Gordon MacMillan Southam Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Gordon MacMillan Southam Observatory clear sky chart from 2005-11-05 to 2017-10-31.

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
35%
14654
8%
47254
7%
14626
38%
14626
13%
35559

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
27%
750
10%
2686
10%
746
29%
746
8%
2184
January16th to endNight
25%
781
8%
2750
8%
781
27%
781
8%
2194
February1st to 15thNight
26%
700
6%
2288
5%
700
28%
700
7%
1884
February16th to endNight
29%
632
11%
2047
8%
632
34%
632
11%
1559
March1st to 15thNight
19%
720
3%
2055
3%
720
20%
720
7%
1777
March16th to endNight
23%
768
6%
2079
3%
768
25%
768
8%
1781
April1st to 15thNight
32%
710
7%
1790
4%
710
33%
710
13%
1454
April16th to endNight
32%
507
8%
1587
5%
507
35%
507
11%
1235
May1st to 15thNight
47%
359
11%
1439
16%
359
50%
359
14%
943
May16th to endNight
46%
374
6%
1451
8%
374
48%
374
12%
1002
June1st to 15thNight
42%
360
2%
1260
1%
360
43%
360
16%
880
June16th to endNight
44%
360
4%
1260
1%
360
45%
360
14%
884
July1st to 15thNight
65%
340
9%
1190
7%
340
67%
340
32%
635
July16th to endNight
69%
376
14%
1320
18%
354
77%
354
34%
658
August1st to 15thNight
71%
434
15%
1507
20%
432
71%
432
35%
749
August16th to endNight
63%
606
18%
1752
14%
606
65%
606
32%
1033
September1st to 15thNight
66%
720
21%
1800
12%
720
68%
720
40%
1070
September16th to endNight
43%
716
15%
1979
11%
716
45%
716
18%
1442
October1st to 15thNight
42%
720
10%
2160
9%
720
44%
720
18%
1531
October16th to endNight
31%
768
7%
2403
6%
768
34%
768
10%
1924
November1st to 15thNight
18%
700
5%
2450
4%
700
20%
700
5%
2120
November16th to endNight
21%
720
5%
2520
3%
720
24%
720
7%
2048
December1st to 15thNight
23%
733
6%
2601
4%
733
25%
733
7%
2138
December16th to endNight
17%
800
6%
2880
3%
800
21%
800
6%
2434

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
24%7%35%424445%32%
48529
Night
28%7%25%423345%31%
47407
Cloud CoverDay
27%8%36%423345%29%
48529
Night
30%8%25%423345%29%
47410
TransparencyDay
59%5%13%14%5%1
48185
Night
58%5%13%14%6%1
47255
SeeingDay
28%11%17%24%13%3
6332
Night
31%16%9%21%17%3
14660

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 15thDay
44%8%36%42334415%
1253
16th to endDay
36%9%36%5%23445%17%
1473
February1st to 15thDay
37%8%36%6%3335%5%16%
1576
16th to endDay
25%10%35%5%24445%25%
1427
March1st to 15thDay
39%11%36%5%3433314%
1903
16th to endDay
32%10%36%7%2433420%
2145
April1st to 15thDay
24%9%37%6%35%5%5%6%21%
2155
16th to endDay
24%8%45%5%24446%27%
2373
May1st to 15thDay
17%7%36%43445%6%36%
2511
16th to endDay
19%7%37%5%35%45%7%30%
2658
June1st to 15thDay
20%5%27%5%25%45%6%33%
2700
16th to endDay
19%7%25%5%3446%7%34%
2700
July1st to 15thDay
10%423413346%55%
2700
16th to endDay
10%312313345%59%
2757
August1st to 15thDay
10%414413245%57%
2442
16th to endDay
10%424324447%51%
2450
September1st to 15thDay
12%5%245%23346%50%
2160
16th to endDay
25%6%25%42345%5%32%
1980
October1st to 15thDay
28%6%35%423446%30%
1800
16th to endDay
36%8%46%423346%19%
1816
November1st to 15thDay
48%6%23412335%17%
1408
16th to endDay
39%8%34423345%18%
1440
December1st to 15thDay
34%11%35%41443322%
1358
16th to endDay
46%8%35%42333316%
1344
January1st to 15thNight
41%8%2441333422%
2686
16th to endNight
34%9%35%424446%20%
2750
February1st to 15thNight
35%9%36%433445%19%
2276
16th to endNight
28%8%35%423336%29%
2047
March1st to 15thNight
40%9%35%5%2433319%
2055
16th to endNight
33%8%36%6%3434420%
2075
April1st to 15thNight
26%9%36%5%35%345%25%
1790
16th to endNight
24%9%36%5%3345%5%26%
1587
May1st to 15thNight
17%325%5%335%5%6%39%
1439
16th to endNight
16%7%36%5%25%45%7%34%
1451
June1st to 15thNight
20%6%26%5%34345%38%
1260
16th to endNight
16%6%25%5%46%5%47%36%
1260
July1st to 15thNight
7%3135%13336%60%
1260
16th to endNight
5%313222346%63%
1411
August1st to 15thNight
6%22313447%61%
1515
16th to endNight
9%32332235%8%52%
1752
September1st to 15thNight
8%212223346%61%
1800
16th to endNight
23%5%25%5%23345%38%
1979
October1st to 15thNight
22%6%25%32334440%
2160
16th to endNight
35%6%25%323346%25%
2403
November1st to 15thNight
43%7%35%5%2333317%
2450
16th to endNight
36%8%36%42432322%
2520
December1st to 15thNight
36%11%35%42333421%
2601
16th to endNight
43%11%25%31322319%
2880

