Marystown
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Marystown clear sky chart from 2005-11-06 to 2025-11-18.

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	25% 15% 5% 10% 5% 4 4 3 4 5% 21%	126864
	Night	24% 15% 5% 11% 6% 5% 4 3 4 5% 19%	123700
Transparency	Day	63% 8% 11% 14% 5%	130334
	Night	64% 8% 10% 12% 4	128341
Seeing	Day	30% 16% 28% 19% 6% 1	19482
	Night	30% 16% 28% 20% 6%	26220

By Month

Percentage Cloud Cover NE (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	21% 19% 9% 13% 5% 4 3 3 3 4 15%	3464
	11th to 20th	Day	19% 18% 10% 18% 6% 5% 4 2 3 4 12%	3591
	21st to end	Day	12% 14% 9% 22% 8% 5% 3 3 3 4 15%	3982
February	1st to 10th	Day	20% 17% 7% 16% 9% 7% 5% 3 3 3 10%	1646
	11th to 20th	Day	22% 16% 6% 17% 7% 6% 5% 2 4 3 12%	1871
	21st to end	Day	15% 17% 6% 16% 8% 4 4 4 4 4 18%	1650
March	1st to 10th	Day	26% 13% 7% 16% 7% 4 3 2 3 4 15%	4474
	11th to 20th	Day	21% 21% 7% 12% 5% 4 3 2 4 4 18%	4842
	21st to end	Day	30% 18% 8% 11% 4 3 3 3 3 4 12%	5667
April	1st to 10th	Day	24% 21% 7% 11% 5% 4 4 2 3 4 16%	2400
	11th to 20th	Day	24% 14% 5% 8% 4 4 4 3 4 5% 26%	2523
	21st to end	Day	27% 17% 5% 8% 4 4 3 2 3 5% 23%	2600
May	1st to 10th	Day	23% 20% 4 9% 4 3 3 2 4 5% 23%	5903
	11th to 20th	Day	23% 16% 4 8% 4 3 3 3 5% 6% 24%	6318
	21st to end	Day	23% 17% 4 8% 4 3 4 3 5% 6% 22%	7332
June	1st to 10th	Day	34% 15% 4 7% 3 3 3 2 4 5% 21%	3000
	11th to 20th	Day	25% 13% 4 6% 4 3 3 3 5% 7% 28%	3000
	21st to end	Day	34% 13% 4 7% 3 3 3 3 4 5% 21%	3000
July	1st to 10th	Day	31% 15% 4 7% 4 3 3 3 5% 6% 19%	6785
	11th to 20th	Day	26% 15% 4 8% 4 3 3 2 4 5% 27%	6360
	21st to end	Day	23% 10% 3 7% 4 4 4 3 5% 7% 30%	6927
August	1st to 10th	Day	30% 11% 3 7% 4 4 4 3 5% 7% 23%	4469
	11th to 20th	Day	25% 11% 4 9% 5% 5% 5% 4 6% 7% 21%	4176
	21st to end	Day	24% 8% 3 6% 4 4 4 3 5% 7% 31%	4595
September	1st to 10th	Day	25% 9% 3 5% 5% 3 5% 3 5% 7% 30%	2380
	11th to 20th	Day	25% 10% 4 7% 5% 3 4 3 4 8% 28%	2200
	21st to end	Day	22% 10% 3 6% 5% 4 4 3 4 6% 34%	2200
October	1st to 10th	Day	22% 10% 3 7% 5% 4 4 4 5% 7% 31%	2180
	11th to 20th	Day	26% 10% 3 10% 5% 4 4 3 5% 6% 24%	1855
	21st to end	Day	26% 14% 4 8% 5% 4 4 4 5% 6% 19%	1980
November	1st to 10th	Day	25% 16% 4 10% 6% 4 4 3 3 5% 20%	1850
	11th to 20th	Day	27% 20% 4 8% 5% 4 4 3 4 5% 16%	1778
	21st to end	Day	27% 18% 5% 10% 5% 3 3 3 3 4 18%	1450
