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 2024-11-06.

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	16% 7% 2 6% 4 4 4 4 5% 7% 39%	101207
	Night	17% 8% 3 6% 4 4 4 3 5% 6% 39%	97871
Transparency	Day	44% 4 11% 23% 15% 2	104758
	Night	45% 4 11% 23% 15% 2	102409
Seeing	Day	21% 16% 34% 20% 7% 2	15084
	Night	21% 14% 34% 24% 7% 1	23950

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	14% 8% 2 6% 4 4 3 3 4 5% 46%	2517
	11th to 20th	Day	17% 7% 3 7% 3 3 5% 4 5% 4 42%	2520
	21st to end	Day	21% 10% 2 7% 5% 4 3 3 5% 6% 34%	2772
February	1st to 10th	Day	20% 7% 2 5% 4 4 5% 4 5% 6% 37%	1777
	11th to 20th	Day	13% 7% 3 6% 4 4 5% 4 6% 7% 41%	1848
	21st to end	Day	17% 9% 2 6% 5% 3 5% 3 6% 5% 38%	1570
March	1st to 10th	Day	25% 13% 2 6% 4 3 4 3 5% 5% 30%	3116
	11th to 20th	Day	12% 7% 2 5% 4 4 5% 4 6% 9% 41%	3351
	21st to end	Day	25% 8% 2 6% 4 4 4 4 6% 5% 34%	3996
April	1st to 10th	Day	18% 9% 3 6% 4 4 4 4 4 7% 36%	2280
	11th to 20th	Day	18% 7% 2 5% 5% 5% 4 4 6% 8% 38%	2280
	21st to end	Day	19% 6% 2 6% 4 4 4 3 6% 7% 37%	2470
May	1st to 10th	Day	22% 12% 3 8% 5% 4 3 4 5% 6% 28%	4106
	11th to 20th	Day	23% 9% 3 7% 5% 3 4 3 5% 6% 32%	4109
	21st to end	Day	14% 5% 2 5% 4 4 4 4 5% 8% 45%	4837
June	1st to 10th	Day	19% 6% 2 5% 4 5% 5% 5% 7% 9% 34%	2660
	11th to 20th	Day	15% 6% 2 6% 5% 4 5% 4 7% 8% 38%	2660
	21st to end	Day	15% 7% 2 7% 5% 5% 5% 5% 7% 9% 35%	2660
July	1st to 10th	Day	11% 7% 3 7% 5% 5% 6% 5% 7% 10% 34%	4440
	11th to 20th	Day	9% 6% 2 7% 5% 5% 5% 4 7% 10% 39%	4440
	21st to end	Day	16% 7% 3 7% 5% 5% 5% 4 6% 7% 35%	4814
August	1st to 10th	Day	14% 7% 3 7% 5% 5% 5% 5% 7% 10% 32%	4115
	11th to 20th	Day	12% 8% 3 7% 4 4 5% 5% 6% 8% 38%	4022
	21st to end	Day	7% 4 2 5% 4 4 5% 5% 7% 10% 46%	4180
September	1st to 10th	Day	13% 6% 2 5% 4 4 4 4 5% 7% 47%	2263
	11th to 20th	Day	14% 6% 2 5% 3 3 2 3 4 6% 52%	2090
	21st to end	Day	18% 6% 3 5% 3 4 4 3 6% 6% 43%	2090
October	1st to 10th	Day	23% 5% 2 5% 3 4 3 2 4 6% 42%	2090
	11th to 20th	Day	14% 6% 2 5% 3 3 4 3 4 6% 48%	1862
	21st to end	Day	18% 7% 2 5% 4 4 4 3 5% 6% 41%	1881
November	1st to 10th	Day	16% 7% 2 5% 4 3 4 3 3 6% 49%	1717
	11th to 20th	Day	17% 8% 2 4 4 3 3 3 5% 5% 46%	1710
	21st to end	Day	17% 8% 2 5% 4 4 4 4 5% 6% 44%	1710
December	1st to 10th	Day	18% 7% 2 6% 4 4 4 3 4 6% 40%	1576
	11th to 20th	Day	21% 8% 2 5% 2 3 3 2 4 5% 44%	1906
	21st to end	Day	21% 10% 3 7% 4 4 4 3 4 4 35%	2772
January	1st to 10th	Night	17% 9% 2 4 3 3 3 3 4 5% 47%	4429
	11th to 20th	Night	15% 9% 3 8% 5% 4 3 3 4 5% 40%	4440
	21st to end	Night	21% 13% 4 8% 4 4 4 3 4 5% 30%	4884
February	1st to 10th	Night	17% 9% 2 6% 5% 4 4 3 5% 7% 38%	2399
	11th to 20th	Night	14% 9% 3 6% 4 4 4 3 5% 6% 40%	2218
	21st to end	Night	18% 8% 2 5% 4 3 4 3 4 6% 42%	1884
March	1st to 10th	Night	29% 10% 2 5% 3 3 3 3 4 5% 32%	3724
