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 2025-02-25.

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	5% 10% 4 8% 4 4 4 4 5% 7% 46%	112832
	Night	5% 12% 4 8% 4 3 3 3 5% 6% 47%	114704
Transparency	Day	33% 8% 13% 21% 17% 7%	112584
	Night	35% 8% 13% 22% 16% 6%	114542
Seeing	Day	11% 14% 30% 28% 14% 2	15914
	Night	11% 8% 26% 38% 17% 1	26146

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	2 16% 9% 17% 6% 4 3 3 4 5% 32%	3638
	11th to 20th	Day	3 19% 6% 19% 10% 4 3 2 3 4 27%	3982
	21st to end	Day	2 16% 8% 11% 6% 4 3 3 3 3 41%	4510
February	1st to 10th	Day	6% 21% 8% 13% 5% 3 3 3 4 5% 29%	1800
	11th to 20th	Day	6% 20% 8% 12% 4 3 4 3 3 5% 31%	1800
	21st to end	Day	4 20% 6% 15% 5% 3 3 3 5% 6% 31%	1610
March	1st to 10th	Day	3 14% 8% 12% 5% 4 4 4 5% 6% 34%	3290
	11th to 20th	Day	3 13% 5% 7% 4 4 4 3 4 6% 49%	3490
	21st to end	Day	4 20% 7% 12% 6% 5% 5% 4 5% 5% 25%	3931
April	1st to 10th	Day	6% 15% 4 9% 5% 4 3 4 5% 7% 39%	2280
	11th to 20th	Day	8% 15% 4 9% 5% 4 4 4 6% 7% 34%	2251
	21st to end	Day	6% 12% 4 7% 5% 5% 5% 4 6% 7% 39%	2280
May	1st to 10th	Day	8% 17% 5% 8% 6% 5% 6% 5% 6% 7% 29%	4098
	11th to 20th	Day	6% 5% 2 6% 4 5% 6% 5% 8% 10% 44%	4388
	21st to end	Day	6% 7% 2 5% 4 4 5% 5% 7% 8% 48%	4825
June	1st to 10th	Day	4 4 1 4 4 4 5% 5% 7% 9% 54%	2660
	11th to 20th	Day	5% 3 1 4 4 4 3 4 5% 7% 61%	2660
	21st to end	Day	3 2 1 3 3 4 4 4 7% 8% 61%	2660
July	1st to 10th	Day	3 2 1 3 3 3 4 3 6% 7% 66%	4430
	11th to 20th	Day	2 2 1 4 3 4 6% 5% 9% 11% 54%	4430
	21st to end	Day	3 3 1 4 4 5% 5% 5% 7% 8% 55%	4518
August	1st to 10th	Day	4 4 2 5% 4 5% 6% 6% 9% 10% 46%	4092
	11th to 20th	Day	5% 3 2 5% 4 6% 5% 4 7% 8% 50%	3914
	21st to end	Day	4 3 2 4 4 4 5% 5% 7% 8% 56%	4180
September	1st to 10th	Day	5% 3 1 3 3 3 4 3 5% 7% 62%	2280
	11th to 20th	Day	6% 4 2 4 4 4 4 4 5% 6% 57%	2280
	21st to end	Day	8% 4 1 4 3 2 4 2 4 5% 63%	2024
October	1st to 10th	Day	5% 5% 1 3 2 2 2 2 3 3 71%	3170
	11th to 20th	Day	6% 7% 2 5% 3 3 4 4 6% 8% 52%	3150
	21st to end	Day	9% 12% 2 5% 2 2 3 3 5% 6% 51%	3438
November	1st to 10th	Day	5% 11% 3 6% 3 3 4 3 4 7% 51%	1787
	11th to 20th	Day	8% 15% 4 8% 3 2 3 2 4 5% 46%	1736
	21st to end	Day	5% 12% 6% 10% 3 2 2 2 3 5% 51%	1600
December	1st to 10th	Day	4 10% 5% 13% 4 2 2 1 2 3 53%	2620
	11th to 20th	Day	4 19% 5% 7% 3 2 2 2 4 6% 47%	3026
	21st to end	Day	6% 18% 6% 11% 6% 4 4 3 5% 7% 31%	4004
January	1st to 10th	Night	3 18% 8% 16% 6% 3 3 2 4 5% 31%	6327
	11th to 20th	Night	3 19% 7% 17% 9% 3 2 2 3 4 29%	6041
	21st to end	Night	3 18% 7% 14% 5% 3 3 2 3 3 41%	6468
February	1st to 10th	Night	7% 19% 8% 13% 6% 3 4 3 4 5% 28%	2600
	11th to 20th	Night	5% 20% 9% 11% 5% 4 3 3 5% 6% 31%	2485
	21st to end	Night	4 18% 8% 13% 4 4 3 3 5% 6% 33%	1938
March	1st to 10th	Night	3 16% 7% 10% 5% 4 4 3 5% 5% 39%	3564
