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 2025-02-06.

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% 6% 44%	108284
	Night	12% 7% 3 6% 4 3 4 3 5% 6% 48%	110139
Transparency	Day	38% 6% 11% 23% 18% 5%	111932
	Night	35% 5% 11% 23% 20% 6%	114631
Seeing	Day	18% 13% 27% 24% 14% 3	15719
	Night	18% 14% 26% 28% 13% 2	25678

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% 11% 4 6% 4 3 3 3 3 5% 46%	4077
	11th to 20th	Day	7% 6% 3 7% 4 4 4 3 4 5% 53%	4080
	21st to end	Day	20% 7% 3 5% 3 3 3 3 3 4 47%	4488
February	1st to 10th	Day	16% 12% 4 5% 5% 4 3 2 4 4 42%	1755
	11th to 20th	Day	15% 10% 3 8% 4 4 3 3 5% 5% 41%	1692
	21st to end	Day	18% 11% 3 7% 3 3 4 2 4 5% 39%	1693
March	1st to 10th	Day	18% 10% 4 6% 3 3 3 3 4 5% 41%	3436
	11th to 20th	Day	13% 7% 2 6% 3 4 4 3 5% 6% 46%	3480
	21st to end	Day	17% 11% 3 7% 5% 4 4 3 4 5% 38%	3814
April	1st to 10th	Day	13% 9% 3 6% 5% 4 4 4 5% 6% 42%	2222
	11th to 20th	Day	17% 8% 2 6% 5% 4 5% 3 5% 6% 38%	2254
	21st to end	Day	20% 10% 3 8% 4 4 3 3 4 5% 37%	2280
May	1st to 10th	Day	19% 9% 3 7% 5% 4 5% 3 5% 5% 34%	3978
	11th to 20th	Day	13% 6% 3 7% 5% 5% 5% 4 6% 7% 38%	4082
	21st to end	Day	16% 8% 3 7% 6% 5% 4 4 6% 7% 34%	4477
June	1st to 10th	Day	9% 7% 2 7% 6% 5% 5% 4 6% 8% 42%	2470
	11th to 20th	Day	7% 5% 2 7% 6% 5% 6% 5% 6% 9% 41%	2470
	21st to end	Day	6% 5% 2 4 4 4 6% 5% 7% 10% 49%	2470
July	1st to 10th	Day	10% 6% 2 6% 4 5% 6% 4 7% 9% 42%	4120
	11th to 20th	Day	5% 4 2 5% 5% 5% 5% 4 6% 9% 51%	4120
	21st to end	Day	8% 6% 2 7% 6% 6% 7% 6% 8% 9% 33%	4465
August	1st to 10th	Day	4 3 1 5% 4 4 5% 5% 6% 10% 53%	4104
	11th to 20th	Day	12% 6% 2 5% 4 4 4 4 6% 8% 44%	3918
	21st to end	Day	5% 3 2 6% 4 5% 5% 4 6% 8% 52%	4180
September	1st to 10th	Day	12% 5% 2 5% 4 4 4 4 5% 6% 49%	2280
	11th to 20th	Day	11% 5% 2 5% 5% 5% 5% 4 7% 9% 43%	2176
	21st to end	Day	8% 4 2 6% 5% 4 4 4 6% 6% 52%	2034
October	1st to 10th	Day	8% 6% 2 6% 3 3 4 3 4 6% 56%	1900
	11th to 20th	Day	13% 6% 2 5% 2 3 3 2 4 5% 56%	1900
	21st to end	Day	19% 8% 2 4 3 3 3 3 2 4 50%	2090
November	1st to 10th	Day	12% 7% 3 5% 3 2 3 2 3 6% 54%	1958
	11th to 20th	Day	10% 8% 2 6% 4 3 3 3 4 6% 52%	2000
	21st to end	Day	16% 9% 3 6% 3 3 3 2 4 5% 46%	1641
December	1st to 10th	Day	12% 10% 2 4 3 3 3 3 3 4 53%	2610
	11th to 20th	Day	13% 10% 4 7% 4 3 3 2 4 5% 45%	3082
	21st to end	Day	17% 15% 6% 7% 3 3 4 3 4 4 33%	4488
January	1st to 10th	Night	13% 7% 4 8% 6% 4 3 3 4 6% 43%	5888
	11th to 20th	Night	8% 6% 3 6% 4 3 3 2 4 6% 55%	5900
	21st to end	Night	17% 10% 3 6% 3 3 3 2 3 5% 46%	6490
February	1st to 10th	Night	14% 10% 3 7% 5% 4 4 3 4 5% 40%	2536
	11th to 20th	Night	14% 9% 4 7% 3 3 3 3 4 6% 44%	2374
	21st to end	Night	14% 10% 3 6% 4 2 4 3 4 5% 44%	1775
March	1st to 10th	Night	19% 11% 3 6% 3 3 3 3 4 5% 40%	3419
