% 42%	123104
Transparency	Day	41% 5% 11% 21% 17% 4	129716
	Night	40% 4 10% 22% 19% 5%	127425
Seeing	Day	19% 10% 22% 22% 18% 8%	15732
	Night	18% 8% 19% 30% 22% 4	25684

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	19% 9% 5% 5% 3 3 3 3 5% 5% 41%	4056
	11th to 20th	Day	18% 5% 5% 9% 3 3 3 2 3 4 45%	4060
	21st to end	Day	22% 9% 3 6% 3 3 3 3 4 6% 38%	4908
February	1st to 10th	Day	19% 11% 3 6% 3 3 3 3 4 7% 38%	2000
	11th to 20th	Day	23% 8% 2 7% 5% 4 4 3 4 5% 36%	1948
	21st to end	Day	23% 8% 2 7% 4 4 4 2 4 5% 38%	1650
March	1st to 10th	Day	21% 9% 3 7% 4 5% 4 3 5% 5% 33%	4480
	11th to 20th	Day	14% 7% 3 6% 3 3 3 3 5% 6% 46%	4842
	21st to end	Day	15% 7% 2 8% 4 3 4 3 5% 7% 42%	5617
April	1st to 10th	Day	14% 6% 2 5% 4 4 5% 4 6% 6% 43%	2400
	11th to 20th	Day	15% 7% 2 6% 4 4 3 3 5% 7% 45%	2374
	21st to end	Day	11% 6% 2 7% 5% 4 5% 4 6% 8% 42%	2445
May	1st to 10th	Day	18% 7% 2 7% 5% 5% 5% 4 6% 8% 31%	5862
	11th to 20th	Day	18% 8% 3 8% 6% 5% 5% 4 7% 8% 28%	5861
	21st to end	Day	17% 7% 2 6% 4 4 4 3 5% 8% 37%	6433
June	1st to 10th	Day	11% 6% 2 7% 5% 6% 6% 5% 8% 10% 34%	2600
	11th to 20th	Day	5% 4 2 6% 5% 5% 6% 6% 8% 11% 41%	2600
	21st to end	Day	7% 5% 2 6% 6% 5% 6% 5% 8% 11% 39%	2600
July	1st to 10th	Day	7% 5% 2 6% 5% 5% 7% 6% 9% 11% 35%	5900
	11th to 20th	Day	7% 5% 2 6% 6% 6% 7% 7% 9% 13% 31%	5900
	21st to end	Day	13% 8% 3 8% 7% 6% 7% 6% 9% 11% 22%	6424
August	1st to 10th	Day	6% 3 2 5% 4 5% 6% 5% 8% 11% 47%	4018
	11th to 20th	Day	6% 4 2 6% 5% 6% 7% 6% 8% 10% 41%	3849
	21st to end	Day	8% 4 2 5% 5% 5% 5% 4 6% 8% 51%	4312
September	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 5% 8% 9% 39%	2325
	11th to 20th	Day	13% 5% 2 6% 5% 5% 5% 4 6% 8% 41%	2178
	21st to end	Day	13% 5% 2 6% 5% 4 4 3 5% 6% 47%	2090
October	1st to 10th	Day	11% 5% 2 5% 4 3 4 2 3 6% 56%	2090
	11th to 20th	Day	9% 5% 2 4 3 4 3 3 5% 7% 54%	2090
	21st to end	Day	15% 6% 2 4 3 3 3 3 4 6% 52%	1924
November	1st to 10th	Day	13% 6% 2 4 4 3 4 3 5% 4 52%	1762
	11th to 20th	Day	13% 6% 2 5% 4 4 4 3 5% 6% 49%	1800
	21st to end	Day	18% 6% 2 6% 3 3 4 3 4 6% 44%	1800
December	1st to 10th	Day	20% 7% 2 5% 4 3 3 3 4 4 45%	2930
	11th to 20th	Day	24% 9% 3 7% 4 4 4 2 4 5% 35%	3373
	21st to end	Day	20% 8% 4 7% 4 4 4 3 6% 7% 33%	4466
January	1st to 10th	Night	17% 11% 3 9% 4 3 3 3 4 5% 40%	5889
	11th to 20th	Night	19% 7% 3 8% 4 3 3 2 3 4 44%	5900
	21st to end	Night	19% 10% 2 5% 3 4 3 3 4 6% 41%	6030
February	1st to 10th	Night	19% 9% 3 6% 4 3 3 2 4 6% 41%	2400
	11th to 20th	Night	22% 8% 3 7% 4 4 3 3 4 4 37%	2338
	21st to end	Night	24% 7% 3 5% 3 4 4 4 4 5% 36%	1980
March	1st to 10th	Night	25% 11% 3 7% 4 4 4 3 4 5% 30%	5373
	11th to 20th	Night	12% 7% 2 6% 3 3 4 4 5% 7% 47%	5118
	21st to end	Night	17% 7% 3 7% 5% 4 4 3 4 5% 41%	5340
