Dalhousie
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dalhousie clear sky chart from 2005-11-06 to 2024-02-05.

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
26%
23098
7%
80735
3%
22987
30%
22987
7%
65324

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
13%
842
4%
4725
0%
817
16%
817
2%
3859
January11th to 20thNight
16%
824
8%
4650
2%
802
21%
802
3%
3731
January21st to endNight
21%
826
8%
4772
4%
814
30%
814
5%
3261
February1st to 10thNight
17%
739
6%
2399
2%
739
23%
739
6%
2021
February11th to 20thNight
14%
703
5%
2276
1%
700
19%
700
5%
1957
February21st to endNight
19%
592
8%
1858
2%
592
26%
592
8%
1549
March1st to 10thNight
20%
720
9%
2080
4%
720
25%
720
7%
1770
March11th to 20thNight
21%
720
8%
1980
4%
720
25%
720
9%
1659
March21st to endNight
21%
792
10%
2178
3%
792
27%
792
10%
1799
April1st to 10thNight
20%
612
7%
1797
4%
612
24%
612
7%
1532
April11th to 20thNight
27%
534
12%
1598
7%
534
31%
534
11%
1272
April21st to endNight
27%
540
12%
1620
6%
540
32%
540
11%
1289
May1st to 10thNight
30%
540
7%
1576
5%
540
32%
540
10%
1293
May11th to 20thNight
33%
532
9%
1417
5%
532
37%
532
13%
1156
May21st to endNight
28%
395
8%
1363
3%
394
34%
394
9%
1067
June1st to 10thNight
38%
360
5%
1260
3%
360
40%
360
8%
1019
June11th to 20thNight
45%
360
7%
1260
5%
360
47%
360
11%
993
June21st to endNight
36%
360
3%
1260
3%
360
38%
360
6%
1091
July1st to 10thNight
37%
350
6%
1225
3%
350
39%
350
9%
971
July11th to 20thNight
47%
348
6%
1218
5%
348
51%
348
10%
977
July21st to endNight
48%
402
6%
1371
6%
380
50%
380
10%
1096
August1st to 10thNight
48%
538
5%
1600
4%
534
50%
534
12%
1284
August11th to 20thNight
48%
532
7%
1596
6%
532
50%
532
12%
1288
August21st to endNight
46%
594
11%
1782
7%
594
48%
594
14%
1361
September1st to 10thNight
39%
540
8%
1764
4%
540
44%
540
13%
1347
September11th to 20thNight
39%
716
12%
1976
7%
716
44%
716
15%
1508
September21st to endNight
38%
720
12%
1980
7%
720
42%
720
15%
1528
October1st to 10thNight
31%
720
12%
2142
5%
720
37%
720
12%
1687
October11th to 20thNight
27%
720
9%
2160
5%
720
30%
720
9%
1766
October21st to endNight
22%
792
7%
2533
3%
792
27%
792
7%
2091
November1st to 10thNight
21%
807
8%
2547
3%
807
25%
807
8%
2071
November11th to 20thNight
19%
850
8%
2660
4%
850
24%
850
6%
2257
November21st to endNight
18%
850
4%
2660
2%
850
21%
850
5%
2336
December1st to 10thNight
15%
850
4%
2817
0%
850
19%
850
4%
2478
December11th to 20thNight
13%
850
4%
3482
1%
849
15%
849
2%
3027
December21st to endNight
14%
928
2%
5153
1%
907
16%
907
2%
3933

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
16%8%37%7%46%6%6%7%30%
77722
Night
16%9%47%7%45%5%6%7%29%
81155
TransparencyDay
34%39%10%12%5%
77410
Night
36%37%10%12%5%
81002
SeeingDay
20%16%30%24%8%1
15197
Night
21%16%30%25%8%1
23129

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 10thDay
13%11%5%8%8%5%5%6%6%7%26%
2207
11th to 20thDay
10%11%48%15%8%9%7%6%6%16%
2274
21st to endDay
