Williamsport
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Williamsport clear sky chart from 2005-11-06 to 2026-08-15.

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% 8% 3 8% 5% 5% 5% 4 6% 7% 33%	144168
	Night	16% 9% 3 8% 5% 5% 5% 4 5% 7% 34%	137679
Transparency	Day	46% 7% 13% 21% 12% 2	147758
	Night	46% 7% 13% 20% 12% 2	142176
Seeing	Day	23% 16% 27% 20% 11% 4	17032
	Night	21% 12% 27% 28% 11% 1	26328

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% 11% 4 10% 7% 7% 5% 4 5% 6% 27%	4661
	11th to 20th	Day	10% 12% 6% 16% 7% 7% 6% 4 5% 6% 20%	4984
	21st to end	Day	14% 11% 4 9% 6% 5% 5% 4 6% 6% 30%	5780
February	1st to 10th	Day	15% 13% 4 9% 7% 5% 6% 5% 4 5% 28%	1890
	11th to 20th	Day	14% 11% 4 11% 6% 5% 4 5% 6% 6% 29%	1865
	21st to end	Day	18% 11% 4 9% 6% 5% 4 3 4 6% 30%	1730
March	1st to 10th	Day	20% 8% 3 8% 5% 4 4 3 4 4 36%	4853
	11th to 20th	Day	13% 9% 3 8% 5% 5% 4 4 5% 6% 37%	5090
	21st to end	Day	14% 9% 3 8% 5% 6% 4 4 5% 7% 35%	5726
April	1st to 10th	Day	17% 10% 3 8% 6% 5% 4 3 4 7% 33%	2520
	11th to 20th	Day	17% 9% 3 6% 5% 4 4 5% 5% 7% 35%	2494
	21st to end	Day	18% 6% 3 7% 5% 5% 4 4 5% 7% 36%	2546
May	1st to 10th	Day	24% 6% 2 7% 5% 5% 5% 4 6% 7% 28%	7778
	11th to 20th	Day	19% 8% 3 8% 5% 5% 5% 4 6% 7% 31%	8221
	21st to end	Day	22% 6% 2 6% 5% 5% 5% 4 6% 7% 32%	9060
June	1st to 10th	Day	13% 6% 2 7% 5% 5% 5% 4 6% 8% 37%	2940
	11th to 20th	Day	15% 6% 2 7% 5% 5% 5% 4 7% 9% 36%	2940
	21st to end	Day	12% 5% 2 6% 5% 5% 5% 5% 7% 10% 38%	2940
July	1st to 10th	Day	10% 6% 3 8% 6% 6% 6% 4 6% 9% 37%	8280
	11th to 20th	Day	7% 6% 3 8% 5% 5% 6% 5% 8% 11% 36%	8280
	21st to end	Day	12% 6% 3 7% 5% 5% 5% 4 7% 9% 38%	8580
August	1st to 10th	Day	19% 7% 2 8% 5% 4 5% 4 6% 8% 32%	4364
	11th to 20th	Day	15% 6% 2 7% 5% 5% 6% 5% 7% 9% 32%	4112
	21st to end	Day	14% 6% 2 5% 3 4 5% 5% 6% 9% 41%	4312
September	1st to 10th	Day	16% 5% 2 5% 4 4 4 4 5% 8% 43%	2400
	11th to 20th	Day	12% 6% 2 5% 5% 3 4 4 5% 7% 49%	2400
	21st to end	Day	23% 6% 3 7% 4 4 4 3 5% 6% 36%	2108
October	1st to 10th	Day	17% 7% 2 6% 4 4 4 3 5% 7% 41%	2000
	11th to 20th	Day	18% 7% 3 7% 4 5% 5% 4 6% 7% 35%	2000
	21st to end	Day	19% 7% 2 7% 5% 5% 4 4 6% 6% 34%	2200
November	1st to 10th	Day	12% 7% 2 7% 5% 4 5% 3 6% 7% 43%	1860
	11th to 20th	Day	16% 10% 2 7% 6% 5% 4 4 6% 7% 34%	1780
	21st to end	Day	17% 9% 3 8% 6% 5% 4 4 6% 6% 33%	1680
December	1st to 10th	Day	16% 12% 5% 11% 7% 6% 6% 4 5% 6% 22%	2680
	11th to 20th	Day	22% 11% 3 7% 4 5% 4 4 4 5% 30%	3066
	21st to end	Day	20% 12% 5% 12% 6% 5% 5% 4 5% 6% 21%	4048
January	1st to 10th	Night	13% 10% 4 14% 7% 6% 5% 4 5% 5% 25%	8234
	11th to 20th	Night	11% 10% 6% 13% 7% 6% 6% 4 6% 7% 25%	7896
	21st to end	Night	15% 11% 3 9% 6% 5% 6% 3 5% 5% 32%	8401
February	1st to 10th	Night	15% 12% 3 8% 5% 5% 5% 4 6% 6% 31%	2730
	11th to 20th	Night	16% 11% 4 8% 5% 5% 5% 4 5% 7% 29%	2639
