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 2026-08-30.

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	16% 6% 2 6% 4 4 4 4 6% 8% 40%	144184
	Night	16% 7% 2 6% 4 4 4 3 5% 7% 43%	138034
Transparency	Day	40% 6% 12% 23% 17% 2	147700
	Night	40% 5% 11% 23% 17% 3	142592
Seeing	Day	22% 14% 29% 22% 10% 3	17330
	Night	19% 12% 31% 28% 9% 1	26528

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	13% 5% 2 6% 4 4 4 4 5% 6% 47%	4661
	11th to 20th	Day	15% 7% 3 7% 4 4 3 3 4 5% 45%	4747
	21st to end	Day	16% 6% 2 5% 4 4 5% 4 6% 7% 40%	5780
February	1st to 10th	Day	16% 6% 3 6% 5% 4 4 5% 5% 7% 40%	1890
	11th to 20th	Day	17% 8% 2 7% 4 3 5% 3 5% 6% 39%	2006
	21st to end	Day	17% 9% 3 6% 5% 3 3 4 4 6% 39%	1730
March	1st to 10th	Day	21% 5% 2 5% 3 3 3 3 5% 5% 44%	4576
	11th to 20th	Day	12% 6% 2 6% 4 5% 5% 4 6% 6% 45%	5078
	21st to end	Day	18% 5% 2 6% 4 4 4 3 5% 6% 43%	5946
April	1st to 10th	Day	16% 7% 2 6% 5% 4 3 3 6% 7% 40%	2520
	11th to 20th	Day	15% 7% 2 5% 4 4 4 4 6% 7% 41%	2494
	21st to end	Day	15% 6% 2 7% 5% 4 4 4 6% 7% 40%	2520
May	1st to 10th	Day	23% 7% 2 6% 4 4 4 4 5% 8% 34%	7565
	11th to 20th	Day	19% 7% 2 6% 4 4 4 3 5% 7% 38%	8221
	21st to end	Day	24% 7% 2 5% 4 4 4 3 5% 7% 34%	9060
June	1st to 10th	Day	12% 5% 2 6% 4 5% 5% 4 7% 8% 41%	2940
	11th to 20th	Day	12% 7% 2 6% 4 5% 5% 4 7% 9% 38%	2940
	21st to end	Day	11% 6% 2 6% 4 4 5% 5% 7% 9% 41%	2940
July	1st to 10th	Day	12% 6% 2 6% 5% 5% 6% 4 6% 10% 38%	8280
	11th to 20th	Day	9% 7% 3 7% 5% 5% 6% 5% 8% 10% 35%	8280
	21st to end	Day	11% 7% 2 6% 5% 5% 5% 4 7% 10% 38%	8885
August	1st to 10th	Day	19% 5% 2 7% 5% 5% 5% 4 6% 8% 34%	4350
	11th to 20th	Day	12% 5% 2 5% 4 4 5% 4 7% 9% 42%	3970
	21st to end	Day	11% 5% 2 5% 3 4 4 3 6% 9% 48%	4420
September	1st to 10th	Day	14% 5% 3 5% 5% 4 4 4 5% 6% 46%	2400
	11th to 20th	Day	13% 5% 2 4 3 3 4 3 5% 8% 51%	2361
	21st to end	Day	17% 8% 3 7% 5% 4 5% 3 5% 6% 37%	2200
October	1st to 10th	Day	17% 6% 2 4 4 3 4 4 5% 6% 45%	2011
	11th to 20th	Day	15% 5% 2 5% 4 3 4 3 5% 7% 48%	2000
	21st to end	Day	18% 6% 2 5% 3 3 3 3 5% 7% 46%	2119
November	1st to 10th	Day	12% 7% 2 4 3 3 2 3 4 6% 53%	1851
	11th to 20th	Day	16% 4 2 6% 3 4 3 3 5% 7% 47%	1890
	21st to end	Day	16% 7% 2 5% 4 3 4 4 5% 8% 44%	1759
December	1st to 10th	Day	13% 8% 2 5% 4 4 4 4 6% 7% 43%	2680
	11th to 20th	Day	20% 9% 2 4 3 3 3 2 4 6% 43%	3066
	21st to end	Day	16% 6% 2 5% 4 5% 5% 4 6% 8% 39%	4048
January	1st to 10th	Night	12% 8% 3 6% 4 4 4 3 5% 6% 47%	8234
	11th to 20th	Night	15% 8% 2 6% 3 3 3 3 4 5% 48%	8133
	21st to end	Night	16% 8% 2 5% 3 3 3 3 5% 5% 46%	8401
February	1st to 10th	Night	16% 9% 2 6% 4 3 5% 3 5% 7% 40%	2730
	11th to 20th	Night	18% 8% 3 6% 4 4 4 3 4 6% 40%	2499
	21st to end	Night	17% 8% 3 6% 4 3 4 3 5% 6% 42%	2076
March	1st to 10th	Night	19% 7% 2 5% 3 3 3 3 4 6% 44%	5484