6%7%48%7%46%6%7%9%34%
2813
February1st to 10thDay
8%10%49%8%5%7%7%7%7%27%
1656
11th to 20thDay
12%14%5%10%7%47%7%6%7%21%
1573
21st to endDay
9%12%5%8%6%46%6%7%7%29%
1394
March1st to 10thDay
14%12%47%7%46%8%5%6%26%
1876
11th to 20thDay
13%11%47%8%46%6%5%7%29%
1980
21st to endDay
12%11%5%8%5%45%5%46%33%
2178
April1st to 10thDay
20%9%37%8%45%5%5%7%27%
2159
11th to 20thDay
16%8%36%5%46%5%6%7%34%
2312
21st to endDay
22%9%38%6%35%45%6%30%
2340
May1st to 10thDay
20%9%47%7%45%45%5%31%
2384
11th to 20thDay
20%8%36%6%45%45%6%33%
2486
21st to endDay
19%7%36%5%345%5%7%34%
2919
June1st to 10thDay
23%8%36%5%45%46%8%29%
2700
11th to 20thDay
19%6%35%5%35%5%6%8%35%
2700
21st to endDay
22%7%36%6%45%7%7%8%26%
2700
July1st to 10thDay
15%6%37%7%47%6%7%9%29%
2700
11th to 20thDay
13%6%346%46%6%6%9%36%
2700
21st to endDay
15%6%36%6%46%6%7%9%33%
2845
August1st to 10thDay
16%5%36%6%5%5%7%7%9%31%
2327
11th to 20thDay
15%7%36%6%47%6%6%7%34%
2304
21st to endDay
12%5%37%6%46%6%7%9%35%
2574
September1st to 10thDay
16%6%26%6%35%45%6%40%
2195
11th to 20thDay
19%6%35%435%44444%
1980
21st to endDay
18%6%36%6%35%46%7%37%
1980
October1st to 10thDay
14%6%27%6%35%5%5%7%39%
1818
11th to 20thDay
20%9%37%7%45%5%5%7%30%
1800
21st to endDay
24%9%37%6%35%5%5%7%26%
1821
November1st to 10thDay
17%9%48%8%46%5%5%6%27%
1529
11th to 20thDay
20%10%48%7%36%7%7%6%22%
1520
21st to endDay
18%13%5%10%8%46%6%5%7%20%
1520
December1st to 10thDay
18%10%6%9%7%5%7%6%6%5%19%
1359
11th to 20thDay
19%10%5%8%7%47%7%7%7%19%
1668
21st to endDay
17%13%49%9%35%5%5%5%25%
2431
January1st to 10thNight
9%11%5%11%13%7%7%7%7%8%16%
4748
11th to 20thNight
14%12%49%11%6%7%7%6%6%18%
4692
21st to endNight
7%10%46%7%5%7%7%7%8%33%
4841
February1st to 10thNight
9%13%5%8%8%5%6%6%6%7%27%
2399
11th to 20thNight
14%15%5%9%6%47%6%6%6%22%
2273
21st to endNight
12%13%5%8%6%46%6%5%6%28%
1858
March1st to 10thNight
14%13%47%8%5%6%445%30%
2071
11th to 20thNight
14%11%5%9%6%46%6%5%7%27%
1979
21st to endNight
14%12%48%6%45%445%34%
2178
April1st to 10thNight
23%12%47%7%3445%5%26%
1797
11th to 20thNight
20%8%36%5%445%5%6%35%
1598
21st to endNight
26%10%45%5%23445%33%
1620
May1st to 10thNight
23%9%36%6%44446%31%
1576
11th to 20thNight
20%7%35%6%45%5%5%6%33%
1416
21st to endNight
20%6%35%6%45%5%6%7%33%
1362
June1st to 10thNight
23%9%35%4445%37%32%
1260
11th to 20thNight
20%6%25%5%345%5%7%38%
1260
21st to endNight
26%7%26%6%45%446%31%
1260
July1st to 10thNight
16%8%37%6%345%6%8%35%
1260
11th to 20thNight
14%5%36%6%35%6%6%8%38%
1260
21st to endNight
16%6%5%6%5%345%7%9%35%
1451
August1st to 10thNight
14%6%27%5%46%47%8%38%
1612
11th to 20thNight
16%6%26%5%36%5%5%9%36%
1596
21st to endNight
12%6%36%5%345%5%8%43%
1782
September1st to 10thNight
17%5%145%35%5%5%8%42%
1764
11th to 20thNight
18%6%25%43345%6%43%
1976
21st to endNight
17%415%5%34446%47%
1980
October1st to 10thNight
17%7%26%5%3445%6%39%
2142
11th to 20thNight
21%6%26%6%35%5%5%6%33%
2160
21st to endNight
22%9%36%6%45%5%6%7%28%
