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 2005-11-05 to 2020-06-03.

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%
18391
18%
57772
7%
18359
37%
18359
18%
37862

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%
900
19%
3136
5%
896
30%
896
15%
2024
January16th to endNight
15%
960
18%
3360
3%
960
33%
960
13%
2182
February1st to 15thNight
14%
884
17%
2746
4%
881
30%
881
14%
1897
February16th to endNight
15%
791
23%
2373
6%
791
36%
791
16%
1579
March1st to 15thNight
15%
900
18%
2685
6%
900
29%
900
14%
1859
March16th to endNight
18%
960
22%
2554
5%
960
32%
960
19%
1753
April1st to 15thNight
17%
898
18%
2246
4%
898
26%
898
18%
1599
April16th to endNight
19%
719
18%
2055
3%
719
30%
719
18%
1408
May1st to 15thNight
20%
675
14%
2025
6%
675
27%
675
15%
1432
May16th to endNight
24%
542
12%
1952
7%
541
31%
541
15%
1285
June1st to 15thNight
27%
425
12%
1701
8%
425
38%
425
14%
1139
June16th to endNight
35%
420
11%
1680
10%
420
43%
420
18%
1021
July1st to 15thNight
40%
400
14%
1600
14%
400
48%
400
22%
920
July16th to endNight
39%
440
15%
1674
14%
418
49%
418
21%
1023
August1st to 15thNight
42%
596
16%
1827
11%
594
47%
594
25%
1139
August16th to endNight
51%
828
20%
2166
10%
828
55%
828
32%
1386
September1st to 15thNight
42%
840
18%
2182
9%
840
47%
840
30%
1423
September16th to endNight
42%
837
25%
2310
15%
837
51%
837
31%
1425
October1st to 15thNight
30%
840
22%
2336
10%
840
40%
840
23%
1544
October16th to endNight
22%
896
17%
2912
6%
896
36%
896
20%
1862
November1st to 15thNight
22%
880
20%
2862
8%
880
37%
880
18%
1841
November16th to endNight
19%
900
21%
2925
7%
900
36%
900
16%
1863
December1st to 15thNight
18%
900
19%
3105
5%
900
34%
900
14%
2042
December16th to endNight
12%
960
19%
3360
4%
960
30%
960
13%
2216

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%
58650
Night
16%6%25%424446%40%
57942
Cloud CoverDay
16%6%2442445%7%40%
58660
Night
16%6%25%424446%41%
57947
TransparencyDay
42%6%12%21%14%2
58338
Night
43%5%11%21%15%2
57772
SeeingDay
20%19%31%19%7%1
9332
Night
18%22%29%22%6%
18391

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%7%25%4245%45%42%
1792
16th to endDay
15%5%25%42335%6%44%
1920
February1st to 15thDay
19%6%2443445%7%36%
2102
16th to endDay
14%6%25%5%2445%6%41%
1980
March1st to 15thDay
19%6%245%25%46%8%35%
2255
16th to endDay
17%7%15%423445%42%
2722
April1st to 15thDay
18%8%35%42445%6%35%
2692
16th to endDay
19%5%2442435%7%39%
2839
May1st to 15thDay
22%7%26%5%2345%6%33%
2917
16th to endDay
15%5%245%2445%7%41%
3231
June1st to 15thDay
22%414425%45%8%33%
2968
16th to endDay
15%5%25%5%35%5%7%8%35%
2940
July1st to 15thDay
10%424435%6%6%10%41%
2940
16th to endDay
11%5%245%36%5%6%8%39%
3102
August1st to 15thDay
11%5%245%35%6%6%9%39%
2691
16th to endDay
9%5%144246%5%8%44%
2738
September1st to 15thDay
15%5%23433447%46%
2437
16th to endDay
14%5%23323345%51%
2310
October1st to 15thDay
22%5%23424336%40%
2282
16th to endDay
16%6%25%433446%43%
2016
November1st to 15thDay
20%7%2342342443%
1986
16th to endDay
17%6%24424346%43%
2025
December1st to 15thDay
18%6%25%334336%40%
1845
16th to endDay
22%7%24325%345%37%
1920
January1st to 15thNight
16%8%25%5%23345%41%
3136
16th to endNight
16%8%345%33446%40%
3360
February1st to 15thNight
16%7%35%5%3445%6%36%
2745
16th to endNight
17%8%24424446%41%
2373
March1st to 15thNight
17%8%25%5%34446%37%
2672
16th to endNight
16%8%26%5%2333442%
2554
April1st to 15thNight
17%8%25%5%2345%6%37%
2246
16th to endNight
21%6%25%423345%40%
2055
May1st to 15thNight
22%7%35%5%24446%33%
2025
16th to endNight
18%7%35%41446%8%36%
1950
June1st to 15thNight
20%7%36%42435%7%35%
1701
16th to endNight
15%4245%245%5%9%40%
1680
July1st to 15thNight
10%425%5%3446%9%42%
1680
16th to endNight
11%5%25%6%36%5%5%9%38%
1770
August1st to 15thNight
8%5%15%5%35%5%8%8%42%
1837
16th to endNight
11%41443447%7%46%
2166
September1st to 15thNight
13%42443445%6%45%
2182
16th to endNight
13%5%23413445%51%
2310
October1st to 15thNight
19%6%24423335%43%
2336
16th to endNight
18%6%25%42435%5%41%
2912
November1st to 15thNight
17%6%24423445%43%
2862
16th to endNight
17%6%34423445%43%
2925
December1st to 15thNight
18%6%25%424446%39%
3105
16th to endNight
17%8%35%433345%39%
3360

