Winthrop
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Winthrop clear sky chart from 2005-11-06 to 2024-03-23.

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
37%
22494
15%
80611
9%
22414
45%
22414
18%
53390

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
23%
754
10%
4373
5%
736
29%
736
5%
3334
January11th to 20thNight
27%
755
17%
4326
7%
740
34%
740
8%
2847
January21st to endNight
30%
827
7%
4862
8%
814
37%
814
6%
4074
February1st to 10thNight
25%
760
11%
2549
6%
760
31%
760
13%
1829
February11th to 20thNight
33%
741
14%
2404
7%
740
42%
740
19%
1589
February21st to endNight
30%
628
14%
1903
8%
628
38%
628
14%
1371
March1st to 10thNight
34%
760
14%
2280
9%
760
41%
760
17%
1631
March11th to 20thNight
36%
760
16%
2156
8%
760
44%
760
22%
1470
March21st to endNight
30%
800
11%
2052
3%
800
35%
800
17%
1531
April1st to 10thNight
36%
720
14%
1800
4%
720
40%
720
21%
1277
April11th to 20thNight
36%
711
16%
1779
5%
711
43%
711
25%
1215
April21st to endNight
31%
453
17%
1533
9%
453
41%
453
22%
927
May1st to 10thNight
39%
360
17%
1440
12%
360
53%
360
22%
803
May11th to 20thNight
48%
354
14%
1416
15%
354
57%
354
20%
839
May21st to endNight
41%
388
10%
1552
12%
388
47%
388
21%
928
June1st to 10thNight
40%
360
9%
1440
7%
360
45%
360
21%
852
June11th to 20thNight
36%
360
8%
1362
3%
360
42%
360
23%
769
June21st to endNight
46%
360
13%
1260
6%
360
53%
360
36%
629
July1st to 10thNight
53%
350
13%
1301
9%
350
61%
350
38%
616
July11th to 20thNight
48%
348
19%
1392
18%
348
64%
348
41%
575
July21st to endNight
58%
394
26%
1488
26%
372
77%
372
55%
534
August1st to 10thNight
64%
358
27%
1424
29%
356
76%
356
44%
594
August11th to 20thNight
62%
432
31%
1500
25%
432
72%
432
52%
643
August21st to endNight
54%
764
28%
1952
14%
764
60%
764
47%
1074
September1st to 10thNight
50%
720
27%
1800
11%
720
57%
720
46%
996
September11th to 20thNight
58%
720
29%
1800
16%
720
63%
720
41%
1064
September21st to endNight
49%
720
25%
2017
15%
720
57%
720
35%
1202
October1st to 10thNight
49%
720
18%
2160
13%
720
56%
720
29%
1320
October11th to 20thNight
42%
720
18%
2168
13%
720
50%
720
24%
1394
October21st to endNight
35%
792
18%
2574
11%
792
46%
792
20%
1694
November1st to 10thNight
28%
740
12%
2475
6%
740
37%
740
16%
1768
November11th to 20thNight
28%
760
13%
2660
7%
760
37%
760
14%
1850
November21st to endNight
26%
760
10%
2660
4%
760
34%
760
12%
1964
December1st to 10thNight
30%
760
14%
2660
7%
760
39%
760
14%
1900
December11th to 20thNight
22%
759
6%
3265
3%
759
26%
759
7%
2613
December21st to endNight
21%
826
9%
4828
5%
817
32%
817
6%
3674

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
6%8%36%6%45%5%6%8%43%
78908
Night
6%10%47%5%35%5%6%7%41%
80930
TransparencyDay
41%9%13%20%14%3
78634
Night
42%10%13%21%12%2
80784
SeeingDay
13%21%34%23%8%2
13729
Night
12%12%33%29%12%2
22529

