Willow Grove Area
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Willow Grove Area clear sky chart from 2005-11-06 to 2024-02-25.

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 5% 5% 3 5% 5% 6% 8% 41%	78382
	Night	16% 6% 2 5% 5% 3 4 4 5% 7% 43%	80907
Transparency	Day	41% 6% 12% 23% 15% 2	78089
	Night	41% 5% 11% 23% 16% 3	80734
Seeing	Day	21% 14% 28% 22% 11% 3	13990
	Night	19% 11% 31% 28% 9% 1	23406

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	15% 6% 2 5% 4 3 3 5% 5% 5% 46%	2517
	11th to 20th	Day	14% 10% 3 7% 6% 4 4 5% 5% 6% 37%	2561
	21st to end	Day	22% 8% 3 6% 6% 3 5% 4 4 6% 35%	3124
February	1st to 10th	Day	17% 6% 3 5% 6% 3 4 6% 4 7% 39%	1710
	11th to 20th	Day	17% 7% 2 5% 6% 2 5% 4 5% 6% 40%	1810
	21st to end	Day	18% 8% 3 6% 5% 3 3 5% 4 6% 38%	1520
March	1st to 10th	Day	13% 6% 2 6% 5% 3 4 4 5% 6% 46%	1797
	11th to 20th	Day	16% 6% 3 5% 5% 4 5% 6% 5% 6% 38%	1969
	21st to end	Day	21% 6% 3 4 5% 3 5% 3 5% 6% 39%	2308
April	1st to 10th	Day	14% 7% 3 5% 6% 3 4 4 5% 7% 41%	2160
	11th to 20th	Day	15% 6% 3 4 5% 3 4 5% 6% 8% 42%	2134
	21st to end	Day	16% 5% 2 5% 6% 3 5% 5% 6% 8% 39%	2160
May	1st to 10th	Day	19% 7% 2 6% 5% 3 5% 6% 5% 7% 35%	2308
	11th to 20th	Day	20% 6% 2 5% 4 4 6% 5% 5% 8% 36%	2478
	21st to end	Day	14% 7% 2 5% 5% 3 5% 5% 5% 9% 39%	2725
June	1st to 10th	Day	14% 5% 2 6% 4 4 5% 6% 7% 9% 39%	2520
	11th to 20th	Day	12% 6% 3 6% 6% 4 6% 6% 6% 9% 37%	2520
	21st to end	Day	11% 5% 2 5% 5% 3 6% 6% 7% 10% 41%	2520
July	1st to 10th	Day	10% 5% 3 5% 6% 4 7% 6% 6% 10% 39%	2520
	11th to 20th	Day	8% 5% 2 5% 6% 3 6% 6% 8% 10% 42%	2520
	21st to end	Day	11% 6% 3 6% 5% 3 6% 6% 6% 9% 38%	2718
August	1st to 10th	Day	9% 5% 2 6% 6% 4 6% 6% 7% 10% 40%	2326
	11th to 20th	Day	11% 5% 2 6% 6% 3 5% 6% 7% 10% 39%	2126
	21st to end	Day	13% 6% 2 5% 5% 3 5% 5% 6% 9% 40%	2376
September	1st to 10th	Day	15% 5% 3 5% 6% 2 5% 5% 5% 7% 43%	2160
	11th to 20th	Day	13% 4 2 3 4 2 3 3 5% 8% 51%	2125
	21st to end	Day	16% 8% 3 5% 6% 3 4 4 5% 7% 39%	1980
October	1st to 10th	Day	18% 6% 2 4 5% 3 3 5% 5% 6% 43%	1811
	11th to 20th	Day	16% 5% 2 4 5% 2 4 5% 5% 7% 46%	1800
	21st to end	Day	19% 6% 2 4 4 2 3 4 5% 7% 43%	1908
November	1st to 10th	Day	13% 7% 3 4 4 2 2 3 4 6% 53%	1671
	11th to 20th	Day	17% 4 2 5% 4 2 4 4 4 7% 48%	1710
	21st to end	Day	15% 5% 2 5% 4 3 3 5% 5% 8% 45%	1592
December	1st to 10th	Day	19% 7% 2 5% 4 2 4 4 6% 8% 38%	1520
	11th to 20th	Day	20% 6% 2 4 4 2 3 4 5% 7% 42%	1906
	21st to end	Day	16% 6% 2 5% 5% 4 5% 6% 7% 7% 35%	2772
January	1st to 10th	Night	16% 6% 2 5% 5% 3 4 3 5% 6% 45%	4429
	11th to 20th	Night	18% 8% 2 5% 4 2 4 3 4 5% 44%	4394
	21st to end	Night	24% 9% 2 5% 5% 2 3 4 4 5% 37%	4532
February	1st to 10th	Night	16% 8% 2 5% 4 2 4 4 5% 7% 41%	2470
	11th to 20th	Night	18% 8% 3 6% 5% 3 4 4 4 6% 41%	2256
	21st to end	Night	18% 7% 3 5% 5% 2 4 3 4 5% 43%	1831
