This page made possible by the sponsorship of: Amateur Telescope Makers of Boston.

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 2024-01-16.

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 TypeMinimum conditions for hour to be usableOut of hours
Clear of CloudsTransparencySeeingSky Brightness
Planetary:50%(no limit)Average(no limit)Nighttime Hours
Deep Sky:70%Average(no limit)5.9Nighttime Hours
CCD Imaging:70%Below AveragePoor(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

PlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
Night
26%
22955
11%
74584
4%
22912
33%
22912
10%
57604

By Month

MonthDays of MonthPlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
January1st to 10thNight
15%
760
12%
2649
2%
757
26%
757
9%
2030
January11th to 20thNight
13%
741
9%
2591
2%
741
22%
741
6%
2120
January21st to endNight
17%
792
12%
2772
3%
792
28%
792
9%
2084
February1st to 10thNight
15%
720
9%
2272
2%
720
24%
720
8%
1793
February11th to 20thNight
13%
703
13%
2100
2%
700
25%
700
9%
1657
February21st to endNight
17%
592
16%
1776
5%
592
30%
592
10%
1381
March1st to 10thNight
18%
720
13%
2160
5%
720
29%
720
10%
1668
March11th to 20thNight
15%
720
11%
2051
3%
720
24%
720
9%
1665
March21st to endNight
19%
792
14%
2075
4%
792
28%
792
12%
1640
April1st to 10thNight
20%
720
13%
1800
3%
720
26%
720
13%
1439
April11th to 20thNight
21%
685
12%
1749
2%
685
27%
685
12%
1408
April21st to endNight
20%
540
13%
1620
3%
540
26%
540
10%
1284
May1st to 10thNight
20%
540
9%
1620
4%
540
25%
540
9%
1320
May11th to 20thNight
21%
532
11%
1594
4%
532
28%
532
10%
1267
May21st to endNight
30%
389
10%
1551
9%
388
35%
388
10%
1153
June1st to 10thNight
30%
360
8%
1440
5%
360
36%
360
7%
1141
June11th to 20thNight
31%
360
9%
1440
9%
360
37%
360
9%
1113
June21st to endNight
37%
360
6%
1440
5%
360
41%
360
9%
1111
July1st to 10thNight
41%
350
6%
1400
7%
350
45%
350
9%
1088
July11th to 20thNight
44%
348
8%
1392
6%
348
50%
348
11%
1055
July21st to endNight
42%
394
12%
1486
9%
372
48%
372
11%
1114
August1st to 10thNight
44%
507
9%
1571
5%
505
47%
505
13%
1217
August11th to 20thNight
46%
538
12%
1602
8%
538
48%
538
13%
1249
August21st to endNight
50%
792
11%
1980
4%
792
52%
792
16%
1561
September1st to 10thNight
44%
720
13%
1814
5%
720
48%
720
17%
1412
September11th to 20thNight
44%
720
17%
1980
8%
720
51%
720
18%
1477
September21st to endNight
39%
720
14%
1980
8%
720
44%
720
16%
1522
October1st to 10thNight
33%
720
12%
1980
5%
720
39%
720
14%
1532
October11th to 20thNight
30%
720
13%
2192
6%
720
38%
720
12%
1692
October21st to endNight
22%
792
11%
2574
4%
792
30%
792
10%
1997
November1st to 10thNight
29%
740
12%
2407
6%
740
37%
740
13%
1796
November11th to 20thNight
22%
760
15%
2470
7%
760
34%
760
10%
1889
November21st to endNight
19%
760
12%
2470
3%
760
30%
760
9%
1919
December1st to 10thNight
17%
760
11%
2603
3%
760
27%
760
8%
2057
December11th to 20thNight
18%
760
12%
3191
1%
759
29%
759
7%
2312
December21st to endNight
14%
828
9%
4792
2%
817
24%
817
4%
3441

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 ForecastData
Points
Cloud CoverDay
16%7%25%5%35%5%5%7%41%
74839
Night
17%7%35%5%3445%7%41%
74945
TransparencyDay
27%39%9%14%9%1
74544
Night
29%38%8%14%9%1
74774
SeeingDay
22%16%33%20%8%2
13965
Night
21%14%34%24%7%1
22963

