New Boston
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the New Boston clear sky chart from 2005-11-06 to 2026-06-01.

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	13% 6% 2 5% 4 4 4 3 5% 7% 47%	132409
	Night	13% 6% 2 5% 3 3 3 3 5% 6% 51%	131029
Transparency	Day	37% 4 10% 23% 20% 6%	136018
	Night	34% 3 9% 21% 23% 9%	135497
Seeing	Day	14% 7% 21% 26% 23% 9%	17290
	Night	14% 6% 24% 34% 20% 2	27160

By Month

Percentage Cloud Cover SE (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	15% 8% 2 4 3 3 3 2 4 6% 48%	5289
	11th to 20th	Day	10% 5% 2 4 3 2 3 3 4 6% 59%	5310
	21st to end	Day	24% 7% 2 6% 3 2 3 2 4 5% 42%	5812
February	1st to 10th	Day	21% 8% 3 6% 4 4 3 2 3 4 41%	1890
	11th to 20th	Day	20% 8% 3 6% 4 3 3 3 4 5% 41%	1965
	21st to end	Day	22% 7% 2 6% 4 3 3 2 3 4 45%	1903
March	1st to 10th	Day	20% 9% 3 6% 3 3 2 2 4 6% 43%	5046
	11th to 20th	Day	13% 6% 2 5% 4 3 3 3 4 5% 52%	5090
	21st to end	Day	16% 8% 3 6% 4 4 4 3 4 5% 42%	5583
April	1st to 10th	Day	15% 6% 3 6% 4 4 4 4 5% 7% 43%	2382
	11th to 20th	Day	16% 6% 3 5% 4 4 4 3 5% 6% 45%	2494
	21st to end	Day	14% 7% 3 6% 4 4 5% 3 5% 6% 42%	2520
May	1st to 10th	Day	20% 8% 3 5% 4 4 4 3 5% 6% 37%	5778
	11th to 20th	Day	15% 7% 3 7% 4 5% 5% 3 5% 7% 39%	5992
	21st to end	Day	14% 7% 3 7% 5% 5% 5% 4 6% 8% 37%	6577
June	1st to 10th	Day	7% 5% 2 6% 5% 4 5% 5% 6% 10% 45%	2601
	11th to 20th	Day	6% 5% 2 4 6% 6% 6% 4 7% 9% 45%	2600
	21st to end	Day	5% 4 1 4 4 4 5% 6% 7% 10% 51%	2600
July	1st to 10th	Day	9% 6% 2 7% 6% 6% 6% 5% 8% 12% 34%	5900
	11th to 20th	Day	5% 3 1 3 3 3 4 4 6% 10% 58%	5900
	21st to end	Day	8% 7% 2 6% 5% 5% 4 5% 7% 9% 42%	6422
August	1st to 10th	Day	2 2 1 3 3 3 4 3 5% 8% 65%	4234
	11th to 20th	Day	6% 2 2 4 4 4 5% 4 7% 10% 52%	4040
	21st to end	Day	6% 4 2 6% 4 5% 5% 5% 8% 9% 46%	4312
September	1st to 10th	Day	10% 6% 2 5% 4 3 5% 4 5% 6% 51%	2370
	11th to 20th	Day	12% 4 2 5% 4 4 5% 4 7% 7% 47%	2200
	21st to end	Day	9% 3 1 4 4 4 4 3 5% 6% 57%	2195
October	1st to 10th	Day	6% 3 2 4 3 3 4 3 5% 6% 61%	2000
	11th to 20th	Day	14% 4 2 4 2 3 3 2 4 4 59%	2000
	21st to end	Day	15% 6% 2 5% 4 3 4 2 3 5% 53%	2200
November	1st to 10th	Day	14% 6% 2 4 4 3 3 3 4 6% 52%	2058
	11th to 20th	Day	10% 5% 2 6% 3 3 3 2 4 7% 55%	2100
