YFOS
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the YFOS clear sky chart from 2005-11-06 to 2024-01-11.

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
25%
22826
18%
74621
7%
22781
38%
22781
18%
48372

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
14%
760
20%
2649
5%
757
33%
757
14%
1638
January11th to 20thNight
15%
724
15%
2532
4%
724
29%
724
11%
1725
January21st to endNight
16%
792
19%
2772
6%
792
33%
792
14%
1759
February1st to 10thNight
10%
720
15%
2329
3%
720
26%
720
12%
1590
February11th to 20thNight
13%
703
19%
2100
3%
700
32%
700
14%
1384
February21st to endNight
15%
592
20%
1776
4%
592
35%
592
15%
1165
March1st to 10thNight
16%
720
20%
2160
6%
720
33%
720
17%
1429
March11th to 20thNight
19%
720
17%
2093
5%
720
31%
720
17%
1404
March21st to endNight
21%
792
22%
2014
6%
792
33%
792
19%
1379
April1st to 10thNight
20%
720
18%
1800
3%
720
29%
720
19%
1278
April11th to 20thNight
22%
711
17%
1775
4%
711
30%
711
22%
1214
April21st to endNight
20%
544
18%
1624
5%
544
31%
544
18%
1106
May1st to 10thNight
20%
540
14%
1620
6%
540
27%
540
15%
1156
May11th to 20thNight
26%
430
17%
1492
10%
430
38%
430
17%
987
May21st to endNight
33%
389
13%
1551
11%
388
42%
388
17%
978
June1st to 10thNight
31%
360
14%
1440
9%
360
45%
360
16%
921
June11th to 20thNight
31%
360
12%
1440
10%
360
40%
360
18%
905
June21st to endNight
36%
360
12%
1440
11%
360
46%
360
19%
902
July1st to 10thNight
42%
350
14%
1400
14%
350
52%
350
23%
821
July11th to 20thNight
48%
348
15%
1392
19%
348
53%
348
24%
810
July21st to endNight
44%
394
19%
1486
18%
372
54%
372
28%
821
August1st to 10thNight
48%
425
18%
1489
12%
423
56%
423
26%
867
August11th to 20thNight
40%
578
17%
1642
9%
578
44%
578
26%
1059
August21st to endNight
46%
792
22%
1980
10%
792
50%
792
34%
1248
September1st to 10thNight
42%
720
21%
1800
7%
720
49%
720
36%
1128
September11th to 20thNight
42%
720
25%
1952
14%
720
49%
720
33%
1190
September21st to endNight
39%
720
25%
1980
14%
720
49%
720
31%
1240
October1st to 10thNight
33%
720
17%
1980
9%
720
40%
720
21%
1363
October11th to 20thNight
29%
720
19%
2229
8%
720
43%
720
23%
1395
October21st to endNight
20%
792
14%
2574
6%
792
32%
792
14%
1788
November1st to 10thNight
25%
740
21%
2407
10%
740
40%
740
20%
1498
November11th to 20thNight
22%
760
23%
2470
9%
760
38%
760
17%
1545
November21st to endNight
21%
760
19%
2521
6%
760
37%
760
17%
1561
December1st to 10thNight
15%
760
17%
2660
4%
760
31%
760
12%
1794
December11th to 20thNight
14%
760
16%
3213
3%
759
29%
759
11%
2063
December21st to endNight
14%
830
12%
4839
3%
817
31%
817
6%
3261

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
16%7%36%6%35%5%5%7%37%
74780
Night
16%7%35%5%35%5%5%7%39%
74912
TransparencyDay
45%5%12%22%14%2
74485
Night
44%5%13%23%13%2
74741
SeeingDay
24%18%30%18%7%2
13790
Night
22%17%29%23%7%1
22832

By Month

Percentage Cloud Cover NE (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
11%8%47%6%35%5%7%9%34%
1517
