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 2005-11-06 to 2024-03-08.

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
58%
24472
29%
79927
22%
24380
66%
24380
38%
42339

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
53%
748
28%
4025
24%
736
64%
736
25%
2049
January11th to 20thNight
51%
756
32%
4016
28%
740
62%
740
27%
1831
January21st to endNight
52%
828
22%
4294
27%
814
63%
814
23%
2236
February1st to 10thNight
47%
760
30%
2470
22%
760
61%
760
42%
1248
February11th to 20thNight
46%
743
29%
2365
21%
740
59%
740
38%
1263
February21st to endNight
42%
628
33%
1846
20%
628
56%
628
39%
1028
March1st to 10thNight
47%
750
31%
2062
18%
750
58%
750
41%
1169
March11th to 20thNight
52%
720
30%
1980
20%
720
59%
720
40%
1147
March21st to endNight
44%
792
27%
2178
12%
792
56%
792
36%
1291
April1st to 10thNight
45%
554
28%
1814
17%
554
59%
554
34%
961
April11th to 20thNight
45%
533
29%
1777
20%
533
60%
533
35%
927
April21st to endNight
46%
540
32%
1800
20%
540
63%
540
40%
895
May1st to 10thNight
51%
540
31%
1674
23%
540
61%
540
41%
859
May11th to 20thNight
55%
531
27%
1594
21%
531
61%
531
37%
903
May21st to endNight
53%
583
27%
1622
17%
582
60%
582
41%
903
June1st to 10thNight
70%
540
33%
1440
23%
540
75%
540
56%
725
June11th to 20thNight
67%
540
34%
1440
25%
540
73%
540
58%
710
June21st to endNight
73%
540
30%
1440
22%
540
78%
540
53%
752
July1st to 10thNight
73%
525
19%
1523
16%
525
74%
525
36%
893
July11th to 20thNight
75%
522
20%
1566
17%
522
76%
522
34%
931
July21st to endNight
70%
591
15%
1674
14%
558
71%
558
25%
1109
August1st to 10thNight
73%
537
19%
1602
17%
534
74%
534
31%
979
August11th to 20thNight
73%
534
25%
1670
25%
534
74%
534
35%
960
August21st to endNight
77%
594
26%
1980
28%
594
78%
594
36%
1068
September1st to 10thNight
74%
540
28%
1800
30%
540
77%
540
34%
999
September11th to 20thNight
69%
576
34%
1836
32%
576
73%
576
38%
996
September21st to endNight
70%
720
35%
2044
26%
720
75%
720
51%
1047
October1st to 10thNight
60%
720
29%
2160
20%
720
66%
720
40%
1185
October11th to 20thNight
68%
720
39%
2160
27%
720
77%
720
58%
990
October21st to endNight
64%
792
41%
2376
27%
792
75%
792
54%
1118
November1st to 10thNight
68%
740
40%
2220
29%
740
76%
740
56%
1025
November11th to 20thNight
60%
879
40%
2403
25%
879
69%
879
59%
1162
November21st to endNight
56%
950
33%
2660
22%
950
64%
950
47%
1400
December1st to 10thNight
51%
950
29%
2660
19%
950
59%
950
43%
1467
December11th to 20thNight
47%
948
26%
3136
17%
948
55%
948
31%
1842
December21st to endNight
45%
1008
16%
4620
17%
998
52%
998
25%
2271

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%5%24435%5%5%7%55%
78740
Night
5%6%2442445%6%58%
80873
TransparencyDay
26%5%9%21%25%14%
78465
Night
27%48%22%25%14%
80669
SeeingDay
9%12%29%30%17%3
14154
Night
9%5%22%40%22%3
24509

