Perth
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Perth clear sky chart from 2005-11-06 to 2024-02-20.

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
27%
22757
13%
80656
8%
22672
35%
22672
14%
57950

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%
754
7%
4415
2%
737
21%
737
4%
3367
January11th to 20thNight
16%
758
7%
4417
4%
740
23%
740
4%
3436
January21st to endNight
19%
828
7%
4685
6%
814
26%
814
6%
3786
February1st to 10thNight
14%
760
10%
2470
3%
760
23%
760
9%
1903
February11th to 20thNight
13%
739
13%
2399
3%
739
26%
739
12%
1765
February21st to endNight
18%
592
15%
1782
6%
592
29%
592
12%
1334
March1st to 10thNight
19%
720
14%
2134
8%
720
28%
720
15%
1585
March11th to 20thNight
20%
720
16%
1979
6%
720
30%
720
17%
1431
March21st to endNight
25%
792
20%
2080
7%
792
35%
792
20%
1483
April1st to 10thNight
20%
720
16%
1800
4%
720
27%
720
19%
1323
April11th to 20thNight
22%
711
17%
1775
5%
711
29%
711
19%
1281
April21st to endNight
26%
631
15%
1711
7%
631
32%
631
18%
1274
May1st to 10thNight
31%
360
16%
1440
12%
360
41%
360
16%
950
May11th to 20thNight
31%
354
21%
1416
15%
354
47%
354
19%
866
May21st to endNight
34%
388
15%
1551
10%
388
46%
388
24%
888
June1st to 10thNight
35%
360
10%
1440
9%
360
45%
360
16%
944
June11th to 20thNight
37%
360
13%
1440
15%
360
46%
360
20%
873
June21st to endNight
37%
360
11%
1440
10%
360
45%
360
17%
944
July1st to 10thNight
46%
350
14%
1400
18%
350
53%
350
24%
789
July11th to 20thNight
50%
348
16%
1392
21%
348
56%
348
24%
826
July21st to endNight
49%
394
15%
1486
16%
372
54%
372
25%
860
August1st to 10thNight
47%
435
17%
1499
12%
433
54%
433
25%
913
August11th to 20thNight
52%
534
17%
1598
12%
534
56%
534
29%
998
August21st to endNight
43%
789
18%
1977
7%
789
48%
789
31%
1306
September1st to 10thNight
41%
720
17%
1800
7%
720
46%
720
31%
1219
September11th to 20thNight
49%
720
23%
1800
12%
720
54%
720
35%
1150
September21st to endNight
41%
720
22%
2042
15%
720
46%
720
26%
1346
October1st to 10thNight
33%
720
16%
2160
11%
720
41%
720
19%
1498
October11th to 20thNight
24%
720
13%
2160
7%
720
32%
720
16%
1573
October21st to endNight
22%
792
13%
2491
5%
792
32%
792
12%
1862
November1st to 10thNight
24%
740
18%
2407
9%
740
35%
740
17%
1638
November11th to 20thNight
20%
760
17%
2658
9%
760
32%
760
13%
1872
November21st to endNight
18%
760
11%
2660
5%
760
25%
760
10%
2040
December1st to 10thNight
13%
760
7%
2660
2%
760
19%
760
7%
2167
December11th to 20thNight
16%
760
11%
3244
5%
759
24%
759
8%
2451
December21st to endNight
13%
828
5%
4848
2%
817
19%
817
4%
4009

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
17%10%47%6%35%5%5%6%32%
78236
Night
17%11%47%6%3445%6%31%
80950
TransparencyDay
52%7%11%19%10%1
77941
Night
53%7%11%18%10%1
80779
SeeingDay
23%17%30%19%8%2
13474
Night
23%14%31%23%8%1
22785

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%16%6%9%7%45%44424%
2517
11th to 20thDay
13%16%6%12%8%46%44422%
2520
21st to endDay