	21st to end	Day	18% 6% 2 5% 4 3 3 2 3 5% 49%	1948
December	1st to 10th	Day	20% 6% 2 4 3 3 2 2 3 5% 51%	3030
	11th to 20th	Day	21% 9% 2 5% 3 3 3 2 4 5% 42%	3481
	21st to end	Day	19% 10% 3 6% 3 3 3 2 3 5% 43%	4587
January	1st to 10th	Night	15% 7% 2 5% 3 3 3 3 4 5% 50%	7634
	11th to 20th	Night	10% 3 1 3 2 2 2 1 3 4 68%	7680
	21st to end	Night	26% 7% 2 6% 3 3 2 2 3 4 42%	8402
February	1st to 10th	Night	18% 6% 2 5% 4 3 3 3 4 5% 47%	2730
	11th to 20th	Night	20% 7% 2 6% 4 3 3 3 3 5% 44%	2559
	21st to end	Night	19% 6% 2 5% 3 2 2 2 3 4 51%	1903
March	1st to 10th	Night	21% 8% 2 5% 3 3 3 2 4 5% 43%	5029
	11th to 20th	Night	13% 4 1 4 4 4 4 3 4 7% 51%	5090
	21st to end	Night	14% 7% 2 6% 3 4 4 3 5% 6% 47%	5588
April	1st to 10th	Night	13% 6% 2 5% 4 4 3 4 5% 6% 50%	2237
	11th to 20th	Night	12% 6% 1 5% 4 3 4 3 5% 7% 49%	2075
	21st to end	Night	12% 6% 2 5% 3 3 3 3 4 8% 51%	2100
May	1st to 10th	Night	19% 7% 2 7% 5% 4 4 3 5% 7% 38%	4331
	11th to 20th	Night	14% 8% 3 7% 4 4 4 3 5% 7% 41%	4143
	21st to end	Night	14% 5% 2 5% 4 3 4 4 6% 8% 43%	4542
June	1st to 10th	Night	5% 4 2 4 4 4 4 4 5% 9% 55%	1800
	11th to 20th	Night	5% 4 1 4 3 3 3 3 5% 9% 59%	1800
	21st to end	Night	4 3 1 3 3 4 3 4 7% 10% 60%	1800
July	1st to 10th	Night	6% 4 2 7% 5% 5% 5% 5% 8% 12% 42%	4078
	11th to 20th	Night	4 3 1 3 3 3 4 3 6% 11% 60%	4077
	21st to end	Night	5% 4 2 5% 4 4 5% 5% 7% 9% 50%	4449
August	1st to 10th	Night	2 2 1 2 2 2 3 3 5% 8% 72%	2926
	11th to 20th	Night	4 3 1 4 3 3 4 4 6% 13% 56%	3015
	21st to end	Night	4 3 2 5% 4 4 5% 5% 7% 9% 52%	3586
September	1st to 10th	Night	7% 4 2 4 3 4 4 3 5% 7% 57%	2025
	11th to 20th	Night	10% 4 2 5% 4 4 4 4 6% 8% 51%	2200
	21st to end	Night	6% 3 2 4 3 3 2 3 5% 6% 63%	2200
October	1st to 10th	Night	6% 3 2 4 2 3 3 3 4 7% 65%	2384
	11th to 20th	Night	10% 4 1 3 3 3 3 2 4 5% 62%	2400
	21st to end	Night	15% 5% 2 4 3 2 2 2 4 4 57%	2640
November	1st to 10th	Night	12% 5% 1 5% 3 2 2 3 3 5% 58%	2460
	11th to 20th	Night	10% 5% 2 4 3 3 3 2 4 5% 58%	2520
	21st to end	Night	14% 6% 2 5% 3 3 3 2 4 5% 52%	2667
December	1st to 10th	Night	16% 7% 2 5% 3 3 3 2 4 4 51%	4380
	11th to 20th	Night	17% 8% 2 6% 4 4 3 2 3 4 46%	4946
	21st to end	Night	17% 9% 3 6% 3 3 3 2 4 4 47%	6633