	11th to 20th	Night	14% 8% 2 5% 4 4 4 3 5% 5% 47%	3480
	21st to end	Night	13% 8% 3 8% 5% 4 5% 3 5% 6% 40%	3817
April	1st to 10th	Night	12% 8% 2 5% 4 4 4 3 5% 6% 47%	1957
	11th to 20th	Night	13% 8% 3 6% 4 3 4 3 5% 7% 46%	1875
	21st to end	Night	18% 6% 2 6% 4 4 4 3 6% 6% 41%	1900
May	1st to 10th	Night	17% 9% 3 6% 3 2 3 3 4 7% 42%	2919
	11th to 20th	Night	13% 7% 3 6% 4 4 5% 4 6% 7% 40%	2823
	21st to end	Night	18% 8% 2 5% 4 4 4 3 6% 8% 39%	3090
June	1st to 10th	Night	6% 4 1 5% 3 4 3 4 6% 10% 53%	1710
	11th to 20th	Night	4 4 1 4 3 4 4 4 8% 11% 55%	1710
	21st to end	Night	5% 3 1 3 4 3 4 3 7% 9% 59%	1710
July	1st to 10th	Night	9% 4 2 6% 4 4 4 4 6% 8% 49%	2850
	11th to 20th	Night	3 3 1 4 3 3 3 3 6% 8% 63%	2850
	21st to end	Night	8% 7% 3 7% 5% 5% 5% 4 6% 7% 43%	3098
August	1st to 10th	Night	3 2 1 4 3 3 5% 4 6% 10% 59%	2836
	11th to 20th	Night	5% 4 2 5% 3 4 5% 4 6% 9% 53%	2916
	21st to end	Night	3 3 1 4 3 3 4 3 5% 9% 61%	3476
September	1st to 10th	Night	8% 4 2 4 3 3 3 3 5% 7% 57%	1900
	11th to 20th	Night	10% 5% 2 5% 4 4 4 4 5% 8% 50%	2000
	21st to end	Night	8% 5% 2 4 4 4 4 3 5% 6% 55%	2125
October	1st to 10th	Night	6% 3 2 4 3 4 4 3 4 8% 60%	2280
	11th to 20th	Night	12% 6% 2 4 3 3 3 2 3 5% 59%	2280
	21st to end	Night	17% 7% 2 5% 3 2 3 2 3 4 52%	2508
November	1st to 10th	Night	14% 6% 2 5% 3 2 3 3 3 5% 56%	2340
	11th to 20th	Night	11% 7% 2 4 3 3 3 3 5% 5% 55%	2400
	21st to end	Night	15% 8% 2 4 3 3 4 2 4 5% 49%	2748
December	1st to 10th	Night	9% 11% 2 5% 3 3 3 3 3 4 52%	4580
	11th to 20th	Night	16% 10% 3 6% 4 3 3 2 3 3 47%	5089
	21st to end	Night	19% 13% 5% 8% 4 3 3 2 3 4 35%	6490

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% 24% 22% 3	4078
	11th to 20th	Day	32% 4 9% 23% 26% 6%	4080
	21st to end	Day	42% 3 6% 18% 21% 11%	4488
February	1st to 10th	Day	46% 4 10% 21% 16% 4	1755
	11th to 20th	Day	44% 5% 9% 20% 19% 4	1701
	21st to end	Day	47% 4 8% 16% 16% 8%	1693
March	1st to 10th	Day	43% 8% 7% 16% 15% 11%	3436
	11th to 20th	Day	37% 7% 11% 21% 20% 4	3480
	21st to end	Day	47% 5% 10% 21% 16% 1	3814
April	1st to 10th	Day	42% 6% 9% 21% 19% 3	2222
	11th to 20th	Day	46% 5% 10% 19% 17% 4	2254
	21st to end	Day	49% 5% 8% 21% 14% 2	2280
May	1st to 10th	Day	49% 9% 9% 18% 13% 1	4014
	11th to 20th	Day	41% 10% 13% 24% 12% 1	4086
	21st to end	Day	46% 10% 12% 17% 13% 1	4489
June	1st to 10th	Day	38% 9% 15% 25% 12% 1	2470
	11th to 20th	Day	35% 9% 14% 25% 14% 2	2470
	21st to end	Day	29% 8% 16% 27% 17% 4	2470
July	1st to 10th	Day	36% 8% 20% 22% 13% 2	4056
	11th to 20th	Day	27% 7% 13% 36% 17% 1	4051
	21st to end	Day	41% 10% 17% 24% 7%	4325
August	1st to 10th	Day	25% 7% 16% 28% 19% 5%	4090
	11th to 20th	Day	36% 7% 16% 28% 12% 1	3957
	21st to end	Day	28% 7% 15% 34% 13% 3	4179
September	1st to 10th	Day	34% 6% 12% 23% 17% 9%	2280
