Northeast Philadelphia Airport
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Northeast Philadelphia Airport clear sky chart from 2005-11-06 to 2025-11-03.

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% 7% 41%	126795
	Night	16% 7% 2 6% 4 4 4 3 5% 6% 43%	123239
Transparency	Day	26% 39% 7% 15% 11% 2	130323
	Night	26% 38% 7% 15% 12% 3	127797
Seeing	Day	21% 15% 29% 22% 10% 3	16305
	Night	19% 12% 31% 28% 9% 1	25522

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% 6% 2 5% 3 4 3 3 5% 6% 52%	3597
	11th to 20th	Day	14% 8% 3 7% 5% 4 4 3 5% 5% 42%	3635
	21st to end	Day	19% 8% 3 5% 5% 3 4 3 4 5% 42%	4466
February	1st to 10th	Day	16% 7% 3 5% 5% 4 4 4 5% 8% 39%	1800
	11th to 20th	Day	16% 8% 2 6% 4 3 4 4 5% 6% 40%	1917
	21st to end	Day	18% 8% 2 6% 5% 4 4 3 5% 6% 39%	1650
March	1st to 10th	Day	21% 5% 2 5% 3 4 4 3 4 6% 45%	4476
	11th to 20th	Day	12% 6% 2 6% 4 4 4 4 6% 6% 45%	4934
	21st to end	Day	19% 5% 2 6% 4 4 4 3 5% 6% 43%	5823
April	1st to 10th	Day	17% 7% 3 6% 5% 4 5% 4 5% 7% 39%	2400
	11th to 20th	Day	15% 6% 2 5% 5% 4 5% 4 5% 7% 41%	2374
	21st to end	Day	16% 6% 2 5% 5% 4 4 5% 5% 8% 40%	2400
May	1st to 10th	Day	27% 7% 2 6% 4 4 4 4 5% 7% 30%	5795
	11th to 20th	Day	24% 8% 2 6% 4 4 4 3 5% 7% 33%	6318
	21st to end	Day	20% 6% 2 5% 3 4 5% 4 6% 8% 36%	6949
June	1st to 10th	Day	14% 5% 2 5% 5% 5% 5% 4 7% 9% 40%	2800
	11th to 20th	Day	14% 6% 2 6% 4 5% 5% 4 7% 9% 39%	2800
	21st to end	Day	11% 5% 2 6% 4 4 5% 5% 7% 9% 42%	2800
July	1st to 10th	Day	9% 5% 2 7% 6% 5% 6% 5% 8% 10% 37%	6360
	11th to 20th	Day	12% 8% 3 7% 5% 6% 6% 5% 8% 10% 31%	6360
	21st to end	Day	12% 6% 2 7% 5% 5% 5% 4 7% 9% 39%	6864
August	1st to 10th	Day	19% 6% 2 8% 4 5% 5% 4 6% 8% 34%	4212
	11th to 20th	Day	13% 6% 2 6% 4 4 5% 4 6% 10% 41%	3850
	21st to end	Day	11% 5% 2 5% 4 3 4 3 6% 9% 47%	4312
September	1st to 10th	Day	14% 5% 2 5% 5% 4 4 4 5% 7% 45%	2400
	11th to 20th	Day	14% 4 2 3 3 3 3 3 5% 7% 52%	2355
	21st to end	Day	19% 7% 3 7% 4 4 5% 3 5% 6% 37%	2200
October	1st to 10th	Day	17% 6% 2 5% 3 3 4 4 5% 6% 46%	2021
	11th to 20th	Day	15% 5% 1 4 4 3 4 4 4 8% 48%	2000
	21st to end	Day	17% 6% 2 4 3 3 3 3 5% 6% 47%	2114
November	1st to 10th	Day	13% 6% 2 5% 3 3 3 2 4 6% 54%	1779
	11th to 20th	Day	16% 6% 1 5% 3 3 3 3 5% 6% 49%	1800
	21st to end	Day	16% 6% 2 5% 4 4 3 4 5% 7% 47%	1688
December	1st to 10th	Day	13% 7% 2 5% 4 4 4 4 6% 7% 44%	2600
	11th to 20th	Day	21% 8% 2 4 3 3 3 2 4 6% 44%	2986
	21st to end	Day	17% 5% 2 5% 4 4 5% 4 6% 7% 41%	3960
January	1st to 10th	Night	13% 6% 2 5% 3 4 4 3 4 6% 51%	6349
	11th to 20th	Night	15% 7% 2 5% 3 3 4 3 4 5% 49%	6246
	21st to end	Night	18% 8% 3 5% 4 3 4 3 4 6% 43%	6490
February	1st to 10th	Night	17% 9% 2 6% 4 4 3 3 6% 6% 40%	2600
	11th to 20th	Night	18% 8% 3 6% 4 4 4 3 5% 5% 41%	2368
	21st to end	Night	17% 9% 2 6% 5% 3 4 3 5% 6% 42%	1980
