Stratford
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stratford clear sky chart from 2005-11-06 to 2024-01-12.

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
32%
23064
12%
74434
9%
23013
37%
23013
16%
54431

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 10thNight
13%
862
7%
2672
2%
860
17%
860
7%
2240
January11th to 20thNight
14%
728
4%
2365
3%
728
16%
728
6%
2049
January21st to endNight
17%
792
8%
2574
4%
792
22%
792
8%
2113
February1st to 10thNight
14%
720
5%
2340
2%
720
18%
720
7%
1953
February11th to 20thNight
19%
700
9%
2275
4%
700
25%
700
10%
1801
February21st to endNight
17%
592
9%
1797
3%
592
24%
592
11%
1429
March1st to 10thNight
23%
720
14%
2024
7%
720
30%
720
16%
1492
March11th to 20thNight
26%
720
17%
1980
8%
720
34%
720
18%
1437
March21st to endNight
28%
792
17%
2178
8%
792
35%
792
18%
1599
April1st to 10thNight
26%
720
13%
1900
6%
720
31%
720
20%
1394
April11th to 20thNight
30%
574
18%
1638
8%
574
37%
574
21%
1130
April21st to endNight
33%
540
16%
1620
7%
540
39%
540
19%
1133
May1st to 10thNight
37%
540
16%
1620
10%
540
43%
540
23%
1106
May11th to 20thNight
41%
489
17%
1551
11%
489
49%
489
24%
987
May21st to endNight
49%
388
18%
1551
17%
388
58%
388
28%
845
June1st to 10thNight
48%
360
14%
1440
14%
360
57%
360
23%
824
June11th to 20thNight
47%
360
17%
1269
16%
360
56%
360
25%
757
June21st to endNight
53%
360
17%
1260
17%
360
60%
360
27%
745
July1st to 10thNight
67%
350
19%
1225
23%
350
72%
350
38%
634
July11th to 20thNight
58%
462
18%
1455
18%
462
63%
462
35%
840
July21st to endNight
59%
591
16%
1672
12%
558
62%
558
32%
1019
August1st to 10thNight
64%
537
18%
1600
15%
534
66%
534
33%
962
August11th to 20thNight
64%
534
22%
1598
17%
534
67%
534
33%
952
August21st to endNight
57%
612
22%
1800
15%
612
60%
612
31%
1118
September1st to 10thNight
53%
720
17%
1825
10%
720
55%
720
30%
1225
September11th to 20thNight
59%
720
25%
1980
18%
720
62%
720
36%
1199
September21st to endNight
46%
720
19%
1980
14%
720
50%
720
25%
1349
October1st to 10thNight
41%
720
16%
2133
14%
720
45%
720
21%
1470
October11th to 20thNight
29%
720
13%
2160
9%
720
35%
720
17%
1588
October21st to endNight
23%
792
13%
2402
8%
792
31%
792
13%
1870
November1st to 10thNight
33%
740
16%
2406
14%
740
37%
740
18%
1663
November11th to 20thNight
19%
760
11%
2655
7%
760
25%
760
11%
2052
November21st to endNight
20%
760
7%
2660
6%
760
23%
760
9%
2120
December1st to 10thNight
13%
760
4%
2660
2%
760
15%
760
5%
2288
December11th to 20thNight
11%
760
4%
3260
1%
759
16%
759
5%
2696
December21st to endNight
9%
849
2%
4909
1%
837
13%
837
2%
4352

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%5%8%6%35%5%5%7%30%
75014
Night
15%11%5%8%6%35%45%6%30%
74647
TransparencyDay
52%8%13%18%9%1
74726
Night
53%8%12%17%9%1
74470
SeeingDay
18%12%29%24%12%5%
13849
Night
19%11%30%28%11%1
23077

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 10thDay
15%17%8%12%10%35%5%35%16%
1474
11th to 20thDay
16%16%9%16%7%35%5%6%5%13%
