Thompson
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Thompson clear sky chart from 2005-11-06 to 2024-01-17.

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
30%
24033
9%
74884
5%
23981
35%
23981
14%
55846

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
24%
950
8%
3026
4%
946
29%
946
10%
2409
January11th to 20thNight
27%
935
8%
2823
4%
935
33%
935
12%
2183
January21st to endNight
23%
990
9%
2970
3%
990
29%
990
10%
2400
February1st to 10thNight
30%
900
12%
2522
5%
900
35%
900
18%
1851
February11th to 20thNight
34%
874
14%
2354
6%
874
38%
874
19%
1717
February21st to endNight
32%
740
17%
1924
6%
740
39%
740
20%
1414
March1st to 10thNight
42%
729
16%
2169
11%
729
47%
729
19%
1579
March11th to 20thNight
29%
554
16%
1994
10%
554
41%
554
17%
1348
March21st to endNight
35%
594
17%
2155
16%
594
44%
594
19%
1411
April1st to 10thNight
36%
540
16%
1671
12%
540
43%
540
20%
1134
April11th to 20thNight
38%
533
16%
1599
9%
533
42%
533
22%
1099
April21st to endNight
34%
540
14%
1548
4%
540
39%
540
28%
984
May1st to 10thNight
28%
540
4%
1260
2%
540
30%
540
23%
940
May11th to 20thNight
31%
531
1%
1241
0%
531
32%
531
28%
887
May21st to endNight
28%
584
0%
1358
0%
582
28%
582
25%
981
June1st to 10thNight
39%
242
0%
962
0%
242
39%
242
17%
619
June11th to 20thNight
37%
180
0%
900
0%
180
37%
180
12%
586
June21st to endNight
41%
180
0%
900
0%
180
41%
180
15%
547
July1st to 10thNight
47%
291
0%
991
0%
291
47%
291
22%
637
July11th to 20thNight
47%
425
0%
1121
0%
425
47%
425
25%
786
July21st to endNight
57%
591
0%
1302
0%
558
58%
558
29%
912
August1st to 10thNight
50%
537
5%
1246
4%
534
51%
534
32%
875
August11th to 20thNight
45%
534
8%
1481
5%
534
47%
534
21%
1061
August21st to endNight
37%
594
9%
1782
4%
594
39%
594
18%
1303
September1st to 10thNight
33%
540
11%
1628
7%
540
39%
540
21%
1132
September11th to 20thNight
30%
540
9%
1969
10%
540
34%
540
13%
1430
September21st to endNight
31%
540
14%
1980
10%
540
40%
540
16%
1361
October1st to 10thNight
28%
678
14%
2118
8%
678
33%
678
14%
1603
October11th to 20thNight
25%
877
9%
2317
5%
877
27%
877
12%
1915
October21st to endNight
15%
990
5%
2672
1%
990
17%
990
7%
2315
November1st to 10thNight
20%
925
7%
2614
3%
925
23%
925
9%
2220
November11th to 20thNight
21%
950
5%
2850
3%
950
24%
950
8%
2367
November21st to endNight
19%
950
6%
2877
2%
950
23%
950
7%
2404
December1st to 10thNight
24%
950
6%
3040
4%
950
27%
950
9%
2489
December11th to 20thNight
28%
950
9%
3868
6%
949
33%
949
8%
3015
December21st to endNight
31%
1035
6%
5652
7%
1026
36%
1026
7%
3932

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
14%10%48%7%46%6%6%7%28%
74546
Night
12%11%5%9%8%46%6%6%7%26%
75280
TransparencyDay
54%7%13%20%7%
74214
Night
56%6%12%19%6%
75131
SeeingDay
19%12%32%24%10%2
14759
Night
18%9%31%29%12%2
24048

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
9%12%6%16%11%5%7%6%7%7%14%
1139
11th to 20thDay
8%13%7%10%9%36%6%9%11%17%
1279
21st to endDay
12%16%5%9%9%46%9%10%8%13%
