Marstown Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Marstown Observatory clear sky chart from 2005-11-06 to 2025-12-03.

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	14% 6% 2 6% 4 4 5% 4 6% 8% 40%	127061
	Night	14% 7% 2 6% 4 4 4 4 5% 7% 42%	123645
Transparency	Day	40% 7% 12% 23% 16% 3	130655
	Night	39% 6% 12% 23% 16% 3	128120
Seeing	Day	18% 13% 27% 23% 13% 4	16322
	Night	17% 11% 31% 28% 11% 1	25790

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	14% 5% 2 5% 4 5% 5% 4 6% 5% 45%	3619
	11th to 20th	Day	13% 9% 3 9% 5% 4 3 3 4 5% 42%	4060
	21st to end	Day	15% 9% 3 5% 4 4 5% 3 5% 6% 41%	4466
February	1st to 10th	Day	14% 9% 3 7% 4 5% 5% 5% 5% 6% 38%	1800
	11th to 20th	Day	17% 8% 2 7% 5% 5% 4 3 5% 6% 39%	1805
	21st to end	Day	17% 10% 3 6% 5% 4 3 3 5% 5% 40%	1650
March	1st to 10th	Day	18% 6% 2 4 4 4 4 3 5% 6% 43%	4817
	11th to 20th	Day	11% 6% 3 6% 4 4 4 4 6% 6% 47%	4980
	21st to end	Day	15% 7% 2 6% 5% 5% 5% 4 6% 7% 39%	5588
April	1st to 10th	Day	14% 9% 2 6% 5% 4 4 4 6% 7% 38%	2400
	11th to 20th	Day	13% 6% 2 6% 4 5% 4 4 5% 6% 45%	2374
	21st to end	Day	17% 7% 2 6% 4 4 4 3 5% 7% 42%	2400
May	1st to 10th	Day	22% 7% 3 6% 4 4 5% 4 7% 9% 30%	5789
	11th to 20th	Day	23% 6% 2 7% 4 4 5% 4 6% 8% 30%	6240
	21st to end	Day	15% 6% 2 6% 4 5% 5% 4 7% 8% 38%	6934
June	1st to 10th	Day	12% 5% 2 5% 4 5% 5% 5% 7% 10% 41%	2800
	11th to 20th	Day	13% 6% 2 6% 5% 5% 5% 5% 6% 9% 40%	2800
	21st to end	Day	10% 4 1 5% 4 4 5% 4 7% 10% 45%	2800
July	1st to 10th	Day	6% 5% 3 6% 5% 5% 6% 5% 8% 10% 43%	6360
	11th to 20th	Day	10% 6% 3 7% 5% 6% 7% 5% 8% 11% 32%	6274
	21st to end	Day	9% 7% 2 6% 5% 4 5% 4 7% 10% 40%	6424
August	1st to 10th	Day	17% 6% 2 6% 4 5% 5% 4 7% 9% 34%	4234
	11th to 20th	Day	11% 5% 2 6% 5% 6% 6% 5% 7% 8% 39%	4010
	21st to end	Day	11% 6% 2 5% 4 4 4 4 6% 8% 47%	4312
September	1st to 10th	Day	14% 6% 2 5% 4 3 4 3 5% 7% 47%	2400
	11th to 20th	Day	12% 5% 2 5% 3 4 4 4 5% 8% 47%	2400
	21st to end	Day	20% 8% 3 6% 4 3 4 3 5% 7% 37%	2105
October	1st to 10th	Day	17% 5% 2 5% 4 3 4 3 5% 7% 46%	2000
	11th to 20th	Day	16% 5% 2 6% 3 4 4 3 5% 5% 48%	2000
	21st to end	Day	13% 7% 2 5% 4 3 4 3 5% 6% 48%	2200
November	1st to 10th	Day	11% 6% 2 4 3 3 3 3 6% 8% 51%	1921
	11th to 20th	Day	14% 6% 2 5% 4 3 4 4 5% 7% 47%	1853
	21st to end	Day	15% 7% 3 5% 3 4 4 4 5% 6% 44%	1680
December	1st to 10th	Day	13% 6% 2 5% 5% 4 5% 4 6% 8% 42%	2616
	11th to 20th	Day	18% 10% 3 7% 4 3 4 3 5% 6% 39%	2990
	21st to end	Day	17% 8% 2 8% 4 5% 4 3 6% 8% 35%	3960
January	1st to 10th	Night	11% 7% 3 6% 4 4 4 3 5% 7% 44%	6325
	11th to 20th	Night	14% 6% 2 5% 4 3 4 3 4 5% 48%	5900
	21st to end	Night	16% 8% 2 5% 4 4 4 3 5% 5% 45%	6490
February	1st to 10th	Night	15% 8% 3 6% 4 4 4 3 6% 7% 40%	2600
	11th to 20th	Night	18% 8% 3 6% 4 3 3 3 5% 6% 41%	2479
	21st to end	Night	16% 9% 2 7% 5% 4 4 4 4 6% 40%	1980
