Mitchell School
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mitchell School clear sky chart from 2005-11-06 to 2024-02-25.

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
51%
22810
16%
80623
16%
22720
54%
22720
21%
52619

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
31%
751
7%
4390
7%
736
36%
736
6%
3355
January11th to 20thNight
39%
755
8%
4416
12%
740
43%
740
8%
3401
January21st to endNight
42%
833
9%
4764
14%
814
47%
814
9%
3560
February1st to 10thNight
35%
760
11%
2470
9%
760
39%
760
13%
1845
February11th to 20thNight
38%
741
13%
2349
12%
740
41%
740
16%
1681
February21st to endNight
31%
610
13%
1829
11%
610
35%
610
13%
1402
March1st to 10thNight
37%
720
11%
2160
9%
720
41%
720
16%
1584
March11th to 20thNight
44%
720
12%
2028
11%
720
45%
720
18%
1516
March21st to endNight
37%
792
9%
2034
5%
792
39%
792
16%
1588
April1st to 10thNight
42%
720
12%
1800
6%
720
43%
720
21%
1322
April11th to 20thNight
44%
711
15%
1779
8%
711
46%
711
24%
1301
April21st to endNight
49%
658
14%
1738
9%
658
51%
658
25%
1213
May1st to 10thNight
56%
360
19%
1440
21%
360
60%
360
24%
820
May11th to 20thNight
60%
354
17%
1416
21%
354
64%
354
23%
820
May21st to endNight
55%
388
12%
1552
16%
388
58%
388
19%
982
June1st to 10thNight
61%
360
12%
1440
18%
360
65%
360
24%
846
June11th to 20thNight
56%
360
16%
1440
23%
360
63%
360
28%
760
June21st to endNight
72%
360
21%
1440
37%
360
74%
360
41%
640
July1st to 10thNight
83%
350
22%
1400
35%
350
86%
350
53%
539
July11th to 20thNight
81%
348
30%
1392
43%
348
86%
348
59%
497
July21st to endNight
85%
394
36%
1488
50%
372
88%
372
62%
515
August1st to 10thNight
85%
381
32%
1447
40%
379
87%
379
53%
578
August11th to 20thNight
85%
588
34%
1656
27%
588
86%
588
64%
763
August21st to endNight
78%
792
37%
1980
24%
792
79%
792
67%
969
September1st to 10thNight
79%
720
37%
1800
23%
720
80%
720
67%
891
September11th to 20thNight
74%
720
34%
1808
23%
720
76%
720
51%
1011
September21st to endNight
67%
720
28%
1998
25%
720
68%
720
41%
1143
October1st to 10thNight
61%
720
21%
2160
21%
720
64%
720
34%
1275
October11th to 20thNight
57%
720
24%
2160
23%
720
60%
720
33%
1288
October21st to endNight
49%
792
18%
2520
18%
792
53%
792
23%
1657
November1st to 10thNight
45%
740
16%
2405
18%
740
47%
740
17%
1715
November11th to 20thNight
39%
760
13%
2586
12%
760
42%
760
15%
1854
November21st to endNight
45%
760
12%
2660
11%
760
49%
760
19%
1785
December1st to 10thNight
43%
760
11%
2660
9%
760
47%
760
17%
1855
December11th to 20thNight
36%
760
11%
3205
9%
759
40%
759
11%
2402
December21st to endNight
25%
832
6%
4813
8%
817
28%
817
6%
3246

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
8%10%46%5%35%5%5%6%44%
78354
Night
8%11%46%5%35%5%5%7%41%
80967
TransparencyDay
40%8%11%19%16%7%
78057
Night
41%10%13%18%14%5%
80785
SeeingDay
13%14%27%26%15%4
13569
Night
14%10%18%33%21%4
22835

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 10thDay
12%17%7%9%7%46%5%6%6%21%
2539
11th to 20thDay
