Marathon
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Marathon 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	15% 11% 4 8% 5% 5% 4 4 6% 8% 30%	138147
	Night	14% 13% 5% 10% 5% 5% 5% 4 6% 7% 26%	134503
Transparency	Day	52% 5% 12% 21% 9% 1	141557
	Night	56% 4 11% 20% 8% 1	139180
Seeing	Day	23% 14% 32% 21% 8% 2	17916
	Night	22% 11% 28% 26% 11% 1	27338

By Month

Percentage Cloud Cover NE (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	8% 19% 6% 14% 7% 6% 6% 4 7% 7% 16%	4426
	11th to 20th	Day	7% 15% 8% 14% 7% 7% 5% 5% 7% 8% 17%	4679
	21st to end	Day	7% 18% 7% 14% 6% 5% 6% 5% 7% 10% 13%	5156
February	1st to 10th	Day	8% 14% 5% 11% 6% 6% 7% 6% 9% 8% 21%	1680
	11th to 20th	Day	8% 15% 6% 13% 7% 5% 5% 4 6% 7% 23%	1927
	21st to end	Day	8% 15% 4 10% 7% 6% 5% 4 6% 6% 27%	1730
March	1st to 10th	Day	10% 13% 4 9% 6% 5% 5% 4 5% 7% 30%	4578
	11th to 20th	Day	10% 15% 5% 11% 7% 5% 5% 3 4 5% 29%	4844
	21st to end	Day	20% 15% 3 9% 4 4 3 3 5% 6% 28%	5974
April	1st to 10th	Day	15% 11% 3 8% 5% 5% 4 4 5% 7% 32%	2520
	11th to 20th	Day	18% 11% 4 8% 5% 4 4 4 6% 8% 28%	2625
	21st to end	Day	17% 10% 4 6% 5% 4 5% 4 5% 7% 35%	2730
May	1st to 10th	Day	13% 9% 3 8% 4 4 4 3 5% 7% 39%	8019
	11th to 20th	Day	17% 8% 3 7% 4 4 4 3 5% 6% 39%	8222
	21st to end	Day	14% 7% 3 5% 3 4 4 4 6% 9% 41%	9635
June	1st to 10th	Day	19% 8% 3 7% 5% 4 4 5% 5% 7% 33%	3150
	11th to 20th	Day	15% 9% 4 8% 5% 4 5% 4 5% 7% 34%	3150
	21st to end	Day	17% 10% 3 7% 5% 4 5% 4 7% 8% 31%	3150
July	1st to 10th	Day	17% 10% 3 7% 4 4 4 3 6% 9% 34%	6820
	11th to 20th	Day	14% 8% 3 7% 5% 4 4 4 6% 9% 35%	6414
	21st to end	Day	15% 8% 3 6% 4 4 4 4 7% 9% 36%	6926
August	1st to 10th	Day	14% 6% 2 7% 6% 6% 5% 5% 8% 11% 30%	4563
	11th to 20th	Day	14% 5% 2 5% 4 4 4 3 6% 9% 44%	4277
	21st to end	Day	20% 7% 2 5% 3 4 4 3 5% 8% 38%	4543
September	1st to 10th	Day	18% 6% 2 6% 4 4 4 4 6% 7% 38%	2400
	11th to 20th	Day	20% 7% 2 6% 4 4 4 4 6% 8% 35%	2237
	21st to end	Day	20% 8% 3 9% 5% 5% 5% 4 7% 7% 29%	2200
October	1st to 10th	Day	20% 10% 3 8% 5% 4 3 3 6% 7% 30%	2164
	11th to 20th	Day	22% 10% 3 7% 6% 5% 6% 4 7% 8% 20%	1810
	21st to end	Day	24% 17% 3 8% 4 4 3 3 5% 6% 22%	1980
November	1st to 10th	Day	19% 15% 4 7% 6% 5% 5% 4 6% 7% 22%	1852
	11th to 20th	Day	20% 18% 5% 12% 7% 5% 4 4 4 6% 17%	1702
	21st to end	Day	19% 20% 4 10% 7% 5% 4 5% 5% 6% 14%	1470
December	1st to 10th	Day	12% 17% 7% 13% 8% 6% 7% 5% 6% 8% 12%	2350
	11th to 20th	Day	14% 18% 8% 12% 6% 4 4 4 5% 8% 14%	2691
	21st to end	Day	22% 24% 8% 11% 4 3 3 2 4 5% 14%	3553
January	1st to 10th	Night	9% 16% 7% 12% 6% 5% 6% 5% 6% 9% 19%	8494
	11th to 20th	Night	7% 17% 8% 16% 7% 6% 5% 4 7% 9% 14%	8236
	21st to end	Night	7% 14% 8% 15% 8% 6% 5% 4 6% 9% 17%	9058
February	1st to 10th	Night	9% 14% 6% 10% 6% 6% 7% 6% 8% 8% 21%	2940
	11th to 20th	Night	9% 17% 5% 10% 7% 6% 6% 5% 7% 6% 23%	2573
