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 2024-10-25.

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	16% 6% 2 6% 4 4 4 4 6% 7% 40%	101218
	Night	17% 7% 2 6% 4 4 4 3 5% 7% 42%	97605
Transparency	Day	41% 7% 12% 23% 15% 2	100975
	Night	41% 6% 12% 22% 16% 3	97467
Seeing	Day	19% 15% 29% 23% 11% 3	14981
	Night	18% 13% 31% 28% 9% 1	24233

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% 7% 2 5% 3 3 4 3 4 6% 47%	2517
	11th to 20th	Day	14% 9% 3 8% 5% 4 3 4 5% 7% 38%	2588
	21st to end	Day	21% 9% 2 7% 4 4 4 3 5% 5% 37%	3124
February	1st to 10th	Day	16% 7% 2 6% 5% 5% 4 3 5% 7% 39%	1710
	11th to 20th	Day	16% 7% 3 5% 4 3 3 3 5% 7% 42%	1811
	21st to end	Day	19% 9% 3 6% 5% 4 3 3 5% 6% 38%	1570
March	1st to 10th	Day	25% 6% 2 6% 4 3 3 3 4 6% 38%	3116
	11th to 20th	Day	10% 5% 2 6% 4 5% 4 4 6% 7% 48%	3475
	21st to end	Day	21% 5% 2 5% 4 3 3 3 4 5% 44%	4055
April	1st to 10th	Day	16% 7% 2 5% 5% 4 4 4 7% 8% 40%	2280
	11th to 20th	Day	14% 7% 2 5% 4 4 5% 3 6% 7% 43%	2254
	21st to end	Day	16% 6% 2 5% 5% 3 4 5% 6% 7% 41%	2280
May	1st to 10th	Day	27% 9% 3 6% 4 4 4 4 5% 7% 28%	4048
	11th to 20th	Day	24% 8% 3 7% 5% 5% 4 3 5% 6% 29%	4398
	21st to end	Day	12% 6% 2 5% 4 4 4 4 6% 9% 44%	4837
June	1st to 10th	Day	13% 5% 2 5% 5% 5% 4 5% 8% 9% 39%	2660
	11th to 20th	Day	12% 6% 2 6% 5% 5% 5% 5% 7% 9% 38%	2660
	21st to end	Day	10% 5% 2 7% 4 4 5% 4 7% 10% 42%	2660
July	1st to 10th	Day	7% 5% 2 7% 6% 7% 7% 5% 8% 10% 36%	4440
	11th to 20th	Day	11% 5% 2 6% 4 6% 5% 5% 7% 10% 39%	4440
	21st to end	Day	12% 7% 3 8% 6% 6% 6% 5% 6% 8% 32%	4734
August	1st to 10th	Day	18% 5% 2 7% 5% 5% 6% 4 6% 8% 33%	4087
	11th to 20th	Day	13% 5% 2 6% 4 4 4 4 6% 10% 41%	3730
	21st to end	Day	10% 5% 2 5% 4 4 4 4 6% 8% 47%	4180
September	1st to 10th	Day	13% 6% 2 6% 5% 4 4 3 5% 6% 45%	2280
	11th to 20th	Day	12% 5% 2 5% 4 3 4 3 5% 8% 50%	2246
	21st to end	Day	19% 8% 2 7% 4 4 4 3 5% 5% 39%	2090
October	1st to 10th	Day	18% 6% 2 5% 3 4 4 4 4 6% 44%	1910
	11th to 20th	Day	15% 5% 2 4 3 3 4 4 4 7% 50%	1900
	21st to end	Day	18% 5% 2 5% 3 4 4 3 4 5% 46%	1962
November	1st to 10th	Day	13% 6% 2 5% 3 2 3 2 5% 6% 52%	1671
	11th to 20th	Day	16% 5% 2 5% 4 4 3 4 5% 7% 47%	1710
	21st to end	Day	15% 5% 1 6% 4 3 4 3 5% 7% 47%	1597
December	1st to 10th	Day	19% 6% 2 6% 4 3 3 4 5% 8% 39%	1520
	11th to 20th	Day	19% 8% 2 5% 4 3 3 3 4 5% 44%	1906
	21st to end	Day	17% 6% 3 6% 5% 4 5% 4 7% 8% 36%	2772
January	1st to 10th	Night	17% 6% 2 6% 3 4 4 3 5% 5% 45%	4429
	11th to 20th	Night	18% 8% 2 6% 3 3 3 3 4 5% 44%	4369
	21st to end	Night	24% 7% 2 6% 4 4 4 3 4 5% 36%	4532
February	1st to 10th	Night	16% 9% 3 5% 4 3 4 3 5% 7% 41%	2470
	11th to 20th	Night	18% 8% 3 6% 4 4 3 3 5% 5% 42%	2253
	21st to end	Night	18% 8% 2 6% 4 3 3 4 5% 6% 42%	1884
March	1st to 10th	Night	23% 10% 2 5% 3 4 3 3 4 5% 38%	3724
