Sandilands Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sandilands 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
28%
25111
12%
74816
6%
25057
34%
25057
18%
53980

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%
950
8%
2837
5%
946
27%
946
11%
2263
January11th to 20thNight
19%
930
7%
2788
3%
930
23%
930
11%
2261
January21st to endNight
18%
990
8%
2818
2%
990
23%
990
11%
2293
February1st to 10thNight
26%
900
12%
2348
5%
900
30%
900
17%
1785
February11th to 20thNight
30%
877
15%
2274
6%
875
35%
875
22%
1631
February21st to endNight
24%
736
15%
1920
4%
736
31%
736
17%
1459
March1st to 10thNight
29%
633
15%
2073
9%
633
35%
633
17%
1487
March11th to 20thNight
28%
540
19%
1980
8%
540
44%
540
20%
1248
March21st to endNight
34%
594
16%
2178
14%
594
42%
594
15%
1488
April1st to 10thNight
30%
540
15%
1784
12%
540
40%
540
20%
1169
April11th to 20thNight
30%
533
18%
1599
11%
533
41%
533
21%
1088
April21st to endNight
30%
540
14%
1620
7%
540
36%
540
21%
1100
May1st to 10thNight
26%
540
14%
1499
4%
540
31%
540
24%
1028
May11th to 20thNight
34%
531
14%
1369
5%
531
38%
531
28%
932
May21st to endNight
28%
584
11%
1358
4%
582
31%
582
27%
963
June1st to 10thNight
37%
540
6%
1260
2%
540
40%
540
30%
880
June11th to 20thNight
35%
540
5%
1260
3%
540
38%
540
29%
885
June21st to endNight
49%
540
5%
1260
4%
540
51%
540
33%
860
July1st to 10thNight
49%
525
10%
1225
5%
525
53%
525
47%
727
July11th to 20thNight
48%
522
10%
1218
5%
522
50%
522
38%
780
July21st to endNight
54%
591
18%
1302
8%
558
57%
558
45%
790
August1st to 10thNight
52%
537
18%
1540
8%
534
57%
534
38%
882
August11th to 20thNight
50%
534
17%
1600
10%
534
57%
534
33%
943
August21st to endNight
48%
594
21%
1782
16%
594
57%
594
34%
1046
September1st to 10thNight
40%
540
17%
1748
15%
540
48%
540
31%
1016
September11th to 20thNight
38%
540
15%
1939
14%
540
45%
540
23%
1204
September21st to endNight
30%
540
19%
1980
14%
540
42%
540
20%
1259
October1st to 10thNight
24%
708
13%
2148
6%
708
30%
708
17%
1543
October11th to 20thNight
24%
720
16%
2160
7%
720
31%
720
17%
1598
October21st to endNight
23%
962
10%
2568
3%
962
27%
962
14%
2000
November1st to 10thNight
18%
925
11%
2591
2%
925
25%
925
14%
2004
November11th to 20thNight
18%
950
11%
2660
4%
950
23%
950
11%
2165
November21st to endNight
14%
950
7%
2695
2%
950
18%
950
8%
2231
December1st to 10thNight
21%
950
6%
2850
2%
950
25%
950
9%
2358
December11th to 20thNight
12%
950
7%
3467
1%
949
17%
949
7%
2732
December21st to endNight
16%
1035
6%
5118
3%
1026
23%
1026
5%
3882

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
13%10%48%7%45%5%6%7%32%
74696
Night
11%10%5%9%7%45%5%6%7%32%
75134
TransparencyDay
50%7%13%22%8%1
74377
Night
51%6%13%20%9%1
74972
SeeingDay
18%16%35%22%8%2
15051
Night
17%12%34%26%10%1
25123

