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 2005-11-05 to 2017-10-16.

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
42%
14640
16%
47189
10%
14615
50%
14615
25%
29041

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
32%
716
8%
2506
5%
716
37%
716
11%
1933
January16th to endNight
38%
768
7%
2688
6%
768
40%
768
12%
2046
February1st to 15thNight
38%
697
9%
2297
7%
696
42%
696
16%
1638
February16th to endNight
41%
632
15%
1960
11%
632
48%
632
21%
1324
March1st to 15thNight
39%
720
12%
2127
7%
720
45%
720
21%
1464
March16th to endNight
38%
768
13%
2022
4%
768
44%
768
24%
1388
April1st to 15thNight
36%
719
16%
1799
6%
719
41%
719
29%
1187
April16th to endNight
36%
585
19%
1665
7%
585
46%
585
32%
976
May1st to 15thNight
41%
359
22%
1439
20%
359
54%
359
29%
708
May16th to endNight
43%
374
13%
1496
12%
374
54%
374
25%
808
June1st to 15thNight
35%
360
11%
1440
7%
360
45%
360
29%
717
June16th to endNight
32%
360
14%
1440
8%
360
48%
360
35%
639
July1st to 15thNight
50%
340
17%
1360
14%
340
65%
340
54%
486
July16th to endNight
45%
376
28%
1416
24%
354
72%
354
61%
466
August1st to 15thNight
58%
396
32%
1468
25%
394
76%
394
62%
527
August16th to endNight
56%
698
34%
1844
17%
698
65%
698
61%
879
September1st to 15thNight
59%
720
31%
1800
13%
720
66%
720
63%
883
September16th to endNight
56%
716
29%
1947
19%
716
63%
716
43%
1066
October1st to 15thNight
50%
720
20%
2160
16%
720
56%
720
33%
1250
October16th to endNight
47%
708
17%
2228
12%
708
55%
708
24%
1418
November1st to 15thNight
41%
700
12%
2359
8%
700
48%
700
18%
1609
November16th to endNight
36%
720
12%
2520
7%
720
44%
720
15%
1790
December1st to 15thNight
38%
720
11%
2520
8%
720
42%
720
14%
1833
December16th to endNight
28%
768
10%
2688
3%
768
36%
768
12%
2006

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
6%6%25%5%3445%7%48%
48256
Night
6%6%25%43445%7%47%
47357
Cloud CoverDay
6%6%25%5%2445%7%48%
48256
Night
7%7%35%42445%7%46%
47361
TransparencyDay
34%7%15%23%15%4
47927
Night
36%7%16%23%12%3
47190
SeeingDay
9%17%30%26%12%2
5579
Night
10%8%30%31%15%2
14653

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 15thDay
9%14%5%9%8%36%5%47%23%
1432
16th to endDay
10%12%5%7%7%45%46%8%26%
1536
February1st to 15thDay
8%12%46%6%45%5%7%8%30%
1567
16th to endDay
6%7%26%6%346%7%8%38%
1514
March1st to 15thDay
10%8%37%7%46%5%6%9%29%
1833
16th to endDay
7%8%47%8%47%5%6%7%32%
2202
April1st to 15thDay
6%8%37%7%47%5%5%9%34%
2149
16th to endDay
46%25%5%35%5%7%8%45%
2290
May1st to 15thDay
5%5%145%345%6%9%49%
2511
16th to endDay
8%6%245%36%6%6%8%42%
2615
June1st to 15thDay
6%3145%345%5%7%51%
2520
16th to endDay
441343435%8%57%
2520
July1st to 15thDay
111213346%72%
2520
16th to endDay
21121223475%
2657
August1st to 15thDay
11221225%7%72%
2483
16th to endDay
112323437%68%
2357
September1st to 15thDay
2112322345%69%
2160
16th to endDay
4413425%46%6%55%
2010
October1st to 15thDay
7%6%245%3347%8%46%
1800
16th to endDay
9%7%26%44445%7%41%
1664
November1st to 15thDay
8%6%36%7%46%5%6%10%33%
1500
16th to endDay
9%11%37%6%5%6%6%6%7%28%
1440
December1st to 15thDay
9%12%7%7%6%35%35%6%31%
1440
16th to endDay
11%12%48%6%34347%32%
1536
January1st to 15thNight
10%13%48%7%346%6%7%26%
2506
16th to endNight
10%10%5%11%7%45%45%7%26%
2688
February1st to 15thNight
10%11%37%5%3445%8%34%
2285
16th to endNight
7%8%35%6%35%5%6%8%40%
1960
March1st to 15thNight
8%7%26%6%35%5%7%9%36%
2127
16th to endNight
5%5%26%5%47%6%8%9%38%
2018
April1st to 15thNight
6%4345%36%5%6%9%44%
1799
16th to endNight
5%4144345%48%53%
1665
May1st to 15thNight
5%32343435%7%57%
1439
16th to endNight
5%425%4345%6%11%47%
1496
June1st to 15thNight
4313325%46%8%55%
1440
16th to endNight
331332445%7%60%
1440
July1st to 15thNight
111111235%77%
1440
16th to endNight
11123382%
1512
August1st to 15thNight
1111346%77%
1476
16th to endNight
11312346%74%
1844
September1st to 15thNight
211212225%77%
1800
16th to endNight
321332445%7%61%
1947
October1st to 15thNight
5%425%5%2445%6%52%
2160
16th to endNight
8%426%6%345%7%8%42%
2228
November1st to 15thNight
9%6%25%5%35%6%7%8%37%
2359
16th to endNight
8%10%5%6%6%445%5%6%35%
2520
December1st to 15thNight
10%11%47%6%35%45%6%33%
2520
16th to endNight
12%15%5%8%434446%29%
2688