	21st to end	Night	7% 16% 6% 12% 7% 5% 5% 4 6% 8% 27%	2076
March	1st to 10th	Night	11% 12% 4 10% 6% 5% 5% 4 5% 7% 30%	5482
	11th to 20th	Night	13% 14% 5% 10% 5% 5% 4 4 5% 6% 29%	5335
	21st to end	Night	16% 14% 4 11% 6% 5% 4 3 4 5% 26%	5242
April	1st to 10th	Night	16% 11% 3 9% 6% 5% 5% 4 5% 7% 29%	2100
	11th to 20th	Night	17% 12% 4 7% 4 3 4 4 6% 7% 33%	1943
	21st to end	Night	17% 10% 3 8% 5% 4 4 3 5% 6% 35%	1890
May	1st to 10th	Night	14% 8% 3 8% 5% 5% 4 3 4 6% 38%	4884
	11th to 20th	Night	17% 7% 3 6% 5% 4 5% 3 6% 6% 36%	4670
	21st to end	Night	14% 9% 2 5% 4 3 4 4 6% 7% 42%	4560
June	1st to 10th	Night	16% 8% 3 7% 5% 5% 4 4 5% 5% 36%	1470
	11th to 20th	Night	19% 9% 3 6% 5% 4 5% 4 5% 7% 34%	1470
	21st to end	Night	18% 10% 2 7% 5% 4 4 4 7% 7% 33%	1470
July	1st to 10th	Night	12% 10% 4 12% 6% 4 4 4 5% 7% 33%	3160
	11th to 20th	Night	15% 9% 3 8% 5% 4 5% 4 6% 8% 32%	3563
	21st to end	Night	17% 7% 3 6% 4 4 4 3 5% 8% 39%	3948
August	1st to 10th	Night	15% 8% 3 7% 5% 4 5% 3 6% 8% 36%	2597
	11th to 20th	Night	12% 5% 2 7% 5% 4 3 3 6% 7% 46%	2775
	21st to end	Night	20% 9% 3 5% 3 3 3 3 5% 6% 41%	3351
September	1st to 10th	Night	14% 8% 2 6% 5% 5% 5% 3 5% 7% 40%	2000
	11th to 20th	Night	18% 8% 2 6% 3 4 4 4 6% 6% 39%	2142
	21st to end	Night	23% 7% 2 7% 4 3 4 3 5% 8% 35%	2200
October	1st to 10th	Night	20% 11% 3 7% 5% 4 4 3 5% 6% 33%	2234
	11th to 20th	Night	22% 12% 3 9% 5% 4 5% 3 5% 6% 26%	2590
	21st to end	Night	22% 15% 4 9% 5% 5% 5% 4 5% 5% 21%	2860
November	1st to 10th	Night	22% 15% 4 8% 6% 5% 5% 4 5% 6% 22%	2666
	11th to 20th	Night	21% 17% 5% 11% 5% 5% 4 4 5% 5% 17%	2911
	21st to end	Night	18% 19% 6% 12% 6% 5% 4 4 6% 6% 15%	3150
December	1st to 10th	Night	10% 17% 7% 14% 6% 6% 7% 5% 6% 7% 14%	5060
	11th to 20th	Night	15% 17% 6% 14% 6% 4 4 4 6% 8% 16%	5736
	21st to end	Night	24% 22% 8% 9% 4 3 3 3 4 6% 13%	7667

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	64% 5% 11% 14% 4 3	4427
	11th to 20th	Day	63% 5% 7% 19% 6%	4710
	21st to end	Day	66% 4 7% 16% 7% 1	5156
February	1st to 10th	Day	55% 5% 12% 18% 9% 1	1680
	11th to 20th	Day	60% 6% 11% 17% 7%	1937
	21st to end	Day	57% 5% 11% 15% 10% 2	1730
March	1st to 10th	Day	52% 6% 9% 15% 14% 4	4580
	11th to 20th	Day	58% 4 9% 18% 9% 1	4844
	21st to end	Day	59% 3 9% 17% 10% 2	5974
April	1st to 10th	Day	51% 6% 11% 21% 10% 1	2520
	11th to 20th	Day	54% 5% 12% 20% 8% 1	2625
	21st to end	Day	50% 5% 10% 18% 14% 3	2730
May	1st to 10th	Day	46% 3 9% 25% 15% 2	8028
	11th to 20th	Day	46% 4 9% 23% 15% 3	8226
	21st to end	Day	41% 4 11% 26% 14% 4	9635
June	1st to 10th	Day	49% 7% 11% 19% 13% 1	3150
	11th to 20th	Day	49% 6% 10% 22% 11% 1	3150
	21st to end	Day	48% 8% 13% 24% 7%	3150
July	1st to 10th	Day	47% 3 12% 30% 7%	6747
	11th to 20th	Day	45% 4 19% 25% 7%	6337
	21st to end	Day	44% 4 17% 29% 6%	6728
August	1st to 10th	Day	46% 6% 16% 26% 5%	4548
