Thomas Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Thomas Observatory clear sky chart from 2005-11-06 to 2024-11-28.

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
60%
25031
39%
96161
27%
24802
73%
24802
55%
35128

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
57%
755
38%
4269
25%
736
70%
736
38%
1422
January11th to 20thNight
60%
754
34%
4196
31%
740
74%
740
36%
1572
January21st to endNight
53%
829
27%
3970
29%
814
68%
814
31%
1818
February1st to 10thNight
54%
760
38%
2280
27%
760
69%
760
48%
1096
February11th to 20thNight
51%
740
40%
2218
27%
740
70%
740
54%
996
February21st to endNight
48%
628
37%
1884
26%
628
66%
628
52%
856
March1st to 10thNight
42%
755
40%
3652
20%
744
65%
744
40%
1229
March11th to 20thNight
48%
755
31%
3681
21%
740
64%
740
37%
1360
March21st to endNight
40%
836
27%
3282
14%
814
60%
814
47%
1216
April1st to 10thNight
40%
760
40%
1900
12%
760
59%
760
55%
938
April11th to 20thNight
43%
751
40%
1879
13%
751
57%
751
60%
887
April21st to endNight
46%
760
40%
1900
12%
760
60%
760
64%
872
May1st to 10thNight
40%
619
51%
2703
18%
609
54%
609
63%
711
May11th to 20thNight
49%
553
37%
2698
23%
541
63%
541
64%
651
May21st to endNight
46%
605
30%
3054
20%
593
60%
593
67%
674
June1st to 10thNight
57%
455
41%
1595
30%
455
72%
455
73%
528
June11th to 20thNight
58%
380
42%
1520
35%
380
77%
380
76%
440
June21st to endNight
67%
380
41%
1520
37%
380
83%
380
78%
439
July1st to 10thNight
72%
360
50%
2064
37%
360
88%
360
77%
429
July11th to 20thNight
81%
361
39%
1881
42%
359
89%
359
58%
551
July21st to endNight
76%
624
36%
2289
28%
583
81%
583
66%
756
August1st to 10thNight
70%
746
45%
2497
21%
733
75%
733
71%
910
August11th to 20thNight
78%
742
38%
2460
21%
731
84%
731
85%
800
August21st to endNight
78%
824
44%
2811
27%
814
83%
814
83%
898
September1st to 10thNight
81%
760
47%
1900
30%
760
87%
760
79%
855
September11th to 20thNight
72%
760
47%
2047
31%
760
84%
760
83%
830
September21st to endNight
75%
760
49%
2090
36%
760
87%
760
83%
817
October1st to 10thNight
69%
740
59%
3042
32%
740
83%
740
75%
862
October11th to 20thNight
72%
747
37%
3115
38%
740
84%
740
72%
871
October21st to endNight
67%
820
48%
3928
38%
813
83%
813
61%
1123
November1st to 10thNight
66%
780
42%
2535
35%
780
82%
780
66%
960
November11th to 20thNight
71%
800
40%
2600
36%
800
82%
800
67%
1009
November21st to endNight
63%
788
40%
2561
34%
788
74%
788
59%
1053
December1st to 10thNight
60%
760
36%
2470
27%
760
72%
760
49%
1117
December11th to 20thNight
59%
759
33%
2935
29%
759
71%
759
38%
1406
December21st to endNight
52%
825
21%
4735
26%
817
65%
817
25%
2176

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
1112222245%78%
105204
Night
1112222246%78%
103075
TransparencyDay
10%35%14%26%43%
104957
Night
10%36%16%28%37%
102912
SeeingDay
5%12%24%31%22%6%
15457
Night
39%26%36%23%4
25205

