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Boston
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Boston clear sky chart from 2005-11-05 to 2018-03-11.

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
24%
15605
18%
49066
7%
15573
36%
15573
18%
32209

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 15thNight
13%
780
18%
2716
5%
776
29%
776
15%
1753
January16th to endNight
13%
832
17%
2912
3%
832
31%
832
13%
1907
February1st to 15thNight
14%
764
16%
2372
4%
761
29%
761
14%
1636
February16th to endNight
15%
683
21%
2049
6%
683
34%
683
16%
1381
March1st to 15thNight
14%
764
16%
2279
6%
764
28%
764
14%
1587
March16th to endNight
18%
768
22%
2043
6%
768
32%
768
19%
1393
April1st to 15thNight
18%
718
18%
1796
4%
718
26%
718
18%
1274
April16th to endNight
20%
582
18%
1662
3%
582
31%
582
18%
1139
May1st to 15thNight
21%
540
15%
1620
6%
540
29%
540
16%
1129
May16th to endNight
21%
431
11%
1553
6%
430
29%
430
14%
1031
June1st to 15thNight
27%
360
11%
1440
8%
360
36%
360
13%
983
June16th to endNight
33%
360
11%
1440
9%
360
41%
360
18%
877
July1st to 15thNight
38%
340
12%
1360
12%
340
45%
340
20%
813
July16th to endNight
37%
376
15%
1418
12%
354
48%
354
20%
887
August1st to 15thNight
41%
520
16%
1591
10%
518
46%
518
24%
997
August16th to endNight
50%
710
20%
1856
10%
710
54%
710
32%
1187
September1st to 15thNight
43%
720
19%
1870
9%
720
48%
720
31%
1202
September16th to endNight
42%
717
26%
1980
15%
717
52%
717
31%
1219
October1st to 15thNight
31%
720
22%
2003
11%
720
42%
720
25%
1298
October16th to endNight
23%
768
17%
2496
6%
768
38%
768
19%
1592
November1st to 15thNight
23%
760
18%
2472
9%
760
37%
760
18%
1599
November16th to endNight
20%
780
21%
2535
8%
780
37%
780
17%
1609
December1st to 15thNight
18%
780
17%
2691
4%
780
33%
780
13%
1785
December16th to endNight
12%
832
18%
2912
4%
832
29%
832
12%
1931

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%6%2442445%7%40%
49572
Night
16%6%25%424446%41%
49236
Cloud CoverDay
16%6%2442445%7%40%
49582
Night
16%6%25%424446%41%
49241
TransparencyDay
42%6%12%21%14%2
49260
Night
43%5%11%21%15%2
49066
SeeingDay
19%19%30%19%8%1
6450
Night
17%23%29%21%6%
15605

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 15thDay
15%8%2432445%5%42%
1552
16th to endDay
17%5%25%423346%43%
1664
February1st to 15thDay
18%6%244345%5%7%36%
1816
16th to endDay
14%6%25%5%2445%6%41%
1710
March1st to 15thDay
20%6%245%2445%7%34%
1908
16th to endDay
17%7%25%423445%42%
2178
April1st to 15thDay
17%7%35%424447%37%
2152
16th to endDay
17%42443435%7%41%
2297
May1st to 15thDay
23%7%26%423345%33%
2332
16th to endDay
16%5%245%2445%8%40%
2571
June1st to 15thDay
24%5%24425%45%8%32%
2520
16th to endDay
14%5%25%5%35%5%7%9%35%
2520
July1st to 15thDay
11%5%25%5%35%6%6%10%37%
2520
16th to endDay
11%5%245%36%5%6%8%39%
2654
August1st to 15thDay
11%5%245%35%6%6%9%38%
2346
16th to endDay
10%5%144246%5%8%45%
2345
September1st to 15thDay
13%424423447%48%
2089
16th to endDay
15%5%24323345%50%
1980
October1st to 15thDay
20%423434336%43%
1956
16th to endDay
16%5%25%434446%43%
1728
November1st to 15thDay
20%7%23423425%42%
1716
16th to endDay
16%5%24424347%44%
1755
December1st to 15thDay
18%7%25%434436%39%
1599
16th to endDay
23%8%24325%34436%
1664
January1st to 15thNight
16%8%25%423345%42%
2716
16th to endNight
16%8%345%23436%40%
2912
February1st to 15thNight
16%7%35%5%35%35%6%36%
2371
16th to endNight
17%8%34424435%41%
2049
March1st to 15thNight
18%8%245%345%46%36%
2266
16th to endNight
16%8%36%42333443%
2043
April1st to 15thNight
16%8%25%5%345%46%37%
1796
16th to endNight
21%5%25%5%23345%40%
1662
May1st to 15thNight
22%6%345%235%46%35%
1620
16th to endNight
19%7%3441346%7%35%
1551
June1st to 15thNight
22%7%35%42435%7%34%
1440
16th to endNight
15%4245%245%5%8%41%
1440
July1st to 15thNight
11%435%6%345%6%9%40%
1440
16th to endNight
11%435%6%37%5%5%8%37%
1514
August1st to 15thNight
9%5%15%5%35%5%7%8%42%
1601
16th to endNight
11%41443447%7%45%
1856
September1st to 15thNight
13%42443445%6%46%
1870
16th to endNight
15%5%13313345%51%
1980
October1st to 15thNight
17%6%24323345%46%
2003
16th to endNight
18%5%25%42435%6%41%
2496
November1st to 15thNight
16%6%25%423345%43%
2472
16th to endNight
16%6%34424446%43%
2535
December1st to 15thNight
19%6%35%424446%39%
2691
16th to endNight
18%8%35%433346%38%
2912

