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 2018-02-22.

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%
16061
32%
48986
23%
16020
70%
16020
44%
24933

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
50%
776
37%
2328
22%
776
64%
776
42%
1212
January16th to endNight
52%
832
40%
2496
24%
832
67%
832
44%
1245
February1st to 15thNight
54%
761
37%
2288
22%
756
69%
756
49%
1099
February16th to endNight
41%
660
36%
1978
18%
660
58%
660
46%
955
March1st to 15thNight
49%
720
36%
2134
21%
720
62%
720
45%
1065
March16th to endNight
49%
653
38%
1997
22%
653
65%
653
46%
967
April1st to 15thNight
47%
539
35%
1798
20%
539
66%
539
41%
861
April16th to endNight
50%
540
37%
1800
27%
540
69%
540
49%
787
May1st to 15thNight
50%
539
29%
1799
23%
539
64%
539
43%
867
May16th to endNight
61%
561
27%
1763
24%
561
70%
561
40%
928
June1st to 15thNight
73%
540
26%
1620
23%
540
78%
540
48%
819
June16th to endNight
76%
540
25%
1620
25%
540
78%
540
43%
864
July1st to 15thNight
77%
510
20%
1692
23%
510
77%
510
38%
913
July16th to endNight
80%
564
22%
1771
25%
531
81%
531
36%
958
August1st to 15thNight
83%
539
23%
1789
30%
536
84%
536
41%
916
August16th to endNight
79%
573
20%
1910
24%
573
79%
573
31%
1103
September1st to 15thNight
75%
540
19%
1806
22%
540
75%
540
33%
1046
September16th to endNight
74%
537
26%
1980
29%
537
76%
537
31%
1102
October1st to 15thNight
72%
567
32%
2007
33%
567
74%
567
43%
952
October16th to endNight
71%
768
39%
2304
28%
768
77%
768
57%
1068
November1st to 15thNight
67%
848
38%
2368
24%
848
73%
848
59%
1119
November16th to endNight
62%
975
41%
2535
22%
975
71%
975
56%
1285
December1st to 15thNight
54%
975
37%
2535
20%
975
64%
975
46%
1408
December16th to endNight
46%
1004
39%
2668
18%
1004
60%
1004
47%
1394

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%
49228
Night
431332445%7%58%
49210
Cloud CoverDay
5%32442445%7%54%
49226
Night
431332445%7%58%
49210
TransparencyDay
25%48%19%24%18%
48948
Night
22%38%19%25%19%
48992
SeeingDay
8%7%20%31%24%7%
5719
Night
6%6%19%36%27%3
16071

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%
1940
16th to endDay
3313323436%64%
2080
February1st to 15thDay
2212324436%66%
1900
16th to endDay
2212223346%67%
1642
March1st to 15thDay
232443435%6%58%
1822
16th to endDay
2312323346%64%
2226
April1st to 15thDay
221332445%6%62%
2150
16th to endDay
222212336%70%
2160
May1st to 15thDay
3213314346%63%
2151
16th to endDay
5%21232446%7%60%
2348
June1st to 15thDay
1123245%8%7%61%
2340
16th to endDay
5%3235%35%6%7%9%47%
2340
July1st to 15thDay
10%425%6%38%8%7%7%35%
2163
16th to endDay
7%425%7%5%8%7%7%10%32%
2277
August1st to 15thDay
6%315%6%38%6%9%11%35%
2160
16th to endDay
9%5%27%7%5%7%6%6%8%32%
2292
September1st to 15thDay
14%425%7%46%5%7%7%33%
2153
16th to endDay
11%6%15%5%35%5%6%6%42%
1980
October1st to 15thDay
7%213323346%60%
1950
16th to endDay
42221233471%
1920
November1st to 15thDay
4312213345%67%
1806
16th to endDay
4312311235%70%
1755
December1st to 15thDay
5%313324337%60%
1755
16th to endDay
3413213447%62%
1918
January1st to 15thNight
3424323346%59%
2328
16th to endNight
5%322313346%62%
2496
February1st to 15thNight
2213323347%64%
2282
16th to endNight
2211323447%64%
1978
March1st to 15thNight
431332345%7%60%
2134
16th to endNight
3323323345%65%
1997
April1st to 15thNight
4213324436%62%
1798
16th to endNight
221322434470%
1800
May1st to 15thNight
3323323446%62%
1799
16th to endNight
5%213335%5%6%7%55%
1762
June1st to 15thNight
21124245%7%9%56%
1620
16th to endNight
4213336%6%7%10%49%
1620
July1st to 15thNight
42245%25%7%7%10%46%
1792
16th to endNight
4224426%6%7%11%44%
1891
August1st to 15thNight
3213435%7%9%15%44%
1800
16th to endNight
43246%36%6%8%12%42%
1910
September1st to 15thNight
8%4145%346%6%11%43%
1806
16th to endNight
8%424325%6%7%8%44%
1980
October1st to 15thNight
6%212422448%60%
2007
16th to endNight
2212213345%70%
2304
November1st to 15thNight
332213246%69%
2368
16th to endNight
3213212235%70%
2535
December1st to 15thNight
42135%23447%59%
2535
16th to endNight
331332334465%
2668

