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 2026-01-02.

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	8% 6% 2 5% 3 3 3 3 5% 9% 51%	135826
	Night	10% 9% 3 7% 4 3 3 3 4 6% 48%	133007
Transparency	Day	28% 7% 11% 19% 21% 15%	130725
	Night	34% 6% 9% 17% 20% 14%	129110
Seeing	Day	11% 10% 24% 30% 19% 7%	16883
	Night	12% 6% 17% 30% 28% 7%	26223

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	14% 6% 2 6% 4 3 4 3 6% 9% 43%	3986
	11th to 20th	Day	15% 7% 3 7% 4 4 3 3 4 5% 46%	4100
	21st to end	Day	19% 7% 3 7% 4 4 3 3 5% 6% 40%	4510
February	1st to 10th	Day	19% 6% 3 4 4 4 4 4 4 4 44%	1800
	11th to 20th	Day	14% 8% 3 7% 6% 4 4 3 5% 9% 38%	1786
	21st to end	Day	16% 7% 2 4 3 4 3 3 5% 7% 46%	1660
March	1st to 10th	Day	14% 10% 3 8% 5% 4 4 3 5% 8% 36%	4952
	11th to 20th	Day	13% 6% 2 5% 3 3 4 4 6% 8% 46%	5000
	21st to end	Day	15% 9% 2 5% 5% 4 4 4 7% 10% 34%	5486
April	1st to 10th	Day	12% 8% 3 6% 5% 4 4 4 5% 6% 42%	2399
	11th to 20th	Day	8% 6% 3 6% 4 4 4 4 5% 7% 50%	2369
	21st to end	Day	7% 5% 2 4 3 3 3 4 6% 12% 53%	2400
May	1st to 10th	Day	5% 3 1 3 2 2 2 2 3 7% 71%	5805
	11th to 20th	Day	3 3 1 4 3 2 3 3 5% 9% 65%	5874
	21st to end	Day	2 5% 2 4 2 2 2 2 3 6% 70%	6902
June	1st to 10th	Day	6% 7% 2 4 3 3 3 3 6% 9% 52%	2800
	11th to 20th	Day	1 4 2 3 2 2 3 3 5% 9% 66%	2800
	21st to end	Day	2 5% 2 5% 3 3 3 2 5% 8% 62%	2800
July	1st to 10th	Day	1 4 2 3 2 2 2 2 4 10% 67%	6238
	11th to 20th	Day	1 6% 3 6% 3 4 4 4 8% 11% 50%	5880
	21st to end	Day	1 13% 6% 7% 4 4 5% 3 6% 11% 40%	6402
August	1st to 10th	Day	1 4 2 6% 3 3 4 4 7% 13% 54%	5777
	11th to 20th	Day	1 8% 3 5% 3 3 4 3 5% 9% 56%	5634
	21st to end	Day	2 5% 3 7% 3 3 4 3 7% 12% 49%	5984
September	1st to 10th	Day	2 7% 1 5% 3 3 3 3 6% 9% 59%	2400
	11th to 20th	Day	5% 7% 3 5% 4 4 4 4 5% 9% 51%	2374
	21st to end	Day	4 4 1 3 2 3 3 3 4 7% 65%	2115
October	1st to 10th	Day	4 4 1 3 2 2 3 2 4 8% 67%	3280
	11th to 20th	Day	9% 6% 2 5% 3 3 3 3 5% 8% 54%	3261
	21st to end	Day	8% 6% 2 5% 3 3 4 4 6% 8% 51%	3558
November	1st to 10th	Day	9% 5% 2 5% 4 3 4 3 5% 9% 51%	2059
	11th to 20th	Day	14% 7% 3 6% 4 3 3 4 5% 7% 43%	1855
	21st to end	Day	11% 6% 2 4 3 4 5% 3 6% 7% 50%	1680
December	1st to 10th	Day	9% 4 2 4 3 3 4 4 7% 10% 49%	2700
	11th to 20th	Day	25% 8% 3 5% 5% 4 4 5% 6% 7% 27%	3108
	21st to end	Day	27% 10% 3 6% 4 3 4 3 5% 7% 28%	4092
January	1st to 10th	Night	12% 7% 2 6% 4 4 4 4 6% 9% 42%	5995
	11th to 20th	Night	15% 7% 3 6% 4 4 4 3 5% 6% 43%	5880
	21st to end	Night	18% 10% 3 6% 4 3 4 3 5% 6% 40%	6468
February	1st to 10th	Night	19% 7% 2 5% 3 4 3 2 5% 5% 46%	2600
	11th to 20th	Night	14% 7% 3 7% 5% 4 4 4 6% 7% 40%	2498
	21st to end	Night	14% 7% 1 4 3 4 3 3 5% 5% 50%	1969
March	1st to 10th	Night	16% 10% 4 8% 4 4 4 3 5% 6% 36%	4942
	11th to 20th	Night	15% 7% 3 6% 3 3 3 2 4 6% 47%	5000