	11th to 20th	Night	11% 10% 3 7% 5% 4 4 4 6% 6% 40%	3607
	21st to end	Night	24% 9% 2 6% 4 4 4 4 5% 5% 33%	3631
April	1st to 10th	Night	18% 9% 3 6% 5% 4 4 4 5% 7% 36%	1900
	11th to 20th	Night	18% 9% 2 6% 4 4 3 3 6% 6% 39%	1847
	21st to end	Night	21% 6% 3 6% 4 4 4 3 6% 5% 39%	1710
May	1st to 10th	Night	23% 11% 3 8% 5% 4 4 4 5% 6% 27%	2834
	11th to 20th	Night	23% 9% 3 7% 5% 5% 5% 4 7% 7% 26%	2813
	21st to end	Night	16% 7% 2 6% 4 4 4 3 6% 8% 40%	2738
June	1st to 10th	Night	18% 8% 3 7% 4 4 5% 4 5% 7% 37%	1520
	11th to 20th	Night	17% 8% 2 7% 5% 3 4 4 5% 9% 37%	1520
	21st to end	Night	16% 6% 2 5% 4 3 6% 4 6% 9% 38%	1520
July	1st to 10th	Night	10% 7% 3 7% 6% 5% 5% 5% 8% 10% 35%	2520
	11th to 20th	Night	9% 7% 3 8% 5% 5% 5% 5% 6% 10% 38%	2520
	21st to end	Night	15% 10% 3 7% 5% 5% 5% 3 6% 7% 35%	2739
August	1st to 10th	Night	18% 7% 2 7% 5% 5% 5% 4 7% 8% 31%	2781
	11th to 20th	Night	9% 7% 3 8% 5% 6% 6% 5% 8% 7% 37%	2802
	21st to end	Night	8% 4 1 4 3 3 4 4 6% 7% 54%	3476
September	1st to 10th	Night	12% 5% 2 6% 4 5% 4 3 6% 7% 46%	1915
	11th to 20th	Night	14% 6% 2 4 2 2 3 3 4 6% 53%	2090
	21st to end	Night	17% 6% 2 6% 5% 3 3 4 6% 5% 43%	2090
October	1st to 10th	Night	20% 6% 2 7% 3 4 4 3 4 6% 42%	2090
	11th to 20th	Night	14% 5% 2 5% 3 4 4 3 5% 7% 47%	2315
	21st to end	Night	20% 7% 2 6% 4 3 3 3 5% 5% 40%	2717
November	1st to 10th	Night	14% 5% 2 6% 4 4 3 3 5% 5% 49%	2483
	11th to 20th	Night	17% 7% 3 5% 4 3 4 3 4 5% 46%	2470
	21st to end	Night	16% 7% 2 5% 3 3 4 4 5% 7% 44%	2470
December	1st to 10th	Night	19% 8% 3 6% 5% 3 3 4 5% 6% 39%	2603
	11th to 20th	Night	19% 7% 2 6% 3 3 4 3 5% 5% 42%	3288
	21st to end	Night	20% 10% 3 7% 4 4 3 3 5% 6% 35%	4884

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	42% 3 7% 33% 13% 2	2518
	11th to 20th	Day	47% 4 9% 26% 13% 2	2520
	21st to end	Day	54% 3 10% 22% 9% 1	2772
February	1st to 10th	Day	49% 4 11% 19% 14% 2	1777
	11th to 20th	Day	43% 4 12% 24% 16% 2	1857
	21st to end	Day	48% 4 10% 20% 15% 3	1570
March	1st to 10th	Day	59% 3 10% 15% 11% 2	3119
	11th to 20th	Day	41% 4 12% 26% 15% 2	3351
	21st to end	Day	52% 3 6% 17% 15% 7%	3996
April	1st to 10th	Day	48% 4 8% 19% 17% 3	2280
	11th to 20th	Day	44% 4 10% 23% 16% 3	2280
	21st to end	Day	46% 3 9% 22% 17% 4	2470
May	1st to 10th	Day	59% 4 10% 16% 10% 1	4120
	11th to 20th	Day	54% 4 13% 17% 10% 2	4113
	21st to end	Day	37% 4 12% 26% 19% 2	4837
June	1st to 10th	Day	45% 6% 13% 24% 12% 1	2660
	11th to 20th	Day	42% 5% 14% 21% 15% 2	2660
	21st to end	Day	44% 6% 14% 22% 13% 1	2660
July	1st to 10th	Day	43% 8% 11% 21% 15% 2	4370
	11th to 20th	Day	39% 6% 14% 25% 14% 2	4365
	21st to end	Day	46% 6% 12% 21% 13% 2	4661
August	1st to 10th	Day	47% 7% 16% 22% 8% 1	4101
	11th to 20th	Day	42% 6% 13% 28% 10% 1	4061
	21st to end	Day	31% 5% 13% 30% 18% 1	4180
