Boston Mountain
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Boston Mountain 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	12% 7% 3 6% 4 4 4 4 5% 7% 44%	131608
	Night	12% 7% 2 6% 4 3 4 3 5% 6% 48%	131796
Transparency	Day	38% 6% 11% 23% 18% 5%	135217
	Night	35% 5% 11% 24% 20% 6%	136264
Seeing	Day	19% 14% 27% 24% 13% 3	17514
	Night	18% 14% 26% 27% 13% 1	27383

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	12% 10% 4 6% 4 4 3 3 4 6% 46%	5289
	11th to 20th	Day	6% 7% 2 6% 4 3 4 3 3 4 57%	5310
	21st to end	Day	17% 8% 3 5% 3 3 3 2 3 4 49%	5812
February	1st to 10th	Day	17% 11% 3 5% 5% 4 3 2 4 4 42%	1890
	11th to 20th	Day	15% 10% 3 7% 4 4 3 3 5% 6% 41%	1876
	21st to end	Day	16% 10% 3 6% 3 3 4 2 4 6% 41%	1866
March	1st to 10th	Day	16% 10% 3 6% 3 3 4 3 4 6% 41%	5046
	11th to 20th	Day	10% 6% 2 6% 3 3 4 3 6% 6% 51%	5090
	21st to end	Day	14% 9% 3 7% 5% 4 4 4 5% 7% 38%	5583
April	1st to 10th	Day	14% 9% 3 6% 5% 3 4 4 4 5% 42%	2456
	11th to 20th	Day	17% 8% 2 6% 5% 5% 5% 3 5% 5% 38%	2494
	21st to end	Day	20% 10% 3 7% 4 4 3 4 5% 5% 36%	2520
May	1st to 10th	Day	19% 8% 3 7% 5% 4 5% 4 5% 6% 34%	5842
	11th to 20th	Day	13% 7% 3 8% 5% 5% 5% 4 6% 7% 37%	5992
	21st to end	Day	20% 8% 3 7% 6% 5% 4 4 5% 6% 32%	6577
June	1st to 10th	Day	9% 7% 2 7% 5% 5% 5% 4 6% 8% 42%	2601
	11th to 20th	Day	8% 5% 2 7% 6% 6% 6% 5% 6% 9% 40%	2600
	21st to end	Day	5% 4 2 4 4 5% 5% 5% 7% 10% 49%	2600
July	1st to 10th	Day	8% 5% 2 6% 5% 5% 6% 5% 8% 11% 39%	5900
	11th to 20th	Day	5% 4 2 5% 4 4 4 4 6% 9% 51%	5900
	21st to end	Day	6% 5% 2 6% 6% 6% 7% 6% 8% 9% 40%	6422
August	1st to 10th	Day	4 3 1 5% 4 4 5% 5% 6% 10% 53%	4234
	11th to 20th	Day	12% 6% 2 5% 4 4 4 4 6% 9% 44%	4044
	21st to end	Day	5% 3 2 6% 5% 5% 5% 4 6% 8% 51%	4312
September	1st to 10th	Day	12% 5% 2 5% 4 4 4 4 5% 6% 50%	2400
	11th to 20th	Day	11% 5% 2 5% 5% 5% 5% 4 7% 10% 42%	2290
	21st to end	Day	8% 4 2 6% 5% 4 4 4 6% 6% 52%	2141
October	1st to 10th	Day	8% 5% 2 5% 4 3 4 3 4 5% 56%	2000
	11th to 20th	Day	12% 6% 2 5% 2 3 2 2 4 5% 57%	2000
	21st to end	Day	20% 8% 2 4 3 3 3 3 2 4 50%	2200
November	1st to 10th	Day	12% 7% 3 4 3 2 3 2 3 6% 56%	2058
	11th to 20th	Day	10% 8% 2 6% 4 3 3 3 4 6% 52%	2100
	21st to end	Day	16% 9% 3 6% 3 3 3 3 4 5% 45%	1722
December	1st to 10th	Day	13% 10% 2 4 3 3 3 3 3 4 52%	2690
	11th to 20th	Day	13% 10% 4 7% 4 3 3 2 3 5% 46%	3164
	21st to end	Day	17% 15% 6% 7% 3 3 4 3 4 4 34%	4587
January	1st to 10th	Night	12% 7% 4 8% 5% 4 3 3 4 6% 44%	7634
	11th to 20th	Night	7% 5% 2 5% 4 3 3 2 4 5% 60%	7680
	21st to end	Night	17% 10% 3 6% 3 3 3 3 4 5% 43%	8402
February	1st to 10th	Night	15% 10% 3 7% 4 4 4 3 4 5% 41%	2730
	11th to 20th	Night	14% 9% 4 7% 3 4 3 3 5% 6% 43%	2630
	21st to end	Night	13% 9% 3 6% 4 2 4 3 4 5% 46%	1955
March	1st to 10th	Night	18% 11% 3 6% 3 3 3 2 3 4 43%	5029