2533
November1st to 10thNight
20%7%37%6%46%6%5%6%30%
2547
11th to 20thNight
19%9%5%9%6%46%46%7%25%
2660
21st to endNight
19%12%5%8%9%45%5%6%6%21%
2660
December1st to 10thNight
20%11%5%9%8%36%5%5%7%22%
2817
11th to 20thNight
20%11%5%9%7%5%6%6%7%6%18%
3531
21st to endNight
16%12%49%8%46%5%6%6%25%
5236

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
42%29%14%13%3
2208
11th to 20thDay
45%35%10%7%3
2274
21st to endDay
30%32%11%18%10%
2813
February1st to 10thDay
35%39%11%10%5%
1656
11th to 20thDay
39%41%9%7%2
1581
21st to endDay
33%40%9%13%5%1
1394
March1st to 10thDay
35%42%9%10%41
1880
11th to 20thDay
33%39%10%12%5%
1980
21st to endDay
33%38%9%12%8%1
2178
April1st to 10thDay
37%39%9%9%5%1
2159
11th to 20thDay
31%39%8%13%8%1
2312
21st to endDay
34%38%7%13%7%2
2340
May1st to 10thDay
37%39%6%10%7%1
2384
11th to 20thDay
33%39%8%13%7%
2490
21st to endDay
30%40%8%14%8%1
2919
June1st to 10thDay
35%39%8%9%7%1
2700
11th to 20thDay
30%39%10%12%8%1
2700
21st to endDay
38%41%8%8%4
2700
July1st to 10thDay
31%43%12%10%4
2625
11th to 20thDay
29%43%12%12%5%
2619
21st to endDay
31%43%12%11%3
2685
August1st to 10thDay
31%40%12%13%3
2314
11th to 20thDay
29%41%14%12%3
2304
21st to endDay
28%41%13%14%3
2574
September1st to 10thDay
29%39%12%15%5%
2195
11th to 20thDay
29%37%9%17%8%
1980
21st to endDay
31%40%10%15%4
1980
October1st to 10thDay
31%39%13%14%4
1818
11th to 20thDay
35%39%11%12%3
1800
21st to endDay
36%41%7%11%4
1821
November1st to 10thDay
31%43%9%13%4
1529
11th to 20thDay
35%44%9%9%3
1520
21st to endDay
39%43%9%7%2
1520
December1st to 10thDay
41%44%6%6%3
1359
11th to 20thDay
42%38%10%7%4
1668
21st to endDay
42%30%13%8%5%1
2431
January1st to 10thNight
47%32%13%8%1
4748
11th to 20thNight
45%31%10%10%4
4692
21st to endNight
35%27%10%15%12%1
4841
February1st to 10thNight
39%37%7%9%5%1
2399
11th to 20thNight
40%40%9%8%3
2276
21st to endNight
37%38%9%11%4
1858
March1st to 10thNight
38%39%7%13%3
2080
11th to 20thNight
35%39%8%12%5%1
1980
21st to endNight
36%37%7%12%8%1
2178
April1st to 10thNight
40%38%7%9%7%1
1797
11th to 20thNight
32%37%8%12%9%2
1598
21st to endNight
32%38%7%12%11%1
1620
May1st to 10thNight
34%38%8%10%8%1
1576
11th to 20thNight
30%40%8%14%7%
1417
21st to endNight
30%40%9%12%8%1
1363
June1st to 10thNight
34%39%8%9%8%1
1260
11th to 20thNight
31%39%8%12%9%1
1260
21st to endNight
41%40%5%9%41
1260
July1st to 10thNight
31%40%10%15%4
1225
11th to 20thNight
31%41%10%12%5%
1218
21st to endNight
29%43%11%14%4
1371
August1st to 10thNight
30%41%11%15%4
1600
11th to 20thNight
30%41%15%12%3
1596
21st to endNight
26%39%11%19%5%
1782
September1st to 10thNight
26%39%11%18%5%
1764
11th to 20thNight
26%37%11%17%8%1
1976
21st to endNight
26%38%11%16%8%
1980
October1st to 10thNight
28%39%11%16%5%1
2142
11th to 20thNight
34%39%11%14%3
2160
21st to endNight
34%40%10%11%4
2533
November1st to 10thNight
31%42%11%11%5%
2547
11th to 20thNight
36%42%9%9%3
2660
21st to endNight
38%44%9%7%11
2660