By Month

Percentage Cloud Cover NW (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
7%15%6%11%8%5%6%6%6%7%23%
2539
11th to 20thDay
7%14%5%10%6%35%446%35%
2516
21st to endDay
14%23%6%8%5%4445%6%22%
2794
February1st to 10thDay
311%5%8%7%37%6%8%10%33%
1629
11th to 20thDay
311%46%5%46%5%8%9%40%
1665
21st to endDay
38%5%9%9%5%6%6%8%11%32%
1551
March1st to 10thDay
5%10%46%6%5%6%6%7%8%36%
1900
11th to 20thDay
410%46%6%46%7%8%9%36%
2019
21st to endDay
5%11%5%8%7%46%7%7%9%32%
2349
April1st to 10thDay
6%9%37%7%5%8%7%6%9%33%
2160
11th to 20thDay
5%7%48%7%46%7%7%9%37%
2132
21st to endDay
47%36%6%46%7%7%10%41%
2424
May1st to 10thDay
5%5%26%6%37%7%7%10%42%
2520
11th to 20thDay
8%6%37%7%5%6%6%7%8%37%
2477
21st to endDay
10%6%37%7%5%7%5%7%8%35%
2723
June1st to 10thDay
9%5%35%6%47%7%7%8%38%
2520
11th to 20thDay
7%5%27%6%47%5%7%9%41%
2592
21st to endDay
5%425%5%35%46%8%53%
2700
July1st to 10thDay
33245%35%46%7%58%
2615
11th to 20thDay
331343445%6%64%
2520
21st to endDay
2112323345%72%
2741
August1st to 10thDay
221343445%6%65%
2507
11th to 20thDay
221332445%6%67%
2397
21st to endDay
23245%35%5%6%8%57%
2403
September1st to 10thDay
431332335%7%66%
2160
11th to 20thDay
5%425%5%35%5%6%8%52%
2160
21st to endDay
7%6%25%5%345%5%6%54%
1940
October1st to 10thDay
7%426%6%36%6%5%6%50%
1800
11th to 20thDay
7%6%25%6%35%6%5%8%47%
1791
21st to endDay
7%8%36%6%35%5%5%9%41%
1782
November1st to 10thDay
9%9%35%6%46%5%8%9%35%
1603
11th to 20thDay
5%13%47%6%45%5%49%37%
1520
21st to endDay
312%6%9%7%47%5%6%8%33%
1520
December1st to 10thDay
5%11%6%8%6%35%5%6%8%38%
1520
11th to 20thDay
7%12%6%8%6%37%5%6%7%32%
1925
21st to endDay
9%18%7%10%7%46%5%5%6%25%
2794
January1st to 10thNight
5%18%7%12%8%5%6%6%6%6%21%
4406
11th to 20thNight
7%16%5%9%5%35%6%6%7%31%
4382
21st to endNight
13%24%7%11%7%34445%20%
4862
February1st to 10thNight
7%12%37%7%5%6%6%6%8%32%
2549
11th to 20thNight
49%46%445%5%7%9%42%
2402
21st to endNight
5%10%5%9%7%35%6%9%9%34%
1903
March1st to 10thNight
410%5%7%7%35%6%8%9%35%
2280
11th to 20thNight
48%35%6%37%7%6%10%42%
2156
21st to endNight
49%36%7%47%7%8%9%37%
2052
April1st to 10thNight
47%37%5%46%6%7%11%40%
1800
11th to 20thNight
35%36%5%37%6%7%8%46%
1779
21st to endNight
46%25%5%345%7%8%50%
1533
May1st to 10thNight
4424435%6%7%9%51%
1440
11th to 20thNight
7%5%245%36%5%8%8%48%
1416
21st to endNight
8%5%35%5%346%6%10%46%
1552
June1st to 10thNight
6%4345%46%5%8%10%45%
1440
11th to 20thNight
44145%46%5%6%9%53%
1362
21st to endNight
331232447%7%62%
1260
July1st to 10thNight
32133235%6%8%65%
1340
11th to 20thNight
111332345%7%70%
1440
21st to endNight
1112212345%79%
1576
August1st to 10thNight
2212314436%73%
1432
11th to 20thNight
1212212348%73%
1500
21st to endNight
32124335%5%8%64%
1952
September1st to 10thNight
3213324447%67%
1800
11th to 20thNight
5%31433446%6%61%
1800
21st to endNight
5%4245%335%7%8%54%
2017
October1st to 10thNight
8%6%244346%6%7%52%
2160
11th to 20thNight
6%5%35%5%35%5%6%6%49%
2168
21st to endNight
8%8%36%6%45%46%8%43%
2574
November1st to 10thNight
10%12%46%5%45%5%7%8%34%
2475
11th to 20thNight
6%13%46%6%46%6%6%8%36%
2660
21st to endNight
414%8%9%7%45%5%6%6%32%
2660
December1st to 10thNight
6%11%5%8%7%36%5%6%7%35%
2660
11th to 20thNight
6%16%7%11%8%5%7%7%6%6%20%
3280
21st to endNight
10%21%7%10%6%345%5%5%24%
4862

