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 2026-07-01.

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	4 9% 3 7% 4 4 4 4 6% 7% 47%	146986
	Night	5% 12% 4 8% 4 3 4 3 5% 6% 46%	142930
Transparency	Day	33% 8% 12% 21% 18% 7%	141877
	Night	36% 7% 12% 22% 17% 6%	139048
Seeing	Day	12% 15% 31% 28% 13% 2	17833
	Night	11% 8% 27% 37% 16% 1	27907

By Month

Percentage Cloud Cover SW (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	5% 17% 8% 18% 5% 3 3 3 4 4 30%	4720
	11th to 20th	Day	2 15% 5% 16% 8% 4 3 2 3 4 38%	5194
	21st to end	Day	2 14% 9% 12% 6% 3 3 3 4 4 41%	5844
February	1st to 10th	Day	6% 21% 8% 13% 5% 3 3 3 4 5% 31%	1890
	11th to 20th	Day	5% 19% 8% 13% 5% 3 4 3 3 5% 30%	1893
	21st to end	Day	4 19% 6% 14% 5% 3 3 3 5% 6% 32%	1730
March	1st to 10th	Day	2 15% 7% 11% 5% 4 4 4 5% 5% 39%	4835
	11th to 20th	Day	2 17% 6% 8% 4 4 4 3 5% 6% 42%	5110
	21st to end	Day	5% 20% 6% 11% 6% 5% 5% 3 5% 6% 29%	5755
April	1st to 10th	Day	6% 14% 4 8% 5% 3 3 3 5% 7% 40%	2520
	11th to 20th	Day	7% 14% 4 9% 5% 4 4 4 6% 7% 35%	2491
	21st to end	Day	5% 12% 4 7% 6% 5% 5% 5% 6% 7% 39%	2520
May	1st to 10th	Day	7% 11% 3 6% 4 4 6% 5% 7% 7% 39%	7602
	11th to 20th	Day	5% 7% 2 6% 5% 5% 5% 4 6% 7% 46%	8191
	21st to end	Day	5% 6% 2 6% 4 5% 6% 5% 8% 9% 45%	9026
June	1st to 10th	Day	4 3 1 4 4 4 5% 4 7% 9% 55%	2940
	11th to 20th	Day	4 3 1 4 4 3 3 4 5% 7% 62%	2940
	21st to end	Day	3 2 1 3 4 4 3 4 7% 7% 61%	2940
July	1st to 10th	Day	3 2 1 3 3 3 4 4 7% 8% 63%	6340
	11th to 20th	Day	1 1 1 4 3 4 6% 5% 8% 10% 56%	6340
	21st to end	Day	2 3 1 5% 4 6% 6% 6% 8% 9% 49%	6493
August	1st to 10th	Day	3 3 1 4 4 5% 5% 5% 8% 10% 52%	5779
	11th to 20th	Day	4 2 2 4 3 5% 5% 4 7% 8% 55%	5632
	21st to end	Day	5% 4 2 7% 5% 5% 5% 4 6% 7% 50%	5984
September	1st to 10th	Day	5% 3 1 3 3 3 4 3 5% 7% 62%	2400
	11th to 20th	Day	6% 4 2 4 4 4 4 4 5% 7% 56%	2400
	21st to end	Day	8% 5% 1 4 3 2 4 2 5% 5% 62%	2130
October	1st to 10th	Day	5% 5% 1 3 2 2 2 2 3 4 71%	3270
	11th to 20th	Day	6% 7% 2 5% 3 3 4 4 6% 8% 52%	3250
	21st to end	Day	9% 12% 2 5% 2 2 3 3 5% 6% 51%	3548
November	1st to 10th	Day	5% 11% 3 6% 3 3 4 2 4 6% 52%	1878
	11th to 20th	Day	7% 15% 4 7% 3 2 3 3 4 5% 46%	1823
	21st to end	Day	5% 13% 6% 10% 3 2 2 2 3 5% 50%	1680