	11th to 20th	Night	3 13% 3 6% 3 3 3 3 4 6% 54%	3490
	21st to end	Night	4 21% 7% 9% 4 5% 4 4 4 6% 31%	3717
April	1st to 10th	Night	8% 15% 3 6% 4 3 4 4 5% 6% 42%	1900
	11th to 20th	Night	8% 15% 4 6% 4 3 4 3 6% 6% 40%	1877
	21st to end	Night	9% 14% 3 5% 4 3 4 3 6% 6% 43%	1900
May	1st to 10th	Night	7% 14% 3 6% 5% 5% 4 4 6% 7% 39%	2855
	11th to 20th	Night	6% 6% 3 6% 5% 6% 4 5% 8% 9% 42%	2517
	21st to end	Night	7% 7% 2 4 4 4 3 4 4 7% 55%	2762
June	1st to 10th	Night	4 4 1 3 3 3 3 4 7% 8% 59%	1520
	11th to 20th	Night	4 4 1 5% 4 3 4 3 5% 7% 61%	1520
	21st to end	Night	3 2 1 3 2 3 4 3 6% 7% 67%	1520
July	1st to 10th	Night	3 2 1 3 3 3 4 4 5% 7% 66%	2540
	11th to 20th	Night	1 2 1 4 3 4 5% 4 9% 11% 54%	2540
	21st to end	Night	1 2 2 4 4 4 5% 4 7% 9% 60%	3040
August	1st to 10th	Night	3 3 1 4 4 5% 6% 5% 9% 9% 52%	2847
	11th to 20th	Night	3 3 1 5% 4 5% 7% 4 7% 9% 50%	2925
	21st to end	Night	3 2 1 4 3 4 4 5% 8% 9% 57%	3487
September	1st to 10th	Night	4 3 1 4 3 2 3 3 6% 7% 63%	1900
	11th to 20th	Night	4 4 2 4 3 4 3 2 5% 5% 63%	1900
	21st to end	Night	7% 4 1 4 2 2 2 3 4 5% 66%	2151
October	1st to 10th	Night	5% 4 1 2 2 2 2 2 3 4 72%	3800
	11th to 20th	Night	5% 6% 2 4 3 3 3 3 4 8% 58%	3778
	21st to end	Night	8% 10% 3 4 2 2 3 2 4 5% 57%	4120
November	1st to 10th	Night	7% 12% 3 5% 2 3 3 3 5% 6% 52%	2490
	11th to 20th	Night	7% 15% 4 6% 3 3 3 2 5% 6% 47%	2660
	21st to end	Night	5% 13% 5% 7% 4 3 2 3 4 6% 47%	2800
December	1st to 10th	Night	4 12% 4 10% 4 2 2 2 2 3 53%	4570
	11th to 20th	Night	4 17% 7% 9% 4 3 3 2 4 6% 41%	5181
	21st to end	Night	7% 22% 5% 11% 4 3 3 3 4 5% 33%	6974

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	51% 8% 14% 16% 11% 1	3639
	11th to 20th	Day	57% 9% 11% 12% 9% 1	3982
	21st to end	Day	45% 8% 12% 17% 13% 4	4510
February	1st to 10th	Day	55% 7% 14% 13% 9% 2	1800
	11th to 20th	Day	53% 7% 12% 18% 8% 2	1809
	21st to end	Day	51% 8% 14% 13% 12% 1	1610
March	1st to 10th	Day	40% 13% 15% 20% 11% 1	3290
	11th to 20th	Day	32% 11% 17% 32% 7% 1	3490
	21st to end	Day	50% 14% 11% 18% 5% 2	3931
April	1st to 10th	Day	41% 9% 14% 19% 13% 4	2280
	11th to 20th	Day	46% 9% 13% 18% 14% 1	2254
	21st to end	Day	40% 9% 14% 27% 11%	2280
May	1st to 10th	Day	45% 13% 12% 16% 13% 1	4098
	11th to 20th	Day	24% 13% 17% 29% 16% 1	4392
	21st to end	Day	25% 11% 13% 28% 21% 1	4825
June	1st to 10th	Day	22% 10% 15% 23% 23% 8%	2660
	11th to 20th	Day	21% 6% 11% 18% 27% 17%	2660
	21st to end	Day	18% 6% 11% 20% 27% 18%	2660
July	1st to 10th	Day	15% 5% 9% 14% 25% 31%	4361
	11th to 20th	Day	19% 7% 12% 19% 26% 17%	4355
	21st to end	Day	21% 9% 12% 16% 22% 22%	4375
August	1st to 10th	Day	27% 10% 14% 20% 15% 13%	4078
	11th to 20th	Day	28% 9% 13% 17% 23% 10%	3953
	21st to end	Day	23% 9% 12% 18% 22% 17%	4179
September	1st to 10th	Day	21% 6% 10% 18% 25% 21%	2280
