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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 2017-05-24.

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%
14601
18%
45824
7%
14569
36%
14569
18%
30136

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%
720
18%
2506
5%
716
29%
716
14%
1635
January16th to endNight
13%
768
18%
2688
3%
768
31%
768
12%
1759
February1st to 15thNight
14%
704
16%
2186
3%
701
29%
701
14%
1496
February16th to endNight
15%
631
22%
1893
6%
631
36%
631
16%
1255
March1st to 15thNight
15%
720
17%
2148
6%
720
28%
720
14%
1482
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%
417
11%
1497
6%
416
29%
416
15%
983
June1st to 15thNight
25%
330
12%
1320
8%
330
35%
330
12%
916
June16th to endNight
35%
330
9%
1320
10%
330
41%
330
18%
804
July1st to 15thNight
38%
310
12%
1240
12%
310
45%
310
20%
736
July16th to endNight
36%
348
14%
1306
11%
326
47%
326
19%
817
August1st to 15thNight
40%
476
17%
1457
11%
474
45%
474
24%
915
August16th to endNight
49%
650
18%
1700
9%
650
52%
650
30%
1101
September1st to 15thNight
44%
660
20%
1714
9%
660
49%
660
30%
1110
September16th to endNight
42%
657
25%
1815
14%
657
52%
657
31%
1110
October1st to 15thNight
29%
660
23%
1836
10%
660
40%
660
23%
1202
October16th to endNight
23%
704
16%
2288
5%
704
36%
704
19%
1467
November1st to 15thNight
23%
700
19%
2277
9%
700
37%
700
18%
1466
November16th to endNight
20%
720
19%
2340
7%
720
35%
720
16%
1511
December1st to 15thNight
18%
720
17%
2484
5%
720
33%
720
13%
1639
December16th to endNight
12%
768
18%
2688
4%
768
29%
768
12%
1797

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%
46459
Night
16%6%25%424446%40%
45994
Cloud CoverDay
16%6%2442445%7%40%
46469
Night
16%6%25%424446%40%
45999
TransparencyDay
42%6%12%21%14%2
46147
Night
43%5%11%21%14%2
45824
SeeingDay
18%20%29%20%8%2
5469
Night
17%24%28%21%6%
14601

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
16%8%243245%5%5%41%
1432
16th to endDay
16%5%25%42335%6%43%
1536
February1st to 15thDay
18%6%244345%5%7%37%
1672
16th to endDay
13%6%25%5%2445%6%42%
1580
March1st to 15thDay
20%6%2442445%7%35%
1802
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
15%4245%2445%8%41%
2470
June1st to 15thDay
24%5%24425%5%5%8%31%
2310
16th to endDay
15%5%245%35%5%7%9%35%
2310
July1st to 15thDay
10%5%25%5%35%6%6%10%37%
2310
16th to endDay
11%4245%36%5%7%8%38%
2458
August1st to 15thDay
11%4245%35%6%6%9%39%
2151
16th to endDay
10%5%14425%6%5%8%43%
2149
September1st to 15thDay
14%424423447%47%
1915
16th to endDay
13%5%24323345%51%
1815
October1st to 15thDay
20%424434336%43%
1793
16th to endDay
16%5%25%434447%42%
1584
November1st to 15thDay
20%7%2342342443%
1581
16th to endDay
16%5%24424347%43%
1620
December1st to 15thDay
18%6%25%434436%40%
1476
16th to endDay
23%8%24325%345%35%
1536
January1st to 15thNight
16%8%25%423335%41%
2506
16th to endNight
17%8%345%23335%40%
2688
February1st to 15thNight
15%8%35%5%3445%7%36%
2185
16th to endNight
16%8%34424435%42%
1893
March1st to 15thNight
18%7%245%34446%37%
2135
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
18%7%3441347%7%37%
1495
June1st to 15thNight
22%7%36%42435%7%32%
1320
16th to endNight
16%4235%245%5%8%41%
1320
July1st to 15thNight
11%435%5%35%5%7%9%39%
1320
16th to endNight
11%435%6%37%5%5%9%37%
1402
August1st to 15thNight
9%5%15%5%345%7%8%42%
1467
16th to endNight
12%5%1443447%7%44%
1700
September1st to 15thNight
13%42343445%7%45%
1714
16th to endNight
14%5%13313346%51%
1815
October1st to 15thNight
19%6%24323335%45%
1836
16th to endNight
18%6%25%42435%6%41%
2288
November1st to 15thNight
16%6%25%423345%44%
2277
16th to endNight
17%6%34424446%41%
2340
December1st to 15thNight
19%5%35%424446%39%
2484
16th to endNight
18%9%35%433346%37%
2688

