The W.M. Keck Observatory at Pacific Lutheran University
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The W.M. Keck Observatory at Pacific Lutheran University clear sky chart from 2005-11-06 to 2024-02-25.

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
41%
22586
10%
80282
10%
22499
43%
22499
13%
60131

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
25%
780
4%
4500
6%
761
27%
761
4%
3995
January11th to 20thNight
32%
751
12%
4358
8%
740
34%
740
6%
3247
January21st to endNight
31%
832
5%
4729
8%
814
33%
814
5%
3993
February1st to 10thNight
29%
760
6%
2470
6%
760
30%
760
9%
2037
February11th to 20thNight
31%
740
8%
2404
7%
740
34%
740
10%
1930
February21st to endNight
31%
610
10%
1920
8%
610
34%
610
10%
1539
March1st to 10thNight
34%
720
8%
2106
7%
720
35%
720
14%
1648
March11th to 20thNight
30%
720
11%
1980
8%
720
32%
720
13%
1588
March21st to endNight
28%
792
5%
2114
3%
792
28%
792
8%
1822
April1st to 10thNight
32%
720
6%
1800
2%
720
33%
720
12%
1508
April11th to 20thNight
36%
689
10%
1757
6%
689
38%
689
16%
1388
April21st to endNight
40%
518
8%
1598
7%
518
41%
518
13%
1238
May1st to 10thNight
45%
360
10%
1440
14%
360
47%
360
15%
985
May11th to 20thNight
49%
354
9%
1416
14%
354
50%
354
13%
1002
May21st to endNight
47%
389
6%
1552
9%
388
49%
388
11%
1130
June1st to 10thNight
47%
360
6%
1434
7%
360
50%
360
15%
976
June11th to 20thNight
44%
360
4%
1260
6%
360
45%
360
14%
922
June21st to endNight
58%
360
11%
1260
13%
360
60%
360
26%
754
July1st to 10thNight
69%
350
13%
1225
17%
350
71%
350
30%
695
July11th to 20thNight
71%
348
18%
1359
29%
348
73%
348
31%
701
July21st to endNight
78%
394
24%
1488
37%
372
81%
372
46%
628
August1st to 10thNight
75%
421
20%
1487
24%
419
76%
419
38%
739
August11th to 20thNight
78%
534
25%
1602
23%
534
79%
534
46%
831
August21st to endNight
67%
702
23%
1890
17%
702
68%
702
37%
1136
September1st to 10thNight
67%
720
21%
1800
13%
720
67%
720
40%
1115
September11th to 20thNight
58%
720
19%
1800
12%
720
59%
720
29%
1253
September21st to endNight
52%
720
17%
2055
15%
720
53%
720
26%
1390
October1st to 10thNight
51%
720
12%
2160
12%
720
52%
720
21%
1518
October11th to 20thNight
45%
720
13%
2160
13%
720
47%
720
18%
1582
October21st to endNight
36%
792
13%
2468
10%
792
41%
792
16%
1814
November1st to 10thNight
32%
740
7%
2540
7%
740
34%
740
10%
2021
November11th to 20thNight
29%
760
8%
2660
8%
760
31%
760
9%
2123
November21st to endNight
31%
760
6%
2660
5%
760
33%
760
10%
2092
December1st to 10thNight
35%
760
9%
2660
9%
760
36%
760
10%
2095
December11th to 20thNight
27%
759
4%
3259
4%
759
27%
759
6%
2629
December21st to endNight
22%
851
3%
4911
4%
837
23%
837
3%
4067

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
22%9%47%6%35%45%6%29%
78648
Night
23%9%37%6%3445%6%30%
80542
TransparencyDay
54%9%13%16%7%1
78368
Night
55%9%13%15%7%1
80402
SeeingDay
23%11%20%25%17%5%
14042
Night
26%8%16%26%21%3
22619