17%18%7%12%8%5%5%435%17%
2964
February1st to 10thDay
16%14%5%11%8%36%45%5%23%
1710
11th to 20thDay
12%14%6%9%8%45%445%29%
1654
21st to endDay
20%14%5%9%7%45%445%23%
1473
March1st to 10thDay
15%12%6%10%8%45%44429%
1822
11th to 20thDay
17%12%47%6%46%345%30%
1980
21st to endDay
20%11%46%5%23345%38%
2276
April1st to 10thDay
21%11%5%6%5%3445%6%32%
2160
11th to 20thDay
21%8%36%5%34345%37%
2134
21st to endDay
24%9%36%5%34445%32%
2264
May1st to 10thDay
20%8%37%5%4445%6%35%
2520
11th to 20thDay
16%7%37%6%2445%7%39%
2478
21st to endDay
14%6%36%5%46%5%6%7%39%
2725
June1st to 10thDay
21%8%36%5%45%5%5%7%30%
2520
11th to 20thDay
16%6%35%5%36%6%5%9%38%
2520
21st to endDay
17%7%36%6%46%6%5%8%33%
2520
July1st to 10thDay
11%5%36%5%35%5%6%9%41%
2520
11th to 20thDay
13%5%36%5%35%5%6%10%38%
2520
21st to endDay
13%5%35%6%46%7%7%8%37%
2744
August1st to 10thDay
13%6%25%7%45%5%6%8%39%
2426
11th to 20thDay
13%5%26%5%36%6%6%9%39%
2287
21st to endDay
14%5%26%6%45%5%6%8%40%
2376
September1st to 10thDay
14%6%25%45%5%5%5%8%41%
2160
11th to 20thDay
13%435%5%345%5%7%46%
2160
21st to endDay
19%5%26%6%335%5%6%40%
1913
October1st to 10thDay
21%8%36%6%33446%36%
1800
11th to 20thDay
21%10%47%6%35%5%5%7%29%
1800
21st to endDay
27%10%46%6%3445%5%26%
1864
November1st to 10thDay
15%8%36%7%46%5%6%6%34%
1671
11th to 20thDay
18%13%48%7%35%45%5%29%
1520
21st to endDay
21%15%48%7%5%5%45%5%21%
1520
December1st to 10thDay
22%16%5%9%7%445%5%5%18%
1520
11th to 20thDay
21%14%6%8%6%35%445%25%
1906
21st to endDay
27%17%6%10%6%2432320%
2772
January1st to 10thNight
17%14%8%10%8%4445%5%21%
4429
11th to 20thNight
13%15%6%12%9%5%6%445%21%
4440
21st to endNight
18%18%6%10%8%5%434420%
4690
February1st to 10thNight
15%15%5%11%7%45%46%5%22%
2470
11th to 20thNight
13%14%5%8%8%45%5%45%29%
2397
21st to endNight
15%14%5%8%7%45%45%5%29%
1782
March1st to 10thNight
16%12%5%9%6%35%5%5%430%
2124
11th to 20thNight
17%12%5%6%5%33346%34%
1979
21st to endNight
17%10%37%6%24445%39%
2080
April1st to 10thNight
21%8%47%6%335%45%35%
1800
11th to 20thNight
20%8%36%6%43445%37%
1775
21st to endNight
20%11%46%6%3445%6%32%
1711
May1st to 10thNight
22%8%25%5%35%5%47%35%
1440
11th to 20thNight
16%7%36%5%245%5%7%41%
1415
21st to endNight
13%5%25%425%45%8%46%
1550
June1st to 10thNight
17%6%36%5%46%5%5%8%35%
1440
11th to 20thNight
14%7%35%5%35%5%5%8%41%
1440
21st to endNight
17%5%25%6%45%5%6%8%36%
1440
July1st to 10thNight
9%5%25%6%35%5%5%9%45%
1440
11th to 20thNight
11%6%25%5%46%6%5%9%42%
1440
21st to endNight
10%4245%35%6%6%9%46%
1574
August1st to 10thNight
9%6%35%6%35%5%6%8%43%
1509
11th to 20thNight
10%6%35%6%4446%8%44%
1598
21st to endNight
10%6%25%4345%5%8%46%
1977
September1st to 10thNight
12%5%35%6%34447%45%
1800
11th to 20thNight
15%5%2342445%8%49%
1800
21st to endNight
17%6%25%43445%6%44%
2042
October1st to 10thNight
18%8%46%5%345%48%36%
2160
11th to 20thNight
22%8%36%7%3445%7%32%
2160
21st to endNight
24%10%47%6%345%46%28%
2491
November1st to 10thNight
