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 2025-07-21.

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% 9% 3 7% 5% 4 4 4 5% 7% 36%	120322
	Night	15% 10% 3 8% 5% 4 4 3 5% 6% 36%	118798
Transparency	Day	46% 6% 11% 22% 14% 2	124002
	Night	46% 5% 11% 22% 14% 2	123255
Seeing	Day	18% 12% 26% 23% 15% 6%	16285
	Night	19% 9% 26% 30% 15% 2	25963

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	16% 17% 8% 12% 4 4 3 3 4 4 25%	3617
	11th to 20th	Day	13% 15% 7% 13% 6% 5% 4 3 3 4 27%	3953
	21st to end	Day	22% 13% 7% 9% 5% 5% 4 3 4 4 24%	4466
February	1st to 10th	Day	19% 15% 5% 9% 4 4 3 3 5% 5% 29%	1900
	11th to 20th	Day	17% 13% 4 11% 5% 4 4 3 3 5% 30%	1948
	21st to end	Day	18% 12% 5% 9% 5% 3 4 3 4 5% 31%	1650
March	1st to 10th	Day	18% 14% 6% 9% 5% 4 3 3 3 4 31%	4521
	11th to 20th	Day	13% 9% 3 6% 4 3 4 4 6% 7% 40%	4980
	21st to end	Day	17% 10% 3 7% 5% 4 5% 3 6% 7% 33%	5465
April	1st to 10th	Day	16% 11% 3 7% 5% 3 5% 4 5% 7% 35%	2357
	11th to 20th	Day	16% 9% 3 7% 4 4 4 3 5% 7% 38%	2432
	21st to end	Day	17% 9% 3 6% 5% 5% 4 3 5% 7% 36%	2600
May	1st to 10th	Day	20% 9% 3 8% 5% 5% 5% 4 6% 8% 28%	5862
	11th to 20th	Day	20% 9% 3 7% 4 4 3 4 5% 7% 34%	5861
	21st to end	Day	13% 7% 3 8% 5% 5% 6% 4 8% 9% 32%	6433
June	1st to 10th	Day	12% 6% 2 7% 5% 4 5% 5% 6% 8% 40%	2600
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 4 6% 9% 42%	2800
	21st to end	Day	8% 6% 2 6% 5% 5% 5% 5% 7% 9% 40%	2800
July	1st to 10th	Day	12% 7% 3 6% 5% 4 6% 4 6% 9% 37%	4580
	11th to 20th	Day	7% 5% 2 6% 4 5% 5% 4 7% 11% 45%	4420
	21st to end	Day	11% 7% 2 8% 6% 6% 6% 6% 7% 10% 31%	4466
August	1st to 10th	Day	9% 5% 2 5% 4 4 5% 5% 7% 11% 43%	4104
	11th to 20th	Day	8% 5% 2 7% 4 4 6% 4 8% 9% 43%	4044
	21st to end	Day	6% 4 2 5% 3 4 4 3 6% 8% 54%	4219
September	1st to 10th	Day	12% 6% 1 5% 4 3 4 3 5% 7% 49%	2161
	11th to 20th	Day	9% 5% 2 6% 4 3 4 4 6% 9% 48%	2090
	21st to end	Day	15% 6% 2 6% 5% 3 3 2 4 6% 46%	2090
October	1st to 10th	Day	10% 5% 2 6% 4 5% 5% 3 4 7% 49%	2090
	11th to 20th	Day	13% 7% 3 6% 4 4 4 3 4 7% 46%	1928
	21st to end	Day	20% 8% 3 6% 3 4 3 3 5% 6% 39%	1947
November	1st to 10th	Day	10% 6% 3 5% 4 3 4 3 4 6% 52%	1762
	11th to 20th	Day	15% 11% 3 8% 4 4 4 3 5% 7% 36%	1800
	21st to end	Day	18% 11% 4 7% 5% 4 3 3 4 5% 35%	1800
December	1st to 10th	Day	18% 10% 4 7% 5% 4 4 3 3 5% 37%	2930
	11th to 20th	Day	17% 15% 5% 10% 5% 5% 4 4 5% 5% 26%	3373
	21st to end	Day	21% 19% 5% 10% 5% 4 4 3 4 5% 18%	4273
January	1st to 10th	Night	15% 17% 6% 14% 7% 5% 4 3 5% 5% 19%	6348
	11th to 20th	Night	14% 17% 7% 12% 8% 5% 6% 2 4 5% 20%	5976
	21st to end	Night	21% 14% 4 8% 5% 4 3 3 3 4 30%	6490
February	1st to 10th	Night	21% 12% 4 9% 5% 4 4 4 4 5% 28%	2497
	11th to 20th	Night	17% 12% 4 10% 5% 5% 4 4 4 6% 29%	2338
	21st to end	Night	21% 14% 4 7% 4 4 4 3 5% 5% 31%	1980