By Month

Percentage Cloud Cover NW (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
37%13%38%8%45%43410%
2409
11th to 20thDay
27%10%48%6%34446%23%
2516
21st to endDay
39%14%49%6%35%44410%
2920
February1st to 10thDay
31%11%5%7%5%35%5%6%6%16%
1710
11th to 20thDay
32%12%5%6%6%34446%17%
1665
21st to endDay
28%12%47%7%345%5%5%19%
1429
March1st to 10thDay
25%11%5%8%7%46%5%5%6%18%
1854
11th to 20thDay
31%11%37%6%44445%19%
1980
21st to endDay
26%12%5%8%8%46%6%6%6%13%
2241
April1st to 10thDay
28%11%47%7%5%5%5%6%7%17%
2160
11th to 20thDay
25%10%49%6%35%5%5%6%24%
2149
21st to endDay
21%11%5%8%7%46%5%5%7%23%
2362
May1st to 10thDay
18%9%36%6%46%5%6%7%31%
2520
11th to 20thDay
19%9%37%6%47%47%7%26%
2477
21st to endDay
22%9%48%7%5%5%6%5%6%25%
2723
June1st to 10thDay
21%9%46%6%35%5%6%7%27%
2521
11th to 20thDay
21%9%48%7%46%7%5%7%22%
2700
21st to endDay
13%7%36%6%345%6%8%40%
2700
July1st to 10thDay
11%6%245%3445%6%50%
2700
11th to 20thDay
11%5%2432445%8%51%
2552
21st to endDay
8%323323347%62%
2741
August1st to 10thDay
8%5%2442445%7%54%
2441
11th to 20thDay
8%5%1432336%9%56%
2298
21st to endDay
10%6%36%5%3436%8%46%
2466
September1st to 10thDay
11%5%25%5%34447%50%
2160
11th to 20thDay
13%6%47%6%345%5%6%39%
2160
21st to endDay
18%7%36%5%3445%8%39%
1900
October1st to 10thDay
20%9%38%5%4445%6%31%
1800
11th to 20thDay
23%8%39%6%3445%6%28%
1800
21st to endDay
30%9%36%7%33446%24%
1886
November1st to 10thDay
31%11%47%5%35%5%5%5%18%
1534
11th to 20thDay
31%11%46%6%35%45%7%19%
1520
21st to endDay
30%10%47%6%3446%6%19%
1520
December1st to 10thDay
27%9%46%8%35%45%5%24%
1520
11th to 20thDay
32%13%47%8%5%6%5%5%5%12%
1925
21st to endDay
26%13%5%9%8%5%5%5%5%5%13%
2689
January1st to 10thNight
34%12%49%7%45%44412%
4507
11th to 20thNight
28%10%47%5%3445%7%23%
4382
21st to endNight
37%12%48%6%44445%13%
4738
February1st to 10thNight
31%11%47%6%44447%19%
2470
11th to 20thNight
27%11%37%7%35%445%24%
2402
21st to endNight
26%12%37%5%45%5%5%5%23%
1920
March1st to 10thNight
26%11%46%6%25%5%5%6%24%
2106
11th to 20thNight
27%10%47%6%3445%6%22%
1979
21st to endNight
27%11%48%8%45%5%5%5%17%
2114
April1st to 10thNight
23%11%47%5%46%5%5%7%24%
1800
11th to 20thNight
18%8%37%6%45%5%6%8%29%
1757
21st to endNight
20%8%37%7%345%5%7%30%
1598
May1st to 10thNight
16%8%27%6%446%5%7%36%
1440
11th to 20thNight
14%8%47%5%46%5%6%8%34%
1416
21st to endNight
14%7%37%8%46%5%6%6%34%
1552
June1st to 10thNight
15%6%26%6%35%5%5%9%38%
1434
11th to 20thNight
17%6%47%5%435%5%9%33%
1260
21st to endNight
10%435%5%2446%6%51%
1260
July1st to 10thNight
7%423433448%58%
1260
11th to 20thNight
6%225%324436%64%
1405
21st to endNight
4221213337%72%
1576
August1st to 10thNight
5%313323349%63%
1495
11th to 20thNight
5%212323346%69%
1602
21st to endNight
9%5%1442435%6%57%
1890
September1st to 10thNight
8%413333446%60%
1800
11th to 20thNight
14%5%35%5%35%346%47%
1800
21st to endNight
16%6%26%5%33447%44%
2055
October1st to 10thNight
18%7%35%6%45%45%6%39%
2160
11th to 20thNight
23%8%36%5%3435%5%35%
2160
21st to endNight
27%7%36%5%3335%6%33%
2468
November1st to 10thNight
32%9%35%6%35%45%5%23%
2540
11th to 20thNight
31%9%36%6%35%446%23%
2660
21st to endNight
27%10%47%6%4446%5%23%
2660
December1st to 10thNight
29%9%47%6%34435%25%
2660
11th to 20thNight
28%12%49%7%45%5%5%417%
3280
21st to endNight
28%13%6%10%7%45%445%15%
4936

