WWYS Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the WWYS Observatory clear sky chart from 2005-11-06 to 2024-01-16.

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
31%
23245
19%
74668
9%
23199
43%
23199
21%
47642

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
15%
760
22%
2649
7%
757
34%
757
16%
1625
January11th to 20thNight
20%
744
19%
2533
6%
744
38%
744
15%
1674
January21st to endNight
16%
792
22%
2574
6%
792
37%
792
18%
1613
February1st to 10thNight
15%
720
17%
2340
5%
720
30%
720
15%
1543
February11th to 20thNight
10%
703
18%
2169
2%
700
29%
700
13%
1433
February21st to endNight
13%
592
24%
1776
6%
592
32%
592
16%
1169
March1st to 10thNight
20%
720
22%
2125
7%
720
38%
720
20%
1316
March11th to 20thNight
19%
720
21%
1980
7%
720
33%
720
19%
1346
March21st to endNight
19%
792
22%
2073
7%
792
31%
792
24%
1401
April1st to 10thNight
17%
720
18%
1800
3%
720
27%
720
20%
1241
April11th to 20thNight
26%
711
19%
1775
6%
711
32%
711
26%
1209
April21st to endNight
26%
720
19%
1800
4%
720
35%
720
25%
1239
May1st to 10thNight
26%
577
15%
1657
6%
577
33%
577
18%
1164
May11th to 20thNight
32%
354
18%
1416
11%
354
43%
354
15%
921
May21st to endNight
45%
388
15%
1551
14%
388
51%
388
19%
962
June1st to 10thNight
44%
360
18%
1440
17%
360
54%
360
21%
867
June11th to 20thNight
47%
360
14%
1440
14%
360
55%
360
21%
881
June21st to endNight
52%
360
16%
1440
18%
360
59%
360
25%
822
July1st to 10thNight
52%
350
13%
1400
14%
350
56%
350
18%
902
July11th to 20thNight
59%
348
16%
1392
16%
348
64%
348
25%
785
July21st to endNight
56%
466
17%
1554
14%
440
61%
440
25%
952
August1st to 10thNight
61%
537
16%
1600
10%
534
64%
534
29%
1018
August11th to 20thNight
51%
660
15%
1724
9%
660
53%
660
27%
1181
August21st to endNight
56%
792
16%
1980
8%
792
58%
792
32%
1316
September1st to 10thNight
45%
720
22%
1800
9%
720
52%
720
31%
1215
September11th to 20thNight
54%
720
29%
1807
19%
720
58%
720
37%
1123
September21st to endNight
45%
720
23%
2011
15%
720
52%
720
26%
1317
October1st to 10thNight
38%
720
16%
2160
11%
720
46%
720
23%
1430
October11th to 20thNight
35%
720
22%
2160
14%
720
47%
720
28%
1317
October21st to endNight
28%
792
25%
2412
13%
792
44%
792
24%
1497
November1st to 10thNight
35%
740
25%
2407
14%
740
51%
740
28%
1376
November11th to 20thNight
28%
760
25%
2470
13%
760
44%
760
20%
1523
November21st to endNight
25%
760
22%
2626
9%
760
43%
760
20%
1560
December1st to 10thNight
20%
760
18%
2660
6%
760
35%
760
14%
1763
December11th to 20thNight
20%
760
19%
3191
6%
759
37%
759
13%
1969
December21st to endNight
20%
827
13%
4776
7%
817
36%
817
9%
2972

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%6%35%5%35%5%5%8%40%
74744
Night
15%6%35%5%345%5%7%43%
75049
TransparencyDay
41%7%12%22%15%2
74454
Night
40%7%12%22%16%3
74873
SeeingDay
22%16%29%20%10%3
13819
Night
20%14%30%26%9%1
23255

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
12%7%36%5%46%45%7%41%
1517
11th to 20thDay