1635
21st to endDay
12%18%9%14%10%45%44414%
1782
February1st to 10thDay
16%18%7%12%12%5%6%44413%
1620
11th to 20thDay
13%12%9%12%10%5%5%5%5%7%17%
1573
21st to endDay
16%15%6%10%9%36%445%21%
1458
March1st to 10thDay
14%12%6%11%9%445%47%26%
1931
11th to 20thDay
13%14%7%9%6%45%45%6%29%
1980
21st to endDay
17%11%6%8%7%4445%5%29%
2178
April1st to 10thDay
17%14%6%7%5%345%6%7%27%
2060
11th to 20thDay
18%11%35%5%2445%7%34%
2285
21st to endDay
20%11%5%7%5%34347%31%
2340
May1st to 10thDay
18%8%36%6%35%46%8%32%
2340
11th to 20thDay
17%8%37%5%445%47%36%
2340
21st to endDay
13%6%35%5%35%6%6%9%39%
2725
June1st to 10thDay
14%6%26%6%35%5%7%8%38%
2520
11th to 20thDay
11%6%26%5%35%6%6%9%40%
2686
21st to endDay
13%6%35%5%36%5%6%9%39%
2700
July1st to 10thDay
9%5%25%5%36%6%6%10%45%
2700
11th to 20thDay
11%6%35%6%37%6%7%9%38%
2450
21st to endDay
10%6%26%6%35%5%7%9%40%
2547
August1st to 10thDay
8%6%36%6%46%5%7%11%37%
2327
11th to 20thDay
10%6%36%6%46%6%7%10%36%
2303
21st to endDay
12%7%35%5%45%47%8%39%
2538
September1st to 10thDay
12%6%25%7%45%6%7%8%37%
2133
11th to 20thDay
12%6%345%347%6%8%41%
1980
21st to endDay
16%6%36%6%45%5%6%8%35%
1980
October1st to 10thDay
18%9%37%6%36%5%5%6%33%
1826
11th to 20thDay
22%11%48%5%35%5%6%6%26%
1800
21st to endDay
23%14%48%7%45%45%6%20%
1954
November1st to 10thDay
14%14%5%7%6%3445%7%31%
1672
11th to 20thDay
14%15%6%8%7%46%5%7%7%22%
1522
21st to endDay
18%16%9%10%6%4446%7%16%
1520
December1st to 10thDay
15%20%10%14%7%45%44413%
1520
11th to 20thDay
18%17%9%14%8%345%3414%
1906
21st to endDay
29%20%8%12%7%344346%
2709
January1st to 10thNight
16%15%8%12%9%5%5%445%17%
2672
11th to 20thNight
18%19%7%11%10%45%44413%
2365
21st to endNight
12%13%9%15%9%45%5%46%18%
2574
February1st to 10thNight
15%17%8%11%10%45%445%16%
2340
11th to 20thNight
13%14%7%11%9%45%46%6%21%
2273
21st to endNight
16%13%6%12%9%45%445%22%
1797
March1st to 10thNight
14%9%49%6%445%6%8%31%
2014
11th to 20thNight
13%11%5%7%6%445%6%6%33%
1979
21st to endNight
16%11%48%6%35%446%33%
2178
April1st to 10thNight
18%9%37%6%45%45%6%32%
1900
11th to 20thNight
16%9%46%6%45%5%5%8%35%
1638
21st to endNight
14%11%36%7%45%45%7%34%
1620
May1st to 10thNight
16%8%36%5%345%48%37%
1620
11th to 20thNight
14%7%36%5%245%6%7%41%
1550
21st to endNight
9%425%43445%9%51%
1550
June1st to 10thNight
11%5%345%45%5%6%7%45%
1440
11th to 20thNight
10%5%25%5%45%46%9%43%
1269
21st to endNight
10%6%25%5%35%5%6%8%46%
1260
July1st to 10thNight
5%325%4245%6%9%57%
1260
11th to 20thNight
8%5%245%4446%9%50%
1502
21st to endNight
9%426%6%25%5%6%8%47%
1771
August1st to 10thNight
8%5%25%5%445%6%8%49%
1611
11th to 20thNight
6%6%25%6%3447%9%48%
1598
21st to endNight
9%5%35%5%25%5%6%7%47%
1800
September1st to 10thNight
10%7%26%5%36%5%6%7%44%
1825
11th to 20thNight
11%42442346%8%52%
1980
21st to endNight
15%7%26%6%3445%7%41%
1980
October1st to 10thNight
15%8%36%6%4445%7%38%
2133
11th to 20thNight
19%11%37%5%3445%7%31%
2160
21st to endNight
23%10%48%6%345%6%6%25%
2402