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
8%12%8%10%7%5%6%9%8%6%21%
1329
11th to 20thDay
8%12%7%12%11%6%6%9%7%7%16%
1295
21st to endDay
9%17%9%12%10%47%7%8%7%9%
1534
February1st to 10thDay
7%11%6%9%9%5%9%7%8%7%21%
1611
11th to 20thDay
7%10%5%8%9%6%8%7%6%9%25%
1574
21st to endDay
6%10%6%13%10%6%8%5%5%5%26%
1350
March1st to 10thDay
12%15%5%8%8%45%5%5%7%26%
1886
11th to 20thDay
8%11%5%10%8%45%6%47%32%
1978
21st to endDay
12%12%5%10%7%45%5%46%31%
2178
April1st to 10thDay
12%12%5%8%6%3445%6%34%
2172
11th to 20thDay
14%10%48%6%45%45%7%34%
2312
21st to endDay
16%10%47%6%45%5%6%6%32%
2340
May1st to 10thDay
16%10%46%5%35%45%6%38%
2461
11th to 20thDay
18%8%36%6%35%5%6%8%33%
2525
21st to endDay
19%8%35%6%345%5%6%37%
2918
June1st to 10thDay
15%7%36%6%46%5%6%8%34%
2700
11th to 20thDay
16%7%37%6%36%5%6%8%32%
2700
21st to endDay
14%6%37%7%46%6%6%9%31%
2700
July1st to 10thDay
8%425%6%46%6%7%9%44%
2700
11th to 20thDay
12%5%25%5%45%5%6%9%43%
2700
21st to endDay
9%5%25%5%35%5%5%8%46%
2939
August1st to 10thDay
9%5%36%6%35%5%7%9%42%
2388
11th to 20thDay
12%5%35%5%45%5%5%8%43%
2301
21st to endDay
14%5%35%5%35%46%8%43%
2574
September1st to 10thDay
13%5%25%5%345%5%7%45%
2211
11th to 20thDay
14%8%36%6%345%6%8%36%
2019
21st to endDay
19%8%36%43345%7%38%
1973
October1st to 10thDay
18%13%47%6%34446%30%
1800
11th to 20thDay
16%15%5%8%6%34446%31%
1800
21st to endDay
17%17%5%9%7%35%5%5%5%23%
1788
November1st to 10thDay
15%16%6%10%7%34445%25%
1487
11th to 20thDay
11%18%6%13%8%44345%23%
1520
21st to endDay
13%15%6%11%12%46%445%20%
1482
December1st to 10thDay
8%13%7%15%10%48%6%7%6%16%
1330
11th to 20thDay
9%15%8%13%10%45%5%6%7%20%
1679
21st to endDay
11%12%6%12%8%46%5%8%7%21%
2442
January1st to 10thNight
6%13%7%12%10%6%7%9%7%6%19%
2837
11th to 20thNight
9%14%6%12%10%6%7%7%7%6%16%
2788
21st to endNight
8%15%7%12%10%5%8%7%8%7%14%
2818
February1st to 10thNight
7%11%49%10%5%8%7%8%8%24%
2348
11th to 20thNight
7%10%48%7%5%8%7%7%7%32%
2273
21st to endNight
7%9%6%11%9%5%6%6%6%7%28%
1920
March1st to 10thNight
9%12%5%11%8%45%45%7%30%
2073
11th to 20thNight
7%9%5%8%7%44448%41%
1980
21st to endNight
13%11%5%7%6%346%5%7%34%
2178
April1st to 10thNight
13%9%46%5%35%5%6%7%38%
1784
11th to 20thNight
14%10%47%5%35%45%7%37%
1599
21st to endNight
12%10%47%6%45%46%7%36%
1620
May1st to 10thNight
15%10%35%5%3446%6%39%
1499
11th to 20thNight
15%7%25%5%25%5%5%9%40%
1369
21st to endNight
18%6%25%5%3336%7%41%
1358
June1st to 10thNight
13%7%36%5%2447%10%40%
1260
11th to 20thNight
13%5%35%6%35%5%5%7%43%
1260
21st to endNight
13%5%245%346%7%8%43%
1260
July1st to 10thNight
6%3234245%6%9%57%
1260
11th to 20thNight
7%425%4246%6%10%49%
1260
21st to endNight
8%42442345%9%56%
1379
August1st to 10thNight
6%425%445%5%5%8%53%
1547
11th to 20thNight
10%5%25%33346%9%51%
1600
21st to endNight
12%6%2433445%7%52%
1782
September1st to 10thNight
12%5%25%43335%6%51%
1748
11th to 20thNight
14%6%36%5%335%5%8%41%
1939
21st to endNight
20%6%345%3445%7%40%
1980
October1st to 10thNight
17%9%37%6%445%5%7%33%
2148
11th to 20thNight
18%11%47%5%45%45%6%30%
2160
21st to endNight
18%14%5%9%6%35%45%6%27%
2568
November1st to 10thNight
14%15%7%8%6%45%45%6%26%
2591
11th to 20thNight
11%16%6%13%8%45%435%23%
2660
21st to endNight
13%15%6%12%10%5%6%445%19%
2695
December1st to 10thNight
8%12%6%14%11%7%7%7%5%6%17%
2850
11th to 20thNight
10%14%8%13%10%46%6%6%6%18%
3518
21st to endNight
10%11%6%13%10%6%6%6%5%5%21%
5225

