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 2024-11-04.

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% 37%	101364
	Night	15% 10% 3 7% 4 4 4 3 5% 7% 38%	97606
Transparency	Day	44% 6% 12% 22% 14% 2	105046
	Night	44% 6% 11% 22% 15% 2	102063
Seeing	Day	18% 12% 26% 23% 15% 6%	15340
	Night	19% 9% 26% 30% 15% 2	25023

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	17% 17% 6% 9% 4 4 4 4 5% 5% 26%	2527
	11th to 20th	Day	16% 14% 7% 11% 6% 6% 5% 3 4 5% 22%	2759
	21st to end	Day	29% 17% 8% 8% 4 3 2 2 3 4 19%	3124
February	1st to 10th	Day	18% 15% 5% 9% 4 4 3 3 5% 5% 29%	1805
	11th to 20th	Day	16% 13% 3 11% 5% 4 4 4 3 5% 31%	1848
	21st to end	Day	19% 13% 5% 9% 5% 3 3 3 4 5% 30%	1570
March	1st to 10th	Day	22% 16% 5% 8% 5% 3 3 3 3 4 26%	3147
	11th to 20th	Day	14% 9% 3 6% 4 3 4 4 5% 7% 41%	3480
	21st to end	Day	18% 9% 3 7% 4 4 5% 4 5% 7% 34%	3815
April	1st to 10th	Day	14% 11% 3 7% 5% 3 5% 4 5% 7% 36%	2239
	11th to 20th	Day	16% 9% 3 7% 4 4 3 3 6% 7% 39%	2309
	21st to end	Day	17% 9% 3 6% 5% 5% 5% 3 5% 6% 36%	2470
May	1st to 10th	Day	21% 10% 3 7% 5% 4 4 4 5% 7% 30%	4082
	11th to 20th	Day	20% 8% 3 6% 4 3 3 3 5% 7% 37%	4081
	21st to end	Day	10% 6% 3 8% 6% 5% 6% 5% 8% 10% 34%	4476
June	1st to 10th	Day	12% 6% 2 7% 5% 4 5% 5% 6% 8% 40%	2470
	11th to 20th	Day	10% 5% 2 6% 5% 4 5% 4 6% 9% 43%	2660
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 5% 7% 9% 40%	2660
July	1st to 10th	Day	12% 7% 3 6% 5% 4 6% 4 6% 9% 37%	4440
	11th to 20th	Day	7% 4 2 6% 4 5% 5% 4 7% 11% 46%	4285
	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	9% 6% 3 5% 4 3 4 3 4 6% 54%	1699
	11th to 20th	Day	15% 11% 3 8% 4 4 4 3 5% 7% 37%	1710
	21st to end	Day	18% 10% 4 6% 5% 4 4 3 4 5% 36%	1710
December	1st to 10th	Day	21% 13% 5% 9% 6% 4 4 3 3 6% 24%	1710
	11th to 20th	Day	15% 15% 5% 9% 5% 4 5% 3 4 5% 29%	2153
	21st to end	Day	21% 21% 6% 10% 6% 4 4 3 4 5% 17%	2996
January	1st to 10th	Night	16% 18% 6% 11% 6% 5% 4 3 4 5% 21%	4428
	11th to 20th	Night	15% 17% 6% 14% 6% 5% 4 2 4 5% 21%	4182
	21st to end	Night	26% 19% 5% 8% 4 3 3 2 3 4 22%	4532
February	1st to 10th	Night	22% 12% 4 8% 5% 4 4 4 4 5% 28%	2373
	11th to 20th	Night	16% 11% 4 9% 5% 5% 4 4 5% 7% 30%	2218
	21st to end	Night	21% 14% 4 7% 4 4 3 3 4 5% 30%	1884
March	1st to 10th	Night	22% 13% 3 5% 4 4 3 3 4 5% 33%	3706
	11th to 20th	Night	13% 8% 2 6% 4 4 4 3 5% 6% 44%	3480
	21st to end	Night	17% 8% 3 8% 4 3 4 4 5% 5% 39%	3817
April	1st to 10th	Night	15% 9% 3 6% 4 5% 5% 4 5% 6% 38%	1956
	11th to 20th	Night	15% 8% 3 5% 3 5% 3 4 6% 7% 40%	1819
	21st to end	Night	17% 8% 2 6% 4 4 4 3 7% 6% 38%	1710
May	1st to 10th	Night	22% 11% 3 8% 5% 5% 5% 4 5% 6% 27%	2813
	11th to 20th	Night	18% 7% 2 6% 4 4 4 3 6% 8% 38%	2812
	21st to end	Night	9% 5% 2 5% 4 4 5% 4 6% 9% 45%	3079
June	1st to 10th	Night	8% 6% 1 6% 4 4 4 3 5% 9% 48%	1705
	11th to 20th	Night	9% 4 2 5% 3 4 5% 3 7% 11% 46%	1520
	21st to end	Night	6% 5% 2 5% 5% 4 5% 4 7% 10% 47%	1520
July	1st to 10th	Night	11% 6% 2 5% 4 4 3 3 5% 8% 49%	2530
	11th to 20th	Night	5% 5% 1 5% 3 4 4 4 7% 9% 52%	2675
	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	13% 8% 2 5% 3 3 4 3 5% 6% 48%	2452
	11th to 20th	Night	16% 9% 3 7% 4 3 4 3 5% 6% 40%	2470
	21st to end	Night	19% 12% 3 7% 5% 3 4 3 4 7% 32%	2470
December	1st to 10th	Night	20% 13% 4 8% 5% 3 3 4 5% 5% 29%	2470
	11th to 20th	Night	19% 13% 5% 9% 5% 4 3 3 5% 6% 29%	3042
	21st to end	Night	21% 18% 6% 12% 6% 5% 4 3 4 5% 17%	4642

