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 2026-07-16.

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	17% 11% 4 9% 6% 5% 5% 4 5% 7% 28%	138547
	Night	16% 11% 4 10% 6% 5% 4 3 5% 6% 28%	134152
Transparency	Day	54% 6% 12% 19% 9% 1	142065
	Night	55% 6% 11% 17% 9% 1	138722
Seeing	Day	25% 18% 28% 17% 8% 3	16841
	Night	23% 16% 28% 23% 9% 1	26043

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	12% 17% 12% 22% 9% 4 3 2 3 4 13%	4661
	11th to 20th	Day	12% 17% 8% 20% 8% 7% 5% 3 4 5% 12%	4650
	21st to end	Day	13% 14% 8% 12% 11% 8% 6% 3 4 4 17%	5757
February	1st to 10th	Day	14% 17% 5% 15% 8% 6% 5% 4 5% 4 17%	1890
	11th to 20th	Day	13% 17% 7% 15% 8% 5% 5% 4 3 5% 18%	1896
	21st to end	Day	19% 14% 4 12% 8% 4 4 3 3 5% 23%	1730
March	1st to 10th	Day	20% 15% 6% 11% 6% 6% 4 3 4 4 22%	4665
	11th to 20th	Day	13% 15% 6% 8% 4 4 3 2 5% 6% 32%	5090
	21st to end	Day	15% 13% 5% 12% 6% 5% 5% 3 5% 5% 27%	5856
April	1st to 10th	Day	19% 14% 4 9% 5% 4 4 3 4 6% 28%	2520
	11th to 20th	Day	17% 12% 4 8% 4 5% 4 3 5% 6% 32%	2494
	21st to end	Day	18% 13% 3 7% 5% 4 4 3 5% 6% 32%	2520
May	1st to 10th	Day	27% 9% 3 7% 4 4 4 3 5% 6% 27%	8108
	11th to 20th	Day	18% 9% 3 8% 5% 4 4 4 5% 7% 32%	8221
	21st to end	Day	21% 8% 3 7% 4 5% 5% 4 7% 7% 29%	9060
June	1st to 10th	Day	17% 8% 3 6% 4 4 5% 4 6% 9% 34%	2940
	11th to 20th	Day	16% 7% 2 6% 5% 5% 6% 5% 8% 8% 33%	2940
	21st to end	Day	15% 7% 2 7% 6% 4 6% 4 7% 9% 33%	2940
July	1st to 10th	Day	9% 6% 2 7% 5% 5% 6% 5% 7% 10% 37%	6500
	11th to 20th	Day	9% 6% 2 8% 5% 6% 6% 5% 9% 11% 32%	6430
	21st to end	Day	13% 6% 2 6% 5% 5% 5% 5% 6% 8% 40%	6926
August	1st to 10th	Day	20% 6% 3 7% 5% 5% 5% 4 6% 9% 32%	4244
	11th to 20th	Day	17% 7% 2 7% 5% 6% 6% 6% 8% 9% 28%	4024
	21st to end	Day	18% 6% 2 6% 4 4 5% 4 6% 9% 36%	4312
September	1st to 10th	Day	16% 7% 2 6% 5% 4 4 3 6% 8% 39%	2400
	11th to 20th	Day	14% 6% 2 6% 4 3 5% 3 5% 7% 44%	2400
	21st to end	Day	21% 7% 3 6% 5% 5% 5% 4 5% 7% 33%	2137
October	1st to 10th	Day	18% 9% 3 7% 5% 4 5% 4 6% 6% 33%	2000
	11th to 20th	Day	20% 9% 3 8% 5% 4 4 4 5% 7% 32%	2000
	21st to end	Day	20% 15% 4 8% 5% 4 5% 3 5% 6% 26%	2116
November	1st to 10th	Day	14% 13% 3 8% 4 5% 5% 4 5% 6% 33%	1851
	11th to 20th	Day	14% 14% 5% 11% 5% 6% 4 3 5% 6% 27%	1795
	21st to end	Day	18% 18% 6% 10% 6% 6% 5% 3 4 6% 19%	1680
December	1st to 10th	Day	18% 18% 9% 17% 7% 4 4 3 5% 4 11%	2680
	11th to 20th	Day	19% 17% 8% 13% 5% 4 3 2 4 3 22%	3066
	21st to end	Day	20% 19% 8% 12% 6% 4 4 2 4 6% 16%	4048
January	1st to 10th	Night	13% 14% 7% 25% 9% 4 4 3 4 4 11%	8234
	11th to 20th	Night	10% 16% 7% 18% 10% 6% 5% 3 5% 5% 15%	8235
	21st to end	Night	13% 11% 5% 13% 9% 9% 7% 4 5% 5% 19%	8420
February	1st to 10th	Night	14% 14% 5% 14% 7% 6% 6% 4 5% 6% 20%	2730
	11th to 20th	Night	13% 12% 7% 13% 9% 5% 6% 4 6% 6% 18%	2608