March	1st to 10th	Night	20% 7% 2 5% 3 3 3 3 4 6% 45%	5364
	11th to 20th	Night	12% 8% 3 5% 3 3 3 3 4 5% 51%	5007
	21st to end	Night	16% 7% 2 6% 4 4 4 3 5% 5% 43%	5108
April	1st to 10th	Night	18% 6% 3 7% 4 4 4 4 5% 6% 40%	2000
	11th to 20th	Night	17% 6% 2 6% 5% 4 4 3 5% 6% 42%	1975
	21st to end	Night	18% 5% 2 5% 3 4 3 3 5% 7% 44%	2000
May	1st to 10th	Night	23% 8% 3 7% 4 4 4 4 6% 8% 28%	4176
	11th to 20th	Night	24% 9% 3 7% 4 3 3 3 5% 7% 33%	3575
	21st to end	Night	22% 6% 2 5% 3 4 4 3 5% 9% 37%	3926
June	1st to 10th	Night	16% 5% 2 5% 5% 4 3 3 6% 7% 43%	1600
	11th to 20th	Night	13% 6% 2 5% 5% 4 5% 4 6% 7% 42%	1600
	21st to end	Night	10% 5% 2 6% 4 5% 4 6% 7% 10% 42%	1600
July	1st to 10th	Night	9% 6% 3 7% 5% 5% 5% 5% 8% 9% 38%	3600
	11th to 20th	Night	14% 8% 3 8% 5% 4 5% 4 7% 9% 32%	3600
	21st to end	Night	12% 5% 2 6% 4 4 4 4 6% 8% 44%	3984
August	1st to 10th	Night	22% 6% 2 6% 4 5% 4 4 7% 8% 33%	2915
	11th to 20th	Night	13% 6% 2 6% 4 4 5% 4 6% 6% 42%	3188
	21st to end	Night	11% 4 2 5% 4 4 4 4 5% 7% 50%	3586
September	1st to 10th	Night	14% 6% 2 6% 4 3 4 3 5% 7% 46%	2000
	11th to 20th	Night	12% 6% 2 4 3 3 4 3 4 6% 52%	2044
	21st to end	Night	19% 6% 2 6% 4 3 3 4 5% 7% 40%	2200
October	1st to 10th	Night	18% 6% 2 5% 4 3 4 3 5% 5% 45%	2375
	11th to 20th	Night	16% 5% 2 5% 3 3 3 3 5% 6% 51%	2400
	21st to end	Night	18% 6% 2 5% 2 4 3 3 5% 6% 48%	2725
November	1st to 10th	Night	11% 5% 2 5% 4 4 3 3 4 6% 54%	2564
	11th to 20th	Night	17% 6% 2 5% 4 3 3 3 4 5% 48%	2600
	21st to end	Night	14% 5% 2 5% 4 4 5% 3 6% 7% 45%	2710
December	1st to 10th	Night	13% 8% 2 5% 3 4 3 3 5% 6% 48%	4580
	11th to 20th	Night	23% 7% 2 5% 3 3 3 3 5% 6% 41%	5208
	21st to end	Night	20% 7% 3 6% 4 4 4 3 5% 5% 41%	6996

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	24% 27% 6% 20% 20% 3	3598
	11th to 20th	Day	31% 27% 6% 18% 13% 4	3665
	21st to end	Day	34% 29% 9% 17% 11% 1	4466
February	1st to 10th	Day	18% 63% 7% 8% 4 1	1800
	11th to 20th	Day	18% 60% 3 9% 9% 2	1926
	21st to end	Day	17% 63% 4 10% 5% 1	1650
March	1st to 10th	Day	31% 30% 5% 15% 12% 7%	4479
	11th to 20th	Day	25% 31% 7% 22% 11% 4	4934
	21st to end	Day	30% 30% 7% 10% 19% 5%	5823
April	1st to 10th	Day	20% 62% 4 8% 6% 2	2400
	11th to 20th	Day	16% 63% 5% 9% 7% 1	2374
	21st to end	Day	16% 62% 4 9% 8% 1	2400
May	1st to 10th	Day	40% 31% 10% 13% 5%	5805
	11th to 20th	Day	38% 30% 7% 17% 8%	6322
	21st to end	Day	33% 30% 8% 17% 10% 1	6949
June	1st to 10th	Day	16% 62% 4 11% 6% 1	2800
	11th to 20th	Day	16% 63% 4 10% 7% 1	2800
	21st to end	Day	15% 61% 5% 10% 9% 1	2800
July	1st to 10th	Day	31% 32% 11% 13% 13% 1	6290
	11th to 20th	Day	36% 30% 14% 13% 7%	6285
	21st to end	Day	30% 28% 7% 16% 18% 1	6667
August	1st to 10th	Day	34% 38% 8% 14% 6%	4199
	11th to 20th	Day	24% 38% 8% 22% 8%	3886
	21st to end	Day	18% 37% 7% 22% 16% 1	4312
