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 2026-08-30.

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 4 3 3 4 3 5% 7% 63%	151441
	Night	4 4 1 3 3 3 3 3 5% 7% 64%	148251
Transparency	Day	21% 4 8% 19% 25% 23%	146555
	Night	21% 4 9% 20% 24% 22%	144145
Seeing	Day	6% 8% 24% 33% 23% 6%	17062
	Night	6% 5% 23% 43% 21% 2	27079

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	2 2 1 3 3 3 2 2 4 6% 73%	5885
	11th to 20th	Day	3 3 1 2 2 2 3 3 3 5% 74%	5910
	21st to end	Day	5% 4 2 4 3 3 3 3 3 6% 63%	6468
February	1st to 10th	Day	3 4 2 3 3 3 3 3 5% 6% 64%	2100
	11th to 20th	Day	4 4 2 4 3 3 3 4 4 6% 64%	2049
	21st to end	Day	4 3 1 3 3 2 3 3 5% 8% 67%	1730
March	1st to 10th	Day	2 3 1 4 3 3 4 3 5% 6% 65%	4600
	11th to 20th	Day	7% 3 1 3 2 2 3 3 5% 6% 66%	4797
	21st to end	Day	2 3 1 4 3 4 4 4 5% 7% 63%	6101
April	1st to 10th	Day	4 3 1 3 3 3 3 3 4 7% 65%	2520
	11th to 20th	Day	5% 4 1 4 3 3 4 3 5% 7% 63%	2491
	21st to end	Day	4 4 1 3 2 3 2 2 4 6% 68%	2520
May	1st to 10th	Day	3 4 2 4 2 3 3 3 4 6% 66%	7057
	11th to 20th	Day	5% 3 1 3 3 3 4 3 5% 7% 62%	7151
	21st to end	Day	4 3 1 3 3 3 5% 4 7% 9% 58%	8404
June	1st to 10th	Day	2 2 1 3 2 3 5% 5% 8% 9% 61%	2730
	11th to 20th	Day	3 3 1 3 3 4 4 4 7% 7% 61%	2730
	21st to end	Day	5% 3 2 4 4 5% 5% 5% 6% 9% 52%	2730
July	1st to 10th	Day	5% 3 1 4 3 4 5% 5% 8% 10% 52%	7364
	11th to 20th	Day	6% 4 1 6% 4 5% 5% 5% 7% 9% 47%	7080
	21st to end	Day	3 2 1 3 3 3 4 3 5% 8% 63%	7686
August	1st to 10th	Day	1 1 1 2 2 3 4 4 5% 9% 68%	5468
	11th to 20th	Day	2 2 2 4 5% 5% 6% 5% 8% 11% 49%	5486
	21st to end	Day	3 3 1 5% 4 6% 5% 4 6% 9% 54%	6093
September	1st to 10th	Day	8% 5% 2 7% 5% 5% 5% 3 6% 8% 46%	2400
	11th to 20th	Day	9% 5% 2 5% 4 5% 4 4 7% 9% 46%	2235
	21st to end	Day	7% 4 2 5% 3 4 4 4 6% 6% 54%	2200
October	1st to 10th	Day	2 2 1 2 1 1 2 2 3 5% 78%	3590
	11th to 20th	Day	5% 3 1 4 2 2 3 2 3 4 69%	3486
	21st to end	Day	3 2 1 1 1 1 1 1 2 4 82%	3548
November	1st to 10th	Day	5% 3 1 1 2 2 3 2 3 4 74%	1928
	11th to 20th	Day	4 4 1 3 3 2 3 3 4 5% 69%	1890
	21st to end	Day	5% 5% 2 4 3 3 3 3 4 5% 64%	1890
December	1st to 10th	Day	10% 12% 1 3 2 3 3 2 3 4 57%	3030
	11th to 20th	Day	2 3 1 4 3 2 3 3 4 6% 70%	3481
	21st to end	Day	3 2 1 2 2 2 2 2 3 5% 77%	4613
January	1st to 10th	Night	2 3 1 3 2 2 2 2 3 5% 73%	7041
	11th to 20th	Night	4 2 1 2 2 2 2 2 4 5% 74%	7080
	21st to end	Night	4 6% 2 6% 3 3 3 3 4 6% 58%	7746
February	1st to 10th	Night	4 5% 2 4 3 3 2 2 4 8% 64%	2520
	11th to 20th	Night	5% 4 2 5% 3 3 4 4 5% 7% 59%	2458
	21st to end	Night	4 5% 1 3 2 2 3 3 5% 5% 66%	2076
March	1st to 10th	Night	2 4 1 4 3 3 4 3 5% 7% 64%	5494
	11th to 20th	Night	7% 4 1 3 2 2 3 2 3 5% 67%	5375
	21st to end	Night	3 3 1 3 2 3 4 3 5% 7% 66%	5094