	21st to end	Night	15% 12% 3 6% 4 3 3 3 4 8% 39%	5489
April	1st to 10th	Night	10% 9% 3 5% 5% 3 3 2 5% 8% 46%	2000
	11th to 20th	Night	9% 9% 3 6% 3 3 3 3 4 6% 52%	1979
	21st to end	Night	6% 6% 2 5% 4 3 3 3 5% 8% 57%	2000
May	1st to 10th	Night	5% 5% 1 3 2 2 1 2 3 5% 71%	4174
	11th to 20th	Night	3 5% 4 8% 3 3 3 2 3 5% 60%	4035
	21st to end	Night	3 6% 2 3 2 2 1 1 2 4 74%	3972
June	1st to 10th	Night	5% 8% 2 6% 3 3 4 4 5% 10% 49%	1600
	11th to 20th	Night	2 5% 2 4 3 1 3 2 4 6% 67%	1600
	21st to end	Night	4 7% 3 5% 2 2 3 2 3 7% 63%	1600
July	1st to 10th	Night	1 8% 4 8% 3 2 2 1 2 5% 63%	3721
	11th to 20th	Night	1 22% 7% 9% 2 3 2 2 3 6% 43%	4100
	21st to end	Night	1 25% 6% 9% 4 4 3 2 4 6% 35%	4472
August	1st to 10th	Night	2 8% 8% 12% 3 3 2 2 3 5% 51%	4035
	11th to 20th	Night	14% 4 9% 4 3 3 2 3 5% 53%	4208
	21st to end	Night	2 11% 7% 11% 4 3 3 3 4 6% 45%	5015
September	1st to 10th	Night	1 7% 3 6% 4 4 3 3 4 7% 57%	2000
	11th to 20th	Night	6% 10% 3 7% 4 4 3 3 4 7% 50%	2022
	21st to end	Night	4 7% 2 6% 3 2 2 3 4 6% 60%	2284
October	1st to 10th	Night	4 4 2 5% 2 2 3 2 5% 6% 66%	3920
	11th to 20th	Night	6% 11% 5% 9% 3 3 3 2 3 5% 52%	3897
	21st to end	Night	10% 5% 1 5% 3 3 3 3 5% 7% 55%	4253
November	1st to 10th	Night	10% 6% 2 6% 4 3 4 3 5% 7% 49%	2460
	11th to 20th	Night	12% 8% 3 5% 4 4 4 3 6% 7% 44%	2757
	21st to end	Night	11% 4 1 6% 4 4 4 4 6% 7% 50%	2940
December	1st to 10th	Night	10% 4 2 5% 4 4 3 3 6% 9% 49%	4700
	11th to 20th	Night	26% 8% 2 6% 4 4 4 4 7% 7% 28%	5316
	21st to end	Night	31% 8% 2 6% 4 3 3 2 3 5% 31%	7106

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	36% 9% 18% 18% 12% 6%	3986
	11th to 20th	Day	40% 6% 8% 12% 19% 15%	4100
	21st to end	Day	44% 5% 8% 12% 14% 17%	4510
February	1st to 10th	Day	41% 6% 8% 19% 18% 8%	1800
	11th to 20th	Day	42% 7% 10% 18% 17% 6%	1795
	21st to end	Day	37% 5% 9% 26% 15% 7%	1660
March	1st to 10th	Day	45% 7% 10% 19% 18% 2	4952
	11th to 20th	Day	33% 8% 17% 24% 9% 8%	5000
	21st to end	Day	41% 10% 17% 22% 9% 1	5486
April	1st to 10th	Day	40% 7% 13% 20% 16% 3	2399
	11th to 20th	Day	32% 7% 14% 23% 19% 5%	2372
	21st to end	Day	24% 6% 17% 28% 20% 4	2400
May	1st to 10th	Day	14% 5% 10% 25% 32% 15%	5805
	11th to 20th	Day	15% 8% 15% 30% 26% 7%	5877
	21st to end	Day	14% 7% 9% 32% 36% 3	6904
June	1st to 10th	Day	26% 7% 13% 26% 23% 5%	2800
	11th to 20th	Day	14% 9% 11% 19% 33% 14%	2800
	21st to end	Day	18% 7% 11% 14% 28% 23%	2800
July	1st to 10th	Day	12% 3 7% 11% 20% 47%	6171
	11th to 20th	Day	18% 4 14% 9% 14% 40%	5811
	21st to end	Day	25% 11% 15% 9% 18% 23%	6217
August	1st to 10th	Day	23% 5% 12% 12% 22% 26%	4206
	11th to 20th	Day	16% 7% 11% 12% 33% 21%	4081
	21st to end	Day	21% 6% 9% 15% 27% 21%	4311
September	1st to 10th	Day	18% 7% 8% 15% 23% 29%	2400
	11th to 20th	Day	26% 9% 10% 20% 25% 10%	2374
	21st to end	Day	17% 7% 9% 19% 28% 19%	2115