September	1st to 10th	Day	16% 58% 5% 11% 9% 1	2400
	11th to 20th	Day	16% 57% 3 12% 9% 2	2355
	21st to end	Day	25% 57% 4 8% 6%	2200
October	1st to 10th	Day	22% 36% 5% 10% 22% 5%	3294
	11th to 20th	Day	12% 36% 6% 16% 24% 6%	3240
	21st to end	Day	11% 35% 9% 19% 22% 5%	3391
November	1st to 10th	Day	17% 56% 7% 10% 9% 2	1779
	11th to 20th	Day	14% 56% 4 12% 13% 2	1800
	21st to end	Day	15% 57% 7% 15% 5% 1	1688
December	1st to 10th	Day	24% 36% 10% 18% 9% 2	2600
	11th to 20th	Day	31% 31% 6% 22% 8% 2	2986
	21st to end	Day	29% 30% 7% 17% 14% 2	3960
January	1st to 10th	Night	25% 27% 5% 19% 18% 6%	6349
	11th to 20th	Night	27% 27% 7% 18% 14% 6%	6291
	21st to end	Night	33% 29% 6% 15% 15% 2	6490
February	1st to 10th	Night	17% 62% 6% 11% 3 1	2600
	11th to 20th	Night	18% 61% 2 9% 9% 2	2370
	21st to end	Night	17% 61% 4 8% 6% 3	1980
March	1st to 10th	Night	32% 30% 5% 16% 8% 9%	5374
	11th to 20th	Night	24% 31% 11% 20% 11% 3	5008
	21st to end	Night	31% 30% 4 11% 20% 3	5108
April	1st to 10th	Night	19% 62% 3 8% 7% 1	2000
	11th to 20th	Night	16% 63% 4 8% 7% 1	1975
	21st to end	Night	16% 62% 4 9% 9% 1	2000
May	1st to 10th	Night	39% 33% 9% 12% 6% 2	4182
	11th to 20th	Night	39% 31% 8% 13% 8%	3576
	21st to end	Night	34% 31% 9% 16% 9% 1	3927
June	1st to 10th	Night	16% 62% 4 8% 10% 1	1600
	11th to 20th	Night	16% 63% 4 11% 7%	1600
	21st to end	Night	15% 61% 6% 10% 7% 1	1600
July	1st to 10th	Night	32% 30% 13% 15% 9% 1	3560
	11th to 20th	Night	37% 30% 14% 14% 5%	3552
	21st to end	Night	30% 27% 9% 17% 16%	3866
August	1st to 10th	Night	36% 38% 11% 9% 5%	2904
	11th to 20th	Night	25% 38% 7% 22% 8%	3218
	21st to end	Night	18% 36% 6% 23% 17% 1	3586
September	1st to 10th	Night	16% 58% 6% 11% 8% 1	2000
	11th to 20th	Night	16% 57% 4 10% 12% 2	2044
	21st to end	Night	23% 57% 4 9% 7%	2200
October	1st to 10th	Night	24% 37% 6% 15% 16% 2	3882
	11th to 20th	Night	11% 35% 6% 20% 23% 4	3899
	21st to end	Night	13% 35% 7% 23% 18% 3	4398
November	1st to 10th	Night	16% 55% 7% 11% 8% 3	2564
	11th to 20th	Night	15% 56% 3 11% 12% 3	2600
	21st to end	Night	15% 57% 6% 13% 7% 1	2710
December	1st to 10th	Night	24% 36% 7% 23% 9% 2	4580
	11th to 20th	Night	32% 32% 6% 17% 10% 3	5208
	21st to end	Night	33% 28% 7% 14% 14% 3	6996

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% 29% 31% 11% 3 1	200
	11th to 20th	Day	35% 27% 27% 10% 1	214
	21st to end	Day	28% 24% 30% 13% 5%	330
February	1st to 10th	Day	28% 25% 34% 10% 2	300
	11th to 20th	Day	31% 25% 32% 10% 2	465
	21st to end	Day	29% 21% 34% 13% 2	414
March	1st to 10th	Day	27% 22% 30% 17% 3	500
	11th to 20th	Day	24% 24% 33% 13% 4 1	500
	21st to end	Day	27% 22% 31% 16% 4 1	550
April	1st to 10th	Day	28% 24% 30% 13% 5% 1	500
	11th to 20th	Day	23% 22% 32% 17% 6% 1	492
	21st to end	Day	26% 16% 32% 19% 5% 1	500