March	1st to 10th	Night	16% 5% 2 4 4 3 3 4 5% 7% 46%	2150
	11th to 20th	Night	17% 7% 3 5% 5% 3 3 4 4 5% 44%	1988
	21st to end	Night	17% 7% 3 5% 4 3 4 3 4 6% 43%	2046
April	1st to 10th	Night	16% 6% 2 6% 5% 3 5% 4 5% 7% 41%	1800
	11th to 20th	Night	15% 6% 2 6% 5% 4 4 5% 5% 6% 42%	1775
	21st to end	Night	20% 5% 2 5% 4 2 5% 3 5% 8% 43%	1800
May	1st to 10th	Night	18% 8% 3 6% 6% 3 5% 5% 4 6% 35%	1665
	11th to 20th	Night	18% 6% 2 6% 6% 2 4 4 6% 7% 38%	1415
	21st to end	Night	16% 6% 2 5% 4 2 5% 5% 5% 9% 41%	1550
June	1st to 10th	Night	17% 5% 2 5% 5% 3 4 3 6% 8% 42%	1440
	11th to 20th	Night	12% 6% 2 6% 6% 3 5% 6% 5% 8% 41%	1440
	21st to end	Night	10% 3 2 5% 5% 4 5% 7% 6% 10% 43%	1440
July	1st to 10th	Night	10% 6% 2 6% 6% 4 6% 5% 8% 9% 38%	1440
	11th to 20th	Night	7% 4 2 6% 5% 4 5% 6% 6% 12% 42%	1440
	21st to end	Night	8% 5% 2 5% 4 4 5% 6% 8% 9% 43%	1597
August	1st to 10th	Night	9% 6% 3 5% 5% 3 5% 6% 7% 9% 42%	1611
	11th to 20th	Night	11% 7% 3 6% 6% 3 5% 6% 6% 8% 39%	1757
	21st to end	Night	14% 5% 2 6% 5% 4 4 5% 6% 9% 41%	1980
September	1st to 10th	Night	14% 7% 2 5% 5% 3 4 4 5% 7% 45%	1800
	11th to 20th	Night	12% 5% 2 4 4 3 4 3 4 6% 53%	1834
	21st to end	Night	17% 6% 2 6% 5% 2 4 5% 5% 7% 42%	1980
October	1st to 10th	Night	19% 6% 2 5% 4 3 4 4 4 5% 43%	2146
	11th to 20th	Night	15% 5% 2 4 4 2 3 5% 4 6% 50%	2160
	21st to end	Night	18% 7% 2 3 4 2 3 4 4 5% 46%	2448
November	1st to 10th	Night	12% 6% 2 4 4 3 3 3 5% 6% 54%	2407
	11th to 20th	Night	17% 5% 3 5% 4 2 4 4 4 5% 48%	2470
	21st to end	Night	14% 4 2 4 4 3 5% 4 6% 7% 45%	2584
December	1st to 10th	Night	20% 7% 3 5% 4 3 4 4 5% 7% 39%	2660
	11th to 20th	Night	18% 6% 2 5% 5% 2 4 4 5% 7% 41%	3288
	21st to end	Night	18% 7% 3 6% 5% 3 4 5% 5% 7% 38%	4884

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	38% 4 14% 27% 13% 4	2518
	11th to 20th	Day	46% 5% 13% 21% 13% 2	2561
	21st to end	Day	50% 5% 12% 22% 10% 1	3124
February	1st to 10th	Day	44% 6% 13% 25% 10% 2	1710
	11th to 20th	Day	43% 4 10% 25% 16% 2	1819
	21st to end	Day	46% 5% 10% 22% 13% 4	1520
March	1st to 10th	Day	38% 4 13% 27% 14% 3	1800
	11th to 20th	Day	43% 5% 12% 21% 16% 3	1969
	21st to end	Day	46% 5% 9% 20% 17% 3	2308
April	1st to 10th	Day	41% 5% 11% 23% 15% 5%	2160
	11th to 20th	Day	39% 6% 11% 24% 16% 4	2134
	21st to end	Day	42% 5% 12% 19% 17% 4	2160
May	1st to 10th	Day	47% 7% 10% 18% 15% 3	2308
	11th to 20th	Day	45% 7% 10% 22% 14% 2	2482
	21st to end	Day	41% 7% 14% 22% 14% 2	2725
June	1st to 10th	Day	39% 7% 15% 24% 14% 1	2520
	11th to 20th	Day	41% 8% 14% 24% 11% 2	2520
	21st to end	Day	37% 8% 13% 24% 16% 1	2520
July	1st to 10th	Day	39% 10% 12% 22% 14% 3	2450
	11th to 20th	Day	34% 9% 13% 26% 15% 3	2445
	21st to end	Day	40% 8% 12% 22% 16% 2	2566
August	1st to 10th	Day	38% 8% 15% 23% 15% 1	2313
	11th to 20th	Day	38% 8% 16% 21% 15% 2	2126