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
62%10%11%10%7%
2539
11th to 20thDay
53%7%12%20%5%3
2540
21st to endDay
65%9%10%11%5%
2794
February1st to 10thDay
50%10%17%16%5%1
1629
11th to 20thDay
42%11%18%18%9%1
1674
21st to endDay
48%12%16%15%7%1
1551
March1st to 10thDay
47%11%15%17%9%2
1900
11th to 20thDay
45%12%14%21%8%
2019
21st to endDay
50%13%17%14%6%
2349
April1st to 10thDay
50%10%15%18%6%1
2160
11th to 20thDay
46%10%16%16%10%2
2132
21st to endDay
41%11%13%21%11%2
2424
May1st to 10thDay
38%11%17%23%9%2
2520
11th to 20thDay
46%10%15%21%9%
2480
21st to endDay
47%9%14%22%7%
2724
June1st to 10thDay
45%10%11%19%12%4
2520
11th to 20thDay
41%11%13%20%13%2
2592
21st to endDay
30%7%15%26%18%4
2700
July1st to 10thDay
26%7%12%24%23%7%
2545
11th to 20thDay
22%5%9%23%28%13%
2445
21st to endDay
14%49%25%34%14%
2589
August1st to 10thDay
20%8%9%26%28%10%
2490
11th to 20thDay
18%7%11%27%30%7%
2397
21st to endDay
26%9%11%29%22%4
2403
September1st to 10thDay
21%6%11%29%27%5%
2160
11th to 20thDay
31%8%11%22%23%5%
2160
21st to endDay
32%8%15%24%17%3
1940
October1st to 10thDay
36%8%12%28%15%1
1800
11th to 20thDay
37%10%14%24%15%1
1791
21st to endDay
42%11%13%23%11%1
1782
November1st to 10thDay
47%12%14%16%10%1
1603
11th to 20thDay
49%9%16%17%8%1
1520
21st to endDay
50%12%13%14%8%2
1520
December1st to 10thDay
47%11%14%18%10%
1520
11th to 20thDay
52%12%18%12%6%
1928
21st to endDay
62%10%9%11%7%1
2794
January1st to 10thNight
60%10%10%13%6%1
4406
11th to 20thNight
52%6%14%19%8%2
4420
21st to endNight
68%10%8%10%41
4862
February1st to 10thNight
49%11%17%16%7%2
2549
11th to 20thNight
41%10%17%21%10%2
2404
21st to endNight
46%12%15%16%10%1
1903
March1st to 10thNight
45%12%14%17%10%1
2280
11th to 20thNight
40%11%14%25%9%
2157
21st to endNight
45%13%18%17%5%1
2052
April1st to 10thNight
40%13%16%22%8%1
1800
11th to 20thNight
37%11%16%21%13%2
1779
21st to endNight
33%12%15%27%13%
1533
May1st to 10thNight
30%11%17%29%12%1
1440
11th to 20thNight
36%10%18%27%9%1
1416
21st to endNight
36%10%17%27%8%1
1552
June1st to 10thNight
35%10%14%25%13%3
1440
11th to 20thNight
31%10%18%27%15%1
1362
21st to endNight
21%8%18%30%20%3
1260
July1st to 10thNight
20%8%12%32%21%8%
1301
11th to 20thNight
16%5%12%31%26%10%
1392
21st to endNight
9%5%8%31%33%14%
1488
August1st to 10thNight
16%6%10%29%26%13%
1424
11th to 20thNight
13%6%13%31%30%6%
1500
21st to endNight
19%7%14%30%27%3
1952
September1st to 10thNight
20%7%11%31%26%5%
1800
11th to 20thNight
25%8%11%28%25%5%
1800
21st to endNight
28%9%13%31%14%5%
2017
October1st to 10thNight
33%9%12%29%15%1
2160
11th to 20thNight
36%10%14%26%13%1
2168
21st to endNight
41%9%12%25%12%1
2574
November1st to 10thNight
48%11%16%14%10%2
2475
11th to 20thNight
46%11%15%18%6%3
2660
21st to endNight
52%11%12%15%8%2
2660
December1st to 10thNight
50%10%14%18%8%
2660
11th to 20thNight
59%14%11%11%6%
3276
21st to endNight
61%9%9%13%8%1
4862

