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 2017-12-02.

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
43%
16021
19%
47708
13%
15977
48%
15977
24%
31757

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
21%
900
13%
2506
5%
895
28%
895
15%
1942
January16th to endNight
22%
846
14%
2574
7%
846
28%
846
15%
1901
February1st to 15thNight
22%
704
13%
2121
6%
701
29%
701
17%
1532
February16th to endNight
25%
631
14%
1893
7%
631
31%
631
16%
1383
March1st to 15thNight
27%
720
15%
2127
9%
720
32%
720
19%
1487
March16th to endNight
33%
747
16%
2091
7%
747
40%
747
21%
1479
April1st to 15thNight
35%
539
18%
1797
13%
539
44%
539
24%
1165
April16th to endNight
41%
540
20%
1685
13%
540
49%
540
27%
1081
May1st to 15thNight
38%
540
15%
1620
9%
540
43%
540
20%
1148
May16th to endNight
57%
561
18%
1684
15%
561
59%
561
27%
1088
June1st to 15thNight
63%
540
19%
1471
16%
540
65%
540
37%
890
June16th to endNight
68%
540
19%
1440
14%
540
72%
540
39%
869
July1st to 15thNight
67%
510
17%
1360
14%
510
68%
510
35%
870
July16th to endNight
73%
564
22%
1515
19%
531
76%
531
39%
891
August1st to 15thNight
72%
539
23%
1609
21%
536
74%
536
38%
930
August16th to endNight
78%
573
24%
1896
25%
573
79%
573
37%
1024
September1st to 15thNight
68%
627
27%
1887
24%
627
71%
627
36%
1081
September16th to endNight
60%
717
28%
1980
21%
717
63%
717
37%
1195
October1st to 15thNight
54%
720
29%
1983
21%
720
60%
720
36%
1194
October16th to endNight
38%
768
23%
2443
18%
768
47%
768
26%
1555
November1st to 15thNight
41%
760
24%
2471
19%
760
50%
760
27%
1500
November16th to endNight
34%
780
20%
2535
16%
780
41%
780
21%
1691
December1st to 15thNight
25%
728
12%
2365
8%
728
30%
728
14%
1782
December16th to endNight
20%
927
12%
2655
5%
927
26%
927
13%
2079

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
14%7%36%5%2445%7%38%
48731
Night
14%7%35%5%2445%7%39%
47887
Cloud CoverDay
15%7%35%5%2445%7%38%
48740
Night
15%7%35%5%2445%7%39%
47892
TransparencyDay
44%6%11%21%13%2
48428
Night
43%5%11%21%14%2
47708
SeeingDay
17%5%22%24%19%10%
6007
Night
17%6%26%30%16%2
16023

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%10%49%6%35%23428%
1432
16th to endDay
13%13%6%7%6%25%446%29%
1648
February1st to 15thDay
16%12%48%5%33446%30%
1741
16th to endDay
15%11%48%7%33445%29%
1580
March1st to 15thDay
23%8%35%424445%32%
1823
16th to endDay
17%10%46%5%25%446%31%
2142
April1st to 15thDay
17%7%35%5%2435%6%38%
2151
16th to endDay
18%6%25%5%2435%7%36%
2274
May1st to 15thDay
22%7%35%5%34447%31%
2331
16th to endDay
12%5%26%5%35%5%6%8%37%
2431
June1st to 15thDay
11%4245%25%5%5%7%44%
2486
16th to endDay
10%4245%35%45%8%43%
2520
July1st to 15thDay
11%5%25%5%35%5%6%9%38%
2520
16th to endDay
7%4245%25%6%8%10%41%
2545
August1st to 15thDay
8%415%5%35%6%6%9%42%
2340
16th to endDay
6%425%5%35%6%6%9%44%
2303
September1st to 15thDay
13%5%25%425%447%43%
2059
16th to endDay
13%424425%35%6%48%
1980
October1st to 15thDay
13%425%5%34346%44%
1976
16th to endDay
17%6%35%42435%6%41%
1779
November1st to 15thDay
12%5%35%5%23346%46%
1717
16th to endDay
18%9%36%423346%38%
1755
December1st to 15thDay
25%11%48%41433328%
1632
16th to endDay
23%12%5%6%42435%6%25%
1566
January1st to 15thNight
19%11%47%7%34445%27%
2506
16th to endNight
17%13%5%7%5%34335%29%
2574
February1st to 15thNight
19%9%5%7%5%3335%6%31%
2119
16th to endNight
16%11%47%5%25%346%30%
1893
March1st to 15thNight
18%8%36%5%3435%6%35%
2114
16th to endNight
17%7%35%43445%8%33%
2091
April1st to 15thNight
15%6%36%4235%46%40%
1797
16th to endNight
14%5%3443435%8%41%
1685
May1st to 15thNight
18%7%46%43445%7%34%
1620
16th to endNight
9%425%6%35%5%6%8%40%
1682
June1st to 15thNight
7%3244345%5%9%48%
1471
16th to endNight
7%2133346%7%11%48%
1440
July1st to 15thNight
9%4244345%5%9%48%
1440
16th to endNight
5%425%33446%8%50%
1620
August1st to 15thNight
5%3234335%6%9%51%
1620
16th to endNight
43135%25%5%7%10%51%
1896
September1st to 15thNight
12%31331345%7%52%
1887
16th to endNight
11%425%323346%51%
1980
October1st to 15thNight
10%325%413446%51%
1983
16th to endNight
17%6%25%5%23345%43%
2443
November1st to 15thNight
13%5%35%423346%46%
2471
16th to endNight
19%8%35%5%23345%38%
2535
December1st to 15thNight
21%12%48%5%33345%27%
2365
16th to endNight
24%12%46%5%23336%25%
2655

