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 2025-04-11.

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% 8% 3 8% 5% 5% 5% 4 6% 7% 33%	113557
	Night	17% 8% 3 8% 5% 5% 4 4 5% 7% 34%	114697
Transparency	Day	46% 7% 13% 21% 11% 2	117191
	Night	45% 7% 13% 20% 12% 2	119226
Seeing	Day	22% 16% 27% 21% 11% 4	15239
	Night	21% 12% 27% 28% 11% 1	24739

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	12% 12% 5% 10% 7% 6% 5% 4 5% 6% 28%	3597
	11th to 20th	Day	12% 13% 7% 16% 7% 7% 6% 4 5% 6% 18%	3820
	21st to end	Day	17% 13% 5% 10% 6% 4 4 4 5% 5% 27%	4466
February	1st to 10th	Day	16% 13% 4 9% 7% 5% 5% 5% 4 5% 26%	1800
	11th to 20th	Day	14% 11% 4 10% 6% 5% 4 5% 6% 6% 30%	1774
	21st to end	Day	18% 10% 4 10% 7% 5% 4 4 4 6% 30%	1650
March	1st to 10th	Day	20% 8% 3 8% 5% 4 4 3 4 5% 36%	4747
	11th to 20th	Day	13% 8% 4 7% 5% 5% 4 4 5% 6% 38%	4980
	21st to end	Day	14% 9% 3 8% 5% 6% 4 4 5% 7% 36%	5602
April	1st to 10th	Day	17% 10% 3 8% 6% 5% 5% 3 4 7% 32%	2400
	11th to 20th	Day	17% 9% 3 6% 5% 4 4 5% 5% 6% 36%	2254
	21st to end	Day	18% 6% 3 7% 5% 6% 4 4 5% 6% 36%	2304
May	1st to 10th	Day	27% 7% 2 6% 4 4 4 3 5% 6% 31%	4190
	11th to 20th	Day	21% 10% 4 9% 5% 5% 5% 3 5% 6% 27%	4398
	21st to end	Day	14% 5% 2 5% 4 5% 5% 4 7% 9% 41%	4837
June	1st to 10th	Day	14% 6% 2 7% 6% 5% 6% 4 6% 8% 35%	2660
	11th to 20th	Day	14% 6% 2 7% 6% 5% 4 4 7% 9% 36%	2660
	21st to end	Day	12% 5% 2 6% 5% 5% 5% 5% 7% 10% 38%	2660
July	1st to 10th	Day	7% 6% 2 7% 6% 6% 6% 5% 7% 10% 37%	4440
	11th to 20th	Day	7% 5% 2 7% 5% 5% 6% 5% 8% 11% 37%	4440
	21st to end	Day	10% 7% 3 8% 7% 6% 6% 5% 8% 7% 34%	4597
August	1st to 10th	Day	19% 7% 2 8% 5% 4 5% 4 6% 8% 31%	4104
	11th to 20th	Day	15% 6% 2 6% 5% 5% 6% 5% 7% 9% 33%	3933
	21st to end	Day	15% 6% 2 5% 3 4 5% 5% 6% 9% 41%	4180
September	1st to 10th	Day	17% 5% 2 5% 4 4 4 4 5% 8% 42%	2280
	11th to 20th	Day	13% 6% 2 5% 4 3 4 4 5% 6% 48%	2280
	21st to end	Day	22% 6% 3 7% 3 4 4 3 5% 6% 37%	2003
October	1st to 10th	Day	18% 7% 2 6% 4 4 4 3 5% 7% 40%	1900
	11th to 20th	Day	18% 6% 2 7% 4 5% 5% 4 6% 7% 35%	1900
	21st to end	Day	19% 7% 2 7% 5% 5% 4 4 6% 6% 34%	2090
November	1st to 10th	Day	13% 7% 2 7% 5% 4 4 3 5% 6% 44%	1770
	11th to 20th	Day	16% 9% 2 7% 6% 4 4 4 6% 7% 35%	1695
	21st to end	Day	17% 9% 2 7% 6% 5% 4 4 6% 6% 33%	1600
December	1st to 10th	Day	17% 12% 6% 11% 7% 6% 6% 4 5% 6% 22%	2600
	11th to 20th	Day	23% 12% 3 7% 4 5% 4 4 4 5% 30%	2986
	21st to end	Day	20% 11% 5% 12% 6% 6% 5% 4 5% 6% 21%	3960
January	1st to 10th	Night	13% 9% 4 13% 8% 6% 5% 4 5% 6% 28%	6349
	11th to 20th	Night	12% 9% 6% 13% 7% 6% 6% 4 7% 7% 24%	6060
	21st to end	Night	18% 10% 4 10% 6% 5% 5% 4 5% 5% 28%	6490
February	1st to 10th	Night	16% 12% 3 8% 5% 4 5% 4 6% 6% 30%	2600
	11th to 20th	Night	16% 10% 3 8% 5% 5% 5% 4 5% 7% 30%	2510
	21st to end	Night	18% 10% 3 8% 6% 5% 4 4 5% 6% 31%	1980
