SETEC Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the SETEC Observatory clear sky chart from 2005-11-05 to 2017-04-26.

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
44%
15419
27%
45605
16%
15374
54%
15374
38%
25342

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
37%
900
30%
2327
14%
895
48%
895
40%
1349
January16th to endNight
37%
960
34%
2496
14%
960
49%
960
43%
1406
February1st to 15thNight
34%
821
23%
2229
9%
817
42%
817
32%
1373
February16th to endNight
34%
632
29%
1894
11%
632
48%
632
34%
1036
March1st to 15thNight
35%
584
25%
2024
15%
584
47%
584
31%
1112
March16th to endNight
30%
576
20%
2112
12%
576
42%
576
25%
1220
April1st to 15thNight
29%
539
23%
1919
14%
539
46%
539
28%
1069
April16th to endNight
32%
528
22%
1584
14%
528
45%
528
29%
944
May1st to 15thNight
36%
494
24%
1485
17%
494
48%
494
31%
880
May16th to endNight
49%
513
17%
1540
15%
513
55%
513
35%
897
June1st to 15thNight
51%
495
20%
1485
11%
495
58%
495
42%
785
June16th to endNight
61%
495
23%
1485
16%
495
68%
495
53%
720
July1st to 15thNight
68%
465
20%
1395
17%
465
71%
465
44%
747
July16th to endNight
75%
522
25%
1468
24%
489
77%
489
46%
756
August1st to 15thNight
75%
494
25%
1475
29%
491
77%
491
43%
789
August16th to endNight
70%
525
24%
1624
24%
525
73%
525
40%
867
September1st to 15thNight
55%
495
25%
1815
24%
495
64%
495
31%
994
September16th to endNight
56%
492
32%
1815
29%
492
65%
492
38%
882
October1st to 15thNight
48%
569
33%
1889
24%
569
57%
569
35%
1015
October16th to endNight
45%
704
35%
2112
17%
704
59%
704
49%
1019
November1st to 15thNight
43%
856
31%
2256
12%
856
55%
856
47%
1210
November16th to endNight
40%
900
31%
2340
12%
900
52%
900
42%
1333
December1st to 15thNight
37%
900
27%
2340
11%
900
47%
900
37%
1398
December16th to endNight
36%
960
30%
2496
14%
960
48%
960
35%
1541

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%5%2442445%7%51%
46055
Night
8%423323347%54%
45801
Cloud CoverDay
9%5%2442445%7%51%
46053
Night
8%423323347%54%
45801
TransparencyDay
32%312%27%18%4
45753
Night
29%311%27%21%5%
45606
SeeingDay
10%6%26%33%19%3
5173
Night
11%7%31%35%13%
15422

By Month

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
7%6%23313338%56%
1611
16th to endDay
7%6%24323347%53%
1728
February1st to 15thDay
7%8%25%424435%50%
1642
16th to endDay
7%5%2341346%7%54%
1580
March1st to 15thDay
10%6%15%4245%46%48%
1935
16th to endDay
16%7%23323446%44%
2112
April1st to 15thDay
12%5%135%2445%7%47%
2026
16th to endDay
14%5%245%24345%45%
2287
May1st to 15thDay
12%5%25%424447%46%
2136
16th to endDay
12%6%25%5%35%45%7%41%
2222
June1st to 15thDay
7%414423448%53%
2145
16th to endDay
5%2124245%6%9%56%
2145
July1st to 15thDay
7%314425%5%6%9%47%
2145
16th to endDay
5%213435%5%6%9%53%
2284
August1st to 15thDay
5%324445%6%6%10%47%
2145
16th to endDay
6%32443446%8%53%
2222
September1st to 15thDay
9%5%24435%5%5%6%47%
1815
16th to endDay
7%313424347%57%
1815
October1st to 15thDay
14%312313445%56%
1730
16th to endDay
7%423323336%59%
1760
November1st to 15thDay
6%313323337%61%
1602
16th to endDay
10%412212235%62%
1620
December1st to 15thDay
13%5%24323446%50%
1620
16th to endDay
12%8%35%323345%47%
1728
January1st to 15thNight
7%424422345%57%
2327
16th to endNight
7%42331333462%
2496
February1st to 15thNight
10%7%34413346%49%
2219
16th to endNight
8%5%13323345%56%
1894
March1st to 15thNight
8%6%25%32445%6%50%
2024
16th to endNight
15%6%24324246%48%
2112
April1st to 15thNight
13%31432345%8%48%
1919
16th to endNight
13%5%2342435%6%48%
1584
May1st to 15thNight
14%415%313348%48%
1484
16th to endNight
9%413325%5%7%9%47%
1539
June1st to 15thNight
5%41232446%10%53%
1485
16th to endNight
4121445%9%62%
1485
July1st to 15thNight
5%213325%5%7%9%52%
1485
16th to endNight
42133245%6%13%53%
1576
August1st to 15thNight
4213335%5%6%9%53%
1485
16th to endNight
442442445%9%55%
1624
September1st to 15thNight
8%4235%2445%6%51%
1815
16th to endNight
6%31332336%7%57%
1815
October1st to 15thNight
11%423323336%56%
1889
16th to endNight
7%212212235%68%
2112
November1st to 15thNight
6%213212337%62%
2256
16th to endNight
10%313213326%60%
2340
December1st to 15thNight
9%6%2431335%7%52%
2340
16th to endNight
10%7%34323347%50%
2496