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
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
14%9%36%5%34445%44%
1517
11th to 20thDay
19%7%35%325%5%6%6%39%
1480
21st to endDay
14%6%36%5%3345%8%43%
1584
February1st to 10thDay
21%8%24435%5%5%7%36%
1684
11th to 20thDay
13%7%36%435%46%7%41%
1748
21st to endDay
18%8%35%6%345%6%6%38%
1480
March1st to 10thDay
17%6%25%5%35%46%8%39%
1797
11th to 20thDay
16%6%35%6%35%5%5%8%40%
1907
21st to endDay
18%8%26%5%3445%5%41%
2276
April1st to 10thDay
18%8%35%435%5%47%37%
2160
11th to 20thDay
17%6%245%35%46%8%39%
2158
21st to endDay
20%6%35%5%35%46%7%36%
2340
May1st to 10thDay
23%8%27%6%2445%6%32%
2340
11th to 20thDay
17%7%25%5%2445%7%40%
2315
21st to endDay
14%5%345%3445%8%44%
2725
June1st to 10thDay
20%5%35%5%36%6%7%9%33%
2520
11th to 20thDay
16%6%25%6%35%5%6%8%38%
2520
21st to endDay
15%6%26%6%35%5%7%9%35%
2520
July1st to 10thDay
12%5%35%5%35%6%6%9%41%
2520
11th to 20thDay
11%6%25%5%45%6%7%10%39%
2520
21st to endDay
11%5%35%5%36%6%6%8%41%
2744
August1st to 10thDay
9%5%25%6%46%7%7%10%38%
2337
11th to 20thDay
12%7%25%5%35%7%6%8%40%
2295
21st to endDay
12%5%25%5%346%5%9%45%
2376
September1st to 10thDay
13%6%244345%5%8%46%
2144
11th to 20thDay
14%6%25%323346%52%
1980
21st to endDay
18%5%2443445%6%46%
1980
October1st to 10thDay
24%5%24434346%42%
1980
11th to 20thDay
15%6%25%424447%47%
1765
21st to endDay
19%7%35%44445%7%39%
1782
November1st to 10thDay
16%6%24423436%49%
1671
11th to 20thDay
17%7%2442335%5%46%
1710
21st to endDay
17%7%24434446%44%
1710
December1st to 10thDay
18%7%26%444447%40%
1576
11th to 20thDay
21%7%34324345%44%
1906
21st to endDay
21%9%36%5%35%34436%
2772
January1st to 10thNight
16%8%34433445%45%
2649
11th to 20thNight
19%9%36%6%3445%7%36%
2591
21st to endNight
14%9%45%5%45%446%41%
2772
February1st to 10thNight
18%8%35%5%35%45%7%37%
2272
11th to 20thNight
15%9%46%5%35%5%46%39%
2098
21st to endNight
18%8%345%24447%42%
1776
March1st to 10thNight
16%7%35%6%345%46%41%
2150
11th to 20thNight
16%9%36%5%35%45%5%39%
2051
21st to endNight
18%9%36%6%33335%42%
2075
April1st to 10thNight
18%8%36%6%335%5%7%37%
1800
11th to 20thNight
18%8%25%5%3445%6%40%
1749
21st to endNight
22%6%36%5%3445%6%38%
1620
May1st to 10thNight
23%8%36%6%245%5%6%32%
1620
11th to 20thNight
18%6%36%435%6%6%8%36%
1593
21st to endNight
17%8%36%5%2336%8%39%
1550
June1st to 10thNight
18%7%37%435%45%7%37%
1440
11th to 20thNight
18%7%36%6%3445%9%36%
1440
21st to endNight
16%5%24425%6%6%10%40%
1440
July1st to 10thNight
13%5%36%6%35%5%7%9%40%
1440
11th to 20thNight
11%6%36%6%46%6%6%10%36%
1440
21st to endNight
12%5%35%6%36%46%9%41%
1574
August1st to 10thNight
9%5%246%445%8%9%44%
1581
11th to 20thNight
10%6%36%6%46%6%8%7%38%
1602
21st to endNight
13%5%245%335%6%7%47%
1980
September1st to 10thNight
13%5%25%5%345%6%7%45%
1814
11th to 20thNight
14%5%24323446%53%
1980
21st to endNight
16%5%25%5%235%6%5%45%
1980
October1st to 10thNight
21%6%25%5%34345%42%
1980
11th to 20thNight
14%5%25%5%3445%7%45%
2192
21st to endNight
21%8%35%5%3345%5%39%
2574
November1st to 10thNight
14%5%25%5%3345%5%48%
2407
11th to 20thNight
17%7%34434445%46%
2470
21st to endNight
16%6%2443445%7%44%
2470
December1st to 10thNight
19%7%35%5%2445%6%39%
2603
11th to 20thNight
19%7%25%42445%6%42%
3288
21st to endNight
20%10%46%5%33446%36%
4884