12%16%6%10%8%5%5%5%46%24%
2540
21st to endDay
10%15%5%8%7%46%5%6%7%27%
2869
February1st to 10thDay
9%14%6%8%7%45%7%7%7%28%
1710
11th to 20thDay
9%14%5%8%5%35%5%5%8%31%
1718
21st to endDay
7%16%7%9%7%46%7%5%6%26%
1521
March1st to 10thDay
8%13%6%10%8%5%7%5%5%7%25%
1800
11th to 20thDay
6%14%5%9%8%46%5%5%7%29%
1930
21st to endDay
8%17%7%10%10%46%6%5%6%21%
2320
April1st to 10thDay
10%15%5%9%7%46%5%6%7%26%
2160
11th to 20thDay
10%13%7%8%6%45%5%5%5%33%
2129
21st to endDay
10%13%36%5%35%5%5%8%37%
2222
May1st to 10thDay
7%9%35%6%36%5%5%6%44%
2518
11th to 20thDay
12%9%37%6%45%5%5%7%38%
2477
21st to endDay
15%9%37%6%35%5%5%7%36%
2723
June1st to 10thDay
13%9%36%5%35%5%5%7%38%
2520
11th to 20thDay
11%7%25%5%2446%8%47%
2520
21st to endDay
5%42332435%6%63%
2520
July1st to 10thDay
2212323445%72%
2520
11th to 20thDay
1212323345%74%
2520
21st to endDay
111221233480%
2741
August1st to 10thDay
2213423446%69%
2482
11th to 20thDay
3212323345%73%
2242
21st to endDay
3313223346%72%
2376
September1st to 10thDay
3212222346%73%
2160
11th to 20thDay
5%4245%2445%7%58%
2152
21st to endDay
9%7%24333446%55%
1961
October1st to 10thDay
12%10%245%33445%48%
1800
11th to 20thDay
8%7%26%5%345%6%8%45%
1800
21st to endDay
11%10%37%6%4445%6%41%
1836
November1st to 10thDay
12%13%5%7%5%345%5%7%34%
1674
11th to 20thDay
10%12%5%8%7%47%5%7%7%28%
1592
21st to endDay
8%12%5%7%7%36%6%6%7%34%
1520
December1st to 10thDay
6%13%5%8%5%45%46%7%37%
1520
11th to 20thDay
8%15%5%10%6%45%6%6%9%28%
1928
21st to endDay
9%15%6%9%7%45%46%7%27%
2794
January1st to 10thNight
7%17%7%10%8%46%6%5%6%22%
4406
11th to 20thNight
15%18%6%8%7%35%45%6%24%
4420
21st to endNight
10%16%49%8%46%5%6%6%25%
4786
February1st to 10thNight
8%14%6%8%6%46%5%5%7%30%
2470
11th to 20thNight
7%14%7%8%6%35%5%6%6%33%
2347
21st to endNight
7%15%7%8%8%5%6%5%5%7%26%
1829
March1st to 10thNight
7%13%5%8%6%48%5%6%8%30%
2160
11th to 20thNight
7%10%5%8%7%5%7%6%8%8%29%
2028
21st to endNight
6%14%7%8%7%47%5%7%9%25%
2034
April1st to 10thNight
10%14%48%6%45%5%5%6%35%
1800
11th to 20thNight
9%13%5%8%7%36%35%6%37%
1779
21st to endNight
9%12%36%5%245%7%9%38%
1738
May1st to 10thNight
8%8%35%43447%7%48%
1440
11th to 20thNight
11%7%245%235%6%8%45%
1416
21st to endNight
12%11%35%435%5%5%7%39%
1552
June1st to 10thNight
12%6%245%345%7%8%46%
1440
11th to 20thNight
11%8%34322336%54%
1440
21st to endNight
5%313323445%66%
1440
July1st to 10thNight
221131345%6%72%
1440
11th to 20thNight
111323235%78%
1440
21st to endNight
1112212235%79%
1576
August1st to 10thNight
2212324447%70%
1455
11th to 20thNight
1213222345%75%
1656
21st to endNight
221231234475%
1980
September1st to 10thNight
221321233477%
1800
11th to 20thNight
6%31242345%6%63%
1808
21st to endNight
7%5%24424446%57%
1998
October1st to 10thNight
9%9%24423346%53%
2160
11th to 20thNight
8%8%25%42345%7%51%
2160
21st to endNight
9%10%36%6%44446%43%
2520
November1st to 10thNight
12%13%46%6%45%6%6%6%34%
2405
11th to 20thNight
11%11%47%6%45%6%7%7%32%
2586
21st to endNight
