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 2025-03-12.

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% 5% 5% 5% 4 5% 7% 28%	108912
	Night	16% 11% 4 10% 6% 4 4 4 5% 6% 28%	110078
Transparency	Day	54% 6% 12% 19% 9% 1	112477
	Night	55% 6% 12% 18% 9% 1	114677
Seeing	Day	25% 17% 28% 18% 9% 3	15056
	Night	23% 16% 28% 23% 9% 1	24429

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	11% 15% 14% 24% 8% 3 3 2 3 4 13%	3597
	11th to 20th	Day	13% 17% 9% 21% 7% 6% 4 3 4 4 12%	3570
	21st to end	Day	15% 16% 9% 11% 8% 6% 4 3 4 4 19%	4443
February	1st to 10th	Day	15% 17% 5% 15% 8% 6% 5% 4 5% 4 17%	1800
	11th to 20th	Day	13% 16% 7% 15% 8% 5% 5% 4 3 5% 18%	1803
	21st to end	Day	20% 14% 4 12% 8% 4 4 3 3 5% 23%	1650
March	1st to 10th	Day	23% 17% 5% 11% 5% 4 3 3 3 4 23%	3277
	11th to 20th	Day	11% 19% 7% 9% 6% 5% 4 2 5% 5% 28%	3491
	21st to end	Day	18% 13% 4 11% 5% 4 4 3 4 5% 29%	3996
April	1st to 10th	Day	19% 14% 3 8% 5% 4 4 3 4 6% 29%	2280
	11th to 20th	Day	18% 11% 4 8% 4 5% 4 3 5% 5% 33%	2254
	21st to end	Day	19% 13% 3 7% 5% 4 4 3 5% 6% 32%	2280
May	1st to 10th	Day	24% 9% 2 6% 5% 4 3 3 5% 6% 32%	4352
	11th to 20th	Day	21% 8% 3 8% 6% 5% 5% 5% 5% 7% 27%	4398
	21st to end	Day	14% 6% 2 6% 4 5% 4 4 7% 8% 40%	4837
June	1st to 10th	Day	18% 8% 3 6% 5% 4 5% 4 7% 9% 32%	2660
	11th to 20th	Day	15% 7% 2 6% 5% 5% 5% 5% 8% 8% 33%	2660
	21st to end	Day	15% 7% 2 7% 6% 4 6% 4 7% 9% 33%	2660
July	1st to 10th	Day	11% 6% 2 6% 5% 5% 6% 5% 8% 12% 35%	4440
	11th to 20th	Day	8% 6% 3 7% 5% 6% 6% 6% 10% 11% 33%	4440
	21st to end	Day	12% 6% 3 7% 6% 5% 6% 6% 7% 8% 34%	4814
August	1st to 10th	Day	20% 6% 3 7% 5% 5% 5% 4 6% 9% 30%	4114
	11th to 20th	Day	17% 7% 2 7% 5% 6% 6% 6% 8% 10% 27%	3898
	21st to end	Day	18% 6% 2 6% 4 4 5% 4 6% 9% 36%	4180
September	1st to 10th	Day	16% 7% 3 6% 5% 5% 4 3 6% 8% 38%	2280
	11th to 20th	Day	15% 7% 2 6% 4 4 4 3 5% 7% 43%	2280
	21st to end	Day	21% 7% 3 6% 5% 5% 5% 4 5% 6% 34%	2031
October	1st to 10th	Day	19% 9% 3 7% 5% 5% 5% 4 6% 6% 31%	1900
	11th to 20th	Day	20% 9% 3 8% 5% 4 4 4 5% 6% 32%	1900
	21st to end	Day	21% 15% 4 8% 5% 4 5% 3 4 6% 25%	2010
November	1st to 10th	Day	13% 12% 3 8% 4 4 4 4 5% 6% 34%	1761
	11th to 20th	Day	15% 14% 4 11% 5% 5% 5% 3 5% 6% 27%	1710
	21st to end	Day	19% 17% 5% 10% 6% 6% 5% 3 4 7% 19%	1600
December	1st to 10th	Day	18% 18% 9% 17% 7% 4 4 3 4 4 11%	2600
	11th to 20th	Day	19% 17% 8% 13% 5% 3 3 2 3 3 22%	2986
	21st to end	Day	20% 19% 8% 12% 6% 4 4 2 4 6% 16%	3960
January	1st to 10th	Night	13% 12% 7% 27% 9% 4 4 3 4 5% 11%	6349
	11th to 20th	Night	10% 15% 7% 17% 10% 6% 5% 4 5% 5% 16%	6315
	21st to end	Night	16% 13% 6% 14% 9% 6% 5% 3 5% 5% 19%	6509
February	1st to 10th	Night	15% 14% 5% 14% 7% 6% 5% 4 4 6% 20%	2600
	11th to 20th	Night	13% 12% 6% 13% 9% 5% 6% 4 6% 6% 19%	2481
	21st to end	Night	18% 12% 4 14% 6% 4 5% 4 6% 6% 21%	1980
