Waterville Public Libarary
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Waterville Public Libarary 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	17% 12% 5% 10% 5% 4 4 4 5% 6% 27%	113898
	Night	17% 13% 5% 12% 6% 4 4 3 4 5% 26%	115051
Transparency	Day	55% 6% 11% 18% 9% 1	117437
	Night	59% 6% 10% 16% 8% 1	119676
Seeing	Day	28% 18% 28% 16% 7% 2	15291
	Night	26% 18% 27% 21% 7% 1	24575

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	10% 20% 9% 23% 9% 3 4 2 3 4 14%	3597
	11th to 20th	Day	13% 19% 11% 22% 8% 4 3 3 3 4 11%	3570
	21st to end	Day	14% 19% 7% 14% 8% 5% 5% 3 4 5% 16%	4304
February	1st to 10th	Day	16% 19% 6% 17% 7% 4 5% 3 5% 3 14%	1800
	11th to 20th	Day	12% 17% 8% 17% 9% 5% 4 3 4 4 17%	1830
	21st to end	Day	18% 17% 5% 16% 8% 4 4 3 3 3 18%	1650
March	1st to 10th	Day	21% 16% 6% 13% 5% 6% 3 3 3 3 20%	4476
	11th to 20th	Day	11% 19% 7% 9% 5% 3 3 2 4 6% 31%	4968
	21st to end	Day	19% 13% 5% 12% 5% 4 3 3 3 5% 28%	5800
April	1st to 10th	Day	19% 17% 4 9% 4 4 4 3 4 5% 28%	2400
	11th to 20th	Day	18% 14% 3 7% 4 4 4 4 5% 5% 32%	2374
	21st to end	Day	22% 14% 3 6% 4 3 3 2 5% 6% 31%	2407
May	1st to 10th	Day	25% 11% 3 7% 4 4 4 3 5% 6% 30%	4523
	11th to 20th	Day	18% 9% 3 8% 5% 5% 5% 4 6% 8% 29%	4398
	21st to end	Day	13% 7% 2 6% 4 4 5% 5% 6% 9% 37%	4837
June	1st to 10th	Day	19% 8% 3 7% 6% 5% 5% 5% 5% 8% 30%	2660
	11th to 20th	Day	20% 6% 2 7% 5% 4 5% 4 7% 9% 32%	2660
	21st to end	Day	19% 7% 3 7% 5% 4 5% 4 7% 9% 30%	2660
July	1st to 10th	Day	13% 6% 3 6% 5% 5% 6% 5% 7% 11% 33%	4440
	11th to 20th	Day	11% 6% 3 7% 6% 6% 6% 5% 9% 10% 32%	4440
	21st to end	Day	13% 6% 3 8% 5% 5% 5% 5% 7% 7% 35%	4814
August	1st to 10th	Day	19% 6% 3 7% 5% 5% 5% 5% 7% 7% 31%	4236
	11th to 20th	Day	16% 8% 3 8% 5% 5% 6% 5% 7% 8% 29%	3863
	21st to end	Day	16% 7% 3 7% 4 4 4 4 6% 8% 37%	4180
September	1st to 10th	Day	16% 6% 2 6% 4 4 6% 4 6% 7% 38%	2280
	11th to 20th	Day	17% 7% 3 6% 4 4 3 3 5% 6% 43%	2261
	21st to end	Day	17% 8% 2 7% 5% 5% 5% 5% 6% 6% 34%	2066
October	1st to 10th	Day	21% 10% 3 7% 4 4 5% 3 5% 6% 31%	1900
	11th to 20th	Day	20% 10% 3 7% 4 4 4 3 5% 7% 32%	1900
	21st to end	Day	26% 15% 3 7% 5% 5% 3 3 6% 5% 23%	1967
November	1st to 10th	Day	16% 14% 3 8% 4 5% 4 4 5% 6% 32%	1761
	11th to 20th	Day	16% 20% 4 8% 5% 4 3 4 4 5% 28%	1730
	21st to end	Day	18% 18% 5% 11% 5% 4 4 4 5% 5% 21%	1600
December	1st to 10th	Day	17% 22% 10% 19% 6% 5% 3 2 3 3 9%	2600
	11th to 20th	Day	18% 18% 10% 15% 5% 5% 3 3 3 4 17%	2986
	21st to end	Day	19% 18% 9% 11% 6% 4 4 3 4 6% 16%	3960
January	1st to 10th	Night	13% 17% 10% 28% 8% 4 4 2 3 3 9%	6349
	11th to 20th	Night	9% 18% 8% 24% 8% 6% 4 3 4 4 12%	6315
	21st to end	Night	14% 15% 6% 16% 10% 7% 4 3 4 4 16%	6651
February	1st to 10th	Night	15% 18% 6% 17% 9% 4 4 3 4 4 17%	2600
	11th to 20th	Night	13% 15% 7% 17% 11% 4 5% 3 4 5% 16%	2455