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% 1 9% 30% 18% 2	5290
	11th to 20th	Day	29% 2 5% 27% 31% 5%	5310
	21st to end	Day	48% 1 4 15% 21% 11%	5812
February	1st to 10th	Day	49% 3 6% 17% 19% 6%	1890
	11th to 20th	Day	47% 3 7% 20% 18% 6%	1974
	21st to end	Day	45% 3 5% 14% 23% 9%	1903
March	1st to 10th	Day	45% 3 6% 14% 21% 13%	5046
	11th to 20th	Day	35% 3 6% 17% 21% 19%	5090
	21st to end	Day	45% 2 9% 22% 18% 3	5585
April	1st to 10th	Day	40% 4 8% 22% 17% 7%	2382
	11th to 20th	Day	41% 4 8% 19% 20% 7%	2494
	21st to end	Day	42% 5% 10% 20% 20% 4	2520
May	1st to 10th	Day	47% 4 10% 25% 11% 2	5813
	11th to 20th	Day	44% 5% 10% 20% 19% 2	5996
	21st to end	Day	45% 6% 12% 21% 14% 3	6590
June	1st to 10th	Day	33% 8% 13% 28% 16% 2	2601
	11th to 20th	Day	34% 7% 15% 24% 19% 2	2600
	21st to end	Day	27% 7% 14% 30% 20% 2	2600
July	1st to 10th	Day	43% 5% 20% 24% 8%	5836
	11th to 20th	Day	23% 3 16% 38% 18% 1	5831
	21st to end	Day	38% 4 14% 32% 12% 1	6240
August	1st to 10th	Day	18% 5% 13% 31% 28% 6%	4220
	11th to 20th	Day	28% 5% 19% 36% 11% 2	4079
	21st to end	Day	33% 5% 16% 30% 14% 2	4311
September	1st to 10th	Day	34% 5% 10% 20% 22% 9%	2370
	11th to 20th	Day	35% 6% 11% 23% 19% 7%	2200
	21st to end	Day	28% 5% 9% 23% 25% 11%	2195
October	1st to 10th	Day	15% 2 7% 12% 28% 36%	3260
	11th to 20th	Day	21% 2 3 18% 34% 22%	3240
	21st to end	Day	31% 2 4 25% 31% 8%	3538
November	1st to 10th	Day	34% 4 8% 20% 26% 8%	2058
	11th to 20th	Day	32% 3 8% 20% 27% 10%	2100
	21st to end	Day	41% 2 5% 18% 26% 7%	1948
December	1st to 10th	Day	40% 2 6% 26% 23% 4	3030
	11th to 20th	Day	46% 2 4 19% 23% 6%	3479
	21st to end	Day	46% 2 5% 19% 23% 4	4587
January	1st to 10th	Night	39% 2 8% 24% 25% 3	7635
	11th to 20th	Night	23% 2 5% 20% 37% 13%	7680
	21st to end	Night	50% 1 4 14% 20% 11%	8402
February	1st to 10th	Night	41% 3 7% 20% 21% 9%	2730
	11th to 20th	Night	45% 3 6% 18% 20% 8%	2560
	21st to end	Night	39% 3 6% 15% 24% 13%	1903
March	1st to 10th	Night	45% 3 4 13% 20% 14%	5029
	11th to 20th	Night	34% 3 6% 16% 19% 21%	5090
	21st to end	Night	40% 3 9% 20% 23% 5%	5588
April	1st to 10th	Night	36% 4 10% 22% 19% 11%	2237
	11th to 20th	Night	35% 5% 9% 19% 21% 11%	2075
	21st to end	Night	35% 5% 10% 23% 23% 5%	2100
May	1st to 10th	Night	46% 5% 11% 22% 12% 4	4361
	11th to 20th	Night	44% 5% 11% 19% 19% 2	4144
	21st to end	Night	39% 5% 15% 21% 14% 6%	4551
June	1st to 10th	Night	25% 8% 13% 30% 20% 4	1800
	11th to 20th	Night	23% 6% 13% 30% 24% 5%	1800
	21st to end	Night	19% 7% 15% 29% 26% 4	1800
July	1st to 10th	Night	34% 5% 20% 31% 9% 1	4035
	11th to 20th	Night	21% 3 18% 37% 19% 2	4026
	21st to end	Night	31% 5% 15% 32% 15% 2	4320
August	1st to 10th	Night	12% 4 14% 27% 36% 6%	2916
	11th to 20th	Night	22% 3 19% 34% 19% 3	3042
	21st to end	Night	27% 6% 14% 25% 26% 2	3586
September	1st to 10th	Night	28% 5% 12% 20% 24% 10%	2025
	11th to 20th	Night	32% 6% 9% 24% 21% 8%	2200
	21st to end	Night	22% 4 9% 25% 27% 14%	2200
October	1st to 10th	Night	14% 2 6% 14% 23% 41%	3904
	11th to 20th	Night	17% 2 4 15% 34% 27%	3899
	21st to end	Night	26% 2 4 22% 30% 16%	4252
November	1st to 10th	Night	31% 4 8% 18% 27% 12%	2460
	11th to 20th	Night	29% 3 8% 18% 28% 13%	2520
	21st to end	Night	37% 3 6% 19% 26% 9%	2667
December	1st to 10th	Night	39% 3 6% 22% 25% 5%	4380
	11th to 20th	Night	45% 2 6% 15% 22% 10%	4947
	21st to end	Night	43% 2 7% 19% 25% 5%	6633

