Central Washington University Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Central Washington University Observatory clear sky chart from 2005-11-06 to 2024-03-01.

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%
22555
16%
80728
10%
22479
48%
22479
21%
50831

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
27%
753
9%
4376
3%
736
33%
736
5%
3306
January11th to 20thNight
29%
754
14%
4338
5%
740
36%
740
7%
2867
January21st to endNight
33%
826
5%
4862
7%
814
37%
814
6%
4114
February1st to 10thNight
29%
760
11%
2485
6%
760
36%
760
15%
1762
February11th to 20thNight
35%
741
14%
2404
9%
740
42%
740
19%
1603
February21st to endNight
35%
628
15%
1894
10%
628
43%
628
18%
1302
March1st to 10thNight
40%
722
16%
2165
10%
722
47%
722
22%
1444
March11th to 20thNight
42%
720
18%
2021
10%
720
50%
720
24%
1328
March21st to endNight
35%
792
14%
2044
5%
792
41%
792
21%
1439
April1st to 10thNight
36%
720
15%
1800
5%
720
40%
720
27%
1200
April11th to 20thNight
33%
711
19%
1779
6%
711
40%
711
30%
1133
April21st to endNight
31%
522
19%
1602
7%
522
42%
522
27%
944
May1st to 10thNight
34%
360
22%
1440
15%
360
53%
360
27%
715
May11th to 20thNight
41%
354
19%
1416
15%
354
56%
354
28%
729
May21st to endNight
36%
389
14%
1552
12%
388
50%
388
26%
820
June1st to 10thNight
33%
360
12%
1440
10%
360
44%
360
26%
755
June11th to 20thNight
24%
360
12%
1440
5%
360
39%
360
27%
676
June21st to endNight
38%
360
18%
1440
13%
360
56%
360
40%
582
July1st to 10thNight
38%
350
18%
1400
10%
350
59%
350
50%
513
July11th to 20thNight
29%
348
26%
1392
14%
348
62%
348
50%
482
July21st to endNight
43%
394
33%
1488
25%
372
76%
372
61%
469
August1st to 10thNight
43%
358
33%
1424
22%
356
69%
356
55%
509
August11th to 20thNight
55%
499
36%
1567
22%
499
68%
499
64%
633
August21st to endNight
46%
779
33%
1967
13%
779
57%
779
55%
983
September1st to 10thNight
48%
720
32%
1800
12%
720
57%
720
57%
900
September11th to 20thNight
55%
720
30%
1800
14%
720
62%
720
47%
1018
September21st to endNight
51%
720
28%
2019
17%
720
61%
720
42%
1085
October1st to 10thNight
50%
720
20%
2160
15%
720
58%
720
34%
1214
October11th to 20thNight
50%
720
23%
2160
18%
720
59%
720
31%
1261
October21st to endNight
41%
792
20%
2545
14%
792
51%
792
26%
1531
November1st to 10thNight
41%
740
15%
2443
10%
740
48%
740
19%
1644
November11th to 20thNight
35%
760
14%
2660
9%
760
43%
760
15%
1825
November21st to endNight
37%
760
12%
2660
6%
760
45%
760
16%
1820
December1st to 10thNight
34%
760
11%
2660
7%
760
41%
760
14%
1919
December11th to 20thNight
30%
759
4%
3276
3%
759
34%
759
7%
2683
December21st to endNight
27%
824
7%
4809
5%
817
35%
817
7%
3623

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
7%7%36%5%345%6%7%47%
78333
Night
8%9%36%5%345%5%7%44%
81040
TransparencyDay
35%8%14%23%16%4
78060
Night
39%9%15%22%13%3
80893
SeeingDay
12%17%30%26%13%3
13417
Night
10%12%32%30%14%2
22594

