Drumheller
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Drumheller clear sky chart from 2005-11-06 to 2024-03-18.

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
43%
23765
13%
80997
9%
23714
47%
23714
20%
53650

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
38%
936
14%
4681
8%
926
42%
926
10%
3224
January11th to 20thNight
39%
940
18%
4617
10%
930
45%
930
12%
3050
January21st to endNight
38%
884
10%
4895
10%
878
45%
878
11%
2940
February1st to 10thNight
41%
760
14%
2627
14%
760
45%
760
16%
1855
February11th to 20thNight
37%
740
12%
2403
10%
740
42%
740
16%
1718
February21st to endNight
49%
628
14%
1985
14%
628
53%
628
21%
1355
March1st to 10thNight
41%
760
14%
2265
11%
760
45%
760
19%
1646
March11th to 20thNight
46%
752
17%
2068
10%
752
51%
752
24%
1460
March21st to endNight
39%
792
9%
2178
5%
792
42%
792
17%
1651
April1st to 10thNight
38%
626
12%
1724
5%
626
43%
626
21%
1248
April11th to 20thNight
36%
533
14%
1601
9%
533
40%
533
19%
1140
April21st to endNight
41%
540
12%
1620
10%
540
44%
540
21%
1106
May1st to 10thNight
42%
497
10%
1469
7%
497
46%
497
21%
1022
May11th to 20thNight
46%
354
13%
1239
11%
354
51%
354
24%
773
May21st to endNight
43%
388
5%
1358
3%
388
45%
388
17%
952
June1st to 10thNight
42%
360
1%
1260
0%
360
42%
360
19%
859
June11th to 20thNight
37%
360
0%
1260
0%
360
37%
360
14%
916
June21st to endNight
51%
360
0%
1260
0%
360
51%
360
24%
780
July1st to 10thNight
60%
350
1%
1225
0%
350
60%
350
26%
741
July11th to 20thNight
56%
348
4%
1218
2%
348
57%
348
25%
752
July21st to endNight
63%
394
12%
1302
11%
372
67%
372
35%
671
August1st to 10thNight
58%
366
10%
1414
14%
364
60%
364
24%
817
August11th to 20thNight
60%
534
14%
1602
13%
534
61%
534
32%
980
August21st to endNight
60%
594
20%
1782
15%
594
63%
594
35%
1032
September1st to 10thNight
54%
566
18%
1697
13%
566
57%
566
30%
1046
September11th to 20thNight
53%
720
18%
1980
13%
720
57%
720
31%
1277
September21st to endNight
49%
720
21%
1980
14%
720
55%
720
36%
1221
October1st to 10thNight
45%
720
19%
2099
10%
720
52%
720
27%
1371
October11th to 20thNight
43%
720
16%
2245
10%
720
50%
720
25%
1464
October21st to endNight
38%
792
15%
2574
9%
792
45%
792
20%
1719
November1st to 10thNight
34%
905
16%
2570
9%
905
40%
905
20%
1831
November11th to 20thNight
35%
950
16%
2688
9%
950
40%
950
20%
1893
November21st to endNight
38%
950
11%
2850
7%
950
42%
950
20%
2013
December1st to 10thNight
41%
950
13%
2850
9%
950
44%
950
17%
2073
December11th to 20thNight
36%
949
11%
3481
6%
949
40%
949
15%
2398
December21st to endNight
36%
1027
10%
4930
8%
1026
41%
1026
15%
2656

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
9%8%47%6%45%6%6%8%37%
78057
Night
7%9%48%7%46%6%6%8%35%
81680
TransparencyDay
43%9%16%22%9%1
77759
Night
44%8%15%21%10%2
81558
SeeingDay
12%7%25%31%19%5%
14476
Night
11%6%28%38%15%2
23829

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
410%6%11%9%5%6%7%8%8%27%
2219
11th to 20thDay
5%8%5%10%7%5%7%10%9%8%27%
2199
21st to endDay