8%13%46%6%35%5%7%8%36%
2660
December1st to 10thNight
8%13%47%6%45%45%7%36%
2660
11th to 20thNight
7%12%5%9%7%5%7%6%7%8%27%
3276
21st to endNight
8%14%6%9%6%45%5%6%6%31%
4862

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
57%16%13%11%3
2539
11th to 20thDay
59%10%12%12%5%2
2540
21st to endDay
54%14%12%13%5%2
2869
February1st to 10thDay
52%11%14%16%7%1
1710
11th to 20thDay
50%11%12%16%9%2
1727
21st to endDay
57%11%13%12%7%1
1521
March1st to 10thDay
57%10%13%12%6%1
1800
11th to 20thDay
53%9%11%19%9%
1930
21st to endDay
62%10%11%13%4
2320
April1st to 10thDay
57%8%12%14%9%1
2160
11th to 20thDay
51%8%12%17%10%3
2132
21st to endDay
46%7%12%22%11%1
2222
May1st to 10thDay
39%8%13%27%14%1
2518
11th to 20thDay
47%7%13%22%11%1
2480
21st to endDay
49%8%13%19%10%1
2724
June1st to 10thDay
45%7%11%27%10%1
2520
11th to 20thDay
34%7%13%28%16%3
2520
21st to endDay
21%5%12%26%26%8%
2520
July1st to 10thDay
15%410%20%28%23%
2451
11th to 20thDay
14%38%12%32%31%
2445
21st to endDay
11%2413%40%29%
2589
August1st to 10thDay
17%48%22%33%17%
2465
11th to 20thDay
14%46%23%35%19%
2242
21st to endDay
15%37%26%34%15%
2376
September1st to 10thDay
14%47%25%32%19%
2160
11th to 20thDay
26%5%9%21%25%13%
2152
21st to endDay
30%6%11%22%21%10%
1961
October1st to 10thDay
39%6%10%19%19%8%
1800
11th to 20thDay
37%7%11%23%18%4
1800
21st to endDay
43%8%12%19%16%2
1836
November1st to 10thDay
50%8%13%19%10%1
1674
11th to 20thDay
52%11%13%16%8%
1592
21st to endDay
48%10%12%14%15%2
1520
December1st to 10thDay
45%13%15%16%10%2
1520
11th to 20thDay
53%11%16%13%7%1
1928
21st to endDay
55%12%15%14%41
2794
January1st to 10thNight
53%17%14%11%4
4406
11th to 20thNight
58%12%11%11%5%3
4420
21st to endNight
53%14%14%11%6%1
4786
February1st to 10thNight
51%12%14%14%9%
2470
11th to 20thNight
48%10%13%17%8%3
2349
21st to endNight
54%11%10%15%8%1
1829
March1st to 10thNight
49%11%14%17%6%2
2160
11th to 20thNight
49%11%15%18%5%1
2028
21st to endNight
50%14%15%16%5%
2034
April1st to 10thNight
49%8%17%18%7%1
1800
11th to 20thNight
48%7%14%21%9%1
1779
21st to endNight
42%9%15%21%11%2
1738
May1st to 10thNight
34%8%15%29%14%
1440
11th to 20thNight
35%8%17%27%11%2
1416
21st to endNight
42%8%17%23%9%1
1552
June1st to 10thNight
36%9%15%28%12%1
1440
11th to 20thNight
31%6%16%30%13%3
1440
21st to endNight
21%5%13%29%25%7%
1440
July1st to 10thNight
13%5%13%25%26%19%
1400
11th to 20thNight
11%39%19%30%28%
1392
21st to endNight
10%3417%41%25%
1488
August1st to 10thNight
14%410%26%33%13%
1447
11th to 20thNight
13%49%23%34%18%
1656
21st to endNight
12%39%26%36%14%
1980
September1st to 10thNight
13%37%27%33%17%
1800
11th to 20thNight
21%6%10%24%26%14%
1808
21st to endNight
28%5%9%24%22%12%
1998
October1st to 10thNight
33%5%10%24%21%7%
2160
11th to 20thNight
32%7%13%21%21%6%
2160
21st to endNight
40%10%12%16%18%4
2520
November1st to 10thNight
48%10%11%15%13%2
2405
11th to 20thNight
48%12%12%18%10%1
2586
21st to endNight
44%10%13%15%18%2
2660
December1st to 10thNight
