This page made possible by the sponsorship of: Walt Lickteig, Ralf Patterson.

McDonald Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the McDonald Observatory clear sky chart from 2005-11-05 to 2020-06-03.

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
61%
18987
32%
57981
23%
18946
70%
18946
44%
29655

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
49%
896
36%
2688
22%
896
64%
896
42%
1398
January16th to endNight
52%
960
39%
2880
24%
960
67%
960
44%
1441
February1st to 15thNight
52%
881
36%
2648
21%
876
67%
876
47%
1283
February16th to endNight
41%
792
35%
2374
17%
792
57%
792
43%
1186
March1st to 15thNight
47%
900
33%
2667
19%
900
59%
900
40%
1404
March16th to endNight
49%
816
38%
2496
21%
816
65%
816
45%
1227
April1st to 15thNight
46%
674
36%
2248
21%
674
65%
674
40%
1084
April16th to endNight
51%
669
38%
2229
27%
669
71%
669
50%
970
May1st to 15thNight
53%
674
30%
2249
25%
674
66%
674
44%
1083
May16th to endNight
64%
705
30%
2216
28%
705
73%
705
45%
1106
June1st to 15thNight
72%
637
27%
1912
25%
637
77%
637
47%
987
June16th to endNight
78%
630
25%
1890
25%
630
80%
630
44%
1000
July1st to 15thNight
79%
600
19%
1990
22%
600
79%
600
39%
1066
July16th to endNight
82%
660
22%
2091
26%
627
83%
627
38%
1095
August1st to 15thNight
84%
620
25%
2058
31%
617
85%
617
43%
1037
August16th to endNight
79%
669
20%
2230
23%
669
79%
669
32%
1265
September1st to 15thNight
75%
630
21%
2106
24%
630
76%
630
32%
1225
September16th to endNight
74%
627
24%
2310
26%
627
76%
627
31%
1283
October1st to 15thNight
71%
661
30%
2341
31%
661
73%
661
40%
1153
October16th to endNight
68%
896
39%
2688
26%
896
75%
896
54%
1277
November1st to 15thNight
66%
981
38%
2741
24%
981
72%
981
55%
1347
November16th to endNight
61%
1125
39%
2925
21%
1125
70%
1125
55%
1495
December1st to 15thNight
53%
1125
37%
2925
20%
1125
62%
1125
45%
1631
December16th to endNight
46%
1159
39%
3079
18%
1159
60%
1159
46%
1612

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
5%31342445%7%55%
58389
Night
431332445%7%58%
58205
Cloud CoverDay
5%3144245%5%7%54%
58387
Night
431332445%7%57%
58205
TransparencyDay
25%48%19%24%18%
58109
Night
23%49%19%25%18%
57987
SeeingDay
7%7%22%30%23%7%
8728
Night
6%6%20%37%26%3
18997

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
4413324337%59%
2240
16th to endDay
3313323435%64%
2400
February1st to 15thDay
2213334446%63%
2200
16th to endDay
2212323346%65%
1980
March1st to 15thDay
3425%43446%6%53%
2278
16th to endDay
231342345%7%61%
2782
April1st to 15thDay
221332445%6%61%
2690
16th to endDay
232212336%70%
2666
May1st to 15thDay
421332445%7%61%
2691
16th to endDay
411132345%7%63%
2950
June1st to 15thDay
21133245%7%7%60%
2757
16th to endDay
4313425%6%7%10%49%
2730
July1st to 15thDay
9%325%6%48%8%7%7%34%
2524
16th to endDay
6%4247%5%8%7%7%10%34%
2661
August1st to 15thDay
6%3146%47%7%9%12%37%
2474
16th to endDay
8%427%6%5%7%7%7%8%34%
2676
September1st to 15thDay
12%425%7%47%5%7%7%34%
2512
16th to endDay
10%6%25%6%35%5%6%7%40%
2310
October1st to 15thDay
6%213323446%58%
2276
16th to endDay
5%2232223469%
2240
November1st to 15thDay
4323213345%65%
2090
16th to endDay
4313312236%68%
2025
December1st to 15thDay
5%313324337%59%
2025
16th to endDay
3412214347%62%
2212
January1st to 15thNight
3424423346%59%
2688
16th to endNight
4323323347%61%
2880
February1st to 15thNight
331332335%7%62%
2642
16th to endNight
33123245%47%61%
2374
March1st to 15thNight
44133245%5%7%55%
2667
16th to endNight
231332345%6%62%
2496
April1st to 15thNight
4313324446%61%
2248
16th to endNight
2212223345%69%
2229
May1st to 15thNight
421332345%6%61%
2249
16th to endNight
421332445%6%60%
2215
June1st to 15thNight
32124245%7%9%54%
1912
16th to endNight
4213336%6%7%10%50%
1890
July1st to 15thNight
32245%25%7%8%11%46%
2090
16th to endNight
4213426%6%7%11%48%
2211
August1st to 15thNight
3212335%6%8%16%45%
2069
16th to endNight
43246%36%6%8%12%42%
2230
September1st to 15thNight
8%4145%347%7%10%43%
2106
16th to endNight
8%414435%6%7%9%44%
2310
October1st to 15thNight
6%213423447%57%
2341
16th to endNight
4212212345%69%
2688
November1st to 15thNight
433223246%67%
2741
16th to endNight
3213322236%68%
2925
December1st to 15thNight
5%213423447%58%
2925
16th to endNight
3313323345%65%
3079

