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 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	17% 12% 4 10% 5% 5% 4 4 5% 6% 28%	149662
	Night	17% 13% 5% 11% 6% 5% 4 3 4 5% 26%	141434
Transparency	Day	55% 6% 11% 19% 9% 1	153155
	Night	59% 6% 10% 16% 9% 1	146030
Seeing	Day	28% 19% 28% 16% 7% 2	17072
	Night	26% 18% 27% 21% 7% 1	26153

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% 20% 8% 24% 10% 3 3 2 2 3 12%	4661
	11th to 20th	Day	11% 18% 11% 22% 9% 6% 4 3 3 3 9%	4650
	21st to end	Day	12% 17% 7% 13% 11% 8% 6% 3 4 5% 14%	5576
February	1st to 10th	Day	15% 19% 6% 17% 8% 5% 5% 3 5% 3 14%	1890
	11th to 20th	Day	13% 18% 8% 17% 9% 5% 4 3 4 4 16%	1924
	21st to end	Day	18% 18% 5% 16% 8% 4 3 3 3 3 18%	1730
March	1st to 10th	Day	21% 16% 6% 13% 5% 5% 3 3 3 3 20%	4576
	11th to 20th	Day	12% 19% 7% 9% 5% 4 3 2 4 5% 30%	5077
	21st to end	Day	19% 13% 5% 12% 5% 4 3 3 3 5% 28%	5928
April	1st to 10th	Day	19% 17% 4 9% 4 4 4 3 4 5% 28%	2520
	11th to 20th	Day	18% 14% 3 7% 5% 4 5% 4 5% 5% 31%	2494
	21st to end	Day	22% 14% 3 7% 4 4 3 2 5% 6% 31%	2527
May	1st to 10th	Day	29% 11% 3 7% 5% 4 4 3 5% 6% 25%	8153
	11th to 20th	Day	18% 10% 3 7% 5% 4 5% 4 5% 7% 31%	8221
	21st to end	Day	23% 9% 3 7% 4 4 5% 4 6% 8% 27%	9060
June	1st to 10th	Day	19% 8% 3 7% 5% 4 5% 4 5% 8% 31%	2940
	11th to 20th	Day	20% 6% 2 7% 5% 4 5% 4 7% 9% 31%	2940
	21st to end	Day	19% 7% 3 7% 5% 4 5% 4 7% 9% 31%	2940
July	1st to 10th	Day	14% 7% 3 6% 5% 5% 5% 5% 7% 10% 34%	8280
	11th to 20th	Day	10% 5% 2 7% 5% 5% 5% 5% 8% 10% 38%	8280
	21st to end	Day	15% 6% 3 7% 5% 5% 4 4 6% 7% 38%	8993
August	1st to 10th	Day	18% 8% 3 8% 6% 5% 5% 4 6% 7% 29%	6206
	11th to 20th	Day	16% 7% 3 7% 5% 5% 6% 4 7% 9% 32%	5695
	21st to end	Day	15% 7% 3 7% 5% 5% 5% 4 7% 8% 34%	6080
September	1st to 10th	Day	16% 6% 2 6% 4 3 5% 4 6% 8% 39%	2520
	11th to 20th	Day	16% 7% 3 6% 4 4 3 3 5% 6% 44%	2416
	21st to end	Day	18% 8% 2 6% 5% 5% 5% 5% 6% 7% 33%	2174
October	1st to 10th	Day	20% 9% 3 7% 4 4 5% 3 5% 6% 33%	2000
	11th to 20th	Day	21% 9% 3 7% 4 5% 4 3 5% 7% 32%	2000
	21st to end	Day	25% 14% 3 7% 5% 5% 3 3 6% 5% 24%	2070
November	1st to 10th	Day	16% 14% 3 8% 4 5% 4 4 5% 5% 31%	1851
	11th to 20th	Day	15% 21% 4 9% 5% 4 3 3 4 5% 27%	1816
	21st to end	Day	18% 19% 6% 11% 5% 4 4 4 5% 4 20%	1680
December	1st to 10th	Day	17% 22% 10% 18% 6% 5% 3 2 3 4 10%	2680
	11th to 20th	Day	18% 18% 9% 15% 5% 5% 3 3 3 4 17%	3066
	21st to end	Day	19% 18% 9% 12% 6% 4 4 3 4 6% 16%	4048
January	1st to 10th	Night	13% 19% 9% 27% 9% 4 4 2 3 3 8%	8234
	11th to 20th	Night	10% 20% 8% 22% 9% 5% 4 3 4 4 11%	8235
	21st to end	Night	11% 13% 5% 14% 12% 10% 6% 3 4 4 16%	8604
February	1st to 10th	Night	14% 17% 5% 17% 9% 4 4 3 3 4 18%	2730
	11th to 20th	Night	13% 15% 7% 17% 11% 4 5% 3 5% 5% 16%	2581