	21st to end	Night	16% 17% 6% 17% 8% 5% 4 3 4 5% 17%	1980
March	1st to 10th	Night	17% 11% 7% 15% 9% 5% 4 3 4 5% 20%	5364
	11th to 20th	Night	17% 17% 6% 9% 4 3 3 3 4 5% 29%	4989
	21st to end	Night	22% 13% 5% 12% 5% 4 3 3 3 4 25%	5131
April	1st to 10th	Night	20% 16% 4 8% 5% 4 4 4 4 6% 27%	2000
	11th to 20th	Night	20% 13% 3 7% 5% 4 4 3 4 5% 33%	1975
	21st to end	Night	21% 11% 4 5% 4 3 3 3 5% 5% 35%	1991
May	1st to 10th	Night	27% 10% 3 7% 4 4 3 3 4 6% 28%	2667
	11th to 20th	Night	15% 10% 3 9% 6% 5% 4 4 5% 6% 33%	2501
	21st to end	Night	13% 8% 3 7% 4 4 5% 5% 6% 7% 38%	2738
June	1st to 10th	Night	18% 7% 3 7% 6% 4 4 4 6% 7% 35%	1520
	11th to 20th	Night	19% 7% 1 5% 4 3 4 4 7% 8% 38%	1520
	21st to end	Night	15% 5% 2 6% 4 5% 3 4 6% 8% 39%	1520
July	1st to 10th	Night	11% 6% 2 8% 6% 6% 5% 4 6% 7% 38%	2520
	11th to 20th	Night	13% 7% 2 6% 4 5% 4 4 5% 7% 42%	2520
	21st to end	Night	14% 7% 3 7% 4 4 3 3 6% 7% 42%	2739
August	1st to 10th	Night	23% 7% 2 6% 4 3 4 3 5% 7% 37%	2689
	11th to 20th	Night	15% 8% 3 8% 5% 4 4 3 5% 6% 38%	2933
	21st to end	Night	14% 7% 2 7% 4 4 4 3 6% 7% 42%	3476
September	1st to 10th	Night	14% 7% 2 6% 4 4 5% 4 4 7% 43%	1900
	11th to 20th	Night	17% 7% 2 4 3 3 3 2 4 6% 48%	1919
	21st to end	Night	18% 7% 3 6% 4 4 4 4 6% 7% 38%	2112
October	1st to 10th	Night	22% 9% 2 8% 4 4 4 4 5% 6% 33%	2280
	11th to 20th	Night	22% 10% 3 7% 4 4 4 3 5% 6% 32%	2280
	21st to end	Night	27% 15% 3 6% 4 4 4 3 4 5% 24%	2631
November	1st to 10th	Night	15% 15% 4 7% 5% 4 4 4 5% 5% 32%	2537
	11th to 20th	Night	18% 15% 4 8% 5% 5% 4 3 4 6% 27%	2665
	21st to end	Night	16% 19% 6% 10% 6% 4 4 3 5% 6% 22%	2800
December	1st to 10th	Night	16% 21% 10% 18% 6% 5% 4 3 4 4 11%	4580
	11th to 20th	Night	22% 18% 7% 14% 5% 3 3 2 4 4 19%	5208
	21st to end	Night	21% 18% 9% 11% 6% 5% 4 2 3 4 17%	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	74% 5% 7% 10% 4	3598
	11th to 20th	Day	78% 4 6% 7% 3 1	3600
	21st to end	Day	71% 5% 7% 11% 5% 1	4304
February	1st to 10th	Day	73% 5% 7% 11% 3	1800
	11th to 20th	Day	72% 5% 7% 12% 4	1839
	21st to end	Day	70% 4 8% 11% 6%	1650
March	1st to 10th	Day	69% 5% 8% 11% 6% 1	4479
	11th to 20th	Day	55% 5% 18% 12% 8% 2	4968
	21st to end	Day	59% 5% 7% 13% 15% 1	5800
April	1st to 10th	Day	59% 4 7% 19% 8% 2	2400
	11th to 20th	Day	53% 6% 10% 18% 11% 1	2374
	21st to end	Day	55% 5% 9% 18% 11% 2	2407
May	1st to 10th	Day	54% 6% 10% 17% 11% 1	4537
	11th to 20th	Day	50% 9% 12% 21% 7% 1	4402
	21st to end	Day	40% 9% 13% 25% 12% 1	4837
June	1st to 10th	Day	51% 8% 11% 18% 11% 1	2660
	11th to 20th	Day	47% 8% 13% 22% 9% 1	2660
	21st to end	Day	49% 8% 14% 19% 9%	2660
July	1st to 10th	Day	42% 9% 14% 24% 11%	4370
	11th to 20th	Day	41% 10% 14% 27% 8%	4365
	21st to end	Day	45% 8% 14% 23% 10%	4661
August	1st to 10th	Day	49% 9% 12% 19% 10% 1	4222