	11th to 20th	Day	37% 5% 17% 34% 7%	4318
	21st to end	Day	46% 5% 13% 28% 8%	4542
September	1st to 10th	Day	45% 8% 15% 23% 9%	2400
	11th to 20th	Day	47% 8% 22% 21% 3	2237
	21st to end	Day	53% 8% 12% 18% 8%	2200
October	1st to 10th	Day	46% 4 16% 20% 13%	3528
	11th to 20th	Day	54% 6% 16% 20% 5%	2922
	21st to end	Day	65% 4 14% 14% 4	3180
November	1st to 10th	Day	61% 7% 13% 14% 5%	1852
	11th to 20th	Day	71% 6% 10% 10% 4	1702
	21st to end	Day	71% 7% 10% 9% 3	1470
December	1st to 10th	Day	69% 4 13% 12% 3	2350
	11th to 20th	Day	67% 3 10% 12% 8%	2691
	21st to end	Day	75% 4 6% 13% 3	3553
January	1st to 10th	Night	60% 5% 9% 17% 5% 3	8494
	11th to 20th	Night	66% 4 7% 19% 5%	8280
	21st to end	Night	64% 3 8% 19% 6% 1	9058
February	1st to 10th	Night	56% 5% 12% 18% 8%	2940
	11th to 20th	Night	59% 6% 10% 19% 6%	2574
	21st to end	Night	55% 5% 13% 18% 9% 1	2076
March	1st to 10th	Night	54% 4 12% 13% 13% 4	5493
	11th to 20th	Night	58% 4 11% 19% 7% 1	5336
	21st to end	Night	60% 4 8% 16% 11% 1	5242
April	1st to 10th	Night	55% 6% 8% 19% 12% 1	2100
	11th to 20th	Night	51% 4 10% 21% 12% 1	1943
	21st to end	Night	50% 4 10% 18% 14% 3	1890
May	1st to 10th	Night	48% 3 9% 26% 12% 3	4891
	11th to 20th	Night	47% 5% 10% 25% 12% 1	4671
	21st to end	Night	42% 4 9% 28% 16% 3	4561
June	1st to 10th	Night	48% 8% 10% 21% 12% 1	1470
	11th to 20th	Night	49% 5% 10% 22% 12% 1	1470
	21st to end	Night	48% 7% 14% 21% 9%	1470
July	1st to 10th	Night	52% 3 12% 24% 9%	3125
	11th to 20th	Night	49% 4 18% 23% 6% 1	3517
	21st to end	Night	45% 4 13% 27% 10%	3832
August	1st to 10th	Night	46% 5% 14% 28% 7%	2588
	11th to 20th	Night	38% 5% 17% 32% 9%	2800
	21st to end	Night	46% 4 11% 29% 10%	3351
September	1st to 10th	Night	44% 7% 15% 26% 8% 1	2000
	11th to 20th	Night	44% 8% 18% 27% 4	2142
	21st to end	Night	48% 7% 14% 24% 7%	2200
October	1st to 10th	Night	43% 3 16% 26% 12%	3650
	11th to 20th	Night	50% 4 17% 24% 5%	4217
	21st to end	Night	64% 4 16% 14% 2	4610
November	1st to 10th	Night	63% 6% 11% 14% 5%	2666
	11th to 20th	Night	68% 5% 10% 12% 4	2911
	21st to end	Night	69% 6% 12% 9% 4	3150
December	1st to 10th	Night	67% 4 14% 13% 2	5060
	11th to 20th	Night	65% 4 10% 16% 6% 1	5735
	21st to end	Night	73% 4 6% 13% 4	7667

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	20% 14% 36% 23% 6% 1	279
	11th to 20th	Day	18% 14% 34% 27% 6% 1	330
	21st to end	Day	18% 15% 39% 19% 7% 1	363
February	1st to 10th	Day	20% 17% 40% 18% 5%	330
	11th to 20th	Day	19% 22% 38% 17% 4 2	330
	21st to end	Day	26% 28% 31% 12% 2	273
March	1st to 10th	Day	22% 22% 34% 18% 4	330
	11th to 20th	Day	21% 23% 34% 15% 5% 1	424
	21st to end	Day	25% 19% 32% 15% 7% 2	594
April	1st to 10th	Day	23% 16% 37% 18% 5% 1	540
	11th to 20th	Day	30% 18% 25% 20% 6% 1	532
	21st to end	Day	26% 18% 27% 17% 10% 2	540
May	1st to 10th	Day	20% 16% 32% 20% 9% 2	674
