Marshall
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Marshall clear sky chart from 2005-11-06 to 2026-01-17.

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	9% 5% 2 5% 4 4 4 3 5% 7% 53%	127453
	Night	9% 5% 2 4 3 3 3 3 5% 7% 57%	124504
Transparency	Day	30% 5% 10% 25% 23% 7%	131170
	Night	27% 4 10% 26% 25% 8%	128850
Seeing	Day	11% 8% 26% 31% 20% 5%	17022
	Night	11% 7% 32% 37% 12% 1	26986

By Month

Percentage Cloud Cover SE (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% 6% 3 8% 4 4 4 3 5% 6% 47%	4167
	11th to 20th	Day	6% 6% 2 6% 3 3 3 3 4 5% 60%	4134
	21st to end	Day	14% 8% 4 6% 3 2 2 2 3 5% 52%	4488
February	1st to 10th	Day	12% 9% 3 7% 4 3 3 2 4 5% 47%	1875
	11th to 20th	Day	10% 7% 2 5% 3 4 3 3 4 6% 53%	1949
	21st to end	Day	10% 9% 3 4 3 3 3 2 4 5% 53%	1650
March	1st to 10th	Day	10% 6% 2 4 3 3 3 2 4 5% 59%	4807
	11th to 20th	Day	6% 5% 2 6% 3 3 3 3 5% 6% 56%	4980
	21st to end	Day	9% 8% 3 7% 5% 4 4 3 5% 8% 44%	5462
April	1st to 10th	Day	9% 4 2 5% 4 4 4 3 6% 7% 51%	2200
	11th to 20th	Day	12% 6% 2 6% 5% 3 3 3 5% 6% 48%	2402
	21st to end	Day	15% 6% 2 5% 4 4 4 4 5% 6% 46%	2600
May	1st to 10th	Day	18% 7% 2 6% 4 4 3 3 5% 6% 42%	5862
	11th to 20th	Day	9% 4 2 5% 4 3 4 3 5% 6% 54%	5862
	21st to end	Day	14% 7% 3 7% 4 5% 5% 3 5% 5% 42%	6434
June	1st to 10th	Day	5% 5% 2 4 4 4 4 5% 7% 9% 50%	2600
	11th to 20th	Day	6% 4 2 4 4 5% 4 4 6% 9% 53%	2600
	21st to end	Day	5% 3 1 4 3 3 3 4 5% 8% 61%	2600
July	1st to 10th	Day	7% 6% 2 6% 5% 5% 5% 5% 7% 9% 44%	5900
	11th to 20th	Day	5% 4 2 5% 4 4 5% 4 6% 9% 53%	5900
	21st to end	Day	3 2 1 4 3 3 4 4 7% 11% 58%	6422
August	1st to 10th	Day	6% 3 2 4 4 4 4 4 6% 9% 55%	4234
	11th to 20th	Day	6% 4 2 5% 3 4 4 4 6% 9% 53%	4171
	21st to end	Day	5% 3 1 4 3 3 3 3 5% 8% 61%	4380
September	1st to 10th	Day	9% 4 2 4 3 3 3 4 4 7% 57%	2200
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 3 5% 7% 50%	2200
	21st to end	Day	8% 4 1 6% 3 4 4 3 4 7% 58%	2200
October	1st to 10th	Day	8% 4 2 4 3 3 4 2 4 5% 62%	2163
	11th to 20th	Day	9% 3 1 3 2 2 2 2 3 5% 68%	2000
	21st to end	Day	13% 4 2 4 3 3 3 2 4 5% 59%	2200
November	1st to 10th	Day	8% 4 2 3 2 3 3 3 4 6% 63%	1933
	11th to 20th	Day	8% 4 1 3 3 3 3 3 3 6% 64%	1890
	21st to end	Day	11% 5% 2 4 3 3 2 3 5% 5% 59%	1890
December	1st to 10th	Day	6% 6% 2 4 3 3 3 2 4 6% 62%	3030
	11th to 20th	Day	8% 5% 2 4 3 3 3 3 4 6% 60%	3481
	21st to end	Day	14% 10% 4 7% 4 4 4 2 3 6% 43%	4587
January	1st to 10th	Night	13% 7% 2 6% 4 3 3 3 5% 5% 50%	6018
	11th to 20th	Night	4 3 1 4 3 4 4 3 5% 6% 62%	5979
	21st to end	Night	15% 7% 2 5% 3 3 3 2 4 5% 52%	6490
February	1st to 10th	Night	13% 8% 2 5% 4 3 3 3 4 5% 50%	2542
	11th to 20th	Night	11% 7% 2 5% 3 3 3 3 4 6% 54%	2338
	21st to end	Night	8% 6% 2 5% 4 3 3 3 4 5% 56%	1980
March	1st to 10th	Night	9% 6% 2 5% 3 3 3 3 4 5% 59%	5052
