Coatesville
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Coatesville 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	15% 6% 2 6% 4 4 4 4 6% 8% 40%	126962
	Night	16% 7% 2 6% 4 4 4 3 5% 7% 43%	123389
Transparency	Day	40% 6% 12% 23% 16% 3	130521
	Night	40% 6% 11% 23% 17% 3	127916
Seeing	Day	21% 17% 29% 21% 10% 3	16309
	Night	19% 16% 30% 25% 9% 1	25611

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	12% 7% 2 6% 4 4 4 3 5% 6% 45%	3597
	11th to 20th	Day	14% 8% 3 10% 5% 4 4 3 4 5% 39%	3775
	21st to end	Day	16% 8% 3 6% 5% 4 4 3 5% 6% 40%	4466
February	1st to 10th	Day	14% 8% 3 7% 5% 4 4 3 6% 8% 38%	1800
	11th to 20th	Day	16% 9% 3 7% 4 5% 4 4 4 7% 38%	1870
	21st to end	Day	17% 11% 3 6% 4 4 3 3 4 5% 39%	1650
March	1st to 10th	Day	20% 5% 1 4 4 4 4 3 4 6% 45%	4586
	11th to 20th	Day	13% 6% 2 6% 4 4 4 4 5% 8% 43%	4980
	21st to end	Day	18% 6% 2 5% 4 4 4 3 5% 6% 42%	5730
April	1st to 10th	Day	15% 8% 2 6% 5% 5% 4 3 6% 7% 39%	2400
	11th to 20th	Day	14% 6% 2 6% 5% 4 5% 4 5% 7% 41%	2374
	21st to end	Day	15% 6% 2 6% 4 5% 4 4 6% 7% 40%	2400
May	1st to 10th	Day	25% 7% 2 5% 4 4 5% 4 5% 8% 30%	5811
	11th to 20th	Day	25% 7% 3 7% 4 4 3 3 5% 8% 32%	6318
	21st to end	Day	18% 6% 2 6% 4 4 4 4 6% 8% 37%	6949
June	1st to 10th	Day	12% 6% 2 6% 5% 5% 5% 4 7% 10% 39%	2800
	11th to 20th	Day	14% 6% 2 6% 5% 4 4 4 7% 9% 38%	2800
	21st to end	Day	11% 5% 2 6% 4 5% 5% 5% 7% 10% 42%	2800
July	1st to 10th	Day	9% 5% 2 6% 5% 5% 6% 5% 7% 10% 40%	6360
	11th to 20th	Day	11% 7% 3 8% 5% 5% 6% 5% 8% 11% 32%	6360
	21st to end	Day	10% 7% 3 6% 5% 5% 5% 5% 6% 9% 40%	6681
August	1st to 10th	Day	19% 5% 2 6% 4 5% 6% 4 7% 8% 34%	4234
	11th to 20th	Day	13% 5% 2 6% 4 4 5% 4 7% 9% 41%	3897
	21st to end	Day	11% 5% 2 5% 3 4 4 4 6% 8% 48%	4312
September	1st to 10th	Day	15% 5% 2 5% 4 4 5% 4 5% 6% 45%	2400
	11th to 20th	Day	13% 5% 2 5% 3 3 4 3 5% 7% 50%	2395
	21st to end	Day	19% 7% 3 6% 4 5% 5% 3 4 6% 37%	2155
October	1st to 10th	Day	16% 6% 2 7% 3 4 4 3 5% 6% 45%	2000
	11th to 20th	Day	15% 6% 2 5% 4 3 3 4 4 7% 48%	2000
	21st to end	Day	17% 5% 2 5% 3 4 4 3 5% 6% 46%	2169
November	1st to 10th	Day	12% 7% 2 5% 2 3 2 3 5% 7% 52%	1851
	11th to 20th	Day	15% 6% 2 4 3 4 3 5% 6% 7% 45%	1863
	21st to end	Day	16% 7% 2 4 4 3 4 4 5% 7% 44%	1633
December	1st to 10th	Day	13% 8% 3 6% 4 4 3 3 5% 6% 44%	2600
	11th to 20th	Day	20% 10% 2 5% 3 3 3 2 4 6% 42%	2986
	21st to end	Day	16% 6% 2 6% 5% 5% 5% 4 5% 7% 39%	3960
January	1st to 10th	Night	12% 6% 2 6% 4 4 4 3 5% 6% 47%	6349
	11th to 20th	Night	15% 7% 2 5% 3 3 4 2 4 6% 48%	6107
	21st to end	Night	16% 8% 3 7% 4 3 4 3 4 5% 43%	6490
February	1st to 10th	Night	17% 8% 2 6% 5% 4 4 4 5% 6% 41%	2600
	11th to 20th	Night	18% 7% 3 6% 4 4 4 3 5% 5% 41%	2416
	21st to end	Night	16% 8% 3 6% 3 4 3 4 6% 6% 40%	1980
March	1st to 10th	Night	17% 7% 3 6% 4 3 3 3 4 6% 44%	5238