	11th to 20th	Day	24% 6% 12% 20% 26% 11%	2280
	21st to end	Day	23% 4 8% 28% 31% 7%	2024
October	1st to 10th	Day	20% 4 7% 17% 35% 18%	3170
	11th to 20th	Day	28% 5% 10% 32% 23% 3	3150
	21st to end	Day	33% 5% 10% 35% 14% 2	3438
November	1st to 10th	Day	33% 5% 13% 24% 23% 1	1787
	11th to 20th	Day	39% 6% 14% 24% 15% 2	1736
	21st to end	Day	37% 4 12% 30% 15% 2	1600
December	1st to 10th	Day	38% 4 14% 34% 8% 1	2620
	11th to 20th	Day	39% 5% 20% 28% 8%	3024
	21st to end	Day	50% 8% 11% 19% 11% 1	4004
January	1st to 10th	Night	52% 8% 11% 17% 9% 3	6327
	11th to 20th	Night	56% 7% 12% 14% 10% 1	6041
	21st to end	Night	45% 9% 11% 16% 17% 1	6468
February	1st to 10th	Night	56% 7% 11% 15% 8% 1	2600
	11th to 20th	Night	51% 8% 12% 19% 9% 2	2486
	21st to end	Night	47% 9% 12% 17% 13% 2	1938
March	1st to 10th	Night	37% 11% 13% 24% 14%	3564
	11th to 20th	Night	29% 9% 17% 32% 12% 2	3490
	21st to end	Night	43% 15% 15% 21% 5% 1	3717
April	1st to 10th	Night	39% 9% 14% 18% 17% 4	1900
	11th to 20th	Night	40% 8% 14% 21% 15% 1	1877
	21st to end	Night	36% 9% 14% 24% 17% 1	1900
May	1st to 10th	Night	33% 14% 14% 22% 14% 2	2855
	11th to 20th	Night	26% 16% 19% 27% 11% 2	2518
	21st to end	Night	24% 9% 12% 29% 24% 2	2762
June	1st to 10th	Night	19% 8% 16% 28% 21% 8%	1520
	11th to 20th	Night	21% 7% 11% 17% 24% 20%	1520
	21st to end	Night	14% 7% 9% 20% 29% 21%	1520
July	1st to 10th	Night	16% 7% 11% 18% 16% 32%	2500
	11th to 20th	Night	19% 7% 12% 26% 18% 17%	2492
	21st to end	Night	18% 7% 14% 21% 21% 18%	2945
August	1st to 10th	Night	24% 9% 17% 22% 12% 16%	2838
	11th to 20th	Night	27% 9% 16% 15% 20% 13%	2952
	21st to end	Night	19% 8% 14% 17% 25% 16%	3487
September	1st to 10th	Night	19% 6% 11% 17% 26% 21%	1900
	11th to 20th	Night	22% 5% 11% 21% 25% 16%	1900
	21st to end	Night	20% 4 9% 24% 33% 9%	2151
October	1st to 10th	Night	17% 5% 7% 17% 37% 17%	3800
	11th to 20th	Night	23% 6% 11% 33% 24% 3	3778
	21st to end	Night	30% 4 10% 33% 20% 2	4120
November	1st to 10th	Night	31% 7% 13% 24% 22% 3	2490
	11th to 20th	Night	37% 5% 14% 26% 16% 2	2660
	21st to end	Night	37% 7% 12% 28% 15% 2	2800
December	1st to 10th	Night	37% 4 11% 36% 11% 1	4570
	11th to 20th	Night	42% 6% 16% 26% 9%	5182
	21st to end	Night	52% 8% 10% 19% 11% 1	6974

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% 26% 29% 11% 2	300
	11th to 20th	Day	16% 15% 34% 26% 9%	300
	21st to end	Day	11% 15% 40% 27% 7% 1	330
February	1st to 10th	Day	19% 22% 38% 17% 4	300
	11th to 20th	Day	17% 21% 34% 23% 4	300
	21st to end	Day	16% 25% 37% 19% 3	237
March	1st to 10th	Day	18% 27% 26% 22% 6% 1	365
	11th to 20th	Day	14% 24% 32% 24% 6% 1	460
	21st to end	Day	19% 30% 34% 15% 3	506
April	1st to 10th	Day	20% 26% 30% 17% 7% 1	460
	11th to 20th	Day	21% 29% 28% 16% 5% 1	452
	21st to end	Day	15% 26% 33% 18% 7% 1	460
May	1st to 10th	Day	19% 23% 30% 18% 9% 1	482