	11th to 20th	Night	7% 3 1 3 2 2 3 2 4 6% 66%	4980
	21st to end	Night	8% 6% 2 6% 4 4 4 4 5% 7% 50%	5467
April	1st to 10th	Night	10% 5% 2 4 2 3 4 3 4 7% 56%	2200
	11th to 20th	Night	14% 5% 1 5% 3 3 3 2 5% 7% 52%	1943
	21st to end	Night	13% 5% 2 5% 3 2 3 3 5% 8% 51%	1800
May	1st to 10th	Night	17% 7% 2 5% 4 3 3 3 4 5% 47%	4053
	11th to 20th	Night	9% 4 2 4 3 4 4 3 7% 8% 52%	4053
	21st to end	Night	11% 6% 2 4 3 3 3 3 5% 8% 53%	4443
June	1st to 10th	Night	7% 3 1 4 3 3 4 4 8% 8% 56%	1800
	11th to 20th	Night	5% 3 1 4 4 4 4 3 7% 10% 56%	1800
	21st to end	Night	4 2 1 4 2 2 3 3 6% 10% 62%	1800
July	1st to 10th	Night	4 4 2 5% 4 4 5% 5% 8% 11% 50%	4078
	11th to 20th	Night	3 4 1 5% 4 3 4 4 6% 11% 57%	4077
	21st to end	Night	4 2 1 3 3 3 3 3 6% 10% 62%	4449
August	1st to 10th	Night	2 3 1 4 3 4 4 5% 6% 10% 59%	2926
	11th to 20th	Night	5% 3 1 4 4 5% 5% 5% 6% 10% 51%	2888
	21st to end	Night	3 3 1 3 3 3 3 4 6% 9% 62%	3494
September	1st to 10th	Night	8% 4 2 4 4 3 2 2 4 6% 61%	2200
	11th to 20th	Night	9% 6% 2 5% 3 2 5% 4 5% 7% 53%	2200
	21st to end	Night	6% 4 1 4 3 3 3 4 4 7% 60%	2200
October	1st to 10th	Night	7% 3 1 4 3 3 3 3 4 6% 63%	2215
	11th to 20th	Night	10% 3 1 4 2 2 2 2 3 5% 66%	2400
	21st to end	Night	12% 3 1 3 2 2 3 2 4 4 63%	2640
November	1st to 10th	Night	9% 3 1 2 2 2 3 2 4 5% 66%	2580
	11th to 20th	Night	7% 4 2 4 2 3 3 3 4 5% 63%	2730
	21st to end	Night	12% 6% 2 3 3 2 2 2 4 7% 58%	2730
December	1st to 10th	Night	6% 4 1 4 2 2 2 2 3 6% 67%	4380
	11th to 20th	Night	8% 4 2 5% 3 3 3 2 4 5% 61%	4946
	21st to end	Night	14% 8% 2 5% 3 3 3 3 4 5% 51%	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	38% 4 11% 26% 19% 3	4168
	11th to 20th	Day	27% 2 11% 21% 31% 7%	4134
	21st to end	Day	38% 2 6% 21% 29% 5%	4488
February	1st to 10th	Day	41% 3 10% 23% 19% 4	1875
	11th to 20th	Day	35% 4 7% 28% 20% 6%	1959
	21st to end	Day	34% 3 5% 23% 24% 10%	1650
March	1st to 10th	Day	29% 3 5% 17% 22% 24%	4807
	11th to 20th	Day	29% 4 8% 18% 24% 17%	4980
	21st to end	Day	38% 5% 11% 29% 15% 3	5464
April	1st to 10th	Day	32% 6% 10% 25% 20% 7%	2200
	11th to 20th	Day	36% 5% 9% 23% 19% 7%	2402
	21st to end	Day	38% 5% 11% 25% 18% 2	2600
May	1st to 10th	Day	43% 6% 10% 18% 18% 4	5900
	11th to 20th	Day	29% 5% 11% 23% 22% 10%	5866
	21st to end	Day	42% 7% 9% 17% 18% 7%	6447
June	1st to 10th	Day	28% 7% 17% 28% 17% 4	2600
	11th to 20th	Day	26% 7% 14% 26% 21% 6%	2600
	21st to end	Day	21% 6% 14% 27% 25% 6%	2600
July	1st to 10th	Day	33% 8% 14% 23% 19% 4	5836
	11th to 20th	Day	26% 6% 10% 30% 25% 4	5831
	21st to end	Day	18% 7% 16% 37% 20% 2	6240
August	1st to 10th	Day	25% 5% 11% 28% 25% 5%	4220
	11th to 20th	Day	26% 7% 16% 29% 19% 3	4210
	21st to end	Day	23% 5% 13% 37% 18% 5%	4379
September	1st to 10th	Day	28% 4 10% 25% 24% 10%	2200