	11th to 20th	Day	34% 7% 12% 21% 21% 4	2176
	21st to end	Day	31% 7% 10% 22% 27% 4	2034
October	1st to 10th	Day	18% 3 6% 14% 36% 23%	3160
	11th to 20th	Day	21% 2 4 20% 35% 18%	3140
	21st to end	Day	31% 4 8% 30% 22% 5%	3428
November	1st to 10th	Day	33% 4 8% 25% 21% 9%	1958
	11th to 20th	Day	34% 4 8% 22% 23% 10%	2000
	21st to end	Day	41% 3 8% 18% 24% 5%	1641
December	1st to 10th	Day	37% 3 6% 27% 23% 3	2610
	11th to 20th	Day	42% 3 5% 22% 24% 3	3079
	21st to end	Day	52% 5% 7% 19% 14% 3	4488
January	1st to 10th	Night	41% 5% 9% 22% 19% 4	5888
	11th to 20th	Night	30% 3 10% 20% 29% 7%	5900
	21st to end	Night	42% 3 7% 18% 23% 7%	6490
February	1st to 10th	Night	45% 4 9% 22% 16% 4	2536
	11th to 20th	Night	40% 4 9% 23% 18% 5%	2375
	21st to end	Night	41% 4 8% 19% 18% 9%	1775
March	1st to 10th	Night	45% 6% 8% 17% 15% 10%	3419
	11th to 20th	Night	37% 8% 9% 24% 18% 4	3480
	21st to end	Night	41% 7% 12% 25% 13% 2	3817
April	1st to 10th	Night	38% 5% 9% 22% 20% 6%	1957
	11th to 20th	Night	38% 5% 8% 23% 19% 7%	1875
	21st to end	Night	42% 7% 10% 23% 16% 2	1900
May	1st to 10th	Night	41% 8% 13% 20% 14% 3	2948
	11th to 20th	Night	38% 8% 17% 23% 12% 1	2824
	21st to end	Night	41% 9% 15% 21% 12% 2	3099
June	1st to 10th	Night	25% 8% 16% 31% 17% 3	1710
	11th to 20th	Night	20% 9% 15% 34% 19% 2	1710
	21st to end	Night	21% 6% 18% 27% 23% 5%	1710
July	1st to 10th	Night	31% 7% 22% 21% 15% 3	2805
	11th to 20th	Night	18% 5% 13% 41% 21% 1	2796
	21st to end	Night	38% 7% 16% 28% 10% 1	2999
August	1st to 10th	Night	18% 7% 19% 33% 22% 1	2826
	11th to 20th	Night	27% 7% 21% 28% 15% 2	2943
	21st to end	Night	20% 6% 15% 38% 19% 2	3476
September	1st to 10th	Night	26% 6% 12% 29% 18% 9%	1900
	11th to 20th	Night	32% 8% 12% 20% 23% 5%	2000
	21st to end	Night	28% 6% 10% 22% 28% 6%	2125
October	1st to 10th	Night	14% 4 8% 13% 33% 28%	3800
	11th to 20th	Night	20% 3 5% 16% 36% 21%	3779
	21st to end	Night	29% 3 7% 29% 24% 9%	4120
November	1st to 10th	Night	33% 4 8% 22% 24% 9%	2340
	11th to 20th	Night	30% 4 8% 23% 25% 9%	2400
	21st to end	Night	37% 3 8% 21% 24% 6%	2748
December	1st to 10th	Night	36% 3 8% 30% 20% 3	4580
	11th to 20th	Night	43% 3 5% 19% 21% 9%	5091
	21st to end	Night	53% 4 7% 20% 12% 4	6490

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% 24% 31% 13% 2	300
	11th to 20th	Day	28% 29% 32% 9% 1	300
	21st to end	Day	29% 26% 31% 11% 2	330
February	1st to 10th	Day	35% 24% 29% 9% 2	285
	11th to 20th	Day	34% 28% 27% 9% 1	270
	21st to end	Day	45% 25% 23% 4 2	225
March	1st to 10th	Day	30% 22% 32% 14% 2	270
	11th to 20th	Day	37% 19% 33% 9% 1	270
	21st to end	Day	34% 30% 26% 7% 3	297
April	1st to 10th	Day	28% 24% 31% 12% 5%	402
	11th to 20th	Day	30% 20% 32% 13% 4	452
	21st to end	Day	28% 22% 30% 13% 6%	460
May	1st to 10th	Day	23% 18% 37% 17% 4	586