	21st to end	Day	39% 8% 12% 21% 17% 3	2376
September	1st to 10th	Day	39% 6% 12% 20% 20% 3	2160
	11th to 20th	Day	32% 5% 10% 25% 24% 4	2125
	21st to end	Day	45% 5% 8% 18% 21% 2	1980
October	1st to 10th	Day	41% 5% 12% 23% 18% 1	1811
	11th to 20th	Day	36% 5% 12% 25% 21% 2	1800
	21st to end	Day	41% 5% 10% 24% 18% 2	1908
November	1st to 10th	Day	35% 3 12% 26% 21% 3	1671
	11th to 20th	Day	38% 5% 9% 25% 21% 3	1710
	21st to end	Day	37% 4 14% 30% 12% 2	1592
December	1st to 10th	Day	43% 5% 15% 23% 13% 1	1520
	11th to 20th	Day	42% 4 14% 27% 11% 2	1906
	21st to end	Day	45% 7% 15% 22% 11% 1	2772
January	1st to 10th	Night	40% 4 10% 29% 13% 4	4429
	11th to 20th	Night	44% 4 11% 25% 13% 3	4394
	21st to end	Night	49% 4 8% 25% 12% 2	4532
February	1st to 10th	Night	44% 4 13% 25% 12% 2	2470
	11th to 20th	Night	45% 4 9% 20% 18% 5%	2258
	21st to end	Night	43% 5% 6% 24% 18% 4	1831
March	1st to 10th	Night	37% 5% 12% 25% 18% 4	2160
	11th to 20th	Night	43% 5% 9% 21% 18% 4	1989
	21st to end	Night	43% 4 10% 18% 19% 6%	2046
April	1st to 10th	Night	43% 5% 11% 20% 17% 5%	1800
	11th to 20th	Night	41% 6% 9% 21% 20% 3	1775
	21st to end	Night	41% 5% 9% 21% 19% 5%	1800
May	1st to 10th	Night	48% 6% 10% 17% 17% 2	1665
	11th to 20th	Night	45% 6% 12% 17% 17% 3	1416
	21st to end	Night	41% 9% 16% 18% 14% 2	1551
June	1st to 10th	Night	40% 6% 11% 21% 19% 2	1440
	11th to 20th	Night	40% 8% 14% 24% 13% 1	1440
	21st to end	Night	33% 9% 15% 25% 15% 2	1440
July	1st to 10th	Night	41% 10% 15% 19% 14% 2	1400
	11th to 20th	Night	32% 10% 16% 25% 14% 3	1392
	21st to end	Night	33% 9% 17% 23% 15% 2	1508
August	1st to 10th	Night	36% 9% 16% 22% 16% 2	1600
	11th to 20th	Night	41% 9% 14% 20% 15% 2	1757
	21st to end	Night	39% 7% 13% 18% 19% 3	1980
September	1st to 10th	Night	39% 7% 12% 20% 20% 3	1800
	11th to 20th	Night	32% 5% 10% 22% 27% 4	1834
	21st to end	Night	41% 6% 7% 22% 22% 2	1980
October	1st to 10th	Night	42% 5% 12% 23% 15% 2	2146
	11th to 20th	Night	35% 4 13% 27% 18% 3	2160
	21st to end	Night	40% 4 9% 26% 18% 2	2448
November	1st to 10th	Night	33% 4 12% 25% 20% 5%	2407
	11th to 20th	Night	39% 4 9% 23% 20% 5%	2470
	21st to end	Night	38% 5% 12% 28% 16% 2	2584
December	1st to 10th	Night	45% 4 12% 23% 14% 1	2660
	11th to 20th	Night	41% 5% 13% 20% 17% 4	3288
	21st to end	Night	45% 6% 13% 24% 11% 1	4884

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	24% 28% 31% 12% 3 2	180
	11th to 20th	Day	35% 26% 26% 11% 2	193
	21st to end	Day	27% 25% 30% 13% 5%	297
February	1st to 10th	Day	30% 22% 34% 10% 4	270
	11th to 20th	Day	30% 23% 34% 11% 2	418
	21st to end	Day	29% 21% 35% 13% 3	362
March	1st to 10th	Day	27% 19% 33% 18% 3	420
	11th to 20th	Day	25% 23% 32% 15% 4 1	420
	21st to end	Day	28% 22% 28% 15% 6% 1	462
April	1st to 10th	Day	27% 20% 32% 14% 6% 1	420
	11th to 20th	Day	21% 23% 30% 17% 7% 2	412
	21st to end	Day	27% 19% 32% 17% 3 2	420
