Tantalus Lookout
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tantalus Lookout clear sky chart from 2005-11-05 to 2017-09-19.

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
29%
14463
8%
46745
5%
14433
32%
14433
12%
36426

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
20%
750
10%
2686
7%
746
26%
746
9%
2180
January16th to endNight
19%
785
8%
2784
5%
785
24%
785
8%
2262
February1st to 15thNight
19%
701
6%
2288
4%
700
22%
700
8%
1887
February16th to endNight
20%
632
9%
2052
5%
632
26%
632
10%
1644
March1st to 15thNight
16%
720
3%
2058
2%
720
18%
720
6%
1820
March16th to endNight
16%
768
5%
2076
2%
768
18%
768
7%
1818
April1st to 15thNight
25%
703
4%
1783
2%
703
26%
703
10%
1519
April16th to endNight
25%
498
6%
1578
4%
498
28%
498
10%
1300
May1st to 15thNight
43%
360
9%
1440
11%
360
47%
360
14%
1029
May16th to endNight
40%
375
5%
1425
7%
374
41%
374
12%
1075
June1st to 15thNight
34%
360
1%
1260
0%
360
35%
360
12%
968
June16th to endNight
36%
360
3%
1260
0%
360
36%
360
12%
955
July1st to 15thNight
64%
340
6%
1190
4%
340
65%
340
30%
669
July16th to endNight
68%
376
12%
1300
17%
354
70%
354
32%
691
August1st to 15thNight
72%
422
13%
1494
17%
420
73%
420
33%
797
August16th to endNight
61%
600
15%
1746
13%
600
62%
600
29%
1104
September1st to 15thNight
55%
720
19%
1800
10%
720
58%
720
38%
1127
September16th to endNight
39%
671
14%
1849
9%
671
41%
671
18%
1379
October1st to 15thNight
30%
660
10%
1980
8%
660
32%
660
16%
1482
October16th to endNight
21%
704
5%
2218
3%
704
24%
704
8%
1859
November1st to 15thNight
9%
700
5%
2450
2%
700
13%
700
4%
2187
November16th to endNight
17%
720
6%
2520
3%
720
21%
720
7%
2067
December1st to 15thNight
14%
738
7%
2628
3%
738
20%
738
7%
2138
December16th to endNight
9%
800
6%
2880
2%
800
14%
800
5%
2469

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
25%15%46%42333425%
48100
Night
25%16%35%423345%25%
46895
Cloud CoverDay
25%16%45%42333425%
48100
Night
25%17%35%423335%25%
46898
TransparencyDay
63%410%13%6%1
47754
Night
62%5%11%13%5%1
46745
SeeingDay
33%13%25%20%6%1
6191
Night
35%6%22%22%11%1
14466

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
35%20%2331233318%
1253
16th to endDay
27%23%46%41323319%
1436
February1st to 15thDay
27%21%46%313325%18%
1576
16th to endDay
15%26%6%7%6%2344320%
1421
March1st to 15thDay
30%27%46%42322311%
1899
16th to endDay
27%28%47%5%2332211%
2148
April1st to 15thDay
23%25%6%6%6%3433412%
2164
16th to endDay
27%20%6%8%6%4433313%
2382
May1st to 15thDay
21%15%47%6%3345%5%22%
2511
16th to endDay
29%11%47%5%3445%6%17%
2684
June1st to 15thDay
33%11%46%6%24445%17%
2700
16th to endDay
29%11%47%5%3323422%
2700
July1st to 15thDay
14%6%25%424446%44%
2700
16th to endDay
16%5%2432435%6%45%
2778
August1st to 15thDay
13%6%24313446%48%
2457
16th to endDay
15%6%25%5%23447%41%
2455
September1st to 15thDay
15%5%24323436%47%
2160
16th to endDay
32%8%35%42323428%
1854
October1st to 15thDay
31%9%25%32343428%
1650
16th to endDay
40%11%3432222419%
1651
November1st to 15thDay
45%19%2331232312%
1408
16th to endDay
31%20%35%32343318%
1440
December1st to 15thDay
25%22%45%41222126%
1329
16th to endDay
28%28%6%5%41221115%
1344
January1st to 15thNight
33%18%25%32233320%
2686
16th to endNight
28%21%35%423245%16%
2784
February1st to 15thNight
27%21%45%42434416%
2276
16th to endNight
17%26%5%7%5%2332422%
2052
March1st to 15thNight
31%25%35%42333213%
2058
16th to endNight
27%25%5%6%42323314%
2072
April1st to 15thNight
22%21%46%6%3444418%
1783
16th to endNight
25%15%46%5%34347%18%
1578
May1st to 15thNight
19%9%25%5%25%5%5%7%31%
1439
16th to endNight
23%10%26%5%345%5%6%24%
1425
June1st to 15thNight
24%10%26%6%24445%26%
1260
16th to endNight
23%8%36%6%35%45%427%
1260
July1st to 15thNight
11%31242435%6%53%
1260
16th to endNight
11%5%23213346%55%
1389
August1st to 15thNight
10%5%1232345%6%55%
1502
16th to endNight
13%42443346%8%44%
1746
September1st to 15thNight
12%5%14213446%53%
1800
16th to endNight
27%10%35%423246%30%
1849
October1st to 15thNight
28%9%35%41433429%
1980
16th to endNight
37%12%36%42333416%
2218
November1st to 15thNight
44%18%3441323310%
2450
16th to endNight
31%19%35%42432319%
2520
December1st to 15thNight
27%20%5%5%42333320%
2628
16th to endNight
28%29%6%5%31221215%
2880