April	1st to 10th	Night	4 6% 2 4 4 3 2 3 5% 6% 61%	2100
	11th to 20th	Night	5% 4 1 4 3 4 3 3 5% 6% 62%	2077
	21st to end	Night	4 5% 2 3 2 2 2 3 5% 7% 66%	2100
May	1st to 10th	Night	5% 7% 2 5% 3 3 3 3 4 5% 59%	5890
	11th to 20th	Night	6% 4 1 4 3 3 4 3 6% 7% 60%	5723
	21st to end	Night	5% 5% 1 4 3 4 4 4 6% 7% 56%	5803
June	1st to 10th	Night	2 2 1 3 4 3 5% 5% 8% 12% 56%	1890
	11th to 20th	Night	3 3 1 4 4 4 4 5% 7% 9% 58%	1890
	21st to end	Night	4 3 1 4 3 3 4 5% 6% 10% 56%	1890
July	1st to 10th	Night	5% 3 1 3 2 3 4 4 8% 13% 52%	5603
	11th to 20th	Night	5% 4 1 4 3 3 4 4 6% 10% 57%	5910
	21st to end	Night	2 2 1 2 3 4 4 4 6% 10% 63%	6423
August	1st to 10th	Night	1 1 1 1 1 2 3 6% 12% 72%	4564
	11th to 20th	Night	2 1 1 3 2 3 4 4 8% 15% 58%	4583
	21st to end	Night	3 2 1 3 2 3 4 3 6% 11% 61%	5083
September	1st to 10th	Night	6% 5% 2 5% 5% 4 4 4 6% 9% 50%	2000
	11th to 20th	Night	7% 4 1 4 3 4 5% 4 6% 10% 50%	2162
	21st to end	Night	6% 4 1 4 3 3 4 4 6% 9% 54%	2200
October	1st to 10th	Night	2 2 1 2 2 2 2 2 3 5% 77%	3590
	11th to 20th	Night	5% 4 1 4 2 2 3 2 4 5% 67%	3650
	21st to end	Night	3 2 1 2 1 2 2 2 3 4 79%	4252
November	1st to 10th	Night	6% 3 1 2 2 2 2 2 3 5% 72%	2565
	11th to 20th	Night	4 3 1 3 3 2 3 3 5% 7% 67%	2730
	21st to end	Night	5% 5% 1 4 3 3 3 2 5% 6% 63%	2730
December	1st to 10th	Night	10% 9% 2 6% 4 3 3 3 4 6% 51%	4380
	11th to 20th	Night	3 4 1 3 2 2 3 2 3 6% 70%	4946
	21st to end	Night	2 3 1 3 2 2 2 2 3 6% 74%	6633

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	15% 2 5% 21% 30% 28%	5888
	11th to 20th	Day	16% 2 4 17% 35% 26%	5910
	21st to end	Day	24% 3 3 16% 29% 25%	6468
February	1st to 10th	Day	20% 3 6% 13% 24% 34%	2100
	11th to 20th	Day	21% 4 6% 15% 26% 28%	2059
	21st to end	Day	17% 3 5% 13% 29% 34%	1730
March	1st to 10th	Day	20% 4 4 8% 20% 44%	4600
	11th to 20th	Day	19% 4 5% 10% 20% 42%	4797
	21st to end	Day	20% 4 5% 9% 24% 38%	6101
April	1st to 10th	Day	19% 3 7% 13% 24% 34%	2520
	11th to 20th	Day	21% 4 6% 12% 24% 32%	2494
	21st to end	Day	18% 3 6% 15% 27% 30%	2520
May	1st to 10th	Day	19% 3 7% 17% 21% 34%	7057
	11th to 20th	Day	20% 4 5% 11% 25% 34%	7154
	21st to end	Day	23% 4 9% 23% 17% 24%	8405
June	1st to 10th	Day	16% 7% 13% 26% 24% 14%	2730
	11th to 20th	Day	20% 5% 9% 18% 31% 16%	2730
	21st to end	Day	26% 8% 12% 21% 21% 11%	2730
July	1st to 10th	Day	26% 7% 15% 30% 21% 2	7306
	11th to 20th	Day	33% 6% 16% 29% 13% 2	7017
	21st to end	Day	19% 5% 14% 31% 26% 5%	7516
August	1st to 10th	Day	16% 6% 10% 32% 32% 4	4012
	11th to 20th	Day	25% 7% 15% 31% 19% 4	4002
	21st to end	Day	28% 7% 15% 26% 20% 3	4420
September	1st to 10th	Day	36% 7% 13% 19% 14% 10%	2400
	11th to 20th	Day	33% 9% 13% 19% 20% 5%	2235
	21st to end	Day	29% 7% 9% 20% 22% 13%	2200