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%13423446%58%
1940
16th to endDay
3323323436%63%
2080
February1st to 15thDay
2213324436%65%
1900
16th to endDay
2213223247%67%
1642
March1st to 15thDay
243442445%5%58%
1822
16th to endDay
3313414346%64%
2224
April1st to 15thDay
3313324347%61%
2150
16th to endDay
2212212247%70%
2160
May1st to 15thDay
3313324345%64%
2151
16th to endDay
42123245%6%6%60%
2348
June1st to 15thDay
11123246%7%7%61%
2340
16th to endDay
5%324427%5%8%9%47%
2340
July1st to 15thDay
9%426%6%48%8%6%9%33%
2163
16th to endDay
7%5%25%7%5%8%8%8%10%31%
2277
August1st to 15thDay
6%325%6%47%7%9%11%33%
2160
16th to endDay
9%5%37%7%5%7%7%6%8%31%
2292
September1st to 15thDay
13%6%27%5%46%5%6%8%33%
2153
16th to endDay
11%6%25%5%36%6%5%6%41%
1980
October1st to 15thDay
6%322323448%58%
1950
16th to endDay
322322235%70%
1920
November1st to 15thDay
3312212445%67%
1806
16th to endDay
4312212235%70%
1755
December1st to 15thDay
5%223324347%59%
1755
16th to endDay
3413323447%60%
1918
January1st to 15thNight
35%2442335%6%59%
2328
16th to endNight
5%323423346%61%
2496
February1st to 15thNight
231332435%7%62%
2282
16th to endNight
231232445%6%63%
1978
March1st to 15thNight
43144235%46%59%
2134
16th to endNight
232332243465%
1997
April1st to 15thNight
4213424446%61%
1798
16th to endNight
221222434469%
1800
May1st to 15thNight
3323424346%61%
1799
16th to endNight
4213435%5%6%6%55%
1762
June1st to 15thNight
2212335%5%6%10%55%
1620
16th to endNight
4114335%6%7%9%50%
1620
July1st to 15thNight
42245%36%6%8%9%47%
1792
16th to endNight
4323436%6%8%12%43%
1891
August1st to 15thNight
3213325%7%9%15%43%
1800
16th to endNight
5%3246%35%6%9%12%40%
1910
September1st to 15thNight
7%415%435%6%7%10%41%
1806
16th to endNight
7%424425%6%7%9%44%
1980
October1st to 15thNight
5%31342335%7%59%
2007
16th to endNight
221221334470%
2304
November1st to 15thNight
322312246%69%
2368
16th to endNight
3313322235%68%
2535
December1st to 15thNight
431342345%6%58%
2535
16th to endNight
341332334463%
2668