410%49%8%46%5%8%9%33%
2707
February1st to 10thDay
8%13%410%11%5%6%5%5%8%25%
1553
11th to 20thDay
6%11%7%10%8%47%7%6%6%25%
1665
21st to endDay
410%7%14%9%5%7%8%5%7%24%
1465
March1st to 10thDay
6%12%5%12%11%5%8%6%5%6%24%
1915
11th to 20thDay
8%8%5%8%8%46%6%8%9%31%
2057
21st to endDay
8%12%7%10%6%36%5%6%8%28%
2178
April1st to 10thDay
9%11%48%7%5%6%6%6%9%31%
2231
11th to 20thDay
11%10%5%8%6%45%6%6%8%31%
2310
21st to endDay
13%8%37%7%45%5%5%8%33%
2340
May1st to 10thDay
14%9%47%7%46%6%7%8%30%
2486
11th to 20thDay
10%5%36%6%45%6%6%9%41%
2654
21st to endDay
18%6%36%5%45%5%5%7%34%
2917
June1st to 10thDay
14%7%27%6%35%5%8%9%34%
2700
11th to 20thDay
19%8%37%6%35%6%7%8%28%
2700
21st to endDay
9%425%5%36%6%8%10%42%
2700
July1st to 10thDay
8%5%25%5%46%6%7%10%43%
2700
11th to 20thDay
10%424436%5%6%10%45%
2700
21st to endDay
6%323435%5%7%9%53%
2938
August1st to 10thDay
8%6%25%4345%7%8%47%
2517
11th to 20thDay
8%425%5%3446%9%50%
2299
21st to endDay
9%4245%345%6%9%49%
2574
September1st to 10thDay
10%6%25%44446%8%47%
2259
11th to 20thDay
16%6%25%4345%5%7%45%
1980
21st to endDay
8%7%35%5%245%6%10%46%
1980
October1st to 10thDay
12%9%37%5%3345%7%41%
1860
11th to 20thDay
9%7%35%7%35%5%5%9%41%
1712
21st to endDay
7%11%47%6%46%6%7%6%36%
1782
November1st to 10thDay
10%14%6%11%7%45%5%5%6%27%
1489
11th to 20thDay
7%12%5%10%8%35%5%5%7%33%
1487
21st to endDay
6%10%5%10%8%46%46%7%34%
1330
December1st to 10thDay
7%12%6%9%9%5%9%6%5%6%27%
1330
11th to 20thDay
310%6%13%10%5%6%6%7%7%27%
1682
21st to endDay
38%48%6%35%46%8%44%
2442
January1st to 10thNight
413%5%11%9%5%6%7%7%9%24%
4726
11th to 20thNight
47%5%11%9%47%8%6%8%29%
4699
21st to endNight
410%5%10%7%46%7%7%8%32%
4978
February1st to 10thNight
5%12%5%12%8%45%6%6%8%28%
2627
11th to 20thNight
5%11%5%11%9%5%7%6%7%7%27%
2402
21st to endNight
48%5%10%7%46%7%7%8%32%
1985
March1st to 10thNight
5%8%5%11%10%6%6%6%6%8%29%
2265
11th to 20thNight
5%8%48%7%5%6%8%7%9%32%
2068
21st to endNight
8%12%6%9%7%5%7%6%7%9%25%
2178
April1st to 10thNight
10%9%48%7%5%6%5%6%8%31%
1724
11th to 20thNight
8%11%5%9%7%5%5%6%5%8%30%
1601
21st to endNight
10%9%46%7%46%6%6%8%34%
1620
May1st to 10thNight
11%9%48%6%46%5%6%9%33%
1469
11th to 20thNight
10%6%36%6%46%46%7%42%
1239
21st to endNight
19%7%35%5%36%6%6%8%32%
1358
June1st to 10thNight
14%8%37%7%36%5%6%9%32%
1260
11th to 20thNight
14%7%48%8%47%6%7%7%28%
1260
21st to endNight
7%7%26%6%35%5%7%9%42%
1260
July1st to 10thNight
7%5%25%6%46%6%7%10%42%
1260
11th to 20thNight
8%3245%47%8%8%9%42%
1260
21st to endNight
5%323436%5%6%9%54%
1379
August1st to 10thNight
8%425%6%45%5%8%10%44%
1421
11th to 20thNight
8%4245%35%5%6%10%47%
1602
21st to endNight
8%425%5%345%6%9%49%
1782
September1st to 10thNight
10%6%16%4345%5%7%48%
1697
11th to 20thNight
13%7%3443445%7%46%
1980
21st to endNight
8%6%2443447%9%48%
1980
October1st to 10thNight
11%8%35%6%35%6%6%7%41%
2099
11th to 20thNight
8%7%38%6%45%6%6%8%39%
2245
21st to endNight
7%9%49%7%36%6%7%10%34%
2574
November1st to 10thNight