Percentage Cloud Cover SE (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
7%6%13412338%56%
1611
16th to endDay
7%6%24323347%53%
1728
February1st to 15thDay
7%7%25%434436%50%
1642
16th to endDay
7%6%2242346%6%54%
1580
March1st to 15thDay
10%6%25%4245%46%47%
1935
16th to endDay
16%7%24323346%44%
2110
April1st to 15thDay
12%424424447%47%
2026
16th to endDay
14%6%245%24345%45%
2287
May1st to 15thDay
12%5%25%424447%45%
2136
16th to endDay
12%5%25%5%36%45%7%41%
2222
June1st to 15thDay
7%323423449%53%
2145
16th to endDay
5%21332446%9%56%
2145
July1st to 15thDay
7%314425%5%7%9%47%
2145
16th to endDay
5%213435%5%6%9%53%
2284
August1st to 15thDay
5%424435%5%6%9%48%
2145
16th to endDay
6%31442446%7%53%
2222
September1st to 15thDay
9%5%244345%5%6%47%
1815
16th to endDay
7%323423346%57%
1815
October1st to 15thDay
13%312223345%56%
1730
16th to endDay
7%423323335%60%
1760
November1st to 15thDay
5%323323437%61%
1602
16th to endDay
10%322212235%62%
1620
December1st to 15thDay
12%6%24313446%50%
1620
16th to endDay
12%8%34324345%47%
1728
January1st to 15thNight
7%424423345%57%
2327
16th to endNight
7%42332333462%
2496
February1st to 15thNight
9%8%35%41335%6%48%
2219
16th to endNight
8%5%23323345%56%
1894
March1st to 15thNight
8%6%15%32445%6%50%
2024
16th to endNight
15%6%24323246%48%
2112
April1st to 15thNight
13%41432345%7%48%
1919
16th to endNight
12%6%2342445%5%48%
1584
May1st to 15thNight
14%5%14413347%48%
1484
16th to endNight
8%4133246%6%9%47%
1539
June1st to 15thNight
6%41232346%10%53%
1485
16th to endNight
4122245%5%9%62%
1485
July1st to 15thNight
5%313325%5%6%9%52%
1485
16th to endNight
42123245%6%12%53%
1576
August1st to 15thNight
42133445%6%8%54%
1485
16th to endNight
431442435%8%55%
1624
September1st to 15thNight
9%42342445%7%51%
1815
16th to endNight
6%31432335%8%57%
1815
October1st to 15thNight
12%423322336%55%
1889
16th to endNight
7%22212235%68%
2112
November1st to 15thNight
6%313212337%62%
2256
16th to endNight
10%313214325%61%
2340
December1st to 15thNight
9%5%3431335%7%51%
2340
16th to endNight
10%7%35%323347%50%
2496