	11th to 20th	Day	49% 9% 17% 20% 6%	3902
	21st to end	Day	44% 7% 12% 30% 7% 1	4180
September	1st to 10th	Day	43% 7% 12% 26% 11% 1	2280
	11th to 20th	Day	41% 5% 12% 24% 17% 1	2261
	21st to end	Day	48% 8% 10% 22% 11% 1	2066
October	1st to 10th	Day	48% 6% 9% 27% 9% 1	3160
	11th to 20th	Day	46% 6% 13% 20% 15%	3140
	21st to end	Day	47% 7% 14% 23% 9% 1	3218
November	1st to 10th	Day	51% 7% 11% 19% 11% 1	1761
	11th to 20th	Day	59% 5% 11% 15% 8% 2	1730
	21st to end	Day	64% 7% 14% 11% 4	1600
December	1st to 10th	Day	81% 5% 6% 6% 2	2600
	11th to 20th	Day	71% 6% 8% 10% 4	2986
	21st to end	Day	69% 5% 8% 11% 7% 1	3960
January	1st to 10th	Night	81% 5% 7% 5% 2	6349
	11th to 20th	Night	75% 6% 6% 7% 5%	6360
	21st to end	Night	71% 4 7% 8% 9% 1	6651
February	1st to 10th	Night	70% 5% 8% 11% 5%	2600
	11th to 20th	Night	71% 5% 9% 10% 5%	2457
	21st to end	Night	69% 6% 9% 10% 6% 1	1980
March	1st to 10th	Night	65% 6% 10% 11% 7%	5374
	11th to 20th	Night	57% 5% 17% 13% 6% 3	4990
	21st to end	Night	62% 4 5% 17% 10% 1	5131
April	1st to 10th	Night	59% 5% 10% 17% 7% 2	2000
	11th to 20th	Night	53% 6% 10% 18% 13% 1	1975
	21st to end	Night	51% 6% 10% 18% 13% 3	1991
May	1st to 10th	Night	56% 7% 10% 15% 10% 2	2673
	11th to 20th	Night	49% 10% 13% 19% 8% 1	2502
	21st to end	Night	42% 9% 12% 20% 16% 1	2739
June	1st to 10th	Night	46% 6% 12% 22% 13% 1	1520
	11th to 20th	Night	41% 9% 15% 23% 11% 1	1520
	21st to end	Night	40% 10% 12% 26% 11% 1	1520
July	1st to 10th	Night	43% 8% 13% 21% 14% 1	2480
	11th to 20th	Night	40% 8% 14% 25% 12% 1	2472
	21st to end	Night	41% 6% 11% 26% 15% 1	2651
August	1st to 10th	Night	47% 7% 13% 25% 8%	2679
	11th to 20th	Night	45% 7% 14% 26% 8%	2960
	21st to end	Night	40% 7% 12% 32% 9%	3476
September	1st to 10th	Night	41% 6% 14% 26% 12% 1	1900
	11th to 20th	Night	37% 5% 12% 27% 17% 1	1919
	21st to end	Night	45% 7% 11% 24% 11% 1	2112
October	1st to 10th	Night	54% 7% 10% 21% 9%	3800
	11th to 20th	Night	48% 6% 12% 17% 15% 3	3779
	21st to end	Night	51% 6% 12% 24% 7%	4330
November	1st to 10th	Night	52% 8% 10% 16% 12% 2	2537
	11th to 20th	Night	58% 5% 9% 17% 8% 2	2665
	21st to end	Night	63% 7% 12% 13% 4	2800
December	1st to 10th	Night	77% 6% 9% 7% 1	4580
	11th to 20th	Night	69% 5% 7% 11% 7%	5208
	21st to end	Night	72% 5% 7% 8% 7% 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	32% 30% 25% 9% 5%	200
	11th to 20th	Day	41% 25% 27% 7% 1 1	200
	21st to end	Day	32% 29% 30% 8% 1	295
February	1st to 10th	Day	45% 28% 21% 6%	300
	11th to 20th	Day	41% 30% 24% 5%	377
	21st to end	Day	40% 24% 28% 7% 1	414
March	1st to 10th	Day	33% 27% 28% 11% 2	500
	11th to 20th	Day	37% 23% 27% 11% 2	500
	21st to end	Day	38% 23% 23% 12% 3 1	550
April	1st to 10th	Day	38% 24% 24% 10% 4	500
	11th to 20th	Day	36% 23% 22% 11% 6% 2	492
	21st to end	Day	36% 19% 27% 11% 5% 2	500
