Willow Grove Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Willow Grove Park clear sky chart from 2005-11-06 to 2026-02-01.

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	19% 10% 3 8% 5% 5% 4 4 5% 6% 32%	132584
	Night	20% 10% 3 8% 5% 4 4 3 4 6% 31%	128126
Transparency	Day	32% 39% 8% 12% 8% 2	132303
	Night	33% 39% 8% 12% 6% 1	127984
Seeing	Day	22% 14% 27% 25% 10% 2	16580
	Night	25% 8% 21% 30% 15% 1	25068

By Month

Percentage Cloud Cover NW (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	35% 17% 4 9% 6% 5% 5% 3 3 4 8%	3719
	11th to 20th	Day	24% 11% 3 9% 6% 5% 5% 3 5% 6% 23%	3696
	21st to end	Day	27% 12% 4 8% 5% 4 4 3 3 4 26%	4447
February	1st to 10th	Day	31% 14% 4 9% 6% 4 5% 4 5% 6% 13%	1801
	11th to 20th	Day	30% 15% 3 10% 7% 4 5% 3 3 3 17%	1755
	21st to end	Day	31% 14% 4 9% 6% 4 3 3 4 4 17%	1538
March	1st to 10th	Day	18% 16% 4 11% 7% 6% 7% 4 6% 4 15%	4755
	11th to 20th	Day	26% 16% 4 10% 5% 4 4 3 3 4 22%	5000
	21st to end	Day	26% 14% 5% 12% 7% 6% 6% 4 5% 4 10%	5573
April	1st to 10th	Day	29% 12% 4 10% 6% 6% 5% 4 5% 6% 14%	2400
	11th to 20th	Day	26% 11% 4 8% 5% 5% 4 4 4 4 24%	2404
	21st to end	Day	23% 11% 3 10% 6% 5% 5% 4 6% 5% 23%	2600
May	1st to 10th	Day	16% 7% 3 7% 5% 5% 4 4 6% 6% 38%	6232
	11th to 20th	Day	15% 11% 3 9% 5% 5% 6% 5% 7% 8% 26%	6277
	21st to end	Day	18% 8% 3 9% 6% 5% 5% 4 5% 8% 30%	6903
June	1st to 10th	Day	22% 9% 4 8% 6% 4 5% 3 5% 6% 30%	2800
	11th to 20th	Day	21% 9% 3 9% 6% 6% 6% 4 6% 7% 22%	2953
	21st to end	Day	12% 8% 3 7% 5% 5% 5% 4 6% 7% 39%	3000
July	1st to 10th	Day	4 4 2 5% 3 3 3 2 4 5% 65%	6762
	11th to 20th	Day	4 2 1 4 2 2 3 3 4 6% 68%	6320
	21st to end	Day	7% 4 2 5% 3 3 3 3 5% 7% 59%	6874
August	1st to 10th	Day	5% 4 1 5% 3 3 3 3 5% 8% 59%	4273
	11th to 20th	Day	8% 5% 2 7% 6% 6% 6% 5% 8% 8% 40%	4156
	21st to end	Day	13% 8% 3 7% 4 4 4 3 5% 6% 44%	4482
September	1st to 10th	Day	8% 6% 3 6% 3 4 4 4 5% 8% 48%	2400
	11th to 20th	Day	12% 9% 3 7% 5% 4 4 4 5% 7% 39%	2385
	21st to end	Day	18% 7% 4 7% 4 4 4 3 5% 7% 39%	2111
October	1st to 10th	Day	18% 7% 2 6% 4 4 4 4 5% 8% 39%	3280
	11th to 20th	Day	21% 9% 4 9% 5% 5% 5% 4 7% 7% 25%	3261
	21st to end	Day	26% 10% 3 10% 6% 6% 5% 4 4 6% 19%	3472
November	1st to 10th	Day	29% 15% 3 11% 5% 6% 5% 4 5% 4 12%	1698
	11th to 20th	Day	34% 12% 4 9% 6% 5% 4 3 4 5% 15%	1680