	11th to 20th	Day	17% 15% 34% 24% 9% 2	675
	21st to end	Day	15% 14% 34% 24% 10% 3	734
June	1st to 10th	Day	11% 7% 25% 33% 21% 2	680
	11th to 20th	Day	7% 5% 29% 36% 20% 3	680
	21st to end	Day	5% 6% 32% 35% 19% 3	680
July	1st to 10th	Day	9% 5% 19% 35% 26% 7%	671
	11th to 20th	Day	6% 3 22% 36% 27% 6%	668
	21st to end	Day	7% 3 14% 36% 31% 9%	739
August	1st to 10th	Day	9% 3 19% 40% 25% 5%	676
	11th to 20th	Day	15% 2 15% 37% 25% 6%	594
	21st to end	Day	8% 2 14% 33% 33% 9%	539
September	1st to 10th	Day	11% 3 17% 33% 29% 7%	490
	11th to 20th	Day	9% 4 24% 35% 23% 6%	386
	21st to end	Day	10% 10% 28% 31% 19% 2	300
October	1st to 10th	Day	10% 10% 34% 29% 15% 2	300
	11th to 20th	Day	17% 16% 42% 21% 5% 1	300
	21st to end	Day	33% 13% 32% 18% 3 1	330
November	1st to 10th	Day	20% 17% 37% 20% 5% 1	300
	11th to 20th	Day	22% 22% 34% 16% 5% 1	300
	21st to end	Day	33% 17% 28% 16% 4 2	300
December	1st to 10th	Day	22% 28% 31% 16% 3 1	300
	11th to 20th	Day	25% 27% 31% 12% 4	300
	21st to end	Day	32% 23% 32% 12% 1	330
January	1st to 10th	Night	22% 22% 32% 20% 4	1000
	11th to 20th	Night	24% 22% 32% 19% 3	1000
	21st to end	Night	22% 22% 31% 22% 3	1100
February	1st to 10th	Night	27% 18% 32% 20% 3	976
	11th to 20th	Night	24% 24% 35% 15% 2	900
	21st to end	Night	29% 24% 29% 17% 1	519
March	1st to 10th	Night	23% 18% 30% 25% 4	570
	11th to 20th	Night	25% 17% 27% 26% 4	570
	21st to end	Night	23% 22% 33% 18% 4	627
April	1st to 10th	Night	20% 22% 29% 24% 4	570
	11th to 20th	Night	20% 20% 30% 24% 5% 1	563
	21st to end	Night	21% 19% 35% 18% 7% 1	570
May	1st to 10th	Night	18% 11% 34% 29% 8%	570
	11th to 20th	Night	17% 11% 27% 29% 15% 1	561
	21st to end	Night	13% 9% 29% 29% 17% 3	617
June	1st to 10th	Night	9% 5% 17% 38% 29% 2	570
	11th to 20th	Night	5% 3 25% 42% 24% 2	570
	21st to end	Night	5% 3 20% 39% 28% 4	570
July	1st to 10th	Night	8% 2 14% 34% 37% 6%	555
	11th to 20th	Night	3 2 16% 36% 36% 7%	552
	21st to end	Night	6% 1 10% 35% 40% 7%	624
August	1st to 10th	Night	5% 1 11% 44% 33% 5%	567
	11th to 20th	Night	7% 1 15% 41% 32% 4	564
	21st to end	Night	7% 2 13% 37% 37% 5%	627
September	1st to 10th	Night	9% 3 15% 35% 36% 2	570
	11th to 20th	Night	11% 3 16% 38% 25% 5%	570
	21st to end	Night	11% 7% 21% 34% 25% 3	605
October	1st to 10th	Night	8% 6% 24% 36% 22% 3	760
	11th to 20th	Night	16% 13% 31% 30% 9%	760
	21st to end	Night	22% 16% 25% 27% 9% 1	836
November	1st to 10th	Night	17% 13% 32% 26% 11% 1	780
	11th to 20th	Night	20% 14% 30% 29% 7%	800
	21st to end	Night	25% 16% 27% 24% 8%	985
December	1st to 10th	Night	22% 18% 33% 23% 4	1000
	11th to 20th	Night	24% 17% 29% 26% 4	1000
	21st to end	Night	25% 21% 28% 23% 3	1100

Page generated at 2025-02-03 02:05:52. Version ac07

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