	21st to end	Night	15% 17% 6% 17% 8% 5% 4 3 3 5% 17%	2076
March	1st to 10th	Night	18% 11% 7% 14% 9% 5% 4 3 4 5% 20%	5484
	11th to 20th	Night	17% 17% 6% 9% 4 3 3 3 4 5% 29%	5100
	21st to end	Night	22% 13% 5% 12% 5% 4 3 3 4 4 25%	5245
April	1st to 10th	Night	20% 16% 4 8% 4 4 4 3 4 6% 27%	2100
	11th to 20th	Night	21% 13% 3 7% 5% 4 4 3 4 5% 32%	2075
	21st to end	Night	21% 11% 3 5% 4 3 3 3 5% 5% 35%	2091
May	1st to 10th	Night	29% 12% 3 7% 4 4 3 3 4 5% 24%	4787
	11th to 20th	Night	16% 11% 3 9% 5% 4 4 4 5% 6% 32%	4639
	21st to end	Night	22% 10% 3 7% 4 4 4 4 6% 6% 30%	5114
June	1st to 10th	Night	19% 6% 3 6% 5% 4 4 4 6% 6% 36%	1680
	11th to 20th	Night	20% 7% 2 5% 4 3 5% 4 6% 8% 36%	1680
	21st to end	Night	15% 5% 2 7% 4 5% 4 4 6% 8% 39%	1680
July	1st to 10th	Night	14% 7% 2 7% 5% 5% 5% 4 6% 9% 36%	4680
	11th to 20th	Night	14% 6% 2 5% 4 5% 4 4 5% 8% 43%	4680
	21st to end	Night	15% 7% 2 6% 4 4 3 3 6% 7% 43%	5084
August	1st to 10th	Night	20% 8% 3 7% 4 4 4 4 5% 7% 34%	3939
	11th to 20th	Night	15% 9% 3 8% 4 4 3 3 5% 6% 38%	4304
	21st to end	Night	14% 8% 2 8% 5% 4 4 4 6% 7% 38%	5045
September	1st to 10th	Night	14% 7% 2 6% 4 4 5% 4 4 7% 43%	2100
	11th to 20th	Night	16% 7% 2 4 4 3 3 3 4 6% 49%	2051
	21st to end	Night	18% 7% 3 6% 4 5% 4 4 6% 7% 38%	2223
October	1st to 10th	Night	21% 9% 2 7% 4 4 4 4 5% 6% 35%	2400
	11th to 20th	Night	22% 10% 3 7% 4 4 4 4 5% 6% 32%	2400
	21st to end	Night	26% 15% 3 6% 4 4 4 3 4 5% 25%	2770
November	1st to 10th	Night	15% 16% 4 7% 5% 4 4 3 5% 5% 32%	2667
	11th to 20th	Night	18% 16% 5% 8% 5% 4 4 3 4 6% 27%	2798
	21st to end	Night	16% 20% 6% 10% 5% 5% 4 3 5% 6% 21%	2940
December	1st to 10th	Night	16% 20% 10% 18% 6% 5% 4 3 4 4 11%	4720
	11th to 20th	Night	22% 18% 7% 14% 6% 3 3 2 4 3 19%	5348
	21st to end	Night	21% 19% 8% 11% 6% 5% 4 2 3 4 17%	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	78% 4 7% 8% 3	4662
	11th to 20th	Day	79% 4 6% 6% 4 1	4680
	21st to end	Day	72% 4 7% 12% 4	5576
February	1st to 10th	Day	73% 5% 7% 11% 4	1890
	11th to 20th	Day	72% 5% 7% 12% 4	1933
	21st to end	Day	71% 4 8% 11% 6%	1730
March	1st to 10th	Day	68% 5% 8% 11% 6% 1	4579
	11th to 20th	Day	56% 4 17% 12% 8% 2	5077
	21st to end	Day	59% 5% 7% 13% 15% 1	5928
April	1st to 10th	Day	59% 4 8% 18% 9% 2	2520
	11th to 20th	Day	53% 6% 10% 18% 11% 1	2494
	21st to end	Day	55% 5% 9% 18% 11% 2	2527
May	1st to 10th	Day	60% 6% 9% 15% 10% 1	8165
	11th to 20th	Day	51% 7% 10% 23% 8%	8225
	21st to end	Day	53% 7% 11% 20% 8%	9061
June	1st to 10th	Day	50% 7% 11% 19% 12% 1	2940
	11th to 20th	Day	48% 8% 13% 22% 9% 1	2940
	21st to end	Day	48% 8% 14% 20% 9% 1	2940
July	1st to 10th	Day	43% 8% 14% 24% 11%	8210
	11th to 20th	Day	37% 8% 15% 30% 10%	8205
	21st to end	Day	43% 6% 11% 28% 11%	8795
August	1st to 10th	Day	52% 8% 11% 16% 11% 1	6192
	11th to 20th	Day	46% 8% 20% 22% 5%	5734