15%7%26%7%35%5%5%7%37%
1548
21st to endDay
14%8%36%6%45%6%46%39%
1782
February1st to 10thDay
16%8%35%6%35%5%5%7%37%
1620
11th to 20thDay
15%8%36%5%4445%6%39%
1678
21st to endDay
20%9%46%5%34346%37%
1480
March1st to 10thDay
14%5%36%5%34447%45%
1828
11th to 20thDay
16%7%36%6%35%5%5%7%38%
1980
21st to endDay
21%7%35%6%24446%38%
2281
April1st to 10thDay
15%7%36%4445%5%8%41%
2160
11th to 20thDay
15%6%25%5%345%5%7%43%
2134
21st to endDay
17%5%26%6%345%6%8%39%
2160
May1st to 10thDay
17%7%36%6%35%5%5%7%35%
2317
11th to 20thDay
21%7%25%435%5%5%7%36%
2478
21st to endDay
14%7%26%6%446%49%40%
2725
June1st to 10thDay
14%5%246%35%6%7%9%38%
2520
11th to 20thDay
13%6%36%6%35%6%6%9%37%
2520
21st to endDay
11%5%25%5%46%6%6%10%41%
2520
July1st to 10thDay
10%5%35%6%46%6%6%10%39%
2520
11th to 20thDay
7%425%5%36%6%9%10%43%
2520
21st to endDay
9%6%36%6%47%6%6%8%37%
2668
August1st to 10thDay
9%6%26%6%46%6%7%8%40%
2327
11th to 20thDay
11%5%36%6%36%5%6%10%39%
2157
21st to endDay
14%5%25%5%35%6%6%8%42%
2376
September1st to 10thDay
16%6%25%435%45%7%43%
2160
11th to 20thDay
13%5%2431435%8%50%
2152
21st to endDay
18%6%37%5%34446%40%
1946
October1st to 10thDay
18%6%3442445%7%43%
1800
11th to 20thDay
16%6%245%3445%8%45%
1800
21st to endDay
20%6%2443345%6%42%
1942
November1st to 10thDay
13%7%24324347%51%
1671
11th to 20thDay
16%4245%3436%6%47%
1710
21st to endDay
15%5%244346%5%7%44%
1549
December1st to 10thDay
17%9%36%5%3446%6%37%
1520
11th to 20thDay
19%7%25%42445%7%41%
1906
21st to endDay
17%6%36%7%46%5%6%6%34%
2772
January1st to 10thNight
14%6%36%5%34447%44%
2649
11th to 20thNight
19%7%25%5%3445%6%38%
2533
21st to endNight
13%7%36%5%345%5%6%44%
2574
February1st to 10thNight
17%8%36%5%3445%6%39%
2340
11th to 20thNight
16%7%36%6%2445%6%40%
2167
21st to endNight
18%7%35%435%5%5%6%40%
1776
March1st to 10thNight
14%5%246%2445%7%46%
2115
11th to 20thNight
17%7%35%5%3445%5%43%
1979
21st to endNight
18%7%36%5%24346%43%
2073
April1st to 10thNight
16%5%36%5%35%5%6%7%40%
1800
11th to 20thNight
14%6%36%5%35%45%8%42%
1775
21st to endNight
19%6%344345%47%42%
1800
May1st to 10thNight
18%8%35%5%35%6%5%5%37%
1657
11th to 20thNight
18%7%35%5%35%6%5%7%38%
1415
21st to endNight
14%6%25%5%2446%9%41%
1550
June1st to 10thNight
15%5%35%5%345%5%8%42%
1440
11th to 20thNight
12%435%5%46%5%6%11%39%
1440
21st to endNight
10%415%6%246%7%10%44%
1440
July1st to 10thNight
9%5%36%7%36%5%6%9%39%
1440
11th to 20thNight
7%4246%45%6%7%10%45%
1440
21st to endNight
10%435%436%6%7%10%41%
1646
August1st to 10thNight
9%5%245%47%6%6%9%43%
1611
11th to 20thNight
12%6%36%6%45%6%6%7%39%
1724
21st to endNight
13%5%25%5%45%5%6%8%43%
1980
September1st to 10thNight
15%5%35%5%2445%7%46%
1800
11th to 20thNight
13%5%233235%5%6%53%
1807
21st to endNight
17%6%25%5%345%5%7%43%
2011
October1st to 10thNight
21%6%245%33336%43%
2160
11th to 20thNight
16%6%2442435%6%48%
2160
21st to endNight
19%6%23423446%47%
2412
November1st to 10thNight
12%5%2442345%6%52%
2407
11th to 20thNight