1386
February1st to 10thDay
5%7%49%9%49%10%7%9%27%
1436
11th to 20thDay
7%11%37%7%410%9%9%8%25%
1493
21st to endDay
47%5%9%11%6%8%9%10%8%24%
1340
March1st to 10thDay
8%11%7%11%8%5%6%6%5%7%27%
1800
11th to 20thDay
10%10%5%8%7%5%5%5%5%7%31%
1964
21st to endDay
8%11%410%7%5%8%6%5%6%30%
2200
April1st to 10thDay
9%9%48%8%46%6%5%8%35%
2289
11th to 20thDay
9%10%5%10%8%36%5%5%5%33%
2312
21st to endDay
7%9%48%6%46%5%6%6%39%
2411
May1st to 10thDay
14%12%5%9%7%345%6%6%28%
2700
11th to 20thDay
14%9%36%6%345%5%5%40%
2655
21st to endDay
19%9%36%435%5%5%6%34%
2918
June1st to 10thDay
18%10%36%6%35%45%5%34%
2993
11th to 20thDay
18%6%37%7%46%6%6%10%28%
3060
21st to endDay
14%7%46%6%5%6%6%6%7%33%
3060
July1st to 10thDay
15%6%36%6%46%6%6%9%34%
2935
11th to 20thDay
14%6%36%6%35%5%7%9%36%
2800
21st to endDay
17%7%36%6%45%5%6%8%33%
2939
August1st to 10thDay
13%6%36%7%36%6%6%9%36%
2686
11th to 20thDay
16%7%37%6%5%6%6%6%8%31%
2417
21st to endDay
24%8%36%6%35%45%7%29%
2574
September1st to 10thDay
16%10%37%6%5%45%5%8%30%
2332
11th to 20thDay
22%12%46%5%45%5%5%6%26%
1990
21st to endDay
17%12%46%6%3446%9%28%
1980
October1st to 10thDay
22%16%5%7%6%35%446%23%
1824
11th to 20thDay
23%19%6%8%6%34445%17%
1639
21st to endDay
19%22%8%11%7%33445%13%
1684
November1st to 10thDay
16%17%7%12%10%46%5%3415%
1462
11th to 20thDay
11%19%6%11%10%5%5%6%6%5%16%
1330
21st to endDay
10%15%8%16%11%5%5%5%45%16%
1302
December1st to 10thDay
5%13%7%12%12%6%6%8%7%8%16%
1140
11th to 20thDay
14%14%7%10%11%45%6%6%8%15%
1263
21st to endDay
7%11%5%15%8%6%7%7%8%6%21%
1814
January1st to 10thNight
8%12%6%12%11%6%7%8%8%7%16%
3026
11th to 20thNight
8%12%6%9%10%5%8%7%8%9%17%
2823
21st to endNight
9%15%7%11%9%5%8%8%7%7%14%
2970
February1st to 10thNight
410%5%7%9%6%8%10%8%9%24%
2522
11th to 20thNight
6%9%48%9%5%8%8%9%9%25%
2353
21st to endNight
411%6%10%9%5%6%6%7%9%26%
1924
March1st to 10thNight
6%10%6%9%8%5%7%7%5%6%32%
2169
11th to 20thNight
11%12%47%6%46%5%5%6%34%
1994
21st to endNight
9%8%5%7%6%5%7%6%7%6%35%
2155
April1st to 10thNight
8%11%5%8%6%45%5%5%6%36%
1671
11th to 20thNight
9%9%47%9%46%45%7%37%
1599
21st to endNight
6%8%46%6%35%6%6%8%42%
1548
May1st to 10thNight
15%12%47%445%5%5%7%32%
1260
11th to 20thNight
15%9%36%435%45%7%40%
1241
21st to endNight
17%9%36%5%24446%39%
1358
June1st to 10thNight
17%6%37%5%3446%7%37%
962
11th to 20thNight
18%5%36%6%36%7%7%8%30%
900
21st to endNight
14%5%37%6%35%6%6%7%38%
900
July1st to 10thNight
12%436%6%45%6%8%8%39%
1021
11th to 20thNight
13%7%25%5%5%5%6%7%10%36%
1157
21st to endNight
11%5%25%345%6%9%10%39%
1379
August1st to 10thNight
14%6%35%44446%6%45%
1253
11th to 20thNight
12%6%27%7%37%6%6%7%37%
1481
21st to endNight
19%8%36%6%445%5%8%32%
1782
September1st to 10thNight
20%8%25%5%245%45%40%
1628
11th to 20thNight
20%12%37%6%335%5%7%30%
1969
21st to endNight
16%11%36%5%45%5%5%7%34%
1980
October1st to 10thNight
21%12%46%5%3445%7%28%
2118
11th to 20thNight
23%16%6%7%5%34445%21%
2317
21st to endNight
21%21%8%11%7%33445%15%