April	1st to 10th	Night	12% 6% 2 5% 4 3 4 4 6% 8% 46%	2000
	11th to 20th	Night	15% 6% 2 5% 3 2 4 3 4 7% 49%	1975
	21st to end	Night	11% 5% 2 5% 4 4 4 4 5% 6% 49%	1953
May	1st to 10th	Night	15% 7% 3 8% 5% 5% 5% 4 6% 7% 33%	4033
	11th to 20th	Night	20% 9% 3 8% 6% 4 5% 4 6% 6% 29%	4032
	21st to end	Night	14% 7% 2 6% 4 4 5% 4 7% 9% 39%	4421
June	1st to 10th	Night	9% 6% 2 6% 5% 4 6% 5% 7% 10% 40%	1800
	11th to 20th	Night	6% 3 2 4 4 4 4 5% 7% 11% 51%	1800
	21st to end	Night	6% 5% 2 5% 5% 4 5% 5% 7% 11% 46%	1800
July	1st to 10th	Night	6% 6% 2 7% 6% 6% 6% 5% 8% 10% 40%	4058
	11th to 20th	Night	8% 5% 2 5% 5% 5% 6% 5% 8% 13% 38%	4057
	21st to end	Night	9% 8% 3 9% 5% 5% 6% 5% 7% 12% 30%	4427
August	1st to 10th	Night	4 3 2 4 3 3 4 4 7% 11% 56%	3126
	11th to 20th	Night	6% 5% 2 5% 4 4 4 3 6% 11% 50%	3200
	21st to end	Night	6% 4 2 5% 3 3 4 4 5% 8% 57%	3586
September	1st to 10th	Night	8% 6% 2 6% 5% 4 5% 4 6% 8% 47%	2053
	11th to 20th	Night	11% 5% 3 6% 4 4 4 3 7% 9% 46%	2179
	21st to end	Night	9% 5% 2 6% 4 3 4 3 5% 7% 51%	2090
October	1st to 10th	Night	10% 3 2 4 3 2 2 3 4 6% 59%	2090
	11th to 20th	Night	12% 6% 2 5% 4 4 3 3 3 5% 52%	2090
	21st to end	Night	15% 6% 1 5% 3 3 3 3 4 5% 52%	2668
November	1st to 10th	Night	14% 5% 2 4 3 3 4 3 5% 6% 51%	2536
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 4 6% 50%	2600
	21st to end	Night	17% 7% 2 5% 3 3 3 3 5% 5% 45%	2600
December	1st to 10th	Night	20% 8% 2 5% 3 3 3 3 4 5% 45%	4250
	11th to 20th	Night	25% 8% 2 5% 3 3 3 3 3 5% 40%	4822
	21st to end	Night	22% 8% 4 8% 4 4 4 4 4 6% 33%	6490

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	46% 3 6% 22% 21% 2	4057
	11th to 20th	Day	44% 2 5% 20% 27% 2	4060
	21st to end	Day	48% 2 7% 21% 18% 4	4908
February	1st to 10th	Day	48% 2 6% 22% 18% 3	2000
	11th to 20th	Day	52% 2 6% 19% 15% 5%	1958
	21st to end	Day	51% 2 7% 15% 20% 6%	1650
March	1st to 10th	Day	52% 3 7% 12% 18% 8%	4480
	11th to 20th	Day	38% 3 6% 19% 21% 12%	4842
	21st to end	Day	43% 3 10% 25% 16% 2	5619
April	1st to 10th	Day	40% 4 10% 24% 17% 5%	2400
	11th to 20th	Day	39% 3 6% 22% 24% 6%	2374
	21st to end	Day	40% 3 10% 29% 15% 3	2445
May	1st to 10th	Day	48% 6% 13% 20% 11% 1	5897
	11th to 20th	Day	52% 6% 14% 19% 8%	5865
	21st to end	Day	45% 5% 9% 23% 17% 2	6447
June	1st to 10th	Day	43% 6% 14% 21% 15% 2	2600
	11th to 20th	Day	32% 6% 19% 25% 15% 2	2600
	21st to end	Day	36% 7% 19% 21% 14% 3	2600
July	1st to 10th	Day	36% 10% 19% 24% 10% 1	5835
	11th to 20th	Day	36% 14% 27% 19% 4 1	5831
	21st to end	Day	49% 12% 20% 15% 4	6240
August	1st to 10th	Day	27% 7% 16% 35% 15% 1	4005
	11th to 20th	Day	33% 9% 17% 26% 13% 2	3885
	21st to end	Day	30% 7% 13% 27% 22% 2	4311
September	1st to 10th	Day	38% 7% 15% 19% 17% 5%	2325
	11th to 20th	Day	40% 6% 12% 19% 20% 3	2178
	21st to end	Day	36% 5% 11% 22% 20% 7%	2090