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	24% 12% 37% 22% 5%	330
	11th to 20th	Day	19% 18% 37% 19% 6%	330
	21st to end	Day	22% 18% 36% 19% 4	363
February	1st to 10th	Day	24% 15% 41% 15% 5% 1	330
	11th to 20th	Day	26% 18% 38% 13% 5%	330
	21st to end	Day	27% 20% 37% 12% 4	273
March	1st to 10th	Day	24% 13% 34% 22% 6% 1	330
	11th to 20th	Day	22% 20% 37% 16% 4 1	330
	21st to end	Day	17% 16% 40% 21% 5%	363
April	1st to 10th	Day	21% 10% 37% 23% 8% 1	402
	11th to 20th	Day	23% 14% 30% 23% 8% 1	532
	21st to end	Day	18% 11% 31% 26% 12% 2	540
May	1st to 10th	Day	17% 11% 31% 30% 11% 1	653
	11th to 20th	Day	15% 7% 24% 31% 19% 3	775
	21st to end	Day	11% 4 22% 36% 20% 6%	844
June	1st to 10th	Day	9% 3 15% 30% 34% 10%	731
	11th to 20th	Day	6% 2 12% 35% 34% 10%	730
	21st to end	Day	5% 1 12% 29% 38% 15%	730
July	1st to 10th	Day	7% 2 8% 29% 39% 15%	721
	11th to 20th	Day	4 2 4 28% 39% 22%	718
	21st to end	Day	7% 2 4 25% 41% 21%	794
August	1st to 10th	Day	4 1 6% 22% 47% 20%	726
	11th to 20th	Day	9% 1 4 25% 43% 17%	636
	21st to end	Day	9% 2 4 20% 41% 23%	583
September	1st to 10th	Day	9% 2 7% 23% 38% 20%	500
	11th to 20th	Day	6% 3 9% 23% 35% 23%	330
	21st to end	Day	12% 3 13% 27% 28% 17%	330
October	1st to 10th	Day	3 18% 43% 28% 8%	330
	11th to 20th	Day	11% 5% 27% 34% 18% 5%	330
	21st to end	Day	21% 10% 27% 25% 14% 3	363
November	1st to 10th	Day	18% 8% 28% 26% 18% 2	330
	11th to 20th	Day	13% 7% 34% 35% 9% 2	330
	21st to end	Day	21% 15% 29% 22% 11% 3	330
December	1st to 10th	Day	21% 10% 36% 22% 7% 3	330
	11th to 20th	Day	25% 11% 34% 20% 8% 2	330
	21st to end	Day	22% 16% 36% 19% 6% 1	363
January	1st to 10th	Night	21% 10% 31% 32% 6%	1050
	11th to 20th	Night	20% 10% 34% 28% 7%	1050
	21st to end	Night	21% 8% 36% 30% 5%	1155
February	1st to 10th	Night	21% 10% 35% 28% 6%	1050
	11th to 20th	Night	25% 11% 34% 25% 4	925
	21st to end	Night	23% 12% 36% 26% 3	519
March	1st to 10th	Night	23% 8% 35% 29% 5%	630
	11th to 20th	Night	19% 8% 34% 33% 7%	630
	21st to end	Night	17% 8% 36% 33% 6%	693
April	1st to 10th	Night	14% 10% 35% 34% 7%	630
	11th to 20th	Night	14% 9% 37% 29% 11%	623
	21st to end	Night	15% 7% 35% 32% 11% 1	630
May	1st to 10th	Night	13% 7% 29% 40% 11%	630
	11th to 20th	Night	13% 5% 27% 35% 18% 2	621
	21st to end	Night	10% 5% 17% 45% 21% 3	683
June	1st to 10th	Night	6% 2 12% 38% 40% 3	600
	11th to 20th	Night	6% 2 12% 44% 35% 3	600
	21st to end	Night	4 1 9% 37% 43% 7%	600
July	1st to 10th	Night	4 1 4 31% 52% 7%	585
	11th to 20th	Night	4 4 30% 53% 8%	582
	21st to end	Night	4 1 5% 27% 54% 10%	657
August	1st to 10th	Night	4 1 4 31% 53% 8%	597
	11th to 20th	Night	7% 1 4 34% 47% 7%	594
	21st to end	Night	6% 1 5% 31% 50% 7%	660
September	1st to 10th	Night	8% 1 5% 36% 44% 6%	600
	11th to 20th	Night	11% 1 7% 34% 42% 5%	600
	21st to end	Night	9% 2 9% 38% 38% 4	600
October	1st to 10th	Night	7% 2 10% 45% 32% 5%	784
	11th to 20th	Night	13% 4 20% 41% 21% 1	800
	21st to end	Night	16% 7% 25% 32% 18% 2	880
November	1st to 10th	Night	14% 4 24% 40% 17% 1	820
	11th to 20th	Night	12% 7% 29% 40% 12% 1	840
	21st to end	Night	18% 9% 30% 30% 12% 1	987
December	1st to 10th	Night	18% 9% 30% 36% 7%	1050
	11th to 20th	Night	22% 8% 28% 34% 7%	1050
	21st to end	Night	20% 10% 32% 31% 7%	1155

Page generated at 2026-05-29 00:53:24. Version ac07

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