2672
November1st to 10thNight
17%15%7%12%10%45%44417%
2614
11th to 20thNight
14%16%7%13%8%5%6%5%5%7%15%
2850
21st to endNight
9%17%8%14%10%5%6%45%6%16%
2877
December1st to 10thNight
6%15%8%15%11%5%7%6%6%7%14%
3040
11th to 20thNight
8%13%6%11%11%6%7%8%6%7%17%
3927
21st to endNight
7%11%5%14%9%5%7%6%6%6%26%
5840

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
67%5%11%15%2
1139
11th to 20thDay
58%6%12%19%5%
1279
21st to endDay
62%6%10%13%8%
1386
February1st to 10thDay
50%6%13%21%10%
1436
11th to 20thDay
51%7%14%19%9%1
1501
21st to endDay
52%6%11%22%9%
1340
March1st to 10thDay
57%5%13%19%7%
1800
11th to 20thDay
52%5%11%22%10%1
1966
21st to endDay
54%6%10%20%9%2
2200
April1st to 10thDay
48%5%12%22%12%
2289
11th to 20thDay
52%410%20%13%1
2312
21st to endDay
44%6%10%26%14%
2411
May1st to 10thDay
55%5%11%20%7%
2700
11th to 20thDay
46%5%10%23%16%1
2659
21st to endDay
50%5%10%22%12%
2918
June1st to 10thDay
51%6%11%22%11%
2993
11th to 20thDay
51%7%11%23%8%
3060
21st to endDay
47%9%12%23%9%
3060
July1st to 10thDay
45%9%13%26%8%
2856
11th to 20thDay
43%9%18%24%6%
2713
21st to endDay
49%9%17%23%3
2776
August1st to 10thDay
44%10%19%23%4
2669
11th to 20thDay
49%11%18%19%3
2417
21st to endDay
55%8%19%15%4
2574
September1st to 10thDay
50%9%18%19%4
2332
11th to 20thDay
57%7%17%17%1
1990
21st to endDay
54%6%19%17%4
1980
October1st to 10thDay
64%6%10%14%5%
1824
11th to 20thDay
70%6%11%12%1
1639
21st to endDay
74%6%8%9%3
1684
November1st to 10thDay
74%5%8%11%1
1462
11th to 20thDay
68%6%13%12%1
1330
21st to endDay
71%5%11%11%2
1302
December1st to 10thDay
63%8%13%15%2
1140
11th to 20thDay
68%410%14%4
1263
21st to endDay
59%6%9%20%5%
1814
January1st to 10thNight
64%6%12%14%3
3026
11th to 20thNight
61%5%12%18%4
2823
21st to endNight
66%5%8%13%8%
2970
February1st to 10thNight
51%7%13%20%7%
2522
11th to 20thNight
52%5%12%23%8%
2355
21st to endNight
53%5%10%22%10%
1924
March1st to 10thNight
52%6%12%20%9%
2169
11th to 20thNight
50%411%23%10%1
1994
21st to endNight
46%5%13%23%11%2
2155
April1st to 10thNight
47%5%10%22%15%1
1671
11th to 20thNight
47%5%8%24%15%1
1599
21st to endNight
38%6%11%30%13%2
1548
May1st to 10thNight
52%5%12%22%9%1
1260
11th to 20thNight
45%5%10%20%18%2
1241
21st to endNight
47%6%8%25%14%
1358
June1st to 10thNight
45%7%13%19%15%1
962
11th to 20thNight
47%9%8%26%9%1
900
21st to endNight
44%7%11%26%12%
900
July1st to 10thNight
42%7%15%27%8%
991
11th to 20thNight
43%9%17%26%5%
1121
21st to endNight
36%13%20%28%3
1302
August1st to 10thNight
39%9%20%28%4
1246
11th to 20thNight
45%11%18%22%4
1481
21st to endNight
50%9%19%17%4
1782
September1st to 10thNight
46%8%18%21%6%1
1628
11th to 20thNight
55%7%14%22%3
1969
21st to endNight
50%7%15%23%5%
1980
October1st to 10thNight
56%8%12%19%5%
2118
11th to 20thNight
66%6%12%14%2
2317
21st to endNight
73%7%9%10%2
2672
November1st to 10thNight
71%5%9%12%3
2614
11th to 20thNight
69%6%13%10%2
2850
21st to endNight
70%5%9%14%2
2877
December1st to 10thNight