March	1st to 10th	Night	16% 6% 2 6% 4 4 4 3 4 5% 46%	5020
	11th to 20th	Night	10% 5% 2 6% 4 4 4 3 5% 6% 50%	4980
	21st to end	Night	12% 8% 3 6% 4 4 4 3 5% 6% 44%	5343
April	1st to 10th	Night	16% 8% 2 7% 5% 5% 5% 3 5% 7% 38%	2000
	11th to 20th	Night	14% 5% 2 6% 4 4 4 3 6% 7% 44%	1975
	21st to end	Night	16% 6% 2 6% 4 3 4 4 7% 7% 43%	2000
May	1st to 10th	Night	24% 9% 3 6% 4 4 5% 4 5% 7% 29%	4135
	11th to 20th	Night	23% 8% 3 6% 4 3 4 4 6% 8% 31%	3649
	21st to end	Night	18% 7% 2 7% 4 4 5% 4 6% 8% 36%	3920
June	1st to 10th	Night	11% 5% 2 6% 4 4 4 4 6% 8% 45%	1600
	11th to 20th	Night	11% 6% 2 6% 5% 4 6% 4 7% 8% 40%	1600
	21st to end	Night	9% 6% 2 5% 4 5% 5% 4 6% 10% 44%	1600
July	1st to 10th	Night	7% 7% 3 7% 5% 5% 6% 4 6% 10% 39%	3598
	11th to 20th	Night	9% 7% 3 10% 6% 6% 6% 5% 8% 10% 30%	3679
	21st to end	Night	9% 6% 2 6% 4 4 5% 4 6% 9% 44%	4427
August	1st to 10th	Night	18% 6% 2 6% 4 4 5% 4 6% 10% 34%	2915
	11th to 20th	Night	10% 8% 3 7% 6% 5% 5% 4 7% 8% 39%	3005
	21st to end	Night	11% 6% 2 5% 4 3 3 4 6% 8% 48%	3586
September	1st to 10th	Night	13% 5% 2 6% 3 3 4 3 6% 7% 47%	2000
	11th to 20th	Night	11% 5% 2 6% 4 3 4 2 4 8% 52%	2000
	21st to end	Night	19% 7% 2 6% 4 4 5% 2 6% 6% 38%	2288
October	1st to 10th	Night	18% 6% 2 5% 4 3 4 3 4 5% 46%	2400
	11th to 20th	Night	15% 6% 2 4 3 3 3 3 5% 6% 51%	2400
	21st to end	Night	15% 5% 2 4 4 3 4 3 5% 7% 48%	2640
November	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 5% 6% 53%	2597
	11th to 20th	Night	14% 7% 2 5% 4 3 3 3 4 6% 47%	2764
	21st to end	Night	13% 5% 2 5% 4 5% 5% 4 5% 7% 45%	2940
December	1st to 10th	Night	11% 6% 2 6% 3 3 4 4 6% 8% 47%	4609
	11th to 20th	Night	20% 7% 2 5% 4 4 4 4 5% 6% 38%	5205
	21st to end	Night	17% 8% 2 6% 5% 4 5% 4 7% 7% 34%	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	36% 5% 11% 27% 17% 4	3620
	11th to 20th	Day	43% 5% 9% 21% 17% 6%	4060
	21st to end	Day	43% 6% 11% 24% 14% 3	4466
February	1st to 10th	Day	45% 6% 13% 25% 10% 2	1800
	11th to 20th	Day	45% 5% 10% 24% 13% 3	1814
	21st to end	Day	46% 4 13% 20% 13% 4	1650
March	1st to 10th	Day	39% 6% 11% 23% 16% 6%	4817
	11th to 20th	Day	35% 6% 12% 26% 17% 5%	4980
	21st to end	Day	43% 6% 10% 19% 18% 5%	5588
April	1st to 10th	Day	43% 7% 12% 18% 16% 4	2400
	11th to 20th	Day	39% 6% 11% 22% 17% 4	2374
	21st to end	Day	42% 6% 10% 21% 18% 3	2400
May	1st to 10th	Day	48% 8% 15% 19% 9% 2	5823
	11th to 20th	Day	48% 9% 12% 21% 9% 1	6244
	21st to end	Day	40% 9% 13% 22% 15% 1	6949
June	1st to 10th	Day	36% 9% 16% 23% 15% 2	2800
	11th to 20th	Day	40% 9% 13% 23% 13% 2	2800
	21st to end	Day	32% 9% 15% 25% 17% 1	2800
July	1st to 10th	Day	35% 10% 16% 20% 16% 3	6290
	11th to 20th	Day	41% 11% 17% 22% 8% 1	6199
	21st to end	Day	37% 7% 12% 22% 21% 1	6240
August	1st to 10th	Day	45% 8% 14% 24% 9%	4221
	11th to 20th	Day	38% 7% 13% 27% 14% 1	4049
	21st to end	Day	34% 6% 13% 27% 18% 2	4312
