Marathon Remote Imaging Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Marathon Remote Imaging Observatory clear sky chart from 2005-11-06 to 2025-04-26.

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	4 3 1 3 3 3 3 3 5% 7% 65%	118093
	Night	4 4 1 3 3 3 3 3 5% 7% 65%	119329
Transparency	Day	19% 5% 8% 19% 25% 25%	117879
	Night	19% 4 9% 19% 24% 24%	119133
Seeing	Day	6% 8% 24% 32% 23% 6%	15189
	Night	6% 5% 24% 42% 21% 2	25400

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 2 1 3 2 3 2 2 4 5% 72%	4537
	11th to 20th	Day	3 3 1 2 2 2 2 3 4 5% 75%	4540
	21st to end	Day	3 4 2 4 4 3 3 3 3 6% 67%	4994
February	1st to 10th	Day	3 4 2 3 3 3 3 3 5% 6% 66%	2000
	11th to 20th	Day	4 4 2 4 3 3 3 4 5% 6% 64%	1949
	21st to end	Day	4 3 1 3 3 3 3 3 4 8% 66%	1650
March	1st to 10th	Day	2 3 1 4 3 3 4 3 5% 6% 65%	4500
	11th to 20th	Day	7% 3 1 3 2 2 3 3 5% 6% 66%	4694
	21st to end	Day	2 3 1 4 3 4 4 4 5% 7% 62%	5969
April	1st to 10th	Day	4 3 1 2 3 3 3 3 5% 7% 67%	2400
	11th to 20th	Day	4 3 1 3 3 3 4 3 5% 7% 64%	2371
	21st to end	Day	4 4 1 3 2 3 2 2 4 6% 69%	2341
May	1st to 10th	Day	3 2 1 3 2 2 2 3 4 6% 71%	3800
	11th to 20th	Day	4 3 1 3 3 3 3 3 6% 7% 65%	3828
	21st to end	Day	3 2 1 2 2 2 3 3 6% 8% 70%	4489
June	1st to 10th	Day	2 2 1 3 2 3 4 5% 8% 9% 62%	2470
	11th to 20th	Day	3 3 1 3 3 3 4 4 6% 8% 62%	2470
	21st to end	Day	5% 3 2 4 4 5% 5% 4 7% 9% 54%	2470
July	1st to 10th	Day	4 3 1 4 3 4 5% 5% 8% 10% 52%	3945
	11th to 20th	Day	2 2 1 3 3 4 4 4 7% 8% 62%	3800
	21st to end	Day	5% 3 1 4 4 4 4 5% 7% 10% 53%	4118
August	1st to 10th	Day	1 2 1 3 3 3 4 4 6% 10% 64%	3784
	11th to 20th	Day	3 3 2 4 4 5% 5% 4 7% 8% 56%	3726
	21st to end	Day	4 3 1 5% 4 6% 6% 4 6% 9% 51%	4180
September	1st to 10th	Day	8% 5% 2 7% 5% 5% 5% 3 6% 8% 46%	2280
	11th to 20th	Day	10% 5% 2 5% 4 5% 4 4 6% 8% 46%	2123
	21st to end	Day	7% 4 2 5% 3 4 4 4 6% 6% 54%	2090
October	1st to 10th	Day	2 2 1 2 1 1 2 2 3 5% 79%	3480
	11th to 20th	Day	6% 3 1 4 2 2 3 2 3 4 69%	3379
	21st to end	Day	3 2 1 1 1 1 1 1 2 4 82%	3438
November	1st to 10th	Day	6% 3 1 1 2 2 3 2 3 5% 72%	1834
	11th to 20th	Day	5% 4 1 3 3 2 3 3 4 5% 68%	1800
	21st to end	Day	5% 5% 2 4 3 3 3 3 4 6% 63%	1800
December	1st to 10th	Day	10% 12% 1 3 2 3 3 2 4 4 56%	2940
	11th to 20th	Day	2 3 1 4 3 2 3 3 4 6% 70%	3391
	21st to end	Day	2 2 1 2 2 2 2 2 3 5% 77%	4513
January	1st to 10th	Night	2 3 2 3 2 2 2 2 3 5% 73%	5431
	11th to 20th	Night	3 2 1 2 2 2 2 2 4 5% 75%	5440
	21st to end	Night	4 5% 2 6% 3 3 3 3 4 6% 62%	5984
February	1st to 10th	Night	4 5% 2 4 3 3 2 2 4 8% 64%	2400
	11th to 20th	Night	5% 4 2 4 3 3 4 4 5% 6% 59%	2338
	21st to end	Night	5% 6% 1 3 3 2 3 3 5% 5% 65%	1980
March	1st to 10th	Night	2 4 1 4 3 3 4 3 5% 7% 65%	5374
	11th to 20th	Night	7% 4 1 3 2 2 3 2 3 5% 67%	5258