September	1st to 10th	Day	37% 5% 10% 25% 21% 2	2263
	11th to 20th	Day	35% 4 8% 29% 23% 3	2090
	21st to end	Day	42% 4 8% 21% 23% 2	2090
October	1st to 10th	Day	40% 5% 9% 26% 17% 3	3480
	11th to 20th	Day	32% 2 6% 30% 25% 4	3065
	21st to end	Day	33% 4 11% 30% 20% 3	3081
November	1st to 10th	Day	40% 3 12% 26% 17% 2	1717
	11th to 20th	Day	41% 3 8% 24% 21% 2	1710
	21st to end	Day	43% 4 13% 24% 13% 2	1710
December	1st to 10th	Day	47% 3 14% 17% 18% 1	1576
	11th to 20th	Day	45% 3 13% 18% 17% 5%	1906
	21st to end	Day	54% 2 10% 22% 11% 1	2772
January	1st to 10th	Night	41% 2 11% 31% 13% 3	4429
	11th to 20th	Night	48% 4 10% 24% 12% 3	4440
	21st to end	Night	60% 2 8% 17% 11% 2	4884
February	1st to 10th	Night	47% 4 12% 21% 14% 2	2399
	11th to 20th	Night	46% 4 8% 21% 16% 4	2220
	21st to end	Night	45% 3 7% 25% 16% 4	1884
March	1st to 10th	Night	57% 2 7% 19% 12% 3	3734
	11th to 20th	Night	45% 3 13% 20% 17% 2	3608
	21st to end	Night	55% 3 4 15% 17% 7%	3631
April	1st to 10th	Night	49% 4 8% 20% 16% 3	1900
	11th to 20th	Night	47% 3 8% 21% 18% 3	1847
	21st to end	Night	48% 3 9% 21% 16% 4	1710
May	1st to 10th	Night	58% 2 12% 17% 8% 2	2840
	11th to 20th	Night	58% 4 14% 14% 8% 2	2814
	21st to end	Night	44% 4 15% 22% 14% 1	2739
June	1st to 10th	Night	46% 5% 14% 23% 12%	1520
	11th to 20th	Night	45% 5% 13% 22% 14% 1	1520
	21st to end	Night	41% 6% 14% 25% 13% 1	1520
July	1st to 10th	Night	41% 7% 13% 21% 16%	2480
	11th to 20th	Night	40% 6% 17% 25% 13% 1	2472
	21st to end	Night	48% 6% 11% 20% 14% 1	2651
August	1st to 10th	Night	51% 6% 14% 19% 10% 1	2771
	11th to 20th	Night	41% 9% 15% 24% 10%	2829
	21st to end	Night	27% 5% 15% 29% 24% 1	3476
September	1st to 10th	Night	37% 5% 11% 25% 19% 2	1915
	11th to 20th	Night	34% 3 11% 24% 26% 2	2090
	21st to end	Night	41% 5% 8% 24% 20% 3	2090
October	1st to 10th	Night	40% 4 12% 25% 15% 3	3480
	11th to 20th	Night	27% 3 10% 28% 29% 3	3851
	21st to end	Night	34% 3 11% 30% 20% 2	4467
November	1st to 10th	Night	38% 4 12% 25% 19% 3	2483
	11th to 20th	Night	42% 3 9% 23% 20% 3	2470
	21st to end	Night	41% 3 13% 27% 13% 2	2470
December	1st to 10th	Night	47% 4 12% 22% 14% 1	2603
	11th to 20th	Night	45% 4 14% 18% 18% 1	3288
	21st to end	Night	51% 4 7% 27% 11% 1	4884

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	28% 24% 28% 16% 4	180
	11th to 20th	Day	22% 29% 37% 9% 2 1	180
	21st to end	Day	22% 25% 39% 11% 3 1	198
February	1st to 10th	Day	28% 22% 35% 12% 2	385
	11th to 20th	Day	26% 28% 35% 10% 1	456
	21st to end	Day	26% 23% 36% 12% 3	382
March	1st to 10th	Day	26% 24% 35% 12% 4	460
	11th to 20th	Day	26% 25% 34% 11% 3 1	460
	21st to end	Day	29% 22% 32% 10% 6% 1	506
April	1st to 10th	Day	30% 22% 30% 12% 6%	460
	11th to 20th	Day	26% 23% 33% 15% 3	452
	21st to end	Day	26% 17% 37% 16% 3 1	460