March	1st to 10th	Night	20% 15% 5% 10% 5% 4 4 2 4 6% 26%	3778
	11th to 20th	Night	13% 15% 6% 9% 6% 4 4 3 5% 6% 27%	3497
	21st to end	Night	17% 13% 3 11% 5% 4 4 3 5% 5% 30%	3636
April	1st to 10th	Night	18% 13% 4 7% 5% 4 4 4 5% 7% 29%	1900
	11th to 20th	Night	19% 11% 3 7% 4 4 4 2 6% 6% 34%	1875
	21st to end	Night	18% 9% 3 6% 4 4 4 3 6% 6% 36%	1900
May	1st to 10th	Night	26% 9% 3 7% 5% 4 3 3 5% 6% 30%	2601
	11th to 20th	Night	19% 8% 3 7% 4 4 5% 4 5% 7% 34%	2495
	21st to end	Night	13% 7% 3 6% 5% 4 5% 4 6% 8% 38%	2738
June	1st to 10th	Night	15% 8% 2 6% 5% 4 3 4 6% 8% 39%	1520
	11th to 20th	Night	15% 6% 2 5% 5% 5% 4 3 7% 10% 38%	1520
	21st to end	Night	14% 5% 2 6% 4 5% 4 5% 7% 7% 42%	1520
July	1st to 10th	Night	9% 7% 3 8% 5% 4 5% 4 7% 9% 40%	2520
	11th to 20th	Night	12% 6% 2 6% 4 5% 5% 4 6% 9% 42%	2520
	21st to end	Night	13% 7% 2 6% 5% 5% 4 5% 6% 7% 41%	2739
August	1st to 10th	Night	20% 6% 2 7% 5% 4 5% 3 5% 7% 36%	2812
	11th to 20th	Night	18% 7% 3 6% 4 3 3 3 5% 6% 41%	2899
	21st to end	Night	15% 5% 2 5% 5% 4 5% 4 6% 7% 42%	3476
September	1st to 10th	Night	15% 6% 2 5% 4 5% 3 4 5% 7% 43%	1900
	11th to 20th	Night	14% 6% 2 5% 3 4 4 3 4 7% 49%	1900
	21st to end	Night	18% 7% 2 6% 5% 5% 5% 4 4 6% 37%	2145
October	1st to 10th	Night	19% 10% 3 7% 4 4 4 3 5% 7% 34%	2280
	11th to 20th	Night	20% 10% 3 7% 4 4 3 3 5% 7% 35%	2280
	21st to end	Night	24% 14% 4 7% 4 4 4 3 5% 5% 26%	2586
November	1st to 10th	Night	11% 13% 4 8% 6% 4 4 3 5% 6% 37%	2537
	11th to 20th	Night	19% 11% 4 8% 5% 5% 5% 4 5% 7% 28%	2686
	21st to end	Night	15% 15% 5% 8% 5% 5% 5% 4 6% 6% 24%	2800
December	1st to 10th	Night	15% 16% 6% 14% 6% 6% 5% 5% 6% 6% 16%	4580
	11th to 20th	Night	22% 14% 6% 10% 5% 4 4 4 4 4 23%	5208
	21st to end	Night	22% 17% 7% 10% 7% 4 4 3 4 5% 17%	6996

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	75% 3 8% 11% 3	3598
	11th to 20th	Day	76% 3 8% 7% 5% 1	3600
	21st to end	Day	66% 5% 10% 14% 5%	4443
February	1st to 10th	Day	70% 5% 9% 12% 4	1800
	11th to 20th	Day	68% 5% 7% 14% 5%	1812
	21st to end	Day	63% 6% 10% 14% 6% 1	1650
March	1st to 10th	Day	65% 4 10% 14% 6% 1	3280
	11th to 20th	Day	56% 6% 14% 15% 8% 1	3491
	21st to end	Day	57% 4 8% 17% 12% 1	3996
April	1st to 10th	Day	56% 6% 7% 21% 9% 2	2280
	11th to 20th	Day	52% 6% 9% 18% 13% 2	2254
	21st to end	Day	53% 6% 7% 18% 12% 3	2280
May	1st to 10th	Day	53% 6% 10% 19% 11% 2	4366
	11th to 20th	Day	53% 8% 11% 21% 6% 1	4402
	21st to end	Day	39% 8% 13% 24% 15% 1	4837
June	1st to 10th	Day	47% 8% 11% 20% 13% 1	2660
	11th to 20th	Day	45% 8% 15% 21% 10% 1	2660
	21st to end	Day	46% 9% 13% 21% 11%	2660
July	1st to 10th	Day	40% 8% 17% 25% 10% 1	4370
	11th to 20th	Day	40% 8% 16% 25% 10%	4365
	21st to end	Day	45% 8% 14% 21% 11%	4661
August	1st to 10th	Day	50% 8% 14% 19% 10%	4100
	11th to 20th	Day	48% 9% 17% 21% 6%	3937