	21st to end	Night	3 3 1 3 2 3 4 3 5% 6% 66%	4984
April	1st to 10th	Night	4 5% 1 4 4 3 3 3 4 6% 62%	2000
	11th to 20th	Night	5% 3 1 4 3 3 3 3 5% 6% 63%	1977
	21st to end	Night	4 4 1 3 2 2 2 2 4 6% 69%	1955
May	1st to 10th	Night	4 4 1 3 2 2 3 2 4 6% 69%	3170
	11th to 20th	Night	5% 5% 1 4 2 3 3 3 5% 5% 64%	3059
	21st to end	Night	3 4 1 3 2 3 3 3 5% 7% 65%	3098
June	1st to 10th	Night	3 2 1 3 3 3 5% 5% 7% 12% 56%	1710
	11th to 20th	Night	3 3 1 3 4 4 3 5% 7% 9% 58%	1710
	21st to end	Night	4 3 1 4 3 3 3 5% 6% 10% 58%	1710
July	1st to 10th	Night	3 2 1 3 3 3 5% 5% 8% 12% 56%	3003
	11th to 20th	Night	2 1 1 1 1 2 3 3 5% 9% 72%	3170
	21st to end	Night	3 3 1 3 3 4 4 4 7% 11% 58%	3445
August	1st to 10th	Night	1 1 2 2 2 3 3 7% 14% 66%	3155
	11th to 20th	Night	2 1 1 3 2 2 3 4 7% 13% 62%	3113
	21st to end	Night	3 3 1 4 3 3 4 3 7% 11% 58%	3487
September	1st to 10th	Night	6% 4 2 5% 5% 4 4 4 6% 9% 52%	1900
	11th to 20th	Night	8% 4 1 4 4 4 5% 4 6% 10% 50%	2054
	21st to end	Night	6% 4 1 4 3 3 4 4 6% 9% 54%	2090
October	1st to 10th	Night	2 2 1 2 2 2 2 2 3 5% 77%	3480
	11th to 20th	Night	5% 4 1 4 2 2 3 2 4 6% 67%	3538
	21st to end	Night	3 2 1 2 1 2 2 2 3 4 79%	4120
November	1st to 10th	Night	6% 4 1 2 2 2 2 2 3 5% 71%	2440
	11th to 20th	Night	4 4 1 3 3 3 3 3 5% 7% 66%	2600
	21st to end	Night	5% 5% 2 4 3 3 3 2 5% 6% 63%	2600
December	1st to 10th	Night	10% 9% 2 6% 4 3 3 3 4 6% 50%	4250
	11th to 20th	Night	3 4 1 3 2 2 3 2 3 6% 70%	4816
	21st to end	Night	2 3 1 2 2 2 2 2 3 6% 74%	6490

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	14% 2 5% 18% 27% 34%	4538
	11th to 20th	Day	14% 2 4 18% 37% 25%	4540
	21st to end	Day	21% 3 4 17% 30% 25%	4994
February	1st to 10th	Day	19% 3 6% 13% 24% 35%	2000
	11th to 20th	Day	21% 4 6% 15% 26% 28%	1959
	21st to end	Day	17% 3 5% 13% 27% 35%	1650
March	1st to 10th	Day	20% 4 4 8% 21% 44%	4500
	11th to 20th	Day	19% 4 5% 10% 19% 42%	4694
	21st to end	Day	20% 4 5% 9% 24% 38%	5969
April	1st to 10th	Day	18% 4 6% 12% 25% 35%	2400
	11th to 20th	Day	20% 4 6% 11% 25% 34%	2374
	21st to end	Day	18% 3 6% 15% 27% 31%	2341
May	1st to 10th	Day	13% 4 7% 14% 19% 42%	3800
	11th to 20th	Day	17% 6% 9% 13% 23% 32%	3831
	21st to end	Day	12% 5% 9% 20% 19% 35%	4489
June	1st to 10th	Day	15% 8% 13% 25% 25% 15%	2470
	11th to 20th	Day	19% 5% 9% 18% 31% 17%	2470
	21st to end	Day	24% 8% 12% 22% 22% 13%	2470
July	1st to 10th	Day	25% 8% 17% 33% 15% 1	3886
	11th to 20th	Day	19% 5% 15% 35% 22% 4	3737
	21st to end	Day	24% 6% 14% 31% 19% 6%	3988
August	1st to 10th	Day	16% 6% 11% 32% 32% 3	3772
	11th to 20th	Day	24% 7% 15% 31% 20% 4	3762
	21st to end	Day	29% 7% 16% 26% 19% 3	4179
September	1st to 10th	Day	36% 8% 13% 19% 15% 10%	2280
	11th to 20th	Day	33% 10% 12% 19% 20% 6%	2123