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%10%48%6%34335%28%
1432
16th to endDay
13%13%6%8%6%25%346%29%
1648
February1st to 15thDay
17%11%5%8%5%32446%30%
1741
16th to endDay
15%11%5%7%7%3445%5%29%
1580
March1st to 15thDay
24%8%35%414445%32%
1833
16th to endDay
17%10%46%5%35%446%31%
2141
April1st to 15thDay
17%7%345%2435%7%38%
2151
16th to endDay
18%6%35%5%2435%7%37%
2274
May1st to 15thDay
22%7%25%5%34447%32%
2331
16th to endDay
12%5%26%5%35%6%6%7%37%
2431
June1st to 15thDay
11%4245%345%5%7%44%
2486
16th to endDay
10%4245%35%5%5%8%43%
2520
July1st to 15thDay
11%5%25%5%35%5%6%9%38%
2520
16th to endDay
7%4245%25%6%8%10%41%
2545
August1st to 15thDay
8%4245%35%6%6%9%43%
2340
16th to endDay
7%424435%6%7%9%44%
2303
September1st to 15thDay
13%5%25%435%447%43%
2059
16th to endDay
13%41442435%6%48%
1980
October1st to 15thDay
13%5%25%434346%45%
1976
16th to endDay
17%6%35%42435%6%41%
1779
November1st to 15thDay
11%5%36%423345%47%
1717
16th to endDay
18%8%36%424246%37%
1755
December1st to 15thDay
25%11%5%7%41333328%
1632
16th to endDay
22%13%5%6%423446%25%
1566
January1st to 15thNight
20%11%48%7%34445%27%
2506
16th to endNight
17%13%5%7%5%24435%29%
2574
February1st to 15thNight
18%10%5%7%433346%31%
2112
16th to endNight
16%11%5%7%5%25%346%30%
1893
March1st to 15thNight
18%8%36%5%34346%35%
2127
16th to endNight
17%8%35%5%3445%8%33%
2091
April1st to 15thNight
15%6%36%42345%6%40%
1797
16th to endNight
14%6%3443346%8%41%
1685
May1st to 15thNight
18%7%36%43445%7%34%
1619
16th to endNight
9%425%6%346%6%9%40%
1682
June1st to 15thNight
7%3244245%5%9%49%
1471
16th to endNight
7%2133346%7%11%48%
1440
July1st to 15thNight
9%4244345%5%8%48%
1440
16th to endNight
5%424435%46%8%50%
1620
August1st to 15thNight
6%3235%2446%9%51%
1620
16th to endNight
4213425%6%6%10%51%
1896
September1st to 15thNight
12%3133235%5%7%52%
1887
16th to endNight
12%324423346%51%
1980
October1st to 15thNight
10%424423447%51%
1983
16th to endNight
17%6%25%5%23345%43%
2443
November1st to 15thNight
14%5%35%423446%46%
2471
16th to endNight
19%8%25%5%23345%38%
2535
December1st to 15thNight
21%12%5%7%5%33345%26%
2365
16th to endNight
24%12%46%5%24345%25%
2655

