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 2024-11-05.

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% 5% 4 5% 4 6% 8% 40%	100875
	Night	14% 7% 2 6% 4 4 4 4 5% 7% 43%	98134
Transparency	Day	41% 5% 11% 22% 18% 4	104666
	Night	39% 4 10% 22% 19% 6%	102481
Seeing	Day	19% 10% 22% 23% 18% 8%	14589
	Night	17% 8% 19% 30% 22% 4	24578

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	20% 11% 4 6% 4 4 3 3 6% 6% 34%	2836
	11th to 20th	Day	22% 7% 4 9% 4 3 3 2 3 4 39%	2840
	21st to end	Day	26% 9% 3 5% 3 3 3 2 4 5% 38%	3426
February	1st to 10th	Day	20% 11% 3 6% 3 3 3 3 4 7% 38%	1900
	11th to 20th	Day	23% 8% 2 7% 5% 3 4 2 4 5% 37%	1848
	21st to end	Day	24% 8% 2 7% 4 4 4 2 4 5% 36%	1570
March	1st to 10th	Day	26% 10% 3 6% 4 4 3 3 5% 5% 30%	3120
	11th to 20th	Day	16% 7% 3 6% 3 3 3 3 4 6% 45%	3384
	21st to end	Day	15% 7% 3 8% 5% 4 5% 3 5% 7% 37%	3927
April	1st to 10th	Day	14% 7% 2 5% 4 4 5% 4 6% 6% 43%	2280
	11th to 20th	Day	15% 7% 2 6% 4 3 4 3 5% 7% 44%	2254
	21st to end	Day	11% 6% 2 7% 4 4 5% 4 7% 7% 43%	2323
May	1st to 10th	Day	14% 7% 2 6% 5% 5% 5% 4 6% 8% 38%	4082
	11th to 20th	Day	17% 8% 3 6% 5% 4 4 3 7% 8% 34%	4081
	21st to end	Day	12% 6% 2 6% 4 4 5% 4 6% 9% 41%	4476
June	1st to 10th	Day	11% 6% 2 7% 5% 6% 6% 5% 8% 10% 34%	2470
	11th to 20th	Day	5% 4 2 6% 5% 5% 5% 7% 8% 11% 43%	2470
	21st to end	Day	7% 5% 2 6% 6% 5% 6% 5% 7% 10% 40%	2470
July	1st to 10th	Day	8% 6% 2 7% 6% 6% 7% 6% 10% 11% 30%	4120
	11th to 20th	Day	9% 5% 2 6% 6% 6% 7% 6% 8% 12% 32%	4120
	21st to end	Day	17% 10% 4 9% 7% 6% 6% 5% 7% 9% 21%	4466
August	1st to 10th	Day	5% 3 2 5% 4 4 6% 5% 8% 11% 48%	3895
	11th to 20th	Day	6% 4 2 6% 5% 6% 7% 6% 8% 10% 41%	3729
	21st to end	Day	8% 3 2 5% 5% 5% 5% 4 6% 8% 52%	4180
September	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 6% 8% 9% 38%	2209
	11th to 20th	Day	13% 5% 2 7% 5% 5% 5% 4 6% 7% 41%	2090
	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	12% 6% 2 4 3 3 4 3 5% 5% 54%	1708
	11th to 20th	Day	13% 6% 2 5% 4 4 3 3 5% 6% 49%	1710
	21st to end	Day	18% 6% 2 6% 3 3 4 3 4 6% 44%	1710
December	1st to 10th	Day	24% 8% 2 6% 5% 4 4 4 5% 5% 35%	1710
	11th to 20th	Day	17% 8% 2 8% 4 5% 4 3 5% 5% 40%	2153
	21st to end	Day	18% 9% 5% 8% 5% 5% 4 3 6% 7% 31%	3124
January	1st to 10th	Night	20% 11% 2 7% 4 3 3 3 4 5% 38%	4109
	11th to 20th	Night	21% 8% 4 7% 4 3 3 3 3 5% 39%	4120
	21st to end	Night	23% 13% 3 5% 3 3 3 3 4 4 36%	4212
February	1st to 10th	Night	18% 9% 3 6% 4 3 3 2 4 6% 40%	2280
	11th to 20th	Night	22% 8% 2 6% 4 4 3 3 4 4 38%	2218
	21st to end	Night	25% 7% 3 6% 3 4 4 4 4 5% 34%	1884