68%5%11%12%3
3040
11th to 20thNight
64%6%11%15%4
3926
21st to endNight
57%5%10%21%6%1
5840

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
12%9%37%30%11%1
180
11th to 20thDay
14%8%38%31%8%1
247
21st to endDay
23%7%33%27%9%1
264
February1st to 10thDay
7%15%41%29%7%2
240
11th to 20thDay
10%13%44%22%9%2
240
21st to endDay
7%12%36%30%12%4
198
March1st to 10thDay
12%14%29%24%14%7%
240
11th to 20thDay
10%17%38%24%8%3
240
21st to endDay
14%14%42%20%7%2
286
April1st to 10thDay
13%12%36%28%9%2
549
11th to 20thDay
12%14%37%23%12%2
590
21st to endDay
12%14%33%27%12%2
600
May1st to 10thDay
22%12%33%23%8%1
603
11th to 20thDay
21%17%30%24%7%1
625
21st to endDay
24%12%33%22%7%1
679
June1st to 10thDay
27%12%27%23%9%3
630
11th to 20thDay
21%12%33%21%9%3
630
21st to endDay
22%8%31%25%12%3
630
July1st to 10thDay
16%12%32%26%11%3
621
11th to 20thDay
18%9%27%26%13%6%
618
21st to endDay
20%9%25%26%15%4
684
August1st to 10thDay
15%12%27%30%13%3
626
11th to 20thDay
19%12%27%25%13%4
615
21st to endDay
25%8%26%25%13%3
693
September1st to 10thDay
23%14%33%21%8%
622
11th to 20thDay
24%13%32%21%10%1
280
21st to endDay
25%11%30%24%9%
270
October1st to 10thDay
28%17%27%17%8%2
270
11th to 20thDay
38%13%28%12%7%3
270
21st to endDay
27%11%32%20%9%1
297
November1st to 10thDay
21%11%26%27%11%3
270
11th to 20thDay
19%14%40%17%7%3
270
21st to endDay
23%12%39%19%7%1
257
December1st to 10thDay
10%11%34%34%8%3
180
11th to 20thDay
20%12%27%25%15%1
122
21st to endDay
12%7%33%31%15%2
123
January1st to 10thNight
14%9%35%32%9%1
950
11th to 20thNight
14%10%36%28%10%2
935
21st to endNight
16%9%35%32%8%1
990
February1st to 10thNight
9%9%42%29%10%2
900
11th to 20thNight
11%8%35%29%15%3
879
21st to endNight
8%10%34%34%13%2
740
March1st to 10thNight
10%7%28%37%16%2
729
11th to 20thNight
19%12%32%25%10%1
554
21st to endNight
12%9%36%27%13%2
594
April1st to 10thNight
14%9%32%31%11%2
540
11th to 20thNight
12%10%31%32%14%2
533
21st to endNight
11%11%38%29%11%1
540
May1st to 10thNight
23%10%32%25%9%1
540
11th to 20thNight
22%11%34%24%9%1
531
21st to endNight
23%10%34%24%9%
584
June1st to 10thNight
21%9%23%33%13%1
242
11th to 20thNight
19%8%27%31%13%2
180
21st to endNight
16%7%29%35%13%
180
July1st to 10thNight
13%5%30%34%17%1
291
11th to 20thNight
16%6%27%28%21%4
425
21st to endNight
15%319%34%25%3
591
August1st to 10thNight
17%5%23%33%21%1
537
11th to 20thNight
16%6%25%34%17%1
534
21st to endNight
23%5%27%32%12%1
594
September1st to 10thNight
25%10%28%26%10%1
540
11th to 20thNight
28%10%26%25%11%1
540
21st to endNight
22%9%32%25%10%2
540
October1st to 10thNight
27%8%30%25%9%1
678
11th to 20thNight
35%7%23%23%10%2
877
21st to endNight
33%11%27%19%8%1
990
November1st to 10thNight
25%10%29%22%11%2
925
11th to 20thNight
22%9%34%24%9%1
950
21st to endNight
19%13%31%27%8%2
950
December1st to 10thNight
13%8%35%32%10%1
950
11th to 20thNight
18%10%26%33%12%1
950
21st to endNight
15%7%28%34%13%4
1045
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