	11th to 20th	Night	11% 6% 3 6% 4 3 4 3 5% 6% 51%	4979
	21st to end	Night	14% 8% 3 6% 4 4 4 3 4 6% 45%	5201
April	1st to 10th	Night	16% 8% 3 6% 4 3 4 4 5% 7% 40%	2000
	11th to 20th	Night	15% 6% 3 6% 4 3 4 3 6% 8% 42%	1975
	21st to end	Night	16% 7% 2 5% 4 4 4 3 5% 7% 44%	2000
May	1st to 10th	Night	24% 8% 2 7% 5% 4 5% 4 6% 7% 28%	4159
	11th to 20th	Night	25% 8% 3 6% 3 3 4 3 5% 7% 31%	3575
	21st to end	Night	21% 7% 2 6% 4 3 4 3 5% 8% 38%	3926
June	1st to 10th	Night	14% 5% 3 5% 5% 4 4 4 5% 8% 43%	1600
	11th to 20th	Night	13% 6% 2 6% 5% 4 5% 5% 6% 9% 40%	1600
	21st to end	Night	11% 5% 1 6% 4 4 4 5% 8% 11% 42%	1600
July	1st to 10th	Night	8% 6% 2 7% 6% 6% 5% 4 6% 10% 39%	3600
	11th to 20th	Night	13% 8% 3 8% 5% 6% 5% 5% 8% 9% 31%	3600
	21st to end	Night	12% 5% 2 6% 4 4 5% 4 7% 8% 44%	4153
August	1st to 10th	Night	20% 6% 2 6% 4 4 5% 3 6% 8% 35%	2915
	11th to 20th	Night	14% 6% 2 7% 5% 4 4 4 6% 6% 42%	3117
	21st to end	Night	11% 5% 2 5% 3 4 4 4 6% 7% 50%	3586
September	1st to 10th	Night	14% 7% 3 5% 4 3 4 3 4 7% 48%	2000
	11th to 20th	Night	12% 6% 2 5% 3 2 3 4 4 6% 52%	2005
	21st to end	Night	19% 7% 2 6% 4 4 4 3 6% 6% 39%	2240
October	1st to 10th	Night	18% 7% 2 7% 4 4 3 3 4 6% 44%	2400
	11th to 20th	Night	17% 5% 1 5% 3 3 3 3 5% 6% 50%	2400
	21st to end	Night	17% 6% 2 4 3 3 3 3 5% 6% 50%	2670
November	1st to 10th	Night	11% 5% 2 5% 3 3 4 4 5% 6% 52%	2667
	11th to 20th	Night	16% 7% 2 5% 3 3 4 4 4 6% 46%	2692
	21st to end	Night	13% 5% 2 5% 5% 4 5% 3 6% 7% 46%	2765
December	1st to 10th	Night	12% 7% 2 7% 4 4 4 3 5% 6% 47%	4580
	11th to 20th	Night	22% 7% 2 5% 3 3 3 3 5% 6% 41%	5208
	21st to end	Night	18% 7% 3 7% 4 4 4 4 5% 6% 39%	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	37% 5% 11% 25% 20% 2	3598
	11th to 20th	Day	46% 5% 8% 20% 16% 6%	3807
	21st to end	Day	43% 6% 11% 24% 14% 1	4466
February	1st to 10th	Day	44% 5% 14% 24% 11% 1	1800
	11th to 20th	Day	46% 5% 9% 23% 14% 2	1879
	21st to end	Day	46% 4 10% 22% 13% 4	1650
March	1st to 10th	Day	39% 5% 9% 24% 17% 6%	4589
	11th to 20th	Day	36% 6% 11% 27% 14% 5%	4980
	21st to end	Day	43% 4 9% 17% 22% 5%	5730
April	1st to 10th	Day	42% 7% 12% 20% 15% 5%	2400
	11th to 20th	Day	39% 7% 12% 21% 17% 4	2374
	21st to end	Day	41% 7% 11% 19% 18% 4	2400
May	1st to 10th	Day	50% 7% 12% 19% 10% 2	5820
	11th to 20th	Day	50% 8% 10% 22% 10% 1	6322
	21st to end	Day	42% 8% 12% 22% 15% 1	6949
June	1st to 10th	Day	38% 9% 14% 22% 16% 1	2800
	11th to 20th	Day	41% 9% 13% 23% 11% 3	2800
	21st to end	Day	35% 9% 14% 24% 16% 1	2800
July	1st to 10th	Day	38% 10% 14% 20% 15% 2	6290
	11th to 20th	Day	44% 9% 18% 20% 9% 1	6285
	21st to end	Day	39% 7% 10% 21% 22% 1	6489
August	1st to 10th	Day	46% 8% 14% 22% 10%	4221
	11th to 20th	Day	38% 7% 12% 30% 13% 1	3934
	21st to end	Day	33% 6% 12% 28% 19% 2	4312