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 10thDay
36%35%4245%5%7%55%
2858
11th to 20thDay
35%245%45%5%6%6%55%
2860
21st to endDay
5%9%35%4345%46%52%
3146
February1st to 10thDay
38%36%5%34446%55%
1710
11th to 20thDay
5%8%36%5%35%45%6%50%
1703
21st to endDay
5%10%36%5%3435%5%50%
1604
March1st to 10thDay
6%8%35%5%335%5%8%48%
2058
11th to 20thDay
47%35%5%35%5%6%8%49%
1980
21st to endDay
6%7%45%6%3445%8%48%
2178
April1st to 10thDay
5%5%245%35%45%7%54%
2144
11th to 20thDay
45%35%435%5%6%8%53%
2131
21st to endDay
5%4245%345%6%8%55%
2160
May1st to 10thDay
6%424435%45%8%56%
2283
11th to 20thDay
5%426%445%5%6%8%51%
2301
21st to endDay
5%32443445%6%59%
2649
June1st to 10thDay
321332445%8%65%
2520
11th to 20thDay
321232444472%
2520
21st to endDay
2213325%5%5%7%64%
2520
July1st to 10thDay
33236%46%8%7%9%48%
2386
11th to 20thDay
3215%5%5%7%7%8%9%46%
2340
21st to endDay
6%5%35%7%48%8%8%10%36%
2545
August1st to 10thDay
5%426%6%46%8%8%9%42%
2328
11th to 20thDay
4425%6%46%5%7%10%46%
2229
21st to endDay
6%4245%35%6%7%11%47%
2376
September1st to 10thDay
7%424435%5%6%8%51%
2160
11th to 20thDay
6%3245%35%5%6%7%55%
2120
21st to endDay
6%313322446%67%
1914
October1st to 10thDay
9%413335%447%57%
1800
11th to 20thDay
5%212323225%73%
1800
21st to endDay
4413323335%68%
1980
November1st to 10thDay
432323336%70%
1859
11th to 20thDay
3313323345%69%
1750
21st to endDay
7%6%2333334463%
1520
December1st to 10thDay
7%7%24413347%57%
1520
11th to 20thDay
8%7%25%5%345%5%7%49%
1926
21st to endDay
5%8%35%424445%55%
2862
January1st to 10thNight
37%37%6%34445%53%
4097
11th to 20thNight
38%33424445%59%
4110
21st to endNight
5%9%35%5%2435%5%54%
4521
February1st to 10thNight
48%24413346%61%
2470
11th to 20thNight
7%7%24424346%58%
2364
21st to endNight
5%10%24333435%57%
1846
March1st to 10thNight
5%6%24434447%56%
2062
11th to 20thNight
6%7%35%42446%5%54%
1980
21st to endNight
7%6%25%434446%54%
2178
April1st to 10thNight
6%5%24435%45%7%55%
1814
11th to 20thNight
6%5%25%4235%5%6%57%
1777
21st to endNight
5%42442345%7%60%
1800
May1st to 10thNight
6%425%423447%59%
1674
11th to 20thNight
7%6%35%32345%7%54%
1594
21st to endNight
8%424424346%59%
1622
June1st to 10thNight
2413324348%67%
1440
11th to 20thNight
4213323345%70%
1440
21st to endNight
3213335%6%7%8%61%
1440
July1st to 10thNight
3435%6%46%7%7%9%48%
1568
11th to 20thNight
45%25%5%47%6%7%9%47%
1620
21st to endNight
5%5%26%7%49%8%9%11%33%
1773
August1st to 10thNight
4425%6%46%7%8%9%45%
1611
11th to 20thNight
5%5%25%5%446%8%9%48%
1670
21st to endNight
44245%36%6%7%10%49%
1980
September1st to 10thNight
6%425%446%6%7%8%48%
1800
11th to 20thNight
6%425%5%35%45%5%57%
1836
21st to endNight
8%31332223468%
2044
October1st to 10thNight
11%42332345%5%59%
2160
11th to 20thNight
421222333474%
2160
21st to endNight
5%313313245%70%
2376
November1st to 10thNight
5%32221325%6%69%
2220
11th to 20thNight
3322213346%72%
2403
21st to endNight
7%5%24322245%64%
2660
December1st to 10thNight
6%6%24323345%61%
2660
11th to 20thNight
7%8%2443445%7%51%
3271
21st to endNight
7%9%35%32333458%
4832

