Philadelphia
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Philadelphia 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% 7% 41%	144136
	Night	16% 7% 2 6% 4 4 4 3 5% 7% 43%	138087
Transparency	Day	40% 6% 12% 23% 17% 3	147666
	Night	40% 6% 11% 23% 17% 3	142644
Seeing	Day	20% 15% 29% 22% 10% 3	17350
	Night	19% 13% 31% 27% 9% 1	26567

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 6% 4 4 5% 3 5% 6% 46%	4661
	11th to 20th	Day	15% 7% 2 7% 4 3 3 3 4 5% 46%	4788
	21st to end	Day	16% 6% 2 6% 4 4 4 3 5% 7% 43%	5780
February	1st to 10th	Day	15% 7% 2 6% 4 5% 4 3 6% 7% 40%	1890
	11th to 20th	Day	17% 8% 3 6% 4 3 3 4 6% 7% 40%	2008
	21st to end	Day	18% 9% 3 6% 5% 4 3 3 5% 6% 39%	1730
March	1st to 10th	Day	21% 5% 2 5% 3 3 3 3 4 5% 45%	4576
	11th to 20th	Day	11% 6% 2 6% 4 4 4 4 5% 6% 47%	5085
	21st to end	Day	18% 5% 2 6% 4 4 4 3 5% 6% 44%	5931
April	1st to 10th	Day	17% 7% 2 4 5% 4 4 4 7% 7% 40%	2520
	11th to 20th	Day	14% 7% 2 5% 4 4 5% 3 6% 7% 43%	2494
	21st to end	Day	16% 6% 2 6% 5% 3 4 5% 6% 7% 41%	2520
May	1st to 10th	Day	23% 7% 2 6% 4 4 4 3 5% 7% 35%	7511
	11th to 20th	Day	20% 7% 2 6% 4 4 4 3 5% 7% 38%	8221
	21st to end	Day	25% 6% 2 5% 4 4 4 3 5% 7% 35%	9060
June	1st to 10th	Day	12% 5% 2 5% 5% 5% 4 5% 7% 9% 40%	2940
	11th to 20th	Day	12% 6% 2 6% 5% 5% 5% 5% 7% 9% 39%	2940
	21st to end	Day	11% 5% 2 7% 4 4 5% 4 7% 10% 42%	2940
July	1st to 10th	Day	12% 5% 2 6% 5% 5% 6% 4 7% 9% 38%	8280
	11th to 20th	Day	11% 5% 2 7% 5% 6% 5% 5% 7% 11% 35%	8280
	21st to end	Day	11% 5% 2 7% 5% 5% 5% 4 6% 9% 39%	8843
August	1st to 10th	Day	18% 5% 2 7% 5% 5% 5% 4 6% 8% 34%	4347
	11th to 20th	Day	13% 5% 2 6% 4 4 4 4 6% 10% 41%	3970
	21st to end	Day	10% 5% 2 5% 4 4 4 4 6% 8% 47%	4420
September	1st to 10th	Day	13% 6% 2 6% 5% 3 4 3 5% 6% 46%	2400
	11th to 20th	Day	13% 5% 2 5% 4 3 4 3 5% 8% 50%	2364
	21st to end	Day	18% 8% 2 7% 4 4 4 3 5% 5% 38%	2200
October	1st to 10th	Day	18% 5% 2 4 3 4 4 3 4 6% 46%	2010
	11th to 20th	Day	16% 5% 2 4 3 3 4 4 5% 7% 49%	2000
	21st to end	Day	17% 5% 2 5% 3 4 3 3 5% 5% 48%	2127
November	1st to 10th	Day	12% 6% 2 5% 3 3 3 2 4 7% 53%	1851
	11th to 20th	Day	15% 5% 2 5% 4 4 3 4 5% 7% 47%	1890
	21st to end	Day	16% 5% 1 6% 4 3 4 3 5% 7% 46%	1765
December	1st to 10th	Day	14% 6% 2 5% 4 4 4 4 6% 7% 45%	2680
	11th to 20th	Day	20% 9% 2 4 3 3 2 2 4 5% 45%	3066
	21st to end	Day	16% 5% 2 5% 4 4 5% 4 6% 7% 40%	4048
January	1st to 10th	Night	13% 7% 2 6% 4 4 4 3 5% 6% 47%	8234
	11th to 20th	Night	15% 7% 2 6% 3 3 3 3 3 5% 49%	8080
	21st to end	Night	16% 7% 2 6% 4 3 4 3 5% 6% 45%	8401
February	1st to 10th	Night	16% 8% 3 6% 4 4 4 3 5% 7% 40%	2730
	11th to 20th	Night	18% 7% 2 6% 4 4 3 3 5% 6% 41%	2496
	21st to end	Night	17% 8% 2 6% 4 4 4 4 5% 6% 42%	2076
March	1st to 10th	Night	19% 8% 2 6% 3 3 3 2 4 5% 45%	5484