	21st to end	Day	29% 7% 9% 19% 22% 13%	2090
October	1st to 10th	Day	10% 4 8% 14% 24% 40%	3480
	11th to 20th	Day	22% 3 6% 18% 19% 32%	3379
	21st to end	Day	11% 1 3 14% 46% 26%	3438
November	1st to 10th	Day	17% 3 6% 19% 30% 25%	1834
	11th to 20th	Day	19% 3 7% 19% 26% 25%	1800
	21st to end	Day	24% 4 6% 16% 27% 24%	1800
December	1st to 10th	Day	33% 2 5% 18% 22% 20%	2940
	11th to 20th	Day	16% 3 4 21% 31% 24%	3389
	21st to end	Day	12% 2 3 13% 35% 34%	4513
January	1st to 10th	Night	15% 2 4 14% 34% 32%	5431
	11th to 20th	Night	13% 2 4 18% 29% 34%	5440
	21st to end	Night	23% 3 4 16% 29% 23%	5984
February	1st to 10th	Night	21% 3 6% 13% 24% 34%	2400
	11th to 20th	Night	26% 4 6% 14% 22% 28%	2338
	21st to end	Night	20% 3 7% 8% 27% 34%	1980
March	1st to 10th	Night	19% 4 4 11% 19% 43%	5374
	11th to 20th	Night	21% 4 6% 10% 17% 43%	5258
	21st to end	Night	17% 4 7% 11% 22% 38%	4984
April	1st to 10th	Night	24% 4 6% 11% 22% 33%	2000
	11th to 20th	Night	21% 4 8% 15% 21% 30%	1977
	21st to end	Night	18% 3 9% 17% 26% 27%	1955
May	1st to 10th	Night	17% 4 8% 16% 17% 38%	3170
	11th to 20th	Night	20% 8% 9% 16% 17% 30%	3060
	21st to end	Night	17% 6% 10% 22% 16% 29%	3099
June	1st to 10th	Night	17% 9% 15% 28% 21% 8%	1710
	11th to 20th	Night	19% 7% 13% 21% 29% 11%	1710
	21st to end	Night	20% 8% 14% 24% 22% 11%	1710
July	1st to 10th	Night	18% 9% 20% 40% 12% 1	2954
	11th to 20th	Night	11% 4 16% 46% 20% 3	3110
	21st to end	Night	19% 6% 16% 36% 20% 3	3336
August	1st to 10th	Night	10% 4 13% 39% 32% 2	3144
	11th to 20th	Night	14% 7% 23% 40% 14% 2	3143
	21st to end	Night	20% 7% 20% 34% 16% 3	3487
September	1st to 10th	Night	28% 7% 17% 24% 14% 9%	1900
	11th to 20th	Night	30% 8% 17% 21% 17% 7%	2054
	21st to end	Night	26% 7% 13% 22% 20% 12%	2090
October	1st to 10th	Night	12% 4 9% 13% 27% 35%	3480
	11th to 20th	Night	23% 3 9% 14% 18% 33%	3538
	21st to end	Night	13% 3 4 18% 39% 24%	4120
November	1st to 10th	Night	18% 3 6% 17% 31% 25%	2440
	11th to 20th	Night	19% 4 7% 19% 26% 25%	2600
	21st to end	Night	23% 3 8% 14% 26% 25%	2600
December	1st to 10th	Night	36% 2 5% 16% 22% 19%	4250
	11th to 20th	Night	19% 2 3 17% 33% 25%	4817
	21st to end	Night	14% 3 4 12% 37% 30%	6490

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	8% 12% 38% 30% 11% 1	300
	11th to 20th	Day	11% 13% 30% 34% 12% 1	300
	21st to end	Day	6% 16% 36% 28% 13% 1	330
February	1st to 10th	Day	9% 14% 28% 26% 18% 5%	300
	11th to 20th	Day	10% 14% 35% 29% 10% 2	300
	21st to end	Day	6% 18% 43% 24% 8% 1	249
March	1st to 10th	Day	8% 13% 42% 25% 10% 1	300
	11th to 20th	Day	10% 20% 38% 23% 7% 2	345
	21st to end	Day	8% 22% 39% 22% 7% 1	550
April	1st to 10th	Day	7% 19% 43% 22% 7% 1	500
	11th to 20th	Day	5% 21% 46% 21% 6% 1	492
	21st to end	Day	6% 17% 42% 30% 6%	481
