Washington
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Washington clear sky chart from 2005-11-05 to 2018-02-22.

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%
15879
21%
48929
12%
15840
49%
15840
25%
30436

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
24%
777
23%
2623
9%
776
41%
776
21%
1594
January16th to endNight
23%
832
24%
2704
10%
832
41%
832
22%
1660
February1st to 15thNight
24%
763
19%
2482
7%
761
41%
761
21%
1534
February16th to endNight
18%
659
25%
1994
6%
659
41%
659
23%
1246
March1st to 15thNight
25%
720
22%
2010
9%
720
39%
720
25%
1278
March16th to endNight
23%
768
21%
2085
8%
768
34%
768
24%
1392
April1st to 15thNight
26%
718
20%
1796
5%
718
35%
718
27%
1180
April16th to endNight
37%
720
20%
1800
7%
720
43%
720
30%
1192
May1st to 15thNight
36%
565
18%
1645
8%
565
43%
565
23%
1106
May16th to endNight
53%
374
17%
1497
15%
374
60%
374
23%
878
June1st to 15thNight
52%
360
19%
1440
19%
360
61%
360
25%
818
June16th to endNight
51%
360
17%
1440
16%
360
59%
360
25%
824
July1st to 15thNight
56%
340
13%
1360
15%
340
59%
340
19%
858
July16th to endNight
70%
546
17%
1577
14%
513
75%
513
37%
898
August1st to 15thNight
65%
539
18%
1609
14%
536
65%
536
31%
987
August16th to endNight
59%
761
19%
1907
12%
761
60%
761
32%
1273
September1st to 15thNight
55%
720
25%
1800
14%
720
59%
720
34%
1192
September16th to endNight
55%
717
24%
1971
20%
717
58%
717
30%
1292
October1st to 15thNight
46%
720
23%
2160
20%
720
53%
720
27%
1385
October16th to endNight
37%
768
31%
2304
18%
768
53%
768
33%
1318
November1st to 15thNight
37%
760
24%
2418
15%
760
51%
760
28%
1441
November16th to endNight
33%
780
24%
2665
13%
780
49%
780
24%
1545
December1st to 15thNight
28%
780
18%
2730
8%
780
42%
780
19%
1724
December16th to endNight
23%
832
22%
2912
8%
832
40%
832
18%
1821

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
13%5%25%435%5%5%7%41%
49314
Night
14%5%2442445%7%43%
49111
Cloud CoverDay
14%5%2443445%7%42%
49324
Night
14%5%2442445%7%44%
49115
TransparencyDay
39%6%11%23%16%2
49014
Night
38%5%11%23%17%3
48929
SeeingDay
17%10%25%25%15%6%
6169
Night
17%10%28%29%13%1
15884

By Month

Percentage Cloud Cover NE (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
16%4244245%46%42%
1643
16th to endDay
13%7%25%5%24347%42%
1872
February1st to 15thDay
15%6%25%6%245%5%7%39%
1710
16th to endDay
14%7%25%5%24446%41%
1624
March1st to 15thDay
15%5%25%5%24345%45%
1939
16th to endDay
16%6%35%5%25%336%41%
2139
April1st to 15thDay
13%5%25%42435%6%45%
2152
16th to endDay
17%42442335%5%44%
2160
May1st to 15thDay
16%6%25%5%345%6%8%35%
2313
16th to endDay
14%5%245%35%5%5%8%38%
2618
June1st to 15thDay
11%4245%25%6%6%9%41%
2520
16th to endDay
9%4245%35%5%6%10%40%
2520
July1st to 15thDay
9%425%5%36%6%7%10%37%
2520
16th to endDay
7%4145%47%7%8%11%37%
2482
August1st to 15thDay
11%6%26%6%36%6%7%9%33%
2328
16th to endDay
11%4145%46%6%7%8%39%
2292
September1st to 15thDay
13%5%24435%46%7%43%
2160
16th to endDay
14%6%25%4245%6%7%41%
1986
October1st to 15thDay
18%423323446%45%
1800
16th to endDay
14%42432445%5%49%
1920
November1st to 15thDay
14%6%2332345%7%46%
1770
16th to endDay
15%5%23324446%48%
1622
December1st to 15thDay
15%5%144245%5%7%43%
1560
16th to endDay
18%6%25%324446%40%
1664
January1st to 15thNight
17%5%24414347%43%
2623
16th to endNight
16%6%24324446%44%
2704
February1st to 15thNight
15%6%145%2445%6%43%
2480
16th to endNight
14%6%25%324346%45%
1994
March1st to 15thNight
16%425%32435%7%44%
1998
16th to endNight
15%6%35%5%2345%6%41%
2085
April1st to 15thNight
14%5%244245%5%6%44%
1796
16th to endNight
16%424325%45%8%42%
1800
May1st to 15thNight
18%7%34424447%39%
1645
16th to endNight
12%5%234245%7%10%41%
1495
June1st to 15thNight
10%424325%5%7%9%44%
1440
16th to endNight
8%415%6%245%6%10%42%
1440
July1st to 15thNight
7%436%7%36%6%7%8%36%
1440
16th to endNight
6%313325%7%9%12%44%
1684
August1st to 15thNight
9%325%5%2447%8%44%
1620
16th to endNight
9%5%245%35%5%5%9%42%
1907
September1st to 15thNight
12%5%24425%45%6%45%
1800
16th to endNight
12%6%245%345%5%7%42%
1971
October1st to 15thNight
18%5%24423345%44%
2160
16th to endNight
13%323323346%52%
2304
November1st to 15thNight
14%5%24423335%50%
2418
16th to endNight
15%41342345%7%47%
2665
December1st to 15thNight
16%5%25%5%3445%6%38%
2730
16th to endNight
16%5%25%5%2445%6%41%
2912

