Westport
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Westport clear sky chart from 2005-11-05 to 2017-09-20.

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
28%
14719
14%
46892
8%
14690
35%
14690
16%
33248

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
14%
720
7%
2506
2%
716
20%
716
7%
2016
January16th to endNight
18%
768
11%
2604
5%
768
26%
768
11%
1969
February1st to 15thNight
15%
702
9%
2287
3%
701
23%
701
10%
1778
February16th to endNight
18%
631
14%
1958
6%
631
29%
631
12%
1458
March1st to 15thNight
18%
720
14%
2076
6%
720
27%
720
16%
1530
March16th to endNight
30%
768
20%
2050
9%
768
39%
768
22%
1429
April1st to 15thNight
21%
718
16%
1796
5%
718
26%
718
19%
1310
April16th to endNight
27%
666
17%
1746
7%
666
34%
666
22%
1222
May1st to 15thNight
34%
360
17%
1440
14%
360
44%
360
17%
945
May16th to endNight
33%
374
14%
1497
11%
374
45%
374
19%
915
June1st to 15thNight
33%
360
10%
1440
8%
360
44%
360
16%
960
June16th to endNight
43%
360
9%
1440
12%
360
48%
360
18%
918
July1st to 15thNight
49%
340
15%
1360
20%
340
55%
340
27%
753
July16th to endNight
55%
376
15%
1418
17%
354
60%
354
28%
800
August1st to 15thNight
48%
485
15%
1556
10%
483
53%
483
26%
955
August16th to endNight
51%
702
22%
1848
11%
702
57%
702
37%
1126
September1st to 15thNight
48%
720
19%
1800
9%
720
52%
720
32%
1198
September16th to endNight
46%
677
23%
1851
14%
677
52%
677
31%
1186
October1st to 15thNight
31%
660
17%
1980
11%
660
40%
660
19%
1393
October16th to endNight
22%
704
10%
2175
4%
704
29%
704
12%
1668
November1st to 15thNight
26%
700
18%
2336
13%
700
35%
700
16%
1613
November16th to endNight
19%
720
12%
2520
6%
720
28%
720
10%
1928
December1st to 15thNight
14%
720
7%
2520
2%
720
19%
720
7%
2009
December16th to endNight
14%
768
9%
2688
4%
768
20%
768
7%
2169

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
17%8%36%5%3445%6%32%
47957
Night
17%9%36%5%34446%33%
47062
Cloud CoverDay
17%8%36%5%34446%33%
47967
Night
17%9%36%5%34446%33%
47067
TransparencyDay
50%5%11%20%10%1
47645
Night
51%5%11%19%10%1
46892
SeeingDay
23%12%28%20%10%4
5483
Night
23%12%29%23%9%
14722