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
45%13323446%58%
2240
16th to endDay
3323423436%62%
2400
February1st to 15thDay
2313434436%62%
2200
16th to endDay
2313323347%64%
1980
March1st to 15thDay
3435%5%3445%6%53%
2278
16th to endDay
231342445%6%61%
2780
April1st to 15thDay
3314324447%60%
2690
16th to endDay
2312212247%69%
2666
May1st to 15thDay
3313324446%61%
2691
16th to endDay
31123245%5%6%63%
2950
June1st to 15thDay
21123246%7%7%60%
2757
16th to endDay
4323326%5%7%9%49%
2730
July1st to 15thDay
8%426%6%48%8%6%9%32%
2524
16th to endDay
6%425%6%5%8%8%8%10%34%
2661
August1st to 15thDay
6%325%6%47%7%10%12%35%
2474
16th to endDay
7%5%36%7%5%7%7%6%9%33%
2676
September1st to 15thDay
12%5%26%6%5%6%6%6%8%33%
2512
16th to endDay
10%5%25%5%36%6%6%7%39%
2310
October1st to 15thDay
5%32332345%7%57%
2276
16th to endDay
431232223468%
2240
November1st to 15thDay
3423212445%64%
2090
16th to endDay
4313312336%67%
2025
December1st to 15thDay
5%324323347%58%
2025
16th to endDay
341332345%7%61%
2212
January1st to 15thNight
35%24423346%58%
2688
16th to endNight
4323423347%61%
2880
February1st to 15thNight
331332336%7%61%
2642
16th to endNight
331232445%7%60%
2374
March1st to 15thNight
43144245%5%7%54%
2667
16th to endNight
2323323445%62%
2496
April1st to 15thNight
4313424447%60%
2248
16th to endNight
2212324345%68%
2229
May1st to 15thNight
3323423347%60%
2249
16th to endNight
321332446%6%60%
2215
June1st to 15thNight
2312435%5%6%10%54%
1912
16th to endNight
4114336%6%6%9%51%
1890
July1st to 15thNight
4214436%6%9%10%46%
2090
16th to endNight
4223435%6%8%11%47%
2211
August1st to 15thNight
3213325%7%9%15%44%
2069
16th to endNight
4325%5%35%6%9%13%40%
2230
September1st to 15thNight
7%414436%6%7%10%41%
2106
16th to endNight
7%424426%6%7%10%43%
2310
October1st to 15thNight
5%31342345%7%57%
2341
16th to endNight
421221334468%
2688
November1st to 15thNight
3312323246%66%
2741
16th to endNight
3313322346%66%
2925
December1st to 15thNight
5%31442345%6%58%
2925
16th to endNight
331332334463%
3079

