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 2024-01-13.

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% 3 5% 5% 3 5% 5% 6% 8% 41%	74685
	Night	14% 6% 2 5% 5% 3 5% 5% 5% 7% 43%	75047
Transparency	Day	40% 7% 13% 22% 15% 3	74395
	Night	39% 7% 12% 23% 16% 3	74856
Seeing	Day	17% 13% 27% 23% 14% 5%	13800
	Night	17% 11% 31% 28% 12% 1	23377

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% 7% 4 5% 5% 3 5% 5% 5% 6% 41%	1525
	11th to 20th	Day	15% 8% 3 6% 5% 3 4 5% 4 6% 40%	1638
	21st to end	Day	14% 10% 4 5% 6% 3 5% 4 4 5% 39%	1782
February	1st to 10th	Day	15% 8% 3 7% 5% 3 6% 6% 5% 6% 36%	1620
	11th to 20th	Day	16% 7% 3 6% 5% 4 4 4 5% 6% 39%	1620
	21st to end	Day	19% 9% 3 6% 5% 3 3 4 4 5% 39%	1480
March	1st to 10th	Day	12% 6% 3 4 6% 3 3 4 5% 6% 47%	1929
	11th to 20th	Day	16% 7% 3 6% 5% 3 4 5% 5% 6% 40%	1980
	21st to end	Day	17% 7% 3 7% 6% 4 5% 4 4 6% 37%	2218
April	1st to 10th	Day	13% 7% 3 5% 6% 3 5% 5% 6% 8% 39%	2160
	11th to 20th	Day	14% 5% 2 5% 5% 3 4 4 4 6% 46%	2134
	21st to end	Day	18% 6% 2 5% 5% 3 4 4 5% 7% 41%	2160
May	1st to 10th	Day	16% 6% 3 6% 5% 3 5% 4 7% 8% 37%	2308
	11th to 20th	Day	20% 6% 2 6% 5% 3 4 5% 6% 8% 36%	2442
	21st to end	Day	13% 6% 2 6% 6% 4 5% 4 6% 8% 40%	2725
June	1st to 10th	Day	13% 4 2 4 5% 3 5% 6% 7% 10% 41%	2520
	11th to 20th	Day	13% 6% 2 5% 5% 3 5% 6% 6% 9% 39%	2520
	21st to end	Day	11% 4 2 4 5% 3 5% 5% 7% 10% 45%	2520
July	1st to 10th	Day	9% 5% 3 5% 6% 4 5% 6% 6% 9% 42%	2520
	11th to 20th	Day	7% 3 2 5% 5% 4 6% 6% 8% 10% 45%	2490
	21st to end	Day	10% 6% 3 6% 6% 4 6% 6% 7% 9% 37%	2547
August	1st to 10th	Day	10% 5% 2 6% 6% 4 6% 6% 8% 9% 38%	2327
	11th to 20th	Day	12% 5% 2 5% 6% 5% 5% 5% 6% 9% 40%	2214
	21st to end	Day	11% 6% 2 5% 6% 4 4 5% 7% 10% 41%	2376
September	1st to 10th	Day	15% 5% 2 5% 5% 3 4 4 5% 8% 44%	2160
	11th to 20th	Day	12% 5% 2 4 4 3 5% 5% 5% 8% 47%	2160
	21st to end	Day	18% 7% 3 5% 5% 2 4 4 5% 7% 40%	1896
October	1st to 10th	Day	18% 5% 2 4 5% 2 3 5% 5% 7% 44%	1800
	11th to 20th	Day	17% 5% 2 5% 4 3 4 4 4 5% 47%	1800
	21st to end	Day	14% 7% 2 4 5% 2 4 4 5% 6% 46%	1980
November	1st to 10th	Day	12% 6% 3 4 4 2 3 3 6% 8% 50%	1735
	11th to 20th	Day	15% 5% 2 4 4 2 4 5% 5% 7% 46%	1677
	21st to end	Day	16% 5% 3 5% 4 3 6% 4 5% 6% 44%	1520
December	1st to 10th	Day	18% 7% 3 5% 4 4 5% 5% 6% 8% 36%	1520
	11th to 20th	Day	16% 7% 3 6% 6% 3 4 4 5% 7% 39%	1910
	21st to end	Day	18% 7% 2 7% 5% 4 5% 5% 6% 8% 35%	2772
January	1st to 10th	Night	14% 7% 3 6% 5% 3 5% 3 5% 7% 43%	2639
	11th to 20th	Night	18% 6% 3 5% 5% 3 4 5% 6% 7% 38%	2367
	21st to end	Night	14% 7% 2 5% 5% 3 5% 5% 5% 6% 43%	2574
February	1st to 10th	Night	15% 8% 3 6% 5% 2 4 4 6% 7% 39%	2340
	11th to 20th	Night	17% 8% 3 6% 4 3 4 4 5% 6% 40%	2224
	21st to end	Night	17% 8% 3 6% 5% 3 4 5% 3 6% 39%	1776
March	1st to 10th	Night	13% 5% 2 5% 4 3 5% 5% 5% 7% 47%	2029