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
29%316%28%18%6%
2859
11th to 20thDay
27%6%14%34%16%4
2860
21st to endDay
31%6%9%31%20%3
3146
February1st to 10thDay
31%412%24%20%8%
1710
11th to 20thDay
34%38%24%26%4
1712
21st to endDay
35%46%18%29%8%
1604
March1st to 10thDay
33%5%6%24%25%7%
2058
11th to 20thDay
31%47%25%26%6%
1980
21st to endDay
33%5%7%22%26%6%
2178
April1st to 10thDay
28%5%6%21%27%14%
2144
11th to 20thDay
27%5%7%20%27%14%
2134
21st to endDay
25%5%7%22%30%11%
2160
May1st to 10thDay
25%47%19%32%11%
2283
11th to 20thDay
28%7%9%19%27%10%
2304
21st to endDay
22%6%6%15%30%20%
2649
June1st to 10thDay
17%5%7%17%30%25%
2520
11th to 20thDay
14%4411%25%41%
2520
21st to endDay
17%5%5%15%24%34%
2520
July1st to 10thDay
25%8%13%13%16%24%
2322
11th to 20thDay
26%10%13%20%17%14%
2271
21st to endDay
35%11%19%19%12%4
2404
August1st to 10thDay
32%11%13%14%14%16%
2313
11th to 20thDay
29%8%12%16%20%16%
2229
21st to endDay
27%7%12%19%23%12%
2376
September1st to 10thDay
28%7%8%16%23%18%
2160
11th to 20thDay
24%7%8%15%23%21%
2120
21st to endDay
19%4417%37%18%
1914
October1st to 10thDay
27%5%8%19%23%18%
1800
11th to 20thDay
16%3318%39%21%
1800
21st to endDay
19%37%24%35%11%
1980
November1st to 10thDay
17%36%28%37%10%
1859
11th to 20thDay
18%27%28%38%7%
1750
21st to endDay
27%26%26%28%11%
1520
December1st to 10thDay
28%314%27%19%8%
1520
11th to 20thDay
34%5%10%28%19%3
1924
21st to endDay
31%412%16%25%11%
2862
January1st to 10thNight
34%49%28%23%3
4097
11th to 20thNight
28%413%31%21%4
4110
21st to endNight
32%5%8%32%18%4
4521
February1st to 10thNight
25%311%27%23%10%
2470
11th to 20thNight
29%38%23%28%9%
2365
21st to endNight
30%36%19%31%11%
1846
March1st to 10thNight
28%45%25%29%9%
2062
11th to 20thNight
30%46%24%25%11%
1980
21st to endNight
32%36%22%28%8%
2178
April1st to 10thNight
29%45%20%27%15%
1814
11th to 20thNight
27%46%18%27%18%
1777
21st to endNight
24%36%20%32%15%
1800
May1st to 10thNight
25%36%19%34%13%
1674
11th to 20thNight
29%5%8%20%25%13%
1595
21st to endNight
26%5%8%15%25%21%
1622
June1st to 10thNight
18%36%15%28%30%
1440
11th to 20thNight
16%35%12%19%45%
1440
21st to endNight
18%6%6%12%23%35%
1440
July1st to 10thNight
30%7%13%13%11%26%
1523
11th to 20thNight
32%7%12%18%13%18%
1566
21st to endNight
38%11%19%16%9%7%
1674
August1st to 10thNight
31%11%14%13%13%17%
1602
11th to 20thNight
30%7%13%16%15%20%
1670
21st to endNight
27%7%13%21%18%14%
1980
September1st to 10thNight
30%7%10%17%16%21%
1800
11th to 20thNight
28%5%7%16%19%26%
1836
21st to endNight
21%3413%33%25%
2044
October1st to 10thNight
26%6%8%16%21%24%
2160
11th to 20thNight
16%3414%37%26%
2160
21st to endNight
18%2422%36%17%
2376
November1st to 10thNight
17%25%25%37%14%
2220
11th to 20thNight
16%25%28%38%10%
2403
21st to endNight
25%25%23%34%12%
2660
December1st to 10thNight
26%310%28%23%8%
2660
11th to 20thNight
34%49%24%24%5%
3272
21st to endNight
32%38%19%26%13%
4832