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
321232335%6%71%
2539
11th to 20thDay
321433447%10%59%
2618
21st to endDay
5%32433336%9%59%
3403
February1st to 10thDay
4213332246%70%
1900
11th to 20thDay
211222246%9%70%
1850
21st to endDay
421323335%5%69%
1570
March1st to 10thDay
1213335%47%9%64%
3138
11th to 20thDay
231322336%8%66%
3259
21st to endDay
21223346%10%67%
4160
April1st to 10thDay
2212223348%73%
2280
11th to 20thDay
1111222236%79%
2249
21st to endDay
11122247%80%
2280
May1st to 10thDay
1111213486%
4034
11th to 20thDay
11111135%85%
4071
21st to endDay
11111112388%
4477
June1st to 10thDay
111112245%81%
2579
11th to 20thDay
111213487%
2660
21st to endDay
11111212288%
2660
July1st to 10thDay
1111112290%
4420
11th to 20thDay
111222223480%
4420
21st to endDay
11211123483%
4414
August1st to 10thDay
111112212384%
3783
11th to 20thDay
111112291%
3724
21st to endDay
111112292%
4180
September1st to 10thDay
11212223483%
2280
11th to 20thDay
111112290%
2133
21st to endDay
111112389%
2090
October1st to 10thDay
1111111488%
3490
11th to 20thDay
11111124385%
3264
21st to endDay
112222245%79%
3273
November1st to 10thDay
111122235%81%
1764
11th to 20thDay
1112123246%77%
1800
21st to endDay
22222335%7%73%
1773
December1st to 10thDay
221323336%9%67%
1710
11th to 20thDay
3424345%46%9%56%
2165
21st to endDay
6%5%25%33446%8%54%
2794
January1st to 10thNight
2313223246%71%
4406
11th to 20thNight
121443437%11%60%
4327
21st to endNight
442423445%8%60%
4268
February1st to 10thNight
3213333347%68%
2280
11th to 20thNight
2213232336%73%
2218
21st to endNight
321222325%7%71%
1884
March1st to 10thNight
121233335%8%69%
3736
11th to 20thNight
321323346%8%65%
3721
21st to endNight
1112233348%72%
3488
April1st to 10thNight
1212222347%75%
1900
11th to 20thNight
1212222235%80%
1879
21st to endNight
11212246%81%
1900
May1st to 10thNight
11122488%
2936
11th to 20thNight
1111125%87%
2834
21st to endNight
111111392%
3110
June1st to 10thNight
1112135%85%
1595
11th to 20thNight
111112390%
1520
21st to endNight
1111112390%
1520
July1st to 10thNight
111111292%
2540
11th to 20thNight
111112223483%
2540
21st to endNight
11111122386%
3130
August1st to 10thNight
111112225%84%
3166
11th to 20thNight
111112292%
3126
21st to endNight
11111293%
3498
September1st to 10thNight
11112112385%
1900
11th to 20thNight
112212289%
2047
21st to endNight
11112390%
2090
October1st to 10thNight
111112390%
3490
11th to 20thNight
1111112488%
3670
21st to endNight
11211223483%
4287
November1st to 10thNight
112112246%80%
2535
11th to 20thNight
112221246%79%
2600
21st to endNight
2212223248%72%
2561
December1st to 10thNight
3214332347%68%
2470
11th to 20thNight
331333448%10%59%
3041
21st to endNight
5%425%34446%8%55%
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
13%49%34%32%8%
2539
11th to 20thDay
18%7%17%26%22%9%
2618
21st to endDay
22%6%10%29%24%9%
3403
February1st to 10thDay
16%47%20%31%21%
1900
11th to 20thDay
13%38%20%33%23%
1860
21st to endDay
17%39%24%31%16%
1570
March1st to 10thDay
13%6%9%25%30%18%
3138
11th to 20thDay
14%6%10%25%28%17%
3259
21st to endDay
11%6%11%24%34%15%
4160
April1st to 10thDay
12%48%21%32%24%
2280
11th to 20thDay
9%36%18%38%27%
2252
21st to endDay
6%36%20%37%28%
2280
May1st to 10thDay
5%2414%34%41%
4034
11th to 20thDay
41311%35%46%
4074
21st to endDay
6%139%36%46%
4477
June1st to 10thDay
7%2412%25%50%
2579
11th to 20thDay
3137%22%64%
2660
21st to endDay
6%11413%75%
2660
July1st to 10thDay
41237%84%
4351
11th to 20thDay
10%125%11%71%
4345
21st to endDay
8%227%24%57%
4274
August1st to 10thDay
9%11421%64%
3770
11th to 20thDay
311412%79%
3760
21st to endDay
311413%77%
4179
September1st to 10thDay
8%235%14%68%
2280
11th to 20thDay
3126%27%60%
2133
21st to endDay
5%127%29%57%
2090
October1st to 10thDay
5%127%21%65%
3490
11th to 20thDay
6%127%28%56%
3264
21st to endDay
10%128%33%47%
3273
November1st to 10thDay
8%25%14%41%30%
1764
11th to 20thDay
10%37%20%32%29%
1800
21st to endDay
13%38%18%31%27%
1773
December1st to 10thDay
16%5%10%23%28%18%
1710
11th to 20thDay
23%9%11%20%26%11%
2164
21st to endDay
27%8%11%26%18%9%
2794
January1st to 10thNight
16%38%25%39%9%
4406
11th to 20thNight
17%5%15%31%22%10%
