Timberline Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Timberline Observatory clear sky chart from 2005-11-05 to 2017-10-31.

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
38%
16158
20%
47428
11%
16111
46%
16111
28%
29952

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
29%
900
18%
2416
7%
895
37%
895
25%
1627
January16th to endNight
24%
960
19%
2496
6%
960
32%
960
22%
1756
February1st to 15thNight
24%
880
12%
2288
4%
876
29%
876
20%
1691
February16th to endNight
27%
724
21%
1986
7%
724
37%
724
27%
1293
March1st to 15thNight
35%
540
22%
1980
15%
540
46%
540
23%
1191
March16th to endNight
24%
576
12%
2112
7%
576
33%
576
14%
1479
April1st to 15thNight
30%
539
18%
1818
11%
539
43%
539
24%
1127
April16th to endNight
33%
540
18%
1699
12%
540
44%
540
21%
1157
May1st to 15thNight
39%
540
16%
1617
11%
540
46%
540
24%
1063
May16th to endNight
43%
563
19%
1498
8%
561
50%
561
30%
966
June1st to 15thNight
53%
540
18%
1440
8%
540
57%
540
37%
884
June16th to endNight
54%
540
19%
1440
9%
540
58%
540
42%
834
July1st to 15thNight
69%
510
18%
1360
11%
510
71%
510
48%
749
July16th to endNight
75%
564
24%
1586
20%
531
77%
531
48%
803
August1st to 15thNight
67%
539
22%
1610
22%
536
71%
536
37%
920
August16th to endNight
65%
573
24%
1809
25%
573
70%
573
35%
1017
September1st to 15thNight
60%
540
26%
1800
24%
540
66%
540
34%
982
September16th to endNight
54%
558
28%
2001
23%
558
64%
558
36%
1034
October1st to 15thNight
40%
720
25%
2160
16%
720
47%
720
30%
1311
October16th to endNight
33%
768
25%
2304
12%
768
44%
768
31%
1363
November1st to 15thNight
37%
784
26%
2280
13%
784
49%
784
35%
1306
November16th to endNight
27%
900
18%
2520
7%
900
37%
900
27%
1618
December1st to 15thNight
23%
900
14%
2520
5%
900
32%
900
20%
1795
December16th to endNight
23%
960
16%
2688
5%
960
33%
960
17%
1986

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
12%7%36%5%3445%7%40%
48319
Night
11%6%25%424446%45%
47608
Cloud CoverDay
12%7%36%5%3445%7%40%
48328
Night
12%6%25%424446%45%
47612
TransparencyDay
42%6%12%23%13%1
48017
Night
38%5%13%25%15%2
47428
SeeingDay
14%7%27%29%16%3
6145
Night
15%8%31%31%12%
16158

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 15thDay
9%8%49%7%35%445%36%
1521
16th to endDay
13%12%5%9%7%35%346%28%
1728
February1st to 15thDay
12%10%5%8%6%44446%33%
1575
16th to endDay
13%8%37%6%25%5%46%35%
1497
March1st to 15thDay
12%11%47%6%25%445%34%
1969
16th to endDay
18%11%36%6%34445%31%
2112
April1st to 15thDay
16%7%45%42435%6%36%
2130
16th to endDay
22%9%35%433446%31%
2259
May1st to 15thDay
15%8%35%5%35%5%5%6%33%
2331
16th to endDay
15%5%36%5%3446%6%37%
2617
June1st to 15thDay
11%7%36%42445%9%41%
2520
16th to endDay
6%415%5%36%5%6%9%46%
2520
July1st to 15thDay
7%315%6%345%6%8%47%
2520
16th to endDay
5%325%425%5%7%9%48%
2472
August1st to 15thDay
7%5%25%5%35%6%7%10%42%
2340
16th to endDay
11%4345%2435%7%46%
2393
September1st to 15thDay
11%5%1432335%7%51%
2158
16th to endDay
10%32332445%7%51%
1947
October1st to 15thDay
16%6%14323335%48%
1800
16th to endDay
13%8%25%4235%5%5%43%
1919
November1st to 15thDay
10%3243245%47%51%
1575
16th to endDay
15%8%36%5%23447%38%
1440
December1st to 15thDay
14%11%49%6%34447%31%
1440
16th to endDay
15%11%410%6%35%345%27%
1536
January1st to 15thNight
9%7%36%6%3446%8%39%
2416
16th to endNight
11%11%37%5%34347%38%
2496
February1st to 15thNight
12%9%48%6%35%436%34%
2285
16th to endNight
10%7%37%434346%44%
1986
March1st to 15thNight
12%7%24423335%47%
1968
16th to endNight
18%10%36%5%24446%32%
2112
April1st to 15thNight
14%6%25%423345%45%
1818
16th to endNight
19%7%25%5%33345%39%
1699
May1st to 15thNight
15%423435%5%5%7%41%
1617
16th to endNight
11%3236%3346%7%46%
1496
June1st to 15thNight
8%5%2342436%7%50%
1440
16th to endNight
4312335%5%6%10%53%
1440
July1st to 15thNight
421222445%10%58%
1440
16th to endNight
32133245%6%9%58%
1693
August1st to 15thNight
5%3245%3445%8%53%
1620
16th to endNight
9%31432345%6%54%
1809
September1st to 15thNight
9%424422446%55%
1800
16th to endNight
10%413212347%56%
2001
October1st to 15thNight
13%6%24323435%50%
2160
16th to endNight
13%5%24423345%51%
2304
November1st to 15thNight
10%5%2331335%7%53%
2280
16th to endNight
14%7%34423335%46%
2520
December1st to 15thNight
13%11%5%7%6%34346%35%
2520
16th to endNight
16%11%48%5%24345%31%
2688