December	1st to 10th	Day	5% 11% 5% 14% 4 2 2 1 2 3 51%	2700
	11th to 20th	Day	4 19% 5% 7% 3 2 2 2 4 6% 47%	3106
	21st to end	Day	6% 18% 6% 11% 6% 4 4 3 5% 7% 32%	4092
January	1st to 10th	Night	5% 20% 8% 16% 5% 3 3 2 4 5% 28%	8201
	11th to 20th	Night	2 16% 6% 14% 7% 3 2 2 3 4 39%	7853
	21st to end	Night	3 18% 6% 13% 5% 3 3 2 3 4 41%	8370
February	1st to 10th	Night	7% 18% 7% 12% 6% 3 4 3 4 5% 30%	2730
	11th to 20th	Night	5% 20% 9% 11% 5% 4 3 3 5% 6% 31%	2612
	21st to end	Night	4 17% 7% 12% 4 4 3 3 5% 6% 34%	2076
March	1st to 10th	Night	3 19% 6% 9% 4 3 3 3 4 5% 40%	5254
	11th to 20th	Night	3 11% 6% 7% 4 3 3 3 4 6% 51%	5110
	21st to end	Night	7% 23% 6% 9% 4 4 4 3 4 5% 32%	5454
April	1st to 10th	Night	8% 15% 4 6% 4 3 4 4 5% 5% 42%	2100
	11th to 20th	Night	8% 15% 4 6% 4 3 4 4 6% 6% 40%	2077
	21st to end	Night	9% 13% 3 5% 4 4 4 3 7% 6% 43%	2100
May	1st to 10th	Night	9% 11% 3 5% 4 4 4 4 6% 7% 44%	5328
	11th to 20th	Night	10% 10% 3 6% 4 4 5% 4 7% 8% 39%	4701
	21st to end	Night	8% 7% 2 5% 4 4 4 4 5% 8% 49%	5181
June	1st to 10th	Night	4 4 1 3 3 3 3 4 6% 8% 59%	1680
	11th to 20th	Night	4 3 1 4 4 3 4 3 5% 7% 63%	1680
	21st to end	Night	3 2 1 3 2 3 4 3 5% 6% 67%	1680
July	1st to 10th	Night	3 2 1 4 4 4 4 4 6% 7% 62%	3640
	11th to 20th	Night	1 2 1 4 3 5% 6% 4 9% 11% 54%	3640
	21st to end	Night	2 2 2 4 4 4 5% 5% 7% 9% 56%	4375
August	1st to 10th	Night	3 3 1 4 4 5% 6% 5% 8% 9% 53%	4033
	11th to 20th	Night	3 2 1 5% 4 4 6% 4 6% 8% 57%	4217
	21st to end	Night	4 3 2 5% 4 4 5% 5% 7% 9% 51%	4993
September	1st to 10th	Night	4 3 1 4 3 2 3 3 6% 7% 63%	2000
	11th to 20th	Night	4 4 2 4 3 4 3 3 6% 6% 62%	2000
	21st to end	Night	7% 4 1 4 2 2 2 3 4 5% 66%	2265
October	1st to 10th	Night	5% 4 1 2 2 2 2 2 4 4 72%	3920
	11th to 20th	Night	6% 6% 2 4 3 3 4 3 4 8% 58%	3898
	21st to end	Night	8% 10% 3 4 2 2 3 2 4 5% 57%	4252
November	1st to 10th	Night	6% 12% 3 5% 2 3 3 2 5% 6% 53%	2618
	11th to 20th	Night	7% 14% 4 6% 3 3 3 2 5% 6% 46%	2793
	21st to end	Night	5% 14% 5% 7% 4 3 3 3 4 6% 46%	2940
December	1st to 10th	Night	5% 13% 4 10% 4 2 2 2 2 3 52%	4710
	11th to 20th	Night	4 17% 7% 9% 4 3 3 2 4 6% 41%	5321
	21st to end	Night	7% 21% 5% 11% 4 3 3 3 4 5% 33%	7128