	21st to end	Day	45% 8% 13% 28% 6%	6080
September	1st to 10th	Day	42% 7% 13% 28% 11% 1	2520
	11th to 20th	Day	40% 5% 12% 25% 18% 1	2416
	21st to end	Day	48% 8% 10% 22% 11% 1	2174
October	1st to 10th	Day	47% 6% 9% 27% 10% 1	3260
	11th to 20th	Day	46% 6% 13% 20% 15%	3240
	21st to end	Day	47% 7% 13% 23% 9% 1	3321
November	1st to 10th	Day	52% 7% 10% 19% 10% 1	1851
	11th to 20th	Day	60% 5% 11% 15% 8% 2	1816
	21st to end	Day	65% 7% 13% 11% 4	1680
December	1st to 10th	Day	80% 5% 6% 6% 2	2680
	11th to 20th	Day	71% 6% 8% 10% 4	3066
	21st to end	Day	70% 5% 8% 11% 6% 1	4048
January	1st to 10th	Night	82% 4 6% 5% 2	8234
	11th to 20th	Night	76% 5% 6% 7% 6%	8280
	21st to end	Night	71% 4 7% 10% 7% 1	8604
February	1st to 10th	Night	70% 5% 8% 11% 6%	2730
	11th to 20th	Night	71% 5% 9% 10% 5%	2583
	21st to end	Night	69% 5% 9% 10% 6% 1	2076
March	1st to 10th	Night	65% 6% 10% 12% 7%	5494
	11th to 20th	Night	57% 5% 16% 13% 5% 3	5101
	21st to end	Night	62% 4 5% 17% 10% 1	5245
April	1st to 10th	Night	59% 5% 10% 16% 8% 1	2100
	11th to 20th	Night	54% 6% 10% 18% 12% 1	2075
	21st to end	Night	51% 5% 10% 18% 13% 3	2091
May	1st to 10th	Night	61% 6% 8% 13% 11% 1	4793
	11th to 20th	Night	50% 8% 12% 21% 9% 1	4640
	21st to end	Night	53% 6% 11% 19% 10% 1	5115
June	1st to 10th	Night	46% 6% 11% 22% 14% 1	1680
	11th to 20th	Night	43% 9% 14% 22% 11% 1	1680
	21st to end	Night	41% 10% 12% 26% 11% 1	1680
July	1st to 10th	Night	44% 7% 14% 22% 12%	4640
	11th to 20th	Night	38% 8% 12% 27% 13% 1	4632
	21st to end	Night	40% 6% 13% 25% 16% 1	4967
August	1st to 10th	Night	49% 7% 12% 22% 10% 1	3929
	11th to 20th	Night	46% 6% 15% 27% 6%	4331
	21st to end	Night	43% 7% 13% 27% 9%	5045
September	1st to 10th	Night	41% 6% 13% 27% 12% 1	2100
	11th to 20th	Night	37% 5% 12% 28% 18% 1	2051
	21st to end	Night	45% 7% 11% 24% 12% 1	2223
October	1st to 10th	Night	53% 7% 10% 21% 10% 1	3920
	11th to 20th	Night	48% 6% 12% 17% 15% 3	3899
	21st to end	Night	50% 6% 12% 24% 8%	4469
November	1st to 10th	Night	53% 7% 10% 16% 12% 2	2667
	11th to 20th	Night	59% 5% 9% 17% 8% 2	2798
	21st to end	Night	64% 7% 12% 13% 4	2940
December	1st to 10th	Night	76% 6% 9% 8% 2	4720
	11th to 20th	Night	69% 5% 7% 11% 7%	5348
	21st to end	Night	72% 5% 7% 8% 7% 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	32% 31% 25% 8% 4	220
	11th to 20th	Day	43% 25% 25% 6% 1	220
	21st to end	Day	34% 29% 28% 8% 1	325
February	1st to 10th	Day	42% 28% 23% 6%	330
	11th to 20th	Day	41% 29% 26% 4	411
	21st to end	Day	39% 26% 27% 7% 1	446
March	1st to 10th	Day	34% 26% 28% 10% 1	540
	11th to 20th	Day	39% 22% 26% 11% 2	540
	21st to end	Day	39% 24% 23% 11% 3 1	594
April	1st to 10th	Day	40% 23% 24% 9% 4	540
	11th to 20th	Day	36% 24% 23% 10% 5% 2	532
	21st to end	Day	36% 19% 27% 11% 5% 1	540
May	1st to 10th	Day	36% 18% 26% 12% 6% 2	543
