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 2024-02-25.

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	15% 6% 2 5% 5% 3 4 5% 6% 8% 41%	78398
	Night	16% 6% 3 5% 5% 3 4 4 5% 7% 43%	80892
Transparency	Day	41% 7% 12% 22% 16% 2	78106
	Night	40% 6% 12% 22% 16% 3	80718
Seeing	Day	19% 15% 29% 23% 11% 3	14006
	Night	18% 13% 31% 28% 9% 1	23439

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 4 4 3 3 4 4 6% 47%	2517
	11th to 20th	Day	14% 9% 3 7% 6% 3 3 5% 5% 7% 38%	2588
	21st to end	Day	22% 8% 2 6% 5% 3 4 3 5% 5% 37%	3124
February	1st to 10th	Day	16% 7% 3 5% 5% 4 4 4 5% 7% 40%	1710
	11th to 20th	Day	17% 7% 3 4 5% 2 4 4 5% 7% 42%	1811
	21st to end	Day	19% 8% 4 5% 5% 3 4 3 5% 6% 38%	1520
March	1st to 10th	Day	13% 5% 3 5% 6% 3 4 4 4 6% 48%	1797
	11th to 20th	Day	17% 5% 3 5% 5% 3 4 6% 6% 6% 40%	1975
	21st to end	Day	20% 7% 2 5% 5% 3 4 4 5% 6% 40%	2307
April	1st to 10th	Day	16% 6% 3 4 5% 3 4 5% 6% 8% 41%	2160
	11th to 20th	Day	14% 6% 2 4 4 3 5% 4 6% 7% 44%	2134
	21st to end	Day	17% 6% 2 4 6% 2 4 5% 6% 7% 40%	2160
May	1st to 10th	Day	19% 7% 3 6% 5% 3 4 5% 6% 7% 36%	2295
	11th to 20th	Day	19% 6% 2 6% 5% 3 5% 4 6% 8% 36%	2478
	21st to end	Day	14% 7% 3 5% 5% 3 4 4 6% 9% 41%	2725
June	1st to 10th	Day	13% 4 2 5% 5% 4 4 7% 7% 10% 38%	2520
	11th to 20th	Day	12% 6% 2 5% 6% 4 5% 6% 7% 9% 37%	2520
	21st to end	Day	11% 4 2 6% 5% 3 5% 5% 6% 11% 41%	2520
July	1st to 10th	Day	10% 5% 2 6% 6% 4 6% 6% 8% 9% 38%	2520
	11th to 20th	Day	7% 4 2 5% 6% 4 6% 6% 7% 11% 42%	2520
	21st to end	Day	10% 6% 3 6% 6% 4 6% 6% 7% 8% 38%	2703
August	1st to 10th	Day	9% 5% 2 6% 7% 4 6% 6% 6% 9% 40%	2323
	11th to 20th	Day	12% 5% 2 5% 6% 4 5% 6% 6% 10% 40%	2126
	21st to end	Day	13% 6% 2 5% 6% 3 5% 5% 6% 9% 41%	2376
September	1st to 10th	Day	13% 6% 2 5% 6% 3 4 5% 5% 7% 44%	2160
	11th to 20th	Day	13% 5% 2 4 4 2 4 4 5% 8% 51%	2128
	21st to end	Day	17% 8% 2 5% 4 3 4 5% 5% 6% 40%	1980
October	1st to 10th	Day	19% 6% 2 4 3 3 5% 4 4 6% 44%	1810
	11th to 20th	Day	16% 5% 2 4 4 3 3 5% 5% 7% 48%	1800
	21st to end	Day	19% 5% 2 5% 4 3 4 3 4 6% 45%	1915
November	1st to 10th	Day	13% 6% 2 4 4 2 3 3 4 6% 52%	1671
	11th to 20th	Day	16% 5% 2 4 5% 3 3 4 4 7% 47%	1710
	21st to end	Day	16% 4 1 5% 4 2 4 3 5% 8% 47%	1597
December	1st to 10th	Day	19% 6% 3 5% 5% 2 4 4 5% 8% 40%	1520
	11th to 20th	Day	19% 7% 2 4 4 2 3 3 4 6% 45%	1906