	21st to end	Day	44% 6% 11% 31% 7% 1	4180
September	1st to 10th	Day	44% 6% 13% 25% 12% 1	2280
	11th to 20th	Day	41% 5% 11% 24% 18% 1	2280
	21st to end	Day	50% 6% 11% 21% 11% 1	2031
October	1st to 10th	Day	46% 6% 10% 24% 14% 1	3160
	11th to 20th	Day	47% 5% 14% 19% 15%	3140
	21st to end	Day	43% 6% 18% 24% 9% 1	3287
November	1st to 10th	Day	48% 7% 14% 19% 11% 2	1761
	11th to 20th	Day	57% 5% 13% 16% 7% 2	1710
	21st to end	Day	64% 6% 12% 13% 4	1600
December	1st to 10th	Day	76% 5% 8% 8% 2	2600
	11th to 20th	Day	67% 4 10% 11% 8%	2986
	21st to end	Day	70% 4 10% 11% 6%	3960
January	1st to 10th	Night	75% 5% 9% 7% 3	6349
	11th to 20th	Night	67% 5% 11% 10% 5% 1	6360
	21st to end	Night	65% 5% 9% 8% 12% 1	6509
February	1st to 10th	Night	63% 7% 11% 13% 6%	2600
	11th to 20th	Night	64% 7% 10% 13% 5% 1	2483
	21st to end	Night	60% 6% 12% 14% 7% 1	1980
March	1st to 10th	Night	60% 5% 11% 16% 8%	3788
	11th to 20th	Night	57% 6% 12% 20% 5%	3498
	21st to end	Night	55% 5% 9% 19% 10% 2	3636
April	1st to 10th	Night	55% 6% 10% 18% 9% 2	1900
	11th to 20th	Night	50% 5% 12% 18% 15% 1	1875
	21st to end	Night	48% 6% 10% 19% 13% 3	1900
May	1st to 10th	Night	55% 6% 9% 19% 9% 2	2607
	11th to 20th	Night	48% 10% 15% 19% 8% 1	2496
	21st to end	Night	41% 10% 13% 20% 16% 1	2739
June	1st to 10th	Night	42% 7% 12% 24% 14% 1	1520
	11th to 20th	Night	38% 9% 15% 24% 11% 1	1520
	21st to end	Night	38% 8% 15% 26% 13% 1	1520
July	1st to 10th	Night	40% 8% 15% 21% 15% 1	2480
	11th to 20th	Night	37% 7% 14% 28% 14%	2472
	21st to end	Night	42% 7% 12% 23% 16% 1	2651
August	1st to 10th	Night	48% 7% 14% 23% 8% 1	2802
	11th to 20th	Night	44% 6% 16% 25% 9%	2926
	21st to end	Night	39% 6% 13% 31% 11% 1	3476
September	1st to 10th	Night	39% 7% 13% 24% 15% 1	1900
	11th to 20th	Night	36% 6% 11% 28% 18% 1	1900
	21st to end	Night	46% 7% 11% 21% 13% 1	2145
October	1st to 10th	Night	51% 6% 10% 22% 11%	3800
	11th to 20th	Night	44% 5% 15% 19% 15% 3	3779
	21st to end	Night	44% 5% 16% 27% 8%	4259
November	1st to 10th	Night	47% 6% 12% 19% 13% 2	2537
	11th to 20th	Night	53% 7% 11% 18% 9% 2	2686
	21st to end	Night	57% 7% 15% 17% 4	2800
December	1st to 10th	Night	66% 6% 12% 14% 2	4580
	11th to 20th	Night	63% 5% 11% 12% 10%	5208
	21st to end	Night	69% 5% 8% 11% 6% 1	6996

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% 32% 9% 2 1	200
	11th to 20th	Day	40% 28% 27% 3 2 1	200
	21st to end	Day	28% 31% 30% 8% 3	325
February	1st to 10th	Day	32% 33% 29% 6%	300
	11th to 20th	Day	38% 30% 27% 4 1	350
	21st to end	Day	36% 24% 30% 9% 1	414
March	1st to 10th	Day	34% 22% 27% 12% 4	500
	11th to 20th	Day	32% 25% 28% 12% 3	464
	21st to end	Day	33% 25% 28% 8% 4 1	506
April	1st to 10th	Day	36% 20% 27% 12% 4 1	460
	11th to 20th	Day	31% 25% 24% 10% 7% 3	452
	21st to end	Day	32% 20% 27% 15% 5% 2	460