December	1st to 10th	Day	18% 16% 5% 11% 6% 6% 6% 4 6% 7% 15%	2290
	11th to 20th	Day	26% 16% 5% 13% 5% 4 4 3 4 5% 16%	2628
	21st to end	Day	24% 18% 8% 12% 5% 4 4 3 4 5% 12%	3498
January	1st to 10th	Night	19% 18% 9% 15% 7% 5% 5% 2 4 3 12%	6498
	11th to 20th	Night	17% 15% 10% 18% 7% 6% 5% 3 4 4 12%	6314
	21st to end	Night	11% 12% 8% 19% 10% 8% 6% 4 6% 5% 13%	6996
February	1st to 10th	Night	22% 13% 7% 16% 10% 6% 4 3 4 3 11%	2753
	11th to 20th	Night	22% 16% 7% 14% 7% 6% 5% 2 3 4 13%	2415
	21st to end	Night	14% 13% 6% 16% 9% 6% 5% 3 5% 5% 18%	1980
March	1st to 10th	Night	24% 13% 5% 13% 7% 5% 4 3 5% 4 16%	5366
	11th to 20th	Night	19% 18% 7% 12% 7% 6% 5% 3 4 5% 15%	5103
	21st to end	Night	25% 15% 8% 12% 5% 4 5% 3 3 4 15%	5250
April	1st to 10th	Night	27% 17% 4 9% 5% 4 4 3 4 5% 18%	2000
	11th to 20th	Night	25% 14% 4 7% 5% 4 3 2 5% 5% 27%	1826
	21st to end	Night	27% 15% 4 8% 5% 4 3 3 4 4 23%	1800
May	1st to 10th	Night	25% 21% 5% 10% 4 3 3 2 3 4 19%	4042
	11th to 20th	Night	28% 17% 4 6% 4 4 4 3 4 5% 21%	3596
	21st to end	Night	26% 18% 5% 7% 4 4 3 2 4 5% 22%	3561
June	1st to 10th	Night	33% 15% 3 7% 3 3 4 3 4 4 21%	1400
	11th to 20th	Night	26% 12% 4 7% 5% 4 4 3 5% 8% 21%	1400
	21st to end	Night	31% 15% 3 7% 4 4 2 3 4 5% 23%	1400
July	1st to 10th	Night	39% 15% 4 7% 3 3 3 2 4 5% 15%	3179
	11th to 20th	Night	28% 18% 3 7% 4 3 3 2 4 5% 23%	3619
	21st to end	Night	25% 14% 4 6% 4 3 3 3 4 6% 29%	3948
August	1st to 10th	Night	24% 13% 4 7% 5% 4 4 3 4 6% 25%	2689
	11th to 20th	Night	27% 12% 3 7% 4 4 4 3 5% 6% 26%	2881
	21st to end	Night	30% 9% 2 6% 2 2 3 2 4 5% 35%	3296
September	1st to 10th	Night	24% 11% 3 5% 4 4 4 3 5% 7% 32%	2019
	11th to 20th	Night	25% 10% 3 7% 4 4 4 3 4 6% 31%	2200
	21st to end	Night	22% 8% 2 6% 4 3 3 3 5% 6% 36%	2200
October	1st to 10th	Night	20% 9% 3 7% 5% 4 5% 4 5% 6% 32%	2220
	11th to 20th	Night	24% 12% 3 8% 5% 4 4 3 5% 6% 27%	2541
	21st to end	Night	26% 13% 4 9% 5% 4 4 3 5% 6% 21%	2860
November	1st to 10th	Night	25% 16% 4 8% 6% 4 4 3 4 5% 21%	2668
	11th to 20th	Night	28% 16% 4 8% 6% 5% 4 3 4 5% 18%	2774
	21st to end	Night	23% 19% 5% 10% 5% 5% 4 3 4 6% 17%	2945
December	1st to 10th	Night	20% 16% 6% 12% 5% 5% 5% 3 4 6% 18%	4900
	11th to 20th	Night	29% 15% 5% 13% 5% 4 4 2 3 3 15%	5581
	21st to end	Night	22% 19% 8% 13% 7% 5% 4 3 3 4 11%	7480