	21st to end	Day	17% 6% 3 5% 6% 3 6% 5% 7% 8% 36%	2772
January	1st to 10th	Night	17% 6% 2 5% 4 3 4 4 4 5% 45%	4429
	11th to 20th	Night	19% 8% 2 5% 4 2 3 3 4 5% 44%	4369
	21st to end	Night	24% 7% 3 5% 5% 3 4 4 4 6% 37%	4532
February	1st to 10th	Night	16% 8% 3 5% 5% 3 4 4 5% 7% 41%	2470
	11th to 20th	Night	18% 7% 3 6% 5% 2 4 3 4 6% 42%	2253
	21st to end	Night	17% 7% 2 5% 5% 3 3 4 5% 6% 42%	1831
March	1st to 10th	Night	15% 6% 2 4 4 3 4 4 5% 7% 47%	2150
	11th to 20th	Night	17% 7% 3 5% 4 3 4 3 4 5% 45%	1982
	21st to end	Night	17% 6% 3 6% 4 2 4 4 4 6% 43%	2048
April	1st to 10th	Night	16% 6% 3 5% 5% 3 5% 5% 5% 6% 41%	1800
	11th to 20th	Night	15% 5% 3 5% 5% 2 5% 4 5% 7% 44%	1775
	21st to end	Night	19% 5% 3 5% 4 3 5% 4 5% 7% 43%	1800
May	1st to 10th	Night	18% 7% 4 6% 5% 3 4 5% 4 6% 37%	1680
	11th to 20th	Night	19% 7% 3 5% 5% 2 4 4 5% 7% 39%	1415
	21st to end	Night	17% 6% 2 5% 5% 3 4 5% 5% 8% 40%	1550
June	1st to 10th	Night	15% 6% 2 5% 5% 3 4 4 5% 9% 43%	1440
	11th to 20th	Night	12% 6% 3 5% 6% 4 5% 5% 5% 9% 40%	1440
	21st to end	Night	9% 4 2 5% 6% 3 5% 6% 8% 10% 43%	1440
July	1st to 10th	Night	10% 5% 3 6% 6% 3 7% 6% 7% 10% 37%	1440
	11th to 20th	Night	7% 4 3 5% 5% 4 5% 7% 7% 10% 43%	1440
	21st to end	Night	9% 5% 3 5% 6% 3 5% 5% 7% 9% 44%	1612
August	1st to 10th	Night	8% 5% 2 5% 6% 4 5% 5% 7% 9% 43%	1611
	11th to 20th	Night	11% 6% 3 7% 5% 3 6% 6% 5% 8% 40%	1760
	21st to end	Night	13% 5% 2 5% 5% 3 5% 5% 6% 8% 41%	1980
September	1st to 10th	Night	15% 5% 3 5% 5% 2 4 4 6% 7% 46%	1800
	11th to 20th	Night	12% 6% 1 4 3 2 4 4 5% 6% 54%	1831
	21st to end	Night	17% 6% 2 6% 4 2 4 5% 4 7% 42%	1980
October	1st to 10th	Night	20% 6% 2 4 4 3 3 4 4 6% 44%	2147
	11th to 20th	Night	16% 4 2 4 4 2 3 4 4 6% 50%	2160
	21st to end	Night	18% 6% 2 4 3 3 3 3 4 5% 48%	2439
November	1st to 10th	Night	12% 5% 2 4 4 2 3 3 4 6% 53%	2407
	11th to 20th	Night	17% 5% 3 5% 4 2 4 3 3 5% 49%	2470
	21st to end	Night	13% 5% 2 4 5% 3 5% 5% 5% 7% 46%	2579
December	1st to 10th	Night	20% 8% 3 4 4 2 3 5% 5% 6% 40%	2660
	11th to 20th	Night	17% 5% 3 5% 4 3 4 4 5% 7% 43%	3288
	21st to end	Night	17% 8% 3 5% 5% 3 5% 5% 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 10% 21% 13% 5%	1520
March	1st to 10th	Day	38% 5% 12% 27% 15% 3	1800
	11th to 20th	Day	42% 6% 11% 22% 16% 4	1975
