Treesdale Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Treesdale Observatory clear sky chart from 2005-11-06 to 2024-01-14.

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
34%
23338
14%
74892
10%
23289
40%
23289
18%
53347

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
12%
757
11%
2469
4%
757
20%
757
8%
1991
January11th to 20thNight
14%
736
7%
2391
3%
736
18%
736
8%
1953
January21st to endNight
17%
792
9%
2574
4%
792
22%
792
9%
2074
February1st to 10thNight
19%
720
10%
2340
5%
720
25%
720
10%
1857
February11th to 20thNight
13%
702
9%
2275
4%
700
20%
700
10%
1757
February21st to endNight
15%
592
12%
1776
4%
592
23%
592
11%
1394
March1st to 10thNight
20%
720
16%
1980
7%
720
28%
720
17%
1452
March11th to 20thNight
25%
720
18%
1980
10%
720
32%
720
19%
1423
March21st to endNight
23%
792
17%
2178
8%
792
31%
792
19%
1596
April1st to 10thNight
26%
720
14%
1882
5%
720
30%
720
20%
1381
April11th to 20thNight
30%
693
16%
1757
7%
693
35%
693
23%
1242
April21st to endNight
28%
540
18%
1620
7%
540
36%
540
22%
1094
May1st to 10thNight
35%
540
14%
1620
8%
540
40%
540
19%
1138
May11th to 20thNight
40%
531
16%
1593
10%
531
45%
531
21%
1109
May21st to endNight
55%
427
17%
1590
15%
427
61%
427
27%
923
June1st to 10thNight
52%
360
15%
1440
17%
360
58%
360
24%
845
June11th to 20thNight
51%
360
16%
1440
19%
360
56%
360
21%
887
June21st to endNight
53%
360
18%
1440
20%
360
60%
360
24%
859
July1st to 10thNight
58%
401
14%
1451
15%
401
62%
401
24%
895
July11th to 20thNight
62%
522
18%
1566
16%
522
65%
522
33%
943
July21st to endNight
62%
591
15%
1672
13%
558
63%
558
26%
1095
August1st to 10thNight
64%
537
18%
1600
15%
534
66%
534
31%
1002
August11th to 20thNight
59%
534
19%
1598
14%
534
62%
534
28%
1018
August21st to endNight
59%
692
20%
1880
14%
692
61%
692
31%
1224
September1st to 10thNight
56%
720
22%
1800
13%
720
60%
720
36%
1155
September11th to 20thNight
63%
720
25%
1968
19%
720
67%
720
38%
1193
September21st to endNight
45%
720
21%
1981
15%
720
49%
720
26%
1335
October1st to 10thNight
46%
720
17%
2160
15%
720
51%
720
25%
1397
October11th to 20thNight
33%
720
16%
2160
11%
720
41%
720
22%
1478
October21st to endNight
29%
792
18%
2376
12%
792
37%
792
19%
1700
November1st to 10thNight
40%
740
21%
2297
17%
740
47%
740
24%
1479
November11th to 20thNight
26%
760
16%
2603
11%
760
35%
760
16%
1822
November21st to endNight
22%
760
14%
2660
9%
760
30%
760
14%
1937
December1st to 10thNight
18%
760
7%
2660
4%
760
23%
760
8%
2128
December11th to 20thNight
18%
759
9%
3250
6%
759
24%
759
8%
2498
December21st to endNight
17%
828
5%
4865
5%
817
22%
817
4%
4073

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
17%10%47%6%35%5%5%7%31%
74611
Night
17%10%47%6%3445%7%33%
75149
TransparencyDay
51%7%12%18%10%1
74330
Night
50%7%12%18%11%2
74949
SeeingDay
22%14%27%21%11%4
13560
Night
21%12%27%27%12%1
23353

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%17%8%11%7%45%345%19%
1691
11th to 20thDay