	11th to 20th	Night	11% 6% 2 6% 4 4 4 3 4 6% 50%	3482
	21st to end	Night	18% 8% 3 6% 3 3 3 2 3 5% 44%	3576
April	1st to 10th	Night	16% 7% 3 6% 4 4 4 5% 5% 6% 40%	1900
	11th to 20th	Night	15% 6% 3 7% 4 3 4 4 5% 7% 43%	1875
	21st to end	Night	18% 5% 2 6% 3 4 4 4 5% 7% 43%	1900
May	1st to 10th	Night	22% 9% 4 7% 4 4 4 4 5% 8% 30%	2922
	11th to 20th	Night	24% 9% 3 7% 4 3 4 4 4 7% 31%	2495
	21st to end	Night	14% 6% 2 6% 4 4 4 4 6% 9% 42%	2738
June	1st to 10th	Night	15% 7% 2 6% 4 4 3 3 5% 9% 43%	1520
	11th to 20th	Night	11% 6% 2 6% 5% 4 5% 4 6% 9% 42%	1520
	21st to end	Night	9% 4 2 6% 5% 4 5% 5% 8% 10% 42%	1520
July	1st to 10th	Night	8% 5% 3 7% 5% 5% 6% 5% 8% 10% 37%	2520
	11th to 20th	Night	12% 5% 3 6% 4 5% 5% 5% 7% 11% 38%	2520
	21st to end	Night	15% 7% 3 7% 5% 4 5% 4 6% 8% 36%	2816
August	1st to 10th	Night	21% 6% 2 6% 4 5% 4 4 6% 8% 33%	2825
	11th to 20th	Night	14% 6% 2 7% 4 4 5% 4 5% 7% 43%	3083
	21st to end	Night	11% 5% 2 5% 4 4 4 3 6% 7% 49%	3476
September	1st to 10th	Night	14% 5% 2 6% 4 3 3 3 6% 6% 47%	1900
	11th to 20th	Night	11% 6% 1 4 2 3 4 4 5% 7% 53%	1933
	21st to end	Night	19% 6% 2 7% 4 3 4 4 5% 7% 40%	2090
October	1st to 10th	Night	19% 6% 3 5% 4 4 3 3 4 6% 44%	2267
	11th to 20th	Night	15% 4 2 5% 3 3 3 3 4 6% 52%	2280
	21st to end	Night	17% 6% 2 5% 3 3 3 3 4 5% 49%	2498
November	1st to 10th	Night	12% 6% 2 5% 3 3 3 3 4 6% 53%	2407
	11th to 20th	Night	17% 6% 2 5% 4 3 3 3 3 5% 48%	2470
	21st to end	Night	13% 6% 1 5% 5% 4 4 4 5% 7% 46%	2579
December	1st to 10th	Night	19% 8% 2 5% 4 3 3 4 5% 6% 40%	2660
	11th to 20th	Night	17% 6% 3 6% 4 4 3 3 5% 7% 42%	3288
	21st to end	Night	17% 8% 3 6% 4 4 5% 4 6% 6% 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	39% 5% 13% 26% 14% 4	2518
	11th to 20th	Day	45% 5% 12% 22% 14% 2	2588
	21st to end	Day	49% 6% 13% 21% 10% 1	3124
February	1st to 10th	Day	44% 6% 13% 24% 12% 2	1710
	11th to 20th	Day	42% 5% 10% 25% 15% 3	1820
	21st to end	Day	47% 4 11% 21% 13% 5%	1570
March	1st to 10th	Day	47% 5% 10% 27% 9% 2	3119
	11th to 20th	Day	31% 7% 12% 30% 17% 3	3475
	21st to end	Day	43% 5% 9% 18% 21% 3	4055
April	1st to 10th	Day	41% 6% 11% 21% 15% 5%	2280
	11th to 20th	Day	38% 7% 12% 22% 16% 4	2254
	21st to end	Day	41% 7% 11% 19% 19% 4	2280
May	1st to 10th	Day	54% 7% 11% 16% 10% 2	4061
	11th to 20th	Day	53% 9% 13% 17% 8% 1	4402
	21st to end	Day	33% 9% 13% 26% 16% 2	4837
June	1st to 10th	Day	38% 9% 12% 23% 15% 2	2660
	11th to 20th	Day	38% 10% 13% 24% 12% 3	2660
	21st to end	Day	36% 8% 15% 23% 17% 1	2660
July	1st to 10th	Day	39% 12% 16% 17% 14% 2	4370
	11th to 20th	Day	39% 9% 16% 22% 13% 1	4365
	21st to end	Day	46% 9% 11% 19% 14% 1	4583
August	1st to 10th	Day	48% 8% 14% 21% 9% 1	4074
	11th to 20th	Day	37% 8% 13% 27% 14% 1	3766
	21st to end	Day	33% 8% 11% 27% 19% 2	4180
