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 2024-03-28.

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% 9% 4 6% 6% 3 5% 5% 5% 7% 36%	79235
	Night	16% 10% 4 7% 5% 3 4 4 5% 7% 37%	80745
Transparency	Day	47% 7% 12% 20% 13% 2	78948
	Night	47% 6% 11% 20% 13% 2	80552
Seeing	Day	18% 12% 26% 23% 15% 6%	14432
	Night	19% 9% 26% 30% 15% 2	24292

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% 16% 7% 8% 5% 3 4 5% 4 5% 26%	2527
	11th to 20th	Day	17% 13% 8% 10% 7% 5% 6% 4 4 4 22%	2759
	21st to end	Day	29% 16% 9% 7% 4 2 3 3 4 4 19%	3124
February	1st to 10th	Day	19% 13% 5% 8% 5% 3 4 4 5% 5% 29%	1805
	11th to 20th	Day	16% 12% 4 10% 6% 3 5% 4 3 6% 31%	1848
	21st to end	Day	19% 12% 6% 8% 6% 3 4 3 4 5% 30%	1570
March	1st to 10th	Day	18% 11% 4 5% 6% 3 5% 4 4 5% 34%	1914
	11th to 20th	Day	18% 11% 4 6% 5% 2 4 4 5% 6% 33%	2090
	21st to end	Day	22% 10% 3 7% 5% 3 5% 5% 5% 6% 30%	2255
April	1st to 10th	Day	14% 11% 3 6% 6% 3 4 5% 5% 7% 37%	2121
	11th to 20th	Day	17% 8% 3 6% 5% 3 3 4 5% 7% 39%	2186
	21st to end	Day	18% 8% 3 6% 6% 3 5% 4 5% 6% 35%	2340
May	1st to 10th	Day	21% 7% 3 6% 5% 3 5% 4 4 6% 34%	2340
	11th to 20th	Day	20% 7% 3 6% 6% 3 4 4 5% 6% 35%	2301
	21st to end	Day	8% 6% 3 7% 7% 4 6% 6% 9% 10% 35%	2531
June	1st to 10th	Day	12% 5% 2 5% 6% 3 5% 6% 6% 8% 41%	2340
	11th to 20th	Day	10% 4 2 5% 6% 4 5% 5% 6% 9% 43%	2520
	21st to end	Day	9% 5% 3 6% 5% 4 6% 6% 6% 10% 40%	2520
July	1st to 10th	Day	11% 7% 3 5% 7% 3 6% 6% 6% 9% 37%	2520
	11th to 20th	Day	8% 5% 2 5% 5% 4 6% 6% 7% 10% 41%	2435
	21st to end	Day	10% 5% 2 5% 5% 3 7% 7% 7% 9% 40%	2547
August	1st to 10th	Day	8% 5% 2 5% 5% 3 6% 6% 7% 11% 43%	2327
	11th to 20th	Day	9% 6% 3 6% 6% 3 5% 6% 7% 10% 40%	2303
	21st to end	Day	8% 5% 3 5% 5% 4 5% 6% 7% 9% 46%	2401
September	1st to 10th	Day	13% 6% 2 4 5% 3 4 5% 5% 8% 47%	2048
	11th to 20th	Day	9% 5% 2 5% 5% 3 4 5% 6% 9% 48%	1980
	21st to end	Day	14% 6% 2 5% 5% 3 3 3 4 6% 49%	1980
October	1st to 10th	Day	11% 5% 2 5% 6% 4 5% 4 5% 7% 47%	1980
	11th to 20th	Day	14% 7% 3 5% 5% 3 4 4 4 7% 45%	1827
	21st to end	Day	22% 8% 3 6% 4 3 4 3 5% 6% 38%	1845
November	1st to 10th	Day	9% 6% 3 5% 4 2 4 5% 4 6% 53%	1672
	11th to 20th	Day	15% 9% 4 7% 5% 3 4 4 5% 7% 37%	1710
	21st to end	Day	18% 9% 5% 6% 5% 4 4 3 4 5% 36%	1710
December	1st to 10th	Day	21% 12% 6% 9% 7% 4 4 5% 3 6% 25%	1710
	11th to 20th	Day	16% 14% 5% 8% 6% 3 4 4 4 5% 29%	2153
	21st to end	Day	21% 20% 6% 9% 7% 3 4 4 4 5% 17%	2996
January	1st to 10th	Night	17% 18% 7% 10% 7% 4 4 4 4 5% 21%	4428
	11th to 20th	Night	15% 16% 6% 12% 8% 4 4 4 4 5% 21%	4182
	21st to end	Night	26% 18% 5% 8% 5% 3 3 3 3 4 22%	4532
February	1st to 10th	Night	22% 11% 4 8% 6% 3 4 5% 4 5% 29%	2373
	11th to 20th	Night	16% 10% 4 8% 6% 4 4 5% 5% 6% 31%	2218
	21st to end	Night	21% 13% 4 6% 5% 3 4 3 4 5% 30%	1884
March	1st to 10th	Night	18% 10% 4 6% 5% 3 4 4 4 5% 38%	2266
	11th to 20th	Night	17% 8% 3 6% 6% 3 4 4 5% 7% 37%	2090
	21st to end	Night	20% 8% 4 7% 5% 2 5% 5% 5% 6% 34%	2256
April	1st to 10th	Night	15% 7% 3 6% 5% 3 5% 5% 5% 7% 39%	1854
	11th to 20th	Night	15% 7% 3 4 4 4 4 5% 6% 7% 40%	1723