Percentage Cloud Cover SE (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
17%5%2442445%7%42%
1643
16th to endDay
14%7%25%424346%43%
1872
February1st to 15thDay
16%7%25%5%2445%6%40%
1710
16th to endDay
14%7%25%433436%42%
1624
March1st to 15thDay
15%6%15%5%24346%45%
1950
16th to endDay
16%6%25%5%34346%40%
2138
April1st to 15thDay
13%435%33445%7%45%
2152
16th to endDay
18%4245%23345%45%
2160
May1st to 15thDay
17%5%25%425%46%8%37%
2313
16th to endDay
14%424445%5%6%7%39%
2618
June1st to 15thDay
12%3244345%6%9%41%
2520
16th to endDay
9%3245%35%6%6%10%41%
2520
July1st to 15thDay
9%425%5%35%6%7%9%38%
2520
16th to endDay
7%3245%37%6%7%12%38%
2482
August1st to 15thDay
11%5%25%7%46%6%6%9%35%
2328
16th to endDay
11%4245%36%6%7%8%39%
2292
September1st to 15thDay
13%41443445%8%44%
2160
16th to endDay
14%5%244246%47%40%
1986
October1st to 15thDay
17%5%1342335%5%45%
1800
16th to endDay
14%414323445%49%
1920
November1st to 15thDay
15%5%2342435%7%46%
1770
16th to endDay
14%5%14323446%48%
1622
December1st to 15thDay
15%41442445%6%44%
1560
16th to endDay
18%6%15%434446%40%
1664
January1st to 15thNight
17%5%24323447%44%
2623
16th to endNight
16%6%24424446%44%
2704
February1st to 15thNight
14%6%23425%446%44%
2473
16th to endNight
15%5%24324347%45%
1994
March1st to 15thNight
16%425%323347%44%
2010
16th to endNight
16%5%26%5%2435%6%41%
2085
April1st to 15thNight
14%425%4335%5%6%44%
1796
16th to endNight
17%42432445%7%44%
1800
May1st to 15thNight
18%5%35%42445%6%39%
1644
16th to endNight
12%6%234245%7%9%41%
1495
June1st to 15thNight
10%424325%6%7%8%45%
1440
16th to endNight
8%315%5%3447%9%43%
1440
July1st to 15thNight
8%436%6%36%6%7%9%37%
1440
16th to endNight
6%322325%6%9%11%45%
1684
August1st to 15thNight
9%425%5%346%6%8%44%
1620
16th to endNight
10%5%245%35%46%8%42%
1907
September1st to 15thNight
12%5%2442446%7%45%
1800
16th to endNight
12%6%25%43445%8%42%
1971
October1st to 15thNight
18%5%24423345%45%
2160
16th to endNight
13%413313346%52%
2304
November1st to 15thNight
15%424423345%50%
2418
16th to endNight
15%4134235%5%6%47%
2665
December1st to 15thNight
15%6%25%43446%5%39%
2730
16th to endNight
17%5%24424446%41%
2912

