CHR
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the CHR clear sky chart from 2005-11-06 to 2024-03-28.

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
36%
23755
21%
80419
11%
23658
47%
23658
22%
49572

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
17%
755
25%
4188
7%
737
39%
737
13%
2227
January11th to 20thNight
23%
759
27%
3954
9%
740
40%
740
12%
2206
January21st to endNight
17%
829
15%
4441
7%
814
38%
814
12%
2681
February1st to 10thNight
25%
760
19%
2470
8%
760
39%
760
19%
1579
February11th to 20thNight
13%
740
19%
2382
4%
740
32%
740
15%
1529
February21st to endNight
16%
628
23%
1884
6%
628
35%
628
18%
1237
March1st to 10thNight
21%
760
25%
2090
7%
760
38%
760
24%
1304
March11th to 20thNight
23%
760
22%
2090
9%
760
36%
760
21%
1359
March21st to endNight
20%
821
20%
2245
6%
821
32%
821
22%
1486
April1st to 10thNight
24%
720
18%
1800
3%
720
34%
720
22%
1222
April11th to 20thNight
31%
711
21%
1775
6%
711
39%
711
30%
1161
April21st to endNight
33%
720
20%
1800
6%
720
41%
720
27%
1200
May1st to 10thNight
39%
558
17%
1638
8%
558
45%
558
24%
1092
May11th to 20thNight
41%
457
16%
1518
11%
457
47%
457
21%
1001
May21st to endNight
52%
388
16%
1551
16%
388
57%
388
21%
932
June1st to 10thNight
53%
360
20%
1440
20%
360
60%
360
24%
832
June11th to 20thNight
55%
360
16%
1440
17%
360
61%
360
21%
860
June21st to endNight
60%
360
18%
1440
19%
360
66%
360
26%
804
July1st to 10thNight
58%
350
15%
1400
17%
350
61%
350
20%
855
July11th to 20thNight
69%
461
16%
1505
15%
461
71%
461
30%
896
July21st to endNight
65%
591
17%
1672
13%
558
69%
558
33%
1021
August1st to 10thNight
63%
537
16%
1600
13%
534
65%
534
27%
1065
August11th to 20thNight
60%
581
18%
1645
12%
581
62%
581
26%
1107
August21st to endNight
60%
792
19%
1980
11%
792
62%
792
32%
1328
September1st to 10thNight
54%
720
22%
1800
12%
720
58%
720
31%
1215
September11th to 20thNight
57%
720
30%
1800
20%
720
61%
720
37%
1131
September21st to endNight
44%
720
20%
2079
14%
720
50%
720
24%
1389
October1st to 10thNight
47%
720
20%
2160
17%
720
54%
720
27%
1367
October11th to 20thNight
41%
720
26%
2160
18%
720
53%
720
29%
1291
October21st to endNight
34%
792
28%
2376
16%
792
50%
792
28%
1430
November1st to 10thNight
36%
740
27%
2281
15%
740
51%
740
30%
1310
November11th to 20thNight
32%
760
26%
2522
13%
760
49%
760
25%
1463
November21st to endNight
30%
760
23%
2660
12%
760
47%
760
22%
1534
December1st to 10thNight
23%
760
20%
2660
6%
760
40%
760
17%
1676
December11th to 20thNight
24%
760
21%
3171
9%
759
40%
759
17%
1898
December21st to endNight
26%
825
14%
4802
9%
817
42%
817
11%
2884

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
14%6%25%5%35%5%6%8%42%
78819
Night
14%6%25%5%345%5%7%44%
81173
TransparencyDay
39%7%12%23%16%3
78531
Night
39%6%12%23%17%3
80980
SeeingDay
20%12%27%23%13%5%
14076
Night
19%12%27%29%13%1
23783

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
18%4245%2445%6%45%
2649
11th to 20thDay
15%9%47%5%3445%5%39%