46%12%16%15%9%2
2660
11th to 20thNight
52%13%13%14%8%
3276
21st to endNight
49%13%17%13%7%1
4862

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
24%21%22%17%13%2
180
11th to 20thDay
21%19%23%19%14%3
180
21st to endDay
14%24%26%21%14%1
247
February1st to 10thDay
23%21%27%20%8%1
370
11th to 20thDay
24%21%29%18%8%1
390
21st to endDay
21%18%26%26%6%2
363
March1st to 10thDay
17%22%33%21%7%
420
11th to 20thDay
18%20%29%21%11%1
420
21st to endDay
20%20%29%24%7%
462
April1st to 10thDay
21%21%25%19%11%3
420
11th to 20thDay
18%21%28%18%11%3
412
21st to endDay
16%21%28%20%13%3
420
May1st to 10thDay
12%17%30%23%15%3
423
11th to 20thDay
22%15%23%22%13%5%
448
21st to endDay
17%10%24%28%14%7%
485
June1st to 10thDay
14%11%26%26%16%7%
450
11th to 20thDay
14%15%32%24%12%3
450
21st to endDay
49%28%34%19%6%
450
July1st to 10thDay
39%32%35%16%5%
446
11th to 20thDay
210%36%30%16%8%
444
21st to endDay
19%31%32%19%8%
487
August1st to 10thDay
46%27%33%18%11%
447
11th to 20thDay
3424%33%24%13%
437
21st to endDay
5%9%25%31%23%7%
495
September1st to 10thDay
47%28%30%28%5%
450
11th to 20thDay
8%7%24%33%22%6%
450
21st to endDay
11%10%24%31%20%3
450
October1st to 10thDay
15%12%20%30%19%4
450
11th to 20thDay
14%14%22%24%21%5%
450
21st to endDay
16%15%26%20%19%5%
351
November1st to 10thDay
16%18%25%27%13%
270
11th to 20thDay
19%15%23%22%18%2
214
21st to endDay
16%17%23%27%13%4
180
December1st to 10thDay
16%22%20%23%13%6%
180
11th to 20thDay
26%15%34%18%7%1
180
21st to endDay
20%21%23%21%13%2
198
January1st to 10thNight
19%17%23%22%15%3
760
11th to 20thNight
21%16%18%24%18%4
760
21st to endNight
19%10%19%31%17%3
836
February1st to 10thNight
17%15%21%30%15%2
760
11th to 20thNight
16%13%23%29%18%2
744
21st to endNight
15%16%25%31%10%1
610
March1st to 10thNight
15%15%24%31%12%3
720
11th to 20thNight
13%13%22%34%16%2
720
21st to endNight
15%14%25%31%14%2
792
April1st to 10thNight
17%13%20%29%17%3
720
11th to 20thNight
15%10%21%30%19%4
711
21st to endNight
15%11%20%34%18%3
658
May1st to 10thNight
10%8%20%37%21%4
360
11th to 20thNight
16%412%38%27%5%
354
21st to endNight
17%6%14%33%25%4
389
June1st to 10thNight
13%416%39%26%3
360
11th to 20thNight
15%8%16%41%20%1
360
21st to endNight
7%313%47%28%3
360
July1st to 10thNight
329%54%29%3
350
11th to 20thNight
1112%55%28%3
348
21st to endNight
218%50%34%5%
394
August1st to 10thNight
217%44%42%4
381
11th to 20thNight
316%35%43%11%
588
21st to endNight
2312%43%34%6%
792
September1st to 10thNight
4210%39%38%6%
720
11th to 20thNight
7%311%35%32%12%
720
21st to endNight
11%313%35%31%8%
720
October1st to 10thNight
15%315%35%27%5%
720
11th to 20thNight
11%9%18%33%22%7%
720
21st to endNight
15%9%18%33%19%7%
792
November1st to 10thNight
18%10%18%35%17%1
740
11th to 20thNight
18%16%19%29%16%2
760
21st to endNight
17%13%19%29%17%6%
760
December1st to 10thNight
17%9%22%30%19%3
760
11th to 20thNight
18%13%24%29%15%2
760
21st to endNight
22%17%27%22%11%1
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