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	69% 9% 6% 9% 6% 1	3465
	11th to 20th	Day	69% 11% 8% 9% 2	3620
	21st to end	Day	68% 8% 10% 9% 4 1	3982
February	1st to 10th	Day	78% 7% 7% 6% 3	1646
	11th to 20th	Day	73% 9% 9% 7% 2 1	1878
	21st to end	Day	66% 8% 9% 9% 6% 1	1650
March	1st to 10th	Day	70% 7% 9% 10% 4	4478
	11th to 20th	Day	67% 9% 8% 9% 6% 2	4842
	21st to end	Day	74% 6% 7% 9% 4 1	5667
April	1st to 10th	Day	71% 6% 8% 9% 5% 1	2400
	11th to 20th	Day	59% 6% 10% 15% 10%	2523
	21st to end	Day	64% 6% 9% 12% 8% 1	2600
May	1st to 10th	Day	61% 8% 9% 14% 6% 1	5915
	11th to 20th	Day	58% 8% 13% 16% 5%	6322
	21st to end	Day	60% 9% 10% 16% 4	7332
June	1st to 10th	Day	67% 6% 8% 13% 6%	3000
	11th to 20th	Day	56% 8% 14% 16% 5%	3000
	21st to end	Day	67% 7% 9% 12% 5%	3000
July	1st to 10th	Day	67% 6% 11% 12% 4	6710
	11th to 20th	Day	61% 6% 11% 16% 6%	6284
	21st to end	Day	53% 7% 18% 17% 5%	6730
August	1st to 10th	Day	61% 7% 13% 15% 4	4455
	11th to 20th	Day	58% 11% 15% 12% 3	4215
	21st to end	Day	51% 7% 15% 24% 4	4595
September	1st to 10th	Day	51% 9% 10% 21% 8% 1	2380
	11th to 20th	Day	54% 7% 13% 19% 7%	2200
	21st to end	Day	50% 7% 14% 21% 8% 1	2200
October	1st to 10th	Day	57% 6% 12% 20% 5% 1	3558
	11th to 20th	Day	51% 7% 17% 17% 6% 2	3003
	21st to end	Day	58% 7% 9% 22% 4	3191
November	1st to 10th	Day	66% 7% 11% 11% 4	1850
	11th to 20th	Day	68% 8% 9% 11% 4	1778
	21st to end	Day	70% 6% 13% 7% 4	1450
December	1st to 10th	Day	63% 9% 13% 11% 3	2290
	11th to 20th	Day	69% 6% 14% 7% 3	2627
	21st to end	Day	73% 8% 9% 8% 2	3498
January	1st to 10th	Night	71% 11% 6% 7% 3 1	6498
	11th to 20th	Night	69% 12% 9% 8% 2	6360
	21st to end	Night	65% 12% 11% 7% 4 1	6996
February	1st to 10th	Night	75% 10% 8% 5% 2	2753
	11th to 20th	Night	71% 11% 9% 7% 2	2419
	21st to end	Night	65% 9% 11% 10% 4	1980
March	1st to 10th	Night	65% 8% 9% 12% 5%	5375
	11th to 20th	Night	67% 9% 11% 7% 5% 1	5104
	21st to end	Night	69% 8% 8% 10% 5%	5250
April	1st to 10th	Night	64% 9% 10% 11% 6% 1	2000
	11th to 20th	Night	58% 6% 9% 18% 8% 1	1826
	21st to end	Night	62% 7% 9% 13% 8% 1	1800
May	1st to 10th	Night	66% 7% 10% 13% 3	4047
	11th to 20th	Night	63% 7% 8% 14% 6% 1	3597
	21st to end	Night	64% 8% 8% 17% 3	3562
June	1st to 10th	Night	67% 6% 8% 13% 6%	1400
	11th to 20th	Night	61% 8% 12% 14% 5%	1400
	21st to end	Night	66% 6% 10% 11% 8%	1400
July	1st to 10th	Night	72% 4 8% 13% 3	3146
	11th to 20th	Night	65% 6% 9% 15% 5%	3572
	21st to end	Night	57% 6% 12% 23% 3	3831
August	1st to 10th	Night	61% 6% 13% 16% 4	2678
	11th to 20th	Night	58% 7% 12% 19% 4	2908
	21st to end	Night	52% 5% 13% 24% 5%	3295
September	1st to 10th	Night	51% 7% 14% 20% 8%	2019
	11th to 20th	Night	54% 7% 14% 17% 8% 1	2200
	21st to end	Night	47% 7% 16% 21% 9% 1	2200
October	1st to 10th	Night	54% 7% 13% 20% 5%	3623
	11th to 20th	Night	51% 8% 17% 14% 10% 1	4142
	21st to end	Night	55% 7% 13% 17% 7%	4610
November	1st to 10th	Night	63% 9% 13% 12% 3	2668
	11th to 20th	Night	67% 8% 10% 11% 4	2774
	21st to end	Night	66% 10% 12% 9% 3	2945
December	1st to 10th	Night	64% 9% 9% 14% 4	4900
	11th to 20th	Night	72% 9% 10% 7% 2	5583
	21st to end	Night	74% 8% 9% 8% 2	7480

