Millard Oakley STEM Center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Millard Oakley STEM Center clear sky chart from 2005-11-06 to 2024-01-12.

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
42%
23605
20%
74517
14%
23548
49%
23548
23%
48691

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 10thNight
21%
760
18%
2459
9%
757
32%
757
17%
1687
January11th to 20thNight
18%
727
15%
2360
8%
727
27%
727
12%
1788
January21st to endNight
18%
792
20%
2489
8%
792
31%
792
18%
1719
February1st to 10thNight
19%
720
16%
2160
7%
720
28%
720
16%
1587
February11th to 20thNight
13%
703
15%
2098
6%
699
22%
699
15%
1544
February21st to endNight
15%
592
18%
1776
7%
592
24%
592
14%
1332
March1st to 10thNight
24%
720
18%
2160
8%
720
32%
720
21%
1471
March11th to 20thNight
23%
720
17%
2006
7%
720
30%
720
18%
1428
March21st to endNight
29%
792
14%
2178
8%
792
34%
792
19%
1562
April1st to 10thNight
35%
540
24%
1800
15%
540
47%
540
26%
1089
April11th to 20thNight
34%
533
26%
1775
15%
533
48%
533
26%
1061
April21st to endNight
38%
540
22%
1684
12%
540
47%
540
25%
1068
May1st to 10thNight
38%
540
19%
1620
11%
540
45%
540
26%
1040
May11th to 20thNight
49%
531
16%
1593
10%
531
53%
531
25%
1034
May21st to endNight
59%
582
17%
1745
12%
582
63%
582
27%
1123
June1st to 10thNight
64%
540
18%
1620
15%
540
66%
540
30%
998
June11th to 20thNight
72%
540
18%
1620
16%
540
74%
540
39%
914
June21st to endNight
67%
540
16%
1576
14%
540
68%
540
31%
982
July1st to 10thNight
64%
525
12%
1571
11%
525
64%
525
25%
1053
July11th to 20thNight
69%
522
17%
1566
15%
522
71%
522
31%
965
July21st to endNight
69%
591
17%
1672
15%
558
71%
558
26%
1098
August1st to 10thNight
70%
537
18%
1600
18%
534
71%
534
30%
986
August11th to 20thNight
69%
534
16%
1767
17%
534
70%
534
26%
1092
August21st to endNight
74%
594
20%
1980
24%
594
74%
594
30%
1166
September1st to 10thNight
65%
608
21%
1868
20%
608
66%
608
28%
1169
September11th to 20thNight
60%
720
23%
1980
20%
720
61%
720
31%
1282
September21st to endNight
61%
720
29%
1980
22%
720
65%
720
37%
1176
October1st to 10thNight
57%
720
32%
1980
26%
720
62%
720
41%
1123
October11th to 20thNight
45%
720
27%
2038
19%
720
52%
720
30%
1268
October21st to endNight
40%
792
28%
2510
22%
792
50%
792
28%
1511
November1st to 10thNight
48%
740
27%
2406
25%
740
55%
740
31%
1382
November11th to 20thNight
34%
760
27%
2470
18%
760
48%
760
28%
1455
November21st to endNight
34%
760
23%
2470
16%
760
45%
760
23%
1555
December1st to 10thNight
25%
760
16%
2470
10%
760
33%
760
15%
1804
December11th to 20thNight
29%
759
20%
2989
14%
759
39%
759
16%
1990
December21st to endNight
22%
831
11%
4481
10%
817
32%
817
8%
3189

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
15%7%36%5%35%5%6%7%38%
74845
Night
15%7%36%5%3445%7%41%
74828
TransparencyDay
44%8%11%19%15%3
74568
Night
42%7%11%20%16%3
74624
SeeingDay
19%12%24%22%16%7%
13517
Night
18%10%24%27%17%3
23613

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 10thDay
