Perth
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Perth 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
28%
14687
14%
46859
8%
14659
35%
14659
16%
33267

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
14%
720
7%
2506
2%
716
21%
716
7%
2007
January16th to endNight
18%
768
11%
2615
5%
768
25%
768
10%
1966
February1st to 15thNight
15%
701
9%
2287
3%
701
23%
701
9%
1773
February16th to endNight
17%
631
14%
1957
6%
631
27%
631
12%
1451
March1st to 15thNight
19%
720
14%
2082
7%
720
27%
720
15%
1535
March16th to endNight
30%
768
20%
2046
9%
768
38%
768
22%
1417
April1st to 15thNight
21%
718
16%
1796
5%
718
27%
718
18%
1315
April16th to endNight
28%
659
17%
1739
7%
659
34%
659
22%
1231
May1st to 15thNight
32%
360
17%
1440
14%
360
43%
360
17%
948
May16th to endNight
32%
374
14%
1497
9%
374
45%
374
20%
917
June1st to 15thNight
35%
360
9%
1440
9%
360
44%
360
15%
965
June16th to endNight
40%
360
9%
1440
10%
360
45%
360
18%
932
July1st to 15thNight
48%
340
14%
1360
20%
340
53%
340
26%
749
July16th to endNight
56%
376
14%
1418
17%
354
59%
354
27%
815
August1st to 15thNight
48%
472
15%
1543
11%
470
54%
470
26%
942
August16th to endNight
52%
703
21%
1849
10%
703
57%
703
37%
1122
September1st to 15thNight
44%
720
17%
1800
8%
720
48%
720
31%
1221
September16th to endNight
46%
665
23%
1818
14%
665
51%
665
31%
1164
October1st to 15thNight
31%
660
17%
1980
11%
660
40%
660
18%
1396
October16th to endNight
21%
704
10%
2183
4%
704
30%
704
12%
1665
November1st to 15thNight
25%
700
18%
2335
12%
700
34%
700
16%
1631
November16th to endNight
19%
720
12%
2520
7%
720
27%
720
10%
1931
December1st to 15thNight
15%
720
6%
2520
2%
720
19%
720
8%
2009
December16th to endNight
12%
768
9%
2688
4%
768
18%
768
8%
2165

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
18%8%36%5%34446%33%
47926
Night
17%9%36%5%34446%33%
47028
Cloud CoverDay
17%9%36%5%34446%32%
47935
Night
17%9%46%5%24446%33%
47034
TransparencyDay
51%6%11%19%10%1
47613
Night
51%6%11%19%10%1
46859
SeeingDay
23%13%29%19%9%4
5470
Night
23%13%29%23%9%
14691

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
19%16%6%7%6%3434423%
1432
16th to endDay
13%15%5%10%9%35%435%22%
1608
February1st to 15thDay
14%13%6%12%7%35%345%22%
1575
16th to endDay
18%14%5%7%6%34335%26%
1515
March1st to 15thDay
17%11%5%8%8%5%5%33326%
1867
16th to endDay
17%8%36%424335%40%
2178
April1st to 15thDay
22%10%35%42334433%
2152
16th to endDay
20%7%36%423345%37%
2230
May1st to 15thDay
19%8%36%5%3345%6%34%
2511
16th to endDay
15%6%26%5%35%46%7%36%
2618
June1st to 15thDay
23%6%35%424448%31%
2520
16th to endDay
17%5%25%6%36%6%48%32%
2520
July1st to 15thDay
12%5%35%435%5%6%9%38%
2520
16th to endDay
12%5%25%5%36%5%7%8%35%
2654
August1st to 15thDay
13%5%26%7%45%45%7%37%
2406
16th to endDay
12%425%5%35%45%8%41%
2339
September1st to 15thDay
17%424433448%42%
2160
16th to endDay
16%4245%33345%45%
1845
October1st to 15thDay
20%8%35%6%33446%34%
1650
16th to endDay
27%10%47%5%2344425%
1689
November1st to 15thDay
14%8%36%7%36%45%5%34%
1521
16th to endDay
22%13%47%42445%5%24%
1440
December1st to 15thDay
23%15%5%7%6%35%435%19%
1440
16th to endDay
24%15%5%7%5%25%42421%
1536
January1st to 15thNight
19%15%6%9%7%3444420%
2506
16th to endNight
13%14%6%10%6%4435%5%25%
2615
February1st to 15thNight
14%14%5%10%6%35%45%5%22%
2285
16th to endNight
15%13%47%6%33446%28%
1957
March1st to 15thNight
19%11%48%5%3344431%
2069
16th to endNight
15%8%46%423345%41%
2046
April1st to 15thNight
22%8%35%5%3343435%
1796
16th to endNight
18%8%35%5%23446%38%
1739
May1st to 15thNight
21%7%35%325%436%36%
1440
16th to endNight
15%6%25%42446%7%39%
1495
June1st to 15thNight
19%7%35%435%45%6%34%
1440
16th to endNight
15%5%245%36%5%5%8%36%
1440
July1st to 15thNight
9%415%5%25%5%48%46%
1440
16th to endNight
9%3144345%6%9%45%
1514
August1st to 15thNight
9%435%5%245%7%8%41%
1553
16th to endNight
9%41432345%8%51%
1849
September1st to 15thNight
15%5%246%23346%45%
1800
16th to endNight
16%5%14323345%48%
1818
October1st to 15thNight
20%6%35%5%24347%34%
1980
16th to endNight
26%10%36%6%23436%26%
2183
November1st to 15thNight
18%10%36%4245%35%34%
2335
16th to endNight
21%12%47%6%24436%25%
2520
December1st to 15thNight
20%16%5%8%6%33345%21%
2520
16th to endNight
24%14%48%5%35%33419%
2688