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
24%2419%30%18%
2241
16th to endDay
21%3415%34%20%
2400
February1st to 15thDay
20%35%17%25%27%
2201
16th to endDay
18%2414%31%28%
1980
March1st to 15thDay
27%2418%24%21%
2278
16th to endDay
21%2414%30%26%
2782
April1st to 15thDay
21%35%11%30%27%
2690
16th to endDay
15%2212%35%31%
2666
May1st to 15thDay
21%3414%26%28%
2691
16th to endDay
17%37%18%24%27%
2953
June1st to 15thDay
17%410%21%26%18%
2757
16th to endDay
26%6%10%21%20%15%
2730
July1st to 15thDay
39%7%14%21%14%1
2395
16th to endDay
35%7%17%28%9%1
2518
August1st to 15thDay
32%7%17%29%11%1
2461
16th to endDay
41%7%17%20%11%1
2676
September1st to 15thDay
42%6%14%18%12%4
2512
16th to endDay
38%49%21%20%6%
2310
October1st to 15thDay
24%39%27%19%16%
2276
16th to endDay
19%1316%30%29%
2240
November1st to 15thDay
20%27%23%27%19%
2090
16th to endDay
18%15%19%31%23%
2025
December1st to 15thDay
24%15%20%31%15%
2025
16th to endDay
20%35%21%31%18%
2212
January1st to 15thNight
25%2315%30%21%
2688
16th to endNight
22%2315%36%19%
2880
February1st to 15thNight
20%26%14%26%30%
2652
16th to endNight
21%3411%29%29%
2374
March1st to 15thNight
25%35%17%25%23%
2667
16th to endNight
21%3414%29%26%
2496
April1st to 15thNight
23%25%12%29%26%
2248
16th to endNight
16%25%14%27%32%
2229
May1st to 15thNight
22%35%16%25%27%
2250
16th to endNight
22%48%19%21%23%
2217
June1st to 15thNight
21%5%11%24%23%12%
1912
16th to endNight
24%414%20%21%15%
1890
July1st to 15thNight
25%6%20%31%13%2
1990
16th to endNight
24%7%20%32%14%1
2091
August1st to 15thNight
21%7%22%32%15%
2058
16th to endNight
29%8%20%26%12%2
2230
September1st to 15thNight
30%8%21%21%12%5%
2106
16th to endNight
32%5%16%22%17%5%
2310
October1st to 15thNight
24%413%23%18%16%
2341
16th to endNight
18%25%14%28%30%
2688
November1st to 15thNight
18%2420%34%18%
2741
16th to endNight
18%2415%33%26%
2925
December1st to 15thNight
24%3420%30%17%
2925
16th to endNight
21%2319%33%19%
3079

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
10%13%31%31%12%
225
16th to endDay
8%19%25%30%12%3
240
February1st to 15thDay
7%13%34%24%19%
225
16th to endDay
9%13%47%21%6%
201
March1st to 15thDay
6%13%47%23%8%
253
16th to endDay
5%17%45%22%7%1
480
April1st to 15thDay
8%17%43%23%6%
449
16th to endDay
7%17%43%26%5%
438
May1st to 15thDay
5%15%32%33%10%1
468
16th to endDay
46%32%38%16%1
653
June1st to 15thDay
3114%43%32%4
656
16th to endDay
6%8%36%38%9%
645
July1st to 15thDay
7%1229%39%20%
429
16th to endDay
5%128%43%19%
455
August1st to 15thDay
7%1222%44%22%
419
16th to endDay
11%1122%42%19%
460
September1st to 15thDay
10%1335%36%12%
427
16th to endDay
16%110%28%31%12%
225
October1st to 15thDay
12%418%32%24%8%
225
16th to endDay
8%425%30%20%11%
240
November1st to 15thDay
9%8%26%32%13%9%
225
16th to endDay
12%13%27%24%15%6%
225
December1st to 15thDay
12%8%28%33%15%1
225
16th to endDay
9%14%38%27%10%
240
January1st to 15thNight
7%7%29%38%15%2
900
16th to endNight
7%10%25%39%15%1
960
February1st to 15thNight
6%9%28%36%17%2
884
16th to endNight
7%10%36%37%7%
792
March1st to 15thNight
7%11%28%36%14%1
900
16th to endNight
5%10%30%39%13%1
816
April1st to 15thNight
6%11%31%39%10%
674
16th to endNight
210%31%43%10%
669
May1st to 15thNight
5%6%30%44%13%
675
16th to endNight
5%421%48%19%
707
June1st to 15thNight
315%51%28%1
637
16th to endNight
5%8%46%37%2
630
July1st to 15thNight
41432%52%4
600
16th to endNight
4232%53%6%
660
August1st to 15thNight
3120%65%8%
620
16th to endNight
5%123%60%8%
669
September1st to 15thNight
9%1223%55%6%
630
16th to endNight
10%15%35%41%5%
627
October1st to 15thNight
8%211%41%32%3
661
16th to endNight
7%315%35%29%8%
896
November1st to 15thNight
6%319%37%27%5%
981
16th to endNight
6%7%20%38%23%3
1125
December1st to 15thNight
7%8%25%38%17%3
1125
16th to endNight
6%12%30%35%13%2
1159
Page generated at 2020-06-04 02:18:55. Version ac04

Copyright 2020 A. Danko


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