May	1st to 10th	Day	30% 24% 31% 11% 3 1	463
	11th to 20th	Day	22% 15% 40% 17% 5% 1	487
	21st to end	Day	18% 13% 35% 28% 5% 1	529
June	1st to 10th	Day	21% 12% 34% 24% 7% 1	490
	11th to 20th	Day	14% 14% 40% 23% 8% 1	490
	21st to end	Day	15% 10% 42% 23% 8% 1	490
July	1st to 10th	Day	16% 8% 32% 28% 13% 2	485
	11th to 20th	Day	14% 6% 32% 35% 12% 2	484
	21st to end	Day	12% 7% 36% 33% 11% 1	532
August	1st to 10th	Day	11% 9% 32% 36% 10% 2	486
	11th to 20th	Day	16% 7% 30% 29% 15% 3	479
	21st to end	Day	14% 7% 33% 27% 15% 4	539
September	1st to 10th	Day	18% 10% 28% 26% 15% 3	490
	11th to 20th	Day	21% 10% 26% 21% 15% 8%	490
	21st to end	Day	22% 10% 32% 22% 9% 4	490
October	1st to 10th	Day	25% 12% 28% 22% 9% 4	490
	11th to 20th	Day	19% 15% 33% 21% 9% 3	427
	21st to end	Day	24% 16% 34% 17% 8% 1	429
November	1st to 10th	Day	19% 16% 34% 19% 11% 1	381
	11th to 20th	Day	24% 22% 33% 16% 5%	370
	21st to end	Day	24% 21% 32% 16% 6% 1	370
December	1st to 10th	Day	25% 24% 33% 14% 3	236
	11th to 20th	Day	32% 26% 34% 7% 2	180
	21st to end	Day	25% 21% 34% 15% 3 3	198
January	1st to 10th	Night	21% 20% 39% 17% 3	760
	11th to 20th	Night	26% 19% 36% 17% 2	760
	21st to end	Night	20% 20% 37% 19% 4	836
February	1st to 10th	Night	23% 19% 37% 19% 1	760
	11th to 20th	Night	24% 20% 41% 14% 1	743
	21st to end	Night	25% 21% 34% 16% 4	628
March	1st to 10th	Night	25% 19% 35% 18% 4	760
	11th to 20th	Night	23% 19% 38% 16% 3 1	760
	21st to end	Night	25% 18% 34% 18% 5%	836
April	1st to 10th	Night	23% 19% 34% 20% 4	760
	11th to 20th	Night	24% 16% 38% 18% 5%	723
	21st to end	Night	24% 15% 36% 23% 2	570
May	1st to 10th	Night	28% 13% 34% 20% 5%	570
	11th to 20th	Night	22% 12% 39% 22% 5%	562
	21st to end	Night	18% 8% 39% 28% 7%	411
June	1st to 10th	Night	20% 7% 33% 33% 6% 1	380
	11th to 20th	Night	17% 8% 36% 33% 6% 1	380
	21st to end	Night	18% 5% 36% 35% 6%	380
July	1st to 10th	Night	13% 3 33% 41% 9% 1	370
	11th to 20th	Night	15% 3 30% 42% 9% 1	368
	21st to end	Night	14% 2 32% 42% 10%	416
August	1st to 10th	Night	11% 5% 32% 40% 11%	536
	11th to 20th	Night	13% 5% 28% 42% 12% 1	569
	21st to end	Night	14% 5% 23% 38% 19% 2	836
September	1st to 10th	Night	14% 4 30% 36% 14% 1	760
	11th to 20th	Night	16% 6% 27% 29% 19% 2	760
	21st to end	Night	22% 6% 26% 31% 13% 3	760
October	1st to 10th	Night	24% 9% 26% 26% 13% 2	760
	11th to 20th	Night	17% 13% 33% 25% 10% 2	760
	21st to end	Night	25% 12% 34% 22% 7%	836
November	1st to 10th	Night	18% 14% 34% 25% 8% 1	764
	11th to 20th	Night	21% 21% 31% 19% 6% 1	760
	21st to end	Night	20% 21% 34% 19% 5% 1	760
December	1st to 10th	Night	25% 18% 35% 18% 4	760
	11th to 20th	Night	23% 20% 33% 20% 3	760
	21st to end	Night	24% 18% 38% 18% 2	836

Page generated at 2024-11-03 13:19:05. Version ac07

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