16%7%3432435%5%47%
2470
21st to endNight
13%4245%45%45%7%46%
2626
December1st to 10thNight
19%6%36%5%35%45%7%38%
2660
11th to 20thNight
18%6%25%5%35%5%5%6%41%
3288
21st to endNight
17%7%36%5%35%5%5%7%36%
4884

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
43%6%13%22%13%3
1518
11th to 20thDay
46%7%13%20%11%2
1548
21st to endDay
45%6%10%23%13%2
1782
February1st to 10thDay
45%7%14%22%10%2
1620
11th to 20thDay
45%6%10%25%12%2
1687
21st to endDay
50%49%22%12%3
1480
March1st to 10thDay
40%5%11%26%14%3
1831
11th to 20thDay
45%6%11%20%14%4
1980
21st to endDay
47%6%8%19%17%3
2281
April1st to 10thDay
41%7%10%21%15%5%
2160
11th to 20thDay
40%7%11%22%16%4
2134
21st to endDay
42%8%11%19%16%4
2160
May1st to 10thDay
46%8%10%18%15%4
2317
11th to 20thDay
46%7%11%20%13%2
2482
21st to endDay
42%9%14%21%13%2
2725
June1st to 10thDay
39%8%15%23%14%1
2520
11th to 20thDay
41%10%15%22%10%2
2520
21st to endDay
37%10%14%25%14%1
2520
July1st to 10thDay
40%10%13%21%14%3
2450
11th to 20thDay
32%9%17%25%14%2
2445
21st to endDay
41%10%12%21%15%2
2519
August1st to 10thDay
39%10%14%22%15%1
2314
11th to 20thDay
38%9%15%21%15%1
2157
21st to endDay
38%9%12%21%16%3
2376
September1st to 10thDay
41%8%12%19%18%3
2160
11th to 20thDay
33%6%11%25%22%4
2152
21st to endDay
46%5%8%18%20%2
1946
October1st to 10thDay
41%6%13%22%16%1
1800
11th to 20thDay
38%6%11%23%19%2
1800
21st to endDay
42%5%10%24%17%2
1942
November1st to 10thDay
34%415%25%18%4
1671
11th to 20thDay
37%6%11%24%20%2
1710
21st to endDay
38%6%14%30%10%2
1549
December1st to 10thDay
47%6%16%19%12%1
1520
11th to 20thDay
42%6%15%27%9%1
1906
21st to endDay
47%8%14%21%9%1
2772
January1st to 10thNight
41%5%12%23%16%3
2649
11th to 20thNight
45%6%14%22%11%2
2533
21st to endNight
40%5%11%25%15%3
2574
February1st to 10thNight
46%5%13%24%10%2
2340
11th to 20thNight
46%5%10%20%16%3
2169
21st to endNight
44%5%9%22%16%4
1776
March1st to 10thNight
37%6%12%25%17%3
2125
11th to 20thNight
42%7%9%22%16%4
1980
21st to endNight
43%5%10%19%18%5%
2073
April1st to 10thNight
42%6%11%20%16%4
1800
11th to 20thNight
40%6%10%21%20%3
1775
21st to endNight
41%6%10%21%17%5%
1800
May1st to 10thNight
47%8%10%16%15%3
1657
11th to 20thNight
45%9%11%14%18%3
1416
21st to endNight
40%9%15%19%15%2
1551
June1st to 10thNight
40%7%12%22%18%1
1440
11th to 20thNight
38%10%16%21%14%1
1440
21st to endNight
33%9%15%28%13%2
1440
July1st to 10thNight
40%12%14%17%14%3
1400
11th to 20thNight
31%11%17%24%15%3
1392
21st to endNight
33%12%15%23%15%2
1554
August1st to 10thNight
35%10%17%22%15%1
1600
11th to 20thNight
41%8%16%18%15%2
1724
21st to endNight
37%8%16%19%18%2
1980
September1st to 10thNight
39%7%10%21%20%3
1800
11th to 20thNight
32%6%11%21%26%4
1807
21st to endNight
40%7%9%21%22%2
2011
October1st to 10thNight
43%6%11%23%15%2
2160
11th to 20thNight
36%6%12%27%17%2
2160
21st to endNight
39%5%10%26%18%2
2412
November1st to 10thNight
32%6%12%26%19%5%
2407
11th to 20thNight
39%5%9%22%19%4
2470
21st to endNight