	21st to end	Day	45% 6% 9% 20% 17% 3	2307
April	1st to 10th	Day	41% 6% 11% 21% 16% 5%	2160
	11th to 20th	Day	38% 7% 12% 23% 16% 4	2134
	21st to end	Day	41% 7% 11% 18% 18% 4	2160
May	1st to 10th	Day	46% 7% 11% 17% 16% 3	2295
	11th to 20th	Day	45% 7% 12% 21% 14% 2	2482
	21st to end	Day	39% 8% 15% 23% 13% 2	2725
June	1st to 10th	Day	39% 9% 12% 23% 14% 1	2520
	11th to 20th	Day	40% 10% 13% 22% 12% 3	2520
	21st to end	Day	37% 8% 15% 22% 17% 1	2520
July	1st to 10th	Day	39% 10% 14% 20% 15% 3	2450
	11th to 20th	Day	34% 10% 15% 24% 15% 2	2445
	21st to end	Day	39% 9% 13% 21% 16% 2	2552
August	1st to 10th	Day	40% 8% 14% 22% 15% 1	2310
	11th to 20th	Day	38% 9% 16% 20% 15% 2	2126
	21st to end	Day	39% 9% 13% 19% 17% 3	2376
September	1st to 10th	Day	39% 7% 11% 20% 20% 3	2160
	11th to 20th	Day	32% 5% 11% 23% 24% 4	2128
	21st to end	Day	44% 6% 8% 18% 21% 2	1980
October	1st to 10th	Day	41% 6% 12% 22% 18% 1	1810
	11th to 20th	Day	36% 5% 11% 25% 22% 2	1800
	21st to end	Day	41% 5% 9% 24% 18% 2	1915
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 9% 22% 16% 5%	1831
March	1st to 10th	Night	37% 5% 12% 24% 18% 4	2160
	11th to 20th	Night	42% 5% 10% 20% 18% 4	1983
	21st to end	Night	43% 5% 9% 18% 18% 6%	2048
April	1st to 10th	Night	42% 6% 11% 19% 18% 5%	1800
	11th to 20th	Night	40% 7% 10% 21% 20% 4	1775
	21st to end	Night	41% 6% 10% 19% 19% 5%	1800
May	1st to 10th	Night	47% 7% 9% 17% 18% 2	1680
	11th to 20th	Night	45% 7% 12% 16% 18% 3	1416
	21st to end	Night	43% 9% 15% 17% 13% 2	1551
June	1st to 10th	Night	38% 8% 11% 22% 19% 2	1440
	11th to 20th	Night	40% 10% 12% 24% 12% 2	1440
	21st to end	Night	33% 10% 15% 24% 16% 2	1440
July	1st to 10th	Night	40% 12% 14% 18% 14% 2	1400
	11th to 20th	Night	32% 11% 17% 24% 14% 2	1392
	21st to end	Night	34% 8% 18% 24% 15% 2	1522
August	1st to 10th	Night	36% 9% 18% 21% 15% 1	1600
	11th to 20th	Night	40% 10% 14% 19% 16% 2	1760
	21st to end	Night	39% 9% 14% 18% 18% 3	1980
September	1st to 10th	Night	39% 7% 13% 19% 19% 3	1800
	11th to 20th	Night	31% 6% 10% 23% 26% 4	1831
	21st to end	Night	41% 7% 9% 20% 21% 2	1980
October	1st to 10th	Night	42% 6% 12% 22% 16% 3	2147
	11th to 20th	Night	34% 6% 12% 27% 18% 3	2160
	21st to end	Night	40% 4 9% 25% 19% 2	2439
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	27% 22% 33% 15% 2	362
March	1st to 10th	Day	20% 23% 33% 20% 3	420