October	1st to 10th	Day	17% 3 7% 13% 25% 36%	3280
	11th to 20th	Day	27% 7% 7% 20% 25% 14%	3261
	21st to end	Day	28% 6% 11% 22% 23% 10%	3558
November	1st to 10th	Day	30% 6% 11% 26% 17% 9%	2059
	11th to 20th	Day	40% 5% 12% 21% 15% 6%	1855
	21st to end	Day	32% 6% 11% 20% 19% 11%	1680
December	1st to 10th	Day	28% 5% 9% 28% 24% 7%	2700
	11th to 20th	Day	54% 6% 9% 15% 14% 3	3108
	21st to end	Day	54% 6% 13% 17% 7% 2	4092
January	1st to 10th	Night	35% 10% 20% 19% 12% 5%	5995
	11th to 20th	Night	39% 7% 9% 15% 17% 13%	5880
	21st to end	Night	42% 6% 8% 10% 18% 16%	6468
February	1st to 10th	Night	40% 5% 9% 19% 17% 11%	2600
	11th to 20th	Night	41% 6% 10% 19% 17% 7%	2500
	21st to end	Night	34% 5% 11% 25% 18% 6%	1969
March	1st to 10th	Night	45% 8% 10% 19% 16% 3	4942
	11th to 20th	Night	36% 7% 11% 21% 17% 8%	5000
	21st to end	Night	40% 11% 14% 22% 11% 2	5489
April	1st to 10th	Night	34% 8% 11% 22% 20% 4	2000
	11th to 20th	Night	32% 6% 11% 23% 20% 8%	1979
	21st to end	Night	25% 5% 10% 31% 23% 6%	2000
May	1st to 10th	Night	17% 3 6% 22% 35% 17%	4171
	11th to 20th	Night	23% 7% 7% 27% 26% 11%	4036
	21st to end	Night	15% 4 7% 30% 40% 4	3972
June	1st to 10th	Night	28% 9% 10% 18% 29% 6%	1600
	11th to 20th	Night	17% 6% 8% 18% 34% 18%	1600
	21st to end	Night	20% 6% 5% 12% 29% 28%	1600
July	1st to 10th	Night	21% 3 6% 7% 19% 44%	3680
	11th to 20th	Night	39% 3 6% 5% 10% 36%	4046
	21st to end	Night	45% 4 8% 6% 13% 23%	4345
August	1st to 10th	Night	42% 4 10% 8% 19% 18%	2929
	11th to 20th	Night	24% 5% 7% 6% 34% 24%	3046
	21st to end	Night	30% 5% 9% 10% 26% 21%	3608
September	1st to 10th	Night	22% 6% 6% 9% 27% 30%	2000
	11th to 20th	Night	31% 6% 6% 19% 25% 12%	2022
	21st to end	Night	22% 5% 6% 19% 25% 22%	2284
October	1st to 10th	Night	18% 3 6% 10% 22% 40%	3920
	11th to 20th	Night	32% 8% 5% 16% 20% 19%	3897
	21st to end	Night	28% 5% 5% 21% 24% 17%	4253
November	1st to 10th	Night	32% 5% 10% 24% 17% 11%	2460
	11th to 20th	Night	37% 7% 11% 23% 15% 7%	2757
	21st to end	Night	32% 5% 11% 19% 22% 11%	2940
December	1st to 10th	Night	30% 5% 7% 24% 23% 10%	4700
	11th to 20th	Night	54% 4 9% 16% 13% 5%	5316
	21st to end	Night	57% 5% 10% 17% 9% 2	7106

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	26% 11% 24% 22% 14% 3	346
	11th to 20th	Day	21% 7% 24% 29% 17% 3	400
	21st to end	Day	16% 8% 24% 29% 20% 3	440
February	1st to 10th	Day	19% 11% 23% 25% 19% 5%	400
	11th to 20th	Day	18% 15% 27% 23% 14% 2	411
	21st to end	Day	17% 10% 27% 29% 16% 1	414
March	1st to 10th	Day	21% 15% 29% 23% 10% 2	500
	11th to 20th	Day	23% 12% 26% 22% 12% 5%	500
	21st to end	Day	18% 13% 34% 24% 9% 2	550
April	1st to 10th	Day	16% 16% 29% 23% 11% 5%	500
	11th to 20th	Day	14% 13% 32% 23% 13% 5%	492
	21st to end	Day	9% 16% 30% 26% 14% 5%	500
May	1st to 10th	Day	9% 15% 30% 28% 14% 4	503