Transparency (when cloud cover <= 30%)
Color Meanings:
Too cloudy to forecast Poor Below Average Average Above average Transparent
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
27%42%7%17%7%
1518
11th to 20thDay
29%43%9%11%7%1
1480
21st to endDay
24%39%9%20%8%1
1584
February1st to 10thDay
31%38%9%11%9%2
1684
11th to 20thDay
26%40%10%15%7%1
1757
21st to endDay
33%38%7%11%9%1
1480
March1st to 10thDay
28%38%10%15%8%2
1800
11th to 20thDay
29%39%9%14%8%2
1907
21st to endDay
27%38%7%12%12%4
2276
April1st to 10thDay
27%39%8%12%12%2
2160
11th to 20thDay
29%38%8%14%9%2
2158
21st to endDay
27%39%7%13%10%3
2340
May1st to 10thDay
34%37%6%13%9%1
2340
11th to 20thDay
28%39%7%12%11%3
2319
21st to endDay
24%40%10%16%9%2
2725
June1st to 10thDay
32%40%10%13%5%
2520
11th to 20thDay
28%40%10%12%9%2
2520
21st to endDay
29%41%11%13%5%
2520
July1st to 10thDay
28%41%10%14%7%1
2450
11th to 20thDay
24%41%11%15%7%1
2445
21st to endDay
23%42%10%15%8%2
2591
August1st to 10thDay
24%43%11%15%7%
2323
11th to 20thDay
26%39%9%15%9%1
2295
21st to endDay
24%40%8%14%12%2
2376
September1st to 10thDay
24%38%8%16%13%1
2144
11th to 20thDay
21%37%8%19%14%1
1980
21st to endDay
27%37%8%13%14%1
1980
October1st to 10thDay
27%37%8%14%12%1
1980
11th to 20thDay
25%39%8%19%8%1
1765
21st to endDay
27%39%9%13%10%2
1782
November1st to 10thDay
23%41%7%17%10%1
1671
11th to 20thDay
24%40%7%16%11%1
1710
21st to endDay
27%42%10%15%6%
1710
December1st to 10thDay
28%41%9%11%9%1
1576
11th to 20thDay
32%32%9%12%11%4
1906
21st to endDay
39%27%10%16%7%
2772
January1st to 10thNight
27%40%9%15%8%
2649
11th to 20thNight
32%42%7%12%5%1
2591
21st to endNight
27%39%8%14%11%1
2772
February1st to 10thNight
31%38%9%14%8%1
2272
11th to 20thNight
30%38%7%15%8%2
2100
21st to endNight
28%38%7%16%9%2
1776
March1st to 10thNight
28%38%7%16%9%2
2160
11th to 20thNight
32%38%9%11%8%1
2052
21st to endNight
30%36%6%12%13%3
2075
April1st to 10thNight
29%38%8%13%11%2
1800
11th to 20thNight
32%36%5%13%11%2
1749
21st to endNight
30%39%8%11%10%3
1620
May1st to 10thNight
34%38%6%10%9%3
1620
11th to 20thNight
30%40%8%10%9%3
1594
21st to endNight
27%39%10%13%10%1
1551
June1st to 10thNight
33%39%10%12%5%
1440
11th to 20thNight
32%38%8%12%9%1
1440
21st to endNight
27%42%11%13%6%
1440
July1st to 10thNight
30%41%10%13%7%
1400
11th to 20thNight
27%40%10%16%6%1
1392
21st to endNight
24%42%9%17%6%1
1486
August1st to 10thNight
27%40%10%16%7%
1571
11th to 20thNight
27%40%10%13%9%
1602
21st to endNight
26%39%8%17%10%1
1980
September1st to 10thNight
26%38%8%15%12%1
1814
11th to 20thNight
22%38%8%16%15%1
1980
21st to endNight
26%38%7%15%12%2
1980
October1st to 10thNight
28%37%8%15%10%2
1980
11th to 20thNight
27%39%10%17%7%
2192
21st to endNight
30%38%8%13%10%1
2574
November1st to 10thNight
21%42%10%14%11%2
2407
11th to 20thNight
26%40%7%15%10%1
2470
21st to endNight
27%41%8%15%9%1
2470
December1st to 10thNight
29%41%8%14%7%1
2603
11th to 20thNight
28%35%12%12%13%
3288
21st to endNight
