Boston
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Boston clear sky chart from 2005-11-06 to 2025-11-03.

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	16% 7% 2 6% 4 4 4 4 5% 7% 39%	126423
	Night	17% 8% 3 6% 4 4 4 3 5% 6% 40%	123621
Transparency	Day	44% 4 11% 23% 15% 3	129956
	Night	44% 4 11% 22% 16% 2	128175
Seeing	Day	22% 16% 33% 20% 7% 2	16360
	Night	21% 14% 34% 24% 7% 1	25199

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	11% 7% 3 8% 4 4 4 4 6% 7% 44%	3597
	11th to 20th	Day	14% 7% 2 6% 3 3 4 3 4 5% 49%	3570
	21st to end	Day	17% 8% 2 5% 4 3 3 3 5% 6% 43%	3960
February	1st to 10th	Day	19% 7% 2 4 4 4 5% 4 5% 7% 37%	1872
	11th to 20th	Day	13% 8% 3 6% 4 4 5% 3 6% 7% 41%	1948
	21st to end	Day	16% 9% 2 6% 5% 3 4 3 6% 6% 39%	1650
March	1st to 10th	Day	20% 10% 2 6% 4 3 4 3 6% 6% 36%	4476
	11th to 20th	Day	14% 9% 3 6% 4 4 5% 4 5% 8% 38%	4795
	21st to end	Day	27% 7% 2 6% 4 4 4 4 6% 6% 30%	5730
April	1st to 10th	Day	19% 9% 3 6% 4 4 4 4 4 7% 36%	2400
	11th to 20th	Day	18% 7% 2 5% 5% 5% 4 3 5% 8% 38%	2402
	21st to end	Day	19% 6% 3 6% 4 4 4 3 6% 7% 38%	2600
May	1st to 10th	Day	28% 11% 3 7% 4 4 3 3 5% 6% 25%	5886
	11th to 20th	Day	19% 9% 3 8% 5% 4 5% 4 5% 7% 31%	5903
	21st to end	Day	18% 7% 3 6% 4 4 4 4 5% 8% 36%	6949
June	1st to 10th	Day	19% 6% 2 5% 4 4 5% 5% 7% 9% 34%	2800
	11th to 20th	Day	15% 6% 2 6% 5% 4 5% 4 6% 8% 38%	2800
	21st to end	Day	15% 7% 2 7% 5% 5% 5% 5% 7% 8% 36%	2800
July	1st to 10th	Day	15% 8% 3 6% 5% 5% 5% 5% 7% 10% 33%	6360
	11th to 20th	Day	8% 6% 3 8% 5% 5% 5% 5% 7% 11% 38%	6360
	21st to end	Day	14% 6% 2 6% 4 4 4 4 5% 8% 44%	6926
August	1st to 10th	Day	14% 6% 3 7% 5% 5% 5% 5% 7% 10% 32%	4245
	11th to 20th	Day	12% 8% 3 7% 4 4 5% 5% 6% 8% 39%	4151
	21st to end	Day	7% 4 2 5% 4 4 5% 5% 7% 10% 47%	4312
September	1st to 10th	Day	13% 6% 1 5% 4 4 4 4 5% 7% 47%	2382
	11th to 20th	Day	14% 6% 2 5% 3 3 2 3 4 6% 52%	2200
	21st to end	Day	18% 6% 2 5% 4 4 4 3 6% 7% 43%	2200
October	1st to 10th	Day	22% 5% 2 5% 3 4 3 2 4 6% 44%	2200
	11th to 20th	Day	15% 6% 2 5% 3 3 4 3 4 7% 47%	1960
	21st to end	Day	18% 7% 2 5% 3 4 4 3 5% 6% 41%	1980
November	1st to 10th	Day	15% 7% 2 4 4 3 3 3 3 6% 50%	1779
	11th to 20th	Day	16% 8% 2 4 4 3 3 3 5% 5% 48%	1800
	21st to end	Day	17% 8% 2 5% 4 4 3 4 5% 6% 44%	1800
December	1st to 10th	Day	15% 8% 2 5% 3 3 4 3 3 6% 49%	2684
	11th to 20th	Day	18% 8% 2 4 2 3 3 2 4 5% 48%	2986
	21st to end	Day	20% 10% 3 7% 4 4 4 2 4 4 38%	3960
January	1st to 10th	Night	14% 7% 2 4 3 3 3 3 4 6% 50%	6349
	11th to 20th	Night	13% 9% 3 8% 5% 4 4 3 5% 6% 40%	6315
	21st to end	Night	15% 10% 3 7% 5% 5% 4 3 4 6% 38%	6996
February	1st to 10th	Night	17% 9% 2 6% 4 4 4 3 5% 7% 38%	2524
	11th to 20th	Night	15% 9% 3 6% 4 4 4 4 5% 6% 39%	2338
	21st to end	Night	17% 8% 2 6% 4 4 4 3 4 6% 42%	1980
