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 2026-09-14.

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 5% 4 6% 8% 39%	149294
	Night	16% 7% 2 6% 4 4 4 4 5% 7% 41%	141793
Transparency	Day	42% 5% 12% 24% 15% 2	152825
	Night	43% 4 11% 24% 16% 3	146350
Seeing	Day	21% 14% 27% 22% 12% 4	17313
	Night	19% 10% 30% 30% 11% 1	26521

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	14% 6% 2 5% 4 4 4 3 6% 7% 44%	4661
	11th to 20th	Day	14% 8% 4 9% 5% 4 4 3 4 6% 38%	4766
	21st to end	Day	15% 6% 3 7% 5% 5% 5% 4 6% 7% 39%	5780
February	1st to 10th	Day	15% 7% 3 6% 6% 4 6% 3 6% 7% 38%	1890
	11th to 20th	Day	15% 9% 3 7% 4 4 4 3 5% 7% 38%	1971
	21st to end	Day	19% 9% 2 6% 4 4 4 4 6% 5% 37%	1730
March	1st to 10th	Day	21% 6% 2 5% 4 4 4 4 4 6% 41%	4617
	11th to 20th	Day	12% 7% 2 6% 5% 4 5% 4 6% 7% 42%	5090
	21st to end	Day	18% 6% 2 5% 3 4 4 4 5% 7% 43%	5903
April	1st to 10th	Day	18% 8% 3 6% 5% 4 4 4 6% 7% 37%	2520
	11th to 20th	Day	16% 7% 2 6% 5% 4 4 4 6% 7% 40%	2494
	21st to end	Day	16% 6% 2 6% 4 4 4 5% 6% 7% 39%	2520
May	1st to 10th	Day	23% 6% 2 6% 4 4 5% 4 5% 8% 33%	7673
	11th to 20th	Day	20% 7% 2 6% 4 4 4 3 5% 8% 37%	8221
	21st to end	Day	23% 6% 2 6% 3 4 4 3 6% 8% 34%	9060
June	1st to 10th	Day	13% 6% 2 6% 5% 6% 4 5% 7% 9% 38%	2940
	11th to 20th	Day	15% 5% 2 5% 5% 5% 5% 5% 7% 9% 37%	2940
	21st to end	Day	12% 6% 2 5% 5% 5% 4 4 6% 9% 42%	2940
July	1st to 10th	Day	11% 6% 2 7% 5% 5% 5% 4 7% 10% 38%	8280
	11th to 20th	Day	9% 7% 2 7% 5% 5% 5% 5% 8% 11% 37%	8280
	21st to end	Day	12% 6% 2 6% 4 4 5% 4 6% 9% 40%	8875
August	1st to 10th	Day	17% 7% 3 7% 5% 5% 6% 5% 8% 10% 28%	6014
	11th to 20th	Day	13% 5% 2 5% 3 4 5% 4 7% 10% 41%	5561
	21st to end	Day	12% 6% 2 5% 3 4 5% 4 6% 9% 43%	6080
September	1st to 10th	Day	15% 6% 2 5% 4 4 5% 3 6% 8% 43%	2520
	11th to 20th	Day	13% 5% 1 5% 4 3 4 3 6% 7% 50%	2416
	21st to end	Day	20% 7% 2 6% 4 4 5% 4 4 6% 39%	2179
October	1st to 10th	Day	18% 6% 2 5% 3 4 4 4 5% 6% 43%	2000
	11th to 20th	Day	16% 6% 2 5% 3 4 4 3 5% 7% 46%	2000
	21st to end	Day	19% 6% 2 5% 3 4 5% 3 6% 6% 42%	2130
November	1st to 10th	Day	13% 7% 2 4 3 3 3 2 4 6% 52%	1851
	11th to 20th	Day	16% 6% 2 5% 3 4 4 4 6% 6% 44%	1890
	21st to end	Day	17% 7% 2 6% 3 3 3 4 6% 8% 41%	1708
December	1st to 10th	Day	16% 7% 3 6% 4 4 4 4 5% 6% 40%	2680
	11th to 20th	Day	22% 8% 2 4 3 3 4 4 6% 5% 40%	3066
	21st to end	Day	18% 8% 2 7% 5% 4 5% 4 6% 7% 34%	4048
January	1st to 10th	Night	12% 8% 3 7% 4 4 4 4 5% 7% 43%	8234
	11th to 20th	Night	13% 9% 3 7% 4 4 4 3 5% 7% 40%	8115
	21st to end	Night	15% 7% 2 5% 3 4 4 4 5% 6% 45%	8401
February	1st to 10th	Night	16% 8% 3 5% 4 3 4 4 5% 7% 40%	2730
	11th to 20th	Night	17% 8% 3 6% 4 5% 4 4 4 5% 39%	2534
	21st to end	Night	17% 8% 2 6% 4 3 4 4 5% 6% 40%	2076
March	1st to 10th	Night	18% 9% 2 6% 3 3 3 3 5% 5% 42%	5441
