Twocan
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Twocan clear sky chart from 2005-11-05 to 2017-10-16.

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
37%
14783
9%
47033
7%
14756
40%
14756
16%
34022

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
24%
742
6%
2643
5%
739
26%
739
7%
2172
January16th to endNight
30%
768
7%
2688
7%
768
32%
768
10%
2089
February1st to 15thNight
30%
696
5%
2354
5%
696
31%
696
9%
1908
February16th to endNight
35%
632
11%
1987
9%
632
39%
632
15%
1477
March1st to 15thNight
25%
720
6%
2136
4%
720
28%
720
11%
1692
March16th to endNight
29%
768
7%
2018
3%
768
31%
768
13%
1602
April1st to 15thNight
34%
719
6%
1799
3%
719
36%
719
15%
1403
April16th to endNight
36%
500
9%
1580
6%
500
39%
500
15%
1158
May1st to 15thNight
45%
359
11%
1439
16%
359
50%
359
19%
912
May16th to endNight
47%
374
6%
1496
9%
374
49%
374
16%
1011
June1st to 15thNight
40%
360
3%
1207
1%
360
41%
360
16%
837
June16th to endNight
41%
360
5%
1134
0%
360
42%
360
23%
733
July1st to 15thNight
61%
340
7%
1190
4%
340
63%
340
36%
607
July16th to endNight
62%
376
15%
1411
18%
354
68%
354
45%
591
August1st to 15thNight
75%
360
19%
1432
28%
358
79%
358
41%
635
August16th to endNight
56%
640
20%
1786
14%
640
60%
640
36%
1054
September1st to 15thNight
57%
720
21%
1800
10%
720
60%
720
41%
1089
September16th to endNight
48%
716
19%
1952
13%
716
51%
716
27%
1292
October1st to 15thNight
43%
720
12%
2160
11%
720
46%
720
21%
1480
October16th to endNight
33%
708
9%
2256
7%
708
37%
708
13%
1687
November1st to 15thNight
22%
700
7%
2412
5%
700
25%
700
9%
1947
November16th to endNight
28%
720
5%
2520
3%
720
31%
720
9%
1997
December1st to 15thNight
29%
892
7%
2692
5%
892
31%
892
9%
2212
December16th to endNight
21%
893
5%
2941
3%
893
24%
893
7%
2437

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
10%10%47%6%35%5%5%7%32%
48393
Night
8%11%47%6%35%5%5%7%33%
47190
Cloud CoverDay
11%12%47%6%35%45%6%31%
48393
Night
9%12%5%7%6%35%5%5%6%32%
47193
TransparencyDay
51%8%14%15%7%1
48053
Night
49%9%15%16%6%1
47034
SeeingDay
18%19%29%20%9%2
6181
Night
14%13%27%26%15%2
14794

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
11%12%6%12%9%37%6%5%7%18%
1293
16th to endDay
8%16%49%6%45%45%8%26%
1536
February1st to 15thDay
7%19%7%11%9%36%5%5%7%17%
1507
16th to endDay
6%16%6%12%7%36%5%5%6%23%
1487
March1st to 15thDay
8%19%6%10%7%46%5%5%5%20%
1821
16th to endDay
9%15%6%10%9%46%6%6%8%16%
2206
April1st to 15thDay
11%16%5%8%8%46%5%5%6%21%
2149
16th to endDay
9%15%5%9%7%36%45%8%22%
2379
May1st to 15thDay
10%11%48%6%35%5%6%7%31%
2511
16th to endDay
15%9%39%8%36%5%6%8%24%
2615
June1st to 15thDay
18%7%47%7%35%446%29%
2747
16th to endDay
13%7%26%6%35%5%5%6%36%
2821
July1st to 15thDay
6%324325%45%7%53%
2700
16th to endDay
6%213424346%59%
2663
August1st to 15thDay
6%4244245%5%7%51%
2520
16th to endDay
8%42443445%7%48%
2413
September1st to 15thDay
9%6%25%423235%52%
2160
16th to endDay
14%7%46%5%4445%7%36%
2005
October1st to 15thDay
11%11%47%6%45%446%33%
1800
16th to endDay
14%11%48%7%445%47%25%
1636
November1st to 15thDay
18%17%5%8%6%35%446%18%
1444
16th to endDay
8%15%5%10%6%35%6%5%7%23%
1434
December1st to 15thDay
7%14%6%10%9%36%5%47%23%
1260
16th to endDay
7%17%7%10%9%47%5%6%7%17%
1286
January1st to 15thNight
11%15%7%12%7%35%5%5%6%18%
2643
16th to endNight
8%16%7%9%7%35%5%6%7%23%
2688
February1st to 15thNight
8%15%6%11%9%46%5%5%6%21%
2342
16th to endNight
411%5%11%8%36%6%47%29%
1987
March1st to 15thNight
6%15%5%9%8%35%6%6%8%24%
2136
16th to endNight
6%14%5%8%7%45%5%5%7%27%
2014
April1st to 15thNight
6%10%5%7%8%5%6%6%6%7%30%
1799
16th to endNight
7%12%47%5%46%6%6%7%31%
1580
May1st to 15thNight
7%7%27%5%245%47%42%
1439
16th to endNight
9%6%25%6%36%6%6%9%36%
1496
June1st to 15thNight
10%7%35%6%46%7%6%7%33%
1207
16th to endNight
8%5%25%5%3447%8%45%
1134
July1st to 15thNight
33332346%7%60%
1260
16th to endNight
411221235%7%66%
1506
August1st to 15thNight
3212223448%65%
1440
16th to endNight
5%3135%245%5%7%55%
1786
September1st to 15thNight
7%32332435%5%58%
1800
16th to endNight
10%6%25%5%235%6%9%41%
1952
October1st to 15thNight
8%10%37%5%3445%6%40%
2160
16th to endNight
13%11%38%6%45%45%7%28%
2256
November1st to 15thNight
14%16%5%8%6%45%45%6%22%
2412
16th to endNight
9%16%6%9%7%345%46%24%
2520
December1st to 15thNight
7%16%6%10%8%35%5%5%7%22%
2692
16th to endNight
7%14%6%11%9%46%5%5%7%20%
2941

