Spradlin Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Spradlin Observatory clear sky chart from 2005-11-05 to 2017-10-06.

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
52%
15370
24%
47178
18%
15327
59%
15327
29%
28828

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
30%
720
25%
2327
12%
716
45%
716
24%
1430
January16th to endNight
29%
768
25%
2334
12%
768
42%
768
23%
1523
February1st to 15thNight
35%
704
25%
2112
12%
701
47%
701
27%
1321
February16th to endNight
31%
631
24%
1893
13%
631
41%
631
27%
1199
March1st to 15thNight
33%
720
22%
2139
12%
720
41%
720
27%
1350
March16th to endNight
41%
735
23%
2079
14%
735
48%
735
30%
1276
April1st to 15thNight
44%
539
25%
1797
19%
539
55%
539
32%
1029
April16th to endNight
50%
540
27%
1800
22%
540
60%
540
34%
992
May1st to 15thNight
56%
540
23%
1623
16%
540
63%
540
31%
1000
May16th to endNight
69%
561
23%
1682
19%
561
73%
561
35%
999
June1st to 15thNight
76%
540
15%
1620
13%
540
77%
540
30%
1014
June16th to endNight
80%
540
19%
1620
17%
540
80%
540
36%
940
July1st to 15thNight
77%
510
13%
1530
13%
510
78%
510
31%
956
July16th to endNight
73%
564
16%
1601
16%
531
74%
531
30%
1032
August1st to 15thNight
77%
539
18%
1788
21%
536
77%
536
30%
1071
August16th to endNight
70%
573
19%
1910
23%
573
70%
573
24%
1258
September1st to 15thNight
66%
633
20%
1893
19%
633
67%
633
26%
1242
September16th to endNight
68%
717
29%
1980
24%
717
71%
717
36%
1194
October1st to 15thNight
61%
684
33%
1881
25%
684
66%
684
41%
1079
October16th to endNight
54%
704
33%
2117
28%
704
61%
704
36%
1207
November1st to 15thNight
54%
700
30%
2276
27%
700
63%
700
37%
1218
November16th to endNight
43%
720
27%
2340
20%
720
53%
720
29%
1357
December1st to 15thNight
39%
720
23%
2340
18%
720
47%
720
25%
1466
December16th to endNight
33%
768
20%
2496
16%
768
41%
768
20%
1675

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%5%5%7%41%
48012
Night
11%5%2442445%7%46%
47378
TransparencyDay
40%6%11%19%17%4
47714
Night
36%5%10%19%20%7%
47180
SeeingDay
13%5%16%23%25%16%
5281
Night
13%6%21%29%23%5%
15372