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	31% 23% 30% 14% 3	400
	11th to 20th	Day	28% 20% 33% 12% 7% 1	400
	21st to end	Day	28% 23% 30% 16% 3	440
February	1st to 10th	Day	31% 24% 28% 15% 3	400
	11th to 20th	Day	30% 28% 31% 10% 1	396
	21st to end	Day	24% 25% 30% 18% 3	331
March	1st to 10th	Day	25% 30% 30% 13% 3	400
	11th to 20th	Day	28% 18% 34% 14% 6% 1	400
	21st to end	Day	31% 19% 27% 17% 5% 1	483
April	1st to 10th	Day	33% 25% 25% 15% 2	500
	11th to 20th	Day	29% 19% 31% 18% 4	641
	21st to end	Day	31% 17% 31% 15% 5% 1	700
May	1st to 10th	Day	31% 14% 27% 20% 6% 2	703
	11th to 20th	Day	26% 11% 31% 23% 9% 1	724
	21st to end	Day	29% 14% 31% 19% 7% 1	790
June	1st to 10th	Day	38% 11% 23% 21% 6% 1	730
	11th to 20th	Day	28% 10% 30% 22% 7% 1	730
	21st to end	Day	38% 9% 25% 21% 6% 1	730
July	1st to 10th	Day	34% 9% 28% 22% 8%	720
	11th to 20th	Day	34% 8% 25% 25% 7% 1	718
	21st to end	Day	33% 10% 25% 22% 9% 1	795
August	1st to 10th	Day	28% 8% 26% 25% 12% 2	725
	11th to 20th	Day	26% 11% 25% 25% 12% 1	717
	21st to end	Day	25% 10% 27% 25% 12% 1	762
September	1st to 10th	Day	23% 12% 28% 27% 8% 1	519
	11th to 20th	Day	31% 15% 25% 20% 7% 1	420
	21st to end	Day	25% 15% 33% 20% 7% 1	420
October	1st to 10th	Day	22% 14% 35% 24% 6%	420
	11th to 20th	Day	28% 17% 28% 17% 6% 3	420
	21st to end	Day	31% 18% 31% 14% 6% 1	462
November	1st to 10th	Day	33% 23% 25% 15% 4	425
	11th to 20th	Day	37% 23% 23% 13% 3 1	421
	21st to end	Day	37% 23% 27% 10% 3	400
December	1st to 10th	Day	34% 18% 30% 14% 4	400
	11th to 20th	Day	33% 26% 28% 11% 2	400
	21st to end	Day	27% 23% 29% 16% 5% 1	440
January	1st to 10th	Night	30% 23% 30% 13% 3 1	900
	11th to 20th	Night	29% 21% 29% 15% 4 1	900
	21st to end	Night	25% 22% 33% 17% 3	990
February	1st to 10th	Night	28% 25% 30% 15% 2	900
	11th to 20th	Night	30% 24% 30% 12% 3	823
	21st to end	Night	21% 19% 35% 21% 3 1	660
March	1st to 10th	Night	27% 22% 33% 14% 4	800
	11th to 20th	Night	22% 20% 29% 23% 5% 1	659
	21st to end	Night	30% 17% 29% 18% 5%	660
April	1st to 10th	Night	31% 17% 28% 19% 4 1	600
	11th to 20th	Night	28% 13% 31% 23% 4	594
	21st to end	Night	33% 12% 29% 21% 5%	600
May	1st to 10th	Night	31% 11% 29% 22% 6% 1	600
	11th to 20th	Night	31% 10% 25% 24% 9% 1	592
	21st to end	Night	29% 10% 28% 25% 7%	649
June	1st to 10th	Night	39% 9% 24% 22% 7%	600
	11th to 20th	Night	31% 7% 28% 25% 9%	600
	21st to end	Night	35% 9% 25% 24% 7% 1	600
July	1st to 10th	Night	36% 7% 24% 26% 6% 1	586
	11th to 20th	Night	34% 9% 24% 27% 6%	582
	21st to end	Night	34% 5% 25% 26% 9%	656
August	1st to 10th	Night	29% 7% 24% 25% 13% 1	598
	11th to 20th	Night	31% 8% 21% 27% 13% 1	592
	21st to end	Night	28% 7% 23% 30% 12%	660
September	1st to 10th	Night	26% 9% 23% 29% 12% 1	600
	11th to 20th	Night	29% 10% 26% 27% 8% 1	600
	21st to end	Night	26% 10% 31% 27% 6% 1	600
October	1st to 10th	Night	22% 13% 32% 25% 7%	611
	11th to 20th	Night	29% 14% 29% 19% 7% 1	851
	21st to end	Night	30% 18% 27% 19% 5% 1	1001
November	1st to 10th	Night	32% 19% 28% 17% 4	930
	11th to 20th	Night	36% 17% 24% 18% 5% 1	936
	21st to end	Night	33% 23% 28% 14% 3	900
December	1st to 10th	Night	29% 21% 29% 16% 5%	900
	11th to 20th	Night	35% 24% 26% 12% 2	900
	21st to end	Night	28% 18% 31% 18% 5% 1	990

Page generated at 2025-11-15 01:55:57. Version ac07

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