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
16%8%25%32445%6%42%
1432
16th to endDay
15%6%25%5%2335%5%43%
1536
February1st to 15thDay
18%6%35%3345%47%37%
1672
16th to endDay
13%7%245%235%5%6%42%
1580
March1st to 15thDay
20%7%2442445%7%35%
1812
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
15%5%145%25%35%8%41%
2470
June1st to 15thDay
25%5%2432446%8%31%
2310
16th to endDay
14%5%25%5%25%5%6%9%36%
2310
July1st to 15thDay
10%435%5%37%5%6%10%38%
2310
16th to endDay
11%4246%36%6%7%9%37%
2458
August1st to 15thDay
11%5%15%5%345%7%9%39%
2151
16th to endDay
11%4244345%6%8%43%
2149
September1st to 15thDay
13%5%24424347%48%
1915
16th to endDay
14%5%1431425%5%51%
1815
October1st to 15thDay
21%413433335%43%
1793
16th to endDay
15%6%25%3235%46%43%
1584
November1st to 15thDay
21%7%3332333443%
1581
16th to endDay
17%5%2442445%6%42%
1620
December1st to 15thDay
17%7%35%434336%40%
1476
16th to endDay
23%9%24325%345%35%
1536
January1st to 15thNight
16%8%35%423345%41%
2506
16th to endNight
16%9%35%5%33335%40%
2688
February1st to 15thNight
15%8%35%5%3445%6%37%
2178
16th to endNight
15%9%34323435%42%
1893
March1st to 15thNight
18%8%246%34446%36%
2148
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%42346%8%36%
1495
June1st to 15thNight
22%6%35%42445%6%32%
1320
16th to endNight
16%3245%245%48%41%
1320
July1st to 15thNight
10%436%5%25%5%5%9%40%
1320
16th to endNight
12%435%6%36%5%5%9%37%
1402
August1st to 15thNight
8%5%16%6%345%6%8%42%
1467
16th to endNight
12%4135%3445%9%44%
1700
September1st to 15thNight
13%4244345%5%7%44%
1714
16th to endNight
14%5%13323345%52%
1815
October1st to 15thNight
20%5%2332334446%
1836
16th to endNight
18%5%25%42435%7%40%
2288
November1st to 15thNight
16%6%35%423345%43%
2277
16th to endNight
17%5%34424446%41%
2340
December1st to 15thNight
18%6%25%424445%40%
2484
16th to endNight
18%9%35%334346%37%
2688