March	1st to 10th	Night	19% 10% 3 9% 5% 4 4 4 4 5% 33%	5089
	11th to 20th	Night	12% 9% 3 7% 4 4 5% 4 5% 7% 40%	4979
	21st to end	Night	16% 9% 3 7% 5% 5% 5% 3 4 5% 38%	5330
April	1st to 10th	Night	19% 11% 3 8% 5% 5% 4 3 5% 6% 31%	2000
	11th to 20th	Night	18% 7% 2 6% 4 4 4 5% 5% 7% 37%	1882
	21st to end	Night	19% 6% 2 6% 5% 3 4 3 4 7% 41%	1891
May	1st to 10th	Night	30% 8% 3 6% 4 4 3 4 4 5% 30%	2750
	11th to 20th	Night	24% 8% 3 8% 5% 5% 4 4 5% 6% 29%	2495
	21st to end	Night	13% 6% 2 6% 5% 4 5% 5% 6% 9% 40%	2738
June	1st to 10th	Night	14% 6% 2 7% 4 4 3 4 6% 8% 42%	1520
	11th to 20th	Night	12% 6% 2 5% 5% 5% 4 4 7% 10% 40%	1520
	21st to end	Night	11% 5% 2 7% 4 4 5% 4 7% 10% 40%	1520
July	1st to 10th	Night	9% 6% 3 8% 4 5% 5% 4 6% 8% 41%	2520
	11th to 20th	Night	7% 5% 2 7% 5% 5% 5% 4 8% 11% 41%	2520
	21st to end	Night	13% 6% 3 6% 6% 4 5% 4 5% 6% 41%	2952
August	1st to 10th	Night	19% 6% 2 7% 5% 5% 4 4 6% 8% 34%	2825
	11th to 20th	Night	17% 8% 2 6% 4 4 3 3 5% 7% 41%	2849
	21st to end	Night	15% 5% 2 5% 5% 4 5% 4 6% 8% 42%	3476
September	1st to 10th	Night	16% 5% 3 5% 4 4 4 3 5% 6% 45%	1900
	11th to 20th	Night	12% 5% 2 5% 4 4 4 3 5% 7% 51%	1900
	21st to end	Night	20% 6% 3 5% 3 4 4 3 6% 7% 40%	2173
October	1st to 10th	Night	20% 7% 2 5% 4 4 3 3 5% 6% 42%	2280
	11th to 20th	Night	17% 6% 2 6% 4 4 4 3 5% 6% 42%	2280
	21st to end	Night	21% 8% 2 6% 4 3 4 3 5% 6% 38%	2508
November	1st to 10th	Night	11% 6% 3 6% 4 3 3 3 5% 6% 48%	2527
	11th to 20th	Night	20% 9% 3 5% 5% 4 4 4 6% 7% 35%	2700
	21st to end	Night	16% 10% 3 8% 5% 4 4 4 6% 7% 33%	2800
December	1st to 10th	Night	15% 12% 4 8% 5% 5% 5% 4 6% 8% 28%	4580
	11th to 20th	Night	23% 9% 3 7% 5% 5% 4 3 5% 5% 30%	5208
	21st to end	Night	21% 13% 4 9% 5% 5% 5% 3 5% 6% 24%	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	50% 7% 13% 19% 9% 2	3598
	11th to 20th	Day	63% 7% 8% 13% 7% 2	3853
	21st to end	Day	55% 6% 11% 18% 9% 1	4466
February	1st to 10th	Day	57% 7% 12% 15% 9%	1800
	11th to 20th	Day	52% 8% 10% 22% 8% 1	1783
	21st to end	Day	55% 5% 10% 20% 8% 2	1650
March	1st to 10th	Day	48% 6% 11% 20% 10% 5%	4750
	11th to 20th	Day	42% 8% 17% 21% 9% 4	4980
	21st to end	Day	46% 6% 11% 18% 17% 3	5602
April	1st to 10th	Day	51% 6% 10% 19% 11% 4	2400
	11th to 20th	Day	46% 8% 11% 18% 14% 2	2254
	21st to end	Day	46% 7% 10% 20% 14% 3	2304
May	1st to 10th	Day	52% 7% 10% 20% 10% 1	4202
	11th to 20th	Day	54% 10% 12% 16% 7% 1	4402
	21st to end	Day	35% 10% 15% 24% 15% 2	4837
June	1st to 10th	Day	43% 9% 13% 20% 13% 2	2660
	11th to 20th	Day	43% 9% 13% 21% 13% 2	2660
	21st to end	Day	37% 10% 15% 26% 12%	2660
July	1st to 10th	Day	39% 9% 15% 24% 11% 1	4370
	11th to 20th	Day	36% 8% 18% 23% 13% 1	4365
	21st to end	Day	44% 8% 13% 19% 14% 1	4450
August	1st to 10th	Day	49% 8% 14% 18% 10%	4091
	11th to 20th	Day	43% 8% 16% 25% 8%	3972