March	1st to 10th	Night	20% 13% 3 6% 4 4 3 3 4 5% 37%	5332
	11th to 20th	Night	12% 7% 2 7% 4 4 4 4 6% 7% 43%	4980
	21st to end	Night	17% 9% 4 9% 4 4 5% 4 5% 6% 34%	5467
April	1st to 10th	Night	16% 9% 3 6% 4 5% 5% 4 5% 6% 37%	2059
	11th to 20th	Night	14% 8% 3 5% 3 5% 3 4 6% 7% 41%	1916
	21st to end	Night	17% 8% 2 6% 4 4 4 3 7% 7% 38%	1800
May	1st to 10th	Night	21% 10% 3 8% 5% 5% 5% 4 6% 6% 28%	4033
	11th to 20th	Night	16% 7% 3 7% 4 4 5% 4 7% 9% 34%	4032
	21st to end	Night	13% 6% 2 7% 5% 4 5% 4 6% 9% 38%	4421
June	1st to 10th	Night	9% 6% 1 6% 4 4 4 3 4 9% 49%	1794
	11th to 20th	Night	10% 5% 2 5% 3 4 5% 4 7% 11% 45%	1600
	21st to end	Night	6% 5% 2 5% 5% 4 5% 4 7% 10% 48%	1600
July	1st to 10th	Night	11% 6% 2 5% 4 4 3 3 5% 8% 49%	2610
	11th to 20th	Night	6% 5% 1 5% 3 5% 4 4 7% 9% 51%	2760
	21st to end	Night	11% 6% 2 7% 5% 5% 5% 5% 7% 10% 37%	3087
August	1st to 10th	Night	8% 6% 2 7% 4 4 5% 4 6% 8% 45%	2825
	11th to 20th	Night	8% 7% 2 6% 4 4 5% 4 6% 8% 46%	2785
	21st to end	Night	4 3 1 4 3 4 4 4 6% 9% 58%	3434
September	1st to 10th	Night	13% 4 1 4 4 4 4 3 5% 7% 51%	2000
	11th to 20th	Night	8% 5% 1 5% 3 3 4 3 5% 6% 55%	2090
	21st to end	Night	14% 6% 2 6% 4 4 3 3 4 6% 49%	2090
October	1st to 10th	Night	9% 4 2 7% 3 4 4 2 5% 7% 55%	2090
	11th to 20th	Night	14% 6% 2 6% 4 4 4 3 5% 6% 45%	2251
	21st to end	Night	18% 8% 3 5% 4 4 4 3 5% 6% 41%	2649
November	1st to 10th	Night	14% 8% 2 5% 3 3 4 3 5% 6% 47%	2536
	11th to 20th	Night	16% 9% 3 7% 4 3 4 3 5% 6% 40%	2600
	21st to end	Night	19% 13% 3 7% 5% 4 4 3 4 7% 31%	2600
December	1st to 10th	Night	17% 11% 4 8% 4 3 3 3 4 5% 38%	4250
	11th to 20th	Night	19% 14% 6% 10% 5% 4 4 3 4 6% 26%	4822
	21st to end	Night	23% 19% 6% 11% 5% 4 4 2 4 5% 17%	6656

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	62% 4 9% 18% 7%	3618
	11th to 20th	Day	62% 4 10% 13% 10% 1	3953
	21st to end	Day	64% 3 10% 14% 7% 2	4466
February	1st to 10th	Day	58% 5% 11% 17% 9% 1	1900
	11th to 20th	Day	57% 4 10% 19% 9% 1	1958
	21st to end	Day	55% 5% 10% 19% 9% 2	1650
March	1st to 10th	Day	57% 4 6% 19% 12% 2	4521
	11th to 20th	Day	42% 5% 9% 27% 15% 1	4980
	21st to end	Day	49% 5% 12% 20% 11% 2	5465
April	1st to 10th	Day	47% 7% 11% 22% 12% 1	2357
	11th to 20th	Day	46% 5% 11% 22% 13% 3	2432
	21st to end	Day	48% 6% 10% 23% 11% 2	2600
May	1st to 10th	Day	53% 7% 12% 17% 10% 1	5897
	11th to 20th	Day	49% 6% 11% 23% 10%	5865
	21st to end	Day	43% 9% 13% 22% 12% 1	6447
June	1st to 10th	Day	40% 8% 11% 22% 17% 2	2600
	11th to 20th	Day	37% 8% 14% 22% 17% 2	2800
	21st to end	Day	38% 9% 15% 20% 15% 4	2800
July	1st to 10th	Day	42% 5% 12% 21% 18% 1	4511
	11th to 20th	Day	33% 6% 12% 30% 18% 1	4345
	21st to end	Day	45% 7% 14% 17% 13% 3	4324
August	1st to 10th	Day	33% 7% 17% 30% 12% 1	4091
	11th to 20th	Day	35% 5% 12% 32% 16% 1	4083
	21st to end	Day	28% 5% 14% 30% 20% 3	4218