	11th to 20th	Night	12% 7% 2 5% 4 4 4 3 4 6% 50%	5089
	21st to end	Night	17% 8% 3 6% 4 4 4 3 4 5% 42%	5271
April	1st to 10th	Night	18% 7% 2 6% 5% 5% 5% 4 5% 7% 38%	2100
	11th to 20th	Night	16% 7% 3 6% 4 4 5% 4 5% 8% 40%	2075
	21st to end	Night	20% 5% 2 4 3 3 4 3 5% 6% 45%	2100
May	1st to 10th	Night	23% 8% 2 6% 4 4 5% 3 6% 7% 31%	5256
	11th to 20th	Night	21% 7% 3 7% 4 4 4 4 5% 8% 35%	4639
	21st to end	Night	24% 7% 2 6% 4 3 4 3 5% 7% 34%	5114
June	1st to 10th	Night	15% 6% 1 6% 5% 4 4 3 6% 6% 44%	1680
	11th to 20th	Night	14% 6% 2 5% 4 5% 4 4 8% 8% 40%	1680
	21st to end	Night	12% 4 2 5% 5% 4 4 4 7% 10% 44%	1680
July	1st to 10th	Night	12% 6% 2 7% 4 5% 5% 4 6% 9% 39%	4680
	11th to 20th	Night	11% 6% 3 8% 5% 5% 6% 5% 8% 9% 34%	4680
	21st to end	Night	13% 6% 2 6% 4 4 5% 4 6% 8% 42%	5198
August	1st to 10th	Night	18% 7% 3 8% 5% 6% 5% 4 6% 7% 31%	4135
	11th to 20th	Night	15% 6% 2 6% 6% 4 4 3 5% 7% 40%	4439
	21st to end	Night	13% 5% 2 7% 4 3 4 4 5% 7% 46%	5045
September	1st to 10th	Night	15% 6% 2 5% 4 3 4 3 5% 6% 46%	2100
	11th to 20th	Night	12% 5% 2 4 4 4 4 3 5% 6% 52%	2050
	21st to end	Night	18% 7% 2 6% 4 3 4 3 5% 6% 43%	2218
October	1st to 10th	Night	20% 6% 2 5% 3 3 3 3 5% 6% 44%	2400
	11th to 20th	Night	17% 5% 2 4 3 3 3 3 5% 6% 49%	2400
	21st to end	Night	19% 6% 1 4 3 3 3 3 4 6% 48%	2709
November	1st to 10th	Night	12% 5% 2 5% 3 4 4 4 5% 6% 52%	2667
	11th to 20th	Night	16% 7% 3 5% 4 3 4 3 5% 5% 46%	2730
	21st to end	Night	15% 7% 2 5% 5% 4 4 3 5% 7% 42%	2909
December	1st to 10th	Night	14% 8% 3 6% 4 4 4 3 5% 6% 43%	4720
	11th to 20th	Night	23% 8% 2 5% 3 3 4 3 4 6% 40%	5348
	21st to end	Night	19% 10% 3 7% 4 4 3 3 5% 5% 37%	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	40% 3 10% 29% 17% 1	4662
	11th to 20th	Day	47% 3 9% 21% 16% 5%	4796
	21st to end	Day	44% 4 13% 23% 14% 1	5780
February	1st to 10th	Day	45% 4 13% 24% 12% 2	1890
	11th to 20th	Day	47% 4 9% 24% 14% 2	1980
	21st to end	Day	47% 4 10% 25% 11% 3	1730
March	1st to 10th	Day	45% 3 8% 25% 13% 6%	4620
	11th to 20th	Day	41% 3 11% 29% 11% 4	5090
	21st to end	Day	42% 3 12% 16% 22% 5%	5903
April	1st to 10th	Day	47% 4 8% 22% 15% 5%	2520
	11th to 20th	Day	42% 4 12% 23% 15% 3	2494
	21st to end	Day	43% 5% 10% 23% 15% 4	2520
May	1st to 10th	Day	49% 5% 12% 22% 11% 1	7683
	11th to 20th	Day	47% 3 10% 26% 12% 1	8225
	21st to end	Day	49% 4 10% 23% 13% 1	9061
June	1st to 10th	Day	41% 6% 12% 24% 15% 1	2940
	11th to 20th	Day	41% 6% 15% 24% 12% 2	2940
	21st to end	Day	39% 6% 14% 26% 14% 1	2940
July	1st to 10th	Day	42% 8% 12% 21% 15% 1	8210
	11th to 20th	Day	39% 6% 15% 28% 11% 1	8205
	21st to end	Day	39% 6% 10% 23% 21% 1	8679
August	1st to 10th	Day	50% 8% 14% 18% 10%	6001
	11th to 20th	Day	37% 6% 16% 31% 10% 1	5598
	21st to end	Day	36% 6% 11% 30% 15% 2	6080