19%10%37%6%4445%5%32%
1706
11th to 20thDay
23%12%49%5%34335%27%
1629
21st to endDay
17%10%36%3245%5%6%38%
1863
February1st to 10thDay
21%10%5%7%5%24445%32%
1800
11th to 20thDay
22%10%5%7%7%3445%5%30%
1748
21st to endDay
23%9%46%5%3335%6%33%
1480
March1st to 10thDay
16%9%35%7%44446%38%
1800
11th to 20thDay
21%9%47%5%34445%32%
1953
21st to endDay
18%9%46%5%35%5%5%6%32%
2191
April1st to 10thDay
14%6%26%5%3445%7%44%
2160
11th to 20thDay
15%8%25%425%5%5%6%42%
2134
21st to endDay
15%7%35%6%35%5%5%7%38%
2273
May1st to 10thDay
16%5%35%5%25%6%6%7%40%
2340
11th to 20thDay
15%7%37%6%35%46%8%36%
2301
21st to endDay
11%5%35%6%47%5%7%8%39%
2531
June1st to 10thDay
11%6%26%6%45%5%6%7%41%
2340
11th to 20thDay
6%425%5%5%5%6%6%11%45%
2340
21st to endDay
8%425%6%47%6%7%11%39%
2340
July1st to 10thDay
11%6%28%7%47%7%7%9%32%
2340
11th to 20thDay
11%5%37%7%47%7%8%9%34%
2340
21st to endDay
11%5%35%7%47%7%8%11%34%
2547
August1st to 10thDay
9%5%37%7%5%6%7%8%10%34%
2327
11th to 20thDay
11%6%36%7%5%5%6%8%8%35%
2132
21st to endDay
11%5%25%6%5%7%8%7%9%34%
2376
September1st to 10thDay
14%5%36%7%36%6%5%7%38%
2072
11th to 20thDay
14%7%36%5%3445%8%42%
1980
21st to endDay
12%5%35%4235%5%7%50%
1980
October1st to 10thDay
10%5%24434446%54%
1980
11th to 20thDay
15%5%24424347%51%
1922
21st to endDay
17%6%25%42346%6%45%
1841
November1st to 10thDay
12%5%24323446%55%
1672
11th to 20thDay
15%7%34334445%48%
1710
21st to endDay
17%8%45%42435%5%44%
1710
December1st to 10thDay
22%11%46%5%34446%31%
1710
11th to 20thDay
17%13%47%6%345%45%33%
2153
21st to endDay
19%14%46%5%3445%8%28%
3124
January1st to 10thNight
20%8%37%5%34346%37%
2459
11th to 20thNight
21%13%5%7%6%3444429%
2360
21st to endNight
17%10%48%5%2345%5%37%
2489
February1st to 10thNight
22%9%47%6%345%5%5%30%
2160
11th to 20thNight
23%8%5%7%5%33335%32%
2098
21st to endNight
24%13%47%42345%6%29%
1776
March1st to 10thNight
17%8%35%5%345%5%6%39%
2160
11th to 20thNight
21%8%36%5%3435%6%36%
2006
21st to endNight
16%8%47%5%35%5%47%35%
2178
April1st to 10thNight
13%5%245%35%45%8%45%
1800
11th to 20thNight
17%6%2342435%6%47%
1775
21st to endNight
15%6%26%5%35%45%7%43%
1684
May1st to 10thNight
14%5%26%5%35%46%8%43%
1620
11th to 20thNight
12%435%5%45%5%5%9%42%
1592
21st to endNight
6%426%6%46%5%8%10%42%
1744
June1st to 10thNight
7%5%35%5%45%6%5%9%46%
1620
11th to 20thNight
43145%35%5%7%11%51%
1620
21st to endNight
5%425%6%46%6%7%13%42%
1576
July1st to 10thNight
10%5%36%6%36%5%7%10%38%
1616
11th to 20thNight
9%4245%36%6%8%10%44%
1620
21st to endNight
8%436%6%47%7%7%11%37%
1771
August1st to 10thNight
8%425%6%35%6%7%11%43%
1611
11th to 20thNight
8%425%5%45%6%8%9%45%
1767
21st to endNight
9%424436%5%6%11%46%
1980
September1st to 10thNight
11%6%35%425%5%6%7%46%
1868
11th to 20thNight
12%6%36%5%35%45%6%46%
1980
21st to endNight
10%5%24324447%54%
1980
October1st to 10thNight
10%424322446%59%
1980
11th to 20thNight
18%5%24423445%49%
2038
21st to endNight
17%7%35%323346%47%
2510
November1st to 10thNight
12%6%24424346%53%