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%
1792
16th to endDay
15%6%25%42335%6%44%
1920
February1st to 15thDay
19%6%25%335%447%36%
2102
16th to endDay
14%7%235%245%5%6%42%
1980
March1st to 15thDay
18%7%2442445%7%36%
2265
16th to endDay
17%7%24423345%42%
2722
April1st to 15thDay
18%7%25%5%245%47%36%
2692
16th to endDay
19%5%2442435%7%40%
2839
May1st to 15thDay
22%7%26%424446%33%
2917
16th to endDay
15%5%25%5%2435%7%41%
3231
June1st to 15thDay
22%42432446%8%34%
2968
16th to endDay
14%5%25%5%25%5%6%9%36%
2940
July1st to 15thDay
10%4245%36%5%5%9%42%
2940
16th to endDay
11%5%245%36%5%6%9%39%
3102
August1st to 15thDay
11%5%25%6%346%7%8%39%
2691
16th to endDay
10%4244345%6%8%44%
2738
September1st to 15thDay
14%5%23424346%47%
2437
16th to endDay
15%41331335%451%
2310
October1st to 15thDay
23%5%24423435%40%
2282
16th to endDay
16%6%25%323445%43%
2016
November1st to 15thDay
20%7%3332333443%
1986
16th to endDay
17%6%24324346%42%
2025
December1st to 15thDay
17%7%35%424346%40%
1845
16th to endDay
22%8%24325%345%37%
1920
January1st to 15thNight
15%8%35%423345%41%
3136
16th to endNight
15%8%25%5%33346%40%
3360
February1st to 15thNight
16%8%35%5%3445%6%37%
2738
16th to endNight
17%8%24324445%41%
2373
March1st to 15thNight
17%8%25%5%34445%36%
2685
16th to endNight
16%8%36%5%2333442%
2553
April1st to 15thNight
17%7%25%5%3345%6%37%
2246
16th to endNight
21%5%25%424245%40%
2055
May1st to 15thNight
21%8%35%5%2435%6%33%
2025
16th to endNight
18%7%35%42346%8%35%
1950
June1st to 15thNight
20%6%35%425%45%6%35%
1701
16th to endNight
15%3245%246%5%9%40%
1680
July1st to 15thNight
9%426%5%245%5%9%43%
1680
16th to endNight
12%435%6%36%5%5%9%38%
1770
August1st to 15thNight
8%5%16%6%345%7%8%43%
1837
16th to endNight
11%41443445%8%47%
2166
September1st to 15thNight
14%42443445%7%45%
2182
16th to endNight
13%5%14323345%51%
2310
October1st to 15thNight
20%5%24323345%44%
2336
16th to endNight
18%6%24424346%41%
2912
November1st to 15thNight
17%6%25%424345%43%
2862
16th to endNight
17%6%34423446%43%
2925
December1st to 15thNight
18%6%25%42445%5%39%
3105
16th to endNight
17%8%35%434346%38%
3360