May	1st to 10th	Day	36% 17% 26% 13% 6% 2	503
	11th to 20th	Day	26% 11% 36% 17% 8% 2	487
	21st to end	Day	19% 15% 31% 22% 11% 3	529
June	1st to 10th	Day	25% 14% 29% 23% 8% 1	490
	11th to 20th	Day	19% 14% 32% 22% 11% 1	490
	21st to end	Day	23% 17% 34% 19% 7% 1	490
July	1st to 10th	Day	14% 12% 32% 26% 12% 4	485
	11th to 20th	Day	14% 10% 31% 31% 12% 3	484
	21st to end	Day	16% 9% 38% 25% 11% 2	532
August	1st to 10th	Day	15% 14% 27% 25% 14% 5%	486
	11th to 20th	Day	21% 7% 28% 27% 12% 5%	479
	21st to end	Day	15% 15% 27% 25% 12% 5%	539
September	1st to 10th	Day	17% 13% 28% 21% 15% 5%	490
	11th to 20th	Day	16% 11% 25% 21% 16% 10%	490
	21st to end	Day	20% 17% 28% 20% 12% 4	490
October	1st to 10th	Day	27% 14% 27% 17% 11% 5%	490
	11th to 20th	Day	27% 19% 29% 15% 7% 2	490
	21st to end	Day	37% 20% 24% 15% 4	474
November	1st to 10th	Day	31% 21% 26% 15% 7%	395
	11th to 20th	Day	32% 28% 23% 12% 3	330
	21st to end	Day	39% 24% 32% 6%	200
December	1st to 10th	Day	37% 24% 29% 10% 1 1	200
	11th to 20th	Day	40% 24% 30% 6% 1	200
	21st to end	Day	35% 27% 29% 7% 1	220
January	1st to 10th	Night	30% 30% 29% 10% 1	800
	11th to 20th	Night	31% 26% 28% 13% 2 1	800
	21st to end	Night	25% 28% 30% 16% 2	880
February	1st to 10th	Night	31% 23% 30% 15% 1	800
	11th to 20th	Night	29% 28% 28% 13% 2	783
	21st to end	Night	32% 25% 28% 14% 1	660
March	1st to 10th	Night	29% 24% 31% 15% 2	800
	11th to 20th	Night	30% 24% 29% 16% 1	800
	21st to end	Night	31% 21% 25% 18% 4	880
April	1st to 10th	Night	29% 21% 31% 17% 2	800
	11th to 20th	Night	30% 20% 27% 16% 7%	791
	21st to end	Night	30% 14% 28% 22% 6%	791
May	1st to 10th	Night	31% 12% 25% 26% 6%	447
	11th to 20th	Night	19% 11% 36% 26% 7% 1	376
	21st to end	Night	16% 8% 35% 32% 9% 1	411
June	1st to 10th	Night	17% 10% 29% 31% 12% 1	380
	11th to 20th	Night	20% 9% 24% 35% 11%	380
	21st to end	Night	17% 8% 28% 38% 9% 1	380
July	1st to 10th	Night	11% 5% 26% 41% 15% 1	370
	11th to 20th	Night	11% 6% 29% 39% 14% 2	368
	21st to end	Night	15% 5% 24% 43% 12% 1	416
August	1st to 10th	Night	15% 5% 27% 35% 18% 1	483
	11th to 20th	Night	15% 5% 22% 33% 21% 3	647
	21st to end	Night	16% 7% 26% 32% 18% 1	836
September	1st to 10th	Night	17% 8% 28% 29% 16% 2	760
	11th to 20th	Night	19% 8% 19% 28% 21% 4	760
	21st to end	Night	21% 12% 24% 27% 15% 1	760
October	1st to 10th	Night	26% 13% 23% 21% 15% 2	760
	11th to 20th	Night	27% 16% 28% 21% 7% 1	760
	21st to end	Night	35% 17% 28% 17% 2	836
November	1st to 10th	Night	27% 20% 30% 18% 6%	780
	11th to 20th	Night	29% 22% 28% 13% 7% 1	800
	21st to end	Night	30% 25% 27% 15% 3	800
December	1st to 10th	Night	35% 25% 26% 11% 3	800
	11th to 20th	Night	33% 25% 26% 12% 3	800
	21st to end	Night	32% 29% 25% 12% 2	880

Page generated at 2025-05-08 09:33:58. Version ac07

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