Allentown(PA)
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Allentown(PA) clear sky chart from 2005-11-06 to 2025-05-11.

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% 7% 2 6% 4 4 4 4 6% 7% 39%	113836
	Night	16% 7% 2 6% 4 4 4 3 5% 6% 42%	115114
Transparency	Day	42% 4 12% 24% 15% 2	117414
	Night	42% 4 11% 24% 16% 3	119700
Seeing	Day	21% 14% 27% 21% 12% 4	15517
	Night	19% 10% 30% 29% 11% 1	24917

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% 4 4 4 3 6% 7% 47%	3597
	11th to 20th	Day	13% 9% 5% 9% 6% 5% 5% 4 4 6% 35%	3658
	21st to end	Day	17% 8% 3 8% 5% 4 4 3 5% 6% 38%	4466
February	1st to 10th	Day	15% 7% 3 6% 6% 4 6% 3 6% 7% 36%	1800
	11th to 20th	Day	15% 9% 2 7% 5% 4 4 3 5% 7% 38%	1875
	21st to end	Day	19% 9% 2 6% 4 4 4 4 6% 5% 37%	1650
March	1st to 10th	Day	21% 6% 2 5% 4 4 4 4 4 6% 41%	4516
	11th to 20th	Day	12% 7% 3 6% 4 4 5% 4 6% 7% 42%	4980
	21st to end	Day	18% 6% 2 5% 3 3 4 4 5% 7% 43%	5776
April	1st to 10th	Day	18% 8% 3 6% 5% 4 4 4 6% 7% 36%	2400
	11th to 20th	Day	17% 7% 2 6% 5% 4 3 4 5% 7% 40%	2374
	21st to end	Day	16% 5% 2 6% 4 4 4 5% 6% 7% 39%	2400
May	1st to 10th	Day	27% 7% 2 7% 4 4 4 3 4 7% 30%	4271
	11th to 20th	Day	25% 8% 3 7% 5% 4 4 4 5% 7% 28%	4398
	21st to end	Day	13% 5% 2 6% 3 4 4 4 6% 10% 43%	4837
June	1st to 10th	Day	13% 6% 2 6% 5% 6% 4 5% 7% 9% 37%	2660
	11th to 20th	Day	15% 5% 2 5% 5% 5% 5% 5% 7% 9% 37%	2660
	21st to end	Day	12% 5% 1 5% 5% 5% 4 4 6% 9% 42%	2660
July	1st to 10th	Day	7% 4 2 7% 6% 6% 6% 5% 8% 10% 37%	4440
	11th to 20th	Day	9% 6% 2 5% 4 5% 5% 4 8% 10% 41%	4440
	21st to end	Day	12% 8% 3 8% 6% 5% 5% 4 7% 8% 34%	4752
August	1st to 10th	Day	20% 6% 3 6% 5% 4 5% 4 7% 8% 32%	4104
	11th to 20th	Day	14% 6% 2 5% 4 4 5% 4 6% 8% 42%	3773
	21st to end	Day	14% 5% 2 4 3 4 4 4 6% 8% 46%	4180
September	1st to 10th	Day	15% 6% 2 5% 4 4 5% 3 6% 7% 43%	2280
	11th to 20th	Day	13% 5% 1 5% 3 3 4 3 6% 7% 50%	2261
	21st to end	Day	20% 6% 2 6% 4 4 5% 4 4 6% 40%	2070
October	1st to 10th	Day	18% 6% 2 5% 3 4 4 4 5% 7% 41%	1900
	11th to 20th	Day	16% 5% 2 5% 3 4 4 3 5% 6% 46%	1900
	21st to end	Day	19% 6% 2 6% 3 4 4 3 5% 6% 41%	2024
November	1st to 10th	Day	13% 7% 2 4 2 2 3 2 4 6% 52%	1761
	11th to 20th	Day	17% 6% 1 5% 3 4 4 4 6% 5% 45%	1800
	21st to end	Day	17% 7% 2 6% 3 3 3 4 6% 8% 41%	1627
December	1st to 10th	Day	16% 7% 2 6% 4 4 4 4 5% 6% 40%	2600
	11th to 20th	Day	22% 8% 2 4 3 3 4 3 6% 5% 39%	2986
	21st to end	Day	18% 8% 2 7% 6% 4 5% 4 6% 7% 34%	3960
January	1st to 10th	Night	13% 6% 2 6% 4 3 4 3 5% 7% 47%	6349
	11th to 20th	Night	13% 8% 3 6% 4 4 5% 4 6% 7% 41%	6223
	21st to end	Night	18% 8% 3 6% 4 3 4 4 5% 6% 41%	6490
February	1st to 10th	Night	17% 8% 3 5% 5% 3 4 4 5% 7% 39%	2600
	11th to 20th	Night	17% 7% 2 6% 4 4 4 4 4 5% 41%	2411
	21st to end	Night	18% 8% 2 6% 4 3 4 3 5% 7% 40%	1980