19%20%8%11%8%3333418%
1647
21st to endDay
18%16%8%13%8%34445%15%
1782
February1st to 10thDay
21%16%6%11%8%35%33420%
1620
11th to 20thDay
17%14%7%12%7%45%5%46%20%
1573
21st to endDay
21%14%5%9%6%33446%24%
1478
March1st to 10thDay
17%12%5%8%7%34447%31%
1980
11th to 20thDay
19%12%5%8%7%34446%29%
1980
21st to endDay
21%11%48%6%35%5%5%6%27%
2178
April1st to 10thDay
16%10%5%7%6%446%6%6%31%
2076
11th to 20thDay
18%10%37%6%45%45%6%32%
2167
21st to endDay
23%9%36%5%3445%6%32%
2340
May1st to 10thDay
19%8%37%6%45%5%6%7%28%
2340
11th to 20thDay
21%7%37%7%45%5%5%7%29%
2301
21st to endDay
13%425%6%46%7%6%10%38%
2681
June1st to 10thDay
15%6%36%6%45%6%6%9%34%
2520
11th to 20thDay
15%6%36%7%35%5%7%9%34%
2520
21st to endDay
14%6%36%6%35%5%6%10%37%
2520
July1st to 10thDay
11%6%35%7%36%5%6%8%41%
2461
11th to 20thDay
12%7%35%6%46%5%7%10%36%
2340
21st to endDay
11%5%37%7%46%7%7%9%34%
2547
August1st to 10thDay
10%5%26%7%5%6%6%8%10%33%
2327
11th to 20thDay
13%6%36%6%46%7%7%9%33%
2303
21st to endDay
12%5%35%6%47%6%7%10%37%
2458
September1st to 10thDay
17%6%345%3445%8%41%
2160
11th to 20thDay
12%6%35%5%445%6%9%43%
1989
21st to endDay
18%7%37%5%45%337%38%
1977
October1st to 10thDay
16%6%25%5%35%5%6%8%38%
1800
11th to 20thDay
20%9%36%6%45%5%5%5%33%
1800
21st to endDay
24%10%36%5%3446%6%30%
1980
November1st to 10thDay
11%9%46%6%35%5%46%42%
1777
11th to 20thDay
18%13%47%5%2445%5%32%
1576
21st to endDay
18%14%6%7%7%3445%6%25%
1520
December1st to 10thDay
21%18%8%11%6%34435%18%
1520
11th to 20thDay
21%16%9%10%5%24446%20%
1910
21st to endDay
25%23%7%9%6%3344413%
2772
January1st to 10thNight
18%17%7%10%6%46%5%4420%
2469
11th to 20thNight
21%15%6%10%8%4434419%
2391
21st to endNight
18%15%7%11%7%4444421%
2574
February1st to 10thNight
22%13%5%9%8%45%44423%
2340
11th to 20thNight
20%12%5%10%6%35%5%45%24%
2273
21st to endNight
22%11%5%11%6%3445%425%
1776
March1st to 10thNight
19%10%5%6%6%3445%5%33%
1980
11th to 20thNight
20%10%47%424347%34%
1980
21st to endNight
19%10%47%6%35%345%33%
2178
April1st to 10thNight
18%11%36%6%3435%7%34%
1882
11th to 20thNight
17%7%36%6%445%5%7%37%
1757
21st to endNight
15%9%37%5%335%5%8%36%
1620
May1st to 10thNight
18%8%47%6%45%45%8%33%
1620
11th to 20thNight
17%8%36%5%3445%8%36%
1592
21st to endNight
10%5%244346%7%10%44%
1589
June1st to 10thNight
11%5%35%5%346%6%8%44%
1440
11th to 20thNight
12%5%25%5%45%6%6%8%41%
1440
21st to endNight
10%425%5%45%6%7%8%44%
1440
July1st to 10thNight
8%6%27%6%35%5%6%8%44%
1494
11th to 20thNight
9%6%35%5%26%47%10%46%
1620
21st to endNight
10%5%36%6%35%6%7%9%39%
1771
August1st to 10thNight
7%5%36%6%35%5%7%10%45%
1611
11th to 20thNight
10%7%25%6%445%6%8%44%
1598
21st to endNight
10%5%245%446%6%9%44%
1880
September1st to 10thNight
13%5%245%346%5%7%48%
1800
11th to 20thNight
11%423435%45%10%51%
1968
21st to endNight
16%7%36%5%345%46%42%
1981
October1st to 10thNight
14%6%344345%47%44%
2160
11th to 20thNight
19%7%26%6%345%46%38%
2160
21st to endNight
24%9%36%5%34446%34%
2376