	21st to end	Night	17% 10% 3 8% 6% 5% 5% 4 5% 6% 31%	2076
March	1st to 10th	Night	19% 10% 3 9% 5% 4 4 3 4 5% 33%	5203
	11th to 20th	Night	13% 9% 3 7% 4 4 5% 4 5% 7% 40%	5089
	21st to end	Night	16% 9% 3 7% 5% 5% 5% 3 4 5% 38%	5448
April	1st to 10th	Night	20% 11% 3 8% 5% 5% 4 3 5% 6% 31%	2100
	11th to 20th	Night	17% 8% 3 6% 5% 4 4 5% 5% 8% 36%	2075
	21st to end	Night	19% 6% 2 6% 5% 3 4 3 4 7% 40%	2090
May	1st to 10th	Night	27% 8% 2 7% 4 4 4 4 5% 6% 30%	5106
	11th to 20th	Night	21% 8% 3 8% 5% 5% 5% 4 5% 6% 33%	4639
	21st to end	Night	22% 7% 2 6% 4 4 4 3 5% 7% 35%	5114
June	1st to 10th	Night	14% 6% 2 7% 4 4 3 4 6% 8% 43%	1680
	11th to 20th	Night	13% 6% 2 6% 5% 5% 4 4 7% 9% 39%	1680
	21st to end	Night	12% 5% 2 7% 5% 4 5% 4 7% 10% 40%	1680
July	1st to 10th	Night	13% 7% 3 8% 5% 5% 5% 4 6% 9% 37%	4680
	11th to 20th	Night	8% 6% 2 8% 5% 6% 6% 5% 8% 11% 36%	4680
	21st to end	Night	11% 6% 3 6% 6% 4 5% 4 5% 7% 42%	5493
August	1st to 10th	Night	18% 6% 2 7% 5% 5% 4 4 6% 8% 35%	3005
	11th to 20th	Night	16% 8% 2 6% 4 4 3 3 5% 7% 41%	2977
	21st to end	Night	15% 5% 2 5% 4 4 5% 4 6% 8% 43%	3586
September	1st to 10th	Night	15% 5% 3 5% 4 4 4 3 5% 6% 45%	2000
	11th to 20th	Night	12% 5% 2 5% 4 4 4 3 5% 7% 51%	2000
	21st to end	Night	20% 6% 3 5% 3 4 4 3 6% 7% 39%	2288
October	1st to 10th	Night	20% 6% 2 5% 4 3 3 3 5% 6% 43%	2400
	11th to 20th	Night	17% 7% 2 6% 4 4 4 3 5% 6% 42%	2400
	21st to end	Night	21% 8% 2 6% 4 3 4 3 5% 6% 38%	2640
November	1st to 10th	Night	11% 6% 3 6% 4 3 3 3 5% 7% 47%	2657
	11th to 20th	Night	19% 9% 3 5% 5% 4 4 4 6% 7% 34%	2835
	21st to end	Night	16% 10% 3 8% 5% 4 4 4 6% 7% 32%	2940
December	1st to 10th	Night	15% 12% 4 8% 5% 5% 5% 4 6% 8% 28%	4720
	11th to 20th	Night	23% 9% 3 7% 5% 5% 4 4 5% 5% 30%	5348
	21st to end	Night	21% 13% 4 9% 5% 5% 5% 3 5% 6% 24%	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	50% 8% 15% 17% 8% 1	4662
	11th to 20th	Day	60% 6% 9% 13% 9% 2	5017
	21st to end	Day	52% 6% 13% 19% 11%	5780
February	1st to 10th	Day	56% 7% 12% 16% 9% 1	1890
	11th to 20th	Day	53% 7% 10% 21% 8% 1	1874
	21st to end	Day	55% 5% 10% 20% 8% 2	1730
March	1st to 10th	Day	48% 6% 11% 20% 10% 4	4856
	11th to 20th	Day	43% 8% 17% 20% 8% 3	5090
	21st to end	Day	46% 6% 11% 18% 17% 3	5726
April	1st to 10th	Day	51% 6% 10% 18% 11% 4	2520
	11th to 20th	Day	46% 8% 13% 17% 14% 2	2494
	21st to end	Day	46% 7% 10% 21% 13% 3	2546
May	1st to 10th	Day	52% 6% 13% 18% 9% 1	7787
	11th to 20th	Day	48% 8% 13% 20% 10% 1	8225
	21st to end	Day	47% 7% 12% 21% 12% 1	9061
June	1st to 10th	Day	42% 9% 13% 21% 15% 2	2940
	11th to 20th	Day	43% 9% 13% 20% 13% 2	2940
	21st to end	Day	37% 10% 15% 25% 12% 1	2940
July	1st to 10th	Day	42% 8% 13% 23% 13% 1	8210
	11th to 20th	Day	38% 7% 18% 26% 11%	8205
	21st to end	Day	40% 7% 13% 22% 17% 1	8391
August	1st to 10th	Day	49% 8% 14% 18% 10%	4351