September	1st to 10th	Day	37% 7% 12% 20% 20% 4	2400
	11th to 20th	Day	35% 5% 10% 26% 21% 3	2400
	21st to end	Day	47% 6% 10% 18% 18% 2	2105
October	1st to 10th	Day	37% 5% 7% 20% 26% 5%	3260
	11th to 20th	Day	29% 4 10% 27% 21% 10%	3240
	21st to end	Day	25% 4 11% 28% 29% 4	3538
November	1st to 10th	Day	32% 4 13% 26% 21% 4	1921
	11th to 20th	Day	36% 5% 11% 23% 21% 4	1853
	21st to end	Day	40% 5% 11% 29% 13% 2	1680
December	1st to 10th	Day	39% 6% 16% 27% 8% 3	2616
	11th to 20th	Day	46% 5% 13% 27% 9% 1	2986
	21st to end	Day	44% 7% 17% 18% 13% 1	3960
January	1st to 10th	Night	36% 6% 10% 25% 20% 3	6325
	11th to 20th	Night	36% 5% 11% 25% 17% 6%	5900
	21st to end	Night	40% 4 9% 27% 17% 2	6490
February	1st to 10th	Night	42% 6% 15% 23% 13% 2	2600
	11th to 20th	Night	44% 5% 10% 22% 16% 4	2481
	21st to end	Night	45% 5% 10% 21% 14% 5%	1980
March	1st to 10th	Night	39% 6% 9% 26% 12% 9%	5020
	11th to 20th	Night	33% 5% 12% 30% 14% 6%	4980
	21st to end	Night	39% 5% 10% 20% 22% 4	5343
April	1st to 10th	Night	45% 6% 11% 19% 17% 3	2000
	11th to 20th	Night	38% 6% 12% 20% 18% 5%	1975
	21st to end	Night	38% 7% 11% 23% 16% 4	2000
May	1st to 10th	Night	53% 6% 11% 15% 11% 3	4163
	11th to 20th	Night	48% 9% 12% 21% 10% 1	3650
	21st to end	Night	43% 9% 14% 23% 9% 1	3927
June	1st to 10th	Night	35% 8% 11% 26% 18% 3	1600
	11th to 20th	Night	39% 10% 12% 24% 14% 2	1600
	21st to end	Night	35% 8% 13% 24% 17% 2	1600
July	1st to 10th	Night	39% 10% 16% 18% 14% 3	3560
	11th to 20th	Night	46% 11% 15% 18% 9% 1	3634
	21st to end	Night	35% 8% 12% 23% 20% 1	4297
August	1st to 10th	Night	44% 8% 18% 21% 8% 1	2904
	11th to 20th	Night	41% 7% 10% 27% 13% 1	3032
	21st to end	Night	33% 6% 12% 30% 16% 2	3586
September	1st to 10th	Night	35% 7% 11% 21% 20% 5%	2000
	11th to 20th	Night	33% 5% 10% 25% 23% 3	2000
	21st to end	Night	45% 7% 10% 18% 18% 3	2288
October	1st to 10th	Night	41% 5% 10% 20% 20% 3	3920
	11th to 20th	Night	25% 4 11% 25% 28% 7%	3899
	21st to end	Night	25% 4 10% 32% 24% 5%	4252
November	1st to 10th	Night	32% 5% 11% 26% 23% 4	2597
	11th to 20th	Night	38% 4 10% 23% 20% 5%	2764
	21st to end	Night	37% 5% 12% 25% 18% 2	2940
December	1st to 10th	Night	35% 5% 15% 31% 13% 1	4609
	11th to 20th	Night	45% 4 13% 19% 16% 3	5208
	21st to end	Night	44% 8% 14% 19% 15%	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	27% 25% 31% 14% 2	205
	11th to 20th	Day	28% 28% 32% 11% 1	300
	21st to end	Day	21% 30% 33% 12% 3 1	330
February	1st to 10th	Day	24% 25% 39% 11% 2	300
	11th to 20th	Day	34% 25% 35% 5% 1	352
	21st to end	Day	28% 22% 31% 16% 2	414
March	1st to 10th	Day	21% 25% 31% 20% 3	500
	11th to 20th	Day	24% 20% 34% 17% 4 1	500
	21st to end	Day	22% 20% 34% 17% 5% 1	550
April	1st to 10th	Day	23% 21% 32% 17% 6% 2	500
	11th to 20th	Day	17% 21% 32% 15% 12% 3	492
	21st to end	Day	23% 15% 32% 20% 8% 2	500
