Old Fort Lewis Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Old Fort Lewis Observatory clear sky chart from 2005-11-05 to 2017-09-21.

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
61%
15767
29%
46990
23%
15726
67%
15726
40%
25588

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
58%
717
30%
2327
26%
716
66%
716
43%
1163
January16th to endNight
57%
768
32%
2496
30%
768
65%
768
40%
1288
February1st to 15thNight
55%
702
28%
2288
24%
699
64%
699
41%
1165
February16th to endNight
50%
632
34%
1905
23%
632
63%
632
40%
1040
March1st to 15thNight
55%
720
33%
1980
22%
720
64%
720
43%
1104
March16th to endNight
52%
768
29%
2112
17%
768
60%
768
41%
1203
April1st to 15thNight
47%
548
25%
1807
16%
548
58%
548
31%
990
April16th to endNight
49%
540
32%
1800
23%
540
64%
540
38%
908
May1st to 15thNight
54%
539
27%
1655
21%
539
60%
539
38%
890
May16th to endNight
53%
562
26%
1600
18%
561
60%
561
39%
902
June1st to 15thNight
68%
540
31%
1440
23%
540
73%
540
56%
728
June16th to endNight
77%
540
30%
1440
24%
540
80%
540
56%
731
July1st to 15thNight
67%
510
14%
1496
12%
510
68%
510
28%
949
July16th to endNight
72%
564
16%
1593
15%
531
73%
531
24%
1065
August1st to 15thNight
75%
539
19%
1611
19%
536
76%
536
31%
999
August16th to endNight
74%
573
25%
1896
27%
573
76%
573
34%
1067
September1st to 15thNight
72%
540
25%
1800
26%
540
74%
540
31%
1044
September16th to endNight
73%
644
38%
1884
31%
644
78%
644
52%
940
October1st to 15thNight
65%
660
33%
1980
24%
660
70%
660
46%
1029
October16th to endNight
70%
704
38%
2112
27%
704
78%
704
54%
1005
November1st to 15thNight
69%
715
37%
2115
30%
715
74%
715
52%
1031
November16th to endNight
59%
900
36%
2463
24%
900
67%
900
50%
1273
December1st to 15thNight
51%
900
27%
2520
18%
900
59%
900
41%
1414
December16th to endNight
48%
942
24%
2670
18%
942
54%
942
31%
1660

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
5%42443445%7%53%
47856
Night
5%5%23324346%58%
47186
TransparencyDay
28%5%9%21%24%11%
47551
Night
27%38%22%25%12%
46992
SeeingDay
9%12%27%31%16%3
5575
Night
8%418%38%25%4
15774

By Month

Percentage Cloud Cover SW (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
7%6%13423335%58%
1611
16th to endDay
6%8%24323335%57%
1728
February1st to 15thDay
38%25%334345%54%
1575
16th to endDay
6%9%25%434345%50%
1566
March1st to 15thDay
47%2442345%7%54%
1980
16th to endDay
5%5%345%2445%7%50%
2112
April1st to 15thDay
6%6%35%5%25%447%50%
2140
16th to endDay
44244345%5%7%54%
2160
May1st to 15thDay
5%425%335%447%52%
2293
16th to endDay
5%31333445%7%56%
2510
June1st to 15thDay
4213223446%65%
2520
16th to endDay
112314446%69%
2520
July1st to 15thDay
43245%47%7%7%9%42%
2370
16th to endDay
6%425%6%47%8%8%10%33%
2467
August1st to 15thDay
5%426%6%46%7%8%8%39%
2340
16th to endDay
5%4145%46%6%7%11%42%
2304
September1st to 15thDay
8%324435%5%6%8%44%
2160
16th to endDay
5%313323435%64%
1863
October1st to 15thDay
8%313223347%58%
1650
16th to endDay
4313311225%68%
1760
November1st to 15thDay
4312223336%66%
1729
16th to endDay
5%5%1232223467%
1495
December1st to 15thDay
8%7%23413345%55%
1440
16th to endDay
8%9%45%5%1433448%
1563
January1st to 15thNight
5%6%2331222464%
2327
16th to endNight
7%5%2241223362%
2496
February1st to 15thNight
5%7%13313245%61%
2277
16th to endNight
5%7%24323235%59%
1905
March1st to 15thNight
5%5%23423345%59%
1980
16th to endNight
6%5%24423445%56%
2112
April1st to 15thNight
7%424434447%52%
1807
16th to endNight
6%423322345%61%
1800
May1st to 15thNight
6%435%323347%56%
1655
16th to endNight
8%423323346%57%
1600
June1st to 15thNight
3312214236%68%
1440
16th to endNight
11322446%7%64%
1440
July1st to 15thNight
3435%6%5%7%7%6%8%41%
1586
16th to endNight
5%425%6%49%8%8%9%34%
1701
August1st to 15thNight
5%415%6%35%7%7%8%44%
1620
16th to endNight
33245%35%7%7%9%46%
1896
September1st to 15thNight
6%425%5%36%5%7%7%45%
1800
16th to endNight
5%2232223369%
1884
October1st to 15thNight
8%31222334463%
1980
16th to endNight
421321224469%
2112
November1st to 15thNight
422221335%6%65%
2115
16th to endNight
6%41321222468%
2463
December1st to 15thNight
7%6%13213345%59%
2520
16th to endNight
9%9%34323335%51%
2670