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
13%12%6%10%7%47%5%5%7%18%
1293
16th to endDay
9%17%5%8%6%35%45%7%25%
1536
February1st to 15thDay
8%20%7%12%8%35%45%6%16%
1507
16th to endDay
7%17%7%12%7%26%5%5%5%21%
1487
March1st to 15thDay
8%21%7%10%6%35%5%5%5%19%
1821
16th to endDay
10%18%8%10%8%46%5%6%6%16%
2206
April1st to 15thDay
11%18%5%9%8%46%446%18%
2149
16th to endDay
10%15%6%9%7%35%5%48%21%
2379
May1st to 15thDay
10%12%47%6%35%45%6%30%
2511
16th to endDay
16%11%48%7%45%5%5%6%23%
2615
June1st to 15thDay
19%9%37%7%34446%28%
2747
16th to endDay
15%7%36%6%35%5%5%6%34%
2821
July1st to 15thDay
7%4244245%48%52%
2700
16th to endDay
6%323324346%58%
2663
August1st to 15thDay
6%425%4345%5%7%50%
2520
16th to endDay
8%5%25%5%2445%7%47%
2413
September1st to 15thDay
11%6%24423335%52%
2160
16th to endDay
15%9%45%5%3345%7%35%
2005
October1st to 15thDay
13%11%47%8%35%445%31%
1800
16th to endDay
16%14%47%5%35%45%6%26%
1636
November1st to 15thDay
19%19%5%8%5%345%5%417%
1444
16th to endDay
9%17%5%10%6%45%5%5%6%23%
1434
December1st to 15thDay
7%15%6%11%9%46%5%45%22%
1260
16th to endDay
8%17%8%11%8%46%5%5%6%17%
1286
January1st to 15thNight
13%17%7%11%7%35%446%18%
2642
16th to endNight
11%15%7%9%7%345%5%6%23%
2688
February1st to 15thNight
8%16%7%10%7%45%5%5%5%21%
2342
16th to endNight
412%7%11%8%45%446%29%
1987
March1st to 15thNight
7%16%6%9%7%35%5%6%7%22%
2136
16th to endNight
7%15%5%8%8%45%45%7%26%
2018
April1st to 15thNight
6%13%5%8%7%46%6%6%6%29%
1799
16th to endNight
7%13%5%6%7%36%6%5%9%29%
1580
May1st to 15thNight
8%9%35%5%25%45%6%42%
1439
16th to endNight
10%7%36%6%35%5%7%9%33%
1496
June1st to 15thNight
11%7%36%6%45%6%7%7%33%
1207
16th to endNight
8%6%35%5%3446%7%44%
1134
July1st to 15thNight
431332345%7%59%
1260
16th to endNight
411231235%6%67%
1506
August1st to 15thNight
32122135%5%7%64%
1440
16th to endNight
5%3234245%5%7%54%
1786
September1st to 15thNight
7%5%1332345%5%57%
1800
16th to endNight
11%7%25%42446%8%41%
1952
October1st to 15thNight
10%10%37%43445%6%40%
2160
16th to endNight
16%12%37%5%36%45%6%27%
2256
November1st to 15thNight
14%17%6%8%5%35%5%5%5%21%
2412
16th to endNight
10%18%7%9%6%3445%6%23%
2520
December1st to 15thNight
7%18%5%9%8%46%5%46%22%
2692
16th to endNight
8%14%7%12%8%46%5%5%5%20%
2941