2840
21st to endDay
17%10%36%5%3445%6%37%
3124
February1st to 10thDay
13%8%35%7%446%6%6%39%
1710
11th to 20thDay
18%7%25%6%35%45%6%39%
1686
21st to endDay
17%9%346%35%446%40%
1570
March1st to 10thDay
13%5%25%435%447%47%
2085
11th to 20thDay
15%6%26%5%45%45%6%42%
2090
21st to endDay
17%6%37%5%3445%7%39%
2265
April1st to 10thDay
11%6%36%6%35%5%5%8%42%
2160
11th to 20thDay
13%6%25%5%34446%47%
2134
21st to endDay
18%6%26%5%35%446%41%
2160
May1st to 10thDay
16%6%25%5%45%6%6%9%38%
2336
11th to 20thDay
19%5%25%6%35%5%6%8%35%
2372
21st to endDay
15%5%25%5%35%5%7%8%41%
2725
June1st to 10thDay
11%5%245%35%6%6%11%43%
2520
11th to 20thDay
10%5%26%5%35%7%7%10%40%
2520
21st to endDay
8%425%5%35%6%8%11%44%
2520
July1st to 10thDay
8%5%26%6%45%6%7%10%41%
2520
11th to 20thDay
6%3247%46%6%8%12%43%
2400
21st to endDay
9%5%36%6%47%6%8%9%37%
2547
August1st to 10thDay
10%426%6%5%7%7%7%9%37%
2327
11th to 20thDay
12%5%26%6%46%6%7%9%38%
2236
21st to endDay
11%5%35%5%35%6%7%9%42%
2376
September1st to 10thDay
15%6%25%5%3445%7%44%
2160
11th to 20thDay
13%5%25%43445%7%49%
2157
21st to endDay
19%6%25%5%3445%7%40%
1879
October1st to 10thDay
17%5%245%3445%7%46%
1800
11th to 20thDay
17%5%245%34445%48%
1800
21st to endDay
15%5%2442445%6%49%
1980
November1st to 10thDay
12%6%1443345%7%51%
1796
11th to 20thDay
15%42432445%6%50%
1653
21st to endDay
16%424435%45%7%47%
1520
December1st to 10thDay
15%7%25%5%35%6%5%6%40%
1520
11th to 20thDay
16%5%25%5%34448%44%
1910
21st to endDay
18%6%35%6%35%6%6%8%35%
2772
January1st to 10thNight
13%7%35%5%34447%46%
4292
11th to 20thNight
19%6%24433346%46%
4120
21st to endNight
19%9%35%423447%40%
4532
February1st to 10thNight
14%7%45%5%35%5%5%7%41%
2470
11th to 20thNight
18%7%25%4245%46%42%
2380
21st to endNight
17%8%35%5%3445%6%41%
1884
March1st to 10thNight
14%5%25%435%45%6%48%
2090
11th to 20thNight
17%6%24424447%45%
2090
21st to endNight
15%6%35%5%3446%7%42%
2245
April1st to 10thNight
14%7%35%435%5%6%7%41%
1800
11th to 20thNight
13%6%345%245%6%9%45%
1775
21st to endNight
17%5%244345%5%7%44%
1800
May1st to 10thNight
17%5%245%35%5%6%8%39%
1638
11th to 20thNight
17%6%45%5%35%5%5%8%37%
1518
21st to endNight
13%5%25%435%6%7%10%41%
1550
June1st to 10thNight
10%5%25%5%35%5%7%8%45%
1440
11th to 20thNight
12%5%25%5%36%7%6%7%42%
1440
21st to endNight
8%435%5%35%6%7%10%45%
1440
July1st to 10thNight
9%6%25%7%45%5%7%11%40%
1440
11th to 20thNight
6%325%5%45%7%6%10%46%
1555
21st to endNight
8%425%5%46%6%8%11%41%
1771
August1st to 10thNight
8%5%27%7%47%7%7%9%37%
1611
11th to 20thNight
10%6%26%7%46%5%7%8%39%
1645
21st to endNight
10%5%25%5%35%6%6%10%41%
1980
September1st to 10thNight
15%425%5%35%5%5%7%44%
1800
11th to 20thNight
10%6%35%434437%51%
1800
21st to endNight
18%7%25%5%34446%42%
2079
October1st to 10thNight
20%5%24324446%46%
2160
11th to 20thNight
15%5%24433445%52%
2160
21st to endNight
15%6%2443335%6%50%
2376
November1st to 10thNight
12%5%35%424446%53%
2281
11th to 20thNight
14%6%3432445%6%50%