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
20%16%5%7%6%3434323%
1432
16th to endDay
13%15%410%10%35%5%35%22%
1608
February1st to 15thDay
14%13%5%12%7%35%445%22%
1575
16th to endDay
18%14%5%6%6%44335%25%
1515
March1st to 15thDay
18%11%5%8%8%46%34326%
1877
16th to endDay
17%8%35%42334441%
2177
April1st to 15thDay
22%10%45%324345%33%
2152
16th to endDay
20%8%35%423445%36%
2230
May1st to 15thDay
19%8%36%5%2345%6%33%
2511
16th to endDay
15%5%35%5%3446%7%36%
2618
June1st to 15thDay
23%6%25%43445%7%31%
2520
16th to endDay
17%6%35%5%36%5%48%32%
2520
July1st to 15thDay
12%5%36%425%5%6%9%38%
2520
16th to endDay
12%5%25%6%36%5%7%9%34%
2654
August1st to 15thDay
12%6%25%7%4445%7%37%
2406
16th to endDay
11%5%146%35%5%5%8%42%
2339
September1st to 15thDay
16%5%25%42435%7%42%
2160
16th to endDay
16%5%2442325%5%44%
1845
October1st to 15thDay
19%9%26%5%33446%34%
1650
16th to endDay
27%10%47%6%2333425%
1689
November1st to 15thDay
14%8%47%6%45%5%46%33%
1521
16th to endDay
22%13%47%5%3345%425%
1440
December1st to 15thDay
23%14%5%8%6%34435%19%
1440
16th to endDay
24%15%5%7%5%35%43421%
1536
January1st to 15thNight
19%15%5%10%7%34335%20%
2506
16th to endNight
14%13%6%9%6%44436%25%
2615
February1st to 15thNight
14%14%6%9%7%35%45%5%23%
2278
16th to endNight
16%13%5%7%6%34445%28%
1957
March1st to 15thNight
19%11%5%6%5%2334431%
2082
16th to endNight
16%8%46%5%2235%5%40%
2046
April1st to 15thNight
22%8%45%5%2333434%
1796
16th to endNight
18%8%25%5%23446%38%
1739
May1st to 15thNight
21%7%26%3245%35%36%
1440
16th to endNight
14%6%35%43445%8%39%
1495
June1st to 15thNight
19%7%35%5%35%46%5%34%
1440
16th to endNight
14%6%15%5%26%5%6%7%37%
1440
July1st to 15thNight
9%4335%25%45%8%46%
1440
16th to endNight
9%315%4246%7%9%45%
1514
August1st to 15thNight
9%5%27%43447%8%41%
1553
16th to endNight
9%32431445%8%50%
1849
September1st to 15thNight
15%5%25%5%33346%45%
1800
16th to endNight
16%5%24322346%48%
1818
October1st to 15thNight
20%6%35%5%2435%6%35%
1980
16th to endNight
26%10%36%5%23436%26%
2183
November1st to 15thNight
18%10%45%424445%34%
2335
16th to endNight
21%12%5%7%6%34336%25%
2520
December1st to 15thNight
20%17%5%9%6%2334421%
2520
16th to endNight
24%14%5%8%6%24435%19%
2688