	11th to 20th	Night	11% 8% 3 5% 3 4 3 3 4 5% 51%	5092
	21st to end	Night	16% 7% 2 6% 4 4 4 3 4 6% 43%	5242
April	1st to 10th	Night	17% 7% 3 6% 4 4 4 4 5% 6% 40%	2100
	11th to 20th	Night	15% 6% 2 7% 4 3 4 4 5% 7% 42%	2075
	21st to end	Night	18% 5% 3 5% 3 4 4 3 5% 7% 44%	2100
May	1st to 10th	Night	19% 7% 3 7% 4 4 4 4 6% 8% 33%	5445
	11th to 20th	Night	20% 7% 3 7% 4 3 4 4 5% 7% 36%	4639
	21st to end	Night	27% 6% 3 7% 3 3 3 3 5% 6% 33%	5114
June	1st to 10th	Night	15% 6% 1 5% 4 4 3 3 5% 9% 44%	1680
	11th to 20th	Night	12% 7% 2 7% 5% 4 5% 4 6% 8% 41%	1680
	21st to end	Night	10% 5% 2 6% 5% 4 5% 5% 8% 10% 41%	1680
July	1st to 10th	Night	13% 6% 3 7% 5% 5% 5% 5% 6% 9% 37%	4680
	11th to 20th	Night	11% 6% 3 7% 4 5% 5% 5% 8% 12% 34%	4680
	21st to end	Night	13% 6% 2 6% 4 4 5% 4 6% 8% 41%	5217
August	1st to 10th	Night	21% 6% 2 6% 4 5% 4 4 6% 8% 34%	3005
	11th to 20th	Night	14% 6% 2 7% 4 4 5% 4 5% 7% 42%	3281
	21st to end	Night	11% 5% 2 5% 4 4 4 3 6% 7% 49%	3677
September	1st to 10th	Night	14% 6% 3 6% 4 3 3 3 6% 6% 47%	2000
	11th to 20th	Night	12% 6% 1 4 3 3 4 3 5% 7% 52%	2035
	21st to end	Night	19% 7% 2 7% 4 3 4 4 5% 7% 39%	2200
October	1st to 10th	Night	18% 6% 3 5% 4 3 3 3 4 6% 46%	2387
	11th to 20th	Night	16% 4 2 5% 3 3 3 3 4 6% 51%	2400
	21st to end	Night	17% 6% 2 5% 3 3 3 3 5% 5% 49%	2711
November	1st to 10th	Night	12% 5% 1 5% 3 3 3 3 4 6% 54%	2667
	11th to 20th	Night	17% 6% 2 5% 4 3 3 3 4 5% 48%	2730
	21st to end	Night	13% 6% 2 5% 5% 4 4 4 5% 6% 45%	2851
December	1st to 10th	Night	13% 8% 2 5% 3 3 3 4 4 6% 48%	4720
	11th to 20th	Night	23% 6% 2 5% 3 3 3 3 5% 7% 41%	5348
	21st to end	Night	19% 7% 3 5% 4 4 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	38% 4 10% 27% 19% 2	4662
	11th to 20th	Day	41% 3 8% 23% 20% 5%	4818
	21st to end	Day	40% 4 14% 27% 13% 1	5780
February	1st to 10th	Day	42% 6% 12% 25% 12% 2	1890
	11th to 20th	Day	44% 5% 10% 24% 15% 2	2017
	21st to end	Day	46% 4 10% 23% 13% 4	1730
March	1st to 10th	Day	41% 4 10% 24% 15% 7%	4579
	11th to 20th	Day	35% 6% 11% 28% 15% 5%	5085
	21st to end	Day	41% 5% 10% 17% 23% 5%	5931
April	1st to 10th	Day	41% 6% 11% 21% 16% 5%	2520
	11th to 20th	Day	38% 6% 13% 22% 16% 4	2494
	21st to end	Day	41% 7% 11% 20% 18% 4	2520
May	1st to 10th	Day	48% 6% 13% 17% 15% 1	7521
	11th to 20th	Day	44% 6% 10% 23% 15% 1	8225
	21st to end	Day	48% 6% 10% 22% 13% 2	9061
June	1st to 10th	Day	38% 9% 12% 23% 17% 2	2940
	11th to 20th	Day	39% 10% 13% 24% 12% 3	2940
	21st to end	Day	37% 8% 14% 22% 17% 1	2940
July	1st to 10th	Day	41% 10% 14% 17% 17% 1	8210
	11th to 20th	Day	41% 8% 15% 24% 12% 1	8205
	21st to end	Day	39% 7% 10% 22% 21% 1	8647
August	1st to 10th	Day	47% 7% 14% 21% 10% 1	4334
	11th to 20th	Day	37% 8% 13% 27% 13% 1	4006
	21st to end	Day	33% 8% 11% 27% 20% 2	4420
September	1st to 10th	Day	38% 7% 11% 20% 21% 3	2400