October	1st to 10th	Day	21% 4 6% 15% 39% 15%	3480
	11th to 20th	Day	21% 3 5% 19% 32% 20%	3458
	21st to end	Day	27% 4 7% 24% 28% 10%	3148
November	1st to 10th	Day	35% 4 7% 19% 28% 8%	1762
	11th to 20th	Day	37% 2 6% 22% 25% 8%	1800
	21st to end	Day	42% 2 6% 22% 24% 5%	1800
December	1st to 10th	Day	44% 2 4 20% 25% 5%	2930
	11th to 20th	Day	53% 3 5% 20% 15% 4	3370
	21st to end	Day	51% 3 9% 25% 11% 2	4466
January	1st to 10th	Night	47% 3 7% 20% 20% 3	5889
	11th to 20th	Night	45% 3 5% 19% 23% 5%	5900
	21st to end	Night	46% 3 5% 19% 22% 7%	6030
February	1st to 10th	Night	46% 2 7% 20% 20% 4	2400
	11th to 20th	Night	51% 2 6% 18% 16% 7%	2339
	21st to end	Night	50% 3 6% 17% 18% 6%	1980
March	1st to 10th	Night	56% 4 6% 14% 17% 5%	5373
	11th to 20th	Night	36% 3 8% 18% 21% 14%	5118
	21st to end	Night	46% 3 7% 23% 18% 4	5340
April	1st to 10th	Night	35% 3 13% 24% 20% 5%	2000
	11th to 20th	Night	36% 3 6% 22% 23% 10%	1975
	21st to end	Night	36% 4 11% 29% 17% 3	1953
May	1st to 10th	Night	47% 6% 14% 19% 12% 1	4060
	11th to 20th	Night	54% 6% 13% 18% 9%	4033
	21st to end	Night	41% 5% 14% 24% 15% 1	4429
June	1st to 10th	Night	37% 7% 15% 23% 17% 2	1800
	11th to 20th	Night	25% 5% 19% 31% 17% 2	1800
	21st to end	Night	30% 8% 18% 23% 16% 4	1800
July	1st to 10th	Night	36% 9% 20% 24% 10% 1	4015
	11th to 20th	Night	33% 11% 28% 21% 5% 2	4006
	21st to end	Night	44% 10% 23% 17% 4 1	4297
August	1st to 10th	Night	21% 6% 15% 38% 18% 1	3115
	11th to 20th	Night	27% 6% 22% 28% 15% 2	3230
	21st to end	Night	25% 6% 15% 27% 26% 2	3586
September	1st to 10th	Night	33% 7% 17% 21% 17% 5%	2053
	11th to 20th	Night	34% 6% 14% 21% 22% 3	2179
	21st to end	Night	32% 5% 9% 23% 22% 9%	2090
October	1st to 10th	Night	18% 4 7% 21% 32% 18%	3480
	11th to 20th	Night	22% 2 4 18% 33% 21%	3461
	21st to end	Night	25% 3 5% 27% 28% 13%	4394
November	1st to 10th	Night	34% 4 7% 22% 23% 10%	2536
	11th to 20th	Night	35% 3 5% 20% 28% 9%	2600
	21st to end	Night	42% 2 5% 22% 23% 6%	2600
December	1st to 10th	Night	43% 3 6% 20% 26% 4	4250
	11th to 20th	Night	48% 3 5% 19% 23% 3	4824
	21st to end	Night	51% 3 7% 25% 12% 1	6490

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	35% 20% 29% 13% 3	300
	11th to 20th	Day	32% 23% 27% 13% 4 1	300
	21st to end	Day	26% 25% 34% 12% 3	330
February	1st to 10th	Day	30% 21% 31% 15% 3	300
	11th to 20th	Day	42% 21% 28% 8% 1	300
	21st to end	Day	39% 22% 28% 9% 2	249
March	1st to 10th	Day	33% 20% 31% 11% 4 1	300
	11th to 20th	Day	30% 18% 28% 16% 6% 1	440
	21st to end	Day	26% 18% 33% 16% 7% 1	550
April	1st to 10th	Day	25% 15% 27% 21% 11% 2	500
	11th to 20th	Day	22% 15% 29% 22% 11% 1	492
	21st to end	Day	21% 11% 32% 22% 13% 1	500
May	1st to 10th	Day	19% 13% 29% 22% 11% 5%	503
	11th to 20th	Day	14% 10% 27% 25% 14% 9%	527