Percentage Cloud Cover SW (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
46%8%45%42223315%
1253
16th to endDay
39%8%35%432345%18%
1473
February1st to 15thDay
38%10%47%424335%15%
1576
16th to endDay
27%12%46%5%35%346%21%
1427
March1st to 15thDay
44%10%46%5%2323312%
1903
16th to endDay
35%12%47%6%3432316%
2145
April1st to 15thDay
29%11%38%6%35%5%45%16%
2155
16th to endDay
28%10%36%5%3445%5%20%
2373
May1st to 15thDay
21%8%37%5%2345%6%30%
2511
16th to endDay
22%9%36%6%35%5%5%6%25%
2658
June1st to 15thDay
24%8%37%5%34446%27%
2700
16th to endDay
22%8%36%6%35%446%28%
2700
July1st to 15thDay
12%424423445%51%
2700
16th to endDay
11%423323345%54%
2757
August1st to 15thDay
10%5%24423335%53%
2442
16th to endDay
13%425%5%2345%7%47%
2450
September1st to 15thDay
14%5%25%332346%48%
2160
16th to endDay
30%6%25%433435%30%
1980
October1st to 15thDay
30%7%3442345%5%29%
1800
16th to endDay
39%9%36%322246%18%
1816
November1st to 15thDay
51%8%35%41222313%
1408
16th to endDay
41%9%36%42333417%
1440
December1st to 15thDay
37%13%4431333222%
1358
16th to endDay
46%11%2442322215%
1344
January1st to 15thNight
43%9%2422233421%
2685
16th to endNight
37%10%35%422346%19%
2750
February1st to 15thNight
38%9%45%423345%18%
2276
16th to endNight
29%11%35%42424427%
2047
March1st to 15thNight
44%10%3442332317%
2055
16th to endNight
37%9%36%5%3434318%
2079
April1st to 15thNight
28%10%35%6%34345%23%
1790
16th to endNight
26%11%36%5%34445%24%
1587
May1st to 15thNight
19%425%5%345%5%5%38%
1439
16th to endNight
17%8%36%425%46%6%33%
1451
June1st to 15thNight
22%7%25%42435%5%35%
1260
16th to endNight
18%6%35%6%35%45%6%34%
1260
July1st to 15thNight
8%413423436%57%
1260
16th to endNight
8%212313345%62%
1411
August1st to 15thNight
7%313223446%61%
1515
16th to endNight
9%42442345%8%50%
1752
September1st to 15thNight
9%213223346%59%
1800
16th to endNight
25%7%24422345%35%
1979
October1st to 15thNight
25%6%2432344438%
2160
16th to endNight
38%7%25%32234423%
2403
November1st to 15thNight
47%9%35%32323315%
2450
16th to endNight
38%9%36%5%2233321%
2520
December1st to 15thNight
37%10%35%42333320%
2601
16th to endNight
45%13%3431223318%
2880