May	1st to 10th	Day	4 15% 39% 31% 10% 1	463
	11th to 20th	Day	5% 8% 36% 36% 15% 1	525
	21st to end	Day	6% 6% 29% 43% 16% 1	734
June	1st to 10th	Day	3 2 21% 40% 29% 5%	680
	11th to 20th	Day	3 1 18% 43% 32% 4	680
	21st to end	Day	5% 1 13% 36% 41% 5%	680
July	1st to 10th	Day	4 1 10% 27% 45% 13%	579
	11th to 20th	Day	2 1 8% 39% 38% 12%	484
	21st to end	Day	2 1 8% 42% 37% 10%	531
August	1st to 10th	Day	2 1 7% 34% 42% 14%	487
	11th to 20th	Day	7% 1 8% 32% 36% 15%	477
	21st to end	Day	6% 1 8% 33% 37% 15%	539
September	1st to 10th	Day	7% 2 8% 41% 33% 10%	490
	11th to 20th	Day	11% 1 5% 27% 43% 15%	333
	21st to end	Day	9% 1 9% 31% 33% 16%	300
October	1st to 10th	Day	5% 2 16% 30% 36% 10%	300
	11th to 20th	Day	8% 5% 19% 37% 22% 9%	300
	21st to end	Day	8% 8% 22% 31% 23% 8%	330
November	1st to 10th	Day	9% 6% 22% 39% 16% 7%	300
	11th to 20th	Day	9% 12% 26% 28% 20% 6%	300
	21st to end	Day	7% 17% 31% 26% 16% 2	300
December	1st to 10th	Day	11% 11% 21% 35% 19% 3	300
	11th to 20th	Day	5% 11% 37% 31% 13% 3	300
	21st to end	Day	5% 12% 35% 34% 13% 2	330
January	1st to 10th	Night	7% 6% 27% 43% 16% 1	803
	11th to 20th	Night	8% 7% 25% 41% 18% 1	800
	21st to end	Night	7% 8% 29% 43% 13% 1	880
February	1st to 10th	Night	8% 9% 25% 41% 17% 1	800
	11th to 20th	Night	8% 7% 32% 42% 11%	784
	21st to end	Night	8% 8% 39% 40% 5%	660
March	1st to 10th	Night	6% 10% 35% 37% 10% 2	800
	11th to 20th	Night	8% 11% 33% 33% 13% 2	779
	21st to end	Night	5% 8% 40% 37% 9% 1	660
April	1st to 10th	Night	6% 9% 38% 38% 9%	600
	11th to 20th	Night	5% 10% 40% 39% 6%	593
	21st to end	Night	4 10% 37% 41% 7% 1	587
May	1st to 10th	Night	4 6% 36% 43% 10%	570
	11th to 20th	Night	7% 5% 33% 43% 12%	561
	21st to end	Night	5% 3 35% 42% 14%	617
June	1st to 10th	Night	2 3 25% 52% 18%	570
	11th to 20th	Night	3 2 23% 49% 23% 1	570
	21st to end	Night	4 1 17% 51% 26%	570
July	1st to 10th	Night	6% 1 12% 44% 36% 2	555
	11th to 20th	Night	3 1 11% 48% 36% 1	552
	21st to end	Night	2 8% 49% 38% 1	624
August	1st to 10th	Night	2 1 6% 45% 44% 3	567
	11th to 20th	Night	4 7% 36% 50% 4	564
	21st to end	Night	4 7% 43% 44% 1	627
September	1st to 10th	Night	8% 1 7% 41% 41% 3	570
	11th to 20th	Night	9% 1 8% 43% 39% 1	570
	21st to end	Night	7% 1 12% 43% 34% 3	570
October	1st to 10th	Night	5% 2 16% 46% 32% 1	570
	11th to 20th	Night	9% 2 18% 47% 21% 3	611
	21st to end	Night	7% 5% 16% 39% 27% 6%	836
November	1st to 10th	Night	8% 3 17% 43% 25% 5%	880
	11th to 20th	Night	6% 4 20% 43% 25% 3	1000
	21st to end	Night	8% 6% 25% 39% 19% 4	1000
December	1st to 10th	Night	9% 4 22% 45% 18% 3	1000
	11th to 20th	Night	6% 7% 26% 44% 16% 1	1000
	21st to end	Night	6% 8% 29% 43% 12% 1	1100

Page generated at 2025-04-23 08:25:02. Version ac07

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