March	1st to 10th	Night	18% 9% 2 6% 3 3 3 3 5% 5% 42%	5322
	11th to 20th	Night	11% 7% 2 5% 4 4 4 3 4 6% 50%	4979
	21st to end	Night	17% 8% 3 6% 4 4 4 3 4 5% 42%	5156
April	1st to 10th	Night	18% 7% 2 6% 5% 5% 5% 4 5% 7% 37%	2000
	11th to 20th	Night	17% 7% 3 5% 4 4 4 4 5% 8% 40%	1975
	21st to end	Night	20% 5% 2 4 3 3 4 4 5% 6% 46%	2000
May	1st to 10th	Night	27% 9% 2 6% 4 4 4 3 5% 7% 30%	2919
	11th to 20th	Night	25% 9% 3 8% 5% 4 4 3 5% 6% 29%	2501
	21st to end	Night	13% 6% 2 6% 4 4 4 4 6% 10% 41%	2738
June	1st to 10th	Night	16% 7% 1 5% 4 4 4 3 5% 6% 44%	1520
	11th to 20th	Night	13% 5% 1 5% 4 5% 4 5% 7% 8% 42%	1520
	21st to end	Night	11% 4 2 5% 5% 4 4 4 6% 10% 44%	1520
July	1st to 10th	Night	9% 5% 2 7% 5% 6% 7% 4 7% 8% 38%	2520
	11th to 20th	Night	10% 6% 2 6% 6% 5% 6% 5% 8% 9% 38%	2520
	21st to end	Night	15% 7% 2 6% 4 4 5% 5% 7% 8% 38%	2797
August	1st to 10th	Night	20% 6% 3 7% 4 5% 5% 4 6% 7% 32%	2825
	11th to 20th	Night	16% 7% 3 6% 5% 4 4 3 5% 6% 42%	3024
	21st to end	Night	13% 4 2 5% 3 3 4 4 6% 8% 48%	3476
September	1st to 10th	Night	15% 6% 2 5% 4 4 3 3 4 6% 48%	1900
	11th to 20th	Night	13% 5% 2 4 3 3 4 3 5% 6% 52%	1918
	21st to end	Night	18% 6% 2 6% 4 3 4 3 5% 6% 43%	2107
October	1st to 10th	Night	20% 6% 2 5% 3 3 3 3 5% 6% 43%	2280
	11th to 20th	Night	16% 5% 2 4 3 3 3 3 5% 6% 49%	2280
	21st to end	Night	20% 6% 1 4 3 3 3 3 4 6% 47%	2573
November	1st to 10th	Night	12% 5% 2 5% 3 4 4 3 5% 5% 52%	2537
	11th to 20th	Night	16% 7% 3 5% 4 3 4 3 5% 5% 47%	2600
	21st to end	Night	15% 6% 2 5% 5% 4 4 3 5% 7% 43%	2770
December	1st to 10th	Night	14% 8% 3 6% 4 4 4 3 4 6% 43%	4580
	11th to 20th	Night	23% 8% 2 5% 3 3 4 3 4 6% 39%	5208
	21st to end	Night	19% 10% 3 7% 4 4 3 3 5% 5% 37%	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	37% 3 10% 29% 19% 2	3598
	11th to 20th	Day	50% 3 9% 20% 14% 4	3688
	21st to end	Day	47% 4 12% 23% 13% 1	4466
February	1st to 10th	Day	46% 4 13% 22% 13% 1	1800
	11th to 20th	Day	46% 4 9% 24% 14% 2	1884
	21st to end	Day	48% 4 10% 25% 11% 3	1650
March	1st to 10th	Day	44% 3 8% 25% 13% 6%	4519
	11th to 20th	Day	41% 4 11% 29% 11% 4	4980
	21st to end	Day	42% 3 11% 16% 22% 4	5776
April	1st to 10th	Day	47% 4 8% 22% 15% 5%	2400
	11th to 20th	Day	43% 4 10% 24% 15% 4	2374
	21st to end	Day	43% 5% 10% 22% 16% 4	2400
May	1st to 10th	Day	55% 3 11% 19% 11% 2	4284
	11th to 20th	Day	57% 3 11% 18% 9% 1	4402
	21st to end	Day	36% 5% 11% 29% 17% 2	4837
June	1st to 10th	Day	42% 6% 13% 25% 14% 1	2660
	11th to 20th	Day	41% 6% 15% 24% 11% 2	2660
	21st to end	Day	38% 6% 14% 27% 14% 1	2660
July	1st to 10th	Day	40% 9% 14% 23% 14% 1	4370
	11th to 20th	Day	36% 7% 17% 26% 13% 1	4365
	21st to end	Day	47% 6% 10% 19% 16% 2	4601
August	1st to 10th	Day	49% 7% 12% 22% 10%	4091
	11th to 20th	Day	39% 5% 13% 31% 13% 1	3810
	21st to end	Day	35% 5% 11% 31% 16% 2	4180