	11th to 20th	Day	21% 14% 35% 21% 7% 2	775
	21st to end	Day	24% 13% 30% 24% 7% 2	844
June	1st to 10th	Day	22% 11% 36% 22% 7% 2	780
	11th to 20th	Day	20% 12% 34% 22% 8% 3	780
	21st to end	Day	21% 10% 34% 22% 10% 3	780
July	1st to 10th	Day	16% 6% 37% 29% 10% 2	722
	11th to 20th	Day	18% 9% 34% 26% 11% 3	718
	21st to end	Day	15% 9% 30% 31% 12% 3	794
August	1st to 10th	Day	19% 8% 32% 27% 11% 3	726
	11th to 20th	Day	19% 8% 27% 27% 14% 5%	586
	21st to end	Day	25% 12% 28% 20% 12% 3	583
September	1st to 10th	Day	24% 10% 27% 23% 13% 3	530
	11th to 20th	Day	25% 13% 27% 22% 10% 3	530
	21st to end	Day	26% 10% 28% 22% 11% 2	530
October	1st to 10th	Day	26% 22% 26% 18% 7% 1	494
	11th to 20th	Day	28% 20% 30% 18% 4	330
	21st to end	Day	35% 18% 29% 14% 3 1	363
November	1st to 10th	Day	29% 19% 31% 15% 5%	330
	11th to 20th	Day	34% 19% 30% 12% 3 2	280
	21st to end	Day	30% 20% 33% 11% 5%	220
December	1st to 10th	Day	26% 11% 33% 21% 8% 1	220
	11th to 20th	Day	28% 19% 33% 15% 5%	220
	21st to end	Day	30% 18% 30% 16% 6% 1	242
January	1st to 10th	Night	23% 14% 35% 22% 6% 1	1050
	11th to 20th	Night	22% 15% 33% 22% 7% 1	1050
	21st to end	Night	17% 13% 37% 26% 8% 1	1155
February	1st to 10th	Night	17% 14% 33% 27% 8% 1	1050
	11th to 20th	Night	19% 13% 35% 22% 9% 1	881
	21st to end	Night	17% 15% 36% 25% 7% 1	692
March	1st to 10th	Night	16% 13% 35% 26% 10% 1	840
	11th to 20th	Night	20% 14% 35% 25% 6%	840
	21st to end	Night	18% 15% 28% 25% 13% 1	772
April	1st to 10th	Night	22% 13% 26% 30% 7% 2	630
	11th to 20th	Night	24% 12% 24% 29% 10% 1	623
	21st to end	Night	20% 10% 27% 30% 12% 2	630
May	1st to 10th	Night	24% 8% 21% 31% 13% 1	630
	11th to 20th	Night	21% 10% 25% 28% 14% 2	621
	21st to end	Night	23% 8% 21% 34% 14%	485
June	1st to 10th	Night	18% 7% 23% 36% 16%	420
	11th to 20th	Night	22% 6% 22% 33% 15% 1	420
	21st to end	Night	22% 4 22% 35% 17% 1	420
July	1st to 10th	Night	14% 3 24% 39% 18% 1	390
	11th to 20th	Night	19% 5% 19% 34% 21% 3	553
	21st to end	Night	17% 4 16% 38% 23% 2	657
August	1st to 10th	Night	17% 5% 18% 35% 23% 2	597
	11th to 20th	Night	14% 4 17% 31% 30% 4	594
	21st to end	Night	23% 5% 22% 29% 20% 2	660
September	1st to 10th	Night	18% 9% 20% 29% 20% 5%	600
	11th to 20th	Night	22% 9% 29% 25% 13% 2	742
	21st to end	Night	26% 9% 23% 27% 14% 2	800
October	1st to 10th	Night	26% 15% 25% 23% 11% 1	800
	11th to 20th	Night	28% 14% 28% 21% 9% 1	800
	21st to end	Night	30% 11% 28% 23% 8%	880
November	1st to 10th	Night	30% 13% 30% 20% 7%	820
	11th to 20th	Night	32% 13% 27% 21% 5% 1	931
	21st to end	Night	29% 16% 28% 21% 5%	1050
December	1st to 10th	Night	20% 14% 35% 22% 8% 1	1050
	11th to 20th	Night	24% 15% 32% 20% 6% 1	1050
	21st to end	Night	31% 13% 28% 20% 7% 1	1155

Page generated at 2026-06-28 02:10:46. Version ac07

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