September	1st to 10th	Day	38% 7% 11% 19% 22% 3	2280
	11th to 20th	Day	32% 5% 11% 23% 24% 4	2246
	21st to end	Day	47% 6% 8% 18% 20% 2	2090
October	1st to 10th	Day	40% 6% 12% 21% 19% 2	1910
	11th to 20th	Day	35% 4 11% 24% 23% 3	1900
	21st to end	Day	40% 5% 9% 24% 20% 3	1962
November	1st to 10th	Day	34% 4 13% 26% 21% 3	1671
	11th to 20th	Day	37% 6% 10% 23% 22% 3	1710
	21st to end	Day	37% 5% 12% 31% 13% 3	1597
December	1st to 10th	Day	43% 5% 16% 21% 14% 1	1520
	11th to 20th	Day	42% 4 12% 28% 11% 2	1906
	21st to end	Day	43% 8% 14% 22% 12% 1	2772
January	1st to 10th	Night	39% 5% 10% 28% 13% 4	4429
	11th to 20th	Night	44% 4 11% 25% 14% 3	4369
	21st to end	Night	48% 5% 9% 24% 12% 2	4532
February	1st to 10th	Night	43% 5% 13% 24% 13% 2	2470
	11th to 20th	Night	44% 5% 8% 20% 18% 5%	2255
	21st to end	Night	43% 4 10% 22% 16% 5%	1884
March	1st to 10th	Night	48% 5% 9% 22% 13% 2	3734
	11th to 20th	Night	33% 7% 12% 27% 17% 3	3483
	21st to end	Night	43% 5% 7% 17% 23% 5%	3576
April	1st to 10th	Night	42% 6% 10% 19% 18% 4	1900
	11th to 20th	Night	41% 7% 9% 20% 19% 4	1875
	21st to end	Night	40% 6% 10% 20% 20% 5%	1900
May	1st to 10th	Night	51% 8% 10% 19% 11% 1	2928
	11th to 20th	Night	53% 8% 14% 14% 10% 2	2496
	21st to end	Night	36% 9% 16% 20% 16% 3	2739
June	1st to 10th	Night	37% 8% 11% 23% 19% 2	1520
	11th to 20th	Night	38% 10% 12% 26% 13% 2	1520
	21st to end	Night	33% 9% 15% 24% 15% 2	1520
July	1st to 10th	Night	40% 11% 19% 17% 12% 2	2480
	11th to 20th	Night	37% 9% 19% 23% 10% 1	2472
	21st to end	Night	45% 8% 15% 18% 14% 1	2726
August	1st to 10th	Night	47% 8% 19% 16% 9% 1	2814
	11th to 20th	Night	40% 7% 12% 27% 14% 1	3113
	21st to end	Night	33% 7% 11% 27% 21% 2	3476
September	1st to 10th	Night	38% 7% 13% 18% 22% 3	1900
	11th to 20th	Night	30% 6% 10% 23% 26% 4	1933
	21st to end	Night	43% 7% 9% 20% 20% 2	2090
October	1st to 10th	Night	41% 6% 12% 21% 17% 3	2267
	11th to 20th	Night	33% 5% 12% 28% 19% 3	2280
	21st to end	Night	39% 4 9% 25% 20% 3	2498
November	1st to 10th	Night	33% 5% 12% 24% 20% 6%	2407
	11th to 20th	Night	39% 5% 9% 21% 20% 5%	2470
	21st to end	Night	37% 6% 12% 26% 17% 2	2579
December	1st to 10th	Night	44% 5% 12% 23% 14% 2	2660
	11th to 20th	Night	41% 5% 12% 20% 18% 4	3288
	21st to end	Night	43% 8% 14% 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	22% 28% 34% 12% 4 1	180
	11th to 20th	Day	26% 35% 27% 11% 2	200
	21st to end	Day	23% 30% 28% 14% 5%	297
February	1st to 10th	Day	25% 26% 34% 11% 4	270
	11th to 20th	Day	29% 23% 35% 11% 2	419
	21st to end	Day	28% 21% 33% 15% 2	382
March	1st to 10th	Day	22% 23% 31% 19% 4 1	460
	11th to 20th	Day	20% 27% 32% 16% 3 2	460
	21st to end	Day	27% 19% 31% 17% 4 1	506
April	1st to 10th	Day	24% 24% 32% 12% 7% 1	460
	11th to 20th	Day	19% 21% 33% 19% 6% 1	452
	21st to end	Day	25% 17% 31% 21% 5% 1	460
