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 2024-09-29.

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 7% 4 4 4 4 6% 7% 45%	101068
	Night	5% 12% 4 8% 4 3 4 3 5% 6% 46%	97388
Transparency	Day	33% 9% 13% 20% 17% 8%	100820
	Night	35% 9% 13% 21% 16% 6%	97226
Seeing	Day	11% 15% 30% 28% 14% 2	15467
	Night	11% 8% 25% 38% 17% 1	25480

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	3 15% 8% 19% 6% 4 3 3 4 5% 30%	2538
	11th to 20th	Day	4 24% 6% 17% 7% 3 3 3 4 4 26%	2770
	21st to end	Day	3 20% 9% 12% 4 4 3 3 4 4 35%	3146
February	1st to 10th	Day	6% 22% 8% 13% 5% 3 3 3 4 5% 29%	1710
	11th to 20th	Day	6% 19% 8% 12% 4 3 4 4 3 6% 33%	1707
	21st to end	Day	4 20% 6% 15% 5% 3 3 3 5% 6% 30%	1570
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%	2004
October	1st to 10th	Day	9% 8% 2 5% 4 4 3 3 4 4 52%	1800
	11th to 20th	Day	8% 9% 2 4 3 2 3 3 5% 7% 56%	1800
	21st to end	Day	9% 10% 3 6% 2 3 4 3 5% 5% 50%	1980
November	1st to 10th	Day	6% 11% 3 5% 3 3 3 3 4 7% 52%	1696
	11th to 20th	Day	8% 16% 4 8% 3 2 3 3 4 5% 45%	1650
	21st to end	Day	5% 12% 6% 10% 3 2 3 2 3 5% 50%	1520
December	1st to 10th	Day	7% 17% 7% 11% 6% 4 3 2 4 5% 35%	1520
	11th to 20th	Day	6% 19% 6% 9% 4 2 3 3 4 6% 38%	1926
	21st to end	Day	4 19% 6% 12% 6% 4 3 3 4 6% 32%	2794
January	1st to 10th	Night	4 17% 8% 18% 7% 3 3 2 4 4 31%	4417
	11th to 20th	Night	4 25% 8% 12% 7% 2 3 2 3 4 30%	4229
	21st to end	Night	4 24% 8% 13% 4 3 3 2 3 4 32%	4521
February	1st to 10th	Night	7% 19% 8% 13% 6% 3 4 3 4 5% 29%	2470
	11th to 20th	Night	5% 19% 8% 10% 5% 3 3 3 5% 6% 32%	2358
	21st to end	Night	4 18% 8% 13% 4 4 3 3 5% 6% 32%	1884
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% 67%	2139
October	1st to 10th	Night	8% 6% 1 4 4 3 3 3 4 6% 57%	2160
	11th to 20th	Night	7% 8% 2 4 2 2 2 2 4 5% 61%	2160
	21st to end	Night	10% 11% 3 5% 2 2 2 3 4 4 54%	2376
November	1st to 10th	Night	7% 11% 3 5% 2 3 3 3 5% 6% 53%	2362
	11th to 20th	Night	8% 15% 4 6% 2 3 3 2 5% 6% 46%	2527
	21st to end	Night	5% 13% 5% 7% 4 3 2 3 4 6% 47%	2660
December	1st to 10th	Night	7% 18% 5% 10% 5% 4 2 2 4 5% 38%	2660
	11th to 20th	Night	5% 17% 6% 9% 4 3 3 2 4 7% 38%	3271
	21st to end	Night	4 16% 6% 13% 5% 4 4 3 5% 6% 36%	4873