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
58%7%12%18%5%
1329
11th to 20thDay
63%6%12%15%5%
1295
21st to endDay
71%5%7%10%7%
1534
February1st to 10thDay
57%7%12%14%9%1
1611
11th to 20thDay
54%5%12%19%9%
1583
21st to endDay
59%6%9%16%9%
1350
March1st to 10thDay
57%6%12%17%7%
1886
11th to 20thDay
51%7%13%21%7%
1980
21st to endDay
54%6%12%20%8%1
2178
April1st to 10thDay
51%5%10%22%11%1
2172
11th to 20thDay
51%411%21%12%1
2312
21st to endDay
52%6%12%20%9%1
2340
May1st to 10thDay
48%5%11%22%12%2
2461
11th to 20thDay
48%6%9%23%12%1
2529
21st to endDay
47%7%11%21%14%1
2918
June1st to 10thDay
46%8%14%21%10%1
2700
11th to 20thDay
48%8%13%21%10%
2700
21st to endDay
48%9%16%21%6%
2700
July1st to 10thDay
32%9%16%31%10%1
2626
11th to 20thDay
36%8%17%29%9%
2619
21st to endDay
36%8%17%30%9%1
2776
August1st to 10thDay
36%10%15%30%8%1
2371
11th to 20thDay
38%8%18%30%6%
2301
21st to endDay
39%6%15%32%8%
2574
September1st to 10thDay
38%7%15%30%9%1
2211
11th to 20thDay
45%9%17%23%5%
2019
21st to endDay
47%6%13%31%4
1973
October1st to 10thDay
55%7%15%18%6%
1800
11th to 20thDay
55%5%13%21%6%1
1800
21st to endDay
63%6%13%15%3
1788
November1st to 10thDay
61%6%12%15%5%1
1487
11th to 20thDay
65%5%13%12%4
1520
21st to endDay
67%5%14%12%2
1482
December1st to 10thDay
66%8%10%12%4
1330
11th to 20thDay
63%5%14%14%3
1680
21st to endDay
60%5%9%21%5%
2442
January1st to 10thNight
63%7%11%15%5%
2837
11th to 20thNight
65%6%9%15%4
2788
21st to endNight
66%5%10%13%5%
2818
February1st to 10thNight
54%6%14%16%10%1
2348
11th to 20thNight
49%5%17%19%10%
2274
21st to endNight
54%7%11%18%10%
1920
March1st to 10thNight
54%412%18%10%1
2073
11th to 20thNight
43%5%13%25%12%1
1980
21st to endNight
49%7%12%18%12%1
2178
April1st to 10thNight
45%6%11%22%15%2
1784
11th to 20thNight
49%39%20%17%2
1599
21st to endNight
47%410%25%12%1
1620
May1st to 10thNight
45%6%11%23%14%1
1499
11th to 20thNight
41%7%12%24%15%1
1369
21st to endNight
43%6%11%21%17%2
1358
June1st to 10thNight
40%8%18%22%12%1
1260
11th to 20thNight
40%8%13%23%14%1
1260
21st to endNight
37%8%20%27%7%1
1260
July1st to 10thNight
23%7%17%39%12%2
1225
11th to 20thNight
28%10%19%29%14%
1218
21st to endNight
26%6%15%38%14%1
1302
August1st to 10thNight
27%7%18%36%11%1
1540
11th to 20thNight
30%8%19%34%8%
1600
21st to endNight
31%6%15%35%12%1
1782
September1st to 10thNight
34%5%15%32%13%
1748
11th to 20thNight
40%7%17%26%9%1
1939
21st to endNight
44%6%14%29%8%
1980
October1st to 10thNight
50%8%14%19%9%
2148
11th to 20thNight
54%6%13%22%5%
2160
21st to endNight
57%7%14%17%4
2568
November1st to 10thNight
59%6%14%15%6%1
2591
11th to 20thNight
64%7%11%13%41
2660
21st to endNight
67%6%12%10%4
2695
December1st to 10thNight
67%7%13%10%3
2850
11th to 20thNight
65%6%13%12%4
3516
21st to endNight
64%5%10%15%5%
5225