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
12%17%6%9%7%25%5%5%6%24%
2539
11th to 20thDay
15%15%47%5%35%5%6%7%28%
2516
21st to endDay
20%18%6%8%6%35%45%6%18%
2794
February1st to 10thDay
8%12%5%8%7%445%5%8%34%
1695
11th to 20thDay
8%10%37%5%446%7%8%38%
1665
21st to endDay
6%9%47%7%35%6%7%9%37%
1560
March1st to 10thDay
9%9%37%7%46%6%6%9%35%
1801
11th to 20thDay
8%9%38%8%5%6%5%5%8%35%
1938
21st to endDay
6%7%49%8%5%7%6%7%8%33%
2311
April1st to 10thDay
7%8%47%7%5%7%7%6%9%34%
2160
11th to 20thDay
6%9%36%6%46%5%7%8%40%
2132
21st to endDay
5%6%26%5%35%5%8%9%47%
2354
May1st to 10thDay
5%4245%35%6%6%9%52%
2520
11th to 20thDay
7%7%25%6%36%6%7%9%42%
2477
21st to endDay
8%7%35%6%35%6%6%8%44%
2723
June1st to 10thDay
7%426%5%45%6%5%7%48%
2520
11th to 20thDay
4424435%5%6%8%55%
2520
21st to endDay
331332346%7%64%
2520
July1st to 10thDay
221232345%6%71%
2520
11th to 20thDay
2212222346%74%
2520
21st to endDay
111121223382%
2741
August1st to 10thDay
2212212346%74%
2507
11th to 20thDay
2212212346%75%
2332
21st to endDay
2213324448%67%
2387
September1st to 10thDay
3213322347%71%
2160
11th to 20thDay
44245%445%6%6%56%
2160
21st to endDay
45%245%2445%7%57%
1936
October1st to 10thDay
7%6%246%3447%7%52%
1800
11th to 20thDay
6%5%25%5%4447%9%49%
1800
21st to endDay
10%8%26%45%45%5%7%44%
1810
November1st to 10thDay
9%9%36%7%46%5%6%9%35%
1636
11th to 20thDay
8%11%5%8%7%5%5%5%6%7%33%
1520
21st to endDay
9%11%48%7%46%7%6%7%33%
1520
December1st to 10thDay
11%12%6%7%6%45%45%8%34%
1520
11th to 20thDay
11%15%6%12%9%46%5%5%7%21%
1925
21st to endDay
10%15%8%11%8%5%5%5%45%24%
2794
January1st to 10thNight
11%18%410%7%45%6%6%6%23%
4406
11th to 20thNight
14%13%47%6%35%45%8%31%
4382
21st to endNight
17%17%6%12%9%35%445%18%
4862
February1st to 10thNight
8%12%48%6%45%5%6%9%34%
2485
11th to 20thNight
8%9%37%5%3446%7%43%
2402
21st to endNight
7%11%47%6%35%6%6%8%37%
1894
March1st to 10thNight
8%10%26%6%45%5%7%8%38%
2165
11th to 20thNight
5%6%36%6%46%6%8%9%41%
2020
21st to endNight
6%7%37%5%5%7%6%8%9%37%
2044
April1st to 10thNight
7%5%445%45%6%7%10%42%
1800
11th to 20thNight
6%5%26%5%25%5%47%52%
1779
21st to endNight
5%5%35%5%446%49%50%
1602
May1st to 10thNight
4324435%45%8%57%
1440
11th to 20thNight
6%4244446%6%10%50%
1416
21st to endNight
5%425%5%345%6%10%49%
1552
June1st to 10thNight
5%423426%5%7%9%52%
1440
11th to 20thNight
43143445%5%8%59%
1440
21st to endNight
221332435%7%67%
1440
July1st to 10thNight
2112212336%76%
1440
11th to 20thNight
111221134480%
1440
21st to endNight
11111223386%
1576
August1st to 10thNight
2212212346%76%
1432
11th to 20thNight
1112212246%79%
1567
21st to endNight
1112323447%72%
1967
September1st to 10thNight
3212212336%75%
1800
11th to 20thNight
33133345%5%6%63%
1800
21st to endNight
431343446%7%62%
2019
October1st to 10thNight
5%5%35%5%345%5%6%56%
2160
11th to 20thNight
6%4246%35%5%7%7%51%
2160
21st to endNight
10%5%26%5%3446%8%47%
2545
November1st to 10thNight
11%8%36%6%35%6%7%8%38%
2443
11th to 20thNight
9%8%47%6%45%5%6%8%39%
2660
21st to endNight
9%11%5%7%7%445%6%7%37%
2660
December1st to 10thNight
11%13%5%8%6%35%5%6%6%34%
2660
11th to 20thNight
13%14%6%13%9%45%5%46%21%
3280
21st to endNight
12%18%7%9%6%345%5%6%25%
4862