	21st to end	Night	17% 7% 3 6% 5% 4 4 4 6% 6% 38%	1620
May	1st to 10th	Night	19% 8% 3 6% 6% 3 5% 4 5% 6% 34%	1620
	11th to 20th	Night	16% 6% 4 6% 5% 3 6% 4 5% 7% 38%	1592
	21st to end	Night	7% 5% 2 5% 5% 4 5% 6% 8% 11% 43%	1744
June	1st to 10th	Night	8% 6% 1 5% 5% 3 5% 4 4 9% 49%	1616
	11th to 20th	Night	9% 4 2 4 3 3 5% 5% 7% 12% 47%	1440
	21st to end	Night	6% 4 3 5% 5% 3 5% 5% 7% 10% 47%	1440
July	1st to 10th	Night	10% 6% 3 5% 5% 2 4 4 6% 8% 47%	1440
	11th to 20th	Night	6% 5% 2 5% 4 3 5% 4 7% 9% 50%	1520
	21st to end	Night	8% 5% 2 5% 5% 3 4 5% 6% 8% 49%	1771
August	1st to 10th	Night	7% 4 2 5% 5% 3 6% 5% 6% 9% 49%	1611
	11th to 20th	Night	7% 5% 2 5% 4 3 6% 6% 6% 8% 47%	1598
	21st to end	Night	6% 4 2 4 3 3 4 4 6% 10% 55%	1952
September	1st to 10th	Night	13% 5% 1 3 5% 3 3 4 5% 7% 50%	1894
	11th to 20th	Night	8% 4 2 5% 4 3 4 4 6% 7% 55%	1980
	21st to end	Night	13% 5% 2 4 5% 3 4 3 4 6% 51%	1980
October	1st to 10th	Night	9% 4 2 6% 4 3 4 3 4 7% 54%	1980
	11th to 20th	Night	15% 6% 2 6% 5% 3 4 4 5% 6% 43%	2132
	21st to end	Night	19% 8% 3 5% 5% 3 4 4 4 5% 40%	2509
November	1st to 10th	Night	14% 7% 3 4 4 2 4 4 5% 6% 48%	2406
	11th to 20th	Night	16% 8% 3 6% 5% 3 4 4 5% 6% 41%	2470
	21st to end	Night	19% 11% 4 6% 6% 3 4 4 4 7% 32%	2470
December	1st to 10th	Night	20% 12% 5% 8% 6% 3 3 4 5% 5% 29%	2470
	11th to 20th	Night	19% 12% 6% 8% 5% 3 4 4 5% 6% 29%	3042
	21st to end	Night	21% 17% 6% 10% 7% 3 5% 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	52% 6% 10% 22% 10% 1	1914
	11th to 20th	Day	52% 6% 11% 19% 11% 1	2090
	21st to end	Day	55% 6% 11% 20% 8% 1	2255
April	1st to 10th	Day	46% 7% 12% 23% 12% 1	2121
	11th to 20th	Day	45% 5% 11% 21% 15% 3	2186
	21st to end	Day	49% 6% 10% 22% 11% 2	2340
May	1st to 10th	Day	51% 8% 12% 20% 9% 1	2340
	11th to 20th	Day	50% 8% 12% 18% 12% 1	2305
	21st to end	Day	40% 12% 15% 21% 12% 1	2531
June	1st to 10th	Day	40% 8% 11% 22% 16% 2	2340
	11th to 20th	Day	35% 9% 14% 22% 17% 2	2520
	21st to end	Day	38% 10% 15% 21% 13% 4	2520
July	1st to 10th	Day	41% 8% 13% 19% 16% 2	2451
	11th to 20th	Day	36% 9% 13% 24% 17% 2	2360
	21st to end	Day	36% 10% 15% 22% 13% 3	2405
August	1st to 10th	Day	32% 10% 16% 26% 14% 2	2314
	11th to 20th	Day	37% 8% 14% 26% 15% 1	2303
	21st to end	Day	33% 8% 15% 22% 17% 5%	2401
September	1st to 10th	Day	35% 6% 12% 23% 18% 5%	2048
	11th to 20th	Day	32% 7% 13% 25% 21% 2	1980
	21st to end	Day	37% 5% 11% 24% 22% 2	1980
October	1st to 10th	Day	37% 7% 10% 24% 20% 2	1980
	11th to 20th	Day	39% 5% 10% 24% 20% 2	1827
	21st to end	Day	47% 5% 11% 21% 14% 2	1845
November	1st to 10th	Day	33% 5% 10% 29% 20% 3	1672
	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	49% 5% 8% 23% 14% 2	2266
	11th to 20th	Night	47% 5% 13% 22% 11% 2	2090
	21st to end	Night	50% 6% 11% 23% 8% 2	2256
April	1st to 10th	Night	43% 6% 12% 25% 12% 2	1854
	11th to 20th	Night	41% 7% 11% 17% 19% 6%	1723
	21st to end	Night	46% 6% 12% 20% 13% 3	1620
May	1st to 10th	Night	50% 7% 10% 20% 10% 3	1620
	11th to 20th	Night	44% 8% 14% 19% 13% 2	1593
	21st to end	Night	31% 10% 19% 22% 14% 4	1745