	21st to end	Day	37% 7% 14% 30% 10% 1	4180
September	1st to 10th	Day	41% 7% 12% 24% 15% 2	2280
	11th to 20th	Day	35% 6% 12% 24% 20% 2	2280
	21st to end	Day	47% 5% 13% 17% 16% 1	2003
October	1st to 10th	Day	40% 6% 11% 23% 17% 4	3160
	11th to 20th	Day	38% 5% 12% 25% 19% 2	3140
	21st to end	Day	33% 5% 18% 29% 14% 1	3428
November	1st to 10th	Day	40% 6% 13% 23% 16% 3	1770
	11th to 20th	Day	46% 7% 13% 21% 11% 2	1695
	21st to end	Day	48% 8% 14% 23% 7% 1	1600
December	1st to 10th	Day	58% 5% 13% 17% 5% 1	2600
	11th to 20th	Day	54% 6% 12% 19% 8%	2986
	21st to end	Day	60% 8% 13% 12% 6% 1	3960
January	1st to 10th	Night	48% 8% 16% 18% 8% 2	6349
	11th to 20th	Night	52% 8% 15% 13% 8% 3	6102
	21st to end	Night	54% 7% 10% 14% 13% 2	6490
February	1st to 10th	Night	51% 8% 13% 20% 8% 1	2600
	11th to 20th	Night	51% 7% 11% 19% 10% 1	2512
	21st to end	Night	51% 7% 11% 18% 11% 2	1980
March	1st to 10th	Night	50% 6% 12% 18% 11% 3	5099
	11th to 20th	Night	40% 7% 17% 23% 9% 4	4980
	21st to end	Night	47% 5% 10% 18% 18% 3	5330
April	1st to 10th	Night	52% 7% 9% 18% 13% 2	2000
	11th to 20th	Night	44% 8% 11% 20% 16% 3	1882
	21st to end	Night	42% 7% 12% 21% 16% 3	1891
May	1st to 10th	Night	55% 6% 9% 21% 7% 2	2757
	11th to 20th	Night	52% 10% 13% 14% 9% 1	2496
	21st to end	Night	37% 11% 17% 18% 13% 4	2739
June	1st to 10th	Night	41% 7% 11% 25% 15% 1	1520
	11th to 20th	Night	37% 9% 15% 23% 13% 2	1520
	21st to end	Night	35% 11% 13% 26% 14% 1	1520
July	1st to 10th	Night	39% 7% 14% 21% 17% 2	2480
	11th to 20th	Night	34% 9% 17% 24% 15% 1	2472
	21st to end	Night	41% 8% 13% 17% 20% 2	2858
August	1st to 10th	Night	46% 9% 15% 21% 9%	2814
	11th to 20th	Night	42% 6% 13% 26% 13%	2876
	21st to end	Night	37% 7% 13% 30% 12% 1	3476
September	1st to 10th	Night	39% 6% 12% 25% 16% 3	1900
	11th to 20th	Night	32% 6% 12% 26% 21% 2	1900
	21st to end	Night	42% 7% 11% 21% 18% 1	2173
October	1st to 10th	Night	43% 6% 13% 20% 19% 1	3800
	11th to 20th	Night	32% 5% 13% 25% 21% 3	3779
	21st to end	Night	33% 5% 16% 28% 17% 2	4120
November	1st to 10th	Night	37% 6% 14% 24% 17% 2	2527
	11th to 20th	Night	46% 6% 13% 21% 11% 3	2700
	21st to end	Night	47% 8% 14% 22% 8% 1	2800
December	1st to 10th	Night	49% 7% 16% 23% 5%	4580
	11th to 20th	Night	53% 6% 14% 18% 8%	5208
	21st to end	Night	56% 9% 12% 15% 8%	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	30% 25% 29% 13% 3 1	200
	11th to 20th	Day	32% 23% 32% 11% 1	255
	21st to end	Day	27% 24% 34% 12% 2 1	330
February	1st to 10th	Day	29% 29% 29% 11% 2	300
	11th to 20th	Day	32% 26% 34% 7% 1	321
	21st to end	Day	31% 21% 30% 16% 2	414
March	1st to 10th	Day	31% 21% 28% 17% 3	500
	11th to 20th	Day	27% 24% 27% 17% 3 1	500
	21st to end	Day	30% 23% 27% 14% 4 2	550
April	1st to 10th	Day	29% 26% 24% 14% 6% 1	500
	11th to 20th	Day	23% 27% 22% 16% 8% 4	452
	21st to end	Day	26% 19% 30% 14% 8% 2	460