September	1st to 10th	Day	39% 7% 11% 20% 20% 3	2400
	11th to 20th	Day	33% 5% 10% 26% 22% 4	2395
	21st to end	Day	48% 6% 9% 17% 18% 2	2155
October	1st to 10th	Day	36% 5% 9% 17% 29% 5%	3260
	11th to 20th	Day	26% 4 12% 25% 25% 8%	3240
	21st to end	Day	24% 3 12% 28% 28% 5%	3483
November	1st to 10th	Day	33% 4 13% 25% 20% 4	1851
	11th to 20th	Day	35% 6% 12% 24% 21% 2	1863
	21st to end	Day	39% 5% 12% 30% 11% 2	1633
December	1st to 10th	Day	41% 4 16% 24% 12% 3	2600
	11th to 20th	Day	44% 4 12% 29% 9% 2	2986
	21st to end	Day	42% 7% 13% 21% 15% 2	3960
January	1st to 10th	Night	37% 4 10% 24% 20% 5%	6349
	11th to 20th	Night	37% 4 11% 24% 18% 6%	6150
	21st to end	Night	42% 4 9% 25% 17% 2	6490
February	1st to 10th	Night	44% 5% 14% 24% 11% 2	2600
	11th to 20th	Night	44% 4 9% 21% 18% 3	2418
	21st to end	Night	43% 5% 10% 21% 15% 5%	1980
March	1st to 10th	Night	40% 5% 9% 24% 13% 8%	5248
	11th to 20th	Night	34% 6% 14% 28% 14% 5%	4980
	21st to end	Night	41% 5% 8% 18% 25% 4	5201
April	1st to 10th	Night	44% 5% 11% 19% 17% 4	2000
	11th to 20th	Night	39% 7% 11% 19% 19% 4	1975
	21st to end	Night	40% 6% 10% 20% 19% 5%	2000
May	1st to 10th	Night	53% 8% 11% 15% 11% 2	4166
	11th to 20th	Night	51% 8% 11% 18% 11% 1	3576
	21st to end	Night	44% 8% 11% 22% 14% 2	3927
June	1st to 10th	Night	38% 9% 11% 22% 19% 1	1600
	11th to 20th	Night	39% 8% 14% 22% 14% 1	1600
	21st to end	Night	33% 11% 13% 25% 15% 2	1600
July	1st to 10th	Night	41% 9% 16% 19% 13% 3	3560
	11th to 20th	Night	47% 8% 17% 17% 8% 1	3552
	21st to end	Night	36% 7% 12% 23% 21% 1	4030
August	1st to 10th	Night	46% 8% 18% 19% 9%	2904
	11th to 20th	Night	41% 7% 11% 26% 14% 1	3146
	21st to end	Night	33% 6% 12% 29% 19% 2	3586
September	1st to 10th	Night	38% 6% 10% 21% 21% 3	2000
	11th to 20th	Night	33% 4 10% 23% 25% 4	2005
	21st to end	Night	44% 7% 9% 18% 20% 2	2240
October	1st to 10th	Night	43% 4 10% 20% 21% 2	3920
	11th to 20th	Night	24% 4 11% 28% 28% 5%	3899
	21st to end	Night	25% 4 11% 32% 23% 5%	4306
November	1st to 10th	Night	31% 6% 11% 26% 20% 5%	2667
	11th to 20th	Night	39% 5% 9% 22% 20% 5%	2692
	21st to end	Night	37% 6% 12% 27% 16% 2	2765
December	1st to 10th	Night	39% 4 12% 30% 13% 2	4580
	11th to 20th	Night	44% 4 12% 23% 13% 3	5208
	21st to end	Night	44% 7% 12% 18% 18% 1	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	28% 31% 26% 13% 1 1	200
	11th to 20th	Day	33% 29% 29% 9%	245
	21st to end	Day	24% 34% 27% 12% 3	330
February	1st to 10th	Day	29% 29% 32% 9% 2	300
	11th to 20th	Day	35% 26% 32% 6% 1	418
	21st to end	Day	31% 27% 27% 12% 3	414
March	1st to 10th	Day	27% 26% 31% 13% 3	500
	11th to 20th	Day	28% 26% 30% 12% 3 1	500
	21st to end	Day	29% 25% 31% 11% 3 1	550
April	1st to 10th	Day	26% 28% 33% 9% 4 1	500
	11th to 20th	Day	25% 24% 30% 15% 5% 2	492
	21st to end	Day	26% 18% 36% 14% 5%	500