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
73%5%6%8%5%1
1253
16th to endDay
69%5%10%10%31
1473
February1st to 15thDay
73%49%6%32
1576
16th to endDay
66%412%10%5%
1427
March1st to 15thDay
79%48%5%1
1903
16th to endDay
77%29%9%1
2145
April1st to 15thDay
71%5%11%9%1
2155
16th to endDay
67%6%12%10%3
2373
May1st to 15thDay
56%5%15%15%5%
2511
16th to endDay
60%7%15%13%2
2662
June1st to 15thDay
60%5%17%12%31
2700
16th to endDay
58%6%15%14%4
2700
July1st to 15thDay
36%6%17%23%12%2
2541
16th to endDay
33%5%20%26%11%1
2585
August1st to 15thDay
35%5%17%29%9%1
2425
16th to endDay
38%6%21%25%6%1
2450
September1st to 15thDay
38%5%14%22%15%2
2160
16th to endDay
58%5%12%14%7%1
1980
October1st to 15thDay
57%6%13%14%7%
1800
16th to endDay
69%411%9%4
1816
November1st to 15thDay
79%36%6%21
1408
16th to endDay
72%36%9%6%
1440
December1st to 15thDay
70%46%9%6%1
1358
16th to endDay
77%27%7%21
1344
January1st to 15thNight
69%46%13%5%
2686
16th to endNight
67%49%12%32
2750
February1st to 15thNight
69%5%10%9%32
2288
16th to endNight
61%311%15%5%1
2047
March1st to 15thNight
75%39%8%2
2055
16th to endNight
72%5%10%10%2
2079
April1st to 15thNight
64%414%13%2
1790
16th to endNight
62%5%13%13%4
1587
May1st to 15thNight
46%7%18%17%8%
1440
16th to endNight
51%7%20%16%3
1451
June1st to 15thNight
52%5%22%13%41
1260
16th to endNight
50%7%18%16%5%1
1260
July1st to 15thNight
29%5%18%28%14%3
1190
16th to endNight
23%7%18%32%14%3
1320
August1st to 15thNight
25%5%25%27%13%3
1507
16th to endNight
31%5%22%25%10%3
1752
September1st to 15thNight
26%6%17%25%19%4
1800
16th to endNight
52%6%12%19%7%2
1979
October1st to 15thNight
50%5%14%17%10%
2160
16th to endNight
64%5%12%11%5%
2403
November1st to 15thNight
76%48%6%22
2450
16th to endNight
70%39%8%7%
2520
December1st to 15thNight
70%49%9%41
2601
16th to endNight
74%37%8%5%1
2880

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
23%18%26%23%16%
60
16th to endDay
63%6%410%13%1
82
February1st to 15thDay
46%12%12%15%12%
236
16th to endDay
34%15%16%18%12%1
213
March1st to 15thDay
54%13%17%12%1
270
16th to endDay
46%15%16%13%5%1
288
April1st to 15thDay
37%15%17%17%11%1
269
16th to endDay
32%17%22%17%8%1
270
May1st to 15thDay
25%15%21%24%11%1
288
16th to endDay
23%13%17%25%16%3
373
June1st to 15thDay
21%16%19%28%11%2
495
16th to endDay
19%15%23%27%12%1
495
July1st to 15thDay
11%12%18%36%19%1
475
16th to endDay
9%13%15%37%19%3
426
August1st to 15thDay
15%6%13%32%23%8%
315
16th to endDay
14%5%21%33%19%4
332
September1st to 15thDay
20%7%21%26%20%3
315
16th to endDay
31%7%17%25%12%5%
315
October1st to 15thDay
36%6%20%21%13%2
315
16th to endDay
50%7%8%19%11%3
256
November1st to 15thDay
63%10%16%6%3
60
16th to endDay
50%16%15%10%16%
60
December1st to 15thDay
65%16%11%13%1
60
16th to endDay
53%19%21%9%4
64
January1st to 15thNight
40%18%9%17%13%2
750
16th to endNight
37%17%12%18%13%1
781
February1st to 15thNight
38%17%9%20%12%
704
16th to endNight
32%18%13%21%12%1
632
March1st to 15thNight
45%18%12%12%10%
720
16th to endNight
38%19%10%17%9%3
768
April1st to 15thNight
31%19%8%25%12%2
710
16th to endNight
30%18%11%24%11%2
507
May1st to 15thNight
20%17%7%26%24%3
360
16th to endNight
19%18%7%26%26%2
375
June1st to 15thNight
21%17%9%22%25%3
360
16th to endNight
19%18%10%25%24%2
360
July1st to 15thNight
11%12%5%26%38%6%
340
16th to endNight
6%17%324%39%9%
376
August1st to 15thNight
10%10%221%42%12%
434
16th to endNight
15%10%6%22%37%7%
606
September1st to 15thNight
11%9%7%27%33%10%
720
16th to endNight
32%9%8%23%21%4
716
October1st to 15thNight
29%9%13%25%18%2
720
16th to endNight
41%9%10%22%12%2
768
November1st to 15thNight
47%16%13%18%4
700
16th to endNight
38%20%11%16%10%2
720
December1st to 15thNight
40%19%10%20%8%
733
16th to endNight
45%19%13%15%6%
800
Page generated at 2017-11-01 14:23:41. Version ac04

Copyright 2017 A. Danko


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