	21st to end	Day	30% 12% 4 9% 6% 4 4 3 4 4 18%	1680
December	1st to 10th	Day	23% 10% 2 8% 7% 6% 5% 4 5% 6% 25%	2700
	11th to 20th	Day	31% 14% 4 11% 7% 6% 5% 3 4 4 9%	3105
	21st to end	Day	33% 17% 4 12% 6% 5% 5% 3 4 3 8%	4092
January	1st to 10th	Night	34% 15% 5% 12% 6% 5% 4 3 3 3 10%	6446
	11th to 20th	Night	22% 11% 3 8% 5% 5% 4 3 5% 5% 28%	6422
	21st to end	Night	29% 12% 3 7% 4 3 3 2 3 4 29%	6763
February	1st to 10th	Night	27% 14% 5% 9% 6% 5% 5% 4 4 4 17%	2600
	11th to 20th	Night	26% 13% 3 8% 5% 5% 4 3 4 5% 23%	2532
	21st to end	Night	27% 15% 3 9% 6% 5% 4 3 4 4 19%	2091
March	1st to 10th	Night	17% 13% 4 11% 7% 6% 7% 5% 6% 6% 18%	5123
	11th to 20th	Night	21% 14% 4 9% 5% 5% 6% 3 4 5% 23%	4999
	21st to end	Night	28% 16% 5% 11% 6% 5% 4 3 4 5% 11%	5389
April	1st to 10th	Night	24% 12% 4 10% 7% 6% 5% 4 4 5% 20%	2000
	11th to 20th	Night	20% 10% 3 7% 6% 5% 4 4 5% 7% 29%	1944
	21st to end	Night	21% 11% 3 7% 5% 5% 5% 3 5% 7% 28%	1800
May	1st to 10th	Night	17% 7% 2 5% 4 3 4 3 4 6% 44%	3717
	11th to 20th	Night	14% 10% 3 8% 5% 5% 5% 4 6% 7% 33%	3616
	21st to end	Night	9% 9% 3 8% 6% 5% 6% 5% 6% 9% 34%	3972
June	1st to 10th	Night	17% 8% 3 6% 6% 4 4 3 4 7% 39%	1600
	11th to 20th	Night	17% 8% 2 6% 4 4 5% 3 6% 8% 37%	1441
	21st to end	Night	9% 5% 3 5% 4 3 3 4 5% 7% 53%	1400
July	1st to 10th	Night	2 1 1 4 3 3 3 3 4 7% 68%	3180
	11th to 20th	Night	2 2 1 3 2 2 2 2 4 6% 73%	3640
	21st to end	Night	3 3 1 3 2 3 3 3 5% 8% 66%	3969
August	1st to 10th	Night	2 2 1 3 3 3 3 4 4 7% 68%	2870
	11th to 20th	Night	8% 4 3 6% 5% 4 4 3 5% 8% 51%	2902
	21st to end	Night	12% 6% 2 6% 3 4 3 3 4 6% 51%	3433
September	1st to 10th	Night	8% 4 1 5% 3 4 3 3 4 7% 58%	2000
	11th to 20th	Night	12% 6% 3 7% 6% 4 4 3 5% 6% 46%	2012
	21st to end	Night	13% 6% 2 6% 4 4 3 4 5% 7% 46%	2288
October	1st to 10th	Night	12% 6% 2 6% 4 4 4 4 5% 8% 45%	3920
	11th to 20th	Night	19% 9% 4 9% 5% 5% 5% 3 4 7% 31%	3897
	21st to end	Night	22% 9% 4 10% 6% 5% 5% 4 5% 6% 25%	4334
November	1st to 10th	Night	32% 10% 3 8% 6% 5% 5% 3 5% 5% 19%	2820
	11th to 20th	Night	34% 11% 4 8% 5% 5% 4 4 4 4 17%	2940
	21st to end	Night	27% 11% 3 9% 6% 5% 5% 4 4 4 22%	2940
December	1st to 10th	Night	21% 10% 3 7% 7% 5% 5% 4 6% 6% 27%	4700
	11th to 20th	Night	32% 16% 5% 12% 6% 4 4 3 3 3 11%	5320
	21st to end	Night	31% 15% 5% 13% 7% 5% 4 3 3 3 10%	7106