37%6%12%28%15%2
2626
December1st to 10thNight
47%6%12%21%13%1
2660
11th to 20thNight
42%6%12%22%15%3
3288
21st to endNight
44%8%14%23%10%1
4884

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
34%23%27%13%3
180
11th to 20thDay
36%27%29%9%1
200
21st to endDay
28%32%25%11%3
264
February1st to 10thDay
32%30%30%6%2
240
11th to 20thDay
32%28%32%7%1
346
21st to endDay
35%24%29%11%1
346
March1st to 10thDay
32%20%30%15%2
420
11th to 20thDay
29%25%30%12%31
420
21st to endDay
31%25%29%11%5%1
462
April1st to 10thDay
26%26%30%13%41
420
11th to 20thDay
24%23%27%17%6%2
412
21st to endDay
29%18%35%13%41
420
May1st to 10thDay
33%18%27%14%6%2
423
11th to 20thDay
25%16%33%18%7%1
447
21st to endDay
18%11%28%24%17%2
485
June1st to 10thDay
17%12%31%25%11%2
450
11th to 20thDay
18%10%33%25%12%1
450
21st to endDay
13%11%32%26%14%3
450
July1st to 10thDay
11%5%25%32%21%6%
445
11th to 20thDay
8%5%27%35%22%4
444
21st to endDay
11%8%30%31%17%3
488
August1st to 10thDay
10%7%28%28%21%7%
446
11th to 20thDay
13%5%22%34%19%7%
439
21st to endDay
12%8%25%29%17%9%
495
September1st to 10thDay
20%9%21%26%16%8%
450
11th to 20thDay
16%6%28%24%16%10%
450
21st to endDay
21%15%25%22%13%4
450
October1st to 10thDay
24%10%26%22%14%5%
450
11th to 20thDay
19%18%26%24%12%1
450
21st to endDay
22%20%32%15%8%2
475
November1st to 10thDay
21%15%30%22%10%3
365
11th to 20thDay
22%22%32%19%5%
370
21st to endDay
22%22%32%21%3
209
December1st to 10thDay
30%26%29%9%41
180
11th to 20thDay
36%23%26%12%3
180
21st to endDay
28%25%27%14%6%1
198
January1st to 10thNight
21%24%36%15%3
760
11th to 20thNight
27%19%29%22%3
744
21st to endNight
18%23%39%18%2
792
February1st to 10thNight
24%22%34%19%1
720
11th to 20thNight
27%26%33%13%1
703
21st to endNight
27%23%35%13%2
592
March1st to 10thNight
22%23%31%21%3
720
11th to 20thNight
25%23%30%17%5%
720
21st to endNight
20%17%40%17%5%1
792
April1st to 10thNight
21%24%34%16%4
720
11th to 20thNight
19%16%36%23%6%
711
21st to endNight
23%16%32%23%6%
720
May1st to 10thNight
24%13%32%24%7%
577
11th to 20thNight
19%9%35%30%6%
354
21st to endNight
15%9%25%38%12%1
389
June1st to 10thNight
15%6%28%39%12%
360
11th to 20thNight
10%8%29%41%10%
360
21st to endNight
10%329%45%13%1
360
July1st to 10thNight
11%5%22%41%20%1
350
11th to 20thNight
7%223%40%25%2
348
21st to endNight
10%424%39%21%2
466
August1st to 10thNight
10%219%42%24%2
537
11th to 20thNight
15%419%36%25%2
660
21st to endNight
12%5%19%39%22%2
792
September1st to 10thNight
17%428%30%18%3
720
11th to 20thNight
14%5%20%35%22%3
720
21st to endNight
19%7%23%31%19%1
720
October1st to 10thNight
23%9%25%24%16%3
720
11th to 20thNight
19%11%31%28%11%
720
21st to endNight
25%14%29%23%8%
792
November1st to 10thNight
16%13%31%27%11%1
740
11th to 20thNight
22%18%27%26%7%1
760
21st to endNight
18%18%35%22%7%
760
December1st to 10thNight
26%19%31%19%5%1
760
11th to 20thNight
21%19%35%21%4
760
21st to endNight
22%20%32%23%2
836
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