December1st to 10thNight
40%43%8%7%1
2817
11th to 20thNight
44%38%8%8%2
3532
21st to endNight
44%29%13%10%32
5236

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
25%22%29%20%31
180
11th to 20thDay
21%31%28%13%6%
180
21st to endDay
13%21%37%20%8%1
226
February1st to 10thDay
18%20%36%19%6%1
352
11th to 20thDay
17%28%33%18%4
336
21st to endDay
13%25%35%20%5%1
308
March1st to 10thDay
21%18%34%22%4
420
11th to 20thDay
17%24%29%21%8%1
420
21st to endDay
18%19%36%17%7%2
462
April1st to 10thDay
25%23%33%16%31
420
11th to 20thDay
18%16%37%22%6%1
412
21st to endDay
24%17%32%19%6%2
420
May1st to 10thDay
27%12%30%25%5%1
467
11th to 20thDay
25%13%27%22%12%1
624
21st to endDay
23%16%31%22%7%1
680
June1st to 10thDay
28%11%29%24%7%1
630
11th to 20thDay
22%12%27%25%13%1
630
21st to endDay
24%10%27%29%8%2
630
July1st to 10thDay
16%11%33%28%11%2
620
11th to 20thDay
13%8%30%37%10%1
618
21st to endDay
16%6%29%36%11%1
615
August1st to 10thDay
14%8%31%31%14%2
446
11th to 20thDay
16%9%28%32%14%1
439
21st to endDay
13%11%27%30%15%3
495
September1st to 10thDay
18%9%33%27%11%2
450
11th to 20thDay
22%12%29%27%8%2
450
21st to endDay
18%19%28%22%11%2
450
October1st to 10thDay
14%17%33%24%11%2
369
11th to 20thDay
24%21%26%20%9%1
360
21st to endDay
30%23%23%16%8%
396
November1st to 10thDay
19%24%30%21%5%1
365
11th to 20thDay
26%23%28%17%5%1
370
21st to endDay
24%27%28%16%41
370
December1st to 10thDay
23%22%32%15%6%1
209
11th to 20thDay
23%29%32%13%2
180
21st to endDay
19%30%36%13%2
198
January1st to 10thNight
20%20%36%19%5%
850
11th to 20thNight
21%23%33%20%4
830
21st to endNight
14%21%36%22%6%
836
February1st to 10thNight
15%22%37%20%5%
739
11th to 20thNight
21%19%38%17%5%
703
21st to endNight
17%15%40%22%5%1
592
March1st to 10thNight
21%18%33%23%5%
720
11th to 20thNight
21%20%31%21%7%1
720
21st to endNight
19%15%36%23%7%1
792
April1st to 10thNight
27%17%29%21%5%
612
11th to 20thNight
24%14%30%22%9%
534
21st to endNight
30%10%27%26%7%1
540
May1st to 10thNight
26%11%27%28%9%
540
11th to 20thNight
23%9%28%30%9%1
532
21st to endNight
21%10%33%28%8%
395
June1st to 10thNight
25%8%25%34%9%
360
11th to 20thNight
22%5%21%37%15%
360
21st to endNight
28%6%23%32%10%1
360
July1st to 10thNight
18%7%29%32%13%1
350
11th to 20thNight
12%428%42%13%1
348
21st to endNight
16%7%22%39%16%1
402
August1st to 10thNight
15%425%40%15%1
538
11th to 20thNight
16%5%23%40%16%
532
21st to endNight
14%6%28%33%17%2
594
September1st to 10thNight
16%8%32%33%10%
540
11th to 20thNight
21%12%24%32%11%1
716
21st to endNight
16%13%30%32%10%
720
October1st to 10thNight
20%14%30%27%9%1
720
11th to 20thNight
22%14%32%22%9%1
720
21st to endNight
27%19%28%21%5%
792
November1st to 10thNight
25%19%31%20%41
807
11th to 20thNight
26%21%28%18%5%1
850
21st to endNight
25%21%28%20%5%
850
December1st to 10thNight
26%17%32%18%6%
850
11th to 20thNight
23%27%30%14%5%
850
21st to endNight
25%24%30%18%22
935
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