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 2017-09-23.

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
45%
15863
27%
46988
16%
15818
55%
15818
38%
26021

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
50%
960
43%
1406
February1st to 15thNight
34%
821
23%
2229
9%
817
42%
817
32%
1373
February16th to endNight
34%
632
30%
1894
11%
632
48%
632
34%
1036
March1st to 15thNight
34%
584
25%
2024
15%
584
46%
584
31%
1112
March16th to endNight
31%
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%
540
22%
1620
14%
540
44%
540
28%
973
May1st to 15thNight
38%
539
23%
1620
16%
539
48%
539
32%
949
May16th to endNight
49%
561
19%
1684
16%
561
55%
561
36%
971
June1st to 15thNight
51%
540
19%
1620
10%
540
58%
540
42%
849
June16th to endNight
60%
540
24%
1620
16%
540
67%
540
52%
794
July1st to 15thNight
70%
510
19%
1530
17%
510
73%
510
45%
813
July16th to endNight
75%
564
26%
1594
26%
531
77%
531
46%
825
August1st to 15thNight
75%
539
24%
1610
27%
536
77%
536
43%
862
August16th to endNight
71%
573
27%
1773
27%
573
74%
573
41%
934
September1st to 15thNight
57%
540
24%
1980
23%
540
65%
540
33%
1052
September16th to endNight
56%
516
33%
1903
29%
516
66%
516
38%
918
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
37%
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%42442445%7%51%
47919
Night
8%423323347%54%
47184
Cloud CoverDay
9%42442445%7%51%
47917
Night
8%413323447%54%
47184
TransparencyDay
31%312%27%19%4
47617
Night
29%311%27%21%5%
46989
SeeingDay
10%5%25%33%20%4
5859
Night
11%7%31%35%13%1
15866

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
15%6%245%24345%44%
2340
May1st to 15thDay
12%5%25%424437%47%
2331
16th to endDay
12%5%25%5%35%46%7%42%
2430
June1st to 15thDay
7%313423448%54%
2340
16th to endDay
5%2124235%6%8%57%
2340
July1st to 15thDay
7%314425%5%6%9%48%
2340
16th to endDay
5%214435%5%6%9%52%
2466
August1st to 15thDay
5%424445%6%6%9%47%
2340
16th to endDay
5%32442446%8%53%
2425
September1st to 15thDay
8%42433445%6%49%
1980
16th to endDay
7%313324347%57%
1893
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%47%
1620
May1st to 15thNight
14%415%313347%49%
1619
16th to endNight
9%3133245%7%9%48%
1683
June1st to 15thNight
5%41232446%10%53%
1620
16th to endNight
4123145%6%9%61%
1620
July1st to 15thNight
5%213325%5%7%9%52%
1620
16th to endNight
421332446%13%53%
1702
August1st to 15thNight
4213345%5%6%9%53%
1620
16th to endNight
442342445%8%56%
1773
September1st to 15thNight
8%4235%23446%53%
1980
16th to endNight
6%31332335%7%58%
1903
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
15%6%245%24345%44%
2340
May1st to 15thDay
11%5%25%424447%47%
2331
16th to endDay
12%5%25%5%35%45%7%42%
2430
June1st to 15thDay
7%323423448%54%
2340
16th to endDay
5%21332446%9%57%
2340
July1st to 15thDay
7%314425%5%7%9%48%
2340
16th to endDay
5%213435%5%7%9%52%
2466
August1st to 15thDay
5%424435%6%6%9%47%
2340
16th to endDay
5%31442446%7%53%
2425
September1st to 15thDay
8%4244345%5%7%49%
1980
16th to endDay
7%323323346%57%
1893
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
13%6%2342445%5%47%
1620
May1st to 15thNight
14%5%14322347%49%
1619
16th to endNight
8%4133245%6%9%48%
1683
June1st to 15thNight
5%31232347%10%53%
1620
16th to endNight
4122245%5%9%61%
1620
July1st to 15thNight
5%313325%5%6%9%52%
1620
16th to endNight
42133245%6%12%53%
1702
August1st to 15thNight
42133445%6%9%53%
1620
16th to endNight
431442435%8%56%
1773
September1st to 15thNight
8%423423346%54%
1980
16th to endNight
6%31432335%8%58%
1903
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
2340
May1st to 15thDay
37%412%21%21%2
2331
16th to endDay
39%414%25%13%2
2434
June1st to 15thDay
27%5%17%26%18%5%
2340
16th to endDay
22%414%28%22%6%
2340
July1st to 15thDay
29%5%14%29%17%2
2201
16th to endDay
24%5%14%29%21%3
2311
August1st to 15thDay
30%6%16%25%16%5%
2326
16th to endDay
27%5%13%28%18%6%
2425
September1st to 15thDay
32%411%27%15%8%
1980
16th to endDay
27%310%32%22%4
1893
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%21%19%4
1620
May1st to 15thNight
34%313%25%19%3
1620
16th to endNight
30%5%20%26%14%2
1685
June1st to 15thNight
24%418%29%19%3
1620
16th to endNight
18%413%32%22%7%
1620
July1st to 15thNight
24%5%13%29%24%3
1530
16th to endNight
22%413%31%23%3
1594
August1st to 15thNight
24%5%16%30%17%6%
1610
16th to endNight
25%5%11%27%22%7%
1773
September1st to 15thNight
31%311%26%17%9%
1980
16th to endNight
24%410%26%28%5%
1903
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
21%10%38%21%7%1
450
May1st to 15thDay
12%10%34%28%10%3
468
16th to endDay
17%7%31%27%12%3
517
June1st to 15thDay
8%327%36%21%3
495
16th to endDay
6%323%41%22%3
495
July1st to 15thDay
8%212%41%30%4
475
16th to endDay
6%9%37%37%8%
515
August1st to 15thDay
6%111%41%32%5%
493
16th to endDay
6%215%38%30%6%
465
September1st to 15thDay
2117%42%29%6%
135
16th to endDay
9%127%28%20%11%
111
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
15%10%38%27%7%
540
May1st to 15thNight
15%7%34%31%9%1
540
16th to endNight
10%430%34%17%2
563
June1st to 15thNight
6%432%36%18%1
540
16th to endNight
5%228%45%17%
540
July1st to 15thNight
6%15%54%21%1
510
16th to endNight
6%11%48%31%1
564
August1st to 15thNight
5%110%50%31%1
539
16th to endNight
5%112%44%32%3
573
September1st to 15thNight
9%123%37%24%3
540
16th to endNight
9%425%35%21%2
516
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 2017-09-24 08:15:49. Version ac04

Copyright 2017 A. Danko


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