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Saint Mary's College Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Saint Mary's College Observatory clear sky chart from 2005-11-05 to 2017-09-10.

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
64%
15339
25%
46842
22%
15303
67%
15303
35%
26681

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
60%
716
18%
2389
17%
716
64%
716
27%
1456
January16th to endNight
57%
768
24%
2496
25%
768
60%
768
27%
1545
February1st to 15thNight
54%
698
22%
2288
22%
698
60%
698
24%
1440
February16th to endNight
53%
632
23%
1900
19%
632
59%
632
27%
1225
March1st to 15thNight
52%
720
18%
2007
16%
720
55%
720
27%
1327
March16th to endNight
49%
768
20%
2088
15%
768
54%
768
29%
1366
April1st to 15thNight
48%
719
22%
1799
11%
719
53%
719
31%
1181
April16th to endNight
56%
720
25%
1800
12%
720
61%
720
43%
1060
May1st to 15thNight
68%
609
25%
1689
18%
609
70%
609
45%
917
May16th to endNight
64%
387
24%
1510
23%
387
70%
387
31%
773
June1st to 15thNight
68%
360
23%
1440
26%
360
74%
360
35%
696
June16th to endNight
81%
360
33%
1440
43%
360
83%
360
53%
551
July1st to 15thNight
87%
373
29%
1393
36%
373
89%
373
56%
554
July16th to endNight
79%
564
29%
1593
28%
531
80%
531
46%
792
August1st to 15thNight
73%
553
26%
1626
21%
550
74%
550
43%
846
August16th to endNight
79%
764
31%
1910
19%
764
79%
764
54%
1039
September1st to 15thNight
81%
700
33%
1750
25%
700
82%
700
57%
936
September16th to endNight
79%
656
36%
1801
33%
656
81%
656
53%
920
October1st to 15thNight
70%
660
30%
1980
30%
660
75%
660
42%
1041
October16th to endNight
70%
704
31%
2112
33%
704
73%
704
38%
1175
November1st to 15thNight
68%
700
27%
2187
27%
700
73%
700
38%
1199
November16th to endNight
62%
720
25%
2436
26%
720
65%
720
31%
1368
December1st to 15thNight
54%
720
20%
2520
18%
720
60%
720
23%
1578
December16th to endNight
47%
768
20%
2688
18%
768
54%
768
22%
1696

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
8%423323346%57%
47786
Night
9%424423347%52%
47028
TransparencyDay
27%48%18%23%16%
47473
Night
30%5%9%17%20%15%
46843
SeeingDay
10%10%26%32%15%3
5976
Night
10%413%33%30%7%
15351