	11th to 20th	Night	12% 8% 3 5% 3 3 3 3 4 5% 50%	5098
	21st to end	Night	15% 8% 3 6% 3 5% 4 3 5% 6% 43%	5226
April	1st to 10th	Night	17% 7% 2 7% 4 4 4 4 5% 6% 40%	2100
	11th to 20th	Night	16% 6% 2 7% 4 5% 4 4 5% 6% 41%	2075
	21st to end	Night	18% 5% 2 6% 3 3 4 3 5% 8% 44%	2100
May	1st to 10th	Night	20% 8% 3 6% 5% 4 4 4 6% 9% 32%	5389
	11th to 20th	Night	20% 8% 3 7% 3 3 4 3 6% 8% 35%	4639
	21st to end	Night	25% 7% 2 6% 4 4 3 3 5% 8% 33%	5114
June	1st to 10th	Night	15% 6% 2 6% 4 4 3 3 6% 8% 43%	1680
	11th to 20th	Night	12% 7% 2 7% 5% 5% 4 5% 6% 7% 41%	1680
	21st to end	Night	11% 4 2 6% 4 5% 6% 5% 6% 10% 41%	1680
July	1st to 10th	Night	12% 6% 3 7% 5% 5% 5% 4 7% 9% 38%	4680
	11th to 20th	Night	11% 8% 3 7% 6% 5% 5% 4 8% 11% 32%	4680
	21st to end	Night	12% 7% 2 6% 4 5% 5% 5% 7% 8% 42%	5175
August	1st to 10th	Night	21% 6% 3 6% 4 4 5% 4 6% 8% 33%	3005
	11th to 20th	Night	13% 7% 2 7% 4 4 4 4 6% 7% 41%	3265
	21st to end	Night	11% 5% 2 5% 3 4 4 3 6% 8% 48%	3677
September	1st to 10th	Night	14% 7% 2 6% 4 3 4 3 5% 7% 46%	2000
	11th to 20th	Night	12% 6% 2 5% 3 4 3 3 4 6% 53%	2038
	21st to end	Night	18% 7% 2 6% 4 3 4 4 5% 7% 39%	2200
October	1st to 10th	Night	18% 6% 2 6% 3 4 4 3 4 5% 45%	2385
	11th to 20th	Night	16% 5% 2 5% 3 2 3 4 4 6% 51%	2400
	21st to end	Night	17% 6% 2 4 3 3 3 3 5% 5% 48%	2720
November	1st to 10th	Night	11% 6% 2 4 4 3 3 3 5% 6% 54%	2667
	11th to 20th	Night	17% 6% 2 5% 3 3 4 3 4 5% 48%	2730
	21st to end	Night	14% 5% 2 5% 4 4 5% 3 6% 7% 45%	2856
December	1st to 10th	Night	13% 8% 2 6% 3 4 4 3 5% 6% 47%	4720
	11th to 20th	Night	22% 7% 2 5% 3 3 4 3 4 6% 41%	5348
	21st to end	Night	19% 7% 3 6% 4 3 4 3 5% 5% 41%	7150

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	37% 4 11% 27% 19% 2	4662
	11th to 20th	Day	40% 4 9% 23% 19% 6%	4777
	21st to end	Day	41% 5% 14% 24% 15% 1	5780
February	1st to 10th	Day	42% 6% 13% 26% 11% 2	1890
	11th to 20th	Day	45% 4 10% 24% 15% 2	2015
	21st to end	Day	45% 4 11% 24% 13% 3	1730
March	1st to 10th	Day	41% 4 9% 24% 15% 7%	4579
	11th to 20th	Day	36% 6% 11% 28% 14% 5%	5078
	21st to end	Day	42% 4 10% 16% 22% 5%	5946
April	1st to 10th	Day	42% 5% 11% 22% 15% 5%	2520
	11th to 20th	Day	39% 6% 13% 23% 16% 3	2494
	21st to end	Day	41% 6% 11% 21% 17% 4	2520
May	1st to 10th	Day	48% 6% 12% 18% 14% 1	7575
	11th to 20th	Day	45% 5% 10% 24% 15% 1	8225
	21st to end	Day	48% 6% 9% 23% 14% 1	9061
June	1st to 10th	Day	38% 7% 14% 23% 17% 1	2940
	11th to 20th	Day	40% 7% 13% 25% 12% 2	2940
	21st to end	Day	37% 8% 13% 24% 16% 1	2940
July	1st to 10th	Day	41% 10% 13% 18% 17% 2	8210
	11th to 20th	Day	40% 9% 14% 24% 12% 1	8205
	21st to end	Day	38% 7% 11% 22% 21% 1	8688
August	1st to 10th	Day	46% 8% 14% 21% 10%	4337
	11th to 20th	Day	36% 7% 13% 29% 14% 1	4006
	21st to end	Day	32% 7% 11% 27% 21% 2	4420