June	1st to 10th	Night	32% 8% 11% 27% 21% 3	1616
	11th to 20th	Night	30% 8% 15% 23% 19% 5%	1440
	21st to end	Night	30% 9% 18% 21% 17% 5%	1440
July	1st to 10th	Night	35% 8% 13% 21% 18% 5%	1400
	11th to 20th	Night	29% 9% 15% 26% 19% 3	1472
	21st to end	Night	31% 8% 12% 30% 16% 2	1672
August	1st to 10th	Night	29% 9% 15% 27% 16% 2	1600
	11th to 20th	Night	31% 9% 15% 28% 16% 1	1598
	21st to end	Night	25% 8% 16% 25% 22% 5%	1952
September	1st to 10th	Night	32% 6% 13% 25% 17% 7%	1894
	11th to 20th	Night	28% 6% 11% 26% 26% 3	1980
	21st to end	Night	35% 5% 9% 22% 26% 3	1980
October	1st to 10th	Night	31% 5% 11% 27% 23% 3	1980
	11th to 20th	Night	40% 6% 9% 24% 20% 2	2132
	21st to end	Night	46% 5% 9% 22% 16% 3	2509
November	1st to 10th	Night	38% 5% 12% 21% 22% 4	2406
	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% 16% 30% 16% 6%	490
April	1st to 10th	Day	20% 17% 30% 22% 9% 1	420
	11th to 20th	Day	22% 18% 25% 19% 12% 4	412
	21st to end	Day	24% 14% 35% 15% 10% 2	420
May	1st to 10th	Day	23% 15% 31% 17% 8% 4	423
	11th to 20th	Day	20% 11% 27% 23% 13% 5%	447
	21st to end	Day	13% 6% 22% 30% 18% 11%	485
June	1st to 10th	Day	14% 5% 22% 30% 21% 8%	450
	11th to 20th	Day	11% 6% 24% 30% 22% 6%	630
	21st to end	Day	10% 6% 24% 33% 19% 7%	630
July	1st to 10th	Day	14% 3 16% 26% 24% 18%	621
	11th to 20th	Day	9% 5% 23% 27% 24% 12%	533
	21st to end	Day	13% 3 18% 27% 26% 14%	488
August	1st to 10th	Day	10% 3 15% 27% 29% 16%	446
	11th to 20th	Day	12% 5% 18% 29% 23% 13%	439
	21st to end	Day	11% 4 13% 29% 28% 14%	495
September	1st to 10th	Day	12% 4 18% 24% 32% 11%	450
	11th to 20th	Day	12% 3 14% 24% 32% 14%	450
	21st to end	Day	16% 9% 19% 26% 21% 9%	450
October	1st to 10th	Day	12% 7% 23% 32% 19% 7%	450
	11th to 20th	Day	16% 15% 30% 25% 12% 2	450
	21st to end	Day	25% 14% 33% 22% 5%	360
November	1st to 10th	Day	13% 13% 34% 26% 10% 3	270
	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	24% 11% 31% 28% 7%	821
April	1st to 10th	Night	18% 12% 36% 26% 9%	594
	11th to 20th	Night	19% 9% 32% 27% 11% 1	533
	21st to end	Night	19% 7% 38% 27% 9% 1	540
May	1st to 10th	Night	22% 9% 30% 27% 12% 1	540
	11th to 20th	Night	18% 4 27% 31% 17% 2	531
	21st to end	Night	9% 3 21% 36% 27% 4	584
June	1st to 10th	Night	10% 3 19% 43% 24% 1	540
	11th to 20th	Night	10% 3 16% 43% 24% 4	540
	21st to end	Night	7% 2 20% 44% 25% 2	540
July	1st to 10th	Night	13% 2 12% 33% 32% 9%	525
	11th to 20th	Night	7% 2 15% 37% 34% 5%	522
	21st to end	Night	10% 3 12% 39% 33% 3	591
August	1st to 10th	Night	7% 3 13% 37% 37% 3	537
	11th to 20th	Night	9% 1 14% 40% 30% 5%	534
	21st to end	Night	7% 1 13% 35% 38% 6%	594
September	1st to 10th	Night	14% 2 13% 33% 33% 4	634
	11th to 20th	Night	9% 4 14% 33% 34% 5%	720
	21st to end	Night	15% 6% 21% 30% 26% 2	720
October	1st to 10th	Night	11% 5% 23% 35% 22% 4	720
	11th to 20th	Night	19% 9% 26% 31% 14% 1	720
	21st to end	Night	23% 10% 31% 26% 10%	792
November	1st to 10th	Night	17% 8% 29% 28% 16% 1	740
	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 2024-03-24 23:37:01. Version ac07

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