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
16%7%37%28%11%1
270
11th to 20thDay
17%14%35%21%12%1
270
21st to endDay
8%9%33%34%14%2
297
February1st to 10thDay
9%16%41%27%6%
270
11th to 20thDay
13%15%40%24%8%
270
21st to endDay
15%19%39%22%5%
259
March1st to 10thDay
12%20%38%22%8%
449
11th to 20thDay
11%21%31%24%11%2
420
21st to endDay
16%28%34%15%6%2
462
April1st to 10thDay
13%24%36%17%8%2
420
11th to 20thDay
17%27%30%18%7%
412
21st to endDay
13%25%37%18%6%
420
May1st to 10thDay
8%27%37%22%6%
423
11th to 20thDay
9%16%35%26%12%2
448
21st to endDay
8%16%32%25%16%3
604
June1st to 10thDay
47%27%39%21%3
630
11th to 20thDay
38%29%36%20%4
630
21st to endDay
36%26%40%21%4
630
July1st to 10thDay
4417%40%30%4
492
11th to 20thDay
4315%36%34%9%
444
21st to endDay
7%310%40%33%7%
487
August1st to 10thDay
7%5%14%39%28%6%
447
11th to 20thDay
5%323%39%23%7%
437
21st to endDay
6%218%43%24%6%
495
September1st to 10thDay
8%321%37%24%7%
450
11th to 20thDay
6%426%42%18%5%
450
21st to endDay
9%8%28%30%20%5%
384
October1st to 10thDay
10%14%20%25%23%9%
270
11th to 20thDay
8%13%25%34%16%4
270
21st to endDay
10%12%29%29%18%2
297
November1st to 10thDay
9%15%28%32%14%1
270
11th to 20thDay
6%10%38%30%14%3
270
21st to endDay
12%8%33%29%15%3
270
December1st to 10thDay
15%9%34%28%12%3
270
11th to 20thDay
11%9%35%30%12%3
270
21st to endDay
17%13%34%28%7%
297
January1st to 10thNight
8%428%39%17%4
760
11th to 20thNight
10%428%38%18%3
760
21st to endNight
10%5%25%38%18%3
836
February1st to 10thNight
10%5%32%39%11%2
760
11th to 20thNight
11%5%34%38%11%1
744
21st to endNight
11%5%38%38%7%
628
March1st to 10thNight
10%6%31%38%13%2
750
11th to 20thNight
9%6%25%35%22%2
720
21st to endNight
10%9%31%35%13%1
792
April1st to 10thNight
10%10%32%39%10%1
554
11th to 20thNight
10%13%24%38%14%1
533
21st to endNight
7%10%33%36%13%1
540
May1st to 10thNight
8%8%27%40%16%1
540
11th to 20thNight
10%423%41%20%2
531
21st to endNight
11%6%25%38%18%2
584
June1st to 10thNight
45%17%48%25%2
540
11th to 20thNight
5%319%46%24%2
540
21st to endNight
4216%43%33%2
540
July1st to 10thNight
5%18%41%41%4
525
11th to 20thNight
7%16%36%42%8%
522
21st to endNight
7%16%33%44%8%
591
August1st to 10thNight
43432%48%9%
537
11th to 20thNight
6%27%40%39%5%
534
21st to endNight
6%16%39%42%6%
594
September1st to 10thNight
7%27%41%37%5%
540
11th to 20thNight
7%29%46%31%5%
576
21st to endNight
8%214%41%29%5%
720
October1st to 10thNight
12%420%40%22%3
720
11th to 20thNight
5%220%47%23%4
720
21st to endNight
7%421%41%23%5%
792
November1st to 10thNight
7%318%46%23%4
740
11th to 20thNight
5%5%26%42%18%5%
879
21st to endNight
11%7%21%36%21%5%
950
December1st to 10thNight
10%6%27%42%13%2
950
11th to 20thNight
14%6%26%36%17%1
950
21st to endNight
14%6%27%40%12%1
1017
Page generated at 2024-03-05 05:23:58. Version ac07

Copyright © 2024 Attilla Danko.


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