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
60%9%12%14%4
2539
11th to 20thDay
55%9%17%17%2
2540
21st to endDay
67%10%12%8%3
2794
February1st to 10thDay
50%11%18%16%41
1695
11th to 20thDay
44%9%18%20%8%1
1674
21st to endDay
40%13%19%19%8%1
1560
March1st to 10thDay
42%10%19%19%10%1
1801
11th to 20thDay
46%6%15%24%8%
1938
21st to endDay
45%12%19%20%4
2311
April1st to 10thDay
44%10%15%20%10%
2160
11th to 20thDay
38%8%15%24%12%2
2132
21st to endDay
31%9%16%26%16%2
2354
May1st to 10thDay
26%7%17%33%15%2
2520
11th to 20thDay
34%8%17%26%14%1
2480
21st to endDay
35%7%17%28%11%1
2724
June1st to 10thDay
33%8%14%27%14%4
2520
11th to 20thDay
25%6%15%33%18%2
2520
21st to endDay
19%5%13%33%22%8%
2520
July1st to 10thDay
14%311%26%29%16%
2451
11th to 20thDay
13%47%20%36%21%
2445
21st to endDay
8%25%21%41%22%
2589
August1st to 10thDay
13%310%24%35%15%
2490
11th to 20thDay
12%39%25%40%10%
2332
21st to endDay
16%6%10%29%33%7%
2387
September1st to 10thDay
15%410%30%30%10%
2160
11th to 20thDay
26%7%13%22%24%7%
2160
21st to endDay
27%6%16%27%19%5%
1936
October1st to 10thDay
30%6%15%31%16%2
1800
11th to 20thDay
31%7%20%28%13%1
1800
21st to endDay
39%7%15%24%14%1
1810
November1st to 10thDay
45%9%16%20%8%1
1636
11th to 20thDay
49%11%18%16%5%1
1520
21st to endDay
48%12%16%15%8%1
1520
December1st to 10thDay
50%9%17%18%6%
1520
11th to 20thDay
60%14%14%10%3
1928
21st to endDay
62%9%12%13%5%
2794
January1st to 10thNight
59%12%14%10%5%
4406
11th to 20thNight
50%9%17%19%4
4420
21st to endNight
68%11%10%9%21
4862
February1st to 10thNight
48%10%20%16%5%
2485
11th to 20thNight
42%9%18%21%9%1
2404
21st to endNight
43%10%17%20%10%
1894
March1st to 10thNight
41%9%17%23%9%1
2165
11th to 20thNight
37%10%17%27%8%1
2021
21st to endNight
39%12%19%23%6%
2044
April1st to 10thNight
36%9%20%23%11%1
1800
11th to 20thNight
32%7%17%26%15%1
1779
21st to endNight
32%8%17%28%13%2
1602
May1st to 10thNight
26%6%17%33%16%2
1440
11th to 20thNight
28%6%19%31%14%1
1416
21st to endNight
29%8%19%32%12%1
1552
June1st to 10thNight
28%8%16%30%14%3
1440
11th to 20thNight
22%7%20%34%15%1
1440
21st to endNight
17%415%37%21%6%
1440
July1st to 10thNight
12%314%28%30%13%
1400
11th to 20thNight
9%39%27%31%20%
1392
21st to endNight
6%36%22%41%22%
1488
August1st to 10thNight
11%310%24%35%17%
1424
11th to 20thNight
9%410%31%34%11%
1567
21st to endNight
12%5%11%29%34%8%
1967
September1st to 10thNight
14%312%30%30%11%
1800
11th to 20thNight
21%6%15%24%25%9%
1800
21st to endNight
22%6%15%31%19%6%
2019
October1st to 10thNight
29%5%13%32%20%1
2160
11th to 20thNight
30%8%16%30%15%2
2160
21st to endNight
35%7%15%27%15%1
2545
November1st to 10thNight
43%10%15%20%10%1
2443
11th to 20thNight
44%12%17%20%6%1
2660
21st to endNight
47%9%17%18%9%
2660
December1st to 10thNight
51%10%15%19%4
2660
11th to 20thNight
59%16%13%8%3
3276
21st to endNight
59%9%13%13%5%
4862