	11th to 20th	Night	17% 6% 2 5% 5% 3 5% 4 4 7% 43%	1980
	21st to end	Night	14% 7% 3 6% 5% 3 4 4 5% 8% 41%	2137
April	1st to 10th	Night	15% 6% 3 6% 5% 3 5% 4 6% 7% 38%	1800
	11th to 20th	Night	14% 5% 3 6% 5% 2 4 4 6% 7% 45%	1775
	21st to end	Night	17% 6% 3 5% 4 2 4 5% 7% 7% 42%	1800
May	1st to 10th	Night	17% 7% 3 5% 4 3 4 5% 5% 7% 39%	1667
	11th to 20th	Night	17% 7% 3 6% 4 2 5% 5% 6% 8% 38%	1447
	21st to end	Night	12% 7% 2 5% 5% 3 5% 5% 6% 9% 41%	1550
June	1st to 10th	Night	12% 5% 2 5% 4 3 5% 4 6% 9% 46%	1440
	11th to 20th	Night	10% 6% 3 6% 6% 3 6% 6% 7% 8% 40%	1440
	21st to end	Night	8% 5% 3 5% 4 3 6% 6% 6% 10% 44%	1440
July	1st to 10th	Night	9% 6% 3 6% 6% 4 6% 6% 6% 10% 37%	1440
	11th to 20th	Night	6% 3 3 5% 5% 3 5% 6% 7% 11% 46%	1466
	21st to end	Night	9% 4 1 4 5% 3 6% 5% 8% 11% 44%	1771
August	1st to 10th	Night	10% 4 2 5% 5% 4 7% 7% 6% 10% 40%	1611
	11th to 20th	Night	10% 6% 2 6% 6% 4 5% 5% 6% 8% 41%	1668
	21st to end	Night	11% 5% 2 5% 6% 3 4 6% 6% 8% 44%	1980
September	1st to 10th	Night	14% 6% 2 5% 4 2 4 5% 5% 7% 46%	1800
	11th to 20th	Night	10% 5% 2 5% 5% 2 3 3 4 8% 53%	1800
	21st to end	Night	17% 6% 2 4 5% 3 5% 3 6% 6% 42%	2058
October	1st to 10th	Night	19% 6% 2 4 5% 2 4 4 4 6% 44%	2160
	11th to 20th	Night	15% 6% 2 4 4 2 4 4 5% 6% 49%	2160
	21st to end	Night	16% 5% 2 4 4 3 3 4 4 7% 47%	2376
November	1st to 10th	Night	12% 5% 2 4 4 2 3 3 5% 6% 53%	2343
	11th to 20th	Night	14% 7% 3 5% 5% 2 3 4 4 6% 48%	2500
	21st to end	Night	13% 4 2 5% 5% 4 5% 5% 4 8% 46%	2660
December	1st to 10th	Night	17% 7% 3 6% 5% 2 4 5% 5% 7% 38%	2660
	11th to 20th	Night	17% 5% 2 5% 5% 4 5% 4 5% 6% 41%	3285
	21st to end	Night	16% 7% 2 5% 6% 3 6% 6% 7% 8% 34%	4884

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	43% 6% 11% 23% 14% 3	1526
	11th to 20th	Day	44% 6% 14% 22% 11% 2	1638
	21st to end	Day	47% 4 11% 21% 12% 4	1782
February	1st to 10th	Day	46% 6% 13% 24% 9% 2	1620
	11th to 20th	Day	45% 5% 11% 25% 12% 3	1629
	21st to end	Day	48% 4 13% 19% 12% 4	1480
March	1st to 10th	Day	38% 5% 12% 25% 18% 2	1929
	11th to 20th	Day	43% 6% 12% 17% 17% 5%	1980
	21st to end	Day	48% 5% 8% 19% 16% 3	2218
April	1st to 10th	Day	41% 7% 13% 19% 16% 4	2160
	11th to 20th	Day	39% 6% 11% 22% 18% 5%	2134
	21st to end	Day	43% 6% 10% 20% 17% 3	2160
May	1st to 10th	Day	43% 7% 13% 21% 14% 2	2308
	11th to 20th	Day	46% 8% 13% 18% 13% 2	2446
	21st to end	Day	40% 8% 15% 22% 12% 2	2725
June	1st to 10th	Day	36% 9% 16% 23% 14% 2	2520
	11th to 20th	Day	40% 10% 13% 21% 13% 2	2520
	21st to end	Day	32% 10% 15% 26% 16% 1	2520
July	1st to 10th	Day	37% 10% 14% 20% 16% 3	2450
	11th to 20th	Day	31% 9% 16% 26% 15% 3	2415
	21st to end	Day	39% 11% 14% 20% 14% 2	2405
August	1st to 10th	Day	39% 10% 15% 23% 12% 1	2314
	11th to 20th	Day	39% 8% 15% 21% 15% 2	2214
	21st to end	Day	37% 8% 15% 23% 14% 3	2376