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
33%21%3331223319%
1253
16th to endDay
25%23%46%423235%18%
1436
February1st to 15thDay
26%24%5%5%322225%19%
1576
16th to endDay
16%27%6%6%5%2334419%
1421
March1st to 15thDay
27%31%5%442223311%
1899
16th to endDay
26%30%6%7%5%3222211%
2148
April1st to 15thDay
23%29%5%7%6%2323411%
2164
16th to endDay
26%26%5%6%43432312%
2382
May1st to 15thDay
20%19%46%5%34345%21%
2511
16th to endDay
30%14%36%5%2345%5%17%
2684
June1st to 15thDay
34%13%36%5%2434416%
2700
16th to endDay
32%10%47%5%3333421%
2700
July1st to 15thDay
15%6%25%424446%43%
2700
16th to endDay
17%5%23323346%45%
2778
August1st to 15thDay
14%5%2432345%7%47%
2457
16th to endDay
16%6%25%5%2445%6%40%
2455
September1st to 15thDay
15%6%23323446%47%
2160
16th to endDay
32%9%35%423345%27%
1854
October1st to 15thDay
31%9%34424235%28%
1650
16th to endDay
39%15%34312225%19%
1651
November1st to 15thDay
42%22%2331232212%
1408
16th to endDay
28%23%4442333418%
1440
December1st to 15thDay
22%23%5%5%42222225%
1329
16th to endDay
26%29%7%6%41221215%
1344
January1st to 15thNight
31%19%35%41233320%
2685
16th to endNight
27%21%35%423336%18%
2784
February1st to 15thNight
26%21%45%423346%17%
2276
16th to endNight
15%27%5%6%42332423%
2052
March1st to 15thNight
29%26%5%5%42332214%
2058
16th to endNight
25%27%6%6%42332414%
2076
April1st to 15thNight
20%24%45%6%24345%17%
1783
16th to endNight
24%19%5%6%5%24337%17%
1578
May1st to 15thNight
18%11%34435%5%5%6%30%
1439
16th to endNight
25%11%26%335%45%5%25%
1425
June1st to 15thNight
26%10%45%5%24436%25%
1260
16th to endNight
25%8%36%6%35%45%5%26%
1260
July1st to 15thNight
12%412323446%53%
1260
16th to endNight
12%5%22222246%54%
1389
August1st to 15thNight
11%41322245%6%55%
1502
16th to endNight
14%5%2442346%8%44%
1746
September1st to 15thNight
12%5%23213448%51%
1800
16th to endNight
28%9%25%423346%30%
1849
October1st to 15thNight
27%9%35%413445%30%
1980
16th to endNight
36%12%46%42333417%
2218
November1st to 15thNight
43%20%3342223310%
2450
16th to endNight
29%21%45%42322320%
2520
December1st to 15thNight
26%19%5%5%42332421%
2628
16th to endNight
27%28%6%6%31222316%
2880