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
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
15%8%25%32445%5%42%
1552
16th to endDay
16%6%25%42335%5%43%
1664
February1st to 15thDay
18%6%35%334447%37%
1816
16th to endDay
14%7%235%245%5%6%42%
1710
March1st to 15thDay
20%7%2442445%7%35%
1918
16th to endDay
17%7%24424345%42%
2178
April1st to 15thDay
17%7%25%5%24447%38%
2152
16th to endDay
17%42442445%7%41%
2297
May1st to 15thDay
23%7%36%423336%33%
2332
16th to endDay
16%5%245%25%35%8%40%
2571
June1st to 15thDay
24%5%2432445%8%32%
2520
16th to endDay
14%5%25%5%25%5%6%9%36%
2520
July1st to 15thDay
11%4245%36%5%6%9%38%
2520
16th to endDay
11%5%245%36%5%6%8%38%
2654
August1st to 15thDay
11%5%25%5%345%7%8%39%
2346
16th to endDay
11%4244345%6%8%44%
2345
September1st to 15thDay
13%5%23423347%48%
2089
16th to endDay
15%5%1331335%450%
1980
October1st to 15thDay
20%414433335%43%
1956
16th to endDay
16%6%25%323446%44%
1728
November1st to 15thDay
20%7%23323435%42%
1716
16th to endDay
16%5%24424346%43%
1755
December1st to 15thDay
17%7%35%434346%39%
1599
16th to endDay
23%8%24325%345%36%
1664
January1st to 15thNight
15%8%35%423345%42%
2716
16th to endNight
16%9%35%5%33336%40%
2912
February1st to 15thNight
16%8%35%5%3445%6%37%
2364
16th to endNight
17%9%34323435%41%
2049
March1st to 15thNight
18%8%245%34446%36%
2279
16th to endNight
16%7%35%5%1333443%
2042
April1st to 15thNight
16%8%35%5%3345%6%37%
1796
16th to endNight
21%425%5%24245%39%
1662
May1st to 15thNight
21%7%245%24346%35%
1620
16th to endNight
18%7%35%42345%8%35%
1551
June1st to 15thNight
22%6%35%42445%6%33%
1440
16th to endNight
15%3245%245%48%41%
1440
July1st to 15thNight
10%436%5%25%5%5%9%40%
1440
16th to endNight
12%436%6%36%5%5%9%37%
1514
August1st to 15thNight
8%5%16%6%345%6%7%43%
1601
16th to endNight
12%41343345%8%46%
1856
September1st to 15thNight
13%42443445%7%45%
1870
16th to endNight
15%5%13323345%51%
1980
October1st to 15thNight
18%5%24323345%46%
2003
16th to endNight
18%5%25%42445%6%41%
2496
November1st to 15thNight
16%6%35%424345%43%
2472
16th to endNight
17%5%34424446%43%
2535
December1st to 15thNight
18%7%35%424445%39%
2691
16th to endNight
18%8%35%324346%37%
2912