	11th to 20th	Day	32% 5% 11% 24% 24% 4	2364
	21st to end	Day	48% 6% 8% 17% 19% 2	2200
October	1st to 10th	Day	35% 5% 9% 17% 29% 5%	3270
	11th to 20th	Day	25% 4 10% 24% 29% 8%	3240
	21st to end	Day	25% 3 12% 27% 28% 5%	3417
November	1st to 10th	Day	32% 4 13% 26% 21% 3	1851
	11th to 20th	Day	37% 6% 10% 24% 21% 3	1890
	21st to end	Day	38% 5% 12% 30% 12% 2	1765
December	1st to 10th	Day	37% 5% 17% 25% 14% 2	2680
	11th to 20th	Day	43% 3 11% 30% 10% 2	3066
	21st to end	Day	40% 7% 12% 22% 17% 1	4048
January	1st to 10th	Night	38% 5% 12% 24% 17% 5%	8234
	11th to 20th	Night	39% 3 9% 22% 19% 8%	8125
	21st to end	Night	40% 4 9% 25% 20% 3	8401
February	1st to 10th	Night	44% 5% 12% 24% 13% 3	2730
	11th to 20th	Night	45% 5% 9% 20% 17% 5%	2498
	21st to end	Night	43% 4 10% 22% 15% 5%	2076
March	1st to 10th	Night	41% 5% 9% 23% 13% 9%	5494
	11th to 20th	Night	35% 7% 14% 25% 15% 5%	5093
	21st to end	Night	43% 5% 8% 16% 24% 5%	5242
April	1st to 10th	Night	43% 6% 10% 19% 18% 4	2100
	11th to 20th	Night	41% 7% 10% 20% 19% 4	2075
	21st to end	Night	39% 6% 10% 20% 19% 5%	2100
May	1st to 10th	Night	47% 7% 11% 20% 13% 2	5451
	11th to 20th	Night	46% 6% 12% 23% 12% 1	4640
	21st to end	Night	51% 6% 10% 19% 12% 1	5115
June	1st to 10th	Night	37% 8% 11% 23% 20% 2	1680
	11th to 20th	Night	40% 10% 12% 24% 13% 2	1680
	21st to end	Night	35% 9% 15% 23% 15% 2	1680
July	1st to 10th	Night	43% 8% 14% 21% 12% 1	4640
	11th to 20th	Night	41% 9% 18% 22% 9% 1	4632
	21st to end	Night	39% 7% 13% 21% 18% 1	5098
August	1st to 10th	Night	47% 8% 18% 17% 9% 1	2994
	11th to 20th	Night	40% 7% 12% 27% 13% 1	3311
	21st to end	Night	33% 7% 11% 27% 21% 2	3677
September	1st to 10th	Night	38% 6% 12% 19% 21% 3	2000
	11th to 20th	Night	31% 6% 10% 23% 25% 4	2035
	21st to end	Night	43% 6% 10% 19% 19% 2	2200
October	1st to 10th	Night	38% 5% 10% 22% 22% 3	3907
	11th to 20th	Night	24% 4 10% 26% 29% 6%	3899
	21st to end	Night	27% 3 11% 30% 24% 4	4371
November	1st to 10th	Night	32% 5% 12% 24% 21% 6%	2667
	11th to 20th	Night	39% 5% 9% 21% 21% 5%	2730
	21st to end	Night	38% 6% 12% 25% 18% 2	2851
December	1st to 10th	Night	37% 4 12% 30% 14% 2	4720
	11th to 20th	Night	44% 4 10% 23% 15% 4	5348
	21st to end	Night	43% 6% 11% 19% 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	25% 27% 34% 10% 3 1	220
	11th to 20th	Day	28% 33% 28% 9% 2	245
	21st to end	Day	23% 31% 29% 13% 4	363
February	1st to 10th	Day	24% 25% 34% 12% 4	330
	11th to 20th	Day	32% 22% 33% 12% 2	496
	21st to end	Day	26% 22% 35% 14% 3	446
March	1st to 10th	Day	26% 22% 30% 18% 4 1	540
	11th to 20th	Day	22% 25% 33% 15% 3 2	540
	21st to end	Day	27% 22% 31% 15% 4 1	594
April	1st to 10th	Day	26% 23% 32% 11% 6% 1	540
	11th to 20th	Day	21% 21% 33% 18% 6% 1	532
	21st to end	Day	24% 17% 31% 21% 5% 1	540