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	54% 7% 14% 15% 9% 1	4723
	11th to 20th	Day	46% 8% 16% 18% 11% 2	5194
	21st to end	Day	44% 7% 10% 21% 14% 4	5844
February	1st to 10th	Day	54% 7% 14% 15% 9% 1	1890
	11th to 20th	Day	54% 7% 11% 18% 8% 2	1902
	21st to end	Day	50% 8% 13% 15% 13% 2	1730
March	1st to 10th	Day	39% 10% 12% 20% 14% 5%	4835
	11th to 20th	Day	38% 9% 15% 27% 10% 1	5110
	21st to end	Day	48% 12% 13% 19% 6% 1	5755
April	1st to 10th	Day	39% 9% 14% 20% 14% 4	2520
	11th to 20th	Day	44% 9% 13% 18% 15% 2	2494
	21st to end	Day	39% 9% 13% 26% 12% 1	2520
May	1st to 10th	Day	35% 10% 12% 21% 18% 3	7602
	11th to 20th	Day	29% 10% 13% 22% 23% 4	8195
	21st to end	Day	27% 9% 13% 27% 22% 1	9027
June	1st to 10th	Day	22% 9% 14% 23% 24% 8%	2940
	11th to 20th	Day	20% 6% 10% 17% 27% 19%	2940
	21st to end	Day	18% 6% 10% 19% 27% 19%	2940
July	1st to 10th	Day	15% 5% 10% 13% 23% 33%	6272
	11th to 20th	Day	17% 6% 11% 20% 30% 15%	6265
	21st to end	Day	23% 8% 13% 15% 24% 16%	6306
August	1st to 10th	Day	27% 10% 14% 20% 16% 14%	4208
	11th to 20th	Day	27% 9% 13% 17% 23% 11%	4079
	21st to end	Day	24% 8% 12% 18% 21% 16%	4311
September	1st to 10th	Day	20% 6% 11% 19% 25% 20%	2400
	11th to 20th	Day	24% 6% 12% 21% 26% 11%	2400
	21st to end	Day	24% 4 8% 27% 30% 7%	2130
October	1st to 10th	Day	20% 4 7% 17% 35% 17%	3270
	11th to 20th	Day	28% 5% 10% 32% 23% 3	3250
	21st to end	Day	33% 5% 10% 35% 15% 2	3548
November	1st to 10th	Day	33% 5% 13% 25% 23% 1	1878
	11th to 20th	Day	39% 6% 15% 25% 14% 2	1823
	21st to end	Day	38% 4 12% 30% 14% 2	1680
December	1st to 10th	Day	40% 4 14% 33% 8% 1	2700
	11th to 20th	Day	39% 5% 20% 28% 8%	3104
	21st to end	Day	49% 7% 11% 19% 11% 1	4092
January	1st to 10th	Night	56% 7% 11% 16% 7% 2	8202
	11th to 20th	Night	47% 6% 15% 20% 11% 1	7853
	21st to end	Night	45% 8% 10% 18% 18% 1	8370
February	1st to 10th	Night	54% 7% 12% 18% 8% 1	2730
	11th to 20th	Night	52% 8% 12% 18% 9% 1	2613
	21st to end	Night	46% 9% 12% 18% 15% 2	2076
March	1st to 10th	Night	40% 8% 11% 22% 14% 5%	5254
	11th to 20th	Night	32% 7% 13% 29% 17% 2	5110
	21st to end	Night	46% 12% 14% 22% 5% 1	5454
April	1st to 10th	Night	39% 9% 13% 19% 16% 3	2100
	11th to 20th	Night	40% 8% 13% 21% 17% 2	2077
	21st to end	Night	36% 8% 13% 23% 18% 1	2100
May	1st to 10th	Night	32% 10% 13% 28% 15% 2	5326
	11th to 20th	Night	34% 11% 14% 21% 16% 4	4702
	21st to end	Night	29% 7% 14% 26% 22% 2	5182
June	1st to 10th	Night	19% 7% 15% 27% 23% 8%	1680
	11th to 20th	Night	20% 7% 10% 17% 25% 22%	1680
	21st to end	Night	14% 6% 9% 19% 30% 23%	1680
July	1st to 10th	Night	20% 6% 9% 16% 15% 34%	3600
	11th to 20th	Night	19% 6% 12% 29% 20% 13%	3592
	21st to end	Night	21% 7% 13% 22% 23% 16%	4250
August	1st to 10th	Night	24% 9% 17% 21% 12% 16%	2928
	11th to 20th	Night	26% 9% 16% 15% 21% 13%	3046
	21st to end	Night	20% 9% 15% 17% 24% 16%	3597
September	1st to 10th	Night	19% 6% 11% 17% 27% 20%	2000
	11th to 20th	Night	22% 5% 11% 22% 24% 15%	2000
	21st to end	Night	21% 4 9% 24% 32% 10%	2265
October	1st to 10th	Night	17% 5% 7% 18% 37% 17%	3920
	11th to 20th	Night	24% 6% 11% 32% 24% 3	3898
	21st to end	Night	30% 4 10% 33% 21% 2	4252
November	1st to 10th	Night	31% 6% 13% 25% 22% 3	2618
	11th to 20th	Night	37% 5% 14% 27% 15% 2	2793
	21st to end	Night	38% 6% 13% 27% 14% 2	2940
December	1st to 10th	Night	38% 4 11% 35% 10% 1	4710
	11th to 20th	Night	42% 6% 16% 27% 9%	5322
	21st to end	Night	52% 8% 10% 19% 11% 1	7128