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	60% 28% 4 5% 3	3719
	11th to 20th	Day	45% 29% 6% 15% 5%	3720
	21st to end	Day	44% 29% 4 11% 10% 2	4447
February	1st to 10th	Day	29% 62% 4 3 2	1801
	11th to 20th	Day	32% 62% 2 2 2	1764
	21st to end	Day	29% 62% 3 3 4	1538
March	1st to 10th	Day	47% 32% 10% 8% 3	4755
	11th to 20th	Day	45% 30% 5% 17% 3	5000
	21st to end	Day	53% 32% 9% 5% 1	5573
April	1st to 10th	Day	27% 64% 4 4 1	2400
	11th to 20th	Day	22% 63% 4 7% 4	2404
	21st to end	Day	24% 63% 4 7% 2	2600
May	1st to 10th	Day	30% 32% 8% 19% 11%	6232
	11th to 20th	Day	38% 32% 15% 12% 2	6280
	21st to end	Day	35% 33% 18% 12% 2	6904
June	1st to 10th	Day	16% 63% 8% 10% 3	2800
	11th to 20th	Day	20% 64% 5% 8% 3	2953
	21st to end	Day	18% 63% 6% 10% 4	3000
July	1st to 10th	Day	14% 26% 9% 20% 15% 16%	6688
	11th to 20th	Day	9% 26% 7% 19% 35% 4	6245
	21st to end	Day	18% 26% 15% 28% 11% 1	6681
August	1st to 10th	Day	12% 37% 5% 18% 21% 8%	4256
	11th to 20th	Day	26% 39% 16% 12% 5% 2	4195
	21st to end	Day	27% 37% 8% 14% 14% 2	4481
September	1st to 10th	Day	18% 59% 6% 11% 5% 1	2400
	11th to 20th	Day	20% 59% 5% 7% 8% 2	2385
	21st to end	Day	19% 58% 7% 9% 6%	2111
October	1st to 10th	Day	24% 37% 6% 24% 9%	3280
	11th to 20th	Day	38% 38% 8% 9% 8%	3261
	21st to end	Day	41% 37% 6% 10% 6%	3472
November	1st to 10th	Day	34% 56% 4 6%	1698
	11th to 20th	Day	33% 55% 4 3 3 1	1680
	21st to end	Day	29% 54% 4 4 9%	1680
December	1st to 10th	Day	36% 36% 4 10% 13%	2700
	11th to 20th	Day	54% 32% 7% 5% 1	3108
	21st to end	Day	58% 30% 7% 4 1	4092
January	1st to 10th	Night	60% 28% 5% 4 3 1	6446
	11th to 20th	Night	40% 30% 6% 18% 6%	6460
	21st to end	Night	42% 29% 3 16% 7% 2	6763
February	1st to 10th	Night	29% 63% 3 4 1	2600
	11th to 20th	Night	30% 61% 3 4 2	2534
	21st to end	Night	24% 63% 4 5% 3	2091
March	1st to 10th	Night	44% 32% 10% 9% 4	5123
	11th to 20th	Night	41% 32% 8% 15% 3	5000
	21st to end	Night	55% 31% 8% 5% 1	5389
April	1st to 10th	Night	25% 64% 5% 4 1	2000
	11th to 20th	Night	19% 64% 6% 7% 4	1944
	21st to end	Night	20% 63% 6% 8% 3	1800
May	1st to 10th	Night	26% 31% 9% 24% 9%	3714
	11th to 20th	Night	35% 33% 15% 12% 5%	3616
	21st to end	Night	31% 36% 19% 14% 1	3972
June	1st to 10th	Night	14% 64% 9% 10% 3	1600
	11th to 20th	Night	13% 65% 6% 14% 2	1441
	21st to end	Night	12% 63% 9% 13% 2	1400
July	1st to 10th	Night	9% 27% 12% 24% 12% 16%	3145
	11th to 20th	Night	8% 26% 6% 24% 31% 5%	3592
	21st to end	Night	13% 27% 18% 29% 11% 1	3857
August	1st to 10th	Night	8% 37% 6% 20% 23% 5%	2862
	11th to 20th	Night	22% 39% 19% 14% 4 2	2929
	21st to end	Night	24% 36% 12% 14% 13% 1	3433
September	1st to 10th	Night	13% 60% 9% 12% 5% 2	2000
	11th to 20th	Night	19% 59% 5% 10% 5% 3	2012
	21st to end	Night	16% 58% 6% 12% 7%	2288
October	1st to 10th	Night	21% 37% 8% 27% 8%	3920
	11th to 20th	Night	35% 37% 10% 13% 4	3897
	21st to end	Night	38% 37% 8% 12% 5%	4334
November	1st to 10th	Night	32% 55% 5% 6% 2	2820
	11th to 20th	Night	33% 55% 4 4 3 1	2940
	21st to end	Night	29% 55% 3 4 8% 1	2940
December	1st to 10th	Night	32% 38% 6% 13% 12%	4700
	11th to 20th	Night	56% 33% 4 5% 2	5316
	21st to end	Night	55% 32% 7% 6% 1	7106