October	1st to 10th	Day	10% 4 8% 14% 24% 39%	3590
	11th to 20th	Day	21% 3 7% 18% 19% 32%	3486
	21st to end	Day	10% 1 3 13% 45% 27%	3548
November	1st to 10th	Day	16% 3 6% 18% 29% 28%	1928
	11th to 20th	Day	19% 3 7% 18% 27% 26%	1890
	21st to end	Day	24% 3 6% 16% 28% 24%	1890
December	1st to 10th	Day	32% 2 5% 18% 22% 21%	3030
	11th to 20th	Day	17% 3 4 21% 31% 25%	3479
	21st to end	Day	12% 2 3 13% 35% 34%	4613
January	1st to 10th	Night	15% 2 4 20% 32% 28%	7041
	11th to 20th	Night	15% 2 4 16% 29% 35%	7080
	21st to end	Night	27% 3 4 16% 27% 23%	7746
February	1st to 10th	Night	21% 3 6% 13% 25% 33%	2520
	11th to 20th	Night	26% 4 6% 14% 22% 28%	2458
	21st to end	Night	19% 3 6% 8% 29% 34%	2076
March	1st to 10th	Night	19% 4 4 11% 19% 43%	5494
	11th to 20th	Night	20% 4 6% 9% 18% 43%	5375
	21st to end	Night	17% 4 7% 12% 22% 38%	5094
April	1st to 10th	Night	25% 4 7% 12% 21% 31%	2100
	11th to 20th	Night	23% 4 9% 15% 20% 28%	2077
	21st to end	Night	19% 3 9% 17% 26% 26%	2100
May	1st to 10th	Night	27% 3 9% 18% 16% 27%	5888
	11th to 20th	Night	23% 5% 6% 17% 24% 25%	5724
	21st to end	Night	26% 5% 12% 22% 13% 21%	5805
June	1st to 10th	Night	19% 9% 16% 29% 21% 8%	1890
	11th to 20th	Night	20% 7% 13% 20% 29% 10%	1890
	21st to end	Night	23% 8% 14% 24% 22% 10%	1890
July	1st to 10th	Night	23% 6% 21% 33% 16% 1	5554
	11th to 20th	Night	25% 6% 19% 34% 14% 2	5850
	21st to end	Night	17% 6% 18% 33% 23% 3	6280
August	1st to 10th	Night	10% 4 12% 39% 32% 2	3344
	11th to 20th	Night	14% 7% 24% 39% 14% 2	3343
	21st to end	Night	20% 7% 19% 34% 17% 3	3687
September	1st to 10th	Night	30% 7% 16% 24% 14% 9%	2000
	11th to 20th	Night	29% 8% 19% 21% 17% 7%	2162
	21st to end	Night	26% 7% 13% 22% 20% 11%	2200
October	1st to 10th	Night	12% 4 9% 14% 27% 35%	3590
	11th to 20th	Night	22% 3 10% 15% 18% 32%	3650
	21st to end	Night	13% 2 4 18% 38% 25%	4252
November	1st to 10th	Night	17% 3 6% 16% 31% 27%	2565
	11th to 20th	Night	19% 4 7% 18% 27% 26%	2730
	21st to end	Night	23% 3 8% 15% 27% 25%	2730
December	1st to 10th	Night	36% 2 5% 16% 22% 19%	4380
	11th to 20th	Night	18% 2 3 17% 33% 26%	4947
	21st to end	Night	14% 3 4 13% 37% 30%	6633

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% 29% 11% 2	330
	11th to 20th	Day	11% 13% 29% 34% 12% 1	330
	21st to end	Day	8% 15% 36% 27% 13% 1	363
February	1st to 10th	Day	8% 14% 28% 26% 19% 5%	330
	11th to 20th	Day	11% 14% 35% 28% 10% 2	330
	21st to end	Day	6% 17% 41% 25% 9% 2	273
March	1st to 10th	Day	9% 13% 39% 26% 12% 2	330
	11th to 20th	Day	10% 18% 37% 24% 8% 3	377
	21st to end	Day	8% 21% 38% 24% 8% 2	594
April	1st to 10th	Day	8% 18% 43% 22% 7% 1	540
	11th to 20th	Day	6% 20% 45% 20% 7% 1	532
	21st to end	Day	6% 16% 42% 30% 5%	540
May	1st to 10th	Day	5% 16% 39% 29% 9% 1	543