November1st to 10thNight
14%10%48%5%345%6%7%34%
2406
11th to 20thNight
16%14%5%10%7%45%5%5%7%23%
2655
21st to endNight
18%15%7%9%6%35%45%7%20%
2660
December1st to 10thNight
17%20%9%12%7%445%45%14%
2660
11th to 20thNight
14%19%8%12%8%4445%5%16%
3288
21st to endNight
26%20%10%11%6%344449%
4917

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 10thDay
68%9%9%10%3
1475
11th to 20thDay
74%8%8%6%41
1635
21st to endDay
73%6%8%8%5%1
1782
February1st to 10thDay
75%7%7%8%2
1620
11th to 20thDay
66%8%9%12%5%
1582
21st to endDay
65%7%10%13%6%1
1458
March1st to 10thDay
58%6%14%15%6%1
1934
11th to 20thDay
56%6%11%15%10%2
1980
21st to endDay
56%6%10%16%10%2
2178
April1st to 10thDay
55%7%12%16%10%
2060
11th to 20thDay
47%8%13%19%12%1
2285
21st to endDay
53%6%11%18%10%2
2340
May1st to 10thDay
48%7%11%21%12%1
2340
11th to 20thDay
46%7%12%22%11%2
2344
21st to endDay
39%9%15%22%13%2
2725
June1st to 10thDay
42%7%14%22%14%1
2520
11th to 20thDay
39%8%13%22%15%1
2686
21st to endDay
41%8%12%24%14%
2700
July1st to 10thDay
33%9%13%25%18%2
2626
11th to 20thDay
39%8%15%24%12%1
2374
21st to endDay
38%8%15%24%13%2
2405
August1st to 10thDay
38%9%19%23%10%1
2314
11th to 20thDay
40%10%18%22%9%1
2303
21st to endDay
40%8%14%25%12%1
2538
September1st to 10thDay
40%10%15%22%10%1
2133
11th to 20thDay
37%8%16%27%12%1
1980
21st to endDay
45%9%14%22%9%1
1980
October1st to 10thDay
50%7%14%16%12%1
1826
11th to 20thDay
57%8%14%14%7%
1800
21st to endDay
64%8%11%11%5%
1954
November1st to 10thDay
54%7%11%19%7%1
1672
11th to 20thDay
58%9%13%12%6%1
1522
21st to endDay
67%8%13%10%3
1520
December1st to 10thDay
73%8%9%8%2
1520
11th to 20thDay
73%8%9%8%2
1906
21st to endDay
79%9%8%41
2709
January1st to 10thNight
69%7%10%10%4
2672
11th to 20thNight
75%6%9%7%3
2365
21st to endNight
68%7%10%10%5%1
2574
February1st to 10thNight
69%7%9%11%3
2340
11th to 20thNight
62%8%11%13%6%
2275
21st to endNight
63%6%11%13%7%
1797
March1st to 10thNight
51%8%13%19%8%1
2024
11th to 20thNight
51%7%11%20%9%2
1980
21st to endNight
52%6%8%19%12%3
2178
April1st to 10thNight
52%5%11%18%13%1
1900
11th to 20thNight
46%7%11%18%16%2
1638
21st to endNight
48%8%10%17%14%3
1620
May1st to 10thNight
45%7%10%21%15%2
1620
11th to 20thNight
42%5%13%22%14%4
1551
21st to endNight
30%9%15%26%17%3
1551
June1st to 10thNight
37%7%13%26%16%2
1440
11th to 20thNight
37%7%13%24%17%3
1269
21st to endNight
35%7%15%23%18%1
1260
July1st to 10thNight
23%9%14%27%23%4
1225
11th to 20thNight
31%7%15%28%18%1
1455
21st to endNight
33%8%16%24%16%3
1672
August1st to 10thNight
32%8%17%30%13%1
1600
11th to 20thNight
31%8%16%29%14%1
1598
21st to endNight
33%9%13%28%14%2
1800
September1st to 10thNight
37%9%13%25%14%2
1825
11th to 20thNight
30%8%14%29%18%1
1980
21st to endNight
42%8%13%24%12%
1980
October1st to 10thNight
45%8%11%21%15%1
2133
11th to 20thNight
52%8%13%19%8%1
2160
21st to endNight
58%8%11%14%9%
2402
November1st to 10thNight
47%8%14%20%10%1
2406
11th to 20thNight
58%10%11%14%6%1
2655
21st to endNight