May	1st to 10th	Day	30% 17% 27% 14% 9% 3	463
	11th to 20th	Day	21% 18% 27% 20% 9% 4	487
	21st to end	Day	19% 11% 26% 23% 16% 6%	529
June	1st to 10th	Day	18% 14% 26% 26% 13% 3	490
	11th to 20th	Day	16% 15% 25% 27% 14% 2	490
	21st to end	Day	15% 12% 29% 27% 12% 4	490
July	1st to 10th	Day	12% 7% 25% 31% 19% 5%	485
	11th to 20th	Day	8% 8% 25% 32% 21% 5%	484
	21st to end	Day	14% 8% 27% 30% 17% 4	532
August	1st to 10th	Day	14% 7% 26% 26% 20% 8%	486
	11th to 20th	Day	16% 6% 23% 27% 18% 9%	479
	21st to end	Day	15% 7% 23% 27% 16% 12%	539
September	1st to 10th	Day	19% 10% 21% 23% 18% 9%	490
	11th to 20th	Day	14% 7% 22% 24% 23% 10%	490
	21st to end	Day	27% 11% 21% 22% 15% 4	490
October	1st to 10th	Day	23% 12% 23% 21% 15% 6%	490
	11th to 20th	Day	22% 14% 27% 24% 10% 2	490
	21st to end	Day	24% 15% 29% 21% 10% 1	539
November	1st to 10th	Day	21% 15% 26% 24% 14% 1	399
	11th to 20th	Day	24% 19% 30% 19% 7% 1	295
	21st to end	Day	26% 23% 34% 15% 3 1	200
December	1st to 10th	Day	28% 27% 30% 11% 5% 1	200
	11th to 20th	Day	34% 21% 29% 13% 4 1	200
	21st to end	Day	30% 19% 32% 15% 4	220
January	1st to 10th	Night	21% 19% 38% 20% 3	800
	11th to 20th	Night	24% 16% 32% 25% 4	800
	21st to end	Night	20% 20% 35% 23% 2	880
February	1st to 10th	Night	21% 19% 33% 26% 1	800
	11th to 20th	Night	25% 21% 36% 17% 1	783
	21st to end	Night	26% 17% 35% 19% 2	660
March	1st to 10th	Night	25% 19% 30% 23% 3	800
	11th to 20th	Night	25% 18% 30% 22% 5%	800
	21st to end	Night	23% 18% 35% 19% 5% 1	880
April	1st to 10th	Night	23% 22% 29% 22% 4	800
	11th to 20th	Night	21% 15% 32% 24% 7% 1	754
	21st to end	Night	22% 13% 29% 30% 7%	751
May	1st to 10th	Night	25% 11% 22% 29% 12%	526
	11th to 20th	Night	18% 9% 31% 33% 9%	374
	21st to end	Night	18% 5% 21% 35% 20% 1	411
June	1st to 10th	Night	14% 6% 23% 40% 16% 1	380
	11th to 20th	Night	16% 4 21% 43% 16% 1	380
	21st to end	Night	12% 4 24% 45% 15%	380
July	1st to 10th	Night	12% 4 20% 40% 23% 2	370
	11th to 20th	Night	10% 2 15% 44% 26% 2	368
	21st to end	Night	14% 3 16% 41% 25% 1	541
August	1st to 10th	Night	14% 2 13% 43% 26% 2	567
	11th to 20th	Night	16% 2 14% 36% 29% 3	602
	21st to end	Night	15% 4 16% 31% 31% 2	836
September	1st to 10th	Night	18% 5% 19% 33% 24% 1	760
	11th to 20th	Night	14% 5% 17% 31% 28% 5%	760
	21st to end	Night	22% 7% 18% 32% 19% 2	760
October	1st to 10th	Night	23% 8% 21% 27% 19% 2	760
	11th to 20th	Night	20% 11% 24% 33% 12%	760
	21st to end	Night	26% 11% 26% 30% 7%	836
November	1st to 10th	Night	16% 11% 30% 33% 11%	780
	11th to 20th	Night	24% 16% 27% 26% 7% 1	800
	21st to end	Night	21% 16% 28% 30% 5% 1	800
December	1st to 10th	Night	26% 17% 30% 23% 4	800
	11th to 20th	Night	24% 15% 30% 26% 5%	800
	21st to end	Night	27% 13% 31% 26% 4	880

Page generated at 2025-04-08 09:17:28. Version ac07

Copyright © 2025 Attilla Danko.