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
11%14%6%8%8%45%6%47%21%
1432
16th to endDay
11%12%5%7%7%36%45%7%27%
1536
February1st to 15thDay
10%12%47%6%26%5%6%7%29%
1567
16th to endDay
7%7%46%5%345%8%8%37%
1514
March1st to 15thDay
12%9%37%5%35%6%6%9%29%
1833
16th to endDay
7%8%36%7%47%6%7%8%32%
2202
April1st to 15thDay
6%8%46%6%47%5%6%9%34%
2149
16th to endDay
45%35%5%35%46%8%46%
2290
May1st to 15thDay
6%3145%345%7%9%49%
2511
16th to endDay
7%5%25%5%35%5%6%9%42%
2615
June1st to 15thDay
6%2145%245%7%7%51%
2520
16th to endDay
4313423448%58%
2520
July1st to 15thDay
111212336%73%
2520
16th to endDay
21121223476%
2657
August1st to 15thDay
111212246%73%
2483
16th to endDay
1212223346%68%
2357
September1st to 15thDay
2212213246%70%
2160
16th to endDay
5%3135%335%6%6%55%
2010
October1st to 15thDay
7%5%245%3447%8%46%
1800
16th to endDay
9%7%26%6%335%6%6%41%
1664
November1st to 15thDay
8%8%36%7%36%6%6%8%33%
1500
16th to endDay
12%10%5%8%7%35%5%5%7%27%
1440
December1st to 15thDay
10%13%5%8%6%35%45%6%31%
1440
16th to endDay
13%13%48%5%2445%7%31%
1536
January1st to 15thNight
13%13%48%6%36%45%7%26%
2506
16th to endNight
11%11%6%9%6%45%5%5%7%26%
2688
February1st to 15thNight
11%12%47%43445%7%33%
2285
16th to endNight
7%9%25%6%35%5%6%7%39%
1960
March1st to 15thNight
8%8%46%5%25%6%8%7%35%
2127
16th to endNight
6%5%45%5%35%5%7%9%39%
2022
April1st to 15thNight
6%5%345%35%6%6%9%43%
1799
16th to endNight
5%5%23325%5%48%52%
1665
May1st to 15thNight
5%313424347%57%
1439
16th to endNight
5%415%4345%6%9%48%
1496
June1st to 15thNight
4423315%5%7%7%55%
1440
16th to endNight
231242445%6%62%
1440
July1st to 15thNight
211112235%77%
1440
16th to endNight
11122483%
1512
August1st to 15thNight
11111346%78%
1476
16th to endNight
111212346%73%
1844
September1st to 15thNight
221211235%75%
1800
16th to endNight
431332345%7%60%
1947
October1st to 15thNight
6%5%3442435%6%52%
2160
16th to endNight
8%5%26%4345%7%9%41%
2228
November1st to 15thNight
10%7%26%6%35%6%6%8%37%
2359
16th to endNight
10%10%5%7%6%35%35%6%34%
2520
December1st to 15thNight
11%10%5%7%6%345%6%6%33%
2520
16th to endNight
14%14%5%7%5%3445%5%28%
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
61%11%11%10%31
1432
16th to endDay
56%10%17%12%3
1536
February1st to 15thDay
51%12%18%14%2
1567
16th to endDay
40%12%21%18%6%
1514
March1st to 15thDay
48%9%19%15%6%
1833
16th to endDay
47%9%19%18%4
2202
April1st to 15thDay
44%8%16%22%7%
2149
16th to endDay
33%9%18%26%10%1
2290
May1st to 15thDay
29%6%14%33%14%2
2511
16th to endDay
36%8%20%25%9%
2619
June1st to 15thDay
29%7%14%30%14%3
2520
16th to endDay
24%5%13%32%19%4
2520
July1st to 15thDay
13%38%25%31%16%
2371
16th to endDay
13%26%22%34%20%
2490
August1st to 15thDay
12%310%28%33%12%
2466
16th to endDay
15%49%28%33%7%
2357
September1st to 15thDay
16%39%27%32%9%
2160
16th to endDay
26%7%15%24%18%6%
2010
October1st to 15thDay
33%7%18%26%12%1
1800
16th to endDay
40%7%17%23%10%
1664
November1st to 15thDay
46%10%17%19%41
1500
16th to endDay
53%10%15%15%5%
1440
December1st to 15thDay