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	18% 14% 28% 28% 10% 2	330
	11th to 20th	Day	15% 15% 35% 26% 9%	330
	21st to end	Day	10% 15% 41% 26% 6% 1	363
February	1st to 10th	Day	18% 21% 38% 18% 5% 1	330
	11th to 20th	Day	17% 20% 35% 23% 5%	330
	21st to end	Day	18% 22% 36% 21% 3	273
March	1st to 10th	Day	17% 25% 29% 22% 7% 1	435
	11th to 20th	Day	17% 24% 30% 23% 5% 1	540
	21st to end	Day	19% 29% 33% 16% 4	594
April	1st to 10th	Day	19% 25% 30% 18% 7% 1	540
	11th to 20th	Day	21% 29% 30% 15% 5% 1	532
	21st to end	Day	15% 27% 31% 19% 7% 1	540
May	1st to 10th	Day	17% 23% 30% 20% 9% 1	564
	11th to 20th	Day	14% 12% 30% 29% 14% 1	775
	21st to end	Day	11% 13% 29% 31% 13% 2	844
June	1st to 10th	Day	6% 11% 29% 31% 19% 4	780
	11th to 20th	Day	7% 15% 34% 29% 13% 2	780
	21st to end	Day	5% 10% 32% 32% 19% 2	780
July	1st to 10th	Day	5% 6% 23% 36% 27% 4	722
	11th to 20th	Day	3 5% 23% 39% 25% 5%	718
	21st to end	Day	4 5% 24% 40% 22% 5%	615
August	1st to 10th	Day	6% 6% 34% 33% 17% 4	527
	11th to 20th	Day	3 5% 31% 36% 20% 5%	517
	21st to end	Day	6% 6% 28% 37% 19% 4	583
September	1st to 10th	Day	6% 8% 32% 30% 19% 5%	530
	11th to 20th	Day	6% 11% 35% 29% 17% 3	530
	21st to end	Day	10% 12% 27% 26% 17% 7%	395
October	1st to 10th	Day	13% 12% 22% 30% 17% 6%	330
	11th to 20th	Day	12% 18% 29% 27% 12% 2	330
	21st to end	Day	16% 18% 33% 25% 7% 1	363
November	1st to 10th	Day	14% 22% 29% 25% 8% 2	330
	11th to 20th	Day	13% 16% 30% 31% 8% 1	330
	21st to end	Day	15% 13% 34% 29% 8% 1	330
December	1st to 10th	Day	17% 17% 31% 22% 11% 2	330
	11th to 20th	Day	16% 11% 39% 27% 7%	330
	21st to end	Day	21% 15% 34% 23% 6% 1	363
January	1st to 10th	Night	16% 12% 28% 33% 10% 1	1050
	11th to 20th	Night	15% 12% 34% 31% 8% 1	913
	21st to end	Night	13% 10% 35% 31% 10% 1	924
February	1st to 10th	Night	16% 10% 38% 29% 7%	840
	11th to 20th	Night	12% 13% 36% 32% 6%	824
	21st to end	Night	13% 13% 37% 34% 3	692
March	1st to 10th	Night	12% 11% 35% 34% 8% 1	840
	11th to 20th	Night	10% 8% 28% 41% 12%	840
	21st to end	Night	13% 9% 37% 32% 9%	858
April	1st to 10th	Night	14% 14% 29% 36% 6%	630
	11th to 20th	Night	16% 10% 32% 33% 9% 1	623
	21st to end	Night	14% 10% 29% 34% 12% 1	630
May	1st to 10th	Night	13% 7% 30% 38% 11% 1	630
	11th to 20th	Night	11% 4 25% 38% 21% 1	621
	21st to end	Night	9% 7% 24% 39% 18% 2	683
June	1st to 10th	Night	6% 5% 21% 42% 25% 1	630
	11th to 20th	Night	5% 6% 27% 45% 16% 1	630
	21st to end	Night	3 4 20% 47% 24% 1	630
July	1st to 10th	Night	3 2 13% 45% 34% 3	585
	11th to 20th	Night	2 2 10% 49% 35% 3	582
	21st to end	Night	3 1 11% 42% 42% 2	657
August	1st to 10th	Night	4 2 14% 41% 38% 1	597
	11th to 20th	Night	4 1 12% 48% 34% 1	594
	21st to end	Night	4 1 11% 45% 35% 4	660
September	1st to 10th	Night	5% 2 12% 48% 30% 3	600
	11th to 20th	Night	5% 4 17% 42% 30% 3	600
	21st to end	Night	8% 4 19% 37% 27% 6%	731
October	1st to 10th	Night	10% 5% 25% 40% 18% 3	800
	11th to 20th	Night	10% 8% 27% 38% 15% 2	800
	21st to end	Night	15% 5% 30% 35% 13% 2	880
November	1st to 10th	Night	12% 10% 31% 32% 13% 1	978
	11th to 20th	Night	15% 10% 31% 35% 8% 1	1050
	21st to end	Night	12% 9% 30% 37% 11%	1050
December	1st to 10th	Night	16% 11% 33% 31% 8%	1050
	11th to 20th	Night	14% 12% 33% 34% 7%	1050
	21st to end	Night	18% 13% 34% 29% 6% 1	1155

Page generated at 2026-06-28 00:10:46. Version ac07

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