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	59% 5% 11% 15% 9%	2528
	11th to 20th	Day	64% 5% 12% 13% 5%	2759
	21st to end	Day	71% 3 9% 13% 3 1	3124
February	1st to 10th	Day	57% 5% 10% 16% 9% 1	1805
	11th to 20th	Day	56% 4 10% 20% 9% 1	1858
	21st to end	Day	56% 5% 9% 19% 8% 2	1570
March	1st to 10th	Day	60% 6% 7% 18% 8% 1	3147
	11th to 20th	Day	41% 6% 11% 24% 16% 1	3480
	21st to end	Day	47% 7% 12% 22% 11% 1	3815
April	1st to 10th	Day	47% 7% 11% 22% 12% 1	2239
	11th to 20th	Day	45% 5% 11% 22% 14% 3	2309
	21st to end	Day	48% 6% 10% 22% 11% 2	2470
May	1st to 10th	Day	52% 9% 12% 20% 7% 1	4120
	11th to 20th	Day	46% 8% 13% 22% 12%	4085
	21st to end	Day	39% 12% 14% 19% 15% 2	4489
June	1st to 10th	Day	40% 9% 11% 22% 16% 2	2470
	11th to 20th	Day	35% 9% 14% 23% 18% 2	2660
	21st to end	Day	38% 9% 15% 20% 14% 4	2660
July	1st to 10th	Day	43% 5% 12% 20% 18% 2	4371
	11th to 20th	Day	32% 6% 11% 31% 19% 1	4210
	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	33% 5% 10% 29% 21% 3	1699
	11th to 20th	Day	47% 4 10% 21% 15% 3	1710
	21st to end	Day	51% 4 10% 22% 13% 1	1710
December	1st to 10th	Day	63% 4 11% 17% 5%	1710
	11th to 20th	Day	57% 5% 10% 19% 9%	2150
	21st to end	Day	69% 5% 9% 10% 6%	2996
January	1st to 10th	Night	63% 7% 9% 14% 7%	4428
	11th to 20th	Night	65% 5% 10% 14% 6%	4182
	21st to end	Night	67% 4 9% 15% 5% 1	4532
February	1st to 10th	Night	58% 5% 10% 19% 7% 1	2373
	11th to 20th	Night	53% 5% 10% 20% 11% 1	2219
	21st to end	Night	57% 4 11% 16% 9% 2	1884
March	1st to 10th	Night	52% 5% 8% 23% 10% 1	3706
	11th to 20th	Night	38% 7% 13% 25% 15% 1	3480
	21st to end	Night	45% 6% 10% 27% 11% 1	3817
April	1st to 10th	Night	44% 6% 12% 24% 12% 2	1956
	11th to 20th	Night	40% 6% 11% 18% 19% 5%	1819
	21st to end	Night	45% 6% 12% 20% 13% 3	1710
May	1st to 10th	Night	56% 9% 8% 18% 7% 2	2840
	11th to 20th	Night	42% 8% 14% 25% 9% 1	2813
	21st to end	Night	29% 10% 18% 24% 13% 5%	3087
June	1st to 10th	Night	32% 8% 11% 26% 21% 2	1705
	11th to 20th	Night	30% 8% 15% 22% 20% 4	1520
	21st to end	Night	30% 8% 17% 22% 17% 5%	1520
July	1st to 10th	Night	35% 6% 11% 25% 18% 6%	2490
	11th to 20th	Night	27% 7% 13% 32% 20% 2	2627
	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	37% 4 11% 21% 22% 4	2452
	11th to 20th	Night	44% 5% 9% 22% 18% 3	2470
	21st to end	Night	52% 5% 11% 21% 10% 1	2470
December	1st to 10th	Night	56% 6% 11% 19% 8% 1	2470
	11th to 20th	Night	56% 5% 10% 20% 9% 1	3044
	21st to end	Night	69% 6% 9% 12% 4	4642