September	1st to 10th	Day	38% 5% 12% 21% 21% 3	2400
	11th to 20th	Day	31% 5% 10% 26% 24% 4	2361
	21st to end	Day	48% 6% 8% 17% 19% 2	2200
October	1st to 10th	Day	35% 4 9% 18% 28% 5%	3271
	11th to 20th	Day	24% 4 12% 25% 29% 7%	3240
	21st to end	Day	25% 3 13% 26% 28% 5%	3396
November	1st to 10th	Day	33% 4 13% 25% 22% 3	1851
	11th to 20th	Day	38% 5% 9% 25% 20% 3	1890
	21st to end	Day	38% 4 14% 29% 12% 2	1759
December	1st to 10th	Day	38% 5% 15% 27% 12% 2	2680
	11th to 20th	Day	43% 3 12% 29% 11% 2	3066
	21st to end	Day	41% 7% 12% 22% 17% 2	4048
January	1st to 10th	Night	38% 4 12% 24% 17% 4	8234
	11th to 20th	Night	39% 3 10% 22% 18% 7%	8178
	21st to end	Night	40% 4 8% 25% 21% 3	8401
February	1st to 10th	Night	43% 4 13% 25% 12% 3	2730
	11th to 20th	Night	45% 4 9% 20% 17% 4	2501
	21st to end	Night	43% 5% 7% 24% 17% 4	2076
March	1st to 10th	Night	42% 4 8% 24% 13% 9%	5494
	11th to 20th	Night	36% 5% 13% 26% 15% 5%	5099
	21st to end	Night	43% 5% 8% 16% 24% 5%	5226
April	1st to 10th	Night	44% 4 11% 19% 17% 4	2100
	11th to 20th	Night	42% 6% 9% 20% 19% 3	2075
	21st to end	Night	39% 6% 9% 22% 19% 5%	2100
May	1st to 10th	Night	48% 7% 12% 19% 13% 2	5395
	11th to 20th	Night	47% 6% 12% 23% 12% 1	4640
	21st to end	Night	50% 7% 11% 19% 12% 1	5115
June	1st to 10th	Night	38% 6% 11% 22% 20% 2	1680
	11th to 20th	Night	40% 8% 13% 25% 13% 2	1680
	21st to end	Night	35% 9% 15% 24% 15% 2	1680
July	1st to 10th	Night	42% 8% 15% 21% 14% 1	4640
	11th to 20th	Night	42% 10% 16% 22% 10% 1	4632
	21st to end	Night	37% 8% 13% 21% 19% 1	5057
August	1st to 10th	Night	47% 7% 17% 18% 10% 1	2994
	11th to 20th	Night	40% 7% 12% 26% 13% 1	3295
	21st to end	Night	33% 6% 11% 28% 21% 2	3677
September	1st to 10th	Night	38% 6% 11% 20% 22% 3	2000
	11th to 20th	Night	33% 5% 9% 23% 27% 4	2038
	21st to end	Night	44% 6% 8% 20% 21% 2	2200
October	1st to 10th	Night	39% 6% 11% 21% 21% 3	3905
	11th to 20th	Night	24% 3 11% 28% 29% 5%	3899
	21st to end	Night	27% 4 11% 31% 24% 4	4380
November	1st to 10th	Night	32% 4 12% 25% 21% 5%	2667
	11th to 20th	Night	39% 4 9% 22% 21% 5%	2730
	21st to end	Night	39% 5% 12% 27% 16% 2	2856
December	1st to 10th	Night	38% 4 12% 30% 14% 2	4720
	11th to 20th	Night	44% 4 10% 25% 14% 4	5348
	21st to end	Night	45% 4 11% 20% 17% 3	7150

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	28% 28% 30% 10% 3 1	220
	11th to 20th	Day	37% 25% 27% 9% 2	237
	21st to end	Day	26% 26% 31% 12% 4	363
February	1st to 10th	Day	28% 22% 35% 11% 3	330
	11th to 20th	Day	32% 22% 33% 12% 1	494
	21st to end	Day	28% 21% 34% 13% 3	446
March	1st to 10th	Day	30% 19% 30% 17% 3 1	540
	11th to 20th	Day	26% 22% 32% 15% 4 1	540
	21st to end	Day	28% 23% 29% 14% 5% 1	594
April	1st to 10th	Day	30% 19% 31% 14% 6% 1	540
	11th to 20th	Day	22% 23% 31% 15% 6% 2	532
	21st to end	Day	25% 19% 31% 20% 4 2	540