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
19%7%35%314345%42%
1611
16th to endDay
18%5%23414435%45%
1889
February1st to 15thDay
16%6%35%5%2434443%
1750
16th to endDay
18%7%24433335%42%
1580
March1st to 15thDay
15%6%36%5%23346%41%
1821
16th to endDay
14%5%35%424348%42%
2153
April1st to 15thDay
13%4244345%46%44%
2151
16th to endDay
9%425%5%35%5%5%8%45%
2160
May1st to 15thDay
10%425%435%5%6%8%44%
2327
16th to endDay
7%5%25%5%35%5%6%8%42%
2431
June1st to 15thDay
8%426%6%5%7%7%7%9%34%
2340
16th to endDay
5%3245%36%7%8%11%41%
2340
July1st to 15thDay
8%336%8%5%7%7%8%11%30%
2340
16th to endDay
6%426%7%49%8%8%11%30%
2456
August1st to 15thDay
7%3247%5%8%8%9%11%31%
2160
16th to endDay
11%5%26%7%46%7%7%10%29%
2292
September1st to 15thDay
11%6%36%6%45%5%6%6%36%
2054
16th to endDay
11%5%24435%5%5%6%45%
1980
October1st to 15thDay
10%424424446%52%
1877
16th to endDay
11%42342322459%
1750
November1st to 15thDay
12%5%2432323454%
1582
16th to endDay
15%5%24313445%49%
1620
December1st to 15thDay
18%6%24423345%41%
1620
16th to endDay
21%9%46%42333436%
1728
January1st to 15thNight
16%6%24423345%45%
2327
16th to endNight
16%7%35%423445%41%
2334
February1st to 15thNight
15%5%25%423345%47%
2103
16th to endNight
16%6%24333335%47%
1893
March1st to 15thNight
16%5%23323447%44%
2139
16th to endNight
12%5%24424348%47%
2079
April1st to 15thNight
10%42441446%7%48%
1797
16th to endNight
9%41442345%6%54%
1800
May1st to 15thNight
8%315%5%35%5%5%7%47%
1622
16th to endNight
4414435%5%6%11%47%
1682
June1st to 15thNight
33245%45%7%8%13%41%
1620
16th to endNight
31145%35%6%8%12%46%
1620
July1st to 15thNight
5%3245%27%6%9%12%39%
1620
16th to endNight
4426%6%46%6%8%10%39%
1709
August1st to 15thNight
44145%45%6%8%11%42%
1800
16th to endNight
8%5%26%7%35%5%6%9%39%
1910
September1st to 15thNight
9%425%6%25%6%6%8%40%
1893
16th to endNight
7%42442445%7%51%
1980
October1st to 15thNight
8%424423336%57%
1881
16th to endNight
10%5%2442322358%
2117
November1st to 15thNight
11%323323345%57%
2276
16th to endNight
11%424424335%51%
2340
December1st to 15thNight
15%5%25%5%23345%44%
2340
16th to endNight
21%7%245%14446%37%
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
43%46%13%26%4
1612
16th to endDay
41%36%20%21%6%
1889
February1st to 15thDay
44%36%21%19%3
1752
16th to endDay
45%35%16%23%6%
1580
March1st to 15thDay
45%48%16%19%5%
1821
16th to endDay
41%411%20%18%3
2154
April1st to 15thDay
39%5%8%19%19%7%
2151
16th to endDay
34%5%10%21%23%3
2160
May1st to 15thDay
37%6%12%18%19%4
2327
16th to endDay
36%7%13%24%14%3
2435
June1st to 15thDay
42%9%15%19%11%1
2340
16th to endDay
33%9%16%22%14%3
2340
July1st to 15thDay
41%11%19%17%8%1
2201
16th to endDay
42%11%18%18%9%
2301
August1st to 15thDay
41%11%16%18%10%2
2148
16th to endDay
47%10%12%17%9%1
2292
September1st to 15thDay
44%8%12%17%12%4
2054
16th to endDay
37%5%11%20%19%5%
1980
October1st to 15thDay
32%49%19%24%9%
1877
16th to endDay
31%2415%34%10%
1750
November1st to 15thDay
33%26%16%29%10%
1582
16th to endDay
36%46%16%26%9%
1620
December1st to 15thDay
45%48%19%15%6%
1620
16th to endDay
53%36%18%12%5%
1728
January1st to 15thNight
40%36%18%24%5%
2327
16th to endNight
45%35%17%20%8%
2334
February1st to 15thNight
40%35%20%20%9%
2112
16th to endNight
41%35%18%24%6%
1893
March1st to 15thNight
39%49%19%19%7%
2139
16th to endNight
35%48%23%23%4
2079
April1st to 15thNight
33%49%22%19%9%
1797
16th to endNight
30%5%10%23%22%8%
1800
May1st to 15thNight
34%7%10%18%25%4
1623
16th to endNight
29%8%14%26%14%5%
1684
June1st to 15thNight
31%12%17%22%15%
1620
16th to endNight
28%8%18%23%17%3
1620
July1st to 15thNight
31%11%22%21%10%2
1530
16th to endNight
34%10%21%20%12%
1601
August1st to 15thNight
31%9%18%23%12%3
1788
16th to endNight
39%9%15%18%13%2
1910
September1st to 15thNight
37%10%13%18%15%5%
1893
16th to endNight
31%6%10%22%21%7%
1980
October1st to 15thNight
29%38%21%23%13%
1881
16th to endNight
32%3411%33%15%
2117
November1st to 15thNight
29%26%18%27%15%
2276
16th to endNight
35%46%17%25%12%
2340
December1st to 15thNight
41%48%18%21%5%
2340
16th to endNight
48%36%18%16%5%
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
27%12%34%16%5%3
90
16th to endDay
19%18%45%10%41
96
February1st to 15thDay
13%26%34%24%1
90
16th to endDay
18%14%34%30%1
81
March1st to 15thDay
19%18%33%20%5%1
111
16th to endDay
17%7%29%27%17%1
288
April1st to 15thDay
16%11%26%22%19%3
270
16th to endDay
11%425%31%21%5%
270
May1st to 15thDay
11%7%22%27%18%12%
288
16th to endDay
11%316%28%21%18%
331
June1st to 15thDay
12%17%22%27%28%
315
16th to endDay
9%9%19%29%32%
315
July1st to 15thDay
6%6%19%27%39%
305
16th to endDay
8%411%34%40%
326
August1st to 15thDay
10%5%23%32%28%
313
16th to endDay
13%321%37%23%
332
September1st to 15thDay
11%29%19%34%22%
315
16th to endDay
11%10%26%37%14%
314
October1st to 15thDay
14%215%30%32%4
278
16th to endDay
13%6%14%28%28%7%
187
November1st to 15thDay
20%8%22%23%23%2
90
16th to endDay
8%15%32%26%12%4
90
December1st to 15thDay
25%11%35%16%10%1
90
16th to endDay
29%10%27%25%7%1
96
January1st to 15thNight
20%12%32%25%7%
720
16th to endNight
20%12%33%27%5%
768
February1st to 15thNight
18%10%32%33%4
704
16th to endNight
18%12%34%29%4
631
March1st to 15thNight
19%11%31%29%6%
720
16th to endNight
14%10%29%28%15%1
735
April1st to 15thNight
12%7%32%28%17%1
539
16th to endNight
10%6%25%36%18%1
540
May1st to 15thNight
8%324%33%26%3
540
16th to endNight
6%114%37%34%6%
563
June1st to 15thNight
5%7%33%42%10%
540
16th to endNight
35%27%45%17%
540
July1st to 15thNight
6%421%49%17%
510
16th to endNight
6%5%22%44%21%
564
August1st to 15thNight
6%422%51%14%
539
16th to endNight
11%5%25%43%13%
573
September1st to 15thNight
12%19%24%41%11%
633
16th to endNight
9%112%34%37%5%
717
October1st to 15thNight
11%217%36%27%4
684
16th to endNight
13%5%21%30%24%3
704
November1st to 15thNight
13%5%22%32%23%3
700
16th to endNight
13%12%26%33%13%1
720
December1st to 15thNight
19%7%28%31%11%1
720
16th to endNight
25%10%26%28%8%
768
Page generated at 2017-10-07 13:07:51. Version ac04

Copyright 2017 A. Danko


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