2406
11th to 20thNight
14%8%24424347%48%
2470
21st to endNight
17%8%34424447%43%
2470
December1st to 10thNight
22%13%46%5%34345%31%
2470
11th to 20thNight
18%11%48%423345%38%
3042
21st to endNight
16%13%48%7%45%45%6%26%
4532

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 10thDay
54%5%7%17%14%3
1707
11th to 20thDay
61%37%16%11%2
1629
21st to endDay
46%5%11%22%15%1
1863
February1st to 10thDay
56%5%7%15%15%2
1800
11th to 20thDay
57%47%17%12%3
1758
21st to endDay
53%410%13%15%5%
1480
March1st to 10thDay
48%47%18%18%5%
1800
11th to 20thDay
54%5%9%15%13%3
1953
21st to endDay
51%7%10%18%12%1
2191
April1st to 10thDay
40%7%11%23%16%3
2160
11th to 20thDay
42%6%7%20%23%3
2134
21st to endDay
44%7%10%25%13%2
2273
May1st to 10thDay
42%8%11%24%12%2
2340
11th to 20thDay
47%8%13%20%12%1
2305
21st to endDay
40%12%14%20%12%2
2531
June1st to 10thDay
40%9%14%22%14%2
2340
11th to 20thDay
32%11%18%22%15%2
2340
21st to endDay
36%12%16%19%14%4
2340
July1st to 10thDay
42%13%15%17%11%3
2275
11th to 20thDay
43%11%16%16%11%3
2271
21st to endDay
40%11%18%20%9%1
2405
August1st to 10thDay
41%13%17%18%9%1
2314
11th to 20thDay
43%10%15%18%13%1
2132
21st to endDay
43%11%14%19%12%2
2376
September1st to 10thDay
42%10%12%18%15%3
2072
11th to 20thDay
41%8%13%20%16%2
1980
21st to endDay
34%7%11%24%20%4
1980
October1st to 10thDay
32%6%10%22%25%5%
1980
11th to 20thDay
34%6%9%20%25%7%
1922
21st to endDay
39%5%8%21%23%4
1841
November1st to 10thDay
32%49%23%27%5%
1672
11th to 20thDay
38%5%7%23%22%5%
1710
21st to endDay
42%5%7%21%21%4
1710
December1st to 10thDay
54%5%6%20%12%2
1710
11th to 20thDay
52%5%8%17%15%2
2150
21st to endDay
54%6%10%19%9%2
3124
January1st to 10thNight
50%410%18%15%2
2459
11th to 20thNight
60%47%14%13%2
2360
21st to endNight
50%5%8%20%15%2
2489
February1st to 10thNight
55%49%17%13%1
2160
11th to 20thNight
56%46%18%12%4
2099
21st to endNight
57%5%6%14%14%4
1776
March1st to 10thNight
46%5%10%14%18%6%
2160
11th to 20thNight
49%6%9%20%14%2
2006
21st to endNight
48%7%11%21%11%2
2178
April1st to 10thNight
37%6%10%24%20%3
1800
11th to 20thNight
37%5%9%20%22%6%
1775
21st to endNight
41%6%10%29%14%1
1684
May1st to 10thNight
40%7%12%23%16%2
1620
11th to 20thNight
39%9%17%22%13%1
1593
21st to endNight
33%13%16%22%13%3
1745
June1st to 10thNight
34%9%13%29%13%2
1620
11th to 20thNight
24%10%21%27%16%2
1620
21st to endNight
31%12%17%24%12%4
1576
July1st to 10thNight
37%12%17%19%12%2
1571
11th to 20thNight
32%10%18%20%14%5%
1566
21st to endNight
36%12%17%22%11%1
1672
August1st to 10thNight
34%9%19%27%12%
1600
11th to 20thNight
34%11%20%19%14%2
1767
21st to endNight
31%10%16%26%14%3
1980
September1st to 10thNight
36%9%13%21%16%5%
1868
11th to 20thNight
37%7%12%21%22%2
1980
21st to endNight
30%7%10%24%24%5%
1980
October1st to 10thNight
28%5%9%22%28%8%
1980
11th to 20thNight
37%5%8%21%22%6%
2038
21st to endNight
38%47%21%24%6%
2510
November1st to 10thNight
34%47%23%25%7%
2406
11th to 20thNight
36%48%20%24%8%
2470
21st to endNight
43%47%23%19%4