March	1st to 10th	Night	28% 9% 3 6% 4 4 4 3 5% 5% 29%	3733
	11th to 20th	Night	12% 7% 2 6% 4 4 4 4 5% 7% 47%	3576
	21st to end	Night	17% 7% 3 8% 5% 4 4 3 5% 5% 39%	3732
April	1st to 10th	Night	12% 6% 2 5% 4 3 4 4 6% 8% 46%	1900
	11th to 20th	Night	15% 6% 2 4 3 2 4 3 4 7% 48%	1875
	21st to end	Night	11% 5% 2 5% 4 5% 4 4 5% 6% 50%	1856
May	1st to 10th	Night	13% 6% 2 6% 5% 4 5% 4 6% 7% 41%	2813
	11th to 20th	Night	16% 6% 3 6% 5% 4 4 4 6% 7% 38%	2812
	21st to end	Night	7% 5% 2 6% 4 5% 5% 4 8% 10% 44%	3079
June	1st to 10th	Night	9% 6% 2 6% 5% 4 6% 5% 7% 10% 40%	1710
	11th to 20th	Night	5% 2 1 3 4 4 4 5% 7% 12% 53%	1710
	21st to end	Night	6% 5% 2 5% 5% 4 5% 5% 7% 11% 46%	1710
July	1st to 10th	Night	7% 6% 2 7% 7% 6% 6% 5% 8% 10% 36%	2840
	11th to 20th	Night	9% 5% 2 5% 5% 4 6% 5% 7% 11% 41%	2840
	21st to end	Night	12% 10% 4 11% 6% 6% 6% 4 6% 9% 25%	3087
August	1st to 10th	Night	3 3 2 3 3 3 4 4 7% 11% 57%	3030
	11th to 20th	Night	6% 5% 2 5% 4 4 4 3 6% 11% 50%	3100
	21st to end	Night	6% 3 2 5% 3 3 4 4 5% 8% 58%	3476
September	1st to 10th	Night	8% 6% 2 6% 4 4 5% 4 6% 8% 48%	1950
	11th to 20th	Night	12% 5% 3 6% 4 4 4 3 7% 8% 45%	2090
	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	13% 5% 2 4 3 3 4 3 5% 6% 52%	2470
	11th to 20th	Night	12% 6% 2 6% 3 3 3 3 4 6% 51%	2470
	21st to end	Night	18% 7% 2 6% 4 3 3 4 5% 5% 44%	2470
December	1st to 10th	Night	20% 8% 2 6% 4 4 4 3 5% 6% 37%	2470
	11th to 20th	Night	18% 7% 2 6% 3 3 3 3 4 6% 45%	3042
	21st to end	Night	20% 8% 4 8% 5% 4 5% 4 5% 7% 29%	4532

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% 2 5% 19% 21% 2	2837
	11th to 20th	Day	51% 2 5% 17% 22% 2	2840
	21st to end	Day	50% 2 7% 22% 17% 2	3426
February	1st to 10th	Day	48% 2 6% 21% 19% 3	1900
	11th to 20th	Day	52% 2 6% 19% 16% 5%	1858
	21st to end	Day	52% 1 7% 15% 19% 6%	1570
March	1st to 10th	Day	56% 3 7% 12% 14% 9%	3120
	11th to 20th	Day	41% 2 7% 23% 20% 7%	3384
	21st to end	Day	46% 3 11% 22% 14% 3	3927
April	1st to 10th	Day	40% 4 10% 23% 17% 5%	2280
	11th to 20th	Day	40% 3 7% 21% 24% 6%	2254
	21st to end	Day	38% 3 10% 30% 16% 3	2323
May	1st to 10th	Day	44% 4 12% 24% 14% 2	4120
	11th to 20th	Day	46% 4 13% 25% 11%	4085
	21st to end	Day	39% 4 11% 25% 19% 1	4489
June	1st to 10th	Day	43% 6% 14% 21% 15% 1	2470
	11th to 20th	Day	31% 6% 19% 26% 16% 2	2470
	21st to end	Day	36% 7% 19% 21% 14% 3	2470
July	1st to 10th	Day	40% 11% 21% 21% 6% 1	4055
	11th to 20th	Day	39% 10% 23% 21% 5% 2	4051
	21st to end	Day	58% 7% 14% 16% 5%	4324
August	1st to 10th	Day	26% 7% 16% 35% 16% 1	3882