53%8%17%15%5%
1440
16th to endDay
53%7%16%16%4
1536
January1st to 15thNight
57%10%14%12%4
2506
16th to endNight
56%9%17%12%2
2688
February1st to 15thNight
48%9%21%15%5%
2297
16th to endNight
41%10%18%22%6%
1960
March1st to 15thNight
42%9%19%20%7%
2127
16th to endNight
37%11%21%24%4
2022
April1st to 15thNight
35%8%20%25%10%
1799
16th to endNight
29%7%20%31%10%1
1665
May1st to 15thNight
27%415%34%16%2
1440
16th to endNight
30%7%21%28%10%
1496
June1st to 15thNight
24%8%18%31%13%2
1440
16th to endNight
21%416%35%17%4
1440
July1st to 15thNight
10%312%27%32%13%
1360
16th to endNight
7%37%26%32%22%
1416
August1st to 15thNight
8%310%30%32%14%
1468
16th to endNight
12%311%31%31%9%
1844
September1st to 15thNight
12%211%28%33%11%
1800
16th to endNight
22%6%16%28%19%7%
1947
October1st to 15thNight
31%5%14%30%15%1
2160
16th to endNight
37%9%16%25%10%
2228
November1st to 15thNight
42%11%17%19%6%1
2359
16th to endNight
49%9%14%18%7%
2520
December1st to 15thNight
50%10%16%18%3
2520
16th to endNight
56%7%16%13%5%
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%311%16%35%6%
60
16th to endDay
28%6%14%29%18%3
64
February1st to 15thDay
17%11%35%23%10%1
227
16th to endDay
14%17%33%24%8%1
227
March1st to 15thDay
18%20%35%21%3
270
16th to endDay
20%26%31%16%5%
288
April1st to 15thDay
10%29%30%20%5%3
269
16th to endDay
8%33%32%13%8%4
270
May1st to 15thDay
14%21%28%27%7%1
288
16th to endDay
8%14%28%27%16%3
330
June1st to 15thDay
7%18%33%23%13%2
315
16th to endDay
7%20%33%22%12%2
315
July1st to 15thDay
211%32%35%16%1
305
16th to endDay
213%34%30%14%3
327
August1st to 15thDay
39%30%32%18%6%
315
16th to endDay
211%25%37%18%5%
332
September1st to 15thDay
316%24%29%19%6%
315
16th to endDay
5%17%27%32%15%1
315
October1st to 15thDay
13%16%30%26%10%1
315
16th to endDay
18%5%27%32%13%1
172
November1st to 15thDay
6%13%35%38%6%
76
16th to endDay
20%10%23%20%21%5%
60
December1st to 15thDay
16%11%30%33%8%
60
16th to endDay
10%323%39%20%3
64
January1st to 15thNight
17%6%26%28%17%2
720
16th to endNight
17%422%33%19%3
768
February1st to 15thNight
17%5%25%33%15%2
704
16th to endNight
10%9%28%32%16%2
632
March1st to 15thNight
12%8%30%35%11%1
720
16th to endNight
8%13%33%31%11%1
768
April1st to 15thNight
8%12%35%29%12%1
719
16th to endNight
9%12%37%25%12%2
585
May1st to 15thNight
7%14%32%31%12%2
360
16th to endNight
7%8%35%33%13%1
375
June1st to 15thNight
6%11%43%28%9%
360
16th to endNight
315%45%21%13%
360
July1st to 15thNight
210%35%37%13%
340
16th to endNight
111%40%34%9%1
376
August1st to 15thNight
15%33%38%16%4
396
16th to endNight
27%31%29%22%6%
698
September1st to 15thNight
39%25%28%25%8%
720
16th to endNight
5%8%26%30%23%6%
716
October1st to 15thNight
9%7%27%32%19%3
720
16th to endNight
11%6%27%32%18%4
708
November1st to 15thNight
13%8%29%35%12%1
700
16th to endNight
14%6%29%27%16%5%
720
December1st to 15thNight
15%7%26%35%13%1
720
16th to endNight
20%8%27%32%11%
768
Page generated at 2017-10-17 13:35:26. Version ac04

Copyright 2017 A. Danko


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