	11th to 20th	Day	33% 7% 8% 24% 22% 6%	2200
	21st to end	Day	26% 4 9% 23% 30% 8%	2200
October	1st to 10th	Day	16% 3 5% 17% 44% 15%	3527
	11th to 20th	Day	15% 3 6% 24% 38% 15%	3240
	21st to end	Day	23% 3 7% 29% 30% 9%	3538
November	1st to 10th	Day	23% 4 8% 25% 31% 11%	1933
	11th to 20th	Day	23% 3 8% 28% 28% 10%	1890
	21st to end	Day	29% 3 7% 30% 24% 7%	1890
December	1st to 10th	Day	25% 3 6% 40% 24% 2	3030
	11th to 20th	Day	26% 3 6% 28% 30% 6%	3479
	21st to end	Day	44% 3 7% 26% 16% 4	4587
January	1st to 10th	Night	36% 3 9% 29% 19% 3	6018
	11th to 20th	Night	22% 4 11% 21% 34% 8%	5979
	21st to end	Night	36% 3 7% 19% 30% 6%	6490
February	1st to 10th	Night	37% 3 9% 23% 21% 6%	2542
	11th to 20th	Night	33% 3 10% 26% 22% 6%	2338
	21st to end	Night	31% 4 6% 20% 27% 12%	1980
March	1st to 10th	Night	29% 3 5% 17% 26% 20%	5052
	11th to 20th	Night	21% 3 6% 22% 24% 24%	4980
	21st to end	Night	32% 5% 11% 31% 19% 3	5467
April	1st to 10th	Night	28% 4 11% 21% 25% 12%	2200
	11th to 20th	Night	32% 5% 8% 26% 19% 10%	1943
	21st to end	Night	32% 5% 12% 26% 22% 4	1800
May	1st to 10th	Night	40% 6% 11% 17% 23% 5%	4080
	11th to 20th	Night	28% 5% 11% 23% 24% 9%	4054
	21st to end	Night	30% 7% 12% 23% 20% 9%	4452
June	1st to 10th	Night	24% 7% 18% 30% 17% 5%	1800
	11th to 20th	Night	23% 6% 14% 31% 21% 5%	1800
	21st to end	Night	17% 5% 14% 31% 21% 10%	1800
July	1st to 10th	Night	25% 8% 19% 28% 17% 3	4035
	11th to 20th	Night	21% 5% 10% 32% 27% 4	4026
	21st to end	Night	18% 5% 15% 37% 23% 2	4320
August	1st to 10th	Night	18% 6% 14% 29% 25% 8%	2916
	11th to 20th	Night	27% 6% 13% 29% 22% 4	2915
	21st to end	Night	19% 7% 13% 36% 20% 5%	3494
September	1st to 10th	Night	25% 4 8% 27% 25% 11%	2200
	11th to 20th	Night	30% 5% 10% 23% 24% 8%	2200
	21st to end	Night	23% 4 7% 25% 32% 9%	2200
October	1st to 10th	Night	16% 3 6% 13% 41% 21%	3618
	11th to 20th	Night	15% 2 5% 22% 37% 18%	3899
	21st to end	Night	21% 2 6% 26% 35% 11%	4252
November	1st to 10th	Night	22% 3 8% 23% 32% 12%	2580
	11th to 20th	Night	25% 3 6% 26% 29% 11%	2730
	21st to end	Night	29% 3 7% 24% 28% 9%	2730
December	1st to 10th	Night	21% 2 9% 41% 24% 3	4380
	11th to 20th	Night	27% 3 7% 26% 29% 8%	4947
	21st to end	Night	36% 4 6% 28% 20% 6%	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	14% 9% 38% 27% 12% 1	330
	11th to 20th	Day	14% 18% 33% 27% 6% 1	318
	21st to end	Day	11% 12% 45% 27% 5%	330
February	1st to 10th	Day	17% 16% 36% 25% 6%	300
	11th to 20th	Day	17% 15% 43% 19% 5%	300
	21st to end	Day	15% 18% 42% 21% 3	249
March	1st to 10th	Day	13% 19% 39% 21% 7% 1	300
	11th to 20th	Day	16% 19% 39% 19% 7%	300
	21st to end	Day	17% 15% 41% 20% 7%	330
April	1st to 10th	Day	14% 19% 37% 22% 9%	300
	11th to 20th	Day	16% 17% 42% 22% 3	520
	21st to end	Day	18% 13% 36% 22% 9% 2	700