62%9%14%11%3
2660
December1st to 10thNight
72%9%9%8%2
2660
11th to 20thNight
69%8%12%10%2
3288
21st to endNight
79%7%9%5%1
4917

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 10thDay
20%23%30%19%7%1
216
11th to 20thDay
19%17%47%12%41
245
21st to endDay
23%15%38%21%31
264
February1st to 10thDay
27%19%41%13%1
240
11th to 20thDay
23%21%43%12%1
240
21st to endDay
28%30%29%12%2
198
March1st to 10thDay
23%14%40%17%6%
374
11th to 20thDay
20%15%36%22%6%1
420
21st to endDay
26%13%34%21%6%1
462
April1st to 10thDay
20%18%35%20%6%2
420
11th to 20thDay
26%14%28%18%10%4
412
21st to endDay
25%13%32%21%7%2
420
May1st to 10thDay
23%10%29%24%10%4
423
11th to 20thDay
19%9%30%23%14%5%
447
21st to endDay
16%6%25%28%18%7%
485
June1st to 10thDay
15%7%32%29%13%4
450
11th to 20thDay
14%9%29%32%15%3
616
21st to endDay
14%8%30%32%13%3
630
July1st to 10thDay
9%5%23%29%25%9%
621
11th to 20thDay
14%5%26%26%20%9%
491
21st to endDay
9%427%34%19%7%
488
August1st to 10thDay
10%5%20%27%25%13%
446
11th to 20thDay
10%418%32%21%15%
439
21st to endDay
15%6%22%26%20%12%
495
September1st to 10thDay
13%6%21%30%18%12%
450
11th to 20thDay
9%6%20%29%26%11%
450
21st to endDay
15%11%27%23%15%9%
450
October1st to 10thDay
21%11%26%20%17%6%
450
11th to 20thDay
22%15%34%22%6%1
450
21st to endDay
27%16%32%20%5%1
469
November1st to 10thDay
21%22%31%16%8%1
270
11th to 20thDay
23%19%34%14%7%1
180
21st to endDay
24%19%36%18%2
180
December1st to 10thDay
26%24%33%11%6%
180
11th to 20thDay
27%27%29%14%2
180
21st to endDay
31%22%34%12%1
198
January1st to 10thNight
22%18%37%17%5%1
864
11th to 20thNight
29%16%32%20%3
728
21st to endNight
18%11%39%25%6%
792
February1st to 10thNight
24%16%38%20%2
720
11th to 20thNight
22%15%35%22%5%
703
21st to endNight
24%17%35%21%3
592
March1st to 10thNight
18%14%38%23%7%
720
11th to 20thNight
17%16%38%22%7%
720
21st to endNight
21%13%31%25%9%1
792
April1st to 10thNight
22%14%31%28%4
720
11th to 20thNight
20%12%33%24%10%1
574
21st to endNight
19%12%30%31%7%1
540
May1st to 10thNight
19%7%29%31%12%1
540
11th to 20thNight
18%6%29%34%12%1
489
21st to endNight
11%430%37%18%1
389
June1st to 10thNight
13%430%39%14%1
360
11th to 20thNight
12%5%30%41%11%
360
21st to endNight
11%6%25%43%14%1
360
July1st to 10thNight
5%221%49%22%1
350
11th to 20thNight
10%224%39%24%2
462
21st to endNight
8%319%48%20%1
591
August1st to 10thNight
10%215%42%28%2
537
11th to 20thNight
7%317%40%28%5%
534
21st to endNight
10%422%37%25%3
612
September1st to 10thNight
13%5%23%33%21%5%
720
11th to 20thNight
12%421%34%25%4
720
21st to endNight
17%7%23%30%17%5%
720
October1st to 10thNight
20%7%27%27%16%2
720
11th to 20thNight
23%13%28%26%8%1
720
21st to endNight
28%11%32%24%4
792
November1st to 10thNight
18%14%30%29%8%1
740
11th to 20thNight
23%14%36%19%6%1
760
21st to endNight
25%14%33%22%5%
760
December1st to 10thNight
26%19%31%18%4
760
11th to 20thNight
23%21%36%16%4
760
21st to endNight
28%17%36%18%2
856
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