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
42%6%11%23%13%2
1793
16th to endDay
40%5%12%25%13%2
1920
February1st to 15thDay
47%6%13%19%11%1
2105
16th to endDay
41%6%12%21%15%2
1980
March1st to 15thDay
46%6%12%20%11%2
2265
16th to endDay
43%5%10%19%16%5%
2722
April1st to 15thDay
45%7%10%18%14%3
2692
16th to endDay
43%5%8%22%16%3
2839
May1st to 15thDay
50%5%8%17%14%2
2917
16th to endDay
40%7%13%22%13%1
3235
June1st to 15thDay
46%7%13%19%11%1
2968
16th to endDay
41%8%14%20%13%1
2940
July1st to 15thDay
34%8%13%24%16%2
2791
16th to endDay
37%8%15%21%12%3
2935
August1st to 15thDay
37%9%15%23%12%1
2677
16th to endDay
33%8%12%25%18%1
2738
September1st to 15thDay
37%6%11%24%19%1
2437
16th to endDay
34%49%25%23%1
2310
October1st to 15thDay
44%5%9%23%13%2
2282
16th to endDay
41%5%10%23%16%2
2016
November1st to 15thDay
44%412%22%15%1
1986
16th to endDay
43%5%11%23%14%2
2025
December1st to 15thDay
45%414%16%17%
1845
16th to endDay
50%412%19%11%1
1920
January1st to 15thNight
45%411%22%14%1
3136
16th to endNight
45%5%11%19%15%2
3360
February1st to 15thNight
48%5%11%20%12%1
2746
16th to endNight
44%5%9%22%15%3
2373
March1st to 15thNight
48%5%9%19%13%3
2685
16th to endNight
45%38%16%20%4
2554
April1st to 15thNight
47%49%18%16%2
2246
16th to endNight
46%49%18%16%4
2055
May1st to 15thNight
50%5%10%15%14%3
2025
16th to endNight
46%7%14%20%10%1
1952
June1st to 15thNight
49%6%11%19%12%
1701
16th to endNight
39%8%12%23%14%1
1680
July1st to 15thNight
36%6%13%25%17%
1600
16th to endNight
39%7%14%26%10%1
1674
August1st to 15thNight
35%8%15%23%14%2
1827
16th to endNight
34%7%12%24%20%
2166
September1st to 15thNight
37%6%12%24%17%1
2182
16th to endNight
34%5%7%24%25%2
2310
October1st to 15thNight
43%411%23%14%3
2336
16th to endNight
43%412%24%14%
2912
November1st to 15thNight
43%5%11%21%15%2
2862
16th to endNight
43%412%23%14%2
2925
December1st to 15thNight
45%6%13%21%12%
3105
16th to endNight
46%5%11%21%13%1
3360

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
17%36%33%10%2
150
16th to endDay
20%25%40%11%2
160
February1st to 15thDay
26%28%33%10%1
396
16th to endDay
17%28%38%13%2
399
March1st to 15thDay
21%28%34%11%3
450
16th to endDay
21%29%33%9%5%
480
April1st to 15thDay
23%26%31%12%4
450
16th to endDay
23%22%31%17%31
439
May1st to 15thDay
27%25%29%11%41
468
16th to endDay
17%17%36%22%5%
523
June1st to 15thDay
20%18%31%22%6%
443
16th to endDay
16%13%40%22%7%
435
July1st to 15thDay
14%8%28%32%16%1
425
16th to endDay
12%9%27%34%13%2
454
August1st to 15thDay
13%11%26%33%12%1
418
16th to endDay
13%7%30%26%18%3
460
September1st to 15thDay
19%11%26%24%15%3
435
16th to endDay
20%9%29%19%12%8%
434
October1st to 15thDay
27%12%25%21%9%3
429
16th to endDay
18%21%34%18%6%
384
November1st to 15thDay
21%23%29%17%6%1
370
16th to endDay
21%29%26%15%6%
375
December1st to 15thDay
25%21%33%14%4
195
16th to endDay
21%28%40%8%1
160
January1st to 15thNight
17%30%35%14%2
900
16th to endNight
18%29%31%17%3
960
February1st to 15thNight
20%27%33%17%1
884
16th to endNight
20%30%31%14%2
791
March1st to 15thNight
22%27%30%16%2
900
16th to endNight
19%27%29%17%5%
960
April1st to 15thNight
18%28%32%17%3
898
16th to endNight
18%25%32%19%4
719
May1st to 15thNight
21%22%30%18%5%
675
16th to endNight
18%20%33%22%5%
542
June1st to 15thNight
17%18%29%27%5%
425
16th to endNight
13%17%30%32%6%
420
July1st to 15thNight
10%12%29%37%9%1
400
16th to endNight
11%14%25%37%9%
440
August1st to 15thNight
8%11%28%39%12%
596
16th to endNight
13%9%20%37%17%2
828
September1st to 15thNight
14%9%26%34%14%1
840
16th to endNight
17%10%22%25%18%3
837
October1st to 15thNight
21%15%25%24%10%1
840
16th to endNight
21%18%34%18%6%
896
November1st to 15thNight
18%25%28%18%7%1
880
16th to endNight
17%31%27%17%5%
900
December1st to 15thNight
21%27%28%17%5%
900
16th to endNight
21%30%30%15%1
960
Page generated at 2020-06-04 03:18:58. Version ac04

Copyright 2020 A. Danko


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