Stratford
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stratford clear sky chart from 2005-11-05 to 2017-09-17.

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
34%
14974
13%
46617
9%
14936
38%
14936
17%
33851

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%
784
7%
2461
3%
782
16%
782
7%
2077
January16th to endNight
14%
768
7%
2496
3%
768
19%
768
7%
2115
February1st to 15thNight
17%
701
5%
2287
1%
701
21%
701
8%
1883
February16th to endNight
22%
631
9%
1967
4%
631
26%
631
11%
1547
March1st to 15thNight
24%
720
12%
2010
6%
720
29%
720
15%
1545
March16th to endNight
31%
768
19%
2112
10%
768
39%
768
21%
1494
April1st to 15thNight
27%
685
14%
1829
7%
685
32%
685
20%
1345
April16th to endNight
36%
540
19%
1620
10%
540
42%
540
23%
1078
May1st to 15thNight
41%
540
16%
1620
10%
540
47%
540
24%
1095
May16th to endNight
45%
402
16%
1525
14%
402
54%
402
25%
882
June1st to 15thNight
48%
360
13%
1386
14%
360
55%
360
24%
815
June16th to endNight
53%
360
15%
1260
15%
360
60%
360
27%
742
July1st to 15thNight
64%
357
19%
1229
22%
357
68%
357
37%
648
July16th to endNight
61%
564
15%
1595
12%
531
64%
531
33%
969
August1st to 15thNight
65%
539
17%
1609
14%
536
66%
536
33%
970
August16th to endNight
60%
585
23%
1731
16%
585
64%
585
33%
1052
September1st to 15thNight
55%
720
20%
1878
13%
720
57%
720
32%
1215
September16th to endNight
55%
665
19%
1837
14%
665
57%
665
31%
1171
October1st to 15thNight
41%
660
16%
1963
13%
660
46%
660
21%
1368
October16th to endNight
22%
704
11%
2128
6%
704
29%
704
13%
1645
November1st to 15thNight
31%
700
16%
2333
15%
700
36%
700
17%
1641
November16th to endNight
22%
720
8%
2520
7%
720
25%
720
9%
2005
December1st to 15thNight
12%
720
4%
2520
2%
720
14%
720
5%
2187
December16th to endNight
10%
781
4%
2701
1%
781
14%
781
4%
2362

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
15%10%47%6%35%45%7%29%
48132
Night
14%10%47%6%3445%6%31%
46793
Cloud CoverDay
15%10%47%6%35%45%7%29%
48142
Night
14%9%47%6%3445%6%31%
46797
TransparencyDay
53%7%12%17%8%
47826
Night
52%6%11%17%9%1
46617
SeeingDay
19%7%26%24%15%6%
5663
Night
19%9%29%27%11%1
14981

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
18%16%7%12%8%34445%14%
1464
16th to endDay
12%18%10%14%8%45%44411%
1728
February1st to 15thDay
14%17%8%12%8%5%6%33413%
1575
16th to endDay
14%15%6%10%8%36%446%17%
1506
March1st to 15thDay
14%13%7%10%7%43436%24%
1939
16th to endDay
15%10%6%8%7%3445%5%29%
2112
April1st to 15thDay
19%11%46%42346%7%27%
2128
16th to endDay
18%10%35%423247%37%
2340
May1st to 15thDay
19%6%36%5%3445%8%33%
2331
16th to endDay
13%6%25%5%345%5%8%37%
2587
June1st to 15thDay
14%6%25%5%35%5%6%8%36%
2571
16th to endDay
13%5%25%5%36%5%6%10%35%
2700
July1st to 15thDay
10%5%25%5%36%5%6%9%40%
2652
16th to endDay
10%6%25%5%36%5%7%9%36%
2465
August1st to 15thDay
9%6%36%6%47%5%7%8%35%
2340
16th to endDay
12%6%345%35%46%8%37%
2458
September1st to 15thDay
13%7%25%6%45%5%5%8%35%
2082
16th to endDay
17%5%25%5%2445%7%39%
1829
October1st to 15thDay
21%8%36%43445%5%31%
1666
16th to endDay
26%12%47%6%34445%20%
1744
November1st to 15thDay
15%12%46%5%3445%6%31%
1524
16th to endDay
17%16%8%9%6%3346%6%16%
1440
December1st to 15thDay
17%18%10%14%7%3343311%
1440
16th to endDay
19%19%9%13%8%243349%
1511
January1st to 15thNight
20%15%6%9%8%44345%15%
2461
16th to endNight
13%15%10%13%9%34445%14%
2496
February1st to 15thNight
14%14%8%11%8%35%44418%
2285
16th to endNight
15%11%5%10%9%35%45%6%22%
1967
March1st to 15thNight
17%10%48%6%3445%7%27%
1998
16th to endNight
13%8%47%5%34346%37%
2112
April1st to 15thNight
19%9%35%5%34447%31%
1829
16th to endNight
12%8%25%6%35%45%7%39%
1620
May1st to 15thNight
15%6%36%5%2345%7%37%
1620
16th to endNight
10%415%5%2445%7%46%
1523
June1st to 15thNight
10%5%244445%5%8%42%
1386
16th to endNight
10%5%15%44446%8%44%
1260
July1st to 15thNight
43144235%5%7%56%
1302
16th to endNight
8%3145%2446%9%47%
1703
August1st to 15thNight
6%4145%345%6%8%47%
1620
16th to endNight
8%4345%23446%51%
1731
September1st to 15thNight
12%5%14425%45%7%47%
1878
16th to endNight
12%5%145%2445%8%45%
1837
October1st to 15thNight
17%7%25%5%3345%8%36%
1963
16th to endNight
24%10%37%5%3346%6%26%
2128
November1st to 15thNight
14%10%46%5%3446%7%32%
2333
16th to endNight
19%14%7%9%5%34447%19%
2520
December1st to 15thNight
18%19%9%11%6%3444413%
2520
16th to endNight
20%16%10%12%7%3444411%
2701

