SFA Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the SFA Observatory clear sky chart from 2005-11-05 to 2018-02-14.

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
47%
16157
25%
48747
17%
16110
55%
16110
31%
28995

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
32%
975
25%
2522
9%
970
42%
970
28%
1634
January16th to endNight
27%
1040
26%
2704
9%
1040
37%
1040
28%
1777
February1st to 15thNight
27%
923
23%
2443
7%
919
37%
919
28%
1589
February16th to endNight
24%
509
27%
1771
10%
509
41%
509
23%
1055
March1st to 15thNight
27%
540
25%
1980
13%
540
42%
540
21%
1219
March16th to endNight
32%
576
21%
2112
15%
576
42%
576
23%
1253
April1st to 15thNight
30%
539
23%
1896
13%
539
44%
539
27%
1084
April16th to endNight
32%
540
22%
1800
14%
540
44%
540
26%
1078
May1st to 15thNight
33%
539
19%
1707
13%
539
43%
539
26%
1051
May16th to endNight
52%
563
24%
1684
21%
561
61%
561
37%
959
June1st to 15thNight
74%
540
21%
1620
21%
540
78%
540
44%
852
June16th to endNight
73%
540
22%
1620
24%
540
76%
540
39%
918
July1st to 15thNight
75%
510
19%
1530
21%
510
77%
510
37%
878
July16th to endNight
82%
564
19%
1594
23%
531
84%
531
41%
888
August1st to 15thNight
87%
539
28%
1613
32%
536
87%
536
51%
787
August16th to endNight
72%
573
17%
1910
20%
573
73%
573
26%
1210
September1st to 15thNight
72%
540
21%
1911
24%
540
73%
540
32%
1079
September16th to endNight
72%
537
30%
1980
33%
537
74%
537
35%
1040
October1st to 15thNight
66%
696
31%
2136
26%
696
70%
696
40%
1167
October16th to endNight
56%
768
37%
2304
24%
768
66%
768
45%
1191
November1st to 15thNight
49%
760
27%
2280
18%
760
55%
760
34%
1313
November16th to endNight
43%
831
32%
2391
18%
831
54%
831
34%
1411
December1st to 15thNight
36%
975
26%
2535
12%
975
45%
975
28%
1680
December16th to endNight
29%
1040
20%
2704
8%
1040
36%
1040
23%
1882

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
12%5%25%5%35%45%7%41%
49324
Night
11%5%24323447%50%
48943
TransparencyDay
40%6%10%18%17%6%
49024
Night
33%5%12%19%19%9%
48748
SeeingDay
12%5%20%27%25%8%
5973
Night
14%7%24%31%18%2
16158

By Month

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
17%8%2422332447%
1746
16th to endDay
20%6%14412235%46%
1872
February1st to 15thDay
18%8%35%323335%42%
1730
16th to endDay
19%7%2442433344%
1703
March1st to 15thDay
22%6%34423335%39%
1980
16th to endDay
15%7%44424345%42%
2110
April1st to 15thDay
13%5%345%24448%43%
2050
16th to endDay
13%5%36%5%345%5%7%39%
2160
May1st to 15thDay
13%6%36%6%36%5%6%6%36%
2241
16th to endDay
10%436%6%46%5%6%10%36%
2430
June1st to 15thDay
6%425%7%46%6%7%10%39%
2340
16th to endDay
7%436%6%47%7%7%10%33%
2340
July1st to 15thDay
9%5%246%47%6%7%10%35%
2340
16th to endDay
7%437%7%47%7%8%10%32%
2466
August1st to 15thDay
33146%35%5%7%9%47%
2334
16th to endDay
10%5%36%8%5%7%7%8%8%29%
2292
September1st to 15thDay
9%5%25%5%36%6%5%7%41%
2046
16th to endDay
7%415%5%35%5%5%6%47%
1980
October1st to 15thDay
9%414323347%53%
1812
16th to endDay
11%413222225%60%
1920
November1st to 15thDay
11%5%24425%45%6%47%
1908
16th to endDay
15%5%2432223452%
1897
December1st to 15thDay
20%7%25%423235%43%
1755
16th to endDay
20%8%45%423445%35%
1872
January1st to 15thNight
16%7%24323345%47%
2522
16th to endNight
18%7%2331233448%
2704
February1st to 15thNight
16%6%24423345%48%
2433
16th to endNight
18%7%24312335%48%
1771
March1st to 15thNight
17%7%2442234444%
1980
16th to endNight
11%6%25%424336%47%
2112
April1st to 15thNight
8%424435%5%47%49%
1896
16th to endNight
12%425%5%24347%47%
1800
May1st to 15thNight
10%6%35%42445%8%43%
1706
16th to endNight
7%31442336%11%50%
1683
June1st to 15thNight
42123135%6%10%57%
1620
16th to endNight
4213335%5%6%12%51%
1620
July1st to 15thNight
5%3144345%6%10%50%
1620
16th to endNight
3213425%5%7%13%49%
1702
August1st to 15thNight
2122235%7%11%59%
1623
16th to endNight
8%424445%7%7%11%38%
1910
September1st to 15thNight
8%423334449%50%
1911
16th to endNight
6%31442345%7%55%
1980
October1st to 15thNight
6%313324336%59%
2136
16th to endNight
11%21221223465%
2304
November1st to 15thNight
11%313323347%53%
2280
16th to endNight
12%6%2432324454%
2391
December1st to 15thNight
15%7%24323346%45%
2535
16th to endNight
20%8%26%42323441%
2704