	11th to 20th	Day	6% 9% 36% 34% 14% 1	609
	21st to end	Day	5% 6% 29% 42% 16% 1	844
June	1st to 10th	Day	3 2 22% 41% 28% 4	780
	11th to 20th	Day	3 1 17% 42% 32% 4	780
	21st to end	Day	5% 1 12% 36% 40% 7%	780
July	1st to 10th	Day	5% 1 10% 28% 43% 13%	669
	11th to 20th	Day	2 1 9% 39% 38% 11%	564
	21st to end	Day	2 1 8% 40% 37% 12%	619
August	1st to 10th	Day	2 1 7% 35% 40% 15%	567
	11th to 20th	Day	6% 1 8% 33% 36% 16%	557
	21st to end	Day	5% 1 7% 33% 38% 16%	620
September	1st to 10th	Day	7% 2 8% 40% 33% 10%	530
	11th to 20th	Day	10% 1 4 27% 43% 15%	365
	21st to end	Day	9% 1 9% 32% 32% 17%	330
October	1st to 10th	Day	5% 2 15% 32% 36% 11%	330
	11th to 20th	Day	7% 5% 18% 36% 23% 11%	330
	21st to end	Day	7% 7% 23% 31% 23% 8%	363
November	1st to 10th	Day	9% 6% 22% 38% 17% 9%	330
	11th to 20th	Day	8% 12% 25% 27% 21% 7%	330
	21st to end	Day	8% 16% 31% 27% 16% 2	330
December	1st to 10th	Day	10% 12% 23% 35% 18% 2	330
	11th to 20th	Day	5% 10% 35% 32% 15% 4	330
	21st to end	Day	5% 11% 33% 34% 14% 4	363
January	1st to 10th	Night	8% 6% 27% 44% 15% 1	843
	11th to 20th	Night	8% 7% 24% 42% 17% 1	840
	21st to end	Night	7% 8% 28% 44% 12% 1	924
February	1st to 10th	Night	8% 9% 24% 42% 17% 1	840
	11th to 20th	Night	8% 6% 33% 42% 11%	824
	21st to end	Night	8% 8% 38% 41% 6%	692
March	1st to 10th	Night	7% 9% 35% 38% 10% 2	840
	11th to 20th	Night	8% 11% 32% 34% 13% 1	818
	21st to end	Night	5% 8% 38% 37% 11% 1	693
April	1st to 10th	Night	6% 9% 38% 39% 9%	630
	11th to 20th	Night	5% 10% 41% 38% 5%	623
	21st to end	Night	5% 10% 37% 41% 7% 1	630
May	1st to 10th	Night	5% 6% 37% 41% 10%	630
	11th to 20th	Night	7% 5% 34% 43% 11%	621
	21st to end	Night	5% 4 35% 42% 14%	683
June	1st to 10th	Night	2 3 25% 51% 19%	630
	11th to 20th	Night	3 1 23% 50% 22%	630
	21st to end	Night	5% 1 17% 50% 27%	630
July	1st to 10th	Night	6% 1 13% 44% 35% 2	615
	11th to 20th	Night	4 1 11% 47% 35% 2	612
	21st to end	Night	2 8% 49% 39% 1	690
August	1st to 10th	Night	2 6% 48% 42% 3	627
	11th to 20th	Night	3 7% 37% 49% 4	624
	21st to end	Night	4 7% 44% 44% 1	687
September	1st to 10th	Night	9% 1 7% 41% 40% 3	600
	11th to 20th	Night	8% 1 9% 43% 39% 1	600
	21st to end	Night	7% 1 12% 44% 34% 3	600
October	1st to 10th	Night	4 2 16% 46% 32% 1	600
	11th to 20th	Night	9% 3 17% 48% 21% 3	643
	21st to end	Night	7% 4 15% 40% 27% 6%	880
November	1st to 10th	Night	8% 3 17% 42% 26% 4	925
	11th to 20th	Night	6% 4 19% 43% 26% 3	1050
	21st to end	Night	8% 6% 25% 39% 18% 4	1050
December	1st to 10th	Night	9% 4 23% 45% 17% 3	1050
	11th to 20th	Night	6% 7% 26% 45% 17% 1	1050
	21st to end	Night	6% 7% 28% 43% 13% 1	1155

Page generated at 2026-08-27 03:07:13. Version ac07

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