	11th to 20th	Day	33% 9% 16% 26% 14% 2	3765
	21st to end	Day	30% 7% 13% 27% 22% 2	4179
September	1st to 10th	Day	39% 7% 15% 18% 17% 5%	2209
	11th to 20th	Day	41% 5% 12% 19% 20% 3	2090
	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	33% 3 7% 19% 29% 8%	1708
	11th to 20th	Day	37% 2 5% 23% 25% 8%	1710
	21st to end	Day	42% 2 6% 21% 24% 5%	1710
December	1st to 10th	Day	52% 2 6% 19% 18% 4	1710
	11th to 20th	Day	47% 2 7% 20% 20% 4	2150
	21st to end	Day	53% 2 8% 23% 12% 3	3124
January	1st to 10th	Night	49% 2 5% 20% 22% 2	4109
	11th to 20th	Night	49% 2 6% 20% 17% 6%	4120
	21st to end	Night	53% 2 5% 18% 19% 4	4212
February	1st to 10th	Night	46% 2 8% 20% 20% 4	2280
	11th to 20th	Night	50% 2 6% 18% 16% 7%	2219
	21st to end	Night	51% 3 6% 17% 17% 6%	1884
March	1st to 10th	Night	56% 4 7% 13% 14% 6%	3733
	11th to 20th	Night	37% 3 9% 22% 20% 10%	3576
	21st to end	Night	48% 3 8% 21% 18% 2	3732
April	1st to 10th	Night	35% 3 13% 24% 21% 4	1900
	11th to 20th	Night	37% 3 6% 21% 23% 10%	1875
	21st to end	Night	35% 4 10% 30% 18% 4	1856
May	1st to 10th	Night	41% 4 12% 24% 17% 2	2840
	11th to 20th	Night	44% 5% 13% 24% 13%	2813
	21st to end	Night	34% 4 15% 27% 18% 2	3087
June	1st to 10th	Night	37% 7% 15% 22% 17% 2	1710
	11th to 20th	Night	23% 5% 20% 32% 18% 2	1710
	21st to end	Night	31% 8% 18% 23% 16% 5%	1710
July	1st to 10th	Night	39% 9% 22% 21% 8% 2	2795
	11th to 20th	Night	33% 9% 24% 24% 7% 2	2786
	21st to end	Night	55% 8% 14% 16% 6% 1	2988
August	1st to 10th	Night	20% 6% 15% 39% 18% 1	3019
	11th to 20th	Night	27% 6% 22% 28% 16% 2	3130
	21st to end	Night	25% 6% 15% 27% 26% 2	3476
September	1st to 10th	Night	33% 7% 17% 21% 17% 5%	1950
	11th to 20th	Night	36% 6% 14% 19% 22% 3	2090
	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	33% 3 7% 22% 24% 10%	2470
	11th to 20th	Night	35% 3 5% 20% 28% 10%	2470
	21st to end	Night	43% 2 5% 21% 24% 6%	2470
December	1st to 10th	Night	49% 3 8% 20% 17% 3	2470
	11th to 20th	Night	41% 3 6% 19% 26% 5%	3044
	21st to end	Night	52% 3 7% 22% 13% 2	4532

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	34% 20% 29% 14% 3	270
	11th to 20th	Day	33% 22% 27% 12% 4 1	270
	21st to end	Day	27% 23% 34% 12% 3	297
February	1st to 10th	Day	31% 21% 30% 15% 3	270
	11th to 20th	Day	40% 22% 28% 9% 1	270
	21st to end	Day	42% 23% 27% 8% 1	225
March	1st to 10th	Day	32% 18% 32% 12% 4 1	270
	11th to 20th	Day	30% 17% 29% 17% 6% 1	403
	21st to end	Day	27% 16% 33% 16% 7% 1	506
April	1st to 10th	Day	26% 13% 25% 22% 11% 2	460
	11th to 20th	Day	23% 15% 27% 23% 11% 1	452
	21st to end	Day	20% 11% 32% 22% 13% 2	460