19%10%36%5%25%5%36%36%
2407
11th to 20thNight
17%11%47%6%35%45%5%32%
2658
21st to endNight
21%13%47%7%35%45%7%24%
2660
December1st to 10thNight
20%18%5%10%7%345%45%18%
2660
11th to 20thNight
22%14%5%8%6%4433427%
3288
21st to endNight
30%16%411%6%3433416%
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
60%8%14%11%6%2
2518
11th to 20thDay
65%6%10%11%8%1
2520
21st to endDay
71%6%9%9%5%1
2964
February1st to 10thDay
61%6%9%18%5%1
1710
11th to 20thDay
57%5%11%16%10%1
1663
21st to endDay
64%5%10%11%10%1
1473
March1st to 10thDay
60%49%16%10%1
1825
11th to 20thDay
56%5%10%15%11%2
1980
21st to endDay
51%48%18%14%6%
2276
April1st to 10thDay
53%5%7%18%13%3
2160
11th to 20thDay
49%48%21%16%2
2134
21st to endDay
54%5%8%15%14%4
2264
May1st to 10thDay
50%5%10%16%16%3
2520
11th to 20thDay
45%6%9%23%15%2
2482
21st to endDay
43%9%13%20%13%2
2725
June1st to 10thDay
53%6%9%18%12%1
2520
11th to 20thDay
42%8%11%24%13%2
2520
21st to endDay
48%9%13%19%11%1
2520
July1st to 10thDay
37%8%12%28%15%1
2450
11th to 20thDay
39%9%16%26%10%
2445
21st to endDay
41%10%14%24%11%
2591
August1st to 10thDay
42%8%14%25%10%
2412
11th to 20thDay
39%8%18%23%11%1
2287
21st to endDay
41%7%14%26%12%1
2376
September1st to 10thDay
40%8%13%27%12%1
2160
11th to 20thDay
37%7%12%29%15%
2160
21st to endDay
44%7%13%26%10%
1913
October1st to 10thDay
49%5%12%22%11%1
1800
11th to 20thDay
54%8%12%18%7%1
1800
21st to endDay
59%6%11%16%7%1
1864
November1st to 10thDay
49%6%16%20%8%1
1671
11th to 20thDay
56%6%12%15%10%1
1520
21st to endDay
64%8%11%10%7%
1520
December1st to 10thDay
68%7%12%10%3
1520
11th to 20thDay
62%7%14%12%4
1906
21st to endDay
71%5%7%14%3
2772
January1st to 10thNight
64%8%13%12%3
4429
11th to 20thNight
65%8%12%11%41
4440
21st to endNight
69%6%9%10%6%1
4690
February1st to 10thNight
61%6%11%16%6%1
2470
11th to 20thNight
57%5%10%20%8%1
2399
21st to endNight
57%6%9%17%10%1
1782
March1st to 10thNight
56%5%11%16%10%1
2134
11th to 20thNight
52%5%11%19%12%2
1980
21st to endNight
48%6%7%20%17%4
2080
April1st to 10thNight
52%5%8%21%14%2
1800
11th to 20thNight
50%5%7%19%17%2
1775
21st to endNight
53%6%8%15%16%3
1711
May1st to 10thNight
49%5%7%18%15%5%
1440
11th to 20thNight
42%6%8%23%18%3
1416
21st to endNight
36%6%17%19%18%3
1551
June1st to 10thNight
46%8%13%20%12%1
1440
11th to 20thNight
40%7%12%21%18%2
1440
21st to endNight
44%9%13%20%12%1
1440
July1st to 10thNight
33%9%11%26%18%3
1400
11th to 20thNight
36%8%13%29%13%
1392
21st to endNight
34%9%16%27%14%1
1486
August1st to 10thNight
37%8%16%25%13%1
1499
11th to 20thNight
37%7%16%27%12%1
1598
21st to endNight
35%9%14%28%13%1
1977
September1st to 10thNight
37%9%12%28%14%1
1800
11th to 20thNight
33%7%14%33%11%1
1800
21st to endNight
41%7%11%31%10%1
2042
October1st to 10thNight
48%7%10%21%14%1
2160
11th to 20thNight
52%6%13%19%8%1
2160
21st to endNight
56%7%12%15%9%
2491
November1st to 10thNight
48%6%14%21%10%1
2407
11th to 20thNight
52%6%12%18%10%2
2658
21st to endNight