	21st to end	Night	17% 12% 4 14% 6% 4 5% 4 6% 6% 21%	2076
March	1st to 10th	Night	17% 12% 5% 12% 8% 5% 4 3 5% 6% 24%	5390
	11th to 20th	Night	16% 12% 5% 8% 5% 4 4 3 5% 6% 33%	5089
	21st to end	Night	19% 12% 4 11% 6% 4 4 3 5% 6% 26%	5318
April	1st to 10th	Night	19% 13% 4 8% 5% 4 4 4 5% 6% 27%	2100
	11th to 20th	Night	18% 12% 3 8% 4 4 4 2 5% 6% 33%	2075
	21st to end	Night	18% 10% 3 6% 4 5% 4 3 6% 6% 36%	2100
May	1st to 10th	Night	27% 10% 3 7% 5% 4 4 3 5% 5% 26%	4831
	11th to 20th	Night	18% 8% 3 7% 4 4 5% 4 5% 7% 35%	4639
	21st to end	Night	21% 8% 3 7% 5% 4 4 4 5% 7% 32%	5114
June	1st to 10th	Night	15% 8% 2 6% 5% 4 4 4 6% 7% 39%	1680
	11th to 20th	Night	15% 6% 2 5% 5% 5% 4 3 7% 9% 37%	1680
	21st to end	Night	14% 5% 2 6% 4 5% 4 5% 7% 7% 42%	1680
July	1st to 10th	Night	9% 6% 3 8% 5% 4 5% 4 6% 10% 40%	3680
	11th to 20th	Night	13% 7% 2 7% 5% 5% 5% 4 6% 9% 40%	3640
	21st to end	Night	11% 6% 2 6% 4 4 4 4 6% 7% 47%	3926
August	1st to 10th	Night	19% 6% 2 7% 5% 4 5% 3 5% 7% 37%	2902
	11th to 20th	Night	18% 7% 3 6% 4 3 3 3 5% 6% 42%	2992
	21st to end	Night	15% 5% 2 5% 5% 4 5% 4 5% 7% 43%	3586
September	1st to 10th	Night	15% 6% 2 5% 4 5% 4 4 5% 7% 44%	2000
	11th to 20th	Night	13% 6% 2 5% 3 4 4 3 5% 7% 49%	2000
	21st to end	Night	18% 7% 2 6% 5% 5% 5% 4 5% 6% 36%	2259
October	1st to 10th	Night	18% 9% 3 7% 4 4 4 3 5% 7% 36%	2400
	11th to 20th	Night	20% 10% 3 7% 4 4 3 3 5% 7% 35%	2400
	21st to end	Night	23% 13% 4 8% 4 4 4 3 5% 5% 27%	2722
November	1st to 10th	Night	12% 13% 3 8% 6% 4 4 3 5% 6% 36%	2667
	11th to 20th	Night	18% 11% 5% 8% 5% 5% 5% 4 5% 7% 28%	2821
	21st to end	Night	15% 16% 6% 8% 5% 5% 5% 4 6% 6% 23%	2940
December	1st to 10th	Night	15% 16% 6% 14% 6% 6% 5% 5% 6% 6% 16%	4720
	11th to 20th	Night	21% 14% 5% 10% 5% 4 4 4 4 5% 23%	5348
	21st to end	Night	22% 17% 7% 10% 7% 4 4 3 4 5% 17%	7150

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	76% 3 9% 9% 3	4662
	11th to 20th	Day	74% 3 8% 7% 6% 1	4680
	21st to end	Day	69% 4 9% 13% 4	5757
February	1st to 10th	Day	69% 5% 9% 13% 5%	1890
	11th to 20th	Day	68% 5% 7% 14% 5%	1905
	21st to end	Day	63% 6% 10% 15% 6% 1	1730
March	1st to 10th	Day	65% 3 11% 13% 8%	4668
	11th to 20th	Day	52% 5% 18% 16% 8% 2	5090
	21st to end	Day	59% 4 9% 14% 13% 2	5856
April	1st to 10th	Day	57% 6% 7% 20% 8% 2	2520
	11th to 20th	Day	53% 6% 10% 17% 13% 1	2494
	21st to end	Day	52% 6% 8% 19% 12% 3	2520
May	1st to 10th	Day	57% 5% 10% 17% 10% 1	8119
	11th to 20th	Day	51% 6% 10% 23% 9% 1	8225
	21st to end	Day	51% 6% 14% 18% 11%	9061
June	1st to 10th	Day	46% 8% 11% 21% 14%	2940
	11th to 20th	Day	45% 8% 15% 21% 10% 1	2940
	21st to end	Day	45% 9% 13% 21% 11% 1	2940
July	1st to 10th	Day	40% 7% 16% 25% 11% 1	6430
	11th to 20th	Day	41% 8% 16% 24% 10% 1	6355
	21st to end	Day	41% 6% 12% 26% 15%	6728
August	1st to 10th	Day	48% 7% 14% 19% 11% 1	4230
