West Chester
Clear Sky Chart
History

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

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	16% 7% 2 6% 4 4 4 4 6% 7% 40%	126819
	Night	16% 7% 2 6% 4 4 4 3 5% 7% 43%	123198
Transparency	Day	41% 5% 11% 23% 17% 3	130364
	Night	40% 5% 11% 23% 17% 3	127740
Seeing	Day	21% 16% 28% 21% 10% 3	16278
	Night	19% 13% 31% 26% 9% 1	25550

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% 5% 4 5% 4 6% 5% 46%	3597
	11th to 20th	Day	14% 8% 3 9% 5% 4 4 3 4 5% 41%	3735
	21st to end	Day	17% 8% 3 5% 4 4 4 3 4 6% 41%	4466
February	1st to 10th	Day	15% 8% 3 7% 4 4 4 3 6% 8% 38%	1800
	11th to 20th	Day	17% 7% 3 6% 5% 5% 4 3 5% 6% 39%	1882
	21st to end	Day	18% 10% 3 6% 4 3 4 2 5% 5% 39%	1650
March	1st to 10th	Day	21% 4 2 4 4 3 4 3 4 5% 46%	4556
	11th to 20th	Day	13% 6% 2 5% 4 4 4 3 5% 7% 45%	4980
	21st to end	Day	18% 6% 2 5% 4 4 3 3 5% 6% 43%	5754
April	1st to 10th	Day	15% 8% 3 6% 4 3 5% 4 5% 6% 40%	2400
	11th to 20th	Day	15% 7% 2 5% 4 5% 5% 3 5% 7% 42%	2374
	21st to end	Day	16% 6% 2 6% 5% 5% 5% 4 6% 7% 40%	2400
May	1st to 10th	Day	26% 8% 2 6% 4 4 4 4 6% 7% 30%	5807
	11th to 20th	Day	25% 8% 3 6% 4 3 4 3 5% 7% 32%	6318
	21st to end	Day	19% 6% 2 6% 5% 4 4 4 6% 8% 37%	6949
June	1st to 10th	Day	12% 6% 2 6% 4 5% 5% 4 7% 10% 39%	2800
	11th to 20th	Day	13% 7% 2 7% 5% 5% 4 4 7% 8% 37%	2800
	21st to end	Day	10% 5% 2 6% 5% 4 5% 4 6% 10% 43%	2800
July	1st to 10th	Day	9% 5% 2 6% 5% 6% 6% 5% 7% 11% 39%	6360
	11th to 20th	Day	11% 8% 2 8% 6% 5% 6% 5% 8% 11% 31%	6360
	21st to end	Day	10% 6% 3 7% 5% 5% 5% 4 6% 10% 39%	6713
August	1st to 10th	Day	19% 6% 2 6% 5% 5% 5% 4 6% 8% 34%	4234
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 4 7% 9% 41%	3884
	21st to end	Day	10% 6% 2 5% 3 4 4 3 6% 8% 49%	4312
September	1st to 10th	Day	15% 5% 2 6% 4 5% 4 4 5% 7% 45%	2400
	11th to 20th	Day	13% 5% 1 4 3 3 4 3 4 7% 51%	2385
	21st to end	Day	19% 7% 3 8% 5% 4 5% 3 4 5% 37%	2173
October	1st to 10th	Day	16% 6% 2 6% 4 4 4 3 5% 7% 44%	2000
	11th to 20th	Day	16% 5% 2 5% 3 3 3 3 5% 7% 49%	2000
	21st to end	Day	17% 6% 2 5% 3 3 2 4 5% 6% 47%	2155
November	1st to 10th	Day	13% 7% 2 3 4 2 3 2 4 6% 53%	1779
	11th to 20th	Day	16% 5% 2 4 3 4 3 4 6% 6% 47%	1800
	21st to end	Day	15% 7% 3 4 4 3 4 2 6% 6% 46%	1650
December	1st to 10th	Day	13% 7% 3 6% 4 5% 4 3 5% 6% 44%	2600
	11th to 20th	Day	20% 10% 2 4 3 3 3 3 4 6% 43%	2986
	21st to end	Day	17% 6% 2 6% 4 5% 5% 4 6% 7% 39%	3960
January	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 4 5% 49%	6349
	11th to 20th	Night	15% 8% 2 5% 3 3 3 3 4 6% 49%	6132
	21st to end	Night	17% 8% 2 6% 3 4 3 3 4 5% 43%	6490
February	1st to 10th	Night	17% 9% 2 6% 4 4 4 3 4 6% 41%	2600
	11th to 20th	Night	19% 7% 3 6% 4 5% 4 3 5% 5% 41%	2402
	21st to end	Night	17% 8% 2 6% 4 3 4 3 5% 6% 42%	1980