September	1st to 10th	Day	40% 5% 10% 22% 20% 2	2280
	11th to 20th	Day	33% 3 10% 25% 25% 3	2261
	21st to end	Day	47% 4 8% 19% 20% 2	2070
October	1st to 10th	Day	38% 4 8% 22% 23% 5%	3160
	11th to 20th	Day	29% 4 10% 29% 25% 4	3140
	21st to end	Day	29% 3 14% 25% 26% 4	3314
November	1st to 10th	Day	35% 3 14% 25% 21% 3	1761
	11th to 20th	Day	39% 4 8% 28% 18% 2	1800
	21st to end	Day	40% 4 14% 30% 10% 2	1627
December	1st to 10th	Day	45% 3 16% 25% 9% 2	2600
	11th to 20th	Day	46% 3 15% 24% 10% 1	2986
	21st to end	Day	49% 3 12% 22% 13% 1	3960
January	1st to 10th	Night	38% 3 9% 27% 20% 4	6349
	11th to 20th	Night	42% 3 13% 23% 14% 6%	6268
	21st to end	Night	44% 2 10% 23% 17% 3	6490
February	1st to 10th	Night	46% 4 13% 26% 10% 2	2600
	11th to 20th	Night	47% 3 9% 22% 16% 4	2413
	21st to end	Night	45% 2 8% 24% 18% 3	1980
March	1st to 10th	Night	45% 3 10% 23% 12% 8%	5332
	11th to 20th	Night	37% 3 15% 28% 14% 4	4980
	21st to end	Night	45% 3 7% 17% 24% 5%	5156
April	1st to 10th	Night	46% 4 10% 22% 15% 4	2000
	11th to 20th	Night	43% 5% 10% 21% 20% 2	1975
	21st to end	Night	39% 5% 9% 23% 19% 5%	2000
May	1st to 10th	Night	54% 4 10% 21% 10% 1	2925
	11th to 20th	Night	57% 4 13% 15% 10% 1	2502
	21st to end	Night	40% 5% 15% 24% 14% 3	2739
June	1st to 10th	Night	40% 5% 11% 23% 19% 2	1520
	11th to 20th	Night	36% 7% 15% 27% 14% 1	1520
	21st to end	Night	34% 7% 14% 30% 14% 1	1520
July	1st to 10th	Night	42% 9% 15% 17% 15% 2	2480
	11th to 20th	Night	39% 7% 14% 26% 13% 1	2472
	21st to end	Night	42% 7% 14% 18% 18% 1	2708
August	1st to 10th	Night	49% 6% 17% 17% 10% 1	2814
	11th to 20th	Night	43% 5% 12% 24% 15% 1	3053
	21st to end	Night	34% 5% 10% 32% 18% 1	3476
September	1st to 10th	Night	39% 5% 10% 24% 21% 3	1900
	11th to 20th	Night	33% 4 10% 24% 26% 4	1918
	21st to end	Night	43% 3 8% 23% 21% 1	2107
October	1st to 10th	Night	44% 4 8% 22% 21% 2	3800
	11th to 20th	Night	28% 2 8% 31% 26% 4	3779
	21st to end	Night	28% 3 12% 31% 24% 2	4233
November	1st to 10th	Night	34% 4 12% 25% 21% 4	2537
	11th to 20th	Night	41% 3 9% 25% 19% 4	2600
	21st to end	Night	41% 4 12% 27% 14% 2	2770
December	1st to 10th	Night	43% 3 13% 31% 9% 1	4580
	11th to 20th	Night	48% 3 11% 23% 13% 2	5208
	21st to end	Night	50% 2 10% 22% 15% 2	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	28% 26% 26% 18% 2 2	200
	11th to 20th	Day	27% 29% 31% 10% 2	218
	21st to end	Day	22% 24% 34% 14% 5% 1	330
February	1st to 10th	Day	22% 27% 36% 11% 3 1	300
	11th to 20th	Day	28% 21% 37% 12% 2	423
	21st to end	Day	30% 17% 30% 19% 3	414
March	1st to 10th	Day	26% 20% 30% 20% 4	500
	11th to 20th	Day	24% 20% 34% 17% 4 1	500
	21st to end	Day	27% 23% 30% 13% 5% 1	550
April	1st to 10th	Day	25% 24% 31% 14% 6% 1	500
	11th to 20th	Day	25% 23% 26% 16% 8% 3	492
	21st to end	Day	25% 17% 31% 18% 6% 3	500