May	1st to 10th	Day	29% 18% 28% 15% 8% 2	463
	11th to 20th	Day	22% 14% 28% 25% 8% 2	487
	21st to end	Day	17% 10% 24% 29% 16% 4	529
June	1st to 10th	Day	14% 13% 29% 27% 14% 3	490
	11th to 20th	Day	15% 10% 31% 28% 13% 3	490
	21st to end	Day	13% 10% 30% 29% 14% 4	490
July	1st to 10th	Day	11% 5% 23% 34% 21% 6%	485
	11th to 20th	Day	6% 3 26% 36% 24% 5%	484
	21st to end	Day	12% 6% 27% 34% 16% 5%	532
August	1st to 10th	Day	12% 6% 25% 32% 19% 7%	486
	11th to 20th	Day	13% 5% 21% 34% 21% 6%	479
	21st to end	Day	13% 7% 29% 27% 16% 9%	539
September	1st to 10th	Day	17% 10% 26% 24% 16% 7%	490
	11th to 20th	Day	14% 7% 24% 30% 16% 8%	490
	21st to end	Day	26% 13% 21% 25% 10% 5%	490
October	1st to 10th	Day	24% 8% 27% 21% 13% 6%	490
	11th to 20th	Day	18% 15% 29% 23% 11% 3	490
	21st to end	Day	20% 13% 35% 18% 10% 3	481
November	1st to 10th	Day	17% 14% 31% 23% 12% 3	365
	11th to 20th	Day	20% 22% 32% 20% 6%	370
	21st to end	Day	17% 23% 35% 19% 6% 1	257
December	1st to 10th	Day	26% 26% 34% 8% 6%	180
	11th to 20th	Day	32% 21% 31% 13% 3	180
	21st to end	Day	27% 20% 31% 14% 7% 1	198
January	1st to 10th	Night	20% 20% 37% 19% 4	760
	11th to 20th	Night	23% 19% 31% 24% 3	760
	21st to end	Night	18% 19% 39% 22% 2	836
February	1st to 10th	Night	21% 17% 38% 23% 2	760
	11th to 20th	Night	24% 22% 36% 17% 1	743
	21st to end	Night	23% 20% 36% 19% 3	628
March	1st to 10th	Night	21% 18% 36% 21% 4	760
	11th to 20th	Night	21% 21% 35% 18% 5%	760
	21st to end	Night	21% 16% 38% 19% 5%	836
April	1st to 10th	Night	20% 20% 37% 18% 5%	760
	11th to 20th	Night	19% 15% 36% 26% 5%	751
	21st to end	Night	20% 13% 34% 27% 5%	760
May	1st to 10th	Night	21% 12% 36% 25% 6%	632
	11th to 20th	Night	19% 11% 36% 28% 6%	374
	21st to end	Night	18% 4 26% 38% 12% 2	411
June	1st to 10th	Night	16% 7% 28% 39% 9% 1	380
	11th to 20th	Night	12% 7% 31% 40% 10% 1	380
	21st to end	Night	10% 4 29% 46% 11%	380
July	1st to 10th	Night	9% 5% 29% 40% 16% 1	370
	11th to 20th	Night	7% 5% 24% 44% 19% 2	368
	21st to end	Night	9% 5% 29% 37% 20% 1	457
August	1st to 10th	Night	11% 3 20% 43% 20% 2	567
	11th to 20th	Night	13% 4 20% 40% 21% 3	736
	21st to end	Night	13% 5% 20% 38% 22% 2	836
September	1st to 10th	Night	16% 4 27% 35% 16% 2	760
	11th to 20th	Night	15% 4 19% 35% 22% 4	760
	21st to end	Night	23% 6% 24% 29% 17% 1	760
October	1st to 10th	Night	20% 10% 24% 26% 16% 3	760
	11th to 20th	Night	18% 10% 28% 31% 13%	760
	21st to end	Night	21% 11% 32% 26% 9%	812
November	1st to 10th	Night	14% 11% 35% 28% 11% 1	740
	11th to 20th	Night	19% 16% 31% 26% 8%	760
	21st to end	Night	15% 17% 34% 25% 8% 1	760
December	1st to 10th	Night	24% 15% 34% 20% 6% 1	760
	11th to 20th	Night	18% 17% 35% 25% 5%	760
	21st to end	Night	21% 15% 35% 27% 2	836

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