	11th to 20th	Day	12% 13% 29% 26% 15% 5%	528
	21st to end	Day	10% 12% 28% 26% 16% 8%	573
June	1st to 10th	Day	7% 11% 28% 34% 14% 7%	530
	11th to 20th	Day	4 14% 29% 35% 14% 4	530
	21st to end	Day	2 6% 27% 35% 24% 7%	530
July	1st to 10th	Day	4 6% 23% 40% 21% 6%	526
	11th to 20th	Day	2 2 19% 42% 26% 10%	524
	21st to end	Day	2 5% 15% 45% 23% 10%	575
August	1st to 10th	Day	3 6% 24% 37% 20% 10%	527
	11th to 20th	Day	4 5% 25% 33% 25% 9%	517
	21st to end	Day	5% 4 17% 38% 26% 10%	583
September	1st to 10th	Day	3 6% 20% 34% 27% 10%	530
	11th to 20th	Day	5% 8% 23% 33% 22% 9%	530
	21st to end	Day	4 8% 21% 31% 25% 10%	530
October	1st to 10th	Day	7% 8% 22% 27% 23% 13%	530
	11th to 20th	Day	8% 9% 18% 26% 26% 12%	530
	21st to end	Day	12% 9% 19% 28% 24% 9%	583
November	1st to 10th	Day	10% 9% 19% 27% 27% 9%	535
	11th to 20th	Day	14% 9% 17% 25% 21% 14%	314
	21st to end	Day	12% 7% 20% 30% 22% 9%	220
December	1st to 10th	Day	18% 9% 15% 26% 21% 10%	220
	11th to 20th	Day	23% 8% 20% 23% 20% 6%	220
	21st to end	Day	23% 14% 19% 25% 17% 1	242
January	1st to 10th	Night	19% 6% 17% 30% 23% 5%	804
	11th to 20th	Night	18% 6% 17% 31% 24% 5%	800
	21st to end	Night	18% 6% 14% 31% 27% 4	880
February	1st to 10th	Night	21% 6% 16% 29% 25% 3	800
	11th to 20th	Night	16% 8% 20% 28% 22% 5%	784
	21st to end	Night	16% 9% 19% 33% 20% 3	660
March	1st to 10th	Night	21% 11% 17% 28% 20% 4	800
	11th to 20th	Night	18% 8% 18% 33% 20% 3	800
	21st to end	Night	17% 10% 26% 29% 16% 3	880
April	1st to 10th	Night	13% 8% 27% 30% 18% 4	800
	11th to 20th	Night	11% 10% 26% 28% 19% 5%	791
	21st to end	Night	7% 10% 30% 26% 22% 6%	800
May	1st to 10th	Night	8% 7% 25% 31% 24% 6%	635
	11th to 20th	Night	7% 4 25% 32% 28% 5%	579
	21st to end	Night	8% 9% 25% 30% 22% 5%	433
June	1st to 10th	Night	8% 5% 23% 36% 26% 3	400
	11th to 20th	Night	2 6% 25% 41% 24% 3	400
	21st to end	Night	6% 4 16% 37% 33% 6%	400
July	1st to 10th	Night	3 3 17% 33% 36% 8%	418
	11th to 20th	Night	5% 1 11% 24% 41% 18%	582
	21st to end	Night	5% 3 11% 26% 41% 13%	657
August	1st to 10th	Night	10% 4 14% 27% 33% 11%	597
	11th to 20th	Night	4 3 15% 30% 37% 11%	659
	21st to end	Night	7% 3 12% 27% 41% 10%	880
September	1st to 10th	Night	4 2 13% 28% 42% 12%	800
	11th to 20th	Night	10% 4 14% 29% 33% 10%	800
	21st to end	Night	6% 4 13% 28% 37% 12%	800
October	1st to 10th	Night	8% 4 15% 29% 34% 12%	800
	11th to 20th	Night	9% 5% 14% 25% 37% 11%	800
	21st to end	Night	11% 4 12% 27% 33% 13%	880
November	1st to 10th	Night	11% 5% 14% 26% 34% 10%	820
	11th to 20th	Night	15% 5% 12% 32% 29% 7%	840
	21st to end	Night	12% 4 16% 36% 28% 6%	840
December	1st to 10th	Night	18% 3 15% 32% 25% 6%	840
	11th to 20th	Night	25% 6% 15% 28% 23% 3	840
	21st to end	Night	23% 8% 20% 30% 17% 2	924

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