2470
December1st to 10thNight
56%5%7%17%12%2
2470
11th to 20thNight
49%5%8%18%18%3
3044
21st to endNight
56%7%11%15%10%1
4532

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 10thDay
25%26%35%10%31
270
11th to 20thDay
39%25%23%10%4
243
21st to endDay
26%28%37%7%3
264
February1st to 10thDay
34%27%25%11%3
240
11th to 20thDay
42%25%28%41
240
21st to endDay
39%26%29%5%1
198
March1st to 10thDay
28%24%34%13%1
240
11th to 20thDay
29%22%29%16%41
393
21st to endDay
29%18%30%17%6%
462
April1st to 10thDay
22%20%28%20%10%1
420
11th to 20thDay
21%16%33%21%9%1
412
21st to endDay
25%12%30%20%11%2
420
May1st to 10thDay
22%13%31%22%8%4
423
11th to 20thDay
15%11%29%25%13%8%
447
21st to endDay
15%7%21%25%19%12%
485
June1st to 10thDay
13%6%21%27%21%12%
450
11th to 20thDay
6%5%18%30%27%13%
450
21st to endDay
11%5%21%29%20%14%
450
July1st to 10thDay
13%312%27%26%18%
445
11th to 20thDay
13%313%25%29%17%
444
21st to endDay
13%211%28%31%16%
488
August1st to 10thDay
13%216%25%30%13%
446
11th to 20thDay
12%315%34%28%8%
439
21st to endDay
14%212%27%30%16%
495
September1st to 10thDay
14%312%27%32%12%
450
11th to 20thDay
14%414%28%28%13%
450
21st to endDay
14%5%19%30%25%7%
450
October1st to 10thDay
12%6%22%32%21%8%
450
11th to 20thDay
16%13%29%24%15%3
450
21st to endDay
24%11%29%24%9%2
356
November1st to 10thDay
20%11%33%26%8%2
270
11th to 20thDay
19%17%35%25%4
270
21st to endDay
21%20%34%15%9%1
270
December1st to 10thDay
33%20%30%13%4
270
11th to 20thDay
30%20%29%14%7%
270
21st to endDay
31%23%31%13%2
297
January1st to 10thNight
26%19%31%20%4
760
11th to 20thNight
29%21%28%17%4
727
21st to endNight
23%18%38%20%2
792
February1st to 10thNight
26%17%34%20%3
720
11th to 20thNight
29%20%35%15%1
703
21st to endNight
32%21%30%15%1
592
March1st to 10thNight
23%16%34%24%3
720
11th to 20thNight
28%16%30%19%6%1
720
21st to endNight
23%15%29%24%7%
792
April1st to 10thNight
16%13%32%30%9%
540
11th to 20thNight
21%11%32%25%11%
533
21st to endNight
17%10%31%31%10%1
540
May1st to 10thNight
16%9%31%29%14%2
540
11th to 20thNight
14%5%24%34%20%3
531
21st to endNight
8%221%35%28%6%
584
June1st to 10thNight
9%415%36%31%4
540
11th to 20thNight
5%114%37%34%8%
540
21st to endNight
6%115%40%32%6%
540
July1st to 10thNight
11%210%28%38%11%
525
11th to 20thNight
9%39%27%43%9%
522
21st to endNight
7%19%34%41%8%
591
August1st to 10thNight
10%26%31%43%7%
537
11th to 20thNight
9%19%39%37%5%
534
21st to endNight
9%27%33%40%9%
594
September1st to 10thNight
11%310%33%37%6%
608
11th to 20thNight
13%314%31%33%6%
720
21st to endNight
12%219%32%30%5%
720
October1st to 10thNight
11%6%21%28%28%7%
720
11th to 20thNight
20%8%22%32%16%2
720
21st to endNight
20%10%27%27%14%2
792
November1st to 10thNight
15%10%24%33%17%2
740
11th to 20thNight
17%14%32%26%10%1
760
21st to endNight
23%13%27%27%9%1
760
December1st to 10thNight
29%16%26%23%5%
760
11th to 20thNight
24%15%29%25%6%1
760
21st to endNight
27%18%28%23%4
836
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