May	1st to 10th	Day	30% 17% 26% 17% 7% 3	503
	11th to 20th	Day	23% 14% 28% 22% 11% 2	487
	21st to end	Day	20% 8% 24% 22% 21% 5%	529
June	1st to 10th	Day	16% 15% 26% 22% 16% 4	490
	11th to 20th	Day	16% 11% 26% 26% 17% 5%	490
	21st to end	Day	15% 10% 29% 27% 15% 4	490
July	1st to 10th	Day	10% 5% 22% 30% 25% 8%	485
	11th to 20th	Day	11% 4 21% 32% 25% 7%	484
	21st to end	Day	15% 5% 25% 32% 16% 7%	532
August	1st to 10th	Day	14% 7% 20% 29% 22% 9%	486
	11th to 20th	Day	16% 5% 18% 30% 22% 9%	479
	21st to end	Day	14% 7% 22% 27% 19% 11%	539
September	1st to 10th	Day	18% 8% 22% 23% 20% 9%	490
	11th to 20th	Day	15% 5% 22% 27% 18% 14%	490
	21st to end	Day	24% 11% 23% 22% 14% 6%	490
October	1st to 10th	Day	24% 8% 25% 22% 15% 7%	490
	11th to 20th	Day	17% 12% 32% 23% 13% 3	490
	21st to end	Day	22% 13% 33% 20% 11% 2	504
November	1st to 10th	Day	17% 13% 30% 26% 12% 3	395
	11th to 20th	Day	21% 19% 31% 23% 6% 1	400
	21st to end	Day	24% 23% 33% 15% 5% 1	227
December	1st to 10th	Day	26% 25% 31% 12% 6% 1	200
	11th to 20th	Day	32% 23% 26% 15% 5%	200
	21st to end	Day	25% 19% 30% 18% 7% 2	220
January	1st to 10th	Night	18% 18% 39% 21% 4	800
	11th to 20th	Night	24% 14% 34% 25% 3	800
	21st to end	Night	18% 14% 40% 25% 2	880
February	1st to 10th	Night	21% 14% 37% 26% 2	800
	11th to 20th	Night	22% 19% 40% 18% 2	783
	21st to end	Night	21% 17% 39% 20% 3	660
March	1st to 10th	Night	22% 14% 36% 25% 4	800
	11th to 20th	Night	22% 15% 36% 21% 6%	800
	21st to end	Night	22% 12% 37% 23% 5% 1	880
April	1st to 10th	Night	22% 17% 36% 21% 5%	800
	11th to 20th	Night	21% 11% 37% 25% 6%	791
	21st to end	Night	23% 9% 33% 29% 7%	800
May	1st to 10th	Night	26% 8% 30% 27% 8%	608
	11th to 20th	Night	20% 6% 33% 31% 10%	376
	21st to end	Night	17% 3 23% 41% 15% 1	411
June	1st to 10th	Night	17% 5% 27% 32% 19%	380
	11th to 20th	Night	12% 7% 24% 43% 14%	380
	21st to end	Night	13% 3 23% 48% 13%	380
July	1st to 10th	Night	13% 3 17% 44% 22% 1	370
	11th to 20th	Night	9% 3 20% 41% 26% 2	368
	21st to end	Night	13% 4 19% 38% 24% 2	450
August	1st to 10th	Night	13% 2 13% 46% 24% 2	567
	11th to 20th	Night	15% 4 15% 37% 28% 2	701
	21st to end	Night	15% 3 14% 41% 25% 2	836
September	1st to 10th	Night	18% 3 21% 36% 20% 2	760
	11th to 20th	Night	14% 3 17% 36% 26% 4	760
	21st to end	Night	21% 4 21% 33% 18% 2	760
October	1st to 10th	Night	22% 5% 24% 29% 17% 3	760
	11th to 20th	Night	17% 8% 29% 34% 11% 1	760
	21st to end	Night	23% 7% 32% 28% 10%	836
November	1st to 10th	Night	15% 6% 35% 31% 12% 1	780
	11th to 20th	Night	20% 13% 30% 29% 7% 1	800
	21st to end	Night	18% 13% 36% 27% 7%	800
December	1st to 10th	Night	24% 11% 35% 24% 6% 1	800
	11th to 20th	Night	23% 11% 36% 26% 5%	800
	21st to end	Night	21% 12% 36% 27% 4	880

Page generated at 2025-05-08 09:33:57. Version ac07

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