May	1st to 10th	Day	18% 11% 30% 23% 12% 5%	463
	11th to 20th	Day	13% 8% 27% 27% 16% 10%	487
	21st to end	Day	16% 5% 19% 27% 19% 14%	529
June	1st to 10th	Day	16% 6% 19% 26% 22% 12%	490
	11th to 20th	Day	5% 4 15% 29% 27% 20%	490
	21st to end	Day	10% 4 17% 26% 27% 16%	490
July	1st to 10th	Day	10% 3 13% 24% 33% 18%	485
	11th to 20th	Day	10% 3 11% 24% 31% 21%	484
	21st to end	Day	11% 5% 8% 24% 33% 20%	532
August	1st to 10th	Day	11% 3 10% 27% 31% 18%	486
	11th to 20th	Day	12% 2 11% 29% 33% 12%	479
	21st to end	Day	15% 3 8% 24% 32% 18%	539
September	1st to 10th	Day	11% 2 14% 28% 28% 16%	490
	11th to 20th	Day	16% 5% 14% 21% 29% 16%	490
	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	21% 11% 27% 25% 14% 2	282
	11th to 20th	Day	14% 18% 38% 24% 6%	270
	21st to end	Day	21% 16% 31% 20% 11% 2	270
December	1st to 10th	Day	34% 15% 26% 18% 6% 1	270
	11th to 20th	Day	26% 20% 32% 14% 7% 1	270
	21st to end	Day	35% 14% 32% 13% 5%	297
January	1st to 10th	Night	28% 15% 27% 23% 6%	760
	11th to 20th	Night	26% 17% 25% 25% 7%	760
	21st to end	Night	23% 14% 31% 29% 3	836
February	1st to 10th	Night	25% 11% 26% 33% 5%	760
	11th to 20th	Night	28% 17% 26% 27% 3	743
	21st to end	Night	32% 13% 28% 24% 3	628
March	1st to 10th	Night	24% 15% 24% 32% 6%	760
	11th to 20th	Night	21% 12% 29% 29% 8% 1	760
	21st to end	Night	20% 12% 29% 28% 11%	789
April	1st to 10th	Night	16% 11% 26% 33% 13%	570
	11th to 20th	Night	20% 9% 25% 30% 15% 2	563
	21st to end	Night	14% 7% 25% 37% 16% 1	570
May	1st to 10th	Night	12% 7% 23% 34% 22% 2	570
	11th to 20th	Night	13% 5% 18% 33% 26% 5%	561
	21st to end	Night	9% 3 9% 36% 36% 6%	617
June	1st to 10th	Night	13% 1 11% 31% 36% 7%	570
	11th to 20th	Night	5% 2 7% 33% 41% 12%	570
	21st to end	Night	8% 2 7% 31% 44% 9%	570
July	1st to 10th	Night	9% 3 5% 25% 41% 16%	555
	11th to 20th	Night	9% 2 4 23% 42% 18%	552
	21st to end	Night	9% 4 5% 23% 47% 13%	624
August	1st to 10th	Night	7% 1 2 27% 48% 15%	567
	11th to 20th	Night	11% 3 6% 25% 46% 9%	564
	21st to end	Night	11% 2 5% 26% 45% 11%	627
September	1st to 10th	Night	10% 2 10% 26% 44% 9%	620
	11th to 20th	Night	13% 4 11% 29% 38% 6%	760
	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% 32% 22% 3	760
	11th to 20th	Night	15% 11% 26% 31% 15% 1	760
	21st to end	Night	23% 9% 22% 33% 13% 1	760
December	1st to 10th	Night	26% 12% 24% 29% 9%	760
	11th to 20th	Night	24% 12% 21% 34% 9%	760
	21st to end	Night	28% 12% 24% 29% 7%	836

Page generated at 2024-11-03 01:11:19. Version ac07

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