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	50% 10% 13% 17% 9% 1	2539
	11th to 20th	Day	56% 11% 14% 11% 6% 1	2770
	21st to end	Day	49% 10% 16% 16% 8% 1	3146
February	1st to 10th	Day	55% 7% 14% 14% 8% 1	1710
	11th to 20th	Day	51% 8% 12% 19% 8% 2	1716
	21st to end	Day	51% 8% 14% 13% 12% 1	1570
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	24% 4 8% 28% 30% 7%	2004
October	1st to 10th	Day	34% 7% 12% 22% 18% 6%	1800
	11th to 20th	Day	28% 5% 11% 30% 21% 4	1800
	21st to end	Day	34% 6% 14% 28% 14% 3	1980
November	1st to 10th	Day	33% 5% 13% 24% 23% 1	1696
	11th to 20th	Day	40% 6% 15% 23% 14% 2	1650
	21st to end	Day	38% 4 12% 29% 15% 2	1520
December	1st to 10th	Day	51% 7% 17% 16% 8% 2	1520
	11th to 20th	Day	44% 7% 14% 26% 8%	1924
	21st to end	Day	49% 9% 10% 17% 14% 1	2794
January	1st to 10th	Night	52% 10% 12% 19% 7% 1	4417
	11th to 20th	Night	55% 9% 13% 13% 8% 1	4229
	21st to end	Night	50% 12% 14% 15% 8% 1	4521
February	1st to 10th	Night	56% 7% 12% 16% 9% 1	2470
	11th to 20th	Night	49% 8% 12% 19% 9% 2	2359
	21st to end	Night	47% 9% 12% 16% 13% 2	1884
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% 34% 9%	2139
October	1st to 10th	Night	27% 8% 11% 25% 20% 9%	2160
	11th to 20th	Night	25% 6% 8% 32% 24% 5%	2160
	21st to end	Night	33% 5% 14% 26% 18% 4	2376
November	1st to 10th	Night	30% 7% 14% 24% 22% 3	2362
	11th to 20th	Night	37% 5% 14% 26% 15% 2	2527
	21st to end	Night	37% 7% 12% 26% 15% 2	2660
December	1st to 10th	Night	48% 7% 14% 18% 12% 2	2660
	11th to 20th	Night	44% 7% 13% 25% 11%	3272
	21st to end	Night	46% 10% 12% 16% 15% 1	4873

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% 24% 29% 12% 2	270
	11th to 20th	Day	17% 16% 33% 25% 8%	270
	21st to end	Day	11% 16% 38% 26% 7% 1	297
February	1st to 10th	Day	18% 24% 36% 19% 4	270
	11th to 20th	Day	16% 21% 33% 24% 5%	270
	21st to end	Day	17% 25% 37% 18% 2	225
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	11% 12% 27% 25% 18% 7%	356
October	1st to 10th	Day	14% 12% 21% 27% 17% 7%	270
	11th to 20th	Day	12% 17% 27% 29% 13% 2	270
	21st to end	Day	17% 19% 32% 24% 7% 1	297
November	1st to 10th	Day	16% 23% 27% 23% 10% 3	270
	11th to 20th	Day	14% 18% 28% 29% 10% 1	270
	21st to end	Day	14% 13% 32% 31% 9% 1	270
December	1st to 10th	Day	16% 16% 33% 23% 9% 1	270
	11th to 20th	Day	16% 12% 36% 27% 8%	270
	21st to end	Day	20% 16% 34% 23% 6% 1	297
January	1st to 10th	Night	16% 12% 25% 35% 11% 1	950
	11th to 20th	Night	16% 12% 32% 30% 8% 1	827
	21st to end	Night	14% 11% 32% 32% 11% 1	836
February	1st to 10th	Night	16% 11% 36% 31% 7%	760
	11th to 20th	Night	12% 12% 34% 34% 7%	744
	21st to end	Night	12% 14% 35% 36% 3	628
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% 36% 28% 6%	690
October	1st to 10th	Night	10% 5% 23% 38% 20% 3	720
	11th to 20th	Night	9% 8% 25% 39% 16% 2	720
	21st to end	Night	15% 6% 26% 36% 15% 2	792
November	1st to 10th	Night	12% 10% 28% 34% 14% 1	882
	11th to 20th	Night	15% 11% 27% 37% 9% 1	950
	21st to end	Night	11% 9% 28% 38% 12%	950
December	1st to 10th	Night	16% 11% 33% 32% 8% 1	950
	11th to 20th	Night	14% 13% 29% 36% 7%	950
	21st to end	Night	17% 12% 32% 31% 7% 1	1045

Page generated at 2024-09-26 07:52:25. Version ac07

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