September	1st to 10th	Day	34% 6% 11% 23% 20% 6%	2161
	11th to 20th	Day	31% 7% 12% 25% 22% 3	2090
	21st to end	Day	39% 5% 10% 23% 21% 2	2090
October	1st to 10th	Day	24% 4 9% 32% 30% 2	3480
	11th to 20th	Day	31% 3 9% 30% 26% 1	3181
	21st to end	Day	37% 3 10% 25% 23% 2	3184
November	1st to 10th	Day	34% 5% 10% 29% 20% 2	1762
	11th to 20th	Day	48% 4 10% 21% 15% 3	1800
	21st to end	Day	52% 4 10% 21% 12% 1	1800
December	1st to 10th	Day	51% 3 9% 26% 10%	2930
	11th to 20th	Day	61% 5% 9% 18% 7%	3370
	21st to end	Day	68% 4 10% 12% 5%	4273
January	1st to 10th	Night	67% 5% 8% 14% 7%	6348
	11th to 20th	Night	67% 4 9% 11% 7% 2	5976
	21st to end	Night	58% 3 9% 20% 9% 1	6490
February	1st to 10th	Night	58% 5% 10% 19% 7% 1	2497
	11th to 20th	Night	55% 5% 10% 19% 11% 1	2339
	21st to end	Night	56% 4 11% 17% 10% 2	1980
March	1st to 10th	Night	50% 4 6% 27% 11% 2	5332
	11th to 20th	Night	38% 6% 12% 27% 16% 1	4980
	21st to end	Night	50% 5% 8% 24% 11% 2	5467
April	1st to 10th	Night	46% 6% 12% 23% 12% 2	2059
	11th to 20th	Night	40% 6% 11% 19% 18% 5%	1916
	21st to end	Night	44% 6% 12% 21% 14% 3	1800
May	1st to 10th	Night	55% 7% 10% 17% 8% 3	4060
	11th to 20th	Night	44% 8% 14% 23% 10% 1	4033
	21st to end	Night	39% 8% 18% 21% 10% 3	4429
June	1st to 10th	Night	33% 7% 10% 26% 21% 2	1794
	11th to 20th	Night	32% 7% 14% 22% 20% 4	1600
	21st to end	Night	30% 8% 17% 21% 18% 5%	1600
July	1st to 10th	Night	35% 6% 11% 25% 18% 6%	2570
	11th to 20th	Night	28% 7% 13% 31% 20% 2	2712
	21st to end	Night	40% 6% 13% 23% 14% 4	2988
August	1st to 10th	Night	36% 6% 14% 30% 12% 1	2814
	11th to 20th	Night	35% 6% 12% 28% 18% 1	2812
	21st to end	Night	22% 5% 16% 31% 22% 3	3434
September	1st to 10th	Night	32% 6% 12% 24% 19% 7%	2000
	11th to 20th	Night	28% 6% 10% 25% 27% 3	2090
	21st to end	Night	37% 5% 9% 21% 25% 2	2090
October	1st to 10th	Night	23% 3 7% 28% 36% 3	3480
	11th to 20th	Night	28% 4 10% 26% 32% 2	3737
	21st to end	Night	34% 4 8% 23% 27% 5%	4362
November	1st to 10th	Night	38% 5% 12% 21% 22% 3	2536
	11th to 20th	Night	45% 5% 9% 22% 17% 3	2600
	21st to end	Night	54% 5% 10% 21% 10% 1	2600
December	1st to 10th	Night	50% 4 12% 25% 10%	4250
	11th to 20th	Night	60% 4 11% 18% 7%	4824
	21st to end	Night	70% 4 9% 13% 3	6656

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	25% 23% 31% 16% 4	300
	11th to 20th	Day	28% 24% 34% 12% 2	300
	21st to end	Day	33% 19% 32% 13% 2	330
February	1st to 10th	Day	29% 22% 31% 16% 2	300
	11th to 20th	Day	30% 26% 29% 12% 2	300
	21st to end	Day	27% 21% 43% 7% 2	249
March	1st to 10th	Day	25% 25% 33% 14% 2	315
	11th to 20th	Day	25% 18% 32% 19% 6%	500
	21st to end	Day	31% 18% 29% 16% 6%	550
April	1st to 10th	Day	23% 18% 30% 20% 8% 1	500
	11th to 20th	Day	22% 20% 27% 18% 10% 3	492
	21st to end	Day	22% 15% 34% 17% 10% 2	500
May	1st to 10th	Day	23% 15% 31% 20% 8% 4	503