September	1st to 10th	Day	39% 5% 12% 22% 20% 2	2520
	11th to 20th	Day	34% 4 10% 26% 24% 3	2416
	21st to end	Day	47% 4 9% 19% 19% 2	2179
October	1st to 10th	Day	38% 4 8% 22% 23% 5%	3260
	11th to 20th	Day	29% 4 10% 29% 24% 4	3240
	21st to end	Day	28% 3 14% 26% 26% 3	3420
November	1st to 10th	Day	35% 3 14% 25% 21% 3	1851
	11th to 20th	Day	39% 4 8% 27% 18% 2	1890
	21st to end	Day	41% 4 13% 29% 10% 2	1708
December	1st to 10th	Day	45% 3 16% 25% 9% 2	2680
	11th to 20th	Day	46% 3 15% 24% 10% 1	3066
	21st to end	Day	49% 3 12% 23% 13% 1	4048
January	1st to 10th	Night	41% 4 13% 25% 15% 3	8234
	11th to 20th	Night	43% 4 11% 21% 16% 5%	8160
	21st to end	Night	40% 3 9% 25% 20% 2	8401
February	1st to 10th	Night	45% 3 12% 26% 11% 2	2730
	11th to 20th	Night	48% 4 9% 21% 16% 3	2536
	21st to end	Night	44% 3 8% 25% 17% 3	2076
March	1st to 10th	Night	45% 3 10% 23% 12% 8%	5451
	11th to 20th	Night	37% 3 15% 28% 14% 4	5090
	21st to end	Night	45% 3 7% 17% 23% 5%	5271
April	1st to 10th	Night	46% 4 10% 22% 15% 4	2100
	11th to 20th	Night	43% 5% 11% 20% 19% 2	2075
	21st to end	Night	39% 5% 10% 24% 18% 5%	2100
May	1st to 10th	Night	52% 4 11% 20% 11% 2	5262
	11th to 20th	Night	49% 4 12% 22% 11% 1	4640
	21st to end	Night	50% 4 11% 22% 11% 2	5115
June	1st to 10th	Night	40% 5% 10% 24% 20% 2	1680
	11th to 20th	Night	38% 7% 15% 25% 14% 1	1680
	21st to end	Night	35% 6% 14% 29% 14% 1	1680
July	1st to 10th	Night	43% 8% 13% 20% 15% 2	4640
	11th to 20th	Night	44% 7% 13% 25% 11% 1	4632
	21st to end	Night	39% 7% 12% 23% 19% 1	5080
August	1st to 10th	Night	51% 7% 14% 18% 10% 1	4124
	11th to 20th	Night	43% 5% 14% 25% 12% 1	4468
	21st to end	Night	37% 5% 11% 29% 16% 1	5045
September	1st to 10th	Night	39% 5% 10% 24% 20% 2	2100
	11th to 20th	Night	33% 4 10% 24% 26% 3	2050
	21st to end	Night	43% 3 8% 23% 21% 1	2218
October	1st to 10th	Night	43% 4 8% 22% 21% 2	3920
	11th to 20th	Night	29% 2 9% 31% 26% 4	3899
	21st to end	Night	28% 3 12% 31% 24% 2	4369
November	1st to 10th	Night	34% 3 12% 25% 21% 4	2667
	11th to 20th	Night	41% 3 9% 25% 18% 4	2730
	21st to end	Night	41% 4 12% 27% 14% 2	2909
December	1st to 10th	Night	42% 3 13% 31% 10% 1	4720
	11th to 20th	Night	47% 3 11% 23% 14% 2	5348
	21st to end	Night	50% 2 10% 22% 15% 2	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	27% 25% 29% 17% 1 1	220
	11th to 20th	Day	28% 31% 30% 9% 2	240
	21st to end	Day	22% 26% 33% 13% 4 1	363
February	1st to 10th	Day	22% 26% 35% 12% 4 1	330
	11th to 20th	Day	28% 20% 37% 13% 2	459
	21st to end	Day	29% 18% 31% 19% 2	446
March	1st to 10th	Day	26% 20% 29% 20% 4	540
	11th to 20th	Day	25% 19% 33% 17% 4 1	540
	21st to end	Day	27% 24% 29% 13% 5% 1	594
April	1st to 10th	Day	25% 23% 31% 14% 6% 1	540
	11th to 20th	Day	23% 23% 28% 16% 8% 3	532
	21st to end	Day	25% 16% 31% 19% 6% 3	540
May	1st to 10th	Day	30% 17% 28% 16% 6% 3	543