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
21%14%5%6%6%34445%23%
1432
16th to endDay
14%14%6%10%9%4444423%
1617
February1st to 15thDay
14%13%5%12%7%35%445%22%
1575
16th to endDay
19%14%48%6%35%34326%
1514
March1st to 15thDay
17%11%5%9%8%45%32326%
1873
16th to endDay
16%9%36%333335%40%
2173
April1st to 15thDay
21%11%35%423345%34%
2152
16th to endDay
19%8%35%424345%37%
2223
May1st to 15thDay
19%7%36%5%33447%33%
2511
16th to endDay
14%6%25%6%35%45%7%37%
2618
June1st to 15thDay
23%6%35%42445%7%31%
2520
16th to endDay
17%5%36%6%25%5%5%8%33%
2520
July1st to 15thDay
12%5%35%445%5%6%9%37%
2520
16th to endDay
12%5%25%5%36%6%7%8%35%
2654
August1st to 15thDay
12%6%27%5%35%6%7%8%36%
2394
16th to endDay
12%5%25%5%345%6%9%40%
2342
September1st to 15thDay
16%5%25%42435%8%41%
2160
16th to endDay
17%5%24424345%45%
1875
October1st to 15thDay
21%7%36%5%33346%34%
1650
16th to endDay
27%8%47%6%24445%24%
1697
November1st to 15thDay
14%8%47%6%3445%7%33%
1521
16th to endDay
21%12%47%5%24445%25%
1440
December1st to 15thDay
23%15%48%6%25%44420%
1440
16th to endDay
24%14%5%8%6%2443420%
1536
January1st to 15thNight
19%14%5%10%7%35%43419%
2506
16th to endNight
14%14%5%9%7%35%445%25%
2604
February1st to 15thNight
14%14%6%8%6%35%45%5%23%
2285
16th to endNight
15%13%47%6%34445%30%
1958
March1st to 15thNight
19%10%46%6%34335%31%
2064
16th to endNight
15%8%46%42334442%
2050
April1st to 15thNight
23%7%45%42334436%
1796
16th to endNight
18%7%25%424446%38%
1746
May1st to 15thNight
20%8%25%4235%5%5%37%
1440
16th to endNight
14%435%5%2445%8%40%
1495
June1st to 15thNight
17%7%36%5%345%5%7%33%
1440
16th to endNight
15%5%25%5%35%5%6%8%36%
1440
July1st to 15thNight
9%424425%35%8%47%
1440
16th to endNight
8%4234245%6%7%49%
1514
August1st to 15thNight
10%425%4346%5%7%43%
1566
16th to endNight
8%41442345%8%50%
1848
September1st to 15thNight
14%5%24423446%46%
1800
16th to endNight
16%5%23313346%48%
1851
October1st to 15thNight
21%6%25%5%34446%35%
1980
16th to endNight
25%9%38%6%24445%25%
2175
November1st to 15thNight
17%9%45%4345%46%33%
2336
16th to endNight
22%11%37%6%35%345%26%
2520
December1st to 15thNight
20%15%410%6%2334422%
2520
16th to endNight
23%14%5%8%6%34435%20%
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
21%15%47%7%34435%24%
1432
16th to endDay
14%14%411%9%4443423%
1617
February1st to 15thDay
14%13%5%11%7%45%445%22%
1575
16th to endDay
19%14%48%6%45%34325%
1514
March1st to 15thDay
18%11%5%9%8%45%43426%
1884
16th to endDay
16%9%35%5%3333440%
2172
April1st to 15thDay
21%10%35%423335%34%
2152
16th to endDay
19%7%36%434335%37%
2223
May1st to 15thDay
19%7%36%5%24447%33%
2511
16th to endDay
14%5%36%6%3446%7%37%
2618
June1st to 15thDay
22%6%35%425%45%7%31%
2520
16th to endDay
17%5%25%5%35%5%5%8%33%
2520
July1st to 15thDay
11%6%25%436%6%6%9%38%
2520
16th to endDay
12%5%25%6%35%6%7%8%35%
2654
August1st to 15thDay
12%6%26%5%35%6%7%8%36%
2394
16th to endDay
12%5%25%5%245%5%9%41%
2342
September1st to 15thDay
16%5%2442445%7%42%
2160
16th to endDay
16%5%25%423346%45%
1875
October1st to 15thDay
20%8%25%434346%35%
1650
16th to endDay
27%8%37%6%24346%24%
1697
November1st to 15thDay
13%9%46%6%35%447%33%
1521
16th to endDay
22%12%48%5%245%35%26%
1440
December1st to 15thDay
23%14%5%8%6%25%345%20%
1440
16th to endDay
24%14%6%8%6%3343420%
1536
January1st to 15thNight
20%13%6%10%7%36%43420%
2506
16th to endNight
13%13%5%10%7%345%46%25%
2604
February1st to 15thNight
14%14%5%8%6%36%5%45%24%
2278
16th to endNight
16%11%48%7%44346%29%
1958
March1st to 15thNight
19%11%46%6%33345%31%
2076
16th to endNight
16%7%36%423336%41%
2050
April1st to 15thNight
23%8%35%33343436%
1796
16th to endNight
19%7%26%424437%38%
1746
May1st to 15thNight
20%7%25%4245%45%36%
1440
16th to endNight
13%436%5%3445%8%40%
1495
June1st to 15thNight
18%6%36%6%235%5%7%34%
1440
16th to endNight
15%6%15%435%5%6%8%36%
1440
July1st to 15thNight
9%4245%25%449%47%
1440
16th to endNight
8%4144245%6%9%48%
1514
August1st to 15thNight
10%5%25%5%345%6%8%42%
1566
16th to endNight
8%4145%2346%8%50%
1848
September1st to 15thNight
13%5%25%433446%47%
1800
16th to endNight
16%5%23323446%48%
1851
October1st to 15thNight
