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 2025-04-26.

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 6% 4 4 4 4 6% 7% 41%	113580
	Night	16% 7% 2 6% 4 4 4 3 5% 6% 44%	115037
Transparency	Day	40% 6% 12% 23% 17% 3	117157
	Night	39% 6% 11% 23% 18% 4	119623
Seeing	Day	20% 15% 29% 22% 11% 3	15549
	Night	18% 13% 31% 27% 9% 1	24964

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 3 4 3 5% 6% 50%	3597
	11th to 20th	Day	14% 8% 3 8% 5% 4 3 3 5% 6% 42%	3680
	21st to end	Day	18% 8% 2 7% 4 4 3 2 4 5% 43%	4466
February	1st to 10th	Day	16% 8% 2 6% 5% 5% 4 3 6% 7% 39%	1800
	11th to 20th	Day	17% 8% 2 6% 4 3 3 4 5% 7% 41%	1910
	21st to end	Day	18% 8% 3 6% 5% 4 3 3 5% 6% 39%	1650
March	1st to 10th	Day	20% 5% 2 5% 3 3 3 3 4 5% 46%	4476
	11th to 20th	Day	11% 6% 2 6% 4 4 4 4 5% 6% 47%	4975
	21st to end	Day	18% 5% 2 6% 4 4 4 3 5% 6% 44%	5803
April	1st to 10th	Day	17% 7% 2 5% 5% 4 4 4 7% 7% 39%	2400
	11th to 20th	Day	14% 7% 2 5% 4 4 5% 3 6% 7% 42%	2374
	21st to end	Day	16% 6% 2 5% 5% 3 4 5% 6% 7% 41%	2340
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 3 3 5% 5% 47%	2021
November	1st to 10th	Day	12% 6% 2 5% 3 3 3 2 4 7% 53%	1761
	11th to 20th	Day	15% 5% 2 5% 4 4 3 4 5% 7% 48%	1800
	21st to end	Day	16% 5% 1 5% 4 3 4 3 5% 7% 47%	1681
December	1st to 10th	Day	14% 6% 2 5% 4 4 4 4 6% 7% 45%	2600
	11th to 20th	Day	20% 9% 2 4 3 3 2 2 3 5% 45%	2986
	21st to end	Day	16% 5% 2 5% 4 4 5% 4 6% 7% 40%	3960
January	1st to 10th	Night	14% 6% 2 5% 3 4 4 3 5% 5% 50%	6349
	11th to 20th	Night	15% 7% 2 6% 3 3 3 3 3 5% 50%	6202
	21st to end	Night	18% 7% 2 6% 4 3 4 3 5% 6% 42%	6490
February	1st to 10th	Night	17% 8% 3 5% 4 3 4 3 5% 7% 40%	2600
	11th to 20th	Night	19% 7% 3 6% 4 4 3 3 5% 6% 41%	2374
	21st to end	Night	17% 8% 2 6% 4 4 4 3 5% 6% 42%	1980
March	1st to 10th	Night	19% 8% 2 6% 3 3 3 3 4 5% 45%	5364
	11th to 20th	Night	11% 8% 3 5% 3 3 3 2 4 5% 51%	4982
	21st to end	Night	16% 7% 2 6% 4 4 4 3 4 6% 43%	5128
April	1st to 10th	Night	17% 7% 3 6% 5% 4 4 4 5% 6% 40%	2000
	11th to 20th	Night	16% 6% 2 6% 4 3 4 4 5% 7% 42%	1975
	21st to end	Night	18% 5% 3 6% 3 4 4 4 5% 7% 43%	1957
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 4 3 3 3 3 5% 5% 49%	2575
November	1st to 10th	Night	12% 5% 2 5% 3 3 3 3 4 6% 54%	2537
	11th to 20th	Night	17% 6% 2 5% 4 3 3 3 4 5% 49%	2600
	21st to end	Night	13% 6% 2 5% 5% 4 4 4 5% 7% 45%	2715
December	1st to 10th	Night	13% 8% 2 5% 3 3 3 4 5% 6% 48%	4580
	11th to 20th	Night	23% 6% 2 5% 3 3 3 3 5% 7% 41%	5208
	21st to end	Night	19% 7% 3 5% 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	35% 4 10% 26% 23% 3	3598
	11th to 20th	Day	43% 4 9% 23% 17% 5%	3710
	21st to end	Day	44% 5% 13% 24% 13% 2	4466
February	1st to 10th	Day	43% 6% 13% 25% 11% 2	1800
	11th to 20th	Day	43% 5% 10% 24% 15% 2	1919
	21st to end	Day	46% 4 10% 22% 13% 4	1650
March	1st to 10th	Day	40% 4 10% 24% 15% 7%	4479
	11th to 20th	Day	35% 6% 11% 28% 16% 5%	4975
	21st to end	Day	42% 4 10% 16% 23% 5%	5803
April	1st to 10th	Day	42% 6% 11% 21% 15% 5%	2400
	11th to 20th	Day	39% 7% 13% 22% 16% 4	2374
	21st to end	Day	41% 7% 11% 19% 18% 4	2340
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	35% 5% 10% 17% 29% 5%	3170
	11th to 20th	Day	24% 4 10% 23% 30% 9%	3140