May	1st to 10th	Day	29% 19% 30% 13% 8% 1	503
	11th to 20th	Day	22% 15% 31% 22% 8% 2	527
	21st to end	Day	21% 10% 24% 27% 16% 3	573
June	1st to 10th	Day	16% 13% 27% 27% 14% 3	530
	11th to 20th	Day	18% 11% 30% 28% 10% 3	530
	21st to end	Day	12% 9% 31% 32% 13% 3	530
July	1st to 10th	Day	10% 6% 23% 34% 22% 6%	525
	11th to 20th	Day	8% 4 25% 34% 24% 5%	524
	21st to end	Day	11% 7% 28% 33% 17% 4	576
August	1st to 10th	Day	12% 5% 29% 32% 17% 6%	526
	11th to 20th	Day	14% 4 21% 36% 19% 7%	519
	21st to end	Day	13% 8% 26% 28% 16% 8%	583
September	1st to 10th	Day	15% 10% 26% 26% 16% 6%	530
	11th to 20th	Day	15% 6% 25% 29% 17% 8%	530
	21st to end	Day	24% 13% 24% 24% 10% 5%	530
October	1st to 10th	Day	22% 7% 26% 23% 13% 8%	530
	11th to 20th	Day	19% 16% 29% 22% 11% 3	530
	21st to end	Day	18% 18% 32% 19% 11% 2	535
November	1st to 10th	Day	18% 12% 33% 23% 12% 2	401
	11th to 20th	Day	22% 19% 33% 20% 6%	400
	21st to end	Day	22% 19% 34% 18% 5% 1	288
December	1st to 10th	Day	24% 26% 34% 10% 7%	200
	11th to 20th	Day	33% 19% 33% 11% 4 1	200
	21st to end	Day	26% 20% 33% 14% 7% 1	220
January	1st to 10th	Night	21% 20% 36% 20% 4	800
	11th to 20th	Night	24% 17% 32% 23% 3	800
	21st to end	Night	17% 19% 40% 22% 2	880
February	1st to 10th	Night	21% 17% 38% 23% 1	800
	11th to 20th	Night	25% 23% 35% 17% 1	783
	21st to end	Night	23% 19% 38% 18% 2	660
March	1st to 10th	Night	22% 19% 35% 21% 4	800
	11th to 20th	Night	21% 20% 36% 18% 5%	800
	21st to end	Night	21% 15% 40% 18% 5%	880
April	1st to 10th	Night	21% 20% 37% 17% 4	800
	11th to 20th	Night	20% 15% 37% 24% 4	791
	21st to end	Night	20% 13% 34% 27% 5%	800
May	1st to 10th	Night	21% 13% 34% 25% 6%	657
	11th to 20th	Night	19% 12% 37% 26% 5% 1	394
	21st to end	Night	18% 6% 26% 38% 11% 1	433
June	1st to 10th	Night	17% 6% 31% 38% 9% 1	400
	11th to 20th	Night	13% 8% 28% 42% 10%	400
	21st to end	Night	10% 4 28% 48% 11%	400
July	1st to 10th	Night	11% 4 26% 41% 17% 2	390
	11th to 20th	Night	7% 3 24% 45% 19% 2	388
	21st to end	Night	9% 4 26% 41% 19%	459
August	1st to 10th	Night	12% 2 19% 43% 22% 2	597
	11th to 20th	Night	13% 4 19% 38% 22% 2	781
	21st to end	Night	14% 4 20% 39% 22% 2	880
September	1st to 10th	Night	15% 5% 27% 35% 16% 2	800
	11th to 20th	Night	14% 5% 22% 35% 21% 4	800
	21st to end	Night	23% 6% 21% 32% 18% 1	800
October	1st to 10th	Night	21% 7% 24% 28% 17% 3	800
	11th to 20th	Night	18% 10% 27% 33% 12% 1	800
	21st to end	Night	20% 12% 33% 26% 9% 1	880
November	1st to 10th	Night	14% 10% 35% 29% 11% 1	789
	11th to 20th	Night	20% 16% 30% 26% 8% 1	800
	21st to end	Night	17% 16% 35% 25% 6% 1	800
December	1st to 10th	Night	24% 16% 33% 20% 5% 1	800
	11th to 20th	Night	20% 15% 35% 24% 5%	800
	21st to end	Night	21% 15% 34% 28% 2	880

Page generated at 2025-10-31 02:02:39. Version ac07

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