Mount Olson Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mount Olson Observatory clear sky chart from 2005-11-05 to 2017-08-12.

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
27%
14558
13%
46493
8%
14528
34%
14528
16%
33195

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
14%
720
6%
2506
2%
716
19%
716
6%
2037
January16th to endNight
18%
768
10%
2628
5%
768
24%
768
10%
2018
February1st to 15thNight
14%
703
9%
2287
3%
701
21%
701
9%
1791
February16th to endNight
17%
631
13%
1940
5%
631
27%
631
12%
1491
March1st to 15thNight
19%
720
14%
2100
6%
720
27%
720
15%
1548
March16th to endNight
27%
768
19%
2034
8%
768
35%
768
21%
1425
April1st to 15thNight
21%
718
16%
1796
5%
718
27%
718
19%
1342
April16th to endNight
27%
669
17%
1749
6%
669
34%
669
22%
1227
May1st to 15thNight
34%
360
17%
1440
15%
360
44%
360
18%
932
May16th to endNight
33%
374
13%
1496
10%
374
44%
374
18%
937
June1st to 15thNight
34%
360
10%
1440
10%
360
44%
360
15%
969
June16th to endNight
39%
360
10%
1440
12%
360
45%
360
19%
891
July1st to 15thNight
50%
340
15%
1360
19%
340
57%
340
26%
755
July16th to endNight
49%
376
14%
1418
15%
354
55%
354
26%
812
August1st to 15thNight
48%
442
14%
1491
11%
440
53%
440
23%
943
August16th to endNight
48%
660
20%
1710
10%
660
53%
660
38%
1056
September1st to 15thNight
45%
660
17%
1650
7%
660
48%
660
31%
1112
September16th to endNight
44%
657
22%
1791
13%
657
49%
657
31%
1137
October1st to 15thNight
33%
660
17%
1980
12%
660
41%
660
19%
1368
October16th to endNight
23%
704
10%
2194
5%
704
30%
704
13%
1657
November1st to 15thNight
25%
700
17%
2315
12%
700
34%
700
16%
1614
November16th to endNight
20%
720
12%
2520
7%
720
28%
720
10%
1929
December1st to 15thNight
14%
720
6%
2520
2%
720
18%
720
7%
2017
December16th to endNight
13%
768
7%
2688
4%
768
17%
768
7%
2187

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
18%9%36%5%34446%32%
47487
Night
18%10%36%5%24446%32%
46663
Cloud CoverDay
18%9%36%5%34446%32%
47496
Night
18%10%46%5%34445%32%
46669
TransparencyDay
52%5%11%20%10%1
47174
Night
53%411%19%9%1
46494
SeeingDay
24%12%28%20%9%3
5359
Night
24%12%28%24%9%
14560