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
28%313%34%17%2
1612
16th to endDay
30%39%29%22%4
1728
February1st to 15thDay
30%313%28%17%5%
1575
16th to endDay
36%36%22%29%2
1566
March1st to 15thDay
28%36%29%28%4
1980
16th to endDay
31%36%23%28%6%
2112
April1st to 15thDay
32%45%22%25%10%
2140
16th to endDay
26%46%23%27%10%
2160
May1st to 15thDay
29%48%19%28%8%
2293
16th to endDay
24%5%7%15%32%13%
2513
June1st to 15thDay
19%4413%28%30%
2520
16th to endDay
15%4312%26%38%
2520
July1st to 15thDay
30%9%14%13%15%17%
2231
16th to endDay
36%10%16%19%10%5%
2312
August1st to 15thDay
34%9%11%15%16%11%
2325
16th to endDay
31%7%12%19%19%9%
2304
September1st to 15thDay
31%8%10%16%20%12%
2160
16th to endDay
21%45%15%35%17%
1863
October1st to 15thDay
25%45%23%30%11%
1650
16th to endDay
20%26%23%36%10%
1760
November1st to 15thDay
20%27%26%33%8%
1729
16th to endDay
22%26%31%28%9%
1495
December1st to 15thDay
30%316%30%15%3
1440
16th to endDay
39%417%22%13%1
1563
January1st to 15thNight
25%210%32%25%3
2327
16th to endNight
27%28%29%26%5%
2496
February1st to 15thNight
26%313%29%19%8%
2288
16th to endNight
29%26%21%34%6%
1905
March1st to 15thNight
27%3429%28%6%
1980
16th to endNight
29%26%21%29%10%
2112
April1st to 15thNight
31%36%19%24%13%
1807
16th to endNight
25%2421%30%15%
1800
May1st to 15thNight
27%37%19%30%10%
1656
16th to endNight
28%47%16%26%16%
1601
June1st to 15thNight
18%25%10%26%36%
1440
16th to endNight
16%45%10%21%40%
1440
July1st to 15thNight
37%8%12%12%9%19%
1496
16th to endNight
40%10%15%17%8%8%
1593
August1st to 15thNight
32%9%14%14%14%15%
1611
16th to endNight
30%6%14%21%17%10%
1896
September1st to 15thNight
34%5%11%18%14%16%
1800
16th to endNight
19%35%15%31%23%
1884
October1st to 15thNight
23%36%19%28%18%
1980
16th to endNight
18%25%18%39%15%
2112
November1st to 15thNight
19%26%25%33%12%
2115
16th to endNight
22%15%27%32%10%
2463
December1st to 15thNight
27%312%30%21%4
2520
16th to endNight
37%312%26%16%2
2670

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
37%8%17%23%11%1
90
16th to endDay
12%11%26%30%17%2
96
February1st to 15thDay
11%11%24%37%14%1
90
16th to endDay
11%23%31%21%11%
102
March1st to 15thDay
9%25%33%19%11%
270
16th to endDay
10%30%34%16%7%1
288
April1st to 15thDay
14%25%35%15%7%1
269
16th to endDay
15%31%34%16%2
270
May1st to 15thDay
7%28%35%22%5%
288
16th to endDay
8%16%32%25%14%3
411
June1st to 15thDay
47%28%34%22%2
495
16th to endDay
1321%42%24%6%
495
July1st to 15thDay
7%420%40%22%4
335
16th to endDay
8%214%40%27%5%
327
August1st to 15thDay
9%218%42%23%3
315
16th to endDay
6%223%43%20%3
332
September1st to 15thDay
7%330%39%14%3
315
16th to endDay
7%6%26%40%12%5%
235
October1st to 15thDay
48%26%27%24%7%
90
16th to endDay
7%10%28%31%18%4
96
November1st to 15thDay
11%12%14%31%27%3
90
16th to endDay
14%15%36%20%13%
90
December1st to 15thDay
11%13%34%36%4
90
16th to endDay
17%15%22%34%9%
96
January1st to 15thNight
8%421%38%21%5%
720
16th to endNight
12%419%37%20%5%
768
February1st to 15thNight
10%324%43%15%2
704
16th to endNight
9%6%30%41%11%
632
March1st to 15thNight
8%6%23%37%21%2
720
16th to endNight
7%9%24%35%20%1
768
April1st to 15thNight
9%12%25%37%14%1
548
16th to endNight
8%11%26%36%15%1
540
May1st to 15thNight
7%7%23%40%18%1
540
16th to endNight
11%5%23%34%21%2
563
June1st to 15thNight
4417%43%27%1
540
16th to endNight
2212%42%36%3
540
July1st to 15thNight
6%18%34%42%6%
510
16th to endNight
8%5%30%46%8%
564
August1st to 15thNight
6%1229%49%10%
539
16th to endNight
5%16%31%46%8%
573
September1st to 15thNight
7%16%40%37%5%
540
16th to endNight
6%211%37%34%8%
644
October1st to 15thNight
8%318%41%23%5%
660
16th to endNight
5%315%40%28%6%
704
November1st to 15thNight
6%315%41%27%4
715
16th to endNight
9%6%21%34%22%6%
900
December1st to 15thNight
12%5%24%41%14%2
900
16th to endNight
15%6%21%38%16%2
942
Page generated at 2017-09-22 05:35:33. Version ac04

Copyright 2017 A. Danko


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