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
41%45%15%24%9%
1747
16th to endDay
41%26%15%24%10%
1872
February1st to 15thDay
45%36%16%18%9%
1731
16th to endDay
45%35%13%21%10%
1703
March1st to 15thDay
48%37%14%20%6%
1980
16th to endDay
43%48%14%24%4
2112
April1st to 15thDay
40%5%8%19%17%8%
2050
16th to endDay
42%7%9%15%19%6%
2160
May1st to 15thDay
47%7%9%13%15%6%
2241
16th to endDay
41%8%12%21%14%1
2434
June1st to 15thDay
37%9%12%23%14%1
2340
16th to endDay
40%10%14%20%10%2
2340
July1st to 15thDay
40%9%19%21%9%
2201
16th to endDay
40%11%17%21%8%
2311
August1st to 15thDay
29%8%18%30%12%
2320
16th to endDay
46%10%17%16%7%1
2292
September1st to 15thDay
39%8%14%17%13%7%
2046
16th to endDay
33%7%11%19%19%8%
1980
October1st to 15thDay
31%5%9%18%19%14%
1812
16th to endDay
27%36%13%33%14%
1920
November1st to 15thDay
36%5%9%20%20%7%
1908
16th to endDay
36%45%14%29%9%
1897
December1st to 15thDay
45%38%15%19%7%
1755
16th to endDay
51%26%15%17%5%
1872
January1st to 15thNight
39%36%15%22%11%
2522
16th to endNight
41%35%13%24%12%
2704
February1st to 15thNight
39%37%16%19%13%
2443
16th to endNight
40%28%16%20%12%
1771
March1st to 15thNight
43%37%15%20%9%
1980
16th to endNight
39%410%16%22%7%
2112
April1st to 15thNight
34%5%11%19%17%10%
1896
16th to endNight
36%5%12%17%17%10%
1800
May1st to 15thNight
38%5%12%17%17%8%
1707
16th to endNight
28%6%19%25%17%2
1685
June1st to 15thNight
20%8%16%31%20%1
1620
16th to endNight
25%8%18%25%20%2
1620
July1st to 15thNight
27%8%24%27%12%
1530
16th to endNight
23%9%28%27%10%
1594
August1st to 15thNight
16%7%24%34%15%2
1613
16th to endNight
35%11%20%21%8%2
1910
September1st to 15thNight
30%7%17%21%14%7%
1911
16th to endNight
27%7%11%20%24%9%
1980
October1st to 15thNight
27%48%21%21%16%
2136
16th to endNight
24%36%14%30%21%
2304
November1st to 15thNight
30%5%10%22%22%8%
2280
16th to endNight
33%36%13%26%15%
2391
December1st to 15thNight
40%49%18%17%9%
2535
16th to endNight
47%37%15%17%8%
2704

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
24%14%37%14%8%
135
16th to endDay
17%27%33%15%4
144
February1st to 15thDay
24%19%39%15%1
130
16th to endDay
17%27%40%14%
81
March1st to 15thDay
28%23%34%10%3
90
16th to endDay
16%13%33%27%9%
96
April1st to 15thDay
18%11%37%17%13%1
169
16th to endDay
22%10%26%27%10%2
270
May1st to 15thDay
18%10%34%25%10%1
378
16th to endDay
14%7%29%29%16%3
517
June1st to 15thDay
8%14%42%28%6%
495
16th to endDay
9%111%35%30%11%
495
July1st to 15thDay
6%112%30%38%10%
475
16th to endDay
5%5%25%47%15%
515
August1st to 15thDay
5%327%49%13%
487
16th to endDay
18%5%21%37%18%
332
September1st to 15thDay
418%25%39%19%
201
16th to endDay
5%238%40%13%
135
October1st to 15thDay
220%40%24%11%
135
16th to endDay
14%9%26%20%21%7%
144
November1st to 15thDay
12%340%33%9%
135
16th to endDay
16%9%34%26%8%4
135
December1st to 15thDay
22%8%34%20%8%6%
135
16th to endDay
33%15%34%12%4
144
January1st to 15thNight
21%15%27%27%8%
975
16th to endNight
23%13%32%25%4
1040
February1st to 15thNight
21%13%35%26%4
923
16th to endNight
24%12%37%23%2
509
March1st to 15thNight
22%11%36%23%5%
540
16th to endNight
14%12%36%27%8%
576
April1st to 15thNight
11%14%41%23%9%
539
16th to endNight
16%7%40%22%13%
540
May1st to 15thNight
13%9%37%29%9%
540
16th to endNight
9%329%35%19%2
563
June1st to 15thNight
5%13%46%30%3
540
16th to endNight
412%44%31%5%
540
July1st to 15thNight
5%10%42%35%5%
510
16th to endNight
41434%47%8%
564
August1st to 15thNight
3436%48%7%
539
16th to endNight
11%435%40%7%
573
September1st to 15thNight
10%18%32%41%5%
540
16th to endNight
7%110%31%41%7%
537
October1st to 15thNight
8%316%36%29%6%
696
16th to endNight
12%5%24%32%23%2
768
November1st to 15thNight
11%7%27%35%16%1
760
16th to endNight
16%9%27%30%15%
831
December1st to 15thNight
20%11%27%30%9%
975
16th to endNight
25%11%29%26%6%
1040
Page generated at 2018-02-15 09:58:47. Version ac04

Copyright 2018 A. Danko


SFA Observatory Clear Sky Chart |Charts Home