May	1st to 10th	Day	28% 17% 31% 15% 7% 2	423
	11th to 20th	Day	23% 13% 30% 24% 8% 2	447
	21st to end	Day	20% 9% 23% 27% 16% 5%	485
June	1st to 10th	Day	16% 11% 30% 26% 14% 4	450
	11th to 20th	Day	16% 12% 29% 28% 12% 3	450
	21st to end	Day	14% 10% 28% 30% 12% 4	450
July	1st to 10th	Day	12% 5% 21% 33% 23% 7%	445
	11th to 20th	Day	9% 2 24% 34% 26% 5%	444
	21st to end	Day	13% 6% 27% 32% 17% 5%	488
August	1st to 10th	Day	11% 5% 23% 35% 20% 7%	446
	11th to 20th	Day	14% 3 21% 35% 20% 7%	439
	21st to end	Day	14% 8% 25% 29% 15% 9%	495
September	1st to 10th	Day	18% 9% 23% 25% 16% 8%	450
	11th to 20th	Day	17% 5% 25% 28% 16% 9%	450
	21st to end	Day	24% 13% 21% 25% 12% 6%	450
October	1st to 10th	Day	26% 5% 27% 21% 14% 8%	450
	11th to 20th	Day	17% 14% 30% 24% 13% 2	450
	21st to end	Day	23% 15% 32% 17% 11% 2	459
November	1st to 10th	Day	21% 12% 30% 23% 12% 2	365
	11th to 20th	Day	22% 18% 32% 23% 5% 1	370
	21st to end	Day	21% 18% 36% 19% 5% 1	252
December	1st to 10th	Day	31% 22% 31% 9% 7%	180
	11th to 20th	Day	35% 17% 34% 10% 4	180
	21st to end	Day	29% 18% 33% 13% 7% 1	198
January	1st to 10th	Night	20% 19% 37% 20% 4	760
	11th to 20th	Night	25% 16% 31% 25% 3	760
	21st to end	Night	19% 18% 39% 23% 2	836
February	1st to 10th	Night	21% 15% 38% 25% 1	760
	11th to 20th	Night	26% 19% 37% 17% 1	743
	21st to end	Night	23% 18% 38% 18% 2	611
March	1st to 10th	Night	22% 16% 36% 23% 3	720
	11th to 20th	Night	23% 17% 36% 18% 5%	720
	21st to end	Night	21% 14% 39% 20% 5%	792
April	1st to 10th	Night	22% 18% 38% 18% 5%	720
	11th to 20th	Night	18% 14% 36% 26% 6%	711
	21st to end	Night	22% 10% 34% 28% 5%	720
May	1st to 10th	Night	23% 12% 34% 26% 6%	585
	11th to 20th	Night	23% 8% 35% 27% 6%	354
	21st to end	Night	18% 5% 24% 37% 14% 1	389
June	1st to 10th	Night	18% 4 28% 38% 11% 1	360
	11th to 20th	Night	12% 7% 29% 41% 11% 1	360
	21st to end	Night	10% 3 28% 49% 11%	360
July	1st to 10th	Night	12% 3 24% 41% 19% 1	350
	11th to 20th	Night	7% 4 23% 43% 21% 1	348
	21st to end	Night	9% 3 27% 41% 19% 1	417
August	1st to 10th	Night	12% 1 16% 45% 23% 2	537
	11th to 20th	Night	14% 3 18% 37% 26% 2	693
	21st to end	Night	15% 4 18% 38% 23% 2	792
September	1st to 10th	Night	17% 4 24% 35% 17% 3	720
	11th to 20th	Night	15% 4 22% 35% 20% 5%	720
	21st to end	Night	21% 5% 22% 32% 18% 1	720
October	1st to 10th	Night	23% 7% 24% 26% 17% 4	720
	11th to 20th	Night	18% 10% 26% 34% 12%	720
	21st to end	Night	23% 11% 33% 25% 8% 1	792
November	1st to 10th	Night	15% 8% 35% 29% 11% 1	740
	11th to 20th	Night	21% 15% 29% 26% 8% 1	760
	21st to end	Night	17% 15% 34% 27% 7% 1	760
December	1st to 10th	Night	24% 16% 33% 21% 6% 1	760
	11th to 20th	Night	21% 14% 35% 25% 5%	760
	21st to end	Night	21% 16% 33% 27% 3	836

Page generated at 2024-02-22 06:05:46. Version ac07

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