November1st to 10thNight
14%8%35%5%3446%7%43%
2297
11th to 20thNight
19%10%37%5%3445%7%31%
2603
21st to endNight
20%13%47%5%345%5%6%29%
2660
December1st to 10thNight
23%15%8%10%6%44345%19%
2660
11th to 20thNight
19%17%8%9%6%34445%23%
3285
21st to endNight
25%18%7%8%7%34345%13%
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
69%6%10%11%5%
1692
11th to 20thDay
72%411%10%2
1647
21st to endDay
71%5%10%10%31
1782
February1st to 10thDay
70%48%12%5%1
1620
11th to 20thDay
66%6%11%12%5%
1582
21st to endDay
62%5%11%14%7%1
1478
March1st to 10thDay
53%6%14%20%7%
1980
11th to 20thDay
57%5%11%16%9%2
1980
21st to endDay
57%6%9%16%10%2
2178
April1st to 10thDay
51%6%13%17%11%2
2076
11th to 20thDay
51%7%11%15%14%2
2167
21st to endDay
51%6%11%20%9%2
2340
May1st to 10thDay
51%9%13%17%9%1
2340
11th to 20thDay
53%8%12%14%10%2
2305
21st to endDay
39%11%17%22%9%2
2681
June1st to 10thDay
45%9%14%21%11%1
2520
11th to 20thDay
44%9%14%20%11%1
2520
21st to endDay
41%9%12%26%11%1
2520
July1st to 10thDay
41%10%13%20%13%3
2394
11th to 20thDay
40%9%15%23%14%1
2271
21st to endDay
42%10%14%20%13%1
2405
August1st to 10thDay
40%11%17%23%8%
2314
11th to 20thDay
43%10%16%22%9%1
2303
21st to endDay
39%10%14%22%12%3
2458
September1st to 10thDay
41%8%11%20%17%4
2160
11th to 20thDay
36%8%15%26%14%1
1989
21st to endDay
47%6%12%21%14%
1977
October1st to 10thDay
41%8%16%19%15%1
1800
11th to 20thDay
50%7%12%22%9%
1800
21st to endDay
53%6%14%16%9%1
1980
November1st to 10thDay
43%6%12%20%16%2
1777
11th to 20thDay
54%5%11%18%9%2
1576
21st to endDay
60%5%12%16%6%1
1520
December1st to 10thDay
70%5%10%12%3
1520
11th to 20thDay
66%5%11%14%3
1906
21st to endDay
74%7%9%8%2
2772
January1st to 10thNight
67%6%9%14%5%
2469
11th to 20thNight
68%6%11%12%3
2391
21st to endNight
65%6%9%14%41
2574
February1st to 10thNight
64%6%10%15%5%
2340
11th to 20thNight
62%5%11%14%7%
2275
21st to endNight
62%6%8%15%9%1
1776
March1st to 10thNight
52%6%11%20%11%1
1980
11th to 20thNight
50%6%13%21%9%2
1980
21st to endNight
52%5%9%17%14%3
2178
April1st to 10thNight
51%6%10%18%14%2
1882
11th to 20thNight
45%8%12%17%16%3
1757
21st to endNight
45%6%12%21%12%3
1620
May1st to 10thNight
49%6%11%19%12%3
1620
11th to 20thNight
46%9%11%17%15%2
1593
21st to endNight
33%9%17%26%13%2
1590
June1st to 10thNight
35%9%15%24%15%2
1440
11th to 20thNight
38%10%14%22%15%1
1440
21st to endNight
35%10%13%24%16%2
1440
July1st to 10thNight
36%10%12%20%17%4
1451
11th to 20thNight
31%9%17%23%19%2
1566
21st to endNight
37%11%15%21%13%3
1672
August1st to 10thNight
32%11%16%25%16%1
1600
11th to 20thNight
36%9%15%24%14%2
1598
21st to endNight
33%11%13%24%15%4
1880
September1st to 10thNight
33%7%14%22%18%5%
1800
11th to 20thNight
30%7%15%26%21%1
1968
21st to endNight
42%7%12%21%17%1
1981
October1st to 10thNight
39%8%14%22%14%2
2160
11th to 20thNight
46%6%11%23%13%1
2160
21st to endNight
50%6%11%19%11%2
2376
November1st to 10thNight
40%7%11%22%16%3
2297
11th to 20thNight
52%5%11%19%10%2
2603