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%12%23%11%3
1433
16th to endDay
41%411%27%12%1
1536
February1st to 15thDay
45%6%13%20%12%1
1675
16th to endDay
41%7%13%21%14%2
1580
March1st to 15thDay
47%6%12%20%10%3
1812
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
40%7%14%22%13%2
2474
June1st to 15thDay
50%8%13%18%8%
2310
16th to endDay
40%9%14%20%13%1
2310
July1st to 15thDay
37%8%14%22%14%1
2161
16th to endDay
38%8%16%21%11%3
2291
August1st to 15thDay
36%10%14%23%13%1
2137
16th to endDay
34%8%12%26%17%1
2149
September1st to 15thDay
35%6%12%23%21%
1915
16th to endDay
33%49%25%23%2
1815
October1st to 15thDay
42%5%9%23%16%2
1793
16th to endDay
40%6%11%23%15%2
1584
November1st to 15thDay
44%5%11%22%14%1
1581
16th to endDay
43%5%12%23%14%1
1620
December1st to 15thDay
46%415%16%16%
1476
16th to endDay
52%413%18%9%
1536
January1st to 15thNight
46%412%21%13%1
2506
16th to endNight
46%5%10%20%15%2
2688
February1st to 15thNight
47%5%12%20%11%1
2186
16th to endNight
43%5%10%22%15%2
1893
March1st to 15thNight
48%5%10%19%13%3
2148
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
44%8%14%20%9%2
1497
June1st to 15thNight
52%7%12%17%9%
1320
16th to endNight
39%8%12%23%14%1
1320
July1st to 15thNight
38%7%14%25%14%
1240
16th to endNight
40%7%14%25%8%2
1306
August1st to 15thNight
37%8%14%22%14%2
1457
16th to endNight
35%7%13%25%17%
1700
September1st to 15thNight
36%6%13%24%18%
1714
16th to endNight
33%5%8%24%26%2
1815
October1st to 15thNight
42%411%22%15%3
1836
16th to endNight
42%5%13%22%14%
2288
November1st to 15thNight
43%5%11%21%15%2
2277
16th to endNight
44%412%21%14%2
2340
December1st to 15thNight
45%6%14%21%11%
2484
16th to endNight
48%413%20%11%
2688

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
21%30%28%16%3
60
16th to endDay
23%25%40%10%
64
February1st to 15thDay
23%27%35%11%2
236
16th to endDay
13%30%35%15%3
239
March1st to 15thDay
21%32%30%10%4
270
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
16%18%34%23%6%
302
June1st to 15thDay
23%22%26%18%7%1
255
16th to endDay
14%16%37%21%9%1
255
July1st to 15thDay
16%13%22%32%14%1
245
16th to endDay
10%14%27%34%11%2
270
August1st to 15thDay
13%12%25%33%11%2
253
16th to endDay
13%11%19%26%23%4
268
September1st to 15thDay
15%12%21%27%18%5%
255
16th to endDay
20%8%28%20%10%11%
254
October1st to 15thDay
25%15%21%20%11%5%
252
16th to endDay
17%17%37%18%7%
240
November1st to 15thDay
23%22%26%17%7%2
235
16th to endDay
18%30%27%14%8%
240
December1st to 15thDay
21%28%32%14%3
96
16th to endDay
25%23%39%10%1
64
January1st to 15thNight
16%31%34%15%1
720
16th to endNight
19%31%30%15%2
768
February1st to 15thNight
17%30%31%18%1
704
16th to endNight
19%32%30%14%3
631
March1st to 15thNight
21%29%30%16%2
720
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
16%23%32%21%5%
417
June1st to 15thNight
18%22%27%25%5%
330
16th to endNight
13%20%26%32%6%
330
July1st to 15thNight
9%14%29%35%10%
310
16th to endNight
10%18%25%35%9%
348
August1st to 15thNight
9%13%26%38%12%
476
16th to endNight
13%11%19%35%17%2
650
September1st to 15thNight
13%11%25%34%14%
660
16th to endNight
17%12%22%24%19%3
657
October1st to 15thNight
21%17%25%24%9%2
660
16th to endNight
20%20%33%19%6%
704
November1st to 15thNight
17%26%26%19%8%1
700
16th to endNight
16%32%26%17%6%
720
December1st to 15thNight
20%29%27%16%4
720
16th to endNight
22%33%28%13%1
768
Page generated at 2017-05-25 13:27:30. Version ac04

Copyright 2017 A. Danko


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