	11th to 20th	Day	14% 11% 28% 29% 16% 2	675
	21st to end	Day	12% 13% 29% 29% 14% 3	734
June	1st to 10th	Day	5% 11% 27% 31% 21% 4	680
	11th to 20th	Day	7% 14% 34% 29% 14% 2	680
	21st to end	Day	4 8% 31% 33% 21% 2	680
July	1st to 10th	Day	5% 5% 22% 36% 28% 4	671
	11th to 20th	Day	4 4 22% 39% 26% 5%	668
	21st to end	Day	5% 4 23% 40% 24% 5%	569
August	1st to 10th	Day	6% 5% 33% 34% 18% 5%	487
	11th to 20th	Day	3 5% 32% 36% 20% 4	477
	21st to end	Day	5% 6% 28% 37% 19% 4	539
September	1st to 10th	Day	6% 9% 31% 30% 19% 6%	490
	11th to 20th	Day	6% 11% 35% 29% 17% 3	490
	21st to end	Day	10% 12% 27% 26% 17% 7%	362
October	1st to 10th	Day	13% 12% 22% 30% 18% 7%	300
	11th to 20th	Day	11% 17% 28% 28% 13% 2	300
	21st to end	Day	17% 18% 33% 24% 7% 1	330
November	1st to 10th	Day	15% 22% 27% 24% 9% 2	300
	11th to 20th	Day	14% 17% 30% 29% 9% 1	300
	21st to end	Day	15% 12% 33% 31% 8% 1	300
December	1st to 10th	Day	15% 16% 32% 24% 12% 2	300
	11th to 20th	Day	16% 12% 38% 27% 7%	300
	21st to end	Day	21% 15% 33% 23% 6% 1	330
January	1st to 10th	Night	16% 12% 27% 35% 11% 1	1000
	11th to 20th	Night	15% 12% 32% 31% 8% 1	870
	21st to end	Night	13% 10% 34% 32% 10% 1	880
February	1st to 10th	Night	16% 11% 37% 30% 7%	800
	11th to 20th	Night	12% 13% 35% 33% 7%	784
	21st to end	Night	13% 14% 35% 35% 3	646
March	1st to 10th	Night	12% 11% 33% 35% 9% 1	760
	11th to 20th	Night	9% 8% 24% 45% 13%	760
	21st to end	Night	13% 10% 34% 33% 10%	776
April	1st to 10th	Night	14% 14% 28% 37% 6%	570
	11th to 20th	Night	17% 10% 29% 34% 9% 1	563
	21st to end	Night	14% 9% 26% 35% 14% 1	570
May	1st to 10th	Night	14% 7% 30% 38% 11% 1	570
	11th to 20th	Night	10% 4 23% 40% 22% 1	561
	21st to end	Night	8% 7% 22% 40% 20% 3	617
June	1st to 10th	Night	5% 5% 19% 43% 27% 1	570
	11th to 20th	Night	6% 6% 23% 46% 18% 1	570
	21st to end	Night	3 3 18% 48% 26% 2	570
July	1st to 10th	Night	3 2 11% 46% 36% 3	555
	11th to 20th	Night	2 2 9% 49% 36% 3	552
	21st to end	Night	3 1 9% 42% 44% 2	624
August	1st to 10th	Night	5% 2 10% 43% 40% 1	567
	11th to 20th	Night	4 1 12% 46% 35% 1	564
	21st to end	Night	4 1 10% 44% 37% 4	627
September	1st to 10th	Night	5% 2 12% 48% 30% 3	570
	11th to 20th	Night	5% 4 15% 41% 31% 3	570
	21st to end	Night	8% 3 19% 35% 28% 6%	694
October	1st to 10th	Night	10% 5% 23% 40% 19% 3	760
	11th to 20th	Night	9% 8% 25% 40% 16% 2	760
	21st to end	Night	15% 6% 28% 35% 14% 2	836
November	1st to 10th	Night	12% 10% 29% 34% 14% 1	930
	11th to 20th	Night	15% 10% 30% 36% 9% 1	1000
	21st to end	Night	12% 9% 30% 37% 12%	1000
December	1st to 10th	Night	15% 11% 33% 32% 8% 1	1000
	11th to 20th	Night	14% 13% 32% 34% 7%	1000
	21st to end	Night	18% 13% 32% 30% 6% 1	1100

Page generated at 2025-02-22 04:51:39. Version ac07

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