May	1st to 10th	Day	26% 15% 29% 19% 8% 3	503
	11th to 20th	Day	20% 13% 29% 23% 11% 4	527
	21st to end	Day	16% 7% 23% 27% 22% 5%	573
June	1st to 10th	Day	13% 11% 29% 25% 16% 6%	530
	11th to 20th	Day	17% 7% 29% 26% 17% 4	530
	21st to end	Day	10% 7% 24% 34% 20% 5%	530
July	1st to 10th	Day	11% 5% 20% 31% 21% 12%	525
	11th to 20th	Day	6% 2 22% 31% 29% 10%	524
	21st to end	Day	11% 3 20% 34% 26% 6%	576
August	1st to 10th	Day	12% 4 19% 30% 25% 10%	526
	11th to 20th	Day	12% 4 19% 34% 23% 9%	519
	21st to end	Day	9% 5% 21% 29% 23% 13%	583
September	1st to 10th	Day	15% 8% 19% 25% 23% 11%	530
	11th to 20th	Day	11% 4 21% 31% 23% 10%	530
	21st to end	Day	26% 11% 21% 24% 13% 4	530
October	1st to 10th	Day	17% 9% 23% 25% 18% 8%	530
	11th to 20th	Day	17% 16% 25% 26% 13% 3	530
	21st to end	Day	16% 13% 36% 24% 9% 2	583
November	1st to 10th	Day	16% 15% 29% 26% 11% 2	463
	11th to 20th	Day	18% 18% 33% 23% 8% 1	393
	21st to end	Day	25% 19% 34% 16% 6%	220
December	1st to 10th	Day	24% 23% 34% 15% 3 1	204
	11th to 20th	Day	25% 24% 27% 20% 5%	200
	21st to end	Day	21% 20% 32% 22% 5%	220
January	1st to 10th	Night	19% 20% 41% 18% 2	800
	11th to 20th	Night	22% 19% 33% 23% 3	800
	21st to end	Night	17% 18% 44% 19% 1	880
February	1st to 10th	Night	19% 17% 38% 24% 2	800
	11th to 20th	Night	26% 20% 38% 15% 1	783
	21st to end	Night	24% 18% 39% 16% 3	660
March	1st to 10th	Night	19% 18% 40% 20% 3	800
	11th to 20th	Night	21% 18% 35% 21% 5%	800
	21st to end	Night	18% 16% 43% 19% 4	880
April	1st to 10th	Night	21% 18% 37% 20% 3	800
	11th to 20th	Night	16% 15% 36% 25% 7% 1	791
	21st to end	Night	20% 11% 34% 28% 7% 1	800
May	1st to 10th	Night	19% 10% 32% 29% 10%	651
	11th to 20th	Night	20% 7% 29% 34% 10%	429
	21st to end	Night	15% 6% 22% 37% 19% 1	433
June	1st to 10th	Night	12% 4 27% 36% 21%	400
	11th to 20th	Night	14% 4 23% 42% 17% 1	400
	21st to end	Night	10% 2 25% 45% 19% 1	400
July	1st to 10th	Night	9% 3 19% 39% 27% 3	390
	11th to 20th	Night	8% 1 17% 44% 29% 2	418
	21st to end	Night	10% 2 16% 42% 28% 3	657
August	1st to 10th	Night	12% 2 12% 45% 27% 3	597
	11th to 20th	Night	11% 4 17% 39% 28% 3	672
	21st to end	Night	11% 4 15% 40% 27% 3	880
September	1st to 10th	Night	14% 3 21% 33% 26% 3	800
	11th to 20th	Night	12% 3 18% 36% 28% 3	800
	21st to end	Night	21% 6% 21% 34% 17% 2	800
October	1st to 10th	Night	19% 6% 23% 30% 19% 4	800
	11th to 20th	Night	16% 10% 29% 32% 12% 1	800
	21st to end	Night	18% 10% 34% 28% 11%	880
November	1st to 10th	Night	14% 10% 34% 33% 9% 1	820
	11th to 20th	Night	18% 16% 33% 26% 6% 1	840
	21st to end	Night	16% 16% 37% 25% 7%	840
December	1st to 10th	Night	23% 12% 39% 20% 6%	809
	11th to 20th	Night	18% 13% 42% 23% 4	800
	21st to end	Night	20% 13% 37% 28% 2	880

Page generated at 2025-11-30 01:00:53. Version ac07

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