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
18%15%6%11%7%35%5%45%14%
1464
16th to endDay
12%19%9%13%9%5%5%44411%
1728
February1st to 15thDay
15%17%9%11%8%47%43413%
1575
16th to endDay
14%15%7%10%6%46%6%46%17%
1506
March1st to 15thDay
15%13%5%11%7%44336%23%
1950
16th to endDay
14%10%6%7%7%45%35%5%29%
2111
April1st to 15thDay
19%11%46%4245%6%6%27%
2128
16th to endDay
17%10%36%323347%36%
2340
May1st to 15thDay
18%7%26%5%3445%8%33%
2331
16th to endDay
12%7%35%5%345%6%8%37%
2587
June1st to 15thDay
13%7%25%435%5%7%8%36%
2571
16th to endDay
13%5%25%6%35%6%6%10%35%
2700
July1st to 15thDay
10%425%5%36%5%6%9%39%
2652
16th to endDay
10%5%36%5%36%5%7%9%35%
2465
August1st to 15thDay
8%5%37%6%47%6%7%8%34%
2340
16th to endDay
12%6%346%35%5%6%8%37%
2458
September1st to 15thDay
13%7%26%6%35%5%6%7%35%
2082
16th to endDay
17%5%25%42445%6%40%
1829
October1st to 15thDay
20%8%36%5%3445%5%31%
1666
16th to endDay
25%11%48%6%34445%20%
1744
November1st to 15thDay
13%11%48%5%35%45%6%31%
1524
16th to endDay
17%15%7%9%6%4445%6%17%
1440
December1st to 15thDay
16%18%11%11%7%35%34312%
1440
16th to endDay
19%19%9%13%7%35%3348%
1511
January1st to 15thNight
20%14%6%9%8%45%445%16%
2461
16th to endNight
13%15%10%12%9%45%445%14%
2496
February1st to 15thNight
13%14%7%11%8%45%5%5%418%
2278
16th to endNight
15%11%5%9%9%4445%6%22%
1967
March1st to 15thNight
17%9%47%6%45%45%7%26%
2010
16th to endNight
13%9%46%5%34446%37%
2112
April1st to 15thNight
20%9%35%5%34347%31%
1829
16th to endNight
12%8%25%6%345%5%7%39%
1620
May1st to 15thNight
14%6%36%5%2445%7%38%
1619
16th to endNight
10%5%245%24448%46%
1523
June1st to 15thNight
9%5%34435%46%8%42%
1386
16th to endNight
9%425%445%45%9%44%
1260
July1st to 15thNight
4215%3245%5%8%55%
1302
16th to endNight
7%4144345%6%9%47%
1703
August1st to 15thNight
6%3145%345%7%8%47%
1620
16th to endNight
7%425%424446%51%
1731
September1st to 15thNight
12%5%2442445%7%47%
1878
16th to endNight
12%5%245%2435%8%45%
1837
October1st to 15thNight
17%6%26%5%34348%36%
1963
16th to endNight
23%10%27%6%3346%6%26%
2128
November1st to 15thNight
14%9%47%5%345%6%7%32%
2333
16th to endNight
19%13%6%9%6%34447%20%
2520
December1st to 15thNight
18%17%8%11%7%35%34414%
2520
16th to endNight
19%16%9%12%6%35%43412%
2701