March	1st to 10th	Night	17% 7% 2 6% 4 3 3 3 4 6% 45%	5282
	11th to 20th	Night	11% 7% 3 6% 4 4 3 3 4 5% 51%	4979
	21st to end	Night	15% 8% 2 6% 4 4 4 3 4 6% 44%	5176
April	1st to 10th	Night	16% 8% 3 6% 4 4 4 4 6% 6% 40%	2000
	11th to 20th	Night	15% 6% 2 7% 4 4 3 3 5% 7% 42%	1975
	21st to end	Night	17% 5% 2 6% 4 4 4 3 6% 7% 43%	2000
May	1st to 10th	Night	24% 9% 3 7% 5% 4 5% 4 5% 7% 28%	4163
	11th to 20th	Night	25% 8% 3 7% 3 3 4 3 5% 7% 31%	3575
	21st to end	Night	21% 7% 2 5% 3 3 4 3 6% 8% 38%	3926
June	1st to 10th	Night	15% 5% 3 6% 4 4 4 2 6% 8% 43%	1600
	11th to 20th	Night	12% 7% 2 5% 5% 5% 6% 4 5% 10% 39%	1600
	21st to end	Night	10% 5% 2 6% 3 4 5% 5% 8% 11% 41%	1600
July	1st to 10th	Night	8% 7% 2 8% 5% 6% 5% 5% 7% 10% 37%	3600
	11th to 20th	Night	13% 9% 3 8% 5% 5% 6% 5% 7% 9% 29%	3600
	21st to end	Night	11% 6% 2 5% 3 4 4 4 7% 8% 44%	4122
August	1st to 10th	Night	21% 6% 2 6% 4 4 5% 4 6% 8% 35%	2915
	11th to 20th	Night	14% 7% 2 6% 4 4 4 3 6% 7% 42%	3130
	21st to end	Night	11% 4 2 5% 3 4 4 4 6% 7% 49%	3586
September	1st to 10th	Night	14% 8% 2 6% 4 3 3 3 4 6% 47%	2000
	11th to 20th	Night	13% 6% 2 4 3 3 4 3 4 6% 52%	2015
	21st to end	Night	20% 7% 2 6% 4 4 4 4 5% 6% 38%	2223
October	1st to 10th	Night	19% 6% 2 5% 4 4 3 3 4 6% 44%	2400
	11th to 20th	Night	17% 5% 1 4 3 3 3 3 5% 6% 50%	2400
	21st to end	Night	17% 7% 2 4 3 3 3 3 4 6% 50%	2684
November	1st to 10th	Night	11% 6% 2 4 3 3 3 3 5% 5% 54%	2564
	11th to 20th	Night	17% 7% 2 4 3 3 4 3 4 5% 47%	2600
	21st to end	Night	12% 6% 2 5% 5% 3 4 4 6% 7% 46%	2746
December	1st to 10th	Night	12% 7% 2 6% 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	19% 7% 2 6% 4 4 4 3 5% 6% 40%	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% 4 10% 26% 21% 3	3598
	11th to 20th	Day	45% 5% 8% 21% 16% 6%	3766
	21st to end	Day	44% 5% 11% 24% 15% 2	4466
February	1st to 10th	Day	44% 5% 14% 24% 12% 2	1800
	11th to 20th	Day	46% 5% 8% 24% 15% 2	1891
	21st to end	Day	48% 3 10% 23% 13% 4	1650
March	1st to 10th	Day	40% 4 8% 24% 18% 7%	4559
	11th to 20th	Day	37% 5% 9% 28% 15% 6%	4980
	21st to end	Day	43% 4 9% 17% 22% 5%	5754
April	1st to 10th	Day	44% 5% 9% 21% 16% 5%	2400
	11th to 20th	Day	41% 6% 11% 22% 16% 4	2374
	21st to end	Day	42% 6% 10% 20% 18% 4	2400
May	1st to 10th	Day	52% 6% 12% 18% 10% 2	5816
	11th to 20th	Day	52% 5% 9% 22% 11% 1	6322
	21st to end	Day	44% 6% 11% 24% 15% 1	6949
June	1st to 10th	Day	39% 7% 13% 24% 16% 1	2800
	11th to 20th	Day	43% 6% 13% 24% 11% 3	2800
	21st to end	Day	36% 6% 15% 25% 17% 1	2800
July	1st to 10th	Day	38% 10% 14% 20% 16% 2	6290
	11th to 20th	Day	44% 8% 17% 20% 9% 1	6285
	21st to end	Day	40% 6% 10% 21% 23% 1	6520
August	1st to 10th	Day	47% 7% 14% 22% 10%	4221
	11th to 20th	Day	38% 6% 12% 30% 14% 1	3921
	21st to end	Day	33% 6% 12% 27% 21% 2	4312