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
76%10%6%6%21
2409
11th to 20thDay
60%10%13%13%41
2540
21st to endDay
75%7%8%8%3
2920
February1st to 10thDay
67%9%10%9%4
1710
11th to 20thDay
69%7%8%11%5%1
1674
21st to endDay
65%10%10%10%4
1429
March1st to 10thDay
65%8%11%13%2
1854
11th to 20thDay
68%7%10%12%21
1980
21st to endDay
70%11%10%8%1
2241
April1st to 10thDay
66%10%13%9%1
2160
11th to 20thDay
61%8%12%12%6%1
2149
21st to endDay
62%9%13%11%5%
2362
May1st to 10thDay
52%9%17%17%5%
2520
11th to 20thDay
55%10%17%14%4
2480
21st to endDay
58%11%15%13%3
2724
June1st to 10thDay
57%10%17%12%5%
2521
11th to 20thDay
61%10%16%11%3
2700
21st to endDay
43%10%18%19%8%2
2700
July1st to 10thDay
36%9%14%25%14%2
2626
11th to 20thDay
32%8%12%30%17%1
2475
21st to endDay
22%6%13%37%18%3
2589
August1st to 10thDay
30%8%16%29%15%2
2424
11th to 20thDay
26%8%18%31%14%3
2298
21st to endDay
37%8%13%31%10%1
2466
September1st to 10thDay
33%6%16%26%13%5%
2160
11th to 20thDay
42%10%14%17%14%3
2160
21st to endDay
44%9%16%18%11%1
1900
October1st to 10thDay
53%7%13%19%8%1
1800
11th to 20thDay
54%7%17%16%6%1
1800
21st to endDay
61%7%10%14%8%
1886
November1st to 10thDay
66%9%9%12%3
1534
11th to 20thDay
65%11%9%7%6%2
1520
21st to endDay
63%9%10%10%9%
1520
December1st to 10thDay
61%8%9%12%8%1
1520
11th to 20thDay
67%9%12%9%21
1928
21st to endDay
68%10%13%7%2
2689
January1st to 10thNight
75%9%7%5%21
4507
11th to 20thNight
58%11%11%16%31
4420
21st to endNight
71%8%9%8%41
4738
February1st to 10thNight
67%7%12%8%5%
2470
11th to 20thNight
62%10%11%10%7%1
2404
21st to endNight
62%10%12%12%5%
1920
March1st to 10thNight
60%7%14%15%4
2106
11th to 20thNight
62%7%11%15%41
1980
21st to endNight
67%12%12%9%1
2114
April1st to 10thNight
60%13%15%10%2
1800
11th to 20thNight
55%11%14%13%6%1
1757
21st to endNight
55%12%15%13%5%1
1598
May1st to 10thNight
48%10%16%17%8%
1440
11th to 20thNight
49%12%18%15%5%2
1416
21st to endNight
50%14%16%16%3
1552
June1st to 10thNight
46%11%18%20%5%
1434
11th to 20thNight
51%11%22%13%2
1260
21st to endNight
35%8%24%21%8%3
1260
July1st to 10thNight
29%10%15%28%14%5%
1225
11th to 20thNight
25%10%11%31%20%2
1359
21st to endNight
14%9%13%38%21%5%
1488
August1st to 10thNight
22%8%16%31%19%3
1487
11th to 20thNight
20%7%20%34%15%4
1602
21st to endNight
29%9%15%34%13%1
1890
September1st to 10thNight
28%9%16%27%16%5%
1800
11th to 20thNight
41%9%11%21%14%5%
1800
21st to endNight
41%9%13%22%12%2
2055
October1st to 10thNight
46%8%12%24%8%1
2160
11th to 20thNight
51%8%14%19%7%1
2160
21st to endNight
54%7%13%17%9%
2468
November1st to 10thNight
62%9%13%11%41
2540
11th to 20thNight
64%7%11%11%5%2
2660
21st to endNight
62%8%11%10%8%
2660
December1st to 10thNight
63%7%10%14%6%1
2660
11th to 20thNight
66%9%15%7%2
3276
21st to endNight
67%12%10%8%2
4936