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
9%9%49%7%35%5%46%36%
1521
16th to endDay
13%12%5%8%7%35%346%28%
1728
February1st to 15thDay
12%11%48%7%34346%33%
1575
16th to endDay
13%7%48%5%25%446%35%
1497
March1st to 15thDay
13%10%48%5%24445%34%
1980
16th to endDay
18%11%36%6%33445%31%
2110
April1st to 15thDay
16%7%45%42435%6%36%
2130
16th to endDay
22%9%35%5%33446%31%
2259
May1st to 15thDay
16%8%25%6%45%5%5%7%33%
2331
16th to endDay
16%5%27%5%2445%7%37%
2617
June1st to 15thDay
11%7%35%42435%9%42%
2520
16th to endDay
6%415%436%5%7%8%46%
2520
July1st to 15thDay
7%315%6%245%5%9%47%
2520
16th to endDay
5%3245%25%5%7%9%48%
2472
August1st to 15thDay
8%4245%35%5%7%9%42%
2340
16th to endDay
11%434424447%46%
2393
September1st to 15thDay
11%41332335%7%51%
2158
16th to endDay
9%4133235%5%8%51%
1947
October1st to 15thDay
16%6%24323236%48%
1800
16th to endDay
14%8%244235%5%5%43%
1919
November1st to 15thDay
10%3243245%47%51%
1575
16th to endDay
15%8%35%5%2345%7%38%
1440
December1st to 15thDay
14%11%5%8%5%34437%31%
1440
16th to endDay
15%11%410%6%45%35%5%28%
1536
January1st to 15thNight
10%6%36%5%35%46%8%39%
2416
16th to endNight
11%11%37%5%34347%38%
2496
February1st to 15thNight
13%9%48%6%35%345%34%
2278
16th to endNight
10%7%36%5%24446%44%
1986
March1st to 15thNight
13%7%24423335%47%
1980
16th to endNight
18%10%37%4245%45%33%
2112
April1st to 15thNight
14%6%35%424345%45%
1818
16th to endNight
19%7%16%5%23345%39%
1699
May1st to 15thNight
16%424335%5%5%8%41%
1616
16th to endNight
11%4135%3346%7%46%
1496
June1st to 15thNight
8%5%2342446%8%49%
1440
16th to endNight
5%213335%5%5%10%53%
1440
July1st to 15thNight
42222445%10%59%
1440
16th to endNight
31123245%6%9%58%
1693
August1st to 15thNight
5%31443445%7%53%
1620
16th to endNight
9%31442445%6%54%
1809
September1st to 15thNight
9%414422436%55%
1800
16th to endNight
11%413312347%57%
2001
October1st to 15thNight
14%5%24223335%50%
2160
16th to endNight
14%5%14423345%51%
2304
November1st to 15thNight
10%5%23223347%53%
2280
16th to endNight
14%7%35%324335%46%
2520
December1st to 15thNight
13%11%5%7%5%34346%35%
2520
16th to endNight
16%11%47%6%24445%31%
2688