	21st to end	Day	25% 3 12% 27% 28% 6%	3311
November	1st to 10th	Day	33% 4 13% 26% 21% 3	1761
	11th to 20th	Day	37% 5% 10% 24% 22% 3	1800
	21st to end	Day	37% 5% 13% 31% 12% 2	1681
December	1st to 10th	Day	37% 5% 17% 25% 14% 2	2600
	11th to 20th	Day	44% 3 11% 29% 10% 2	2986
	21st to end	Day	39% 7% 13% 22% 17% 1	3960
January	1st to 10th	Night	35% 5% 8% 24% 21% 6%	6349
	11th to 20th	Night	38% 3 10% 24% 18% 7%	6247
	21st to end	Night	43% 4 10% 23% 18% 2	6490
February	1st to 10th	Night	44% 5% 13% 24% 12% 2	2600
	11th to 20th	Night	44% 5% 8% 21% 18% 5%	2376
	21st to end	Night	43% 4 10% 21% 16% 6%	1980
March	1st to 10th	Night	41% 5% 9% 23% 13% 9%	5374
	11th to 20th	Night	35% 7% 14% 25% 15% 5%	4983
	21st to end	Night	43% 5% 7% 16% 24% 5%	5128
April	1st to 10th	Night	44% 6% 10% 19% 18% 4	2000
	11th to 20th	Night	41% 7% 9% 20% 19% 4	1975
	21st to end	Night	39% 6% 10% 20% 20% 5%	1957
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	39% 5% 10% 22% 21% 3	3787
	11th to 20th	Night	23% 4 10% 27% 29% 6%	3779
	21st to end	Night	26% 3 11% 30% 24% 4	4235
November	1st to 10th	Night	32% 5% 12% 24% 21% 6%	2537
	11th to 20th	Night	39% 5% 9% 21% 21% 5%	2600
	21st to end	Night	37% 6% 12% 26% 17% 2	2715
December	1st to 10th	Night	37% 4 12% 30% 14% 3	4580
	11th to 20th	Night	44% 4 11% 23% 14% 4	5208
	21st to end	Night	43% 6% 11% 19% 17% 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	25% 28% 33% 11% 4 1	200
	11th to 20th	Day	27% 33% 28% 10% 2	223
	21st to end	Day	23% 29% 30% 14% 5%	330
February	1st to 10th	Day	25% 27% 34% 11% 3	300
	11th to 20th	Day	33% 22% 32% 11% 2	458
	21st to end	Day	27% 22% 34% 14% 2	414
March	1st to 10th	Day	24% 23% 31% 18% 3 1	500
	11th to 20th	Day	20% 26% 32% 16% 3 2	500
	21st to end	Day	27% 21% 31% 16% 4 1	550
April	1st to 10th	Day	27% 23% 32% 11% 6% 1	500
	11th to 20th	Day	21% 21% 33% 18% 6% 1	492
	21st to end	Day	24% 17% 31% 21% 6% 1	480
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% 19% 12% 2	502
November	1st to 10th	Day	16% 15% 32% 23% 12% 3	395
	11th to 20th	Day	19% 22% 33% 21% 6%	400
	21st to end	Day	19% 24% 32% 18% 6% 1	281
December	1st to 10th	Day	25% 26% 36% 8% 6%	200
	11th to 20th	Day	32% 22% 31% 13% 4	200
	21st to end	Day	26% 20% 32% 14% 7%	220
January	1st to 10th	Night	20% 21% 36% 19% 4	800
	11th to 20th	Night	23% 20% 31% 23% 3	800
	21st to end	Night	18% 19% 40% 22% 2	880
February	1st to 10th	Night	22% 17% 38% 22% 2	800
	11th to 20th	Night	26% 22% 35% 16% 1	783
	21st to end	Night	23% 20% 36% 19% 2	660
March	1st to 10th	Night	21% 19% 35% 21% 4	800
	11th to 20th	Night	20% 20% 36% 18% 5%	800
	21st to end	Night	21% 16% 39% 19% 5%	880
April	1st to 10th	Night	21% 20% 37% 18% 5%	800
	11th to 20th	Night	20% 16% 35% 25% 4	791
	21st to end	Night	20% 13% 34% 27% 5%	783
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	20% 11% 33% 26% 9%	836
November	1st to 10th	Night	14% 12% 34% 29% 10% 1	780
	11th to 20th	Night	19% 16% 32% 25% 8%	800
	21st to end	Night	16% 18% 35% 24% 8% 1	800
December	1st to 10th	Night	24% 14% 34% 20% 6% 1	800
	11th to 20th	Night	20% 16% 35% 25% 5%	800
	21st to end	Night	21% 15% 34% 27% 2	880

Page generated at 2025-04-23 09:25:02. Version ac07

Copyright © 2025 Attilla Danko.