May	1st to 10th	Day	29% 20% 29% 14% 7% 1	543
	11th to 20th	Day	21% 15% 30% 23% 8% 2	567
	21st to end	Day	23% 10% 24% 26% 14% 3	617
June	1st to 10th	Day	14% 14% 28% 28% 13% 3	570
	11th to 20th	Day	18% 12% 30% 26% 11% 3	570
	21st to end	Day	13% 11% 30% 29% 15% 3	570
July	1st to 10th	Day	11% 5% 23% 35% 21% 5%	565
	11th to 20th	Day	7% 5% 27% 33% 24% 5%	564
	21st to end	Day	11% 6% 27% 34% 16% 5%	620
August	1st to 10th	Day	11% 7% 26% 31% 18% 6%	566
	11th to 20th	Day	14% 5% 22% 33% 20% 6%	559
	21st to end	Day	12% 7% 30% 27% 16% 8%	619
September	1st to 10th	Day	16% 9% 28% 24% 16% 7%	530
	11th to 20th	Day	15% 7% 24% 31% 15% 8%	530
	21st to end	Day	26% 13% 22% 25% 10% 5%	530
October	1st to 10th	Day	23% 8% 27% 21% 14% 7%	530
	11th to 20th	Day	20% 15% 28% 22% 11% 3	530
	21st to end	Day	20% 13% 34% 19% 11% 2	542
November	1st to 10th	Day	16% 17% 31% 23% 11% 2	425
	11th to 20th	Day	20% 22% 32% 20% 5%	430
	21st to end	Day	20% 24% 32% 18% 5% 1	305
December	1st to 10th	Day	25% 25% 36% 8% 5%	220
	11th to 20th	Day	32% 21% 32% 12% 4	220
	21st to end	Day	28% 21% 31% 13% 7%	242
January	1st to 10th	Night	20% 21% 37% 18% 4	840
	11th to 20th	Night	23% 21% 30% 23% 3	840
	21st to end	Night	18% 19% 39% 22% 2	924
February	1st to 10th	Night	21% 17% 38% 23% 2	840
	11th to 20th	Night	26% 21% 35% 18% 1	823
	21st to end	Night	22% 19% 38% 19% 2	692
March	1st to 10th	Night	21% 18% 35% 21% 4	840
	11th to 20th	Night	20% 20% 36% 18% 5%	840
	21st to end	Night	22% 17% 39% 18% 5%	924
April	1st to 10th	Night	21% 20% 37% 17% 5%	840
	11th to 20th	Night	19% 16% 36% 25% 4	831
	21st to end	Night	20% 13% 34% 28% 5%	840
May	1st to 10th	Night	20% 13% 37% 24% 6%	699
	11th to 20th	Night	19% 12% 36% 28% 5%	414
	21st to end	Night	20% 6% 27% 35% 11% 2	455
June	1st to 10th	Night	15% 7% 30% 38% 9% 1	420
	11th to 20th	Night	13% 8% 31% 39% 9% 1	420
	21st to end	Night	11% 5% 29% 45% 11%	420
July	1st to 10th	Night	10% 5% 29% 40% 16% 1	410
	11th to 20th	Night	7% 5% 24% 44% 19% 1	408
	21st to end	Night	10% 4 30% 36% 20% 1	505
August	1st to 10th	Night	12% 4 18% 43% 21% 2	627
	11th to 20th	Night	14% 4 20% 40% 20% 2	814
	21st to end	Night	13% 5% 21% 38% 21% 2	917
September	1st to 10th	Night	16% 4 27% 35% 16% 2	800
	11th to 20th	Night	15% 5% 20% 36% 21% 4	800
	21st to end	Night	23% 6% 23% 30% 17% 1	800
October	1st to 10th	Night	20% 10% 24% 27% 17% 4	800
	11th to 20th	Night	19% 11% 27% 31% 12%	800
	21st to end	Night	20% 12% 33% 26% 9%	880
November	1st to 10th	Night	14% 12% 34% 29% 10% 1	820
	11th to 20th	Night	19% 17% 31% 25% 7%	840
	21st to end	Night	16% 18% 34% 24% 7% 1	840
December	1st to 10th	Night	24% 15% 34% 20% 6% 1	840
	11th to 20th	Night	20% 17% 34% 24% 5%	840
	21st to end	Night	22% 16% 34% 27% 2	924

Page generated at 2026-08-27 04:07:10. Version ac07

Copyright © 2026 Attilla Danko.