	11th to 20th	Day	47% 8% 16% 22% 6%	4063
	21st to end	Day	43% 6% 11% 31% 7% 1	4312
September	1st to 10th	Day	42% 6% 12% 26% 12% 1	2400
	11th to 20th	Day	40% 5% 11% 25% 18% 1	2400
	21st to end	Day	50% 6% 11% 21% 11% 1	2137
October	1st to 10th	Day	45% 6% 10% 24% 14% 1	3260
	11th to 20th	Day	47% 5% 14% 19% 15%	3240
	21st to end	Day	43% 6% 17% 24% 10% 1	3393
November	1st to 10th	Day	48% 7% 13% 19% 11% 1	1851
	11th to 20th	Day	58% 5% 12% 16% 7% 2	1795
	21st to end	Day	65% 6% 12% 13% 4	1680
December	1st to 10th	Day	76% 5% 8% 9% 2	2680
	11th to 20th	Day	67% 4 10% 11% 8%	3066
	21st to end	Day	70% 4 10% 11% 5%	4048
January	1st to 10th	Night	76% 5% 9% 8% 4	8234
	11th to 20th	Night	69% 5% 9% 9% 6% 1	8280
	21st to end	Night	65% 4 9% 10% 11% 1	8420
February	1st to 10th	Night	63% 7% 10% 13% 7%	2730
	11th to 20th	Night	65% 6% 10% 13% 5% 1	2610
	21st to end	Night	60% 5% 12% 14% 7% 1	2076
March	1st to 10th	Night	62% 4 12% 14% 8% 1	5400
	11th to 20th	Night	52% 5% 17% 17% 6% 3	5090
	21st to end	Night	58% 4 7% 18% 11% 1	5318
April	1st to 10th	Night	57% 6% 10% 17% 9% 1	2100
	11th to 20th	Night	51% 5% 12% 17% 14%	2075
	21st to end	Night	48% 6% 11% 20% 13% 3	2100
May	1st to 10th	Night	59% 5% 9% 16% 10% 2	4837
	11th to 20th	Night	47% 7% 13% 21% 10% 1	4640
	21st to end	Night	51% 6% 11% 20% 11% 1	5115
June	1st to 10th	Night	42% 7% 11% 24% 15% 1	1680
	11th to 20th	Night	40% 9% 14% 23% 12% 1	1680
	21st to end	Night	38% 8% 15% 26% 12% 1	1680
July	1st to 10th	Night	39% 8% 16% 22% 14% 1	3640
	11th to 20th	Night	41% 7% 13% 24% 13% 1	3592
	21st to end	Night	36% 6% 12% 27% 18% 1	3809
August	1st to 10th	Night	47% 7% 14% 23% 9% 1	2892
	11th to 20th	Night	43% 6% 16% 26% 9%	3019
	21st to end	Night	39% 6% 13% 31% 11% 1	3586
September	1st to 10th	Night	39% 7% 13% 24% 15% 1	2000
	11th to 20th	Night	35% 6% 11% 30% 17% 1	2000
	21st to end	Night	46% 7% 11% 20% 13% 1	2259
October	1st to 10th	Night	50% 6% 10% 23% 11% 1	3920
	11th to 20th	Night	44% 5% 15% 19% 14% 3	3899
	21st to end	Night	44% 5% 15% 26% 9%	4395
November	1st to 10th	Night	48% 6% 12% 19% 13% 2	2667
	11th to 20th	Night	54% 7% 11% 18% 8% 2	2821
	21st to end	Night	58% 7% 15% 16% 4	2940
December	1st to 10th	Night	66% 6% 12% 14% 2	4720
	11th to 20th	Night	63% 4 11% 12% 9%	5348
	21st to end	Night	70% 5% 8% 11% 6% 1	7150

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	32% 25% 31% 8% 2 1	220
	11th to 20th	Day	40% 28% 27% 3 2	220
	21st to end	Day	31% 31% 28% 8% 3	358
February	1st to 10th	Day	32% 32% 29% 7% 1	330
	11th to 20th	Day	38% 30% 28% 4 1	383
	21st to end	Day	35% 25% 30% 8% 1	446
March	1st to 10th	Day	35% 21% 28% 12% 4	540
	11th to 20th	Day	35% 24% 26% 12% 3	540
	21st to end	Day	33% 26% 29% 8% 4 1	594
April	1st to 10th	Day	39% 20% 25% 11% 4 1	540
	11th to 20th	Day	32% 25% 25% 10% 6% 3	532
	21st to end	Day	30% 22% 27% 15% 4 1	540