	11th to 20th	Day	21% 12% 25% 22% 15% 5%	527
	21st to end	Day	14% 8% 24% 28% 16% 9%	573
June	1st to 10th	Day	13% 7% 23% 29% 20% 7%	530
	11th to 20th	Day	11% 6% 23% 31% 23% 6%	730
	21st to end	Day	10% 6% 24% 32% 20% 8%	730
July	1st to 10th	Day	14% 3 17% 26% 24% 17%	721
	11th to 20th	Day	10% 4 23% 26% 25% 11%	623
	21st to end	Day	13% 2 18% 28% 25% 13%	532
August	1st to 10th	Day	10% 3 16% 27% 28% 15%	486
	11th to 20th	Day	12% 5% 19% 29% 23% 12%	479
	21st to end	Day	10% 4 12% 29% 29% 15%	539
September	1st to 10th	Day	11% 4 17% 24% 32% 11%	490
	11th to 20th	Day	11% 3 14% 25% 33% 14%	490
	21st to end	Day	18% 9% 20% 25% 20% 9%	490
October	1st to 10th	Day	11% 6% 22% 33% 21% 7%	490
	11th to 20th	Day	16% 14% 29% 23% 13% 4	490
	21st to end	Day	23% 14% 34% 23% 6% 1	396
November	1st to 10th	Day	14% 13% 35% 24% 10% 3	300
	11th to 20th	Day	22% 20% 28% 20% 7% 2	300
	21st to end	Day	25% 15% 39% 18% 3	300
December	1st to 10th	Day	21% 20% 36% 17% 5% 1	300
	11th to 20th	Day	22% 23% 37% 13% 5%	300
	21st to end	Day	30% 20% 32% 15% 3	330
January	1st to 10th	Night	22% 18% 31% 23% 5%	1000
	11th to 20th	Night	29% 15% 33% 21% 3	837
	21st to end	Night	23% 15% 34% 25% 3	880
February	1st to 10th	Night	29% 11% 32% 26% 2	800
	11th to 20th	Night	23% 17% 36% 22% 2	783
	21st to end	Night	28% 13% 32% 25% 2	660
March	1st to 10th	Night	23% 13% 34% 25% 5%	800
	11th to 20th	Night	23% 13% 33% 25% 6% 1	800
	21st to end	Night	23% 11% 32% 27% 6%	880
April	1st to 10th	Night	19% 13% 35% 24% 8%	659
	11th to 20th	Night	19% 9% 34% 26% 10% 1	593
	21st to end	Night	18% 7% 39% 27% 9% 1	600
May	1st to 10th	Night	22% 8% 30% 28% 12% 1	600
	11th to 20th	Night	17% 5% 26% 32% 17% 2	591
	21st to end	Night	10% 3 23% 35% 24% 3	650
June	1st to 10th	Night	11% 3 19% 43% 23% 1	600
	11th to 20th	Night	11% 3 15% 44% 24% 3	600
	21st to end	Night	7% 2 20% 42% 27% 3	600
July	1st to 10th	Night	13% 2 12% 34% 31% 8%	585
	11th to 20th	Night	7% 2 15% 38% 34% 4	582
	21st to end	Night	11% 3 12% 40% 31% 3	624
August	1st to 10th	Night	8% 3 14% 36% 36% 3	567
	11th to 20th	Night	9% 1 14% 41% 29% 5%	564
	21st to end	Night	7% 1 12% 34% 39% 6%	627
September	1st to 10th	Night	14% 2 14% 34% 33% 4	670
	11th to 20th	Night	9% 4 13% 35% 34% 5%	760
	21st to end	Night	16% 6% 21% 30% 25% 2	760
October	1st to 10th	Night	11% 5% 23% 35% 23% 4	760
	11th to 20th	Night	18% 9% 26% 31% 15% 1	760
	21st to end	Night	22% 9% 31% 26% 11%	836
November	1st to 10th	Night	18% 8% 29% 28% 16% 1	780
	11th to 20th	Night	21% 13% 30% 26% 10%	800
	21st to end	Night	25% 10% 27% 31% 6% 1	800
December	1st to 10th	Night	27% 11% 32% 24% 5%	800
	11th to 20th	Night	25% 10% 33% 25% 6% 1	800
	21st to end	Night	30% 15% 29% 23% 3	955

Page generated at 2025-07-18 01:09:58. Version ac07

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