2522
21st to endNight
14%424435%45%7%48%
2660
December1st to 10thNight
16%6%36%5%3445%6%42%
2660
11th to 20thNight
17%6%25%5%35%35%7%43%
3285
21st to endNight
16%7%35%5%35%6%6%8%36%
4884

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
38%6%8%30%14%4
2650
11th to 20thDay
45%5%9%20%15%5%
2840
21st to endDay
47%6%10%23%12%2
3124
February1st to 10thDay
43%6%13%23%12%3
1710
11th to 20thDay
44%5%10%23%16%3
1695
21st to endDay
45%6%11%19%15%4
1570
March1st to 10thDay
38%5%11%24%19%2
2085
11th to 20thDay
41%6%10%19%18%6%
2090
21st to endDay
46%6%10%19%16%4
2265
April1st to 10thDay
38%7%11%19%19%5%
2160
11th to 20thDay
38%5%12%21%20%6%
2134
21st to endDay
44%6%10%21%15%4
2160
May1st to 10thDay
42%8%14%20%15%2
2336
11th to 20thDay
45%8%13%19%13%2
2376
21st to endDay
39%9%15%26%11%1
2725
June1st to 10thDay
35%8%15%25%16%1
2520
11th to 20thDay
37%11%15%22%13%2
2520
21st to endDay
31%10%17%25%16%1
2520
July1st to 10thDay
36%10%13%22%16%2
2450
11th to 20thDay
30%10%16%26%15%2
2327
21st to endDay
38%10%13%20%17%2
2405
August1st to 10thDay
39%10%16%23%11%1
2314
11th to 20thDay
39%10%14%21%14%1
2236
21st to endDay
36%9%15%24%13%3
2376
September1st to 10thDay
38%7%12%19%18%4
2160
11th to 20thDay
34%6%10%27%21%3
2157
21st to endDay
43%7%10%20%18%2
1879
October1st to 10thDay
38%6%11%26%16%3
1800
11th to 20thDay
38%5%10%24%21%3
1800
21st to endDay
36%49%25%22%3
1980
November1st to 10thDay
33%5%11%26%20%5%
1796
11th to 20thDay
35%411%23%21%5%
1653
21st to endDay
38%5%10%28%17%2
1520
December1st to 10thDay
43%7%12%25%11%3
1520
11th to 20thDay
39%5%11%31%11%4
1906
21st to endDay
43%8%15%24%10%1
2772
January1st to 10thNight
36%6%9%27%18%3
4292
11th to 20thNight
40%410%23%19%3
4120
21st to endNight
44%5%9%26%15%1
4532
February1st to 10thNight
42%6%13%24%14%2
2470
11th to 20thNight
43%5%9%22%16%5%
2382
21st to endNight
43%5%10%21%15%5%
1884
March1st to 10thNight
36%5%11%27%18%3
2090
11th to 20thNight
39%5%11%21%20%4
2090
21st to endNight
41%6%11%20%16%5%
2245
April1st to 10thNight
41%6%9%20%19%4
1800
11th to 20thNight
35%7%12%20%19%6%
1775
21st to endNight
39%6%13%25%13%5%
1800
May1st to 10thNight
41%8%13%20%13%5%
1638
11th to 20thNight
44%7%13%19%15%2
1519
21st to endNight
36%10%17%23%12%1
1551
June1st to 10thNight
35%9%13%25%17%2
1440
11th to 20thNight
38%9%15%22%14%2
1440
21st to endNight
33%8%16%23%18%3
1440
July1st to 10thNight
38%10%16%20%13%4
1400
11th to 20thNight
32%10%17%22%17%2
1505
21st to endNight
32%9%19%24%15%1
1672
August1st to 10thNight
38%11%15%22%14%1
1600
11th to 20thNight
40%8%14%23%13%2
1645
21st to endNight
36%9%15%23%14%3
1980
September1st to 10thNight
39%7%11%21%18%5%
1800
11th to 20thNight
34%5%10%27%22%3
1800
21st to endNight
43%6%10%20%18%3
2079
October1st to 10thNight
40%5%11%24%18%3
2160
11th to 20thNight
35%5%10%26%22%2
2160
21st to endNight
36%5%10%25%22%4
2376
November1st to 10thNight
33%5%10%25%22%6%
2281
11th to 20thNight
35%5%7%24%22%6%
2522
21st to endNight
36%5%11%25%20%4
2660