May	1st to 10th	Day	32% 18% 31% 11% 4 3	543
	11th to 20th	Day	24% 16% 32% 16% 10% 2	567
	21st to end	Day	22% 13% 30% 21% 10% 3	617
June	1st to 10th	Day	21% 16% 28% 23% 10% 2	570
	11th to 20th	Day	19% 15% 29% 23% 11% 3	570
	21st to end	Day	19% 14% 35% 23% 7% 2	570
July	1st to 10th	Day	11% 10% 31% 28% 15% 4	565
	11th to 20th	Day	11% 8% 30% 30% 15% 5%	544
	21st to end	Day	14% 7% 36% 27% 12% 3	576
August	1st to 10th	Day	13% 8% 28% 25% 19% 6%	526
	11th to 20th	Day	16% 10% 23% 27% 16% 7%	519
	21st to end	Day	14% 11% 29% 26% 13% 8%	583
September	1st to 10th	Day	18% 12% 21% 25% 17% 7%	530
	11th to 20th	Day	14% 8% 24% 22% 20% 12%	530
	21st to end	Day	25% 12% 26% 20% 13% 5%	530
October	1st to 10th	Day	24% 13% 25% 18% 14% 6%	530
	11th to 20th	Day	26% 19% 27% 19% 7% 2	530
	21st to end	Day	32% 17% 27% 18% 6%	536
November	1st to 10th	Day	26% 20% 26% 19% 8% 1	425
	11th to 20th	Day	30% 24% 25% 17% 4	335
	21st to end	Day	35% 25% 29% 9% 2	220
December	1st to 10th	Day	34% 26% 28% 8% 4	220
	11th to 20th	Day	38% 24% 27% 10% 1	220
	21st to end	Day	36% 21% 33% 7% 2	242
January	1st to 10th	Night	26% 27% 32% 14% 2	840
	11th to 20th	Night	28% 22% 29% 18% 3	840
	21st to end	Night	21% 23% 36% 17% 3	924
February	1st to 10th	Night	27% 21% 32% 18% 1	840
	11th to 20th	Night	27% 24% 35% 14% 1	823
	21st to end	Night	29% 21% 32% 15% 3	692
March	1st to 10th	Night	26% 23% 31% 17% 3	840
	11th to 20th	Night	27% 21% 30% 18% 3	840
	21st to end	Night	25% 23% 31% 15% 6% 1	924
April	1st to 10th	Night	31% 19% 30% 18% 3	840
	11th to 20th	Night	27% 19% 28% 19% 7%	831
	21st to end	Night	26% 16% 30% 23% 5%	840
May	1st to 10th	Night	28% 11% 26% 26% 8%	487
	11th to 20th	Night	17% 11% 34% 30% 8% 1	414
	21st to end	Night	18% 7% 28% 34% 13% 1	455
June	1st to 10th	Night	19% 8% 26% 36% 11% 1	420
	11th to 20th	Night	18% 9% 28% 31% 14%	420
	21st to end	Night	13% 6% 27% 41% 12%	420
July	1st to 10th	Night	9% 4 22% 44% 19% 1	410
	11th to 20th	Night	11% 4 22% 39% 23% 1	398
	21st to end	Night	13% 4 21% 43% 18% 2	438
August	1st to 10th	Night	12% 5% 23% 35% 23% 2	584
	11th to 20th	Night	15% 5% 16% 33% 28% 3	659
	21st to end	Night	17% 6% 17% 34% 24% 1	880
September	1st to 10th	Night	18% 6% 23% 32% 18% 2	800
	11th to 20th	Night	14% 8% 19% 28% 27% 4	800
	21st to end	Night	20% 10% 25% 26% 17% 2	800
October	1st to 10th	Night	24% 11% 23% 24% 16% 2	800
	11th to 20th	Night	25% 13% 29% 25% 7% 1	800
	21st to end	Night	31% 15% 27% 21% 5%	880
November	1st to 10th	Night	22% 16% 29% 25% 7%	820
	11th to 20th	Night	27% 18% 31% 17% 6% 1	840
	21st to end	Night	26% 21% 29% 19% 5%	840
December	1st to 10th	Night	28% 23% 29% 16% 4	840
	11th to 20th	Night	27% 25% 28% 17% 3	840
	21st to end	Night	31% 21% 30% 15% 2	924

Page generated at 2026-07-13 03:14:51. Version ac07

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