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
71%28%10%5%2
1253
16th to endDay
71%38%10%32
1436
February1st to 15thDay
70%5%9%7%5%1
1576
16th to endDay
70%29%12%4
1421
March1st to 15thDay
79%46%7%1
1899
16th to endDay
81%37%7%
2148
April1st to 15thDay
78%47%8%1
2164
16th to endDay
79%47%6%2
2382
May1st to 15thDay
65%5%12%11%4
2511
16th to endDay
69%7%11%9%2
2688
June1st to 15thDay
71%412%8%2
2700
16th to endDay
67%310%13%4
2700
July1st to 15thDay
43%414%22%13%1
2541
16th to endDay
38%5%16%24%13%1
2604
August1st to 15thDay
38%5%15%27%11%2
2440
16th to endDay
42%6%18%23%7%1
2455
September1st to 15thDay
38%6%12%21%18%3
2160
16th to endDay
60%6%7%14%7%2
1854
October1st to 15thDay
60%412%13%8%
1650
16th to endDay
70%311%10%4
1651
November1st to 15thDay
78%46%5%21
1408
16th to endDay
71%46%8%6%2
1440
December1st to 15thDay
67%38%11%7%1
1329
16th to endDay
79%25%8%21
1344
January1st to 15thNight
69%36%12%5%2
2686
16th to endNight
68%5%7%10%43
2784
February1st to 15thNight
69%5%9%9%5%1
2288
16th to endNight
67%39%12%5%1
2052
March1st to 15thNight
77%46%8%2
2058
16th to endNight
76%410%7%1
2076
April1st to 15thNight
70%6%10%11%1
1783
16th to endNight
67%5%12%10%2
1578
May1st to 15thNight
53%7%17%16%4
1440
16th to endNight
59%6%16%14%2
1425
June1st to 15thNight
61%5%18%8%31
1260
16th to endNight
57%8%12%16%31
1260
July1st to 15thNight
33%5%21%24%13%2
1190
16th to endNight
29%6%18%31%11%1
1300
August1st to 15thNight
29%6%22%27%11%2
1494
16th to endNight
36%8%21%23%8%2
1746
September1st to 15thNight
31%7%16%24%17%3
1800
16th to endNight
56%411%17%7%3
1849
October1st to 15thNight
56%6%11%15%9%
1980
16th to endNight
71%48%10%3
2218
November1st to 15thNight
79%5%5%5%21
2450
16th to endNight
71%38%7%8%1
2520
December1st to 15thNight
69%5%8%10%5%1
2628
16th to endNight
76%35%6%5%1
2880

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%8%30%28%13%3
60
16th to endDay
49%7%17%17%7%
67
February1st to 15thDay
44%12%20%16%5%
236
16th to endDay
34%18%22%18%4
211
March1st to 15thDay
49%18%21%9%
270
16th to endDay
49%14%21%7%5%
288
April1st to 15thDay
44%15%17%17%4
269
16th to endDay
40%19%17%19%2
270
May1st to 15thDay
34%15%26%19%4
288
16th to endDay
33%8%24%23%9%1
399
June1st to 15thDay
38%15%22%15%7%
495
16th to endDay
32%14%27%20%41
495
July1st to 15thDay
19%12%28%31%5%1
475
16th to endDay
17%10%33%31%5%2
447
August1st to 15thDay
15%8%26%31%14%3
315
16th to endDay
13%15%34%25%9%2
332
September1st to 15thDay
22%14%29%25%6%1
315
16th to endDay
33%11%26%18%7%2
268
October1st to 15thDay
42%8%27%16%3
255
16th to endDay
47%9%19%15%5%1
192
November1st to 15thDay
58%6%25%6%3
60
16th to endDay
40%8%15%11%15%10%
60
December1st to 15thDay
50%323%20%3
60
16th to endDay
35%339%15%41
64
January1st to 15thNight
43%6%23%20%4
750
16th to endNight
38%9%26%19%5%
785
February1st to 15thNight
40%5%28%22%2
704
16th to endNight
31%10%29%21%7%
632
March1st to 15thNight
43%11%23%15%6%
720
16th to endNight
42%8%24%18%5%
768
April1st to 15thNight
37%6%20%26%8%
703
16th to endNight
35%325%27%8%
498
May1st to 15thNight
24%320%33%16%1
360
16th to endNight
31%218%29%17%
375
June1st to 15thNight
29%323%23%17%1
360
16th to endNight
27%418%32%15%
360
July1st to 15thNight
13%212%40%29%1
340
16th to endNight
13%211%31%36%4
376
August1st to 15thNight
13%8%31%40%4
422
16th to endNight
19%213%31%32%2
600
September1st to 15thNight
15%220%28%29%2
720
16th to endNight
34%416%27%16%1
671
October1st to 15thNight
33%5%26%21%12%1
660
16th to endNight
44%5%23%19%5%
704
November1st to 15thNight
56%7%22%11%1
700
16th to endNight
39%10%25%16%5%1
720
December1st to 15thNight
38%9%32%16%2
738
16th to endNight
45%9%31%12%
800
Page generated at 2017-09-20 01:49:54. Version ac04

Copyright 2017 A. Danko


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