	11th to 20th	Day	20% 25% 33% 16% 3 2	420
	21st to end	Day	27% 20% 29% 18% 5% 1	462
April	1st to 10th	Day	22% 25% 32% 13% 7% 1	420
	11th to 20th	Day	19% 20% 34% 20% 7% 1	412
	21st to end	Day	26% 18% 31% 19% 5% 1	420
May	1st to 10th	Day	28% 19% 28% 15% 8% 2	423
	11th to 20th	Day	21% 15% 29% 25% 9% 2	447
	21st to end	Day	19% 9% 22% 30% 16% 4	485
June	1st to 10th	Day	14% 13% 28% 28% 14% 4	450
	11th to 20th	Day	16% 10% 30% 28% 12% 3	450
	21st to end	Day	13% 10% 28% 30% 15% 4	450
July	1st to 10th	Day	11% 5% 22% 33% 22% 7%	445
	11th to 20th	Day	6% 3 25% 36% 25% 6%	444
	21st to end	Day	12% 6% 26% 34% 16% 6%	488
August	1st to 10th	Day	10% 6% 25% 33% 19% 7%	446
	11th to 20th	Day	15% 4 20% 34% 22% 6%	439
	21st to end	Day	13% 7% 28% 27% 16% 9%	495
September	1st to 10th	Day	18% 9% 25% 24% 17% 8%	450
	11th to 20th	Day	15% 8% 25% 28% 16% 8%	450
	21st to end	Day	24% 13% 21% 26% 11% 5%	450
October	1st to 10th	Day	25% 8% 26% 21% 14% 6%	450
	11th to 20th	Day	18% 15% 29% 24% 12% 2	450
	21st to end	Day	21% 14% 36% 18% 10% 2	462
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	22% 20% 37% 18% 2	611
March	1st to 10th	Night	20% 18% 36% 22% 4	720
	11th to 20th	Night	21% 19% 35% 18% 5%	720
	21st to end	Night	21% 16% 38% 20% 5%	792
April	1st to 10th	Night	20% 20% 37% 18% 5%	720
	11th to 20th	Night	18% 15% 35% 27% 5%	711
	21st to end	Night	21% 13% 34% 26% 5%	720
May	1st to 10th	Night	21% 11% 37% 25% 6%	600
	11th to 20th	Night	19% 11% 37% 27% 6%	354
	21st to end	Night	19% 4 25% 39% 12% 2	389
June	1st to 10th	Night	16% 7% 28% 38% 10% 1	360
	11th to 20th	Night	12% 7% 31% 40% 10% 1	360
	21st to end	Night	10% 3 29% 47% 11%	360
July	1st to 10th	Night	9% 5% 28% 40% 16% 1	350
	11th to 20th	Night	7% 4 24% 43% 20% 2	348
	21st to end	Night	8% 5% 28% 38% 21% 1	432
August	1st to 10th	Night	10% 3 20% 44% 21% 2	537
	11th to 20th	Night	13% 4 20% 40% 21% 3	696
	21st to end	Night	13% 5% 20% 38% 22% 2	792
September	1st to 10th	Night	17% 4 25% 35% 16% 2	720
	11th to 20th	Night	15% 4 20% 36% 21% 4	720
	21st to end	Night	22% 5% 24% 30% 18% 1	720
October	1st to 10th	Night	21% 9% 24% 26% 16% 3	720
	11th to 20th	Night	19% 10% 27% 32% 12%	720
	21st to end	Night	22% 12% 32% 25% 9% 1	792
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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