May	1st to 10th	Day	28% 19% 31% 14% 6% 1	543
	11th to 20th	Day	23% 13% 32% 23% 7% 2	567
	21st to end	Day	24% 11% 24% 24% 14% 4	617
June	1st to 10th	Day	16% 12% 29% 27% 13% 3	570
	11th to 20th	Day	17% 14% 29% 26% 12% 3	570
	21st to end	Day	14% 11% 30% 30% 11% 4	570
July	1st to 10th	Day	12% 6% 22% 33% 22% 6%	565
	11th to 20th	Day	9% 3 28% 30% 25% 5%	564
	21st to end	Day	12% 6% 29% 34% 15% 4	620
August	1st to 10th	Day	12% 6% 25% 34% 18% 6%	566
	11th to 20th	Day	14% 5% 21% 34% 19% 6%	559
	21st to end	Day	13% 8% 28% 28% 15% 8%	619
September	1st to 10th	Day	17% 10% 24% 26% 15% 6%	530
	11th to 20th	Day	16% 5% 24% 31% 15% 8%	530
	21st to end	Day	25% 13% 22% 24% 12% 5%	530
October	1st to 10th	Day	23% 6% 27% 21% 14% 8%	530
	11th to 20th	Day	19% 15% 29% 22% 12% 3	530
	21st to end	Day	22% 16% 31% 18% 11% 1	538
November	1st to 10th	Day	20% 13% 31% 22% 11% 2	425
	11th to 20th	Day	23% 18% 33% 21% 4	430
	21st to end	Day	25% 20% 34% 16% 4 1	299
December	1st to 10th	Day	31% 21% 33% 9% 6%	220
	11th to 20th	Day	36% 16% 34% 10% 4	220
	21st to end	Day	29% 19% 32% 13% 7%	242
January	1st to 10th	Night	20% 19% 37% 20% 4	840
	11th to 20th	Night	25% 18% 31% 24% 3	840
	21st to end	Night	19% 18% 39% 22% 2	924
February	1st to 10th	Night	21% 15% 38% 24% 1	840
	11th to 20th	Night	27% 19% 36% 17% 1	823
	21st to end	Night	23% 18% 38% 18% 3	692
March	1st to 10th	Night	23% 17% 34% 22% 4	840
	11th to 20th	Night	22% 18% 36% 19% 5%	840
	21st to end	Night	21% 16% 39% 19% 5%	924
April	1st to 10th	Night	22% 19% 37% 17% 5%	840
	11th to 20th	Night	19% 16% 37% 24% 5%	831
	21st to end	Night	21% 11% 34% 29% 5%	840
May	1st to 10th	Night	22% 12% 35% 25% 6%	681
	11th to 20th	Night	23% 9% 35% 28% 5%	414
	21st to end	Night	19% 7% 25% 35% 13% 1	455
June	1st to 10th	Night	17% 5% 30% 37% 10% 1	420
	11th to 20th	Night	12% 8% 30% 40% 10%	420
	21st to end	Night	10% 5% 27% 47% 11%	420
July	1st to 10th	Night	11% 4 24% 43% 17% 1	410
	11th to 20th	Night	7% 5% 25% 42% 20% 1	408
	21st to end	Night	9% 4 26% 40% 20% 1	488
August	1st to 10th	Night	13% 2 16% 44% 23% 1	627
	11th to 20th	Night	13% 4 19% 38% 24% 2	810
	21st to end	Night	15% 4 20% 37% 21% 2	917
September	1st to 10th	Night	16% 4 25% 35% 17% 3	800
	11th to 20th	Night	14% 4 22% 36% 20% 4	800
	21st to end	Night	23% 5% 21% 33% 17% 1	800
October	1st to 10th	Night	21% 8% 24% 26% 18% 3	800
	11th to 20th	Night	19% 10% 26% 32% 12%	800
	21st to end	Night	21% 12% 33% 26% 8% 1	880
November	1st to 10th	Night	15% 9% 35% 29% 11% 1	820
	11th to 20th	Night	21% 17% 30% 25% 7% 1	840
	21st to end	Night	18% 16% 33% 26% 6%	840
December	1st to 10th	Night	24% 16% 34% 20% 5% 1	840
	11th to 20th	Night	22% 14% 34% 24% 5%	840
	21st to end	Night	22% 15% 33% 27% 3	924

Page generated at 2026-08-27 04:07:09. Version ac07

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