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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 2017-09-10.

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%
14877
18%
46742
7%
14845
36%
14845
18%
30698

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%
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
42%
700
19%
1814
9%
700
48%
700
31%
1169
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%
47895
Night
16%6%25%424446%40%
46912
Cloud CoverDay
16%6%2442445%7%40%
47905
Night
16%6%25%424446%40%
46917
TransparencyDay
42%6%12%21%14%2
47583
Night
43%5%11%21%14%2
46742
SeeingDay
19%19%30%20%8%2
5897
Night
17%24%28%21%6%
14877

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
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
14%423333447%47%
2033
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
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%
1814
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
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%
2033
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%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%41443445%7%45%
1814
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
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%24%21%
2033
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
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%
1814
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
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%25%25%15%4
294
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
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
13%11%27%33%14%
700
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 2017-09-11 19:35:28. Version ac04

Copyright 2017 A. Danko


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