May	1st to 10th	Day	28% 21% 27% 18% 5% 1	503
	11th to 20th	Day	23% 16% 35% 19% 7% 1	527
	21st to end	Day	20% 11% 27% 26% 15% 2	573
June	1st to 10th	Day	14% 14% 33% 24% 12% 3	530
	11th to 20th	Day	19% 11% 31% 26% 12% 2	530
	21st to end	Day	13% 12% 27% 32% 11% 4	530
July	1st to 10th	Day	13% 7% 23% 32% 20% 6%	525
	11th to 20th	Day	7% 4 25% 37% 22% 4	524
	21st to end	Day	10% 7% 29% 34% 16% 3	576
August	1st to 10th	Day	13% 6% 26% 30% 19% 6%	526
	11th to 20th	Day	13% 5% 24% 35% 17% 7%	519
	21st to end	Day	11% 9% 26% 30% 17% 9%	583
September	1st to 10th	Day	19% 11% 23% 26% 16% 6%	530
	11th to 20th	Day	15% 5% 28% 28% 16% 8%	530
	21st to end	Day	24% 13% 24% 26% 10% 2	530
October	1st to 10th	Day	21% 9% 30% 21% 12% 7%	530
	11th to 20th	Day	21% 15% 30% 22% 10% 2	530
	21st to end	Day	19% 20% 33% 17% 9% 2	565
November	1st to 10th	Day	19% 18% 32% 20% 9% 3	425
	11th to 20th	Day	20% 24% 31% 20% 5%	421
	21st to end	Day	25% 23% 30% 19% 3	233
December	1st to 10th	Day	28% 25% 31% 13% 4	200
	11th to 20th	Day	35% 24% 29% 10% 4	200
	21st to end	Day	25% 25% 30% 15% 5%	220
January	1st to 10th	Night	20% 28% 35% 15% 2	800
	11th to 20th	Night	25% 23% 31% 19% 2	800
	21st to end	Night	18% 26% 39% 16% 1	880
February	1st to 10th	Night	22% 23% 36% 19% 1	800
	11th to 20th	Night	27% 29% 31% 11% 1	783
	21st to end	Night	24% 24% 36% 14% 2	660
March	1st to 10th	Night	22% 25% 33% 17% 3	800
	11th to 20th	Night	22% 23% 33% 16% 5% 1	800
	21st to end	Night	21% 19% 39% 17% 4	880
April	1st to 10th	Night	22% 24% 34% 17% 4	800
	11th to 20th	Night	20% 16% 35% 23% 5%	791
	21st to end	Night	20% 19% 33% 23% 6%	800
May	1st to 10th	Night	22% 13% 34% 25% 6%	654
	11th to 20th	Night	21% 12% 33% 28% 6%	394
	21st to end	Night	17% 8% 24% 38% 12% 1	433
June	1st to 10th	Night	15% 7% 28% 40% 11%	400
	11th to 20th	Night	14% 5% 29% 43% 10%	400
	21st to end	Night	11% 3 30% 43% 13% 1	400
July	1st to 10th	Night	12% 4 24% 39% 21% 1	390
	11th to 20th	Night	7% 3 19% 44% 26% 1	388
	21st to end	Night	11% 3 23% 43% 19% 2	538
August	1st to 10th	Night	12% 2 20% 39% 24% 3	597
	11th to 20th	Night	16% 3 18% 36% 25% 2	734
	21st to end	Night	13% 5% 20% 39% 21% 2	880
September	1st to 10th	Night	17% 5% 25% 32% 19% 2	800
	11th to 20th	Night	13% 5% 21% 35% 23% 4	800
	21st to end	Night	22% 6% 25% 29% 17% 1	800
October	1st to 10th	Night	19% 10% 26% 25% 16% 4	800
	11th to 20th	Night	19% 13% 29% 28% 11%	800
	21st to end	Night	21% 15% 32% 24% 8%	880
November	1st to 10th	Night	15% 14% 34% 26% 10% 1	820
	11th to 20th	Night	20% 21% 30% 22% 6% 1	829
	21st to end	Night	19% 21% 33% 22% 6% 1	800
December	1st to 10th	Night	24% 21% 33% 16% 6% 1	800
	11th to 20th	Night	21% 20% 36% 20% 4	800
	21st to end	Night	21% 21% 33% 24% 1	880

Page generated at 2025-11-15 00:56:00. Version ac07

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