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
34%13%16%18%14%5%
180
11th to 20thDay
33%13%20%17%14%3
180
21st to endDay
36%13%18%15%13%4
272
February1st to 10thDay
36%12%18%19%12%3
370
11th to 20thDay
42%14%18%14%11%1
365
21st to endDay
32%17%22%20%7%2
321
March1st to 10thDay
31%15%24%19%9%1
420
11th to 20thDay
29%14%19%22%15%2
420
21st to endDay
31%15%24%21%8%2
462
April1st to 10thDay
34%14%19%20%10%4
420
11th to 20thDay
27%16%26%17%7%7%
412
21st to endDay
22%16%27%21%11%2
420
May1st to 10thDay
21%11%26%24%15%4
423
11th to 20thDay
27%9%19%24%15%5%
448
21st to endDay
23%9%23%25%16%4
485
June1st to 10thDay
24%9%23%23%16%6%
451
11th to 20thDay
21%12%25%27%12%3
630
21st to endDay
12%8%20%32%24%4
630
July1st to 10thDay
12%6%18%33%27%5%
621
11th to 20thDay
9%422%34%27%5%
474
21st to endDay
7%414%37%33%6%
487
August1st to 10thDay
11%413%33%29%10%
447
11th to 20thDay
6%313%38%30%10%
437
21st to endDay
13%8%20%32%20%7%
495
September1st to 10thDay
12%6%25%32%19%5%
450
11th to 20thDay
16%8%18%31%21%6%
450
21st to endDay
18%10%23%27%19%4
450
October1st to 10thDay
27%10%17%21%17%8%
450
11th to 20thDay
28%12%14%22%17%7%
450
21st to endDay
32%15%16%19%12%6%
401
November1st to 10thDay
35%16%14%19%13%2
203
11th to 20thDay
33%18%15%11%15%8%
180
21st to endDay
29%13%22%16%9%11%
180
December1st to 10thDay
42%13%12%12%9%12%
180
11th to 20thDay
38%17%14%18%11%2
180
21st to endDay
36%13%16%18%15%2
198
January1st to 10thNight
41%13%14%16%13%3
788
11th to 20thNight
36%10%12%21%18%3
760
21st to endNight
33%8%15%23%17%3
836
February1st to 10thNight
37%8%15%24%14%2
760
11th to 20thNight
33%10%15%23%17%2
744
21st to endNight
33%10%19%24%12%1
610
March1st to 10thNight
31%12%16%23%15%3
720
11th to 20thNight
29%13%17%24%15%2
720
21st to endNight
30%13%19%26%11%2
792
April1st to 10thNight
28%8%21%24%16%2
720
11th to 20thNight
23%11%21%26%17%2
689
21st to endNight
22%11%17%34%15%2
518
May1st to 10thNight
18%9%19%32%20%2
360
11th to 20thNight
20%5%18%30%26%2
354
21st to endNight
17%8%17%28%27%4
389
June1st to 10thNight
15%8%21%29%26%1
360
11th to 20thNight
19%8%19%31%21%1
360
21st to endNight
11%5%20%34%29%2
360
July1st to 10thNight
9%314%33%37%5%
350
11th to 20thNight
6%411%43%33%3
348
21st to endNight
5%211%34%47%1
394
August1st to 10thNight
7%410%30%43%7%
421
11th to 20thNight
6%29%27%47%8%
534
21st to endNight
11%412%34%35%4
702
September1st to 10thNight
11%313%35%33%4
720
11th to 20thNight
16%5%13%25%34%9%
720
21st to endNight
18%6%15%31%26%3
720
October1st to 10thNight
21%6%13%28%27%5%
720
11th to 20thNight
26%8%14%25%21%6%
720
21st to endNight
31%7%18%23%18%3
792
November1st to 10thNight
36%8%19%25%11%1
740
11th to 20thNight
34%12%16%22%13%3
760
21st to endNight
32%10%19%20%15%4
760
December1st to 10thNight
31%8%16%26%17%3
760
11th to 20thNight
37%10%17%23%12%2
760
21st to endNight
43%11%17%19%10%1
858
Page generated at 2024-02-22 03:05:44. Version ac07

Copyright 2024 Attilla Danko


The W.M. Keck Observatory at Pacific Lutheran University Clear Sky Chart |Charts Home