December1st to 10thNight
43%5%11%24%14%2
2660
11th to 20thNight
41%5%10%24%17%4
3288
21st to endNight
41%9%16%20%12%1
4884

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
31%23%28%14%2
218
11th to 20thDay
36%21%28%14%1
270
21st to endDay
29%29%27%11%31
297
February1st to 10thDay
23%26%36%11%4
270
11th to 20thDay
36%26%31%5%2
293
21st to endDay
30%19%32%16%2
382
March1st to 10thDay
27%21%30%18%3
460
11th to 20thDay
26%23%30%16%42
460
21st to endDay
23%20%34%17%5%1
490
April1st to 10thDay
22%22%30%17%8%1
420
11th to 20thDay
19%16%29%20%13%3
412
21st to endDay
25%16%31%17%9%2
420
May1st to 10thDay
23%17%29%18%9%4
423
11th to 20thDay
21%11%30%22%13%3
447
21st to endDay
21%6%22%24%21%7%
485
June1st to 10thDay
14%8%26%27%19%6%
450
11th to 20thDay
17%7%27%28%15%6%
450
21st to endDay
11%5%30%30%16%7%
450
July1st to 10thDay
13%316%29%28%11%
445
11th to 20thDay
6%221%33%30%9%
444
21st to endDay
14%222%32%19%9%
488
August1st to 10thDay
13%419%28%23%13%
446
11th to 20thDay
13%318%30%23%12%
439
21st to endDay
10%221%32%21%14%
495
September1st to 10thDay
20%6%18%24%22%11%
450
11th to 20thDay
16%419%32%18%12%
450
21st to endDay
24%8%23%26%15%4
450
October1st to 10thDay
20%5%20%27%20%7%
450
11th to 20thDay
19%14%24%28%13%2
450
21st to endDay
19%13%33%24%9%2
495
November1st to 10thDay
19%11%31%25%10%4
426
11th to 20thDay
20%14%35%19%10%1
313
21st to endDay
21%19%35%16%8%1
180
December1st to 10thDay
20%23%37%16%41
180
11th to 20thDay
22%28%27%18%4
180
21st to endDay
23%21%32%17%7%1
198
January1st to 10thNight
22%21%35%19%3
760
11th to 20thNight
25%16%30%25%5%
760
21st to endNight
20%20%37%22%2
836
February1st to 10thNight
19%16%34%28%3
760
11th to 20thNight
25%24%33%16%2
743
21st to endNight
26%18%35%18%3
628
March1st to 10thNight
22%19%35%21%3
760
11th to 20thNight
24%19%30%21%5%1
760
21st to endNight
20%18%36%20%5%
821
April1st to 10thNight
19%20%33%23%4
720
11th to 20thNight
17%12%34%28%8%1
711
21st to endNight
21%12%28%29%9%
720
May1st to 10thNight
22%7%25%31%13%1
558
11th to 20thNight
21%5%23%39%12%1
457
21st to endNight
17%420%40%19%1
389
June1st to 10thNight
11%424%38%22%1
360
11th to 20thNight
15%320%39%21%2
360
21st to endNight
9%320%48%19%1
360
July1st to 10thNight
12%214%38%28%5%
350
11th to 20thNight
7%111%41%35%4
461
21st to endNight
10%215%41%28%4
591
August1st to 10thNight
12%111%42%30%4
537
11th to 20thNight
11%410%41%32%3
581
21st to endNight
13%215%39%27%4
792
September1st to 10thNight
18%217%32%26%5%
720
11th to 20thNight
13%418%32%29%4
720
21st to endNight
23%5%20%29%22%1
720
October1st to 10thNight
21%5%20%30%20%4
720
11th to 20thNight
17%12%24%32%14%1
720
21st to endNight
20%12%30%26%12%1
792
November1st to 10thNight
15%11%31%30%11%1
740
11th to 20thNight
18%15%30%28%8%1
760
21st to endNight
17%15%32%28%8%1
760
December1st to 10thNight
22%18%33%21%7%1
760
11th to 20thNight
19%14%38%25%4
760
21st to endNight
21%14%33%29%3
836
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