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
71%8%9%8%2
1465
16th to endDay
75%6%7%6%3
1728
February1st to 15thDay
74%5%6%10%2
1577
16th to endDay
66%7%9%11%4
1506
March1st to 15thDay
62%5%11%13%6%
1950
16th to endDay
55%5%12%15%10%1
2112
April1st to 15thDay
53%7%11%18%8%
2128
16th to endDay
47%412%20%12%1
2340
May1st to 15thDay
48%6%11%22%10%
2331
16th to endDay
42%7%14%21%12%1
2591
June1st to 15thDay
42%7%14%22%11%
2571
16th to endDay
42%8%13%22%12%
2700
July1st to 15thDay
37%8%12%24%15%1
2496
16th to endDay
41%8%15%21%11%1
2310
August1st to 15thDay
42%8%17%18%11%1
2327
16th to endDay
41%8%15%23%10%1
2458
September1st to 15thDay
44%8%15%20%10%
2082
16th to endDay
43%6%13%25%9%
1829
October1st to 15thDay
53%6%13%14%12%1
1666
16th to endDay
64%7%12%10%4
1744
November1st to 15thDay
53%7%12%16%8%
1524
16th to endDay
67%7%12%10%2
1440
December1st to 15thDay
76%7%9%5%
1440
16th to endDay
79%6%6%5%1
1511
January1st to 15thNight
70%6%9%9%4
2461
16th to endNight
71%6%9%8%2
2496
February1st to 15thNight
67%6%10%11%3
2287
16th to endNight
61%6%12%11%6%
1967
March1st to 15thNight
56%7%11%17%6%
2010
16th to endNight
48%5%9%21%11%2
2112
April1st to 15thNight
52%5%9%19%12%
1829
16th to endNight
43%6%9%18%18%3
1620
May1st to 15thNight
44%5%10%22%13%2
1620
16th to endNight
35%7%13%26%13%3
1525
June1st to 15thNight
37%6%13%26%14%1
1386
16th to endNight
36%6%15%23%16%1
1260
July1st to 15thNight
25%7%13%29%22%2
1229
16th to endNight
32%7%16%23%17%2
1595
August1st to 15thNight
32%8%16%27%13%1
1609
16th to endNight
32%7%10%30%16%2
1731
September1st to 15thNight
35%7%12%27%15%1
1878
16th to endNight
36%7%14%27%13%
1837
October1st to 15thNight
47%7%10%20%13%
1963
16th to endNight
58%7%13%13%6%
2128
November1st to 15thNight
49%7%12%18%10%1
2333
16th to endNight
64%6%12%12%3
2520
December1st to 15thNight
73%7%9%7%2
2520
16th to endNight
76%6%9%5%1
2701

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
23%16%43%7%7%1
78
16th to endDay
25%17%41%15%
96
February1st to 15thDay
23%17%47%11%
90
16th to endDay
22%22%40%12%2
81
March1st to 15thDay
24%13%34%21%7%
240
16th to endDay
21%8%34%22%10%2
288
April1st to 15thDay
25%11%30%20%9%2
270
16th to endDay
22%10%28%21%10%5%
270
May1st to 15thDay
25%7%28%21%10%6%
288
16th to endDay
18%6%29%25%13%6%
331
June1st to 15thDay
17%5%25%30%16%3
366
16th to endDay
16%6%29%29%14%3
495
July1st to 15thDay
13%324%27%22%8%
450
16th to endDay
9%124%28%24%10%
326
August1st to 15thDay
10%217%30%23%16%
313
16th to endDay
18%415%21%21%18%
332
September1st to 15thDay
11%417%26%27%13%
315
16th to endDay
18%416%26%19%14%
259
October1st to 15thDay
20%6%25%25%16%4
255
16th to endDay
29%11%30%23%5%
256
November1st to 15thDay
15%17%30%21%12%3
80
16th to endDay
30%18%35%8%8%
60
December1st to 15thDay
31%26%25%13%3
60
16th to endDay
35%21%35%6%
64
January1st to 15thNight
27%15%36%15%4
787
16th to endNight
22%13%36%22%4
768
February1st to 15thNight
21%15%36%22%3
704
16th to endNight
21%12%34%25%5%
631
March1st to 15thNight
21%14%35%21%6%
720
16th to endNight
18%10%34%24%11%1
768
April1st to 15thNight
23%12%31%26%5%
685
16th to endNight
16%8%32%30%9%1
540
May1st to 15thNight
20%6%25%32%14%1
540
16th to endNight
12%5%29%34%16%
403
June1st to 15thNight
12%5%27%40%14%
360
16th to endNight
13%325%43%13%
360
July1st to 15thNight
5%122%46%22%1
357
16th to endNight
9%120%45%21%2
564
August1st to 15thNight
9%115%42%28%3
539
16th to endNight
10%219%38%25%4
585
September1st to 15thNight
15%420%31%22%6%
720
16th to endNight
14%420%32%22%6%
665
October1st to 15thNight
20%6%25%29%15%2
660
16th to endNight
29%10%31%22%5%
704
November1st to 15thNight
19%11%32%26%9%1
700
16th to endNight
26%14%30%21%6%
720
December1st to 15thNight
28%16%34%16%3
720
16th to endNight
27%16%36%16%3
781
Page generated at 2017-09-18 08:38:23. Version ac04

Copyright 2017 A. Danko


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