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
16%23%23%22%12%4
180
11th to 20thDay
17%17%27%24%12%3
180
21st to endDay
20%15%31%23%11%
198
February1st to 10thDay
15%27%38%15%5%
289
11th to 20thDay
17%23%36%18%5%1
365
21st to endDay
13%25%37%20%6%
373
March1st to 10thDay
18%23%39%15%5%1
460
11th to 20thDay
15%26%28%21%9%1
460
21st to endDay
17%27%35%15%5%
471
April1st to 10thDay
24%22%33%14%6%1
420
11th to 20thDay
18%31%26%18%5%2
412
21st to endDay
13%29%32%18%6%1
420
May1st to 10thDay
13%23%35%24%41
423
11th to 20thDay
18%17%27%26%9%2
448
21st to endDay
15%20%31%23%9%2
485
June1st to 10thDay
15%25%34%19%7%
450
11th to 20thDay
16%27%35%15%5%1
522
21st to endDay
9%20%37%24%9%1
630
July1st to 10thDay
6%20%34%33%7%
540
11th to 20thDay
8%19%37%27%9%2
444
21st to endDay
316%36%31%12%2
487
August1st to 10thDay
6%13%36%31%12%2
447
11th to 20thDay
312%33%31%16%4
437
21st to endDay
8%18%36%27%7%3
495
September1st to 10thDay
10%18%38%22%10%1
450
11th to 20thDay
10%19%32%25%11%2
450
21st to endDay
14%23%31%23%9%2
450
October1st to 10thDay
14%18%34%24%10%
450
11th to 20thDay
15%21%31%21%10%2
441
21st to endDay
19%20%34%18%8%2
297
November1st to 10thDay
13%24%35%20%8%
237
11th to 20thDay
16%22%31%18%11%3
180
21st to endDay
16%14%34%19%12%4
180
December1st to 10thDay
17%14%23%22%18%6%
180
11th to 20thDay
9%17%36%29%7%2
180
21st to endDay
10%12%38%26%14%1
198
January1st to 10thNight
17%15%32%23%11%1
760
11th to 20thNight
17%11%33%27%10%1
760
21st to endNight
15%11%32%30%11%1
836
February1st to 10thNight
15%14%38%23%8%2
760
11th to 20thNight
11%11%36%29%11%
744
21st to endNight
14%14%32%25%13%2
628
March1st to 10thNight
11%12%35%31%11%1
760
11th to 20thNight
11%12%37%28%12%1
760
21st to endNight
12%14%38%28%8%1
800
April1st to 10thNight
10%13%34%31%10%1
720
11th to 20thNight
8%16%35%29%9%2
711
21st to endNight
10%17%40%25%8%1
453
May1st to 10thNight
10%13%35%31%11%
360
11th to 20thNight
11%9%29%33%17%1
354
21st to endNight
11%11%32%29%16%
389
June1st to 10thNight
11%13%35%31%11%1
360
11th to 20thNight
8%17%36%29%10%
360
21st to endNight
6%11%35%31%18%
360
July1st to 10thNight
49%31%41%15%1
350
11th to 20thNight
412%32%36%15%
348
21st to endNight
27%32%39%20%1
394
August1st to 10thNight
48%21%43%23%
358
11th to 20thNight
36%27%36%26%2
432
21st to endNight
6%9%29%36%18%2
764
September1st to 10thNight
6%10%31%33%18%3
720
11th to 20thNight
7%8%23%29%26%6%
720
21st to endNight
10%10%28%32%17%4
720
October1st to 10thNight
13%9%27%32%18%2
720
11th to 20thNight
13%12%28%30%14%2
720
21st to endNight
16%15%32%26%9%3
792
November1st to 10thNight
20%12%36%25%7%
740
11th to 20thNight
19%11%37%21%10%2
760
21st to endNight
14%13%34%25%10%4
760
December1st to 10thNight
15%9%37%28%10%3
760
11th to 20thNight
18%10%37%29%5%1
760
21st to endNight
17%14%36%28%6%
836
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