4327
21st to endNight
21%7%11%28%23%10%
4268
February1st to 10thNight
18%48%19%28%24%
2280
11th to 20thNight
14%36%19%32%25%
2219
21st to endNight
14%47%25%32%18%
1884
March1st to 10thNight
11%6%9%27%28%19%
3736
11th to 20thNight
13%7%12%25%28%15%
3721
21st to endNight
11%49%31%35%10%
3488
April1st to 10thNight
12%46%26%31%20%
1900
11th to 20thNight
9%35%23%36%24%
1879
21st to endNight
7%35%21%44%19%
1900
May1st to 10thNight
325%14%43%33%
2936
11th to 20thNight
42311%42%39%
2835
21st to endNight
31311%45%37%
3110
June1st to 10thNight
5%2315%30%46%
1595
11th to 20thNight
32310%21%61%
1520
21st to endNight
412415%74%
1520
July1st to 10thNight
3128%85%
2500
11th to 20thNight
8%12411%73%
2492
21st to endNight
7%125%26%58%
3033
August1st to 10thNight
7%126%23%62%
3156
11th to 20thNight
312312%79%
3156
21st to endNight
41217%76%
3498
September1st to 10thNight
7%125%19%66%
1900
11th to 20thNight
4227%29%57%
2047
21st to endNight
3126%30%59%
2090
October1st to 10thNight
4136%16%70%
3490
11th to 20thNight
4126%30%57%
3670
21st to endNight
8%136%33%50%
4287
November1st to 10thNight
8%2416%39%31%
2535
11th to 20thNight
9%26%17%31%34%
2600
21st to endNight
12%37%20%30%28%
2561
December1st to 10thNight
17%48%20%29%22%
2470
11th to 20thNight
20%8%13%22%26%12%
3041
21st to endNight
24%8%12%27%21%8%
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
9%11%26%35%18%2
180
11th to 20thDay
10%16%20%25%21%8%
232
21st to endDay
10%13%24%31%18%4
480
February1st to 10thDay
9%17%24%30%15%6%
460
11th to 20thDay
9%17%29%28%13%3
455
21st to endDay
10%18%32%31%7%2
382
March1st to 10thDay
13%22%28%23%12%1
460
11th to 20thDay
8%21%31%25%12%2
460
21st to endDay
10%25%33%25%7%
506
April1st to 10thDay
10%20%28%25%14%3
460
11th to 20thDay
8%22%31%27%10%2
452
21st to endDay
7%17%31%25%16%4
460
May1st to 10thDay
7%20%33%22%15%3
463
11th to 20thDay
416%26%32%17%5%
488
21st to endDay
6%14%23%31%22%5%
529
June1st to 10thDay
410%25%31%23%8%
490
11th to 20thDay
310%25%31%26%5%
490
21st to endDay
18%26%32%24%9%
490
July1st to 10thDay
1421%34%30%9%
486
11th to 20thDay
1115%34%33%15%
484
21st to endDay
2213%40%31%12%
531
August1st to 10thDay
25%22%34%28%10%
487
11th to 20thDay
1417%35%33%10%
477
21st to endDay
1319%36%32%9%
539
September1st to 10thDay
1420%36%30%9%
490
11th to 20thDay
210%21%35%28%6%
490
21st to endDay
29%18%35%31%5%
490
October1st to 10thDay
38%23%30%29%7%
490
11th to 20thDay
39%21%33%28%6%
490
21st to endDay
39%23%32%26%8%
439
November1st to 10thDay
5%12%19%33%26%6%
300
11th to 20thDay
29%21%36%24%7%
300
21st to endDay
412%21%35%23%6%
291
December1st to 10thDay
5%9%23%38%21%4
270
11th to 20thDay
410%22%37%21%7%
268
21st to endDay
12%16%21%32%16%4
198
January1st to 10thNight
6%8%24%32%23%7%
760
11th to 20thNight
47%25%40%21%4
760
21st to endNight
7%9%26%36%19%4
836
February1st to 10thNight
410%26%35%21%4
760
11th to 20thNight
411%31%32%18%4
744
21st to endNight
6%11%33%35%13%3
628
March1st to 10thNight
416%33%30%14%2
760
11th to 20thNight
5%15%29%29%18%4
760
21st to endNight
319%36%28%12%1
836
April1st to 10thNight
318%35%25%16%3
760
11th to 20thNight
417%35%30%13%1
751
21st to endNight
316%35%30%14%3
760
May1st to 10thNight
315%39%28%12%2
620
11th to 20thNight
213%32%35%16%1
561
21st to endNight
316%33%33%15%
617
June1st to 10thNight
18%32%43%14%1
455
11th to 20thNight
7%34%41%17%1
380
21st to endNight
1426%48%19%1
380
July1st to 10thNight
1221%43%31%2
370
11th to 20thNight
214%54%29%2
368
21st to endNight
218%48%31%2
635
August1st to 10thNight
2123%44%28%3
756
11th to 20thNight
1117%46%30%5%
752
21st to endNight
2117%40%34%6%
836
September1st to 10thNight
1313%38%39%6%
760
11th to 20thNight
423%38%31%4
760
21st to endNight
16%17%36%33%7%
760
October1st to 10thNight
15%22%42%27%3
760
11th to 20thNight
16%18%40%28%8%
760
21st to endNight
27%20%31%31%9%
836
November1st to 10thNight
110%20%34%27%7%
780
11th to 20thNight
25%18%42%27%7%
800
21st to endNight
36%24%37%25%5%
788
December1st to 10thNight
5%7%23%37%24%4
760
11th to 20thNight
48%24%40%19%4
760
21st to endNight
5%9%29%33%21%4
836
Page generated at 2024-11-24 21:22:19. Version ac07

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