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
41%5%12%22%14%4
1644
16th to endDay
41%411%24%14%3
1872
February1st to 15thDay
43%410%24%13%2
1712
16th to endDay
42%410%26%14%1
1624
March1st to 15thDay
41%410%24%16%3
1950
16th to endDay
44%410%19%17%3
2139
April1st to 15thDay
38%49%24%18%4
2152
16th to endDay
39%410%18%20%6%
2160
May1st to 15thDay
43%7%10%20%15%2
2313
16th to endDay
40%7%13%23%12%2
2622
June1st to 15thDay
36%9%11%26%14%1
2520
16th to endDay
35%8%14%21%17%2
2520
July1st to 15thDay
38%8%13%20%15%3
2371
16th to endDay
35%9%14%23%15%2
2326
August1st to 15thDay
43%7%11%20%16%
2315
16th to endDay
39%8%12%21%15%2
2292
September1st to 15thDay
36%5%12%21%20%3
2160
16th to endDay
40%6%8%23%19%2
1986
October1st to 15thDay
39%5%9%25%17%3
1800
16th to endDay
35%38%28%22%2
1920
November1st to 15thDay
37%411%23%17%5%
1770
16th to endDay
36%39%28%19%2
1622
December1st to 15thDay
39%5%13%25%12%3
1560
16th to endDay
44%411%25%12%1
1664
January1st to 15thNight
40%410%21%18%5%
2623
16th to endNight
40%49%26%18%1
2704
February1st to 15thNight
40%411%24%16%3
2482
16th to endNight
39%49%26%18%2
1994
March1st to 15thNight
38%411%26%14%2
2010
16th to endNight
42%5%10%21%16%3
2085
April1st to 15thNight
38%5%9%20%21%3
1796
16th to endNight
38%5%10%20%17%7%
1800
May1st to 15thNight
44%6%11%20%14%3
1645
16th to endNight
35%8%17%20%14%2
1497
June1st to 15thNight
33%8%12%27%16%1
1440
16th to endNight
34%7%15%21%17%2
1440
July1st to 15thNight
41%8%14%17%12%4
1360
16th to endNight
27%7%17%27%17%1
1577
August1st to 15thNight
34%7%14%23%18%1
1609
16th to endNight
37%7%11%24%16%3
1907
September1st to 15thNight
37%6%10%22%20%4
1800
16th to endNight
39%6%8%22%20%3
1971
October1st to 15thNight
41%47%24%18%3
2160
16th to endNight
31%38%29%22%3
2304
November1st to 15thNight
37%39%23%20%6%
2418
16th to endNight
35%5%10%24%20%4
2665
December1st to 15thNight
44%412%22%14%2
2730
16th to endNight
42%5%12%24%12%2
2912

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
20%24%36%14%3
111
16th to endDay
18%29%29%19%3
144
February1st to 15thDay
30%20%40%7%2
135
16th to endDay
20%17%40%20%2
245
March1st to 15thDay
18%17%32%23%6%
270
16th to endDay
20%13%37%20%5%2
288
April1st to 15thDay
16%16%31%22%10%2
270
16th to endDay
20%10%26%25%10%5%
270
May1st to 15thDay
26%18%22%18%11%3
288
16th to endDay
19%422%30%18%4
331
June1st to 15thDay
14%8%22%28%16%9%
315
16th to endDay
11%428%33%16%4
315
July1st to 15thDay
14%120%35%21%5%
305
16th to endDay
8%115%34%30%10%
326
August1st to 15thDay
15%115%31%24%10%
313
16th to endDay
9%114%34%25%14%
332
September1st to 15thDay
13%5%20%25%17%17%
315
16th to endDay
20%316%28%17%13%
314
October1st to 15thDay
24%5%18%22%23%6%
315
16th to endDay
13%12%30%25%15%2
336
November1st to 15thDay
17%10%27%24%13%4
293
16th to endDay
13%17%38%19%9%1
152
December1st to 15thDay
16%27%23%16%12%3
90
16th to endDay
29%19%31%16%3
96
January1st to 15thNight
20%15%35%23%5%
780
16th to endNight
19%16%36%25%2
832
February1st to 15thNight
20%15%33%28%1
764
16th to endNight
19%20%37%20%1
659
March1st to 15thNight
20%13%36%25%3
720
16th to endNight
18%14%40%20%5%1
768
April1st to 15thNight
16%16%36%24%4
718
16th to endNight
19%9%29%30%11%
720
May1st to 15thNight
21%8%28%29%11%
565
16th to endNight
13%5%20%39%19%1
375
June1st to 15thNight
10%228%36%20%1
360
16th to endNight
10%323%43%17%
360
July1st to 15thNight
9%221%40%25%
340
16th to endNight
7%115%41%31%3
546
August1st to 15thNight
11%111%42%29%1
539
16th to endNight
13%215%35%29%3
761
September1st to 15thNight
14%219%32%27%3
720
16th to endNight
14%318%33%26%4
717
October1st to 15thNight
19%6%20%27%21%4
720
16th to endNight
15%11%32%25%14%
768
November1st to 15thNight
16%9%31%28%12%1
760
16th to endNight
17%14%30%26%10%
780
December1st to 15thNight
20%11%32%26%8%
780
16th to endNight
21%12%36%26%3
832
Page generated at 2018-02-23 11:21:35. Version ac04

Copyright 2018 A. Danko


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