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
64%49%13%6%1
1433
16th to endDay
64%5%8%12%8%
1608
February1st to 15thDay
63%5%9%16%4
1577
16th to endDay
61%410%11%10%1
1515
March1st to 15thDay
63%49%14%7%1
1877
16th to endDay
47%49%18%15%5%
2178
April1st to 15thDay
53%47%17%14%2
2152
16th to endDay
49%47%19%16%3
2230
May1st to 15thDay
50%410%17%14%2
2511
16th to endDay
45%7%12%20%11%2
2622
June1st to 15thDay
51%6%10%19%10%
2520
16th to endDay
47%7%12%20%11%
2520
July1st to 15thDay
38%7%13%28%11%
2371
16th to endDay
43%9%14%22%9%
2487
August1st to 15thDay
44%8%14%21%10%
2392
16th to endDay
38%6%16%26%11%
2339
September1st to 15thDay
41%6%12%26%12%
2160
16th to endDay
41%5%10%30%11%
1845
October1st to 15thDay
50%5%13%20%9%
1650
16th to endDay
62%6%10%15%5%
1689
November1st to 15thDay
51%5%15%18%9%
1521
16th to endDay
61%6%10%13%7%1
1440
December1st to 15thDay
67%5%12%11%2
1440
16th to endDay
67%6%10%10%5%
1536
January1st to 15thNight
66%5%9%12%4
2506
16th to endNight
60%5%10%16%7%
2615
February1st to 15thNight
60%7%11%15%4
2287
16th to endNight
56%5%10%18%9%
1957
March1st to 15thNight
55%411%19%7%
2082
16th to endNight
45%5%9%19%16%3
2046
April1st to 15thNight
52%47%20%14%1
1796
16th to endNight
46%5%7%18%18%3
1739
May1st to 15thNight
48%5%8%18%14%4
1440
16th to endNight
41%6%14%19%15%1
1497
June1st to 15thNight
48%7%13%19%11%1
1440
16th to endNight
43%8%13%20%13%
1440
July1st to 15thNight
32%7%13%28%16%1
1360
16th to endNight
32%9%16%25%15%
1418
August1st to 15thNight
37%7%17%23%13%
1543
16th to endNight
29%7%16%32%13%
1849
September1st to 15thNight
38%7%11%30%11%1
1800
16th to endNight
37%5%11%33%11%
1818
October1st to 15thNight
49%5%10%20%12%
1980
16th to endNight
59%5%12%16%6%
2183
November1st to 15thNight
50%6%13%17%11%1
2335
16th to endNight
61%5%10%17%4
2520
December1st to 15thNight
65%5%12%12%3
2520
16th to endNight
67%5%10%11%5%
2688

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
28%30%18%16%6%
60
16th to endDay
44%14%25%10%5%
76
February1st to 15thDay
37%24%32%5%
90
16th to endDay
32%19%29%12%5%
174
March1st to 15thDay
26%17%30%20%4
270
16th to endDay
24%18%30%15%8%1
288
April1st to 15thDay
28%24%25%11%10%
270
16th to endDay
23%18%28%16%7%5%
270
May1st to 15thDay
29%14%27%17%6%4
288
16th to endDay
22%13%35%16%9%2
331
June1st to 15thDay
26%10%30%21%9%1
315
16th to endDay
20%12%30%21%12%2
315
July1st to 15thDay
14%7%31%26%14%4
305
16th to endDay
13%7%33%24%15%5%
326
August1st to 15thDay
15%5%30%30%11%6%
313
16th to endDay
14%5%28%26%12%12%
332
September1st to 15thDay
17%10%24%20%13%12%
315
16th to endDay
22%6%30%18%13%9%
259
October1st to 15thDay
26%12%26%21%8%3
255
16th to endDay
31%18%27%17%5%
259
November1st to 15thDay
18%14%30%27%6%1
175
16th to endDay
30%16%30%13%10%
60
December1st to 15thDay
28%21%33%10%6%
60
16th to endDay
48%25%25%1
64
January1st to 15thNight
28%20%29%17%3
720
16th to endNight
22%15%35%21%4
768
February1st to 15thNight
21%21%34%18%3
704
16th to endNight
23%16%36%17%4
631
March1st to 15thNight
27%16%32%20%2
720
16th to endNight
19%17%28%23%9%1
768
April1st to 15thNight
27%15%31%21%3
718
16th to endNight
22%16%30%20%8%1
659
May1st to 15thNight
25%8%28%26%8%1
360
16th to endNight
16%8%38%26%10%
375
June1st to 15thNight
21%8%28%32%9%
360
16th to endNight
17%432%31%14%
360
July1st to 15thNight
10%5%28%39%15%
340
16th to endNight
7%228%44%14%1
376
August1st to 15thNight
13%426%33%20%1
472
16th to endNight
11%5%26%34%20%2
703
September1st to 15thNight
17%5%28%27%19%2
720
16th to endNight
19%7%23%25%20%3
665
October1st to 15thNight
24%12%26%23%12%1
660
16th to endNight
31%13%26%22%5%
704
November1st to 15thNight
26%12%31%19%10%
700
16th to endNight
30%17%25%17%8%
720
December1st to 15thNight
31%19%29%16%3
720
16th to endNight
33%15%32%16%1
768
Page generated at 2017-09-18 08:38:23. Version ac04

Copyright 2017 A. Danko


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