May	1st to 10th	Day	31% 17% 31% 12% 5% 4	463
	11th to 20th	Day	23% 15% 33% 16% 10% 2	487
	21st to end	Day	18% 13% 30% 23% 12% 4	529
June	1st to 10th	Day	21% 16% 30% 21% 10% 2	490
	11th to 20th	Day	17% 14% 28% 26% 12% 3	490
	21st to end	Day	19% 14% 35% 22% 7% 2	490
July	1st to 10th	Day	11% 11% 30% 28% 16% 5%	485
	11th to 20th	Day	12% 8% 30% 30% 15% 5%	484
	21st to end	Day	14% 8% 36% 27% 12% 3	532
August	1st to 10th	Day	14% 8% 28% 25% 17% 6%	486
	11th to 20th	Day	16% 10% 24% 26% 15% 8%	479
	21st to end	Day	15% 11% 28% 26% 13% 8%	539
September	1st to 10th	Day	19% 12% 21% 24% 17% 7%	490
	11th to 20th	Day	15% 8% 24% 23% 19% 11%	490
	21st to end	Day	25% 12% 26% 19% 13% 5%	490
October	1st to 10th	Day	25% 13% 26% 17% 14% 5%	490
	11th to 20th	Day	25% 17% 28% 20% 8% 2	490
	21st to end	Day	33% 18% 26% 17% 6%	496
November	1st to 10th	Day	24% 19% 27% 20% 9% 1	395
	11th to 20th	Day	31% 23% 25% 17% 5%	310
	21st to end	Day	35% 23% 31% 10% 2	200
December	1st to 10th	Day	35% 24% 29% 9% 4 1	200
	11th to 20th	Day	38% 24% 27% 10% 2 1	200
	21st to end	Day	34% 22% 34% 8% 2	220
January	1st to 10th	Night	26% 26% 32% 14% 2	800
	11th to 20th	Night	27% 22% 30% 18% 3	800
	21st to end	Night	20% 23% 37% 17% 3	880
February	1st to 10th	Night	27% 22% 33% 17% 1	800
	11th to 20th	Night	27% 24% 34% 13% 1	783
	21st to end	Night	30% 21% 32% 15% 3	660
March	1st to 10th	Night	26% 23% 31% 17% 3	800
	11th to 20th	Night	26% 21% 31% 19% 4	767
	21st to end	Night	24% 23% 30% 16% 6% 1	836
April	1st to 10th	Night	29% 19% 30% 19% 3	760
	11th to 20th	Night	27% 18% 29% 18% 7%	751
	21st to end	Night	25% 16% 30% 23% 5%	760
May	1st to 10th	Night	27% 11% 26% 27% 9%	441
	11th to 20th	Night	17% 11% 34% 29% 9% 1	374
	21st to end	Night	15% 7% 28% 35% 14% 1	411
June	1st to 10th	Night	18% 9% 26% 36% 11% 1	380
	11th to 20th	Night	17% 8% 27% 32% 15%	380
	21st to end	Night	12% 7% 29% 41% 12%	380
July	1st to 10th	Night	8% 4 22% 44% 19% 2	370
	11th to 20th	Night	11% 3 22% 40% 22% 1	368
	21st to end	Night	13% 4 21% 42% 18% 2	416
August	1st to 10th	Night	12% 5% 24% 34% 22% 3	554
	11th to 20th	Night	15% 5% 17% 33% 27% 4	626
	21st to end	Night	17% 6% 17% 34% 25% 1	836
September	1st to 10th	Night	19% 6% 23% 32% 17% 3	760
	11th to 20th	Night	15% 8% 19% 28% 25% 4	760
	21st to end	Night	19% 10% 26% 26% 17% 2	760
October	1st to 10th	Night	25% 11% 23% 24% 15% 2	760
	11th to 20th	Night	26% 13% 28% 25% 7% 1	760
	21st to end	Night	32% 15% 28% 21% 5%	836
November	1st to 10th	Night	21% 16% 29% 26% 7%	780
	11th to 20th	Night	27% 17% 31% 17% 6% 1	800
	21st to end	Night	25% 21% 30% 19% 5%	800
December	1st to 10th	Night	28% 23% 29% 16% 4	800
	11th to 20th	Night	27% 25% 28% 17% 3	800
	21st to end	Night	30% 21% 31% 16% 2	880

Page generated at 2025-03-09 08:08:48. Version ac07

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