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 15thDay
41%6%11%23%12%3
1553
16th to endDay
42%412%26%12%1
1664
February1st to 15thDay
46%6%13%20%12%1
1819
16th to endDay
41%7%13%21%14%2
1710
March1st to 15thDay
48%5%12%20%9%2
1918
16th to endDay
42%5%11%20%14%5%
2178
April1st to 15thDay
43%6%11%19%16%2
2152
16th to endDay
41%5%9%23%16%4
2297
May1st to 15thDay
51%5%7%17%14%2
2332
16th to endDay
41%7%14%22%12%2
2575
June1st to 15thDay
49%7%12%19%9%1
2520
16th to endDay
40%8%14%20%14%1
2520
July1st to 15thDay
37%9%13%22%14%2
2371
16th to endDay
38%8%15%21%11%3
2487
August1st to 15thDay
37%9%14%23%12%1
2332
16th to endDay
33%8%11%26%18%1
2345
September1st to 15thDay
35%6%11%23%21%1
2089
16th to endDay
35%49%24%23%2
1980
October1st to 15thDay
41%5%9%24%15%3
1956
16th to endDay
40%5%11%24%16%2
1728
November1st to 15thDay
44%5%12%22%14%1
1716
16th to endDay
42%5%11%23%14%1
1755
December1st to 15thDay
46%415%16%16%
1599
16th to endDay
51%412%19%10%1
1664
January1st to 15thNight
45%5%12%21%14%1
2716
16th to endNight
46%5%11%19%15%2
2912
February1st to 15thNight
48%5%11%20%12%1
2372
16th to endNight
45%5%10%21%14%2
2049
March1st to 15thNight
49%5%10%19%12%3
2279
16th to endNight
44%48%18%19%4
2043
April1st to 15thNight
47%49%18%16%2
1796
16th to endNight
46%5%9%18%15%3
1662
May1st to 15thNight
49%5%10%16%14%3
1620
16th to endNight
46%7%14%19%9%2
1553
June1st to 15thNight
50%6%11%19%10%1
1440
16th to endNight
39%8%12%22%14%1
1440
July1st to 15thNight
39%7%13%24%14%
1360
16th to endNight
41%7%13%25%9%2
1418
August1st to 15thNight
36%8%14%22%14%2
1591
16th to endNight
34%7%12%25%19%
1856
September1st to 15thNight
35%6%12%25%18%1
1870
16th to endNight
34%5%8%23%26%1
1980
October1st to 15thNight
40%411%24%15%3
2003
16th to endNight
42%5%13%23%14%
2496
November1st to 15thNight
43%5%11%21%15%2
2472
16th to endNight
42%412%22%14%2
2535
December1st to 15thNight
46%6%14%20%11%
2691
16th to endNight
47%412%20%13%
2912

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 15thDay
20%34%27%14%3
90
16th to endDay
26%22%40%10%
96
February1st to 15thDay
23%28%35%10%2
290
16th to endDay
16%28%37%13%3
291
March1st to 15thDay
24%31%29%10%3
312
16th to endDay
19%31%31%9%6%1
288
April1st to 15thDay
19%24%33%15%6%
270
16th to endDay
18%21%31%22%41
270
May1st to 15thDay
29%25%23%13%5%2
288
16th to endDay
19%17%34%22%5%
331
June1st to 15thDay
23%19%29%20%6%
315
16th to endDay
13%16%39%21%8%
315
July1st to 15thDay
17%11%27%30%11%
305
16th to endDay
12%12%26%32%13%2
326
August1st to 15thDay
13%13%25%33%12%2
313
16th to endDay
13%9%25%25%21%3
332
September1st to 15thDay
16%12%26%25%16%4
315
16th to endDay
23%8%26%18%13%10%
314
October1st to 15thDay
23%14%22%23%11%5%
311
16th to endDay
17%18%35%19%8%1
288
November1st to 15thDay
22%21%28%17%8%1
280
16th to endDay
17%30%26%16%8%
285
December1st to 15thDay
24%22%34%13%5%
129
16th to endDay
22%29%38%8%1
96
January1st to 15thNight
16%31%35%15%1
780
16th to endNight
18%30%32%15%2
832
February1st to 15thNight
18%29%32%17%1
764
16th to endNight
20%30%31%14%2
683
March1st to 15thNight
22%28%31%15%2
764
16th to endNight
18%29%28%17%5%
768
April1st to 15thNight
16%30%31%17%4
718
16th to endNight
16%26%31%19%4
582
May1st to 15thNight
20%24%29%18%6%
540
16th to endNight
17%23%32%21%5%
431
June1st to 15thNight
18%20%27%26%5%
360
16th to endNight
12%19%30%31%6%
360
July1st to 15thNight
10%13%29%36%9%
340
16th to endNight
10%17%25%36%9%
376
August1st to 15thNight
9%12%26%39%12%
520
16th to endNight
13%10%19%36%17%2
710
September1st to 15thNight
12%10%27%33%13%
720
16th to endNight
18%12%21%23%19%4
717
October1st to 15thNight
20%16%25%25%10%2
720
16th to endNight
20%19%33%18%7%1
768
November1st to 15thNight
17%25%27%19%8%1
760
16th to endNight
15%31%27%17%6%
780
December1st to 15thNight
21%28%27%17%4
780
16th to endNight
22%32%29%13%1
832
Page generated at 2018-03-12 13:28:12. Version ac04

Copyright 2018 A. Danko


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