20%6%345%34446%34%
1980
16th to endNight
25%10%37%6%2444426%
2175
November1st to 15thNight
17%9%35%4245%46%33%
2336
16th to endNight
22%11%37%6%35%335%26%
2520
December1st to 15thNight
20%15%49%7%2334422%
2520
16th to endNight
24%14%5%8%6%34445%19%
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
64%49%13%6%
1433
16th to endDay
64%5%8%11%8%
1617
February1st to 15thDay
63%5%9%16%5%
1577
16th to endDay
62%49%12%10%1
1514
March1st to 15thDay
63%39%14%7%1
1884
16th to endDay
47%38%19%15%5%
2173
April1st to 15thDay
53%37%18%13%2
2152
16th to endDay
49%47%20%15%3
2223
May1st to 15thDay
50%5%9%18%14%2
2511
16th to endDay
44%7%12%21%11%2
2622
June1st to 15thDay
51%5%10%20%10%
2520
16th to endDay
46%7%12%20%11%
2520
July1st to 15thDay
38%7%13%28%11%
2371
16th to endDay
43%7%15%23%9%
2487
August1st to 15thDay
42%7%15%22%10%
2380
16th to endDay
37%6%15%29%11%
2342
September1st to 15thDay
40%6%12%26%12%
2160
16th to endDay
39%6%10%30%12%
1875
October1st to 15thDay
49%412%22%10%
1650
16th to endDay
61%5%9%17%5%
1697
November1st to 15thDay
49%7%13%18%10%
1521
16th to endDay
60%6%10%14%8%1
1440
December1st to 15thDay
65%7%12%11%3
1440
16th to endDay
69%5%9%10%5%
1536
January1st to 15thNight
68%5%9%11%5%
2506
16th to endNight
60%5%9%15%8%
2604
February1st to 15thNight
61%6%10%15%5%
2287
16th to endNight
57%49%17%10%
1958
March1st to 15thNight
56%310%20%8%
2076
16th to endNight
46%48%19%17%3
2050
April1st to 15thNight
50%5%6%20%15%1
1796
16th to endNight
45%5%8%19%17%3
1746
May1st to 15thNight
47%5%8%18%14%4
1440
16th to endNight
42%6%13%19%15%2
1497
June1st to 15thNight
48%6%12%20%11%
1440
16th to endNight
43%8%12%20%14%
1440
July1st to 15thNight
32%7%11%28%18%1
1360
16th to endNight
31%8%15%28%14%1
1418
August1st to 15thNight
37%7%16%25%13%
1556
16th to endNight
30%6%14%32%14%
1848
September1st to 15thNight
37%6%11%30%12%1
1800
16th to endNight
36%5%10%33%12%
1851
October1st to 15thNight
50%5%10%21%12%
1980
16th to endNight
60%412%14%6%
2175
November1st to 15thNight
50%5%13%17%11%1
2336
16th to endNight
61%5%9%17%5%1
2520
December1st to 15thNight
65%5%11%13%3
2520
16th to endNight
67%5%9%11%5%
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
28%25%25%13%8%
60
16th to endDay
38%15%27%14%3
77
February1st to 15thDay
37%24%32%5%
90
16th to endDay
34%18%28%14%4
173
March1st to 15thDay
28%17%28%20%5%
270
16th to endDay
26%15%31%14%9%1
288
April1st to 15thDay
28%21%27%12%10%
270
16th to endDay
22%17%31%14%8%5%
270
May1st to 15thDay
28%12%26%20%7%3
288
16th to endDay
21%13%33%20%8%3
331
June1st to 15thDay
25%10%28%23%9%2
315
16th to endDay
20%10%29%25%11%3
315
July1st to 15thDay
14%8%27%29%14%5%
305
16th to endDay
14%6%31%27%14%6%
326
August1st to 15thDay
14%5%25%34%13%6%
313
16th to endDay
15%7%25%25%15%12%
332
September1st to 15thDay
14%9%26%20%14%13%
315
16th to endDay
20%8%26%19%16%10%
270
October1st to 15thDay
25%12%27%20%10%3
255
16th to endDay
31%16%28%18%4
261
November1st to 15thDay
17%12%34%25%9%1
175
16th to endDay
31%6%35%15%11%
60
December1st to 15thDay
35%15%33%6%10%
60
16th to endDay
46%28%25%
64
January1st to 15thNight
30%17%30%17%4
720
16th to endNight
22%14%35%22%5%
768
February1st to 15thNight
22%18%37%17%3
704
16th to endNight
24%16%35%18%5%
631
March1st to 15thNight
29%15%30%21%2
720
16th to endNight
21%17%27%23%9%
768
April1st to 15thNight
27%16%30%20%4
718
16th to endNight
24%15%28%21%9%
666
May1st to 15thNight
23%9%26%29%9%1
360
16th to endNight
14%7%39%26%11%
375
June1st to 15thNight
19%10%29%28%11%1
360
16th to endNight
17%328%35%15%
360
July1st to 15thNight
11%427%39%16%
340
16th to endNight
7%328%42%16%1
376
August1st to 15thNight
14%425%32%21%1
485
16th to endNight
11%425%34%21%2
702
September1st to 15thNight
16%5%25%30%20%2
720
16th to endNight
18%7%23%24%21%3
677
October1st to 15thNight
25%11%26%21%13%1
660
16th to endNight
32%12%25%23%5%
704
November1st to 15thNight
26%11%30%20%10%
700
16th to endNight
31%16%26%16%8%
720
December1st to 15thNight
30%20%29%16%3
720
16th to endNight
33%15%30%17%11
768
Page generated at 2017-09-21 07:18:17. Version ac04

Copyright 2017 A. Danko


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