	11th to 20th	Day	26% 14% 35% 16% 7% 2	567
	21st to end	Day	22% 15% 31% 20% 9% 3	617
June	1st to 10th	Day	24% 14% 29% 24% 8% 1	570
	11th to 20th	Day	22% 15% 32% 21% 10% 1	570
	21st to end	Day	23% 16% 33% 20% 6% 1	570
July	1st to 10th	Day	14% 13% 32% 26% 11% 3	565
	11th to 20th	Day	14% 11% 32% 30% 12% 2	564
	21st to end	Day	17% 9% 36% 25% 10% 3	620
August	1st to 10th	Day	15% 13% 26% 27% 15% 4	566
	11th to 20th	Day	20% 8% 28% 28% 12% 4	559
	21st to end	Day	15% 16% 29% 24% 11% 5%	627
September	1st to 10th	Day	18% 14% 28% 21% 14% 5%	570
	11th to 20th	Day	15% 11% 25% 21% 17% 10%	542
	21st to end	Day	21% 16% 27% 21% 12% 4	530
October	1st to 10th	Day	25% 14% 26% 18% 12% 5%	530
	11th to 20th	Day	28% 21% 28% 15% 7% 2	530
	21st to end	Day	37% 21% 24% 15% 4	511
November	1st to 10th	Day	32% 21% 25% 15% 7%	425
	11th to 20th	Day	33% 28% 22% 13% 3 1	356
	21st to end	Day	40% 25% 30% 5%	220
December	1st to 10th	Day	37% 25% 28% 9% 1	220
	11th to 20th	Day	40% 24% 29% 5% 1	220
	21st to end	Day	38% 26% 28% 7% 1	242
January	1st to 10th	Night	30% 29% 29% 10% 1	840
	11th to 20th	Night	33% 25% 27% 13% 2 1	840
	21st to end	Night	26% 28% 29% 16% 2	924
February	1st to 10th	Night	30% 23% 30% 15% 1	840
	11th to 20th	Night	29% 27% 29% 13% 2	823
	21st to end	Night	32% 26% 28% 13% 1	692
March	1st to 10th	Night	29% 23% 31% 15% 2	840
	11th to 20th	Night	31% 24% 28% 16% 1	840
	21st to end	Night	32% 22% 24% 18% 4	924
April	1st to 10th	Night	29% 21% 31% 17% 2	840
	11th to 20th	Night	31% 19% 27% 16% 7%	831
	21st to end	Night	30% 14% 28% 23% 5%	831
May	1st to 10th	Night	31% 12% 25% 26% 6%	469
	11th to 20th	Night	19% 11% 36% 26% 7%	414
	21st to end	Night	18% 8% 35% 31% 8% 1	455
June	1st to 10th	Night	18% 10% 29% 30% 12% 1	420
	11th to 20th	Night	21% 11% 24% 34% 10%	420
	21st to end	Night	16% 8% 28% 37% 10%	420
July	1st to 10th	Night	12% 5% 27% 40% 15% 1	410
	11th to 20th	Night	12% 6% 29% 38% 13% 1	408
	21st to end	Night	15% 5% 24% 43% 12% 1	460
August	1st to 10th	Night	14% 5% 27% 36% 17% 1	535
	11th to 20th	Night	15% 5% 22% 35% 20% 3	716
	21st to end	Night	16% 8% 26% 31% 18% 1	924
September	1st to 10th	Night	17% 8% 27% 30% 16% 2	840
	11th to 20th	Night	19% 8% 19% 29% 22% 4	813
	21st to end	Night	21% 12% 24% 27% 15% 1	800
October	1st to 10th	Night	25% 13% 22% 22% 16% 2	800
	11th to 20th	Night	27% 16% 29% 21% 6% 1	800
	21st to end	Night	35% 17% 28% 18% 2	880
November	1st to 10th	Night	28% 20% 30% 17% 5%	820
	11th to 20th	Night	30% 22% 27% 13% 6% 1	840
	21st to end	Night	30% 24% 27% 15% 3	840
December	1st to 10th	Night	35% 25% 27% 11% 3	840
	11th to 20th	Night	33% 26% 26% 12% 3	840
	21st to end	Night	34% 28% 25% 11% 2	924

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

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