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
60%39%16%10%
1433
16th to endDay
55%413%20%5%
1648
February1st to 15thDay
55%411%18%9%
1743
16th to endDay
55%412%21%6%
1580
March1st to 15thDay
52%6%11%20%8%1
1833
16th to endDay
53%5%12%20%8%
2142
April1st to 15thDay
44%6%11%23%13%1
2151
16th to endDay
45%6%10%21%12%4
2274
May1st to 15thDay
52%6%11%17%10%
2331
16th to endDay
43%8%13%18%14%2
2435
June1st to 15thDay
36%7%13%23%17%2
2486
16th to endDay
37%7%13%21%17%2
2520
July1st to 15thDay
41%7%13%17%17%3
2371
16th to endDay
33%10%17%23%12%2
2387
August1st to 15thDay
34%8%12%23%17%3
2327
16th to endDay
33%7%13%22%16%6%
2303
September1st to 15thDay
39%5%10%20%17%6%
2059
16th to endDay
36%5%9%21%24%1
1980
October1st to 15thDay
40%48%24%20%2
1976
16th to endDay
43%310%25%15%1
1779
November1st to 15thDay
38%410%24%17%5%
1717
16th to endDay
47%310%22%14%1
1755
December1st to 15thDay
59%49%16%9%
1632
16th to endDay
59%5%12%17%4
1566
January1st to 15thNight
58%6%7%18%8%
2506
16th to endNight
57%311%18%7%
2574
February1st to 15thNight
54%412%19%7%1
2121
16th to endNight
54%5%12%18%8%
1893
March1st to 15thNight
50%410%20%11%2
2127
16th to endNight
48%6%12%23%7%1
2091
April1st to 15thNight
43%5%9%21%17%2
1797
16th to endNight
40%5%11%20%15%6%
1685
May1st to 15thNight
47%8%11%20%10%1
1620
16th to endNight
38%8%13%19%15%4
1684
June1st to 15thNight
30%6%12%27%19%3
1471
16th to endNight
27%9%16%21%20%4
1440
July1st to 15thNight
33%7%13%20%18%5%
1360
16th to endNight
28%6%13%30%17%3
1515
August1st to 15thNight
27%8%15%26%19%3
1609
16th to endNight
26%7%14%28%18%5%
1896
September1st to 15thNight
29%6%11%22%22%7%
1887
16th to endNight
33%310%23%25%3
1980
October1st to 15thNight
32%5%9%26%20%5%
1983
16th to endNight
42%38%25%17%2
2443
November1st to 15thNight
38%410%20%20%5%
2471
16th to endNight
48%39%21%15%
2535
December1st to 15thNight
59%49%16%9%
2365
16th to endNight
61%6%8%15%7%
2655

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
34%11%33%15%5%
90
16th to endDay
22%16%44%13%2
96
February1st to 15thDay
25%23%37%11%2
90
16th to endDay
13%16%49%14%6%
81
March1st to 15thDay
30%17%29%19%2
123
16th to endDay
25%10%31%23%6%2
288
April1st to 15thDay
21%11%28%26%9%2
270
16th to endDay
21%432%20%15%5%
270
May1st to 15thDay
28%10%23%19%12%5%
288
16th to endDay
18%423%22%15%14%
331
June1st to 15thDay
11%317%34%24%8%
461
16th to endDay
12%323%30%22%6%
495
July1st to 15thDay
15%217%30%20%13%
475
16th to endDay
7%111%28%29%21%
405
August1st to 15thDay
13%9%18%27%30%
313
16th to endDay
9%112%24%31%20%
332
September1st to 15thDay
13%213%24%29%17%
315
16th to endDay
15%213%21%32%15%
314
October1st to 15thDay
15%321%31%20%7%
315
16th to endDay
17%5%32%31%11%1
195
November1st to 15thDay
18%11%22%23%16%7%
135
16th to endDay
22%8%41%22%5%
135
December1st to 15thDay
27%7%34%18%9%3
94
16th to endDay
25%18%44%8%3
96
January1st to 15thNight
24%9%38%23%4
900
16th to endNight
22%12%37%25%3
846
February1st to 15thNight
23%11%37%24%2
704
16th to endNight
22%10%36%27%2
631
March1st to 15thNight
23%9%35%25%5%
720
16th to endNight
20%6%33%30%8%
747
April1st to 15thNight
17%9%34%28%9%
539
16th to endNight
17%5%31%33%11%1
540
May1st to 15thNight
20%5%28%28%15%1
540
16th to endNight
10%121%36%25%5%
563
June1st to 15thNight
7%118%44%25%2
540
16th to endNight
6%17%44%28%1
540
July1st to 15thNight
11%210%37%33%4
510
16th to endNight
7%9%35%38%8%
564
August1st to 15thNight
7%9%35%38%7%
539
16th to endNight
49%36%42%6%
573
September1st to 15thNight
13%111%36%30%7%
627
16th to endNight
13%216%27%34%5%
717
October1st to 15thNight
11%423%31%23%5%
720
16th to endNight
20%6%30%29%11%1
768
November1st to 15thNight
15%7%30%27%16%1
760
16th to endNight
22%9%27%30%8%
780
December1st to 15thNight
27%8%32%21%8%
728
16th to endNight
30%9%33%22%4
927
Page generated at 2017-12-03 11:16:35. Version ac04

Copyright 2017 A. Danko


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