September	1st to 10th	Day	40% 5% 10% 21% 21% 3	2400
	11th to 20th	Day	33% 5% 9% 26% 23% 4	2385
	21st to end	Day	49% 5% 8% 17% 19% 2	2173
October	1st to 10th	Day	36% 4 9% 18% 28% 5%	3260
	11th to 20th	Day	25% 3 11% 25% 28% 7%	3240
	21st to end	Day	24% 3 11% 28% 29% 5%	3457
November	1st to 10th	Day	33% 4 14% 26% 20% 4	1779
	11th to 20th	Day	36% 5% 10% 25% 21% 3	1800
	21st to end	Day	38% 4 11% 32% 12% 2	1650
December	1st to 10th	Day	41% 4 15% 25% 12% 3	2600
	11th to 20th	Day	44% 3 11% 30% 10% 2	2986
	21st to end	Day	42% 6% 12% 22% 17% 2	3960
January	1st to 10th	Night	37% 3 9% 25% 21% 5%	6349
	11th to 20th	Night	38% 3 11% 24% 18% 7%	6176
	21st to end	Night	44% 3 9% 24% 18% 3	6490
February	1st to 10th	Night	45% 4 12% 25% 12% 2	2600
	11th to 20th	Night	45% 4 9% 21% 18% 4	2404
	21st to end	Night	44% 4 9% 21% 16% 5%	1980
March	1st to 10th	Night	41% 4 8% 25% 12% 9%	5292
	11th to 20th	Night	36% 4 13% 26% 15% 5%	4980
	21st to end	Night	43% 3 7% 18% 25% 4	5176
April	1st to 10th	Night	44% 4 9% 20% 18% 4	2000
	11th to 20th	Night	42% 4 10% 20% 20% 4	1975
	21st to end	Night	40% 5% 9% 20% 20% 5%	2000
May	1st to 10th	Night	55% 5% 11% 15% 11% 2	4170
	11th to 20th	Night	52% 6% 10% 19% 11% 1	3576
	21st to end	Night	45% 5% 12% 22% 14% 2	3927
June	1st to 10th	Night	40% 6% 11% 22% 21% 1	1600
	11th to 20th	Night	39% 7% 14% 25% 14% 1	1600
	21st to end	Night	34% 9% 15% 26% 16% 2	1600
July	1st to 10th	Night	40% 9% 16% 20% 13% 2	3560
	11th to 20th	Night	48% 8% 17% 18% 8% 1	3552
	21st to end	Night	36% 7% 13% 24% 20% 1	4001
August	1st to 10th	Night	47% 7% 18% 19% 9% 1	2904
	11th to 20th	Night	40% 6% 12% 26% 15% 1	3159
	21st to end	Night	33% 5% 12% 29% 20% 2	3586
September	1st to 10th	Night	39% 5% 11% 21% 21% 3	2000
	11th to 20th	Night	33% 5% 9% 24% 26% 4	2015
	21st to end	Night	44% 6% 8% 20% 20% 2	2223
October	1st to 10th	Night	41% 5% 9% 21% 22% 2	3920
	11th to 20th	Night	24% 3 10% 28% 28% 5%	3899
	21st to end	Night	26% 3 12% 32% 24% 5%	4332
November	1st to 10th	Night	32% 4 12% 26% 21% 5%	2564
	11th to 20th	Night	40% 4 9% 22% 21% 5%	2600
	21st to end	Night	37% 4 13% 27% 17% 2	2746
December	1st to 10th	Night	39% 3 12% 30% 14% 2	4580
	11th to 20th	Night	44% 3 10% 24% 14% 4	5208
	21st to end	Night	45% 4 11% 20% 18% 2	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% 30% 29% 12% 1 2	200
	11th to 20th	Day	27% 37% 27% 8% 1	237
	21st to end	Day	23% 33% 28% 13% 3	330
February	1st to 10th	Day	27% 28% 34% 8% 3	300
	11th to 20th	Day	34% 23% 34% 8% 1	430
	21st to end	Day	31% 23% 30% 12% 3	414
March	1st to 10th	Day	27% 24% 29% 15% 4	500
	11th to 20th	Day	25% 27% 29% 15% 2 1	500
	21st to end	Day	27% 26% 30% 12% 4 1	550
April	1st to 10th	Day	26% 27% 32% 11% 4 1	500
	11th to 20th	Day	25% 22% 30% 14% 6% 2	492
	21st to end	Day	25% 21% 32% 17% 5% 1	500
May	1st to 10th	Day	29% 20% 29% 13% 7% 1	503