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	26% 21% 31% 16% 5%	270
	11th to 20th	Day	29% 24% 33% 12% 2	270
	21st to end	Day	35% 19% 30% 13% 3	297
February	1st to 10th	Day	29% 23% 28% 17% 2	270
	11th to 20th	Day	29% 25% 31% 13% 2	270
	21st to end	Day	30% 20% 41% 7% 2	225
March	1st to 10th	Day	26% 26% 32% 14% 2	284
	11th to 20th	Day	26% 17% 32% 19% 6%	460
	21st to end	Day	31% 17% 30% 16% 6%	506
April	1st to 10th	Day	21% 19% 29% 21% 8% 1	460
	11th to 20th	Day	22% 19% 25% 19% 11% 3	452
	21st to end	Day	23% 15% 35% 16% 10% 2	460
May	1st to 10th	Day	23% 15% 31% 19% 8% 4	463
	11th to 20th	Day	20% 11% 25% 23% 15% 6%	487
	21st to end	Day	13% 7% 24% 30% 17% 10%	529
June	1st to 10th	Day	13% 7% 23% 29% 20% 8%	490
	11th to 20th	Day	11% 6% 23% 31% 23% 6%	680
	21st to end	Day	10% 6% 25% 33% 19% 8%	680
July	1st to 10th	Day	14% 3 17% 26% 23% 17%	671
	11th to 20th	Day	9% 4 24% 27% 25% 11%	578
	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	13% 13% 35% 25% 11% 3	279
	11th to 20th	Day	22% 21% 26% 21% 8% 2	270
	21st to end	Day	24% 15% 39% 19% 4	270
December	1st to 10th	Day	21% 21% 34% 17% 6% 1	270
	11th to 20th	Day	19% 24% 37% 14% 6%	270
	21st to end	Day	29% 22% 32% 14% 3	297
January	1st to 10th	Night	23% 18% 31% 23% 5%	950
	11th to 20th	Night	29% 15% 32% 21% 3	796
	21st to end	Night	23% 15% 33% 26% 3	836
February	1st to 10th	Night	29% 11% 31% 27% 2	760
	11th to 20th	Night	22% 17% 36% 23% 3	743
	21st to end	Night	29% 13% 32% 24% 2	628
March	1st to 10th	Night	23% 13% 34% 24% 5%	760
	11th to 20th	Night	23% 13% 33% 25% 6% 1	760
	21st to end	Night	23% 11% 32% 28% 7%	836
April	1st to 10th	Night	18% 13% 35% 25% 8%	626
	11th to 20th	Night	19% 9% 33% 26% 10% 1	563
	21st to end	Night	18% 7% 39% 26% 8% 1	570
May	1st to 10th	Night	22% 8% 30% 27% 12% 1	570
	11th to 20th	Night	17% 5% 26% 32% 17% 2	561
	21st to end	Night	10% 3 23% 35% 26% 4	617
June	1st to 10th	Night	10% 3 20% 42% 24% 1	570
	11th to 20th	Night	10% 3 16% 44% 24% 3	570
	21st to end	Night	7% 2 21% 44% 25% 2	570
July	1st to 10th	Night	13% 2 12% 34% 30% 8%	555
	11th to 20th	Night	7% 3 15% 38% 33% 4	552
	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	17% 8% 29% 28% 16% 1	755
	11th to 20th	Night	21% 13% 30% 26% 11%	760
	21st to end	Night	24% 9% 27% 31% 7% 1	760
December	1st to 10th	Night	27% 12% 31% 25% 6%	760
	11th to 20th	Night	25% 9% 34% 25% 6% 1	760
	21st to end	Night	29% 15% 30% 23% 3	907

Page generated at 2024-11-01 22:47:39. Version ac07

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