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
13%16%39%21%9%2
270
11th to 20thDay
15%12%40%28%5%
255
21st to endDay
16%14%42%21%5%2
264
February1st to 10thDay
16%18%42%20%31
240
11th to 20thDay
19%18%39%16%7%1
240
21st to endDay
15%21%41%17%5%1
198
March1st to 10thDay
14%19%40%16%8%2
240
11th to 20thDay
17%18%36%20%7%2
240
21st to endDay
12%22%38%22%41
264
April1st to 10thDay
19%18%36%21%6%
432
11th to 20thDay
22%19%33%18%6%1
590
21st to endDay
20%20%34%19%6%1
600
May1st to 10thDay
23%20%32%19%5%1
603
11th to 20thDay
23%17%30%20%8%2
625
21st to endDay
22%14%34%24%5%1
679
June1st to 10thDay
19%16%33%20%10%3
630
11th to 20thDay
21%16%37%18%6%2
630
21st to endDay
16%13%30%27%11%3
630
July1st to 10thDay
10%14%36%23%12%4
621
11th to 20thDay
13%14%34%24%13%2
618
21st to endDay
10%12%33%26%14%6%
684
August1st to 10thDay
10%11%32%28%16%3
626
11th to 20thDay
14%10%30%29%12%6%
615
21st to endDay
15%12%32%26%13%1
693
September1st to 10thDay
16%14%33%25%11%1
501
11th to 20thDay
18%17%39%18%7%
309
21st to endDay
26%17%36%19%3
270
October1st to 10thDay
26%14%32%18%8%2
270
11th to 20thDay
22%16%36%22%4
270
21st to endDay
30%18%30%18%31
297
November1st to 10thDay
25%15%37%18%5%
270
11th to 20thDay
22%23%39%14%3
270
21st to endDay
27%23%33%14%2
270
December1st to 10thDay
15%16%38%23%7%1
270
11th to 20thDay
18%19%47%14%3
270
21st to endDay
16%12%42%17%10%2
297
January1st to 10thNight
12%16%35%25%10%1
950
11th to 20thNight
17%12%39%24%7%1
930
21st to endNight
16%14%39%24%6%1
990
February1st to 10thNight
14%13%36%29%9%
900
11th to 20thNight
12%13%36%27%10%2
879
21st to endNight
10%15%40%24%9%1
736
March1st to 10thNight
15%13%33%28%9%1
633
11th to 20thNight
11%14%43%22%9%1
540
21st to endNight
20%12%28%27%12%2
594
April1st to 10thNight
17%12%35%27%9%1
540
11th to 20thNight
21%11%31%27%10%1
533
21st to endNight
18%11%35%29%6%
540
May1st to 10thNight
20%10%38%25%7%
540
11th to 20thNight
20%9%34%26%10%1
531
21st to endNight
21%11%38%23%8%1
584
June1st to 10thNight
17%9%31%30%14%
540
11th to 20thNight
16%9%33%29%13%1
540
21st to endNight
15%6%25%35%17%1
540
July1st to 10thNight
6%6%34%35%18%1
525
11th to 20thNight
8%8%31%36%16%1
522
21st to endNight
9%428%38%18%3
591
August1st to 10thNight
9%430%37%17%1
537
11th to 20thNight
11%8%25%32%20%4
534
21st to endNight
17%6%26%35%16%1
594
September1st to 10thNight
15%6%35%31%11%2
540
11th to 20thNight
17%9%32%31%11%
540
21st to endNight
23%9%33%25%9%1
540
October1st to 10thNight
23%12%35%22%7%
708
11th to 20thNight
24%10%35%22%8%1
720
21st to endNight
28%13%31%22%6%1
962
November1st to 10thNight
22%16%35%19%8%1
925
11th to 20thNight
21%15%35%21%7%1
950
21st to endNight
22%19%38%17%41
950
December1st to 10thNight
14%13%38%26%7%1
950
11th to 20thNight
19%16%39%19%5%1
950
21st to endNight
20%13%36%23%8%1
1045
Page generated at 2024-01-13 08:37:54. Version ac07

Copyright 2024 Attilla Danko


Sandilands Observatory Clear Sky Chart |Charts Home