	11th to 20th	Day	23% 15% 33% 20% 7% 2	527
	21st to end	Day	22% 10% 25% 25% 16% 2	573
June	1st to 10th	Day	17% 12% 30% 27% 12% 2	530
	11th to 20th	Day	16% 11% 31% 27% 12% 3	530
	21st to end	Day	12% 10% 29% 31% 13% 4	530
July	1st to 10th	Day	14% 4 23% 31% 22% 6%	525
	11th to 20th	Day	9% 4 24% 34% 24% 5%	524
	21st to end	Day	12% 6% 25% 35% 17% 4	576
August	1st to 10th	Day	14% 4 28% 27% 20% 7%	526
	11th to 20th	Day	14% 4 20% 36% 18% 8%	519
	21st to end	Day	13% 7% 25% 28% 19% 8%	583
September	1st to 10th	Day	18% 12% 21% 26% 15% 8%	530
	11th to 20th	Day	15% 5% 26% 31% 15% 8%	530
	21st to end	Day	26% 12% 20% 27% 12% 3	530
October	1st to 10th	Day	22% 8% 28% 21% 14% 7%	530
	11th to 20th	Day	19% 17% 29% 21% 12% 2	530
	21st to end	Day	21% 16% 33% 18% 9% 3	558
November	1st to 10th	Day	18% 14% 35% 19% 10% 3	401
	11th to 20th	Day	19% 20% 35% 20% 5%	400
	21st to end	Day	23% 22% 34% 16% 4 1	250
December	1st to 10th	Day	25% 30% 30% 11% 5%	200
	11th to 20th	Day	32% 27% 27% 11% 4	200
	21st to end	Day	25% 24% 29% 14% 8%	220
January	1st to 10th	Night	21% 23% 37% 17% 2	800
	11th to 20th	Night	25% 19% 32% 21% 3	800
	21st to end	Night	19% 22% 38% 19% 1	880
February	1st to 10th	Night	21% 19% 40% 19% 2	800
	11th to 20th	Night	26% 25% 35% 13% 1	783
	21st to end	Night	25% 21% 38% 15% 2	660
March	1st to 10th	Night	21% 23% 33% 20% 4	800
	11th to 20th	Night	22% 21% 35% 17% 5% 1	800
	21st to end	Night	20% 18% 40% 17% 5%	880
April	1st to 10th	Night	23% 20% 37% 17% 4	800
	11th to 20th	Night	21% 14% 36% 23% 6%	791
	21st to end	Night	21% 15% 34% 25% 6%	800
May	1st to 10th	Night	25% 12% 31% 26% 6%	658
	11th to 20th	Night	20% 10% 36% 28% 6%	394
	21st to end	Night	18% 6% 24% 37% 14% 1	433
June	1st to 10th	Night	18% 6% 29% 36% 11% 1	400
	11th to 20th	Night	14% 5% 29% 40% 13%	400
	21st to end	Night	9% 5% 27% 46% 13% 1	400
July	1st to 10th	Night	12% 3 23% 41% 20% 2	390
	11th to 20th	Night	7% 4 20% 43% 25% 1	388
	21st to end	Night	11% 4 21% 42% 20% 2	520
August	1st to 10th	Night	12% 3 18% 42% 22% 3	597
	11th to 20th	Night	15% 4 16% 39% 24% 2	747
	21st to end	Night	14% 4 20% 39% 22% 2	880
September	1st to 10th	Night	17% 5% 24% 35% 17% 2	800
	11th to 20th	Night	15% 4 20% 35% 23% 4	800
	21st to end	Night	22% 5% 24% 31% 16% 1	800
October	1st to 10th	Night	21% 8% 26% 25% 17% 3	800
	11th to 20th	Night	20% 11% 28% 30% 12%	800
	21st to end	Night	20% 13% 34% 24% 8%	880
November	1st to 10th	Night	15% 12% 36% 26% 10% 1	789
	11th to 20th	Night	21% 18% 28% 25% 7% 1	800
	21st to end	Night	18% 18% 34% 23% 6% 1	800
December	1st to 10th	Night	25% 17% 34% 17% 6% 1	800
	11th to 20th	Night	20% 17% 36% 22% 5%	800
	21st to end	Night	21% 18% 35% 24% 2	880

Page generated at 2025-10-31 02:02:37. Version ac07

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