	11th to 20th	Day	43% 8% 16% 25% 7%	4151
	21st to end	Day	37% 7% 14% 30% 10% 1	4312
September	1st to 10th	Day	40% 7% 12% 25% 16% 2	2400
	11th to 20th	Day	35% 6% 12% 25% 20% 2	2400
	21st to end	Day	48% 5% 12% 16% 17% 1	2108
October	1st to 10th	Day	39% 6% 11% 23% 17% 4	3260
	11th to 20th	Day	38% 5% 12% 24% 19% 2	3240
	21st to end	Day	33% 5% 17% 29% 14% 1	3538
November	1st to 10th	Day	40% 6% 13% 23% 16% 2	1860
	11th to 20th	Day	47% 7% 13% 21% 11% 2	1780
	21st to end	Day	49% 8% 14% 22% 7% 1	1680
December	1st to 10th	Day	58% 5% 13% 17% 5% 1	2680
	11th to 20th	Day	54% 6% 13% 19% 8%	3066
	21st to end	Day	60% 8% 13% 12% 6% 1	4048
January	1st to 10th	Night	51% 8% 16% 17% 7% 1	8234
	11th to 20th	Night	52% 7% 13% 13% 11% 3	7938
	21st to end	Night	50% 6% 9% 18% 15% 1	8401
February	1st to 10th	Night	50% 8% 13% 20% 9% 1	2730
	11th to 20th	Night	52% 7% 11% 19% 10% 1	2641
	21st to end	Night	51% 6% 12% 18% 11% 2	2076
March	1st to 10th	Night	50% 6% 12% 18% 11% 3	5213
	11th to 20th	Night	40% 7% 17% 23% 9% 4	5090
	21st to end	Night	47% 5% 10% 18% 18% 3	5448
April	1st to 10th	Night	52% 7% 9% 17% 13% 2	2100
	11th to 20th	Night	44% 8% 11% 20% 15% 3	2075
	21st to end	Night	42% 7% 12% 22% 15% 3	2090
May	1st to 10th	Night	54% 5% 11% 19% 9% 2	5113
	11th to 20th	Night	48% 8% 14% 18% 11% 1	4640
	21st to end	Night	47% 7% 13% 20% 12% 2	5115
June	1st to 10th	Night	40% 7% 10% 26% 16% 1	1680
	11th to 20th	Night	39% 8% 15% 22% 14% 2	1680
	21st to end	Night	36% 10% 14% 25% 14% 1	1680
July	1st to 10th	Night	42% 7% 13% 21% 15% 1	4640
	11th to 20th	Night	39% 8% 16% 25% 11%	4632
	21st to end	Night	39% 7% 13% 21% 19% 1	5367
August	1st to 10th	Night	46% 8% 15% 21% 9% 1	2994
	11th to 20th	Night	42% 6% 14% 25% 13%	3004
	21st to end	Night	37% 7% 13% 31% 12% 1	3586
September	1st to 10th	Night	38% 6% 12% 26% 16% 3	2000
	11th to 20th	Night	32% 7% 13% 27% 20% 2	2000
	21st to end	Night	43% 6% 11% 20% 17% 1	2288
October	1st to 10th	Night	42% 6% 13% 20% 19% 1	3920
	11th to 20th	Night	33% 5% 13% 25% 21% 3	3899
	21st to end	Night	33% 5% 16% 27% 17% 2	4252
November	1st to 10th	Night	37% 6% 14% 24% 17% 2	2657
	11th to 20th	Night	47% 6% 13% 20% 11% 3	2835
	21st to end	Night	48% 8% 14% 21% 8% 1	2940
December	1st to 10th	Night	49% 7% 16% 23% 5%	4720
	11th to 20th	Night	53% 6% 14% 18% 8% 1	5348
	21st to end	Night	56% 9% 12% 15% 8% 1	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	29% 25% 29% 13% 3 1	220
	11th to 20th	Day	33% 24% 31% 11% 1	281
	21st to end	Day	29% 25% 32% 10% 2 1	363
February	1st to 10th	Day	29% 28% 30% 11% 2	330
	11th to 20th	Day	33% 25% 33% 9% 1	352
	21st to end	Day	31% 21% 31% 16% 2	446
March	1st to 10th	Day	32% 20% 27% 17% 3	540
	11th to 20th	Day	29% 23% 26% 17% 3 1	540
	21st to end	Day	30% 24% 27% 14% 4 2	594
April	1st to 10th	Day	30% 24% 24% 15% 5% 1	540
	11th to 20th	Day	25% 26% 24% 14% 7% 3	532