11%13%5%7%6%45%6%5%7%32%
2570
11th to 20thNight
5%9%49%8%5%7%7%7%9%32%
2688
21st to endNight
6%10%5%9%8%46%7%7%9%32%
2850
December1st to 10thNight
6%12%6%10%8%5%6%5%5%8%29%
2850
11th to 20thNight
310%5%11%9%5%7%7%6%9%29%
3510
21st to endNight
47%47%7%45%5%5%9%43%
5214

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
49%10%12%20%8%1
2219
11th to 20thDay
46%9%13%15%15%2
2220
21st to endDay
42%9%16%26%6%
2707
February1st to 10thDay
56%7%13%16%8%1
1553
11th to 20thDay
55%8%13%15%8%1
1674
21st to endDay
56%6%11%17%9%
1465
March1st to 10thDay
58%10%13%14%5%
1915
11th to 20thDay
45%9%15%22%9%1
2058
21st to endDay
52%10%17%15%6%
2178
April1st to 10thDay
48%9%18%18%8%1
2231
11th to 20thDay
49%9%13%20%8%1
2310
21st to endDay
48%9%14%21%7%1
2340
May1st to 10thDay
49%10%14%18%9%
2486
11th to 20thDay
38%9%15%26%11%1
2658
21st to endDay
48%8%15%20%8%1
2918
June1st to 10thDay
45%10%16%21%8%
2700
11th to 20thDay
52%9%16%17%6%1
2700
21st to endDay
34%12%20%26%7%2
2700
July1st to 10thDay
33%10%21%27%8%1
2626
11th to 20thDay
33%11%20%27%9%
2619
21st to endDay
26%10%20%31%13%1
2775
August1st to 10thDay
33%11%20%27%9%1
2499
11th to 20thDay
30%7%24%27%10%1
2299
21st to endDay
30%9%19%28%13%1
2574
September1st to 10thDay
34%8%17%26%11%4
2259
11th to 20thDay
39%7%14%29%10%1
1980
21st to endDay
33%8%17%27%11%3
1980
October1st to 10thDay
43%6%16%24%10%1
1860
11th to 20thDay
39%8%17%25%10%1
1712
21st to endDay
45%9%13%22%11%1
1782
November1st to 10thDay
58%6%13%16%6%1
1489
11th to 20thDay
49%8%13%21%8%1
1487
21st to endDay
47%8%17%19%8%1
1330
December1st to 10thDay
55%7%13%16%7%2
1330
11th to 20thDay
54%8%14%20%41
1684
21st to endDay
36%5%13%25%15%5%
2442
January1st to 10thNight
50%10%13%20%5%1
4726
11th to 20thNight
49%6%12%17%13%3
4740
21st to endNight
44%8%14%26%8%1
4978
February1st to 10thNight
52%6%12%19%10%
2627
11th to 20thNight
51%8%14%15%10%1
2403
21st to endNight
46%8%14%21%10%1
1985
March1st to 10thNight
48%10%16%19%6%
2265
11th to 20thNight
44%9%15%22%9%1
2068
21st to endNight
52%9%15%17%6%1
2178
April1st to 10thNight
48%8%16%16%11%1
1724
11th to 20thNight
51%6%13%22%7%1
1601
21st to endNight
45%9%13%24%8%2
1620
May1st to 10thNight
46%9%13%21%10%1
1469
11th to 20thNight
40%8%13%25%15%
1239
21st to endNight
49%9%13%20%7%1
1358
June1st to 10thNight
48%7%15%21%9%
1260
11th to 20thNight
50%12%14%15%7%1
1260
21st to endNight
36%10%19%25%8%1
1260
July1st to 10thNight
33%12%25%22%8%1
1225
11th to 20thNight
33%13%22%22%9%1
1218
21st to endNight
24%9%22%30%14%1
1302
August1st to 10thNight
33%11%22%24%8%1
1414
11th to 20thNight
30%11%19%28%11%1
1602
21st to endNight
30%7%17%30%13%3
1782
September1st to 10thNight
33%9%16%24%13%6%
1697
11th to 20thNight
37%7%14%27%12%4
1980
21st to endNight
31%7%17%28%14%2
1980
October1st to 10thNight
41%7%15%24%11%2
2099
11th to 20thNight
41%7%17%22%10%2
2245
21st to endNight
44%9%16%23%7%1
2574
November1st to 10thNight
49%7%13%21%8%1
2570
11th to 20thNight
47%8%16%19%10%1
2688
21st to endNight
46%10%17%19%6%2