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
21%16%5%8%6%4433420%
1432
16th to endDay
15%14%6%9%7%45%445%22%
1593
February1st to 15thDay
15%14%6%11%7%35%44322%
1575
16th to endDay
20%14%5%8%6%3443424%
1533
March1st to 15thDay
18%13%5%9%7%4433326%
1848
16th to endDay
18%10%35%42323440%
2190
April1st to 15thDay
23%11%35%42434433%
2152
16th to endDay
19%8%26%5%24445%36%
2220
May1st to 15thDay
20%8%36%5%24447%33%
2511
16th to endDay
18%6%25%4245%5%8%35%
2618
June1st to 15thDay
23%6%35%5%3445%8%30%
2520
16th to endDay
18%6%25%5%35%45%7%33%
2520
July1st to 15thDay
13%5%25%5%35%5%6%8%39%
2520
16th to endDay
12%5%24446%6%8%8%35%
2654
August1st to 15thDay
12%7%38%5%35%45%7%35%
2372
16th to endDay
11%425%5%35%45%8%42%
2129
September1st to 15thDay
18%5%25%5%34447%40%
1980
16th to endDay
18%5%14433347%43%
1835
October1st to 15thDay
21%9%25%5%23346%34%
1650
16th to endDay
27%10%37%5%23335%24%
1677
November1st to 15thDay
14%8%26%7%45%445%34%
1542
16th to endDay
22%13%46%5%24345%25%
1440
December1st to 15thDay
25%15%5%8%7%2433419%
1440
16th to endDay
26%14%5%9%5%3343319%
1536
January1st to 15thNight
20%16%6%10%6%3443419%
2506
16th to endNight
14%15%5%9%7%35%445%23%
2628
February1st to 15thNight
16%13%6%12%7%34445%22%
2285
16th to endNight
18%13%5%8%5%44335%26%
1940
March1st to 15thNight
20%12%46%5%23335%31%
2087
16th to endNight
18%8%46%423245%39%
2034
April1st to 15thNight
23%8%36%42434434%
1796
16th to endNight
20%7%24424346%38%
1749
May1st to 15thNight
20%7%25%414446%37%
1440
16th to endNight
15%6%27%42445%7%38%
1495
June1st to 15thNight
20%7%25%6%245%5%7%33%
1440
16th to endNight
15%6%245%246%6%7%37%
1440
July1st to 15thNight
10%425%43445%9%45%
1440
16th to endNight
8%415%4345%8%9%44%
1514
August1st to 15thNight
11%6%26%5%35%46%9%39%
1501
16th to endNight
11%413423448%49%
1710
September1st to 15thNight
15%5%25%6%23346%44%
1650
16th to endNight
15%5%24323446%47%
1791
October1st to 15thNight
19%7%45%433448%34%
1980
16th to endNight
27%10%36%5%24346%26%
2194
November1st to 15thNight
19%9%35%5%34445%34%
2315
16th to endNight
22%11%47%6%35%345%25%
2520
December1st to 15thNight
22%17%5%7%6%2434421%
2520
16th to endNight
25%15%5%8%6%3433418%
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
21%15%5%9%7%35%34320%
1432
16th to endDay
15%13%6%9%8%45%44422%
1593
February1st to 15thDay
15%14%5%11%7%36%44421%
1575
16th to endDay
19%15%6%7%7%3443423%
1533
March1st to 15thDay
19%12%5%9%6%35%24325%
1858
16th to endDay
19%9%35%423235%40%
2189
April1st to 15thDay
23%11%35%42424433%
2152
16th to endDay
19%8%36%5%24345%37%
2220
May1st to 15thDay
20%7%35%5%24446%33%
2511
16th to endDay
17%5%35%4245%5%8%36%
2618
June1st to 15thDay
23%6%345%3445%8%30%
2520
16th to endDay
19%5%25%5%35%5%5%8%33%
2520
July1st to 15thDay
12%5%25%436%5%6%7%40%
2520
16th to endDay
12%5%15%5%45%5%7%9%36%
2654
August1st to 15thDay
12%6%37%6%35%5%5%7%36%
2372
16th to endDay
12%4145%35%46%8%42%
2129
September1st to 15thDay
18%5%24434346%40%
1980
16th to endDay
18%5%24323346%44%
1835
October1st to 15thDay
22%8%26%423347%34%
1650
16th to endDay
27%11%36%5%23335%24%
1677
November1st to 15thDay
14%7%37%7%46%45%5%33%
1542
16th to endDay
22%13%38%5%14345%25%
1440
December1st to 15thDay
24%16%5%8%7%23335%18%
1440
16th to endDay
26%15%6%8%6%3333319%
1536
January1st to 15thNight
20%17%6%9%6%35%34319%
2506
16th to endNight
14%16%6%9%7%34445%23%
2628
February1st to 15thNight
16%13%5%12%7%25%34422%
2278
16th to endNight
18%13%6%8%5%4443425%
1940
March1st to 15thNight
20%12%5%6%5%2334431%
2100
16th to endNight
19%9%36%423335%38%
2034
April1st to 15thNight
23%8%35%42334434%
1796
16th to endNight
20%7%244335%45%38%
1749
May1st to 15thNight
21%6%25%42435%7%36%
1440
16th to endNight
16%6%26%5%3445%7%37%
1495
June1st to 15thNight
20%6%36%5%2445%8%32%
1440
16th to endNight
15%6%25%5%35%5%6%7%37%
1440
July1st to 15thNight
11%424435%45%8%45%
1440
16th to endNight
8%5%235%246%7%10%43%
1514
August1st to 15thNight
12%5%25%5%245%6%8%39%
1501
16th to endNight
11%315%423447%49%
1710
September1st to 15thNight
15%5%35%5%23345%45%
1650
16th to endNight
15%5%23324446%46%
1791
October1st to 15thNight