11th to 20thDay
15%8%36%6%35%5%6%9%34%
1440
21st to endDay
10%7%46%6%45%6%7%8%37%
1584
February1st to 10thDay
15%10%46%5%48%5%6%8%30%
1627
11th to 20thDay
12%7%38%6%46%5%5%8%35%
1748
21st to endDay
16%11%48%6%345%5%5%33%
1480
March1st to 10thDay
12%9%45%7%45%6%6%7%35%
1797
11th to 20thDay
15%8%46%6%345%5%7%36%
1867
21st to endDay
17%8%46%6%44445%38%
2340
April1st to 10thDay
16%9%36%6%45%35%6%36%
2160
11th to 20thDay
16%6%35%6%45%6%6%7%38%
2134
21st to endDay
20%9%37%6%35%5%46%32%
2335
May1st to 10thDay
22%8%37%6%44446%32%
2340
11th to 20thDay
18%6%36%5%35%448%38%
2417
21st to endDay
15%6%36%6%35%45%7%41%
2725
June1st to 10thDay
19%6%36%7%46%5%6%8%32%
2520
11th to 20thDay
17%5%36%6%35%45%10%36%
2520
21st to endDay
19%7%36%6%45%6%6%7%31%
2520
July1st to 10thDay
15%6%35%5%35%6%7%8%39%
2520
11th to 20thDay
12%6%26%6%46%47%9%37%
2520
21st to endDay
14%5%35%6%46%6%7%7%38%
2744
August1st to 10thDay
11%5%25%6%5%6%7%6%9%38%
2421
11th to 20thDay
13%5%35%7%5%5%6%7%9%36%
2255
21st to endDay
12%325%6%45%5%6%8%44%
2376
September1st to 10thDay
12%6%35%5%345%6%7%44%
2160
11th to 20thDay
14%5%34423347%49%
2008
21st to endDay
16%5%15%5%3445%7%45%
1980
October1st to 10thDay
22%7%36%6%34445%37%
1980
11th to 20thDay
14%7%25%5%35%5%5%8%40%
1726
21st to endDay
23%7%35%5%35%5%5%7%32%
1782
November1st to 10thDay
12%7%36%5%24347%45%
1671
11th to 20thDay
17%6%26%5%25%45%6%42%
1710
21st to endDay
17%8%26%5%345%6%7%37%
1658
December1st to 10thDay
16%9%37%6%45%5%6%7%31%
1520
11th to 20thDay
20%9%25%435%45%5%38%
1906
21st to endDay
21%12%47%6%35%5%5%6%28%
2772
January1st to 10thNight
13%9%35%5%2446%8%40%
2649
11th to 20thNight
14%9%36%6%35%6%7%8%32%
2532
21st to endNight
11%7%46%6%46%5%6%7%39%
2772
February1st to 10thNight
14%10%46%5%45%5%6%7%34%
2329
11th to 20thNight
11%9%36%6%36%6%5%8%37%
2098
21st to endNight
14%9%36%5%346%6%7%36%
1776
March1st to 10thNight
13%9%37%5%35%5%6%7%38%
2150
11th to 20thNight
15%8%445%45%6%7%7%36%
2092
21st to endNight
16%8%25%43445%7%40%
2014
April1st to 10thNight
17%8%35%5%35%5%5%7%36%
1800
11th to 20thNight
16%8%35%425%5%6%7%39%
1775
21st to endNight
19%9%37%5%3445%7%36%
1624
May1st to 10thNight
21%10%37%6%35%45%6%31%
1620
11th to 20thNight
19%5%36%5%46%5%5%7%36%
1491
21st to endNight
16%6%36%6%25%46%7%38%
1550
June1st to 10thNight
19%7%36%6%3445%8%36%
1440
11th to 20thNight
20%6%34435%46%7%38%
1440
21st to endNight
17%5%26%5%25%46%8%40%
1440
July1st to 10thNight
15%5%344345%5%9%43%
1440
11th to 20thNight
12%6%245%346%7%9%41%
1440
21st to endNight
12%5%243345%6%10%45%
1574
August1st to 10thNight
10%6%35%5%3446%8%46%
1499
11th to 20thNight
13%6%26%6%445%6%9%40%
1642
21st to endNight
15%42442345%8%48%
1980
September1st to 10thNight
12%5%25%42445%7%51%
1800
11th to 20thNight
14%5%23325%446%52%
1952
21st to endNight
14%5%2443445%7%47%
1980
October1st to 10thNight
21%7%25%6%3445%6%39%
1980
11th to 20thNight
16%6%25%5%35%5%6%7%41%
2229
21st to endNight
23%8%35%5%345%5%6%33%
2574
November1st to 10thNight