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
25%12%22%23%14%4
180
11th to 20thDay
23%13%23%25%12%4
180
21st to endDay
19%8%27%28%16%2
198
February1st to 10thDay
21%18%33%21%7%1
355
11th to 20thDay
20%19%36%19%5%1
365
21st to endDay
15%19%36%21%8%1
377
March1st to 10thDay
15%22%33%22%7%1
421
11th to 20thDay
15%20%30%24%11%1
420
21st to endDay
18%23%31%22%6%
462
April1st to 10thDay
17%28%26%21%6%2
420
11th to 20thDay
13%29%28%19%7%4
412
21st to endDay
13%27%30%22%7%1
420
May1st to 10thDay
13%19%35%24%8%1
423
11th to 20thDay
15%16%28%26%12%3
448
21st to endDay
12%16%31%26%12%2
485
June1st to 10thDay
12%21%30%22%13%2
450
11th to 20thDay
12%25%38%18%6%1
450
21st to endDay
6%20%29%26%16%3
450
July1st to 10thDay
5%13%34%35%11%1
446
11th to 20thDay
417%35%29%13%2
444
21st to endDay
212%31%33%19%3
487
August1st to 10thDay
5%12%28%34%18%4
447
11th to 20thDay
39%28%34%21%5%
437
21st to endDay
5%16%27%32%14%5%
495
September1st to 10thDay
5%15%31%28%16%5%
450
11th to 20thDay
6%14%28%30%20%2
450
21st to endDay
10%18%28%25%18%2
450
October1st to 10thDay
13%14%30%23%16%4
450
11th to 20thDay
12%16%25%26%15%6%
450
21st to endDay
16%15%26%26%13%4
325
November1st to 10thDay
17%16%29%27%10%1
252
11th to 20thDay
16%15%36%13%16%5%
180
21st to endDay
16%11%28%23%18%3
180
December1st to 10thDay
21%9%22%21%16%10%
180
11th to 20thDay
15%11%32%31%11%1
180
21st to endDay
15%8%34%25%17%2
198
January1st to 10thNight
19%8%28%28%15%2
760
11th to 20thNight
16%8%28%30%17%2
760
21st to endNight
18%7%24%33%16%3
836
February1st to 10thNight
14%12%33%27%12%1
760
11th to 20thNight
13%12%31%31%13%1
744
21st to endNight
13%14%29%29%13%2
628
March1st to 10thNight
12%9%31%35%12%1
722
11th to 20thNight
8%12%31%33%16%1
720
21st to endNight
10%16%33%30%9%1
792
April1st to 10thNight
10%14%35%29%12%1
720
11th to 20thNight
9%18%35%28%9%2
711
21st to endNight
8%18%39%24%10%1
522
May1st to 10thNight
7%19%37%29%8%1
360
11th to 20thNight
7%11%36%31%13%1
354
21st to endNight
8%11%39%29%12%1
389
June1st to 10thNight
9%13%43%25%10%
360
11th to 20thNight
7%23%43%23%4
360
21st to endNight
416%38%28%13%1
360
July1st to 10thNight
315%42%31%8%
350
11th to 20thNight
119%50%22%6%1
348
21st to endNight
217%37%33%10%2
394
August1st to 10thNight
211%42%30%14%1
358
11th to 20thNight
18%33%34%18%5%
499
21st to endNight
315%34%28%17%3
779
September1st to 10thNight
414%31%29%19%3
720
11th to 20thNight
5%9%27%28%24%6%
720
21st to endNight
6%11%29%29%21%4
720
October1st to 10thNight
8%10%27%32%21%2
720
11th to 20thNight
9%10%25%32%21%4
720
21st to endNight
13%12%29%29%13%4
792
November1st to 10thNight
14%7%33%34%12%1
740
11th to 20thNight
14%11%31%27%14%2
760
21st to endNight
13%8%28%31%16%4
760
December1st to 10thNight
15%9%26%32%15%3
760
11th to 20thNight
19%7%28%33%12%1
760
21st to endNight
18%10%28%34%9%1
836
Page generated at 2024-02-27 03:30:17. Version ac07

Copyright © 2024 Attilla Danko.


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