20%7%35%43445%6%34%
1980
16th to endNight
26%10%36%5%24335%26%
2194
November1st to 15thNight
18%9%36%5%34445%33%
2315
16th to endNight
21%12%38%6%35%345%25%
2520
December1st to 15thNight
22%17%5%8%6%3333421%
2520
16th to endNight
25%16%6%8%5%3423418%
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
69%29%11%5%1
1433
16th to endDay
64%410%11%7%
1593
February1st to 15thDay
65%47%17%3
1577
16th to endDay
63%49%11%9%1
1533
March1st to 15thDay
63%310%14%7%1
1858
16th to endDay
49%29%18%15%5%
2190
April1st to 15thDay
55%36%19%13%2
2152
16th to endDay
49%37%19%15%3
2220
May1st to 15thDay
50%49%20%14%1
2511
16th to endDay
45%6%12%21%11%1
2622
June1st to 15thDay
51%5%11%21%10%
2520
16th to endDay
48%6%11%20%11%
2520
July1st to 15thDay
38%6%12%29%12%
2371
16th to endDay
42%7%16%24%9%
2487
August1st to 15thDay
44%7%15%22%10%
2358
16th to endDay
37%417%28%11%
2129
September1st to 15thDay
43%5%11%27%12%
1980
16th to endDay
41%410%31%11%
1835
October1st to 15thDay
51%311%22%10%
1650
16th to endDay
62%411%14%6%
1677
November1st to 15thDay
50%414%21%8%
1542
16th to endDay
61%5%10%15%6%
1440
December1st to 15thDay
69%5%11%12%2
1440
16th to endDay
70%48%10%5%
1536
January1st to 15thNight
69%410%10%4
2506
16th to endNight
63%49%15%6%
2628
February1st to 15thNight
65%310%15%4
2287
16th to endNight
62%38%17%8%
1940
March1st to 15thNight
56%311%18%8%
2100
16th to endNight
49%38%19%16%3
2034
April1st to 15thNight
53%37%19%15%1
1796
16th to endNight
47%49%19%16%3
1749
May1st to 15thNight
47%5%8%20%14%4
1440
16th to endNight
44%5%13%19%14%2
1497
June1st to 15thNight
49%6%13%20%9%
1440
16th to endNight
44%5%13%21%14%
1440
July1st to 15thNight
34%5%13%31%15%
1360
16th to endNight
33%8%16%25%14%
1418
August1st to 15thNight
40%7%16%22%12%
1491
16th to endNight
33%414%32%14%
1710
September1st to 15thNight
41%412%30%11%
1650
16th to endNight
38%412%31%13%
1791
October1st to 15thNight
49%411%22%11%
1980
16th to endNight
60%313%15%5%
2194
November1st to 15thNight
52%413%17%10%1
2315
16th to endNight
61%5%10%16%4
2520
December1st to 15thNight
67%411%13%2
2520
16th to endNight
70%310%10%3
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
26%25%28%11%8%
60
16th to endDay
35%13%32%12%5%
73
February1st to 15thDay
32%30%32%5%
90
16th to endDay
31%20%28%17%3
192
March1st to 15thDay
28%16%30%18%5%
270
16th to endDay
28%17%29%14%7%3
288
April1st to 15thDay
34%18%24%12%7%1
270
16th to endDay
25%17%29%15%6%5%
270
May1st to 15thDay
30%12%26%18%9%2
288
16th to endDay
22%16%31%16%10%2
331
June1st to 15thDay
26%12%29%20%9%1
315
16th to endDay
23%9%32%20%11%2
315
July1st to 15thDay
15%8%29%27%15%3
305
16th to endDay
14%6%30%31%12%4
326
August1st to 15thDay
17%5%23%31%15%5%
297
16th to endDay
13%329%27%13%13%
268
September1st to 15thDay
20%5%23%23%13%13%
255
16th to endDay
19%8%29%20%15%6%
254
October1st to 15thDay
27%11%25%19%11%5%
255
16th to endDay
36%13%26%18%4
257
November1st to 15thDay
19%12%35%21%8%2
196
16th to endDay
36%8%26%25%3
60
December1st to 15thDay
30%18%35%10%6%
60
16th to endDay
43%32%20%11
64
January1st to 15thNight
31%17%28%18%4
720
16th to endNight
24%17%31%21%4
768
February1st to 15thNight
24%19%34%18%2
704
16th to endNight
26%15%34%19%4
631
March1st to 15thNight
29%16%30%20%3
720
16th to endNight
24%15%27%23%8%
768
April1st to 15thNight
27%16%29%22%4
718
16th to endNight
27%13%28%22%8%
669
May1st to 15thNight
23%9%27%29%8%1
360
16th to endNight
17%9%35%26%10%
375
June1st to 15thNight
20%9%25%33%9%
360
16th to endNight
17%5%31%30%14%
360
July1st to 15thNight
11%6%25%43%13%
340
16th to endNight
8%429%40%15%1
376
August1st to 15thNight
14%325%35%19%1
442
16th to endNight
12%327%35%19%1
660
September1st to 15thNight
18%5%25%31%17%2
660
16th to endNight
17%8%25%25%20%1
657
October1st to 15thNight
23%10%27%23%13%1
660
16th to endNight
32%11%25%23%6%
704
November1st to 15thNight
25%12%29%21%10%
700
16th to endNight
30%16%25%17%9%
720
December1st to 15thNight
35%17%26%15%3
720
16th to endNight
35%16%27%17%21
768
Page generated at 2017-08-13 14:16:42. Version ac04

Copyright 2017 A. Danko


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