14%7%35%6%2445%6%45%
2407
11th to 20thNight
16%6%35%5%345%5%5%43%
2470
21st to endNight
14%7%26%5%35%5%6%8%40%
2521
December1st to 10thNight
16%8%47%6%35%45%7%33%
2660
11th to 20thNight
18%7%36%5%35%5%5%7%36%
3288
21st to endNight
21%11%5%7%5%3445%6%29%
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
45%5%10%27%12%1
1518
11th to 20thDay
46%6%15%19%10%3
1440
21st to endDay
43%5%13%24%12%3
1584
February1st to 10thDay
52%414%18%9%1
1627
11th to 20thDay
47%5%12%21%12%3
1757
21st to endDay
53%310%19%13%2
1480
March1st to 10thDay
47%5%13%19%12%4
1800
11th to 20thDay
47%312%18%16%4
1867
21st to endDay
48%48%19%17%5%
2340
April1st to 10thDay
49%48%21%14%3
2160
11th to 20thDay
44%411%22%17%2
2134
21st to endDay
54%48%18%14%3
2335
May1st to 10thDay
54%38%20%13%2
2340
11th to 20thDay
45%49%23%16%3
2421
21st to endDay
43%6%14%23%13%2
2725
June1st to 10thDay
48%6%12%22%11%1
2520
11th to 20thDay
44%6%12%19%16%2
2520
21st to endDay
49%7%12%19%12%1
2520
July1st to 10thDay
41%6%12%22%17%1
2450
11th to 20thDay
42%6%15%26%11%
2445
21st to endDay
41%6%14%24%14%1
2591
August1st to 10thDay
40%7%15%24%13%1
2407
11th to 20thDay
43%5%15%24%12%1
2255
21st to endDay
37%6%12%27%16%2
2376
September1st to 10thDay
37%5%14%26%16%2
2160
11th to 20thDay
36%49%27%23%2
2008
21st to endDay
38%411%27%18%3
1980
October1st to 10thDay
50%48%22%13%3
1980
11th to 20thDay
40%415%30%10%1
1726
21st to endDay
51%411%21%10%3
1782
November1st to 10thDay
39%414%25%16%2
1671
11th to 20thDay
42%412%23%17%2
1710
21st to endDay
46%417%23%10%1
1658
December1st to 10thDay
51%5%15%18%10%1
1520
11th to 20thDay
47%314%20%15%1
1906
21st to endDay
57%412%17%10%1
2772
January1st to 10thNight
42%414%24%14%2
2649
11th to 20thNight
49%6%14%21%7%2
2532
21st to endNight
44%411%22%16%2
2772
February1st to 10thNight
48%5%14%20%11%2
2329
11th to 20thNight
44%5%13%23%14%2
2100
21st to endNight
44%5%13%24%11%3
1776
March1st to 10thNight
46%411%23%12%3
2160
11th to 20thNight
46%413%20%14%3
2093
21st to endNight
44%410%18%19%5%
2014
April1st to 10thNight
47%411%21%15%2
1800
11th to 20thNight
43%411%23%17%1
1775
21st to endNight
49%311%20%14%4
1624
May1st to 10thNight
54%38%19%13%2
1620
11th to 20thNight
46%6%11%19%14%3
1492
21st to endNight
44%7%14%23%11%2
1551
June1st to 10thNight
47%5%14%22%12%1
1440
11th to 20thNight
44%6%15%20%13%2
1440
21st to endNight
43%7%15%22%12%1
1440
July1st to 10thNight
38%7%16%25%13%1
1400
11th to 20thNight
36%8%16%28%12%
1392
21st to endNight
33%6%17%30%12%2
1486
August1st to 10thNight
35%7%16%28%14%1
1489
11th to 20thNight
40%6%17%24%12%1
1642
21st to endNight
35%5%12%31%17%1
1980
September1st to 10thNight
33%5%15%28%18%1
1800
11th to 20thNight
34%411%27%22%2
1952
21st to endNight
35%6%10%31%16%2
1980
October1st to 10thNight
47%5%11%23%12%3
1980
11th to 20thNight
41%416%28%11%1
2229
21st to endNight
51%5%12%20%10%2
2574
November1st to 10thNight
42%412%23%17%3
2407
11th to 20thNight
42%411%24%16%2
2470
21st to endNight