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%2420%31%16%
1941
16th to endDay
21%2415%33%21%
2080
February1st to 15thDay
18%2417%26%29%
1901
16th to endDay
16%2415%34%26%
1642
March1st to 15thDay
24%2317%27%24%
1822
16th to endDay
19%2413%30%29%
2226
April1st to 15thDay
20%25%10%30%29%
2150
16th to endDay
14%2210%35%34%
2160
May1st to 15thDay
20%2314%26%31%
2151
16th to endDay
19%48%18%23%25%
2351
June1st to 15thDay
16%410%21%26%20%
2340
16th to endDay
28%6%10%22%20%12%
2340
July1st to 15thDay
39%6%13%23%14%2
2034
16th to endDay
38%8%17%26%8%1
2134
August1st to 15thDay
33%7%17%27%11%1
2147
16th to endDay
43%7%16%18%11%2
2292
September1st to 15thDay
43%6%14%19%12%3
2153
16th to endDay
37%49%21%20%7%
1980
October1st to 15thDay
23%38%25%20%18%
1950
16th to endDay
17%1317%30%28%
1920
November1st to 15thDay
18%26%22%28%21%
1806
16th to endDay
18%1418%33%24%
1755
December1st to 15thDay
23%15%19%33%16%
1755
16th to endDay
20%35%22%31%16%
1918
January1st to 15thNight
25%2415%32%19%
2328
16th to endNight
22%2316%34%20%
2496
February1st to 15thNight
19%25%13%27%31%
2292
16th to endNight
18%35%11%31%29%
1978
March1st to 15thNight
21%2317%28%26%
2134
16th to endNight
20%2314%29%28%
1997
April1st to 15thNight
22%35%12%29%26%
1798
16th to endNight
17%15%13%26%35%
1800
May1st to 15thNight
22%35%14%23%30%
1800
16th to endNight
25%48%18%18%22%
1764
June1st to 15thNight
19%6%11%24%24%14%
1620
16th to endNight
25%415%21%20%13%
1620
July1st to 15thNight
27%5%17%31%14%3
1692
16th to endNight
26%7%20%31%12%1
1771
August1st to 15thNight
22%7%23%29%16%
1789
16th to endNight
30%8%19%25%12%3
1910
September1st to 15thNight
30%7%20%22%12%5%
1806
16th to endNight
32%5%14%22%17%6%
1980
October1st to 15thNight
22%312%24%18%17%
2007
16th to endNight
16%25%15%29%31%
2304
November1st to 15thNight
15%2421%35%20%
2368
16th to endNight
17%1314%33%28%
2535
December1st to 15thNight
24%2420%29%18%
2535
16th to endNight
21%2420%33%17%
2668

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%15%24%35%13%
135
16th to endDay
9%18%25%29%11%4
144
February1st to 15thDay
38%29%31%25%1
135
16th to endDay
9%14%38%28%9%2
100
March1st to 15thDay
9%44%30%13%1
112
16th to endDay
5%17%45%20%9%2
288
April1st to 15thDay
8%18%42%22%7%
269
16th to endDay
6%20%41%24%5%
270
May1st to 15thDay
321%38%30%5%
288
16th to endDay
6%5%35%35%14%1
435
June1st to 15thDay
2213%42%33%5%
495
16th to endDay
7%6%36%40%8%
495
July1st to 15thDay
8%1232%37%16%
308
16th to endDay
7%229%41%17%
327
August1st to 15thDay
8%1223%42%21%
315
16th to endDay
15%1228%37%14%
332
September1st to 15thDay
11%1336%34%11%
308
16th to endDay
17%13%31%28%7%
135
October1st to 15thDay
18%111%32%26%8%
135
16th to endDay
4222%29%24%16%
144
November1st to 15thDay
8%7%25%33%15%9%
135
16th to endDay
13%9%25%27%16%8%
135
December1st to 15thDay
14%328%36%15%2
135
16th to endDay
10%12%34%30%11%
144
January1st to 15thNight
7%6%29%36%16%2
780
16th to endNight
7%10%25%37%16%2
832
February1st to 15thNight
49%27%36%19%2
764
16th to endNight
6%11%36%36%8%
660
March1st to 15thNight
6%11%27%35%17%1
720
16th to endNight
411%28%37%15%1
653
April1st to 15thNight
412%31%39%10%
539
16th to endNight
210%31%43%10%1
540
May1st to 15thNight
47%32%41%12%
540
16th to endNight
6%422%45%20%
563
June1st to 15thNight
215%49%29%1
540
16th to endNight
5%8%44%37%2
540
July1st to 15thNight
5%15%34%49%4
510
16th to endNight
41232%52%6%
564
August1st to 15thNight
3119%65%10%
539
16th to endNight
6%123%59%8%
573
September1st to 15thNight
10%1222%55%7%
540
16th to endNight
10%15%36%40%5%
537
October1st to 15thNight
8%29%40%35%3
567
16th to endNight
5%315%35%31%9%
768
November1st to 15thNight
4320%35%30%5%
848
16th to endNight
5%7%19%37%25%4
975
December1st to 15thNight
7%8%24%38%18%3
975
16th to endNight
6%11%29%34%14%2
1004
Page generated at 2018-02-23 10:21:32. Version ac04

Copyright 2018 A. Danko


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