60%7%12%16%5%
2660
December1st to 10thNight
68%6%10%12%4
2660
11th to 20thNight
61%6%10%15%8%1
3288
21st to endNight
72%6%8%12%2
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
26%22%36%12%4
180
11th to 20thDay
32%22%28%16%2
180
21st to endDay
27%18%34%18%3
252
February1st to 10thDay
31%27%28%12%1
270
11th to 20thDay
26%33%35%5%1
267
21st to endDay
34%23%31%11%2
339
March1st to 10thDay
26%21%33%17%3
420
11th to 20thDay
30%18%33%15%4
420
21st to endDay
32%22%26%14%6%1
462
April1st to 10thDay
31%22%28%13%6%
420
11th to 20thDay
31%20%22%17%8%3
412
21st to endDay
30%22%28%13%6%2
420
May1st to 10thDay
25%17%32%17%6%2
423
11th to 20thDay
21%16%35%19%7%2
447
21st to endDay
19%15%33%20%10%2
485
June1st to 10thDay
24%13%35%20%8%1
450
11th to 20thDay
16%17%33%22%10%2
450
21st to endDay
20%15%31%20%12%2
450
July1st to 10thDay
13%8%30%32%15%3
445
11th to 20thDay
12%12%33%24%14%4
444
21st to endDay
15%8%34%30%10%4
488
August1st to 10thDay
15%9%30%30%11%4
446
11th to 20thDay
13%7%29%29%15%7%
439
21st to endDay
14%12%33%23%11%7%
495
September1st to 10thDay
15%13%29%23%12%7%
450
11th to 20thDay
13%10%27%24%17%9%
450
21st to endDay
19%13%29%23%11%5%
450
October1st to 10thDay
25%16%26%18%11%5%
450
11th to 20thDay
27%22%28%16%5%2
450
21st to endDay
31%19%27%20%3
437
November1st to 10thDay
27%17%30%19%6%
365
11th to 20thDay
29%24%29%12%6%1
180
21st to endDay
29%23%33%11%3
180
December1st to 10thDay
30%21%35%12%2
180
11th to 20thDay
34%26%28%9%21
180
21st to endDay
33%23%28%12%31
198
January1st to 10thNight
28%20%31%17%4
760
11th to 20thNight
31%17%29%19%41
760
21st to endNight
21%15%37%23%5%
836
February1st to 10thNight
25%22%33%18%3
760
11th to 20thNight
22%20%39%16%3
742
21st to endNight
24%22%33%18%4
592
March1st to 10thNight
25%17%34%21%3
720
11th to 20thNight
25%20%30%19%5%1
720
21st to endNight
23%15%32%22%8%1
792
April1st to 10thNight
27%16%33%22%3
720
11th to 20thNight
26%18%29%19%6%1
711
21st to endNight
26%14%29%23%8%
631
May1st to 10thNight
26%10%27%25%11%1
360
11th to 20thNight
18%12%34%30%6%1
354
21st to endNight
15%8%39%25%13%1
389
June1st to 10thNight
18%9%31%34%8%
360
11th to 20thNight
17%9%32%32%10%1
360
21st to endNight
18%6%33%29%14%
360
July1st to 10thNight
8%6%33%39%13%1
350
11th to 20thNight
11%6%28%39%14%1
348
21st to endNight
10%430%43%12%1
394
August1st to 10thNight
12%5%30%33%20%
435
11th to 20thNight
11%424%35%24%2
534
21st to endNight
12%8%31%30%16%2
789
September1st to 10thNight
14%7%33%29%16%2
720
11th to 20thNight
18%7%22%30%20%4
720
21st to endNight
19%10%26%24%19%2
720
October1st to 10thNight
23%13%27%22%14%2
720
11th to 20thNight
26%14%31%22%8%1
720
21st to endNight
29%16%27%23%5%
792
November1st to 10thNight
27%15%30%20%8%
740
11th to 20thNight
25%19%31%16%9%1
760
21st to endNight
29%18%29%18%6%
760
December1st to 10thNight
31%18%32%17%3
760
11th to 20thNight
28%18%31%18%31
760
21st to endNight
35%17%29%18%2
836
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