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	39% 18% 24% 11% 6% 2	220
	11th to 20th	Day	30% 17% 25% 17% 8% 3	220
	21st to end	Day	30% 14% 20% 24% 9% 2	402
February	1st to 10th	Day	43% 14% 16% 17% 9% 1	401
	11th to 20th	Day	39% 17% 24% 16% 3 1	395
	21st to end	Day	38% 18% 25% 14% 6%	359
March	1st to 10th	Day	32% 18% 27% 16% 7%	500
	11th to 20th	Day	31% 18% 24% 18% 8% 2	500
	21st to end	Day	32% 22% 23% 16% 7% 1	550
April	1st to 10th	Day	30% 20% 23% 18% 7% 2	500
	11th to 20th	Day	28% 21% 26% 15% 7% 2	492
	21st to end	Day	24% 18% 30% 18% 8% 2	500
May	1st to 10th	Day	17% 19% 28% 25% 9% 3	503
	11th to 20th	Day	27% 14% 26% 21% 9% 2	528
	21st to end	Day	21% 13% 29% 24% 10% 3	573
June	1st to 10th	Day	23% 13% 27% 26% 10% 1	530
	11th to 20th	Day	18% 22% 33% 22% 6% 1	683
	21st to end	Day	9% 13% 32% 34% 11% 2	730
July	1st to 10th	Day	9% 8% 37% 34% 10% 2	716
	11th to 20th	Day	8% 9% 31% 36% 14% 2	524
	21st to end	Day	6% 5% 29% 38% 18% 3	575
August	1st to 10th	Day	7% 7% 26% 42% 14% 4	527
	11th to 20th	Day	6% 5% 25% 40% 19% 5%	517
	21st to end	Day	9% 10% 30% 33% 13% 4	583
September	1st to 10th	Day	9% 8% 29% 36% 15% 3	530
	11th to 20th	Day	15% 10% 26% 33% 14% 3	530
	21st to end	Day	18% 14% 23% 29% 13% 2	530
October	1st to 10th	Day	23% 12% 24% 25% 14% 2	530
	11th to 20th	Day	24% 14% 24% 21% 16% 2	530
	21st to end	Day	32% 16% 24% 16% 10% 2	533
November	1st to 10th	Day	30% 21% 25% 17% 6% 1	247
	11th to 20th	Day	40% 14% 19% 18% 9%	220
	21st to end	Day	29% 20% 21% 19% 11%	220
December	1st to 10th	Day	34% 16% 19% 20% 9% 3	220
	11th to 20th	Day	36% 19% 28% 12% 5%	220
	21st to end	Day	38% 17% 24% 14% 7% 1	242
January	1st to 10th	Night	41% 10% 20% 18% 9% 2	840
	11th to 20th	Night	35% 10% 16% 23% 14% 1	840
	21st to end	Night	33% 7% 15% 28% 15% 2	924
February	1st to 10th	Night	33% 10% 19% 27% 11% 1	800
	11th to 20th	Night	30% 10% 20% 27% 12% 1	784
	21st to end	Night	32% 11% 22% 25% 11%	660
March	1st to 10th	Night	29% 11% 22% 25% 13% 1	800
	11th to 20th	Night	32% 10% 20% 26% 12%	800
	21st to end	Night	34% 13% 18% 25% 9% 1	880
April	1st to 10th	Night	28% 12% 22% 24% 12% 1	800
	11th to 20th	Night	22% 12% 25% 28% 11% 1	756
	21st to end	Night	25% 9% 20% 31% 14% 1	600
May	1st to 10th	Night	17% 10% 25% 32% 16%	438
	11th to 20th	Night	18% 6% 24% 34% 16% 2	394
	21st to end	Night	12% 7% 26% 35% 18% 1	433
June	1st to 10th	Night	17% 5% 27% 40% 11%	400
	11th to 20th	Night	15% 9% 29% 37% 10% 1	400
	21st to end	Night	10% 4 29% 41% 16%	400
July	1st to 10th	Night	5% 3 27% 52% 13%	390
	11th to 20th	Night	4 2 35% 45% 13% 1	388
	21st to end	Night	4 2 26% 50% 17%	438
August	1st to 10th	Night	5% 2 18% 48% 25% 2	555
	11th to 20th	Night	6% 2 15% 42% 31% 3	594
	21st to end	Night	11% 4 22% 41% 20% 1	770
September	1st to 10th	Night	11% 4 21% 42% 21% 1	800
	11th to 20th	Night	12% 5% 18% 35% 27% 4	800
	21st to end	Night	14% 6% 21% 38% 19% 2	800
October	1st to 10th	Night	19% 5% 19% 36% 20% 2	800
	11th to 20th	Night	23% 7% 17% 30% 21% 3	800
	21st to end	Night	28% 9% 17% 28% 16% 1	880
November	1st to 10th	Night	32% 10% 20% 27% 10%	820
	11th to 20th	Night	37% 11% 17% 24% 10% 1	840
	21st to end	Night	31% 10% 17% 26% 13% 2	840
December	1st to 10th	Night	31% 9% 18% 25% 15% 2	840
	11th to 20th	Night	38% 12% 20% 19% 9% 1	840
	21st to end	Night	40% 14% 22% 17% 7% 1	924

Page generated at 2026-01-28 22:21:41. Version ac07

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