2850
December1st to 10thNight
53%7%12%20%8%1
2850
11th to 20thNight
47%10%20%17%6%
3507
21st to endNight
37%6%14%23%16%4
5214

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
8%5%24%32%23%8%
180
11th to 20thDay
9%333%28%22%4
180
21st to endDay
10%9%25%29%22%5%
198
February1st to 10thDay
13%7%27%35%13%5%
195
11th to 20thDay
11%6%30%31%18%4
270
21st to endDay
6%6%22%40%20%5%
277
March1st to 10thDay
11%7%32%33%15%3
460
11th to 20thDay
8%5%32%33%17%5%
449
21st to endDay
10%7%31%30%18%5%
462
April1st to 10thDay
11%13%35%30%9%4
420
11th to 20thDay
13%9%29%29%17%3
412
21st to endDay
17%9%29%29%14%2
420
May1st to 10thDay
14%10%22%29%20%5%
529
11th to 20thDay
11%6%22%34%21%5%
625
21st to endDay
17%5%22%28%21%7%
679
June1st to 10thDay
16%6%25%33%17%3
630
11th to 20thDay
19%8%27%28%14%2
630
21st to endDay
12%5%23%28%24%8%
630
July1st to 10thDay
9%5%18%36%24%9%
621
11th to 20thDay
11%5%21%32%24%7%
618
21st to endDay
5%420%32%30%9%
684
August1st to 10thDay
9%5%23%32%22%9%
465
11th to 20thDay
10%322%32%27%7%
437
21st to endDay
8%9%20%31%22%10%
495
September1st to 10thDay
13%7%29%29%19%4
450
11th to 20thDay
17%8%22%31%19%3
450
21st to endDay
12%11%27%30%15%4
450
October1st to 10thDay
17%6%23%30%17%6%
330
11th to 20thDay
12%7%31%33%14%3
270
21st to endDay
12%12%33%30%12%1
297
November1st to 10thDay
19%9%27%29%13%2
270
11th to 20thDay
13%11%29%35%11%2
255
21st to endDay
7%8%31%32%11%11%
180
December1st to 10thDay
7%5%30%34%17%7%
180
11th to 20thDay
7%7%32%39%13%2
180
21st to endDay
9%420%37%24%6%
198
January1st to 10thNight
9%7%32%35%15%2
950
11th to 20thNight
10%6%30%39%14%1
950
21st to endNight
10%8%28%39%13%2
900
February1st to 10thNight
10%7%25%36%18%3
760
11th to 20thNight
10%5%31%38%16%1
744
21st to endNight
8%420%43%20%5%
628
March1st to 10thNight
8%431%39%16%3
760
11th to 20thNight
8%5%28%41%15%2
752
21st to endNight
12%6%29%35%16%2
792
April1st to 10thNight
12%5%31%33%17%1
626
11th to 20thNight
12%9%32%35%10%1
533
21st to endNight
14%6%29%37%13%1
540
May1st to 10thNight
14%7%24%36%17%1
497
11th to 20thNight
11%6%31%34%18%1
354
21st to endNight
19%6%24%31%18%2
389
June1st to 10thNight
19%8%24%37%11%2
360
11th to 20thNight
17%6%28%30%18%1
360
21st to endNight
11%327%36%20%3
360
July1st to 10thNight
9%319%48%21%1
350
11th to 20thNight
10%224%43%19%1
348
21st to endNight
5%422%44%24%2
394
August1st to 10thNight
8%323%39%24%3
366
11th to 20thNight
10%319%45%21%2
534
21st to endNight
8%5%21%43%22%1
594
September1st to 10thNight
13%422%43%16%2
566
11th to 20thNight
16%6%19%41%17%1
720
21st to endNight
11%5%30%38%14%2
720
October1st to 10thNight
14%328%36%16%2
720
11th to 20thNight
12%5%30%36%14%2
720
21st to endNight
10%8%33%35%13%
792
November1st to 10thNight
16%8%31%35%9%1
905
11th to 20thNight
8%8%37%34%10%3
950
21st to endNight
9%5%35%37%12%2
950
December1st to 10thNight
12%7%27%35%16%3
950
11th to 20thNight
9%6%33%38%13%2
950
21st to endNight
10%430%37%16%2
1045
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