39%27%8%19%7%
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
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
28%24%28%16%4
180
11th to 20thDay
22%29%36%10%21
170
21st to endDay
21%27%38%10%31
176
February1st to 10thDay
30%22%34%12%2
352
11th to 20thDay
26%27%36%10%1
416
21st to endDay
25%23%38%11%2
346
March1st to 10thDay
23%24%37%12%4
420
11th to 20thDay
25%25%34%11%31
420
21st to endDay
27%22%33%11%6%1
462
April1st to 10thDay
29%22%29%13%7%
420
11th to 20thDay
25%23%33%16%3
412
21st to endDay
28%16%36%16%31
420
May1st to 10thDay
32%23%30%11%31
423
11th to 20thDay
21%16%40%17%5%
447
21st to endDay
18%13%34%28%6%1
485
June1st to 10thDay
22%11%34%24%7%2
450
11th to 20thDay
16%14%41%21%7%1
450
21st to endDay
15%9%41%25%9%1
450
July1st to 10thDay
17%7%30%29%14%2
445
11th to 20thDay
15%5%30%36%12%2
444
21st to endDay
11%7%35%34%11%1
488
August1st to 10thDay
11%8%32%37%11%2
446
11th to 20thDay
16%7%29%29%15%3
439
21st to endDay
16%7%32%26%15%4
495
September1st to 10thDay
18%9%27%27%15%4
450
11th to 20thDay
21%10%28%21%13%6%
450
21st to endDay
22%10%32%22%10%4
450
October1st to 10thDay
26%12%27%22%9%4
450
11th to 20thDay
19%14%33%22%10%2
394
21st to endDay
26%16%33%17%8%1
396
November1st to 10thDay
20%16%33%19%11%1
365
11th to 20thDay
24%22%33%16%5%
370
21st to endDay
24%21%32%16%6%1
370
December1st to 10thDay
25%24%33%14%3
236
11th to 20thDay
32%26%34%7%2
180
21st to endDay
25%21%34%15%33
198
January1st to 10thNight
21%20%39%17%3
760
11th to 20thNight
26%20%36%17%2
741
21st to endNight
19%20%37%20%4
792
February1st to 10thNight
24%19%37%19%1
720
11th to 20thNight
25%19%40%15%1
703
21st to endNight
24%21%34%17%3
592
March1st to 10thNight
24%19%36%18%3
720
11th to 20thNight
23%19%37%17%31
720
21st to endNight
24%17%34%19%5%
792
April1st to 10thNight
23%19%34%20%4
720
11th to 20thNight
23%16%38%19%5%
685
21st to endNight
25%14%36%22%2
540
May1st to 10thNight
28%12%34%20%5%
540
11th to 20thNight
22%13%39%21%5%
532
21st to endNight
19%8%38%28%7%
389
June1st to 10thNight
21%6%32%34%7%1
360
11th to 20thNight
17%8%36%33%6%1
360
21st to endNight
17%437%36%6%
360
July1st to 10thNight
13%332%42%9%1
350
11th to 20thNight
16%229%42%9%1
348
21st to endNight
13%233%42%10%
394
August1st to 10thNight
9%5%33%41%12%
507
11th to 20thNight
14%428%42%12%1
538
21st to endNight
15%5%22%38%18%2
792
September1st to 10thNight
15%429%37%14%1
720
11th to 20thNight
16%7%28%29%18%2
720
21st to endNight
21%6%27%31%13%3
720
October1st to 10thNight
25%9%26%25%13%2
720
11th to 20thNight
18%13%34%25%10%2
720
21st to endNight
26%12%34%22%6%1
792
November1st to 10thNight
18%14%33%25%8%1
740
11th to 20thNight
21%21%31%19%6%1
760
21st to endNight
20%21%34%19%5%1
760
December1st to 10thNight
25%18%35%18%4
760
11th to 20thNight
23%20%33%20%3
760
21st to endNight
24%18%38%18%2
836
Page generated at 2024-01-13 01:38:08. Version ac07

Copyright 2024 Attilla Danko


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