September	1st to 10th	Day	39% 8% 12% 20% 18% 4	2160
	11th to 20th	Day	34% 5% 11% 25% 21% 4	2160
	21st to end	Day	44% 6% 10% 19% 19% 2	1896
October	1st to 10th	Day	39% 7% 10% 25% 16% 2	1800
	11th to 20th	Day	39% 6% 11% 22% 20% 2	1800
	21st to end	Day	38% 6% 11% 25% 19% 2	1980
November	1st to 10th	Day	33% 4 13% 26% 19% 4	1735
	11th to 20th	Day	36% 6% 12% 23% 20% 4	1677
	21st to end	Day	40% 5% 12% 29% 12% 2	1520
December	1st to 10th	Day	45% 6% 14% 23% 10% 1	1520
	11th to 20th	Day	41% 6% 13% 30% 9% 2	1906
	21st to end	Day	44% 8% 17% 20% 10%	2772
January	1st to 10th	Night	42% 4 10% 25% 15% 4	2639
	11th to 20th	Night	44% 6% 14% 23% 12% 1	2367
	21st to end	Night	40% 5% 10% 26% 15% 3	2574
February	1st to 10th	Night	43% 6% 15% 22% 12% 2	2340
	11th to 20th	Night	44% 5% 10% 21% 17% 4	2226
	21st to end	Night	47% 5% 9% 21% 14% 5%	1776
March	1st to 10th	Night	37% 6% 11% 26% 17% 2	2029
	11th to 20th	Night	42% 5% 11% 22% 17% 3	1980
	21st to end	Night	42% 6% 11% 21% 15% 5%	2137
April	1st to 10th	Night	43% 6% 11% 19% 17% 4	1800
	11th to 20th	Night	37% 6% 12% 20% 18% 6%	1775
	21st to end	Night	39% 7% 11% 23% 15% 5%	1800
May	1st to 10th	Night	43% 6% 12% 22% 13% 4	1667
	11th to 20th	Night	43% 8% 12% 17% 17% 2	1448
	21st to end	Night	37% 10% 18% 21% 12% 2	1551
June	1st to 10th	Night	34% 9% 11% 26% 17% 3	1440
	11th to 20th	Night	39% 10% 13% 23% 14% 2	1440
	21st to end	Night	34% 8% 13% 25% 17% 2	1440
July	1st to 10th	Night	41% 11% 16% 18% 12% 3	1400
	11th to 20th	Night	30% 11% 16% 25% 16% 3	1418
	21st to end	Night	30% 10% 18% 23% 16% 2	1672
August	1st to 10th	Night	36% 10% 17% 23% 13% 1	1600
	11th to 20th	Night	40% 10% 13% 21% 15% 1	1668
	21st to end	Night	35% 8% 14% 24% 15% 4	1980
September	1st to 10th	Night	37% 7% 12% 20% 19% 5%	1800
	11th to 20th	Night	32% 5% 11% 25% 23% 3	1800
	21st to end	Night	41% 7% 10% 19% 19% 3	2058
October	1st to 10th	Night	42% 6% 11% 22% 16% 2	2160
	11th to 20th	Night	35% 5% 11% 28% 19% 2	2160
	21st to end	Night	36% 6% 10% 26% 19% 2	2376
November	1st to 10th	Night	32% 5% 10% 27% 21% 4	2343
	11th to 20th	Night	38% 4 10% 24% 20% 4	2500
	21st to end	Night	37% 5% 12% 25% 18% 2	2660
December	1st to 10th	Night	45% 6% 12% 24% 12% 1	2660
	11th to 20th	Night	42% 5% 12% 22% 16% 3	3288
	21st to end	Night	42% 10% 16% 21% 10% 1	4884

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	23% 25% 33% 16% 3 1	184
	11th to 20th	Day	22% 30% 33% 13% 2	246
	21st to end	Day	22% 31% 31% 12% 3 1	264
February	1st to 10th	Day	24% 25% 38% 10% 3	240
	11th to 20th	Day	31% 25% 39% 5%	287
	21st to end	Day	30% 23% 31% 15% 1	346
March	1st to 10th	Day	18% 27% 30% 22% 3	420
	11th to 20th	Day	25% 19% 33% 17% 4 2	420
	21st to end	Day	21% 21% 34% 17% 6% 1	462
April	1st to 10th	Day	20% 21% 33% 18% 6% 2	420
	11th to 20th	Day	15% 19% 33% 16% 13% 3	412