	11th to 20th	Day	21% 15% 29% 22% 11% 2	567
	21st to end	Day	23% 9% 24% 21% 18% 4	617
June	1st to 10th	Day	15% 15% 26% 23% 17% 4	570
	11th to 20th	Day	18% 12% 27% 24% 15% 4	570
	21st to end	Day	15% 10% 29% 27% 14% 4	570
July	1st to 10th	Day	10% 5% 22% 30% 26% 7%	565
	11th to 20th	Day	10% 5% 23% 30% 25% 8%	564
	21st to end	Day	14% 5% 24% 33% 16% 7%	620
August	1st to 10th	Day	13% 7% 20% 28% 23% 9%	566
	11th to 20th	Day	15% 5% 19% 31% 22% 9%	559
	21st to end	Day	13% 7% 24% 28% 18% 10%	627
September	1st to 10th	Day	17% 8% 24% 24% 19% 8%	570
	11th to 20th	Day	15% 4 21% 28% 19% 13%	542
	21st to end	Day	23% 10% 23% 24% 14% 6%	530
October	1st to 10th	Day	22% 8% 24% 22% 16% 8%	530
	11th to 20th	Day	19% 13% 32% 22% 13% 3	530
	21st to end	Day	22% 13% 33% 21% 10% 2	544
November	1st to 10th	Day	17% 14% 29% 26% 11% 3	425
	11th to 20th	Day	22% 20% 29% 23% 6% 1	430
	21st to end	Day	26% 21% 33% 13% 5% 1	248
December	1st to 10th	Day	26% 25% 31% 11% 6% 1	220
	11th to 20th	Day	31% 23% 27% 14% 5%	220
	21st to end	Day	26% 19% 30% 17% 6% 2	242
January	1st to 10th	Night	18% 18% 38% 22% 4	840
	11th to 20th	Night	24% 15% 34% 24% 3	840
	21st to end	Night	18% 15% 40% 25% 2	924
February	1st to 10th	Night	20% 14% 38% 26% 3	840
	11th to 20th	Night	23% 18% 39% 19% 1	823
	21st to end	Night	20% 17% 39% 21% 3	692
March	1st to 10th	Night	22% 13% 36% 25% 4	840
	11th to 20th	Night	22% 15% 35% 22% 6%	840
	21st to end	Night	22% 12% 38% 23% 5% 1	924
April	1st to 10th	Night	22% 17% 36% 21% 5%	840
	11th to 20th	Night	20% 11% 37% 26% 6%	831
	21st to end	Night	23% 9% 33% 30% 6%	840
May	1st to 10th	Night	25% 9% 31% 27% 8%	639
	11th to 20th	Night	20% 7% 32% 32% 10%	414
	21st to end	Night	19% 4 22% 40% 14% 1	455
June	1st to 10th	Night	17% 5% 26% 33% 19%	420
	11th to 20th	Night	13% 7% 24% 43% 13%	420
	21st to end	Night	13% 4 22% 47% 14%	420
July	1st to 10th	Night	13% 3 16% 43% 23% 1	410
	11th to 20th	Night	9% 4 20% 40% 25% 2	408
	21st to end	Night	12% 4 18% 38% 25% 2	498
August	1st to 10th	Night	13% 2 13% 44% 26% 2	627
	11th to 20th	Night	15% 4 14% 38% 27% 2	775
	21st to end	Night	15% 3 15% 42% 24% 2	924
September	1st to 10th	Night	18% 3 20% 37% 20% 2	840
	11th to 20th	Night	14% 3 16% 37% 25% 4	813
	21st to end	Night	21% 4 21% 33% 19% 2	800
October	1st to 10th	Night	22% 5% 23% 29% 18% 3	800
	11th to 20th	Night	18% 8% 29% 34% 11% 1	800
	21st to end	Night	23% 7% 32% 29% 9%	880
November	1st to 10th	Night	16% 6% 34% 31% 11% 1	820
	11th to 20th	Night	19% 14% 30% 28% 7% 1	840
	21st to end	Night	19% 13% 35% 27% 7%	840
December	1st to 10th	Night	23% 11% 36% 24% 5% 1	840
	11th to 20th	Night	23% 12% 35% 26% 5%	840
	21st to end	Night	22% 12% 36% 26% 4	924

Page generated at 2026-09-11 04:11:55. Version ac07

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