	11th to 20th	Night	10% 9% 2 5% 3 4 4 3 5% 6% 49%	5090
	21st to end	Night	13% 7% 2 7% 4 4 4 3 5% 8% 43%	5588
April	1st to 10th	Night	13% 8% 2 5% 4 4 4 3 5% 6% 46%	2163
	11th to 20th	Night	15% 8% 3 5% 4 3 4 2 4 7% 46%	2075
	21st to end	Night	18% 7% 2 6% 4 4 4 3 5% 6% 41%	2100
May	1st to 10th	Night	17% 8% 2 6% 3 3 4 3 5% 7% 41%	4281
	11th to 20th	Night	14% 7% 3 6% 5% 4 5% 4 6% 7% 38%	4143
	21st to end	Night	19% 8% 2 6% 5% 4 4 4 5% 7% 36%	4542
June	1st to 10th	Night	7% 4 1 5% 3 4 3 4 5% 10% 52%	1800
	11th to 20th	Night	5% 4 1 4 3 4 4 4 8% 11% 53%	1800
	21st to end	Night	4 3 1 3 4 3 4 3 7% 9% 60%	1800
July	1st to 10th	Night	7% 5% 2 6% 4 5% 5% 5% 7% 10% 44%	4078
	11th to 20th	Night	4 3 1 4 3 3 3 3 5% 9% 62%	4077
	21st to end	Night	6% 5% 2 6% 4 4 4 4 6% 8% 50%	4449
August	1st to 10th	Night	3 2 1 4 3 3 5% 4 6% 9% 59%	2926
	11th to 20th	Night	5% 4 2 5% 3 4 5% 4 6% 9% 53%	3010
	21st to end	Night	4 3 1 4 3 3 4 3 5% 9% 60%	3586
September	1st to 10th	Night	8% 4 2 4 3 4 3 4 6% 7% 56%	2000
	11th to 20th	Night	9% 5% 2 5% 4 4 4 4 5% 8% 49%	2106
	21st to end	Night	9% 5% 2 5% 4 4 4 3 5% 6% 54%	2237
October	1st to 10th	Night	6% 3 2 4 3 4 4 3 4 7% 61%	2400
	11th to 20th	Night	11% 5% 2 4 3 3 3 2 3 5% 59%	2400
	21st to end	Night	17% 8% 2 5% 3 2 3 2 3 4 51%	2640
November	1st to 10th	Night	13% 6% 2 4 3 2 3 3 3 5% 57%	2460
	11th to 20th	Night	11% 7% 1 4 3 3 3 3 5% 5% 55%	2520
	21st to end	Night	15% 8% 2 5% 3 3 4 2 4 5% 48%	2885
December	1st to 10th	Night	9% 11% 2 6% 3 3 3 3 3 4 52%	4720
	11th to 20th	Night	16% 10% 3 5% 4 3 3 2 3 4 47%	5227
	21st to end	Night	19% 13% 5% 7% 4 3 3 2 3 4 35%	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	41% 3 7% 25% 22% 3	5290
	11th to 20th	Day	31% 3 7% 24% 29% 6%	5310
	21st to end	Day	40% 2 5% 20% 23% 10%	5812
February	1st to 10th	Day	46% 4 9% 21% 16% 4	1890
	11th to 20th	Day	44% 4 10% 19% 19% 4	1885
	21st to end	Day	44% 4 7% 16% 20% 8%	1866
March	1st to 10th	Day	43% 6% 6% 17% 13% 15%	5046
	11th to 20th	Day	32% 5% 9% 19% 23% 12%	5090
	21st to end	Day	44% 5% 10% 26% 14% 1	5585
April	1st to 10th	Day	43% 6% 10% 20% 19% 3	2456
	11th to 20th	Day	46% 5% 10% 18% 16% 4	2494
	21st to end	Day	49% 6% 9% 21% 13% 2	2520
May	1st to 10th	Day	49% 8% 9% 19% 14% 1	5878
	11th to 20th	Day	44% 8% 12% 22% 13% 1	5996
	21st to end	Day	50% 8% 10% 16% 16% 1	6590
June	1st to 10th	Day	37% 9% 15% 25% 13% 1	2601
	11th to 20th	Day	37% 9% 14% 24% 14% 2	2600
	21st to end	Day	29% 8% 16% 27% 16% 4	2600
July	1st to 10th	Day	35% 8% 19% 25% 11% 2	5836
	11th to 20th	Day	28% 7% 16% 35% 13% 1	5831
	21st to end	Day	35% 10% 16% 28% 10% 1	6240
August	1st to 10th	Day	25% 7% 16% 29% 18% 5%	4220
	11th to 20th	Day	36% 7% 16% 28% 11% 1	4083
	21st to end	Day	29% 7% 15% 34% 13% 3	4311
September	1st to 10th	Day	34% 6% 12% 24% 16% 8%	2400