March	1st to 10th	Night	25% 9% 2 5% 3 3 3 3 4 5% 37%	5364
	11th to 20th	Night	15% 11% 3 7% 4 4 4 4 5% 5% 38%	5163
	21st to end	Night	25% 10% 2 6% 4 4 4 4 4 5% 32%	5197
April	1st to 10th	Night	19% 9% 3 6% 5% 4 4 4 5% 6% 36%	2000
	11th to 20th	Night	18% 9% 2 6% 4 4 3 3 6% 6% 39%	1945
	21st to end	Night	20% 6% 3 7% 4 4 4 3 6% 5% 39%	1800
May	1st to 10th	Night	29% 10% 3 7% 4 4 4 3 4 6% 26%	4054
	11th to 20th	Night	18% 8% 3 7% 5% 5% 5% 5% 7% 7% 28%	4033
	21st to end	Night	19% 9% 3 8% 5% 4 4 3 6% 7% 32%	3926
June	1st to 10th	Night	17% 8% 3 7% 4 4 5% 4 5% 6% 37%	1600
	11th to 20th	Night	17% 8% 2 7% 5% 4 4 4 5% 9% 36%	1600
	21st to end	Night	16% 6% 2 5% 4 3 6% 4 6% 9% 39%	1600
July	1st to 10th	Night	11% 8% 3 7% 5% 4 5% 4 7% 10% 37%	3600
	11th to 20th	Night	10% 7% 2 7% 5% 4 5% 4 7% 10% 38%	3600
	21st to end	Night	11% 7% 2 6% 5% 4 5% 3 6% 7% 44%	3926
August	1st to 10th	Night	18% 7% 2 7% 5% 5% 5% 4 7% 8% 32%	2870
	11th to 20th	Night	9% 7% 3 8% 5% 6% 6% 5% 8% 7% 37%	2893
	21st to end	Night	8% 4 1 4 3 3 4 3 7% 7% 54%	3586
September	1st to 10th	Night	12% 5% 2 6% 4 5% 4 3 6% 7% 46%	2016
	11th to 20th	Night	13% 5% 2 4 3 2 3 3 5% 6% 53%	2200
	21st to end	Night	17% 6% 2 7% 5% 3 3 4 6% 5% 43%	2200
October	1st to 10th	Night	19% 6% 2 6% 3 4 3 2 4 6% 44%	2200
	11th to 20th	Night	15% 5% 2 5% 3 4 4 3 5% 7% 47%	2437
	21st to end	Night	20% 8% 2 6% 4 3 3 3 5% 5% 41%	2860
November	1st to 10th	Night	13% 5% 2 5% 4 4 3 3 5% 5% 49%	2564
	11th to 20th	Night	16% 7% 3 4 4 3 4 3 4 5% 47%	2600
	21st to end	Night	16% 7% 2 5% 3 4 4 3 5% 6% 44%	2600
December	1st to 10th	Night	18% 8% 2 5% 4 3 4 3 4 6% 43%	4481
	11th to 20th	Night	20% 8% 2 6% 3 3 3 3 4 4 42%	5208
	21st to end	Night	17% 10% 3 6% 3 4 3 3 4 6% 40%	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	40% 3 6% 28% 20% 2	3598
	11th to 20th	Day	40% 3 10% 23% 20% 4	3600
	21st to end	Day	45% 3 10% 22% 19% 2	3960
February	1st to 10th	Day	49% 4 11% 19% 15% 2	1872
	11th to 20th	Day	43% 4 11% 24% 16% 3	1957
	21st to end	Day	47% 4 10% 21% 15% 3	1650
March	1st to 10th	Day	51% 3 10% 15% 13% 9%	4479
	11th to 20th	Day	45% 4 11% 27% 11% 2	4795
	21st to end	Day	55% 3 6% 16% 13% 7%	5730
April	1st to 10th	Day	48% 4 8% 19% 17% 4	2400
	11th to 20th	Day	44% 4 10% 23% 17% 3	2402
	21st to end	Day	45% 3 8% 22% 17% 4	2600
May	1st to 10th	Day	60% 4 13% 15% 7% 1	5897
	11th to 20th	Day	52% 4 13% 18% 11% 2	5907
	21st to end	Day	46% 5% 11% 23% 13% 1	6949
June	1st to 10th	Day	45% 6% 13% 23% 12% 1	2800
	11th to 20th	Day	42% 5% 14% 21% 15% 2	2800
	21st to end	Day	43% 6% 13% 23% 14% 1	2800
July	1st to 10th	Day	45% 8% 11% 21% 14% 1	6290
	11th to 20th	Day	39% 7% 16% 25% 12% 1	6285
	21st to end	Day	39% 5% 12% 27% 16% 2	6728
August	1st to 10th	Day	46% 7% 16% 22% 9% 1	4231
	11th to 20th	Day	42% 6% 13% 28% 10% 1	4190
	21st to end	Day	31% 5% 13% 31% 19% 1	4312