	21st to end	Day	18% 6% 19% 26% 18% 13%	573
June	1st to 10th	Day	15% 6% 20% 26% 22% 12%	530
	11th to 20th	Day	6% 5% 15% 29% 27% 19%	530
	21st to end	Day	10% 4 16% 25% 28% 17%	530
July	1st to 10th	Day	9% 3 12% 25% 33% 17%	525
	11th to 20th	Day	9% 3 10% 24% 33% 20%	524
	21st to end	Day	10% 4 7% 24% 34% 20%	576
August	1st to 10th	Day	13% 3 10% 27% 30% 17%	526
	11th to 20th	Day	12% 3 10% 29% 33% 14%	519
	21st to end	Day	14% 3 10% 24% 31% 17%	583
September	1st to 10th	Day	12% 2 15% 28% 28% 15%	530
	11th to 20th	Day	15% 5% 14% 22% 29% 15%	522
	21st to end	Day	16% 6% 15% 25% 31% 8%	490
October	1st to 10th	Day	13% 4 22% 30% 22% 8%	490
	11th to 20th	Day	13% 10% 27% 30% 18% 2	490
	21st to end	Day	23% 8% 23% 29% 15% 3	373
November	1st to 10th	Day	23% 11% 26% 25% 13% 2	300
	11th to 20th	Day	15% 18% 37% 23% 6%	300
	21st to end	Day	22% 17% 30% 20% 10% 2	300
December	1st to 10th	Day	32% 17% 27% 17% 5% 1	300
	11th to 20th	Day	29% 20% 30% 14% 7% 1	300
	21st to end	Day	35% 13% 31% 14% 6%	330
January	1st to 10th	Night	29% 16% 27% 23% 5%	800
	11th to 20th	Night	26% 17% 25% 25% 7%	800
	21st to end	Night	23% 14% 31% 29% 3	880
February	1st to 10th	Night	25% 11% 26% 33% 5%	800
	11th to 20th	Night	29% 17% 25% 26% 3	783
	21st to end	Night	31% 13% 28% 25% 3	660
March	1st to 10th	Night	24% 15% 24% 31% 5%	800
	11th to 20th	Night	22% 12% 29% 29% 8% 1	800
	21st to end	Night	20% 12% 29% 28% 11%	830
April	1st to 10th	Night	16% 12% 27% 33% 13%	600
	11th to 20th	Night	19% 9% 26% 31% 14% 2	593
	21st to end	Night	14% 8% 25% 38% 15% 1	600
May	1st to 10th	Night	13% 7% 23% 34% 21% 2	600
	11th to 20th	Night	14% 5% 18% 33% 25% 5%	591
	21st to end	Night	10% 4 11% 36% 34% 6%	650
June	1st to 10th	Night	13% 1 12% 31% 36% 7%	600
	11th to 20th	Night	6% 2 7% 33% 40% 12%	600
	21st to end	Night	7% 2 7% 29% 45% 10%	600
July	1st to 10th	Night	9% 3 5% 25% 41% 16%	585
	11th to 20th	Night	9% 2 4 23% 42% 19%	582
	21st to end	Night	8% 3 5% 22% 47% 14%	657
August	1st to 10th	Night	8% 2 2 26% 47% 15%	597
	11th to 20th	Night	11% 3 6% 24% 46% 10%	594
	21st to end	Night	12% 2 6% 27% 43% 10%	660
September	1st to 10th	Night	9% 2 10% 27% 43% 8%	653
	11th to 20th	Night	12% 4 10% 29% 39% 6%	793
	21st to end	Night	10% 5% 13% 28% 41% 3	760
October	1st to 10th	Night	12% 4 15% 33% 30% 6%	760
	11th to 20th	Night	15% 8% 20% 34% 22% 1	760
	21st to end	Night	18% 7% 17% 37% 19% 2	836
November	1st to 10th	Night	16% 8% 19% 33% 22% 2	780
	11th to 20th	Night	16% 11% 26% 31% 15% 1	800
	21st to end	Night	23% 10% 22% 32% 13% 1	800
December	1st to 10th	Night	26% 13% 24% 29% 9%	800
	11th to 20th	Night	25% 12% 22% 33% 9%	800
	21st to end	Night	28% 12% 24% 29% 7%	880

Page generated at 2025-09-16 00:49:10. Version ac07

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