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
48%413%20%12%
1522
16th to endDay
57%414%16%7%
1728
February1st to 15thDay
52%5%12%21%8%
1576
16th to endDay
48%5%12%20%11%1
1497
March1st to 15thDay
51%6%12%19%10%1
1980
16th to endDay
54%5%13%18%6%1
2112
April1st to 15thDay
47%6%9%26%9%
2130
16th to endDay
53%5%11%19%8%
2259
May1st to 15thDay
50%6%11%19%10%
2331
16th to endDay
45%7%13%21%10%1
2621
June1st to 15thDay
39%6%15%21%14%2
2520
16th to endDay
32%8%14%26%16%2
2520
July1st to 15thDay
33%7%15%27%12%3
2371
16th to endDay
28%8%16%26%18%1
2316
August1st to 15thDay
33%9%15%25%14%
2326
16th to endDay
36%5%11%25%16%4
2393
September1st to 15thDay
32%5%11%27%19%3
2158
16th to endDay
30%412%32%18%
1947
October1st to 15thDay
38%49%26%19%2
1800
16th to endDay
40%5%10%25%15%2
1919
November1st to 15thDay
30%5%13%26%21%2
1575
16th to endDay
45%5%13%24%10%
1440
December1st to 15thDay
53%5%14%18%7%
1440
16th to endDay
58%5%11%15%8%1
1536
January1st to 15thNight
42%6%14%22%14%
2416
16th to endNight
46%5%15%21%10%
2496
February1st to 15thNight
52%412%21%7%1
2288
16th to endNight
40%413%23%15%2
1986
March1st to 15thNight
39%5%11%26%15%1
1980
16th to endNight
51%7%12%19%8%1
2112
April1st to 15thNight
41%410%28%13%2
1818
16th to endNight
46%6%12%19%12%1
1699
May1st to 15thNight
39%8%13%23%13%1
1617
16th to endNight
35%8%14%25%13%2
1498
June1st to 15thNight
31%6%15%28%14%3
1440
16th to endNight
24%8%14%27%21%4
1440
July1st to 15thNight
21%6%18%34%14%4
1360
16th to endNight
19%5%15%36%20%1
1586
August1st to 15thNight
27%8%13%33%15%2
1610
16th to endNight
29%6%10%28%17%7%
1809
September1st to 15thNight
29%5%9%27%23%3
1800
16th to endNight
27%411%34%20%1
2001
October1st to 15thNight
36%47%24%22%3
2160
16th to endNight
34%412%24%21%2
2304
November1st to 15thNight
30%314%25%21%4
2280
16th to endNight
40%313%27%12%1
2520
December1st to 15thNight
50%414%21%8%
2520
16th to endNight
53%412%17%10%1
2688

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
18%12%41%24%12
90
16th to endDay
25%9%40%22%2
96
February1st to 15thDay
11%21%36%26%4
90
16th to endDay
22%7%41%19%43
81
March1st to 15thDay
25%16%37%18%1
90
16th to endDay
36%11%33%14%4
96
April1st to 15thDay
20%13%30%26%8%
249
16th to endDay
28%11%28%20%8%1
369
May1st to 15thDay
19%14%31%23%10%1
468
16th to endDay
18%9%31%24%13%2
517
June1st to 15thDay
12%5%25%35%17%3
495
16th to endDay
6%8%34%31%16%3
495
July1st to 15thDay
8%223%34%21%10%
475
16th to endDay
6%116%43%26%5%
515
August1st to 15thDay
8%219%36%28%4
493
16th to endDay
11%319%33%24%7%
433
September1st to 15thDay
11%221%30%24%8%
313
16th to endDay
8%6%23%39%19%2
135
October1st to 15thDay
22%12%27%22%8%5%
135
16th to endDay
13%13%38%24%10%
144
November1st to 15thDay
5%6%37%27%15%6%
90
16th to endDay
36%11%35%13%3
90
December1st to 15thDay
20%22%28%22%5%1
90
16th to endDay
32%19%36%9%2
96
January1st to 15thNight
13%12%39%25%8%
900
16th to endNight
17%10%39%25%5%
960
February1st to 15thNight
19%9%38%26%5%
880
16th to endNight
14%12%41%24%6%1
724
March1st to 15thNight
17%8%33%28%10%
540
16th to endNight
23%10%36%22%6%
576
April1st to 15thNight
17%12%34%30%4
539
16th to endNight
23%7%29%28%9%
540
May1st to 15thNight
16%8%31%34%7%
540
16th to endNight
14%6%30%34%14%
563
June1st to 15thNight
10%426%41%16%
540
16th to endNight
6%430%44%14%
540
July1st to 15thNight
6%19%45%27%1
510
16th to endNight
315%49%28%1
564
August1st to 15thNight
7%115%49%24%2
539
16th to endNight
10%216%40%27%3
573
September1st to 15thNight
11%5%17%35%27%2
540
16th to endNight
12%424%33%24%1
558
October1st to 15thNight
18%7%30%30%12%1
720
16th to endNight
17%8%34%28%10%
768
November1st to 15thNight
13%11%34%30%9%
784
16th to endNight
17%12%39%21%8%
900
December1st to 15thNight
18%14%37%24%5%
900
16th to endNight
21%11%34%27%5%
960
Page generated at 2017-11-01 16:23:34. Version ac04

Copyright 2017 A. Danko


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