September	1st to 10th	Day	37% 5% 10% 25% 21% 2	2382
	11th to 20th	Day	34% 4 8% 29% 23% 3	2200
	21st to end	Day	43% 4 8% 22% 22% 2	2200
October	1st to 10th	Day	39% 5% 9% 26% 18% 4	3590
	11th to 20th	Day	33% 2 6% 30% 24% 4	3163
	21st to end	Day	33% 4 11% 30% 20% 3	3180
November	1st to 10th	Day	38% 3 12% 26% 18% 2	1779
	11th to 20th	Day	39% 3 8% 24% 23% 3	1800
	21st to end	Day	43% 5% 13% 25% 13% 2	1800
December	1st to 10th	Day	40% 3 14% 28% 14% 1	2684
	11th to 20th	Day	40% 3 14% 17% 20% 5%	2986
	21st to end	Day	51% 2 7% 22% 13% 4	3960
January	1st to 10th	Night	35% 2 9% 35% 17% 2	6349
	11th to 20th	Night	47% 3 11% 22% 13% 4	6360
	21st to end	Night	51% 2 8% 17% 17% 4	6996
February	1st to 10th	Night	47% 4 12% 21% 14% 2	2524
	11th to 20th	Night	46% 4 8% 21% 16% 4	2340
	21st to end	Night	45% 3 7% 24% 16% 4	1980
March	1st to 10th	Night	51% 2 8% 16% 16% 7%	5374
	11th to 20th	Night	49% 3 15% 19% 13% 2	5164
	21st to end	Night	56% 2 4 13% 19% 6%	5197
April	1st to 10th	Night	49% 4 8% 20% 16% 4	2000
	11th to 20th	Night	47% 4 8% 20% 18% 3	1945
	21st to end	Night	48% 3 9% 20% 17% 4	1800
May	1st to 10th	Night	60% 3 13% 14% 8% 2	4060
	11th to 20th	Night	54% 5% 15% 16% 9% 2	4034
	21st to end	Night	53% 4 12% 19% 10% 1	3927
June	1st to 10th	Night	47% 5% 13% 22% 12% 1	1600
	11th to 20th	Night	46% 6% 13% 22% 14% 1	1600
	21st to end	Night	41% 6% 14% 24% 14% 1	1600
July	1st to 10th	Night	42% 6% 12% 22% 17%	3560
	11th to 20th	Night	40% 6% 19% 23% 12% 1	3552
	21st to end	Night	40% 5% 11% 28% 14% 1	3809
August	1st to 10th	Night	50% 6% 13% 19% 10% 1	2860
	11th to 20th	Night	41% 8% 15% 25% 10%	2920
	21st to end	Night	27% 5% 15% 29% 24% 1	3586
September	1st to 10th	Night	37% 5% 11% 25% 20% 2	2016
	11th to 20th	Night	33% 4 10% 24% 26% 2	2200
	21st to end	Night	41% 5% 8% 24% 19% 2	2200
October	1st to 10th	Night	39% 4 12% 26% 16% 3	3590
	11th to 20th	Night	28% 3 10% 28% 29% 3	3973
	21st to end	Night	34% 3 11% 30% 20% 2	4610
November	1st to 10th	Night	38% 4 11% 25% 19% 3	2564
	11th to 20th	Night	41% 3 9% 22% 21% 4	2600
	21st to end	Night	41% 3 13% 27% 13% 2	2600
December	1st to 10th	Night	43% 3 11% 28% 14% 1	4481
	11th to 20th	Night	47% 3 14% 15% 20% 1	5208
	21st to end	Night	46% 4 7% 24% 17% 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	30% 25% 28% 14% 4	200
	11th to 20th	Day	22% 30% 38% 10% 2 1	200
	21st to end	Day	22% 26% 39% 10% 3	220
February	1st to 10th	Day	28% 23% 35% 12% 2	420
	11th to 20th	Day	27% 28% 34% 9% 1	496
	21st to end	Day	25% 23% 37% 12% 3	414
March	1st to 10th	Day	27% 25% 34% 11% 3	500
	11th to 20th	Day	26% 25% 33% 12% 3 1	500
	21st to end	Day	29% 23% 33% 10% 5% 1	550
April	1st to 10th	Day	30% 22% 30% 12% 6%	500
	11th to 20th	Day	27% 24% 32% 14% 3	492
	21st to end	Day	26% 17% 37% 17% 3 1	500