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
62%11%10%10%3
1293
16th to endDay
57%8%13%15%41
1536
February1st to 15thDay
66%11%10%7%4
1507
16th to endDay
61%9%12%11%4
1487
March1st to 15thDay
65%9%13%10%1
1821
16th to endDay
66%10%13%8%1
2206
April1st to 15thDay
64%10%11%10%2
2149
16th to endDay
59%11%14%10%3
2379
May1st to 15thDay
51%8%14%16%8%
2511
16th to endDay
59%9%15%13%2
2619
June1st to 15thDay
56%7%13%15%5%1
2747
16th to endDay
47%9%16%17%8%
2821
July1st to 15thDay
30%7%15%22%18%4
2541
16th to endDay
26%7%15%25%20%4
2495
August1st to 15thDay
30%7%18%25%13%4
2503
16th to endDay
33%8%16%25%12%2
2413
September1st to 15thDay
35%5%13%23%19%2
2160
16th to endDay
48%7%14%17%9%3
2005
October1st to 15thDay
53%8%13%17%6%
1800
16th to endDay
57%8%15%15%3
1636
November1st to 15thDay
66%9%11%9%21
1444
16th to endDay
59%8%13%12%41
1434
December1st to 15thDay
61%11%13%10%2
1260
16th to endDay
66%10%10%10%2
1286
January1st to 15thNight
65%9%11%9%3
2643
16th to endNight
59%9%12%11%41
2688
February1st to 15thNight
62%10%12%11%3
2354
16th to endNight
53%9%14%17%5%
1987
March1st to 15thNight
57%10%16%11%2
2136
16th to endNight
55%11%16%13%2
2018
April1st to 15thNight
52%10%17%15%3
1799
16th to endNight
49%11%19%16%3
1580
May1st to 15thNight
41%9%18%20%10%
1440
16th to endNight
44%11%20%18%3
1496
June1st to 15thNight
47%9%17%18%5%1
1207
16th to endNight
37%10%23%20%7%
1134
July1st to 15thNight
23%8%17%28%17%4
1190
16th to endNight
17%420%31%21%4
1411
August1st to 15thNight
17%7%22%30%17%3
1432
16th to endNight
27%9%18%27%14%2
1786
September1st to 15thNight
26%7%16%26%18%4
1800
16th to endNight
38%9%15%22%10%3
1952
October1st to 15thNight
43%8%17%20%9%
2160
16th to endNight
55%7%14%18%3
2256
November1st to 15thNight
62%10%12%10%41
2412
16th to endNight
60%10%12%11%41
2520
December1st to 15thNight
62%11%9%13%2
2692
16th to endNight
63%11%12%9%2
2941

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
16%28%33%23%6%
60
16th to endDay
14%15%37%26%6%
64
February1st to 15thDay
23%18%30%20%5%1
167
16th to endDay
14%23%32%21%8%
223
March1st to 15thDay
32%30%25%10%1
270
16th to endDay
25%34%22%15%3
288
April1st to 15thDay
28%23%30%11%41
269
16th to endDay
21%29%27%12%5%1
270
May1st to 15thDay
21%20%32%18%42
288
16th to endDay
25%15%20%20%12%3
330
June1st to 15thDay
19%17%32%17%11%1
542
16th to endDay
16%18%31%19%12%1
616
July1st to 15thDay
9%15%37%28%8%
475
16th to endDay
9%15%36%26%9%3
332
August1st to 15thDay
7%11%25%31%17%6%
315
16th to endDay
10%14%35%22%12%3
332
September1st to 15thDay
15%17%25%21%17%1
315
16th to endDay
20%19%21%22%12%3
315
October1st to 15thDay
22%18%28%16%11%2
315
16th to endDay
22%11%25%29%11%
144
November1st to 15thDay
25%23%38%11%
67
16th to endDay
10%15%28%23%11%11%
60
December1st to 15thDay
20%13%33%18%11%3
60
16th to endDay
18%12%42%15%7%3
64
January1st to 15thNight
22%15%29%20%11%1
743
16th to endNight
15%14%31%24%11%2
768
February1st to 15thNight
18%12%24%29%12%2
704
16th to endNight
9%15%26%29%17%1
632
March1st to 15thNight
17%20%29%21%9%1
720
16th to endNight
15%18%30%26%8%1
768
April1st to 15thNight
13%15%28%27%13%2
719
16th to endNight
11%12%32%30%12%1
500
May1st to 15thNight
11%9%27%31%17%2
360
16th to endNight
14%5%25%31%22%1
375
June1st to 15thNight
12%11%26%31%16%1
360
16th to endNight
10%11%30%31%14%1
360
July1st to 15thNight
5%8%21%40%22%2
340
16th to endNight
45%24%39%24%1
376
August1st to 15thNight
5%215%37%33%6%
360
16th to endNight
8%6%25%30%27%3
640
September1st to 15thNight
10%6%20%24%27%9%
720
16th to endNight
16%8%21%24%24%5%
716
October1st to 15thNight
14%11%24%28%19%2
720
16th to endNight
21%15%24%22%14%2
708
November1st to 15thNight
23%17%34%18%5%1
700
16th to endNight
19%17%26%22%11%2
720
December1st to 15thNight
17%15%28%28%10%
892
16th to endNight
14%18%35%25%6%
893
Page generated at 2017-10-17 13:35:30. Version ac04

Copyright 2017 A. Danko


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