May	1st to 10th	Day	15% 10% 38% 26% 10% 1	703
	11th to 20th	Day	16% 10% 29% 26% 17% 2	725
	21st to end	Day	15% 7% 29% 29% 17% 3	789
June	1st to 10th	Day	7% 4 20% 38% 25% 6%	730
	11th to 20th	Day	6% 3 25% 40% 22% 3	730
	21st to end	Day	4 2 22% 45% 23% 5%	730
July	1st to 10th	Day	7% 2 13% 33% 34% 12%	721
	11th to 20th	Day	3 2 12% 40% 35% 8%	718
	21st to end	Day	6% 1 6% 37% 38% 12%	794
August	1st to 10th	Day	7% 1 11% 43% 31% 7%	726
	11th to 20th	Day	6% 2 13% 34% 37% 9%	715
	21st to end	Day	6% 1 11% 33% 39% 10%	638
September	1st to 10th	Day	8% 1 10% 34% 36% 11%	330
	11th to 20th	Day	7% 2 13% 36% 32% 10%	330
	21st to end	Day	11% 5% 18% 35% 22% 9%	330
October	1st to 10th	Day	8% 4 25% 33% 22% 8%	330
	11th to 20th	Day	8% 9% 34% 33% 15% 2	330
	21st to end	Day	20% 11% 27% 24% 14% 3	363
November	1st to 10th	Day	11% 8% 35% 32% 12% 2	330
	11th to 20th	Day	11% 13% 34% 27% 12% 3	330
	21st to end	Day	16% 13% 33% 25% 12% 2	330
December	1st to 10th	Day	10% 10% 39% 30% 8% 2	330
	11th to 20th	Day	13% 10% 38% 26% 12% 2	330
	21st to end	Day	13% 16% 38% 25% 7%	363
January	1st to 10th	Night	11% 9% 39% 34% 7%	1050
	11th to 20th	Night	10% 11% 39% 34% 6%	1031
	21st to end	Night	12% 9% 38% 36% 5%	1100
February	1st to 10th	Night	16% 10% 39% 30% 4	942
	11th to 20th	Night	14% 13% 42% 28% 4	784
	21st to end	Night	12% 13% 40% 31% 3	660
March	1st to 10th	Night	12% 10% 39% 36% 3	660
	11th to 20th	Night	17% 10% 36% 31% 5%	600
	21st to end	Night	15% 12% 40% 27% 6%	660
April	1st to 10th	Night	13% 11% 43% 29% 5%	600
	11th to 20th	Night	16% 11% 42% 27% 4	593
	21st to end	Night	15% 13% 39% 27% 6% 1	600
May	1st to 10th	Night	11% 10% 40% 31% 7%	600
	11th to 20th	Night	12% 6% 33% 33% 15% 1	591
	21st to end	Night	9% 6% 33% 37% 14% 2	650
June	1st to 10th	Night	9% 3 26% 43% 19% 2	600
	11th to 20th	Night	5% 4 30% 45% 14% 1	600
	21st to end	Night	2 2 28% 52% 16% 1	600
July	1st to 10th	Night	5% 2 15% 46% 31% 2	585
	11th to 20th	Night	3 1 15% 48% 32% 2	582
	21st to end	Night	6% 1 10% 51% 32% 2	657
August	1st to 10th	Night	3 15% 52% 27% 2	597
	11th to 20th	Night	6% 1 13% 49% 29% 2	594
	21st to end	Night	3 1 11% 44% 38% 2	660
September	1st to 10th	Night	9% 2 15% 47% 26% 2	600
	11th to 20th	Night	12% 1 21% 41% 24% 2	600
	21st to end	Night	8% 3 24% 42% 21% 3	600
October	1st to 10th	Night	9% 3 28% 39% 20% 2	615
	11th to 20th	Night	10% 5% 39% 34% 10% 1	800
	21st to end	Night	13% 9% 33% 34% 11% 1	880
November	1st to 10th	Night	10% 6% 36% 37% 10% 1	940
	11th to 20th	Night	9% 10% 35% 36% 8% 1	1050
	21st to end	Night	14% 11% 32% 33% 10%	1050
December	1st to 10th	Night	12% 8% 37% 35% 8%	1050
	11th to 20th	Night	14% 8% 39% 33% 6%	1050
	21st to end	Night	13% 11% 36% 36% 5%	1155

Page generated at 2026-01-14 00:14:26. Version ac07

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