May	1st to 10th	Day	30% 23% 31% 11% 3 1	503
	11th to 20th	Day	21% 16% 40% 17% 5% 1	527
	21st to end	Day	19% 13% 35% 28% 5% 1	573
June	1st to 10th	Day	21% 12% 35% 24% 7% 1	530
	11th to 20th	Day	16% 15% 39% 22% 7% 1	530
	21st to end	Day	15% 10% 41% 23% 10% 1	530
July	1st to 10th	Day	17% 9% 32% 28% 13% 2	525
	11th to 20th	Day	14% 6% 31% 34% 12% 3	524
	21st to end	Day	11% 7% 37% 33% 11% 1	576
August	1st to 10th	Day	10% 10% 31% 36% 11% 2	526
	11th to 20th	Day	16% 7% 29% 31% 14% 3	519
	21st to end	Day	14% 8% 35% 26% 14% 4	583
September	1st to 10th	Day	17% 10% 29% 26% 14% 3	530
	11th to 20th	Day	19% 9% 27% 23% 15% 7%	530
	21st to end	Day	22% 10% 33% 22% 9% 5%	530
October	1st to 10th	Day	24% 12% 27% 22% 10% 5%	530
	11th to 20th	Day	20% 15% 33% 21% 9% 2	461
	21st to end	Day	24% 17% 33% 17% 8% 1	462
November	1st to 10th	Day	20% 16% 34% 18% 10% 1	401
	11th to 20th	Day	22% 23% 35% 15% 5%	400
	21st to end	Day	25% 22% 30% 16% 6% 1	400
December	1st to 10th	Day	25% 22% 33% 16% 3	258
	11th to 20th	Day	31% 28% 32% 8% 2	200
	21st to end	Day	24% 20% 35% 17% 3 3	220
January	1st to 10th	Night	22% 21% 39% 16% 3	800
	11th to 20th	Night	26% 20% 36% 17% 2	800
	21st to end	Night	19% 20% 37% 19% 4	880
February	1st to 10th	Night	23% 20% 36% 19% 1	800
	11th to 20th	Night	25% 20% 40% 14% 1	783
	21st to end	Night	24% 21% 35% 17% 3	660
March	1st to 10th	Night	25% 20% 35% 17% 4	800
	11th to 20th	Night	23% 19% 37% 17% 4 1	800
	21st to end	Night	26% 17% 34% 18% 5%	880
April	1st to 10th	Night	24% 19% 34% 20% 3	800
	11th to 20th	Night	24% 17% 37% 17% 5%	761
	21st to end	Night	24% 17% 35% 22% 2	600
May	1st to 10th	Night	28% 13% 35% 20% 5%	600
	11th to 20th	Night	21% 13% 40% 21% 5%	592
	21st to end	Night	19% 9% 38% 28% 7%	433
June	1st to 10th	Night	20% 7% 33% 33% 6% 1	400
	11th to 20th	Night	17% 8% 37% 33% 6% 1	400
	21st to end	Night	17% 5% 35% 36% 7%	400
July	1st to 10th	Night	13% 3 34% 41% 8% 1	390
	11th to 20th	Night	15% 3 31% 41% 9% 1	388
	21st to end	Night	13% 3 33% 41% 9%	438
August	1st to 10th	Night	11% 5% 32% 41% 11%	565
	11th to 20th	Night	13% 5% 27% 43% 12% 1	600
	21st to end	Night	14% 5% 24% 38% 18% 2	880
September	1st to 10th	Night	14% 5% 30% 36% 14% 1	800
	11th to 20th	Night	16% 6% 27% 30% 19% 2	800
	21st to end	Night	22% 6% 26% 31% 13% 2	800
October	1st to 10th	Night	23% 10% 26% 27% 13% 2	800
	11th to 20th	Night	18% 13% 33% 25% 10% 2	800
	21st to end	Night	25% 12% 34% 22% 6%	880
November	1st to 10th	Night	18% 14% 35% 24% 8% 1	789
	11th to 20th	Night	21% 22% 31% 19% 6% 1	800
	21st to end	Night	21% 22% 34% 19% 5% 1	800
December	1st to 10th	Night	25% 18% 35% 18% 5%	800
	11th to 20th	Night	24% 20% 33% 19% 4	800
	21st to end	Night	24% 18% 38% 18% 2	880

Page generated at 2025-10-31 02:02:43. Version ac07

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