	11th to 20th	Day	34% 8% 12% 22% 20% 4	2290
	21st to end	Day	31% 6% 10% 22% 27% 4	2141
October	1st to 10th	Day	18% 3 6% 14% 36% 23%	3260
	11th to 20th	Day	20% 2 4 21% 35% 17%	3240
	21st to end	Day	32% 4 8% 29% 22% 5%	3538
November	1st to 10th	Day	32% 4 8% 25% 22% 9%	2058
	11th to 20th	Day	34% 4 8% 23% 22% 9%	2100
	21st to end	Day	41% 3 9% 19% 23% 5%	1722
December	1st to 10th	Day	37% 3 6% 27% 24% 3	2690
	11th to 20th	Day	42% 3 5% 23% 24% 3	3161
	21st to end	Day	51% 5% 7% 20% 14% 2	4587
January	1st to 10th	Night	40% 4 8% 27% 17% 3	7635
	11th to 20th	Night	26% 2 9% 19% 35% 9%	7680
	21st to end	Night	44% 3 7% 18% 21% 8%	8402
February	1st to 10th	Night	45% 4 9% 22% 16% 4	2730
	11th to 20th	Night	41% 4 9% 22% 19% 5%	2631
	21st to end	Night	39% 4 8% 18% 20% 10%	1955
March	1st to 10th	Night	45% 4 6% 15% 16% 13%	5029
	11th to 20th	Night	34% 6% 7% 22% 20% 11%	5090
	21st to end	Night	38% 6% 13% 27% 14% 2	5588
April	1st to 10th	Night	38% 5% 10% 22% 18% 6%	2163
	11th to 20th	Night	39% 5% 8% 23% 19% 6%	2075
	21st to end	Night	43% 6% 10% 23% 16% 2	2100
May	1st to 10th	Night	41% 7% 14% 23% 12% 2	4310
	11th to 20th	Night	42% 8% 15% 21% 11% 2	4144
	21st to end	Night	46% 8% 13% 17% 13% 3	4551
June	1st to 10th	Night	26% 8% 16% 31% 17% 3	1800
	11th to 20th	Night	22% 9% 15% 33% 19% 2	1800
	21st to end	Night	21% 6% 19% 27% 23% 5%	1800
July	1st to 10th	Night	33% 8% 20% 24% 12% 2	4035
	11th to 20th	Night	19% 6% 17% 41% 16% 1	4026
	21st to end	Night	31% 7% 16% 32% 14% 1	4320
August	1st to 10th	Night	18% 6% 19% 33% 22% 1	2916
	11th to 20th	Night	27% 7% 22% 28% 15% 2	3037
	21st to end	Night	21% 6% 15% 37% 18% 2	3586
September	1st to 10th	Night	27% 6% 12% 29% 18% 9%	2000
	11th to 20th	Night	32% 8% 13% 21% 22% 5%	2106
	21st to end	Night	29% 6% 10% 21% 28% 6%	2237
October	1st to 10th	Night	14% 4 8% 13% 33% 28%	3920
	11th to 20th	Night	20% 3 5% 17% 35% 20%	3899
	21st to end	Night	30% 3 7% 28% 24% 8%	4252
November	1st to 10th	Night	32% 4 8% 22% 25% 9%	2460
	11th to 20th	Night	30% 4 8% 25% 25% 9%	2520
	21st to end	Night	38% 3 8% 21% 24% 6%	2885
December	1st to 10th	Night	36% 3 8% 29% 20% 3	4720
	11th to 20th	Night	42% 3 5% 20% 21% 9%	5229
	21st to end	Night	53% 4 7% 20% 12% 4	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	29% 24% 32% 12% 2	330
	11th to 20th	Day	28% 29% 33% 8% 1	330
	21st to end	Day	28% 27% 32% 10% 2	363
February	1st to 10th	Day	36% 24% 28% 9% 2	330
	11th to 20th	Day	36% 28% 27% 7% 1	330
	21st to end	Day	41% 25% 26% 6% 3	273
March	1st to 10th	Day	34% 22% 30% 12% 2	330
	11th to 20th	Day	38% 21% 32% 9% 1	330
	21st to end	Day	31% 30% 28% 7% 2	363
April	1st to 10th	Day	29% 22% 31% 13% 5%	476
	11th to 20th	Day	32% 20% 32% 12% 4	532
	21st to end	Day	28% 22% 30% 14% 5%	540