21st to endNight
56%6%12%20%6%1
2660
December1st to 10thNight
66%6%12%12%4
2660
11th to 20thNight
64%5%12%15%3
3288
21st to endNight
71%8%11%9%2
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
29%26%29%11%41
263
11th to 20thDay
31%28%29%8%3
249
21st to endDay
26%32%30%8%4
264
February1st to 10thDay
34%23%38%5%
240
11th to 20thDay
34%30%28%7%
240
21st to endDay
41%22%28%9%
224
March1st to 10thDay
28%22%30%16%4
420
11th to 20thDay
31%19%32%12%5%1
420
21st to endDay
30%22%27%16%5%1
462
April1st to 10thDay
27%22%25%16%8%1
420
11th to 20thDay
23%19%27%17%9%4
412
21st to endDay
30%18%28%17%5%2
420
May1st to 10thDay
30%17%25%17%7%3
423
11th to 20thDay
23%11%33%20%9%4
447
21st to endDay
14%9%26%27%18%7%
485
June1st to 10thDay
14%10%30%29%14%4
450
11th to 20thDay
18%12%25%26%15%4
450
21st to endDay
14%10%32%28%14%3
450
July1st to 10thDay
13%7%19%28%23%11%
445
11th to 20thDay
14%5%25%29%18%9%
444
21st to endDay
13%5%26%30%19%7%
488
August1st to 10thDay
10%7%22%30%21%10%
446
11th to 20thDay
16%422%30%16%11%
439
21st to endDay
12%425%27%21%11%
495
September1st to 10thDay
16%8%22%27%19%8%
450
11th to 20thDay
12%5%20%32%21%11%
450
21st to endDay
23%11%23%23%14%6%
450
October1st to 10thDay
17%10%25%25%18%5%
450
11th to 20thDay
22%16%29%23%8%2
450
21st to endDay
31%15%29%18%7%
495
November1st to 10thDay
19%18%31%19%10%3
375
11th to 20thDay
29%20%29%14%5%2
206
21st to endDay
23%23%42%9%3
180
December1st to 10thDay
31%27%26%11%42
180
11th to 20thDay
32%21%33%11%3
180
21st to endDay
35%18%31%13%3
198
January1st to 10thNight
27%21%34%15%3
760
11th to 20thNight
32%15%32%18%3
736
21st to endNight
26%19%34%19%3
792
February1st to 10thNight
29%16%29%23%2
720
11th to 20thNight
30%19%35%15%1
703
21st to endNight
28%16%36%17%3
592
March1st to 10thNight
24%18%33%21%3
720
11th to 20thNight
26%18%28%21%6%1
720
21st to endNight
24%14%33%23%5%1
792
April1st to 10thNight
25%16%30%24%5%
720
11th to 20thNight
21%13%33%23%10%1
693
21st to endNight
19%10%36%29%6%
540
May1st to 10thNight
21%9%28%32%9%1
540
11th to 20thNight
21%8%27%33%11%
531
21st to endNight
11%326%34%22%3
429
June1st to 10thNight
11%5%25%41%18%1
360
11th to 20thNight
12%6%24%38%20%1
360
21st to endNight
11%328%36%20%1
360
July1st to 10thNight
9%120%42%23%5%
401
11th to 20thNight
9%5%18%35%28%5%
522
21st to endNight
11%314%42%28%2
591
August1st to 10thNight
10%115%43%28%2
537
11th to 20thNight
12%416%34%30%4
534
21st to endNight
12%320%33%27%5%
692
September1st to 10thNight
15%319%32%26%4
720
11th to 20thNight
12%416%36%29%4
720
21st to endNight
17%9%25%25%20%5%
720
October1st to 10thNight
17%8%23%29%22%1
720
11th to 20thNight
22%10%31%27%9%1
720
21st to endNight
29%12%26%26%7%
792
November1st to 10thNight
18%12%26%31%12%
740
11th to 20thNight
25%15%28%24%6%1
760
21st to endNight
26%16%30%21%6%
760
December1st to 10thNight
31%16%28%19%4
760
11th to 20thNight
27%19%30%21%3
760
21st to endNight
28%17%32%21%2
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