	21st to end	Day	26% 20% 30% 15% 7% 2	540
May	1st to 10th	Day	29% 18% 29% 13% 8% 3	543
	11th to 20th	Day	22% 18% 29% 19% 8% 4	567
	21st to end	Day	22% 12% 26% 21% 14% 5%	617
June	1st to 10th	Day	17% 15% 27% 25% 13% 3	570
	11th to 20th	Day	18% 16% 26% 25% 12% 2	570
	21st to end	Day	15% 12% 28% 28% 12% 4	570
July	1st to 10th	Day	12% 6% 27% 32% 18% 5%	565
	11th to 20th	Day	9% 8% 26% 31% 20% 6%	564
	21st to end	Day	14% 8% 27% 30% 17% 4	620
August	1st to 10th	Day	14% 6% 25% 26% 20% 8%	566
	11th to 20th	Day	16% 6% 24% 27% 17% 9%	535
	21st to end	Day	15% 8% 24% 26% 16% 12%	583
September	1st to 10th	Day	19% 10% 20% 24% 18% 9%	530
	11th to 20th	Day	13% 6% 22% 24% 23% 12%	530
	21st to end	Day	27% 10% 22% 22% 16% 4	530
October	1st to 10th	Day	22% 12% 22% 21% 16% 8%	530
	11th to 20th	Day	23% 15% 27% 24% 9% 2	530
	21st to end	Day	24% 14% 30% 21% 10% 1	583
November	1st to 10th	Day	22% 15% 27% 23% 13% 1	429
	11th to 20th	Day	26% 18% 29% 18% 7% 1	320
	21st to end	Day	26% 23% 34% 14% 2	220
December	1st to 10th	Day	28% 26% 30% 10% 5%	220
	11th to 20th	Day	34% 20% 30% 13% 3	220
	21st to end	Day	33% 18% 32% 13% 3	242
January	1st to 10th	Night	21% 19% 38% 20% 3	840
	11th to 20th	Night	25% 16% 32% 24% 3	840
	21st to end	Night	20% 20% 35% 23% 2	924
February	1st to 10th	Night	21% 19% 33% 26% 2	840
	11th to 20th	Night	25% 20% 36% 17% 1	823
	21st to end	Night	26% 17% 36% 18% 2	692
March	1st to 10th	Night	26% 19% 29% 23% 3	840
	11th to 20th	Night	26% 18% 29% 22% 5%	840
	21st to end	Night	23% 18% 36% 18% 5% 1	924
April	1st to 10th	Night	24% 21% 29% 22% 4	840
	11th to 20th	Night	21% 16% 33% 23% 7%	831
	21st to end	Night	21% 12% 29% 30% 6%	830
May	1st to 10th	Night	25% 12% 23% 28% 11%	581
	11th to 20th	Night	17% 9% 30% 35% 8%	414
	21st to end	Night	20% 5% 22% 35% 18% 1	455
June	1st to 10th	Night	14% 6% 23% 40% 16%	420
	11th to 20th	Night	17% 5% 22% 41% 14%	420
	21st to end	Night	12% 4 23% 45% 15%	420
July	1st to 10th	Night	12% 4 19% 42% 22% 1	410
	11th to 20th	Night	10% 3 15% 44% 25% 2	408
	21st to end	Night	13% 3 16% 41% 26% 2	599
August	1st to 10th	Night	14% 2 12% 42% 28% 2	627
	11th to 20th	Night	15% 2 15% 35% 29% 3	646
	21st to end	Night	15% 4 16% 32% 31% 2	880
September	1st to 10th	Night	17% 5% 19% 34% 24% 1	800
	11th to 20th	Night	14% 5% 16% 32% 29% 5%	800
	21st to end	Night	23% 7% 18% 32% 18% 2	800
October	1st to 10th	Night	22% 8% 21% 27% 21% 2	800
	11th to 20th	Night	20% 11% 25% 33% 11%	800
	21st to end	Night	25% 11% 27% 30% 6%	880
November	1st to 10th	Night	16% 11% 30% 32% 10%	820
	11th to 20th	Night	24% 16% 27% 25% 7% 1	840
	21st to end	Night	23% 16% 27% 29% 4	840
December	1st to 10th	Night	25% 17% 30% 23% 4	840
	11th to 20th	Night	25% 16% 29% 26% 4	840
	21st to end	Night	27% 13% 31% 25% 3	924

Page generated at 2026-08-12 03:37:17. Version ac07

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