May	1st to 10th	Day	24% 20% 36% 16% 4	679
	11th to 20th	Day	19% 16% 33% 22% 8% 2	775
	21st to end	Day	16% 14% 34% 23% 10% 3	844
June	1st to 10th	Day	11% 9% 26% 31% 20% 2	731
	11th to 20th	Day	8% 5% 28% 36% 20% 2	730
	21st to end	Day	5% 6% 32% 36% 19% 3	730
July	1st to 10th	Day	9% 5% 19% 35% 26% 7%	721
	11th to 20th	Day	6% 4 22% 37% 26% 6%	718
	21st to end	Day	6% 3 14% 37% 30% 9%	794
August	1st to 10th	Day	9% 3 20% 41% 23% 5%	726
	11th to 20th	Day	14% 2 15% 36% 26% 7%	640
	21st to end	Day	9% 3 16% 32% 32% 9%	583
September	1st to 10th	Day	11% 4 17% 34% 28% 6%	530
	11th to 20th	Day	8% 4 23% 34% 24% 6%	420
	21st to end	Day	11% 10% 26% 32% 18% 3	330
October	1st to 10th	Day	9% 10% 34% 31% 15% 2	330
	11th to 20th	Day	16% 16% 40% 21% 6% 1	330
	21st to end	Day	36% 12% 32% 17% 2 1	363
November	1st to 10th	Day	20% 17% 38% 19% 5% 1	330
	11th to 20th	Day	22% 24% 33% 15% 5% 1	330
	21st to end	Day	34% 18% 27% 16% 4 2	330
December	1st to 10th	Day	23% 28% 30% 15% 3 1	330
	11th to 20th	Day	25% 27% 33% 12% 4	330
	21st to end	Day	31% 22% 33% 12% 1	363
January	1st to 10th	Night	22% 21% 33% 20% 4	1050
	11th to 20th	Night	24% 22% 32% 19% 3	1050
	21st to end	Night	23% 22% 32% 22% 3	1155
February	1st to 10th	Night	27% 19% 32% 20% 3	1050
	11th to 20th	Night	27% 23% 34% 15% 2	996
	21st to end	Night	28% 23% 30% 19% 1	571
March	1st to 10th	Night	24% 20% 29% 23% 3	630
	11th to 20th	Night	25% 19% 27% 25% 4	630
	21st to end	Night	22% 22% 33% 18% 4	693
April	1st to 10th	Night	22% 21% 30% 23% 4	630
	11th to 20th	Night	22% 20% 29% 24% 5%	623
	21st to end	Night	21% 19% 34% 18% 7%	630
May	1st to 10th	Night	18% 11% 34% 29% 8%	630
	11th to 20th	Night	18% 13% 26% 28% 14% 1	621
	21st to end	Night	14% 10% 29% 28% 17% 3	683
June	1st to 10th	Night	9% 6% 17% 38% 28% 2	600
	11th to 20th	Night	7% 3 24% 41% 24% 2	600
	21st to end	Night	5% 3 21% 40% 28% 4	600
July	1st to 10th	Night	7% 2 14% 34% 37% 6%	585
	11th to 20th	Night	4 2 16% 36% 36% 6%	582
	21st to end	Night	6% 2 10% 35% 39% 8%	657
August	1st to 10th	Night	5% 2 13% 44% 32% 5%	597
	11th to 20th	Night	7% 1 14% 40% 33% 4	594
	21st to end	Night	7% 2 13% 37% 36% 5%	660
September	1st to 10th	Night	9% 4 15% 35% 35% 2	600
	11th to 20th	Night	11% 3 16% 38% 27% 6%	600
	21st to end	Night	11% 7% 21% 33% 25% 3	637
October	1st to 10th	Night	8% 6% 25% 37% 22% 3	800
	11th to 20th	Night	15% 13% 31% 31% 10%	800
	21st to end	Night	23% 17% 25% 26% 9% 1	880
November	1st to 10th	Night	17% 14% 32% 27% 10%	820
	11th to 20th	Night	20% 13% 30% 29% 7%	840
	21st to end	Night	26% 17% 27% 24% 7%	1034
December	1st to 10th	Night	21% 19% 33% 23% 4	1050
	11th to 20th	Night	24% 17% 30% 26% 3	1050
	21st to end	Night	25% 21% 28% 22% 3	1155

Page generated at 2026-05-29 00:53:30. Version ac07

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