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
28%212%32%21%2
1612
16th to endDay
30%213%31%17%3
1728
February1st to 15thDay
36%314%27%13%4
1643
16th to endDay
28%313%30%19%4
1580
March1st to 15thDay
36%310%24%20%4
1935
16th to endDay
40%410%25%15%2
2112
April1st to 15thDay
35%313%27%16%3
2026
16th to endDay
41%310%28%13%2
2287
May1st to 15thDay
38%413%20%20%2
2136
16th to endDay
40%415%23%13%2
2226
June1st to 15thDay
28%5%17%24%18%5%
2145
16th to endDay
22%5%15%27%22%6%
2145
July1st to 15thDay
30%5%14%29%17%2
2006
16th to endDay
24%5%14%30%20%3
2129
August1st to 15thDay
29%6%15%25%17%5%
2131
16th to endDay
27%5%13%29%16%7%
2222
September1st to 15thDay
34%5%11%24%15%8%
1815
16th to endDay
27%310%31%23%4
1815
October1st to 15thDay
30%39%26%26%5%
1730
16th to endDay
26%38%29%26%5%
1760
November1st to 15thDay
24%39%26%27%9%
1602
16th to endDay
26%210%30%24%5%
1620
December1st to 15thDay
35%312%30%15%1
1620
16th to endDay
39%210%26%17%3
1728
January1st to 15thNight
28%29%31%24%3
2327
16th to endNight
26%210%30%24%4
2496
February1st to 15thNight
35%313%25%16%4
2229
16th to endNight
29%211%28%19%6%
1894
March1st to 15thNight
34%210%24%21%7%
2024
16th to endNight
38%311%26%16%3
2112
April1st to 15thNight
33%310%25%22%3
1919
16th to endNight
36%313%22%18%5%
1584
May1st to 15thNight
35%314%24%19%2
1485
16th to endNight
30%6%21%25%13%2
1541
June1st to 15thNight
25%418%28%19%3
1485
16th to endNight
17%414%32%23%8%
1485
July1st to 15thNight
24%5%13%29%22%3
1395
16th to endNight
22%413%32%23%3
1468
August1st to 15thNight
24%5%15%29%18%6%
1475
16th to endNight
25%5%12%27%20%7%
1624
September1st to 15thNight
33%311%23%17%10%
1815
16th to endNight
24%410%25%28%6%
1815
October1st to 15thNight
31%29%22%25%9%
1889
16th to endNight
20%27%29%32%7%
2112
November1st to 15thNight
22%39%24%29%10%
2256
16th to endNight
27%26%30%25%7%
2340
December1st to 15thNight
31%312%30%19%2
2340
16th to endNight
35%212%27%15%6%
2496

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
11%11%34%34%6%2
90
16th to endDay
14%10%40%32%11
96
February1st to 15thDay
6%12%58%14%7%
90
16th to endDay
6%9%46%28%7%1
81
March1st to 15thDay
6%16%50%22%4
90
16th to endDay
19%13%33%27%6%
96
April1st to 15thDay
14%13%35%26%10%
145
16th to endDay
19%11%38%21%7%1
430
May1st to 15thDay
12%10%35%28%10%2
393
16th to endDay
18%6%28%27%14%3
437
June1st to 15thDay
8%328%38%19%1
420
16th to endDay
5%322%40%22%4
420
July1st to 15thDay
9%213%44%28%2
400
16th to endDay
7%10%38%35%7%
445
August1st to 15thDay
5%10%42%33%6%
418
16th to endDay
7%215%38%29%6%
390
September1st to 15thDay
3220%53%17%3
90
16th to endDay
10%225%25%22%14%
90
October1st to 15thDay
11%10%20%30%22%6%
90
16th to endDay
6%439%26%16%7%
96
November1st to 15thDay
6%325%32%21%11%
90
16th to endDay
21%20%33%17%7%
90
December1st to 15thDay
14%10%35%30%8%1
90
16th to endDay
18%12%39%22%6%
96
January1st to 15thNight
10%12%35%33%8%
900
16th to endNight
10%10%38%32%7%
960
February1st to 15thNight
14%10%34%35%4
821
16th to endNight
10%13%35%33%5%
632
March1st to 15thNight
11%10%38%32%6%
584
16th to endNight
17%11%36%28%6%
576
April1st to 15thNight
17%10%40%25%7%
539
16th to endNight
14%10%39%28%7%
528
May1st to 15thNight
15%7%34%31%9%
495
16th to endNight
10%430%33%17%2
515
June1st to 15thNight
6%432%37%16%
495
16th to endNight
5%228%45%18%
495
July1st to 15thNight
6%16%53%20%1
465
16th to endNight
5%12%48%31%1
522
August1st to 15thNight
5%111%48%32%1
494
16th to endNight
5%112%43%32%3
525
September1st to 15thNight
9%123%37%23%3
495
16th to endNight
10%423%35%22%2
492
October1st to 15thNight
14%430%28%20%1
569
16th to endNight
8%7%35%34%12%
704
November1st to 15thNight
9%9%33%33%13%
856
16th to endNight
13%10%33%32%9%
900
December1st to 15thNight
14%10%33%34%6%
900
16th to endNight
14%8%34%35%5%
960
Page generated at 2017-04-27 16:48:56. Version ac04

Copyright 2017 A. Danko


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