	21st to end	Day	24% 15% 33% 19% 7% 2	420
May	1st to 10th	Day	25% 15% 29% 19% 9% 3	423
	11th to 20th	Day	20% 12% 30% 23% 11% 3	447
	21st to end	Day	15% 5% 22% 28% 24% 6%	485
June	1st to 10th	Day	13% 10% 28% 27% 17% 6%	450
	11th to 20th	Day	16% 7% 30% 24% 18% 4	450
	21st to end	Day	11% 6% 24% 34% 20% 5%	450
July	1st to 10th	Day	12% 4 19% 29% 22% 13%	445
	11th to 20th	Day	5% 2 20% 32% 30% 11%	444
	21st to end	Day	12% 3 21% 31% 26% 7%	488
August	1st to 10th	Day	11% 4 17% 29% 28% 11%	446
	11th to 20th	Day	11% 4 18% 32% 25% 10%	439
	21st to end	Day	9% 4 20% 30% 24% 14%	495
September	1st to 10th	Day	17% 7% 18% 23% 23% 12%	450
	11th to 20th	Day	10% 5% 21% 29% 23% 11%	450
	21st to end	Day	21% 12% 22% 26% 15% 4	450
October	1st to 10th	Day	19% 8% 22% 24% 19% 7%	450
	11th to 20th	Day	17% 14% 25% 28% 14% 2	450
	21st to end	Day	16% 12% 37% 23% 8% 2	495
November	1st to 10th	Day	16% 12% 29% 28% 12% 3	397
	11th to 20th	Day	18% 18% 31% 23% 9% 1	337
	21st to end	Day	20% 19% 34% 19% 7%	180
December	1st to 10th	Day	24% 22% 33% 16% 3 2	180
	11th to 20th	Day	24% 24% 27% 20% 5%	180
	21st to end	Day	20% 19% 33% 22% 6%	198
January	1st to 10th	Night	18% 19% 42% 19% 3	760
	11th to 20th	Night	22% 16% 34% 25% 3	729
	21st to end	Night	17% 18% 43% 20% 2	792
February	1st to 10th	Night	20% 17% 38% 24% 1	720
	11th to 20th	Night	24% 20% 39% 15% 1	703
	21st to end	Night	24% 19% 39% 15% 2	592
March	1st to 10th	Night	18% 17% 41% 21% 3	720
	11th to 20th	Night	22% 16% 36% 21% 5%	720
	21st to end	Night	18% 14% 44% 19% 5% 1	792
April	1st to 10th	Night	20% 18% 38% 21% 4	720
	11th to 20th	Night	15% 14% 36% 26% 8% 1	711
	21st to end	Night	21% 11% 34% 27% 7% 1	720
May	1st to 10th	Night	18% 9% 32% 29% 10%	587
	11th to 20th	Night	20% 7% 29% 34% 10% 1	386
	21st to end	Night	14% 5% 22% 38% 20% 1	389
June	1st to 10th	Night	12% 3 27% 35% 22%	360
	11th to 20th	Night	13% 4 24% 41% 19% 1	360
	21st to end	Night	9% 2 24% 48% 17%	360
July	1st to 10th	Night	9% 3 18% 40% 27% 4	350
	11th to 20th	Night	7% 1 16% 44% 30% 2	374
	21st to end	Night	9% 2 16% 43% 28% 3	591
August	1st to 10th	Night	11% 2 12% 45% 28% 2	537
	11th to 20th	Night	11% 4 16% 38% 28% 3	604
	21st to end	Night	12% 4 15% 39% 28% 3	792
September	1st to 10th	Night	15% 3 20% 33% 27% 3	720
	11th to 20th	Night	11% 3 19% 35% 28% 4	720
	21st to end	Night	19% 5% 22% 34% 18% 2	720
October	1st to 10th	Night	20% 6% 23% 30% 17% 4	720
	11th to 20th	Night	16% 10% 29% 31% 13% 1	720
	21st to end	Night	19% 9% 34% 27% 11%	792
November	1st to 10th	Night	14% 9% 34% 33% 9% 2	740
	11th to 20th	Night	18% 15% 33% 27% 6% 1	760
	21st to end	Night	15% 15% 37% 26% 7%	760
December	1st to 10th	Night	23% 12% 38% 19% 6%	760
	11th to 20th	Night	18% 12% 43% 22% 5%	760
	21st to end	Night	20% 13% 38% 27% 2	836

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