By Month

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
16%5%24425%35%6%43%
1546
16th to endDay
18%5%25%434345%42%
1728
February1st to 15thDay
18%8%24323345%41%
1576
16th to endDay
18%7%35%42426%6%38%
1574
March1st to 15thDay
16%6%36%42446%9%36%
1950
16th to endDay
13%6%35%5%2445%6%40%
2136
April1st to 15thDay
15%6%2442345%6%42%
2149
16th to endDay
7%413313347%58%
2160
May1st to 15thDay
4312223348%64%
2259
16th to endDay
6%414312347%58%
2601
June1st to 15thDay
5%412224347%61%
2520
16th to endDay
212212246%74%
2520
July1st to 15thDay
1112212236%73%
2482
16th to endDay
112212236%74%
2468
August1st to 15thDay
1213212236%70%
2325
16th to endDay
212212236%74%
2292
September1st to 15thDay
1212212235%74%
2097
16th to endDay
3312213235%71%
1826
October1st to 15thDay
6%313213236%64%
1650
16th to endDay
8%413422335%59%
1760
November1st to 15thDay
10%423413447%51%
1670
16th to endDay
14%6%35%323335%47%
1521
December1st to 15thDay
18%8%25%324335%42%
1440
16th to endDay
20%6%24413347%40%
1536
January1st to 15thNight
14%5%143245%48%43%
2389
16th to endNight
17%5%24323346%45%
2496
February1st to 15thNight
15%6%245%34445%43%
2276
16th to endNight
16%6%2342435%6%43%
1900
March1st to 15thNight
16%6%245%2445%6%41%
2007
16th to endNight
15%5%35%423346%45%
2088
April1st to 15thNight
12%5%25%42446%7%45%
1799
16th to endNight
7%42332335%6%58%
1800
May1st to 15thNight
432442425%8%59%
1689
16th to endNight
5%335%5%2435%7%55%
1509
June1st to 15thNight
432442336%7%57%
1440
16th to endNight
112313336%71%
1440
July1st to 15thNight
11122242410%65%
1475
16th to endNight
1215%445%45%9%53%
1701
August1st to 15thNight
1326%6%3446%11%49%
1635
16th to endNight
2146%35%46%10%56%
1910
September1st to 15thNight
1213414347%63%
1750
16th to endNight
223323247%65%
1801
October1st to 15thNight
7%413313347%60%
1980
16th to endNight
7%41433345%5%56%
2112
November1st to 15thNight
8%42322335%7%54%
2187
16th to endNight
10%5%14423446%52%
2436
December1st to 15thNight
16%7%24324346%43%
2520
16th to endNight
19%6%24423336%43%
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
40%5%13%17%16%6%
1546
16th to endDay
44%48%13%19%10%
1728
February1st to 15thDay
43%38%19%20%3
1576
16th to endDay
44%48%24%12%3
1574
March1st to 15thDay
44%5%11%19%15%5%
1950
16th to endDay
41%6%12%18%17%3
2136
April1st to 15thDay
40%5%13%19%18%2
2149
16th to endDay
26%411%26%23%7%
2160
May1st to 15thDay
19%5%6%28%31%8%
2259
16th to endDay
25%6%10%27%24%6%
2605
June1st to 15thDay
23%6%9%20%23%16%
2520
16th to endDay
12%3315%33%30%
2520
July1st to 15thDay
11%447%23%48%
2336
16th to endDay
11%35%7%24%47%
2313
August1st to 15thDay
15%4410%29%36%
2309
16th to endDay
11%4412%37%29%
2292
September1st to 15thDay
13%4411%33%33%
2097
16th to endDay
16%3323%31%20%
1826
October1st to 15thDay
22%46%21%27%17%
1650
16th to endDay
27%38%23%24%13%
1760
November1st to 15thDay
31%5%11%28%15%7%
1670
16th to endDay
39%48%19%23%4
1521
December1st to 15thDay
45%49%19%18%3
1440
16th to endDay
44%412%18%16%4
1536
January1st to 15thNight
38%5%10%19%16%8%
2389
16th to endNight
40%48%14%17%14%
2496
February1st to 15thNight
41%48%19%17%7%
2288
16th to endNight
40%5%11%24%15%2
1900
March1st to 15thNight
42%411%19%15%6%
2007
16th to endNight
40%5%11%19%19%5%
2088
April1st to 15thNight
35%6%13%20%18%4
1799
16th to endNight
25%5%11%25%21%9%
1800
May1st to 15thNight
22%6%8%21%28%12%
1690
16th to endNight
27%7%10%24%22%7%
1510
June1st to 15thNight
23%6%10%18%22%17%
1440
16th to endNight
15%25%12%30%32%
1440
July1st to 15thNight
13%38%5%19%49%
1393
16th to endNight
21%412%6%15%38%
1593
August1st to 15thNight
24%411%6%20%32%
1626
16th to endNight
18%6%9%8%25%31%
1910
September1st to 15thNight
18%47%6%26%36%
1750
16th to endNight
18%4420%27%24%
1801
October1st to 15thNight
24%46%21%23%18%
1980
16th to endNight
26%6%6%20%23%15%
2112
November1st to 15thNight
28%5%12%28%16%8%
2187
16th to endNight
32%48%21%24%8%
2436
December1st to 15thNight
42%48%17%19%6%
2520
16th to endNight
42%5%11%15%18%6%
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
24%7%27%25%14%1
174
16th to endDay
19%10%20%28%17%3
256
February1st to 15thDay
23%10%19%27%18%1
236
16th to endDay
21%11%27%24%12%1
237
March1st to 15thDay
21%14%26%18%16%3
270
16th to endDay
15%20%31%21%9%1
288
April1st to 15thDay
19%21%27%20%7%1
269
16th to endDay
14%25%25%25%7%1
270
May1st to 15thDay
8%16%38%27%7%
288
16th to endDay
10%14%26%33%13%1
330
June1st to 15thDay
6%16%37%31%6%1
315
16th to endDay
15%29%41%20%1
315
July1st to 15thDay
1126%48%19%1
305
16th to endDay
226%48%14%7%
327
August1st to 15thDay
25%25%43%18%4
315
16th to endDay
224%43%21%7%
332
September1st to 15thDay
3419%44%21%7%
294
16th to endDay
49%21%33%21%9%
255
October1st to 15thDay
7%8%29%33%15%5%
255
16th to endDay
12%9%27%27%20%2
272
November1st to 15thDay
9%8%20%22%34%5%
176
16th to endDay
8%19%26%30%16%
73
December1st to 15thDay
23%18%28%20%10%
60
16th to endDay
17%14%26%14%25%3
64
January1st to 15thNight
16%310%32%28%8%
720
16th to endNight
18%312%31%26%8%
768
February1st to 15thNight
16%6%12%32%24%7%
704
16th to endNight
16%7%15%31%22%5%
632
March1st to 15thNight
18%7%12%28%29%4
720
16th to endNight
15%7%19%33%22%2
768
April1st to 15thNight
11%10%21%32%21%3
719
16th to endNight
7%8%22%32%24%5%
720
May1st to 15thNight
4315%43%27%4
610
16th to endNight
45%19%38%29%3
388
June1st to 15thNight
3218%48%24%1
360
16th to endNight
1111%50%33%1
360
July1st to 15thNight
18%39%46%4
373
16th to endNight
217%30%48%9%
564
August1st to 15thNight
2112%32%42%7%
553
16th to endNight
19%37%43%8%
764
September1st to 15thNight
217%31%44%11%
700
16th to endNight
318%33%40%12%
656
October1st to 15thNight
7%310%34%33%9%
660
16th to endNight
8%310%30%32%13%
704
November1st to 15thNight
8%214%32%32%10%
700
16th to endNight
11%5%11%31%29%9%
720
December1st to 15thNight
18%6%11%32%23%6%
720
16th to endNight
19%6%18%31%22%2
768
Page generated at 2017-09-11 16:35:31. Version ac04

Copyright 2017 A. Danko


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