41%5%18%25%11%1
2521
December1st to 10thNight
50%414%22%9%1
2660
11th to 20thNight
47%5%14%19%14%2
3288
21st to endNight
55%412%21%7%
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%22%30%14%3
180
11th to 20thDay
32%23%33%11%11
160
21st to endDay
23%28%35%10%4
176
February1st to 10thDay
35%32%23%8%2
279
11th to 20thDay
32%25%29%13%1
416
21st to endDay
29%26%29%13%2
346
March1st to 10thDay
29%21%35%11%3
420
11th to 20thDay
30%27%28%13%21
420
21st to endDay
29%23%30%11%6%2
462
April1st to 10thDay
34%21%27%10%7%2
420
11th to 20thDay
30%24%27%11%6%2
412
21st to endDay
30%19%31%12%7%1
420
May1st to 10thDay
32%22%27%13%5%2
423
11th to 20thDay
21%20%38%15%5%2
447
21st to endDay
20%14%34%24%7%1
485
June1st to 10thDay
22%16%35%17%8%2
450
11th to 20thDay
20%16%34%20%8%2
450
21st to endDay
21%15%34%20%7%3
450
July1st to 10thDay
18%11%30%26%12%3
445
11th to 20thDay
16%9%32%31%10%2
444
21st to endDay
15%10%36%29%8%2
488
August1st to 10thDay
13%12%32%28%11%4
446
11th to 20thDay
16%8%36%23%12%5%
439
21st to endDay
15%11%32%25%12%5%
495
September1st to 10thDay
17%13%26%24%15%5%
450
11th to 20thDay
20%15%27%18%13%8%
450
21st to endDay
19%14%31%21%10%6%
450
October1st to 10thDay
29%14%23%20%10%4
450
11th to 20thDay
22%18%29%22%7%2
410
21st to endDay
35%14%27%18%6%1
396
November1st to 10thDay
24%21%27%18%10%1
365
11th to 20thDay
31%24%22%17%5%1
370
21st to endDay
31%21%28%14%41
318
December1st to 10thDay
25%32%26%14%31
180
11th to 20thDay
37%25%29%8%2
180
21st to endDay
33%21%30%12%41
198
January1st to 10thNight
24%24%32%18%3
760
11th to 20thNight
26%24%30%18%2
724
21st to endNight
19%26%32%19%4
792
February1st to 10thNight
25%23%34%16%2
720
11th to 20thNight
23%26%33%17%2
703
21st to endNight
27%25%29%16%2
592
March1st to 10thNight
28%18%34%17%3
720
11th to 20thNight
27%22%28%20%4
720
21st to endNight
25%20%29%19%6%
792
April1st to 10thNight
26%19%30%22%4
720
11th to 20thNight
22%21%30%20%6%
711
21st to endNight
29%17%28%23%4
544
May1st to 10thNight
28%15%31%21%4
540
11th to 20thNight
19%17%33%26%5%
430
21st to endNight
21%8%32%30%9%1
389
June1st to 10thNight
22%9%31%28%9%1
360
11th to 20thNight
22%11%29%29%8%
360
21st to endNight
18%7%31%34%9%1
360
July1st to 10thNight
16%8%29%37%10%
350
11th to 20thNight
13%328%42%13%2
348
21st to endNight
13%5%34%36%11%1
394
August1st to 10thNight
12%6%26%40%15%1
425
11th to 20thNight
15%6%29%31%16%2
578
21st to endNight
16%7%23%36%15%2
792
September1st to 10thNight
14%8%30%30%17%1
720
11th to 20thNight
18%10%26%26%17%3
720
21st to endNight
17%10%28%30%13%2
720
October1st to 10thNight
26%13%23%26%11%2
720
11th to 20thNight
19%16%30%24%10%1
720
21st to endNight
28%19%26%20%6%
792
November1st to 10thNight
19%19%30%23%7%1
740
11th to 20thNight
23%25%27%18%6%1
760
21st to endNight
24%22